Effect of Deposition Temperature in RF Sputtered ZnO Thin Films on ZnO TFT Performance

Effect of Deposition Temperature in RF Sputtered ZnO Thin Films on ZnO TFT Performance | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effect of Deposition Temperature in RF Sputtered ZnO Thin Films on ZnO TFT Performance Sasikala Muthusamy, Sudhakar Bharatan, Sinthamani Sivaprakasam, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4599511/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract ZnO thin films are deposited using RF magnetron sputtering by varying argon: oxygen gas flow rates and substrate temperatures. The structural and optical characterization of ZnO thin films are systematically carried out using X-ray diffraction (XRD), SEM, UV-visible spectroscopy and X-ray photoelectron spectroscopy (XPS). Dominant (002) Grazing incidence (GI) XRD peak on samples deposited at 300°C with Ar:O 2 (16:4) ratio suggest c-axis orientation both on the bulk and surface of ZnO thin film. Increase in the crystallite grain size were observed as the deposition temperature is increased from Room temperature (RT) to 300°C, leading to the reduction in grain boundaries. Absorption analyses show the reduction in band-tail states within the bandgap, supporting annihilation of defects, on the samples deposited at 250°C and 300°C. XPS spectra confirm the improved O 2 incorporation and reduction in oxygen vacancies in sample deposited at 300°C. Highest hall mobility of 46.09 cm 2 /V-sec has been observed on sample deposited at RT, and is dominated by defects. Whereas, films deposited at 250°C and 300°C exhibit Hall bulk mobilities of 20.43 cm 2 /V-sec and 31.63 cm 2 /V-sec, respectively. Further, bottom-gate ZnO thin film transistors (TFTs) are also fabricated on SiO 2 /p-Si substrate. Variation in substrate temperature showed performance enhancement in terms of leakage current, threshold voltage, sub-threshold swing and I ON /I OFF ratio. Devices deposited at 300°C resulted in O 2 -rich surface through chemisorption, which led to the reduction in leakage current of upto 10 -12 A and 10-fold reduction in sub-threshold swing from 30V to 2.8V. Highest field-effect mobility of 1.1 cm 2 /V-sec has been achieved when the ZnO thickness in the TFT is reduced to 50 nm. RF sputtering ZnO X-Ray diffraction X-ray photoelectron Spectroscopy Thin film transistor Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Introduction Enormous research efforts on metal oxide semiconductors have been carried out in the recent past, due to which, it has become a promising candidate in the field of solar cells [1–4], thin film transistors [5–8], photodetectors [9–11], memory [12–13], gas sensors [14–17], bio sensors [18–20] and CMOS circuits [21,22]. Key properties such as wide bandgap, high mobility, transparency, tunability and substrate compatibility play a vital role in deciding the applications of the metal oxide semiconductor material system. In most of the traditional electronic applications, Si based TFTs are widely used in pixel and peripheral driver circuits, because of its ease of integration with CMOS. The advancements in the field of IOT, AI and ML, requires the large memory and high bandwidth, which leads to the scaling down of devices and that would be possible only in 3D integration of devices. Even though Si technology has the advanced manufacturing processes, it suffers due to poor mobility, bias stress, threshold voltage instability and reliability issues. Hence, metal oxide based TFTs are the potential alternative to Si-based TFTs in the areas of flexible display, health care, environment and automotive applications. Process level advantage of oxide semiconductor is the ability to realize large area device properties on any substrate using a non-equilibrium deposition process such as sputtering. Among the various metal oxide semiconductor cations, Zn and Sn are considered non-toxic and abundantly available elements. ZnO with its wide band gap (3.37eV), low cost, excellent electrical and optical properties, makes it a promising candidate for numerous applications in the electronics field. Traditionally, ZnO thin films are deposited using a variety of processes, such as spin coating [23], spray analysis [24, 25], Pulsed laser deposition [26], Molecular beam epitaxy [27,28], Atomic layer deposition [29, 30] and RF sputtering [31,32,33]. Highest mobility of 300 cm 2 /V-sec, has been achieved [28] in Mg doped ZnO heterostructures using Molecular beam epitaxy system which require ultra-high vacuum, leading to high cost. However, RF sputtering method provides versatility, scalability, uniformity and high quality thin films in controlled environment even at low temperature, and has paved the way for large area and low cost device applications. Even though the mobilities of RF sputtered ZnO thin films have reached the of range 70 cm 2 /V-sec, defects in the ZnO thin films play the crucial role in realizing high quality thin film transistor [34, 35] especially in flexible display, optoelectronic and gas sensing applications. Oxygen vacancies, interstitials, grain boundaries, dislocations, surface defects are reported to play a major role in forming n-type semiconductor [36, 37]. Among these, oxygen vacancies can have significant impact; they can act as shallow donors and/or deep acceptors depending on the charge state, which significantly affects the optical and electrical properties. Advanced characterization techniques like X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and UV-visible spectroscopy can be employed to understand and manipulate the defects in thin film optimization. Singh S et al. reported field effect mobility of 0.6134 cm 2 /V-sec and threshold voltage of 3.1V in their ZnO TFT using RF sputtering with SiO 2 as gate insulator [32]. Similarly, Jong Hoon Lee et al. fabricated ZnO TFT with MgO as gate insulator and reported lower field-effect mobility of 0.0235 cm 2 /V-sec, I ON /I OFF ratio of ∼10 5 , threshold voltage of 2.2V and SS value of 1.18 V/decade [33]. Brandon Walker et al. compared the performance of ZnO TFT with various gate dielectric materials namely, Al 2 O 3 , HfO 2 and ZrO 2 , and achieved the highest on/off ratio of greater than 10 5 [38]. B. -S. Wang et al., achieved the mobility of 84.22 cm 2 /V-sec and I ON /I OFF ratio of 3 × 10 6 by means of MgZnO/ZnO heterostructure TFT. Thus the presence of native defects in ZnO material system such as O 2 vacancies and interstitials, significantly affects the threshold voltage, which is crucial for the realization of a normally-off device (enhancement type transistor) [39]. Hence, in this paper, the optimization of ZnO thin film is systematically carried out using RF sputtering technique with different argon: oxygen flow rate and different substrate temperature. Various properties of ZnO thin films are characterized by X-ray diffraction, XPS, Hall measurement, SEM imaging and UV-visible spectrophotometer. Based on these properties, ZnO TFTs were fabricated with different substrate temperatures and were electrically characterized. Various parameters such as threshold voltage, ON/OFF ratio, field effect mobility and subthreshold swing were determined for different W/L ratios of TFT. Experimental Details Deposition of ZnO Thin films ZnO thin films are deposited using RF magnetron sputtering on p-Si substrate. Prior to ZnO deposition, substrates were cleaned using RCA1 and RCA2 methods to remove organic and metal contaminations, followed by HF dip to remove the native oxide. During ZnO deposition, target to substrate distance was maintained at 7.5 cm and chamber was evacuated to 5.5 x 10 − 6 mbar. In order to maintain the contamination free source material, the target was pre-sputtered for 10 mins prior with the shutter closed. In order to optimize the Ar/O 2 gas flow rate, a series of films are deposited at room temperature. Samples A, B and C represents 180 nm thick ZnO films deposited at different Ar:O2 flow rates as described in Table 1 . Sample A, B and C represents films deposited at room temperature followed by post-deposition annealing at 300⁰C in N 2 ambient, with Ar:O 2 flow rates of 20:0, 18:2, and 16:4, respectively. The gas flow rates are measured using the Aalborg mass flow controllers. Upon completion of the flow rate optimization, deposition temperature optimization was carried out at room temperature (RT), 250°C and 300°C. Sample C, D and E represents 180 nm thick ZnO deposited at RT, 250°C and 300°C, respectively, with a constant Ar:O 2 flow rate ratio of 16:4. Deposition rates of ZnO films deposited at RT, 250°C and 300°C, are determined to be 3 nm/min, 6 nm/min and 7 nm/min, respectively, by step profile measurements using Bruker profilometer (Fig S1 ). Table 1 shows the ZnO thin film deposition temperature, Ar/O 2 flow rate and annealing temperatures. Table 1 Deposition parameters of ZnO thin films Sample Deposition Temperature Ar/O 2 flow rate Annealing temperature A RT 20/0 N2 ambient 300°C B RT 18/2 N2 ambient 300°C C RT 16/4 N2 ambient 300°C D 250°C 16/4 - E 300°C 16/4 - Grazing Incidence (GI) θ/2θ X-ray diffraction was carried out on Samples A, B, C, D and E using Bruker/PAN Analytical X-ray Diffractometer. Based on the FWHM of (002) XRD peak, grain size, strain and dislocation density values are determined. Hall measurements were also carried out on the above samples at a magnetic field of 0.51T using Ecopia Van der Pauw HMS 3000 system. Surface morphology of ZnO thin films was studied using Zeiss ULTRA 55 scanning electron microscopy (SEM) system on all the above samples. X-ray photoelectron spectroscopy (XPS) analysis of the ZnO thin films was performed using a Kratos Axis Ultra spectrometer, employing a monochromatic Al-Kα source. Absorption edge of all the above samples are derived from the transmission data measured in UV-1650PC Shimadzu spectrophotometer. Device fabrication and characterization Commercially available SiO 2 /Si substrate with oxide thickness of 100 nm is used for thin film transistor device fabrication. With the optimized flow rate of 16:4 (Ar:O 2 ) and 60W RF power, thin film transistors C1, D1 and E1 were fabricated with deposition temperatures as listed in Table 2 . Post-deposition annealing was carried out only on Sample C1 at 300°C in N 2 ambient. Table 2 Deposition parameters and Hall performance of ZnO TFT Sample Deposition Temperature Annealing temperature and ambient Ar/O 2 flow rate Post contact annealing Carrier concentration (cm − 3 ) C1 RT 300°C in N 2 ambient 16/4 220°C 2.6 \(\times\) 10 17 D1 250°C - 1 \(\times\) 10 16 E1 300°C - 1.6 \(\times\) 10 17 Mask for patterning the channel and electrode layer was written on the chrome glass using the mask writer (Fig S2). Exposure and pattering were carried on Karl SussMA6-BA6 mask aligner. Thin film transistor (TFT) fabrication was carried in two step lithographic process. In the first lithography process, 300µm x 300µm ZnO mesa structure was created by wet etching process. In the second lithography process, the standard lithography technique is used to pattern the source and drain electrodes. 100 nm Al metal was deposited at room temperature using thermal evaporation method, followed by lift-off process, to get the patterned Source/Drain. Finally, back side SiO 2 was selectively removed using HF dip, and blanket Al metal (100 nm) as gate electrode was deposited by thermal evaporation to get the final fabricated device (Fig. 1 ). In order to improve the contact resistance, all the devices were annealed at 220°C in N 2 ambient for 10 mins. Electrical properties of ZnO TFTs with Width to Length (W/L) ratio of 50µm/50µm were examined using I-V transfer and output characteristics. I-V characteristics were carried out on Summit 11000B-M (Cascade Microtech) precision 4-axis semi-automated Probe station platform. Results and discussion X-Ray Diffraction (XRD) Figure 2 represents the grazing incident x-ray diffraction of Samples A, B and C, deposited at varying Ar/O 2 flow rates. Sample A showed high intensity (002) XRD peak at 34.47° and even higher (103) peak at 62°. As the O 2 ratio is increased, the overall intensity of ZnO (002) peak has decreased with the suppression of (103) ZnO peak in Samples B and C. The crystallite size, dislocation density and micro-strain where calculated using equations ( 1 – 3 ) and are listed in Table 3 . $$D=\frac{0.9\lambda }{\beta Cos\theta }$$ 1 $$\delta =\frac{1}{{D}^{2}}$$ 2 $$\epsilon =\frac{\beta }{4\text{tan}\theta }$$ 3 Where λ is the wavelength of X-Ray (1.54Ȧ), θ the Bragg’s Diffraction angle and β is the full width of half maximum (FWHM) of the peak, δ is the dislocation density, D is the representation crystallite size. Table 3 ZnO Thin Films parameter extracted from XRD Sample FWHM (β) (radians) 2θ (deg) Crystallite size D (nm) Dislocation density𝛿 ( lines/m 2 ) lattice parameter spacing (nm) Micro Strain ε A 0.57 34.53 14.596 0.0047 0.260 0.0008 B 0.7 34.25 11.876 0.0071 0.262 0.0009 C 0.76 34.3 10.940 0.0084 0.261 0.0010 D 0.6 34.31 13.858 0.0052 0.261 0.0008 E 0.73 34.15 11.385 0.0077 0.262 0.0010 Figure 3 compares the GIXRD of Samples C, D and E deposited at RT, 250°C and 300°C, respectively. By maintaining the flow rate at 16:4, Samples D and E exhibited notable improvement in (002) peak as compared to the Sample C, indicative of improvement in crystalline quality. All the samples except A sample exhibit preferred orientation along the (002) plane. Hence, all our TFT devices were grown at the optimized gas flow ratio of 16: 4 (Ar:O 2 ) at room temperature, 250°C and 300°C. It may be noted that Sample D exhibits polycrystalline property with distinct peaks at (002), (101), (102), (110), (103) and (112). Table 3 lists the crystalline size from the FWHM values of XRD spectrum. Sample A exhibited the lowest (002) XRD FWHM and highest crystalline size, whereas Sample C exhibited the highest FWHM and lowest crystalline size. Even though Sample A exhibits lowest strain in the microstructure, the presence of dominant (103) XRD peak suggests that the surface is affected by the non-equilibrium growth condition of RF sputtering. Similar (103) ZnO GIXRD peaks are reported by various groups [40]. At higher deposition temperatures of 250°C and 300°C in Samples D and E, the grain size increased to 13.85 nm and 11.38 nm, respectively. Additionally, reduction in dislocation density and micro strain has been observed in Samples D and E, suggesting overall improvement in the crystal quality. Figure S3 shows the conventional x-ray diffraction spectra of Sample C, which exhibits dominant (002) ZnO peak at 34°, and a relatively low intensity (103) peak at 62°, indicative of preferential (002) crystal formation in the bulk layer. The intensity ratio of (002)/(103) peak has substantially increased from 3 (GIXRD) to 9.52 (Conventional XRD), indicative of c-axis orientation in the bulk layer. The sharp (002) peak in conventional XRD is the evidence for c-axis orientation, and relatively higher (103) peak in GIXRD is the evidence of surface re-structuring during the final stages of RF sputtering. Usually, (103) XRD peak are reported in the ZnO thin films prepared by electro-deposition [41] and sol-gel [42] processes. However, RF sputtering being a non-equilibrium growth technique, during the final stages of deposition, the re-orientation of atoms by diffusion may happen on the surface, which could be the reason for the presence of dominant (103) peak. Yunlan Wang et al. [40] reported in their RF sputtered ZnO thin films that the appearance of (103) GIXRD orientation is due to native property of ZnO. It is observed that Sample A has the highest (103) peak intensity compared to any other sample. This could be attributed to the relatively low oxygen reactive species, and making the Zn atoms to move freely along the surface. As the O 2 flow rates are increased in Samples B, C, D and E, the influx of O 2 species suppressed the free movement or diffusion of Zn atoms leading to low intensity (103) peak. Hence, O 2 over-pressure during the sputtering process becomes vital in depositing smooth surface. SEM Figure 4 a-e. show the SEM images of ZnO films A, B, C, D, and E respectively. Sample A deposited at an Ar/O 2 flow rate of 20/0 shows larger grain size with distinct contours. The presence of bigger grain crystalloids in SEM image corroborates with the calculated grain size of 14.59 nm for Sample A based on the XRD data. As the O 2 ambient was introduced at 2 sccm, the grain size started to decrease with highly dense surface morphology in Sample B. Both the samples exhibit granular and void free surface with high packing density. With further increase in O 2 flow rate to 4 sccm in Sample C, grain size has nominally remained the same as Sample B. However small voids between the grains started to appear as pointed in the Fig. 4 c. Both samples B and C, with higher O 2 content, distinct and isolated grains are formed and more structured morphology was observed. The voids disappear when the films are deposited at 250⁰C and 300⁰C as observed in Samples D and E, respectively. The conventional XRD (Fig.S3) shows dominant (002) XRD peak orienting along the c-axis, while the GIXRD spectra (Fig. 3 ) shows multiple XRD peaks on the surface of Sample D matching the asymmetric contours in the corresponding SEM image (Fig. 4 d). Optical Characterization The Tauc plot (Fig. 5 ) is obtained between (αhν) 2 and band gap(E g ) to determine the absorption edge of the ZnO thin film Samples C, D and E. All the samples C, D and E show bulk-like absorption edge, attesting good quality ZnO thin films. Sample C deposited at RT exhibits absorption edge at 3.28 eV with comparatively low absorption. Whereas, Sample D deposited at 250°C shows a redshift in the bandgap of 3.25 eV with relatively higher absorption at higher energy levels attesting to possible absence of high energy band states above the conduction band minimum. Sample E deposited at 300°C shows further redshift to 3.15 eV with faster roll-off. Redshift in the absorption edge from 3.28 eV to 3.25 eV to 3.15 eV has been observed as the deposition temperature is changed from RT to 250°C and 300°C, respectively, which may be attributed increase in the grain size observed in SEM image and XRD calculation, leading to the increase in the tensile stress [43]. Intrinsically, ZnO thin films have both oxygen vacancies and Zn interstitials exhibiting n-type conductivity [44]. The observed absorption spectra of our ZnO thin films can be separated into the following transitions: (i) ZnO band-to-band transition; (ii) transitions to localized band states (widely known as Urbach tail) [45]. There are several reports on Urbach tail states in ZnO thin films, due to the structural disorders. Several groups have reported that shallow donors (oxygen vacancies) and deep acceptors (Zn interstitials) act as the crystal disorders. [46, 47]. In order to delineate various sub-bands of our samples, natural log of the absorption edge and the bandgap was plotted [48]. Figure 6 shows the plot between ln (α) and bandgap for samples C, D and E. Sample C exhibits Urbach tail bands at 2.8 eV and 1.53 eV, indicating the presence of localized defects. Our Room temperature hall data shows a high carrier concentration of 2.6 x 10 17 cm −3 for Sample C (Table 2 ), which could be attributed to the presence of defect-induced transport. All our hall measurements showed that ZnO thin films are n-type based on the hall coefficient. Sample C shows the higher mobility of 46.09 cm 2 /V-sec, whereas Sample D deposited at 250°C exhibited a hall bulk mobility of 20.43 cm 2 /V-sec, and Sample E1 exhibited an intermediate mobility of 31.63 cm 2 /V-sec. As shown in Fig. 6 , Sample D exhibits a weak band-tail at 2.42 eV and a strong tail state at 1.87 eV. Samples C and D shows more than one band-tail states, which could be attributed to possible presence of the combination of oxygen vacancies and Zn interstitials leading to deep donor and acceptor levels within the bandgap. Whereas, Sample E shows band-to-band absorption at 3.14 eV and a weak tail state at 2.47 eV representative of possible direct band transition and band-to-tail transition. Presence of only one transition shows that the sample E deposited at 300°C resulted in reduction of vacancy and interstitial related defects and the presence of shallow tail-band within the bandgap. The reduction in the tail states has led to the higher absorption in Sample D and even higher absorption in Sample E, attesting to the annihilation of defects. Yet, Samples D and E exhibits a slightly lower mobility as compared to defect induced transport in Sample C. Our SEM image shows the coalescing nature of ZnO denoting the wrinkled nature of the surface, which is also confirmed by the polycrystalline property from XRD spectrum (Fig. 3 ). The reason for the drop in the Hall mobility will be analyzed from the XPS results. Even though the Sample D exhibits a mobility of 20.43 cm 2 /V-sec, as compared to Sample C, our values are comparable to the other ZnO thin films in the literature [49]. Further improvement in the bulk mobility of Sample E has been observed, which is the highest among the other reports [50]. XPS XPS analyses on the Samples C, D and E describes the origin of oxygen related defects and vacancies when the samples are deposited at RT, 250°C and 300°C, respectively (Figs. 7 (a-c)). The O 1s peaks for various films are shown in Figs. 7 (a-c). XPS analyses on the Samples C, D and E describes the origin of oxygen related defects and vacancies when the samples are deposited at RT, 250°C and 300°C, respectively. O 1s peak is resolved into 2 peaks at ~ 530eV and ~ 532eV, representative of O 1s peak and oxygen vacancies, respectively. Sample C exhibits narrower O 1s peak at 530.3eV, and broader O 2 vacancy peak at 531.7 eV. Since Sample C which was deposited at RT and undergo post-deposition annealing at 300°C, the strain in the thin film has remained at 0.001% (Table 2 ), which explains the narrower O 1s peak. However, Sample C shows lower area (~ 58.62%) under the O 1s curve, indicative of lower O 2 concentration, as compared to Samples D (~ 61%) and E (~ 70%), which was deposited at higher temperatures (Table 2 ). Sample E1 exhibits substantial increase in the area under the O 1s curve to as compared to Sample C1 Alternatively, Samples D and E exhibits relatively broader O 1s peak, and narrower oxygen vacancy peak. In particular, Sample D shows a broad O 1s peak with FHWM of 1.6 eV, suggesting the presence of O 2 complexes in addition to the O 2 vacancies, which also supports the drop in the Hall mobility. Overall, our XPS data shows that depositing at higher temperatures 300°C assists in the better O 2 incorporation in the ZnO crystal. The voids in the SEM image (Fig. 4 c) and the presence of tail-bands at 2.8 eV and 1.53 eV in the tauc plot are clear evidences of high oxygen related defects in Sample C due to room temperature deposition. Additionally, annealing of Sample C at 300°C also did not help in the annihilation of defects. However, the percentage of O 2 vacancies in Samples D and E has reduced with a narrower O 2 vacancy peak resolved at 531.88 eV. Hence, based on the XPS analysis, it is clear that higher deposition temperatures not only enhances the O 2 incorporation, but also reduces the oxygen related defects. Table 4 Oxygen Vacancies obtained from XPS Sample O 2 vacancy O 1s peak Binding Energy (eV) Area FWHM Binding Energy (eV) FWHM Area C 531.77 41.38 1.99 530.354 1.25 58.62 D 532.38 39.1 1.54 530.43 1.6 60.9 E 531.88 30.26 1.57 529.88 1.7 69.74 Device Characterization Thin film transistors were fabricated on 100 nm SiO 2 /Si substrates with ZnO channel layer. TFT Device C1 was fabricated at room temperature followed by 300°C post deposition annealing (similar to Sample C thin film). And Devices D1 and E1 represent TFTs deposited at 250°C and 300°C, respectively. All the TFTs exhibit n-type behavior as represented by the I D – V D curves. The staggered bottom gate TFT structure has been fabricated. The drain current expressions of the enhancement mode TFT are: In Linear region $${I}_{D}=\frac{W}{L}{C}_{ox}{\mu }_{fe}\left[\left({V}_{GS}-{V}_{th}\right){V}_{DS}-\frac{1}{2}{V}_{DS}^{2}\right] \left(4\right)$$ Saturation region $${I}_{D}=\frac{W}{2L}{C}_{ox}{\mu }_{fe}{\left({V}_{GS}-{V}_{th}\right)}^{2} \left(5\right)$$ Field effect mobility: $${ \mu }_{FE}=\frac{{g}_{m}}{{C}_{ox}\frac{W}{L}{V}_{DS}} \left(6\right)$$ $$SS=\frac{{dV}_{GS}}{d\left(\text{log}{I}_{D}\right)} \left(7\right)$$ where W-Width of the channel, L-Length of the channel, C ox -Capacitance per unit area. Drain Characteristics The drain characteristics are measured for TFT devices with the same channel length and width of 50 µm. Gate-Source voltage (V G ) is varied from 0 to 40V in steps of 5V. At V G of 40V, Device C1 reaches the highest drain current of 0.6µA, whereas, Device D1 and E1 reaches a maximum drain current of 0.11µA. The variation in the drain currents match well with the variation of bulk hall mobility on Samples C, D and E, as represented in Table 5 . Since the thickness of these devices are 180 nm, bulk mobility dominates I D , where Sample C exhibits highest hall mobility of 46.09 cm 2 /V-sec corroborating with the high drain current of 0.6µA in Device C1 (Fig. 8 (a)). Transfer Characteristics Figures 9 (a-c) represents the transfer characteristics of the above TFT devices C1, D1 and E1 for W = 50 µm and L = 50 µm at V D = 5V. The gate voltage (V G ) is swept from − 40V to + 40V. The effect of deposition temperature on the transfer curves shows that all the TFTs operate in the enhancement mode. Table 5 displays the electrical parameters of TFTs deposited at different temperatures. The I off current for Device C1 is of the order of nanoampere (~ 1.2x10 − 10 A), indicative of a leaky device. The presence of voids between the crystallites observed in the SEM image, smaller crystallite size (10.94 nm), and grain boundaries, could be the reason for nano ampere range leakage current in Sample C. Even though the bulk hall mobility is determined to be the highest in Sample C, we speculate the high carrier concentration of 2.6x10 17 cm − 3 is predominantly defect induced transport. The defect is confirmed by the presence of two band-tail states in the absorption data (Fig. 6 .) at 1.53 eV and 2.8 eV. As the deposition temperature is increased to 250°C and 300°C in Devices D1 and E1, respectively, the leakage current substantially reduces to pico amperes of 9.96 x 10 − 12 A and 9.35 x 10 − 12 A, respectively. With all the device structural parameters remaining the same, only the deposition temperature is varied, we speculate the effect of surface modification due to O 2 over-pressure in Devices D and E may play a major role in the variation of the device parameters. It may be noted that the Devices D1 and E1 are subjected to oxygen over-pressure for ~ 2 hours prior to breaking the vacuum. During the ramping down of the deposition temperature to RT, we speculate the surface layer is modified due to chemisorption of O 2 atoms. The effect of chemisorption is also confirmed in our GIXRD data, where the intensity of ZnO (103) peak is increased in Sample D and E, due to the O 2 over-pressure during the post-deposition ramp down. Our XPS analyses also support the theory of O 2 -rich layers in Samples D and E, where relatively high O 1s peaks are observed. Hence, the surface modification due to chemisorption is strongly believed to increase in the crystallite size (13.83 nm and 11.38 nm), reduce the grain boundaries and oxygen vacancies, which are combined indications of the reduced defects in high temperature deposited TFTs (D1 and E1). Table 5 Electrical parameters of TFTs deposited at different temperatures Sample Bulk Mobility (cm 2 /V-sec) I ON (µA) V ON (V) V Th (V) SS (V/dec) I ON /I OFF ratio µ FE (cm 2 /V-sec) C1 46.09 0.223 -34. 6 23.1 30 10 3 0.062 D1 20.43 1.58 -3.33 20.3 13 10 4 0.64 E1 31.6 0.127 10.8 21.7 4.43 10 4 0.10 The turn-ON voltage (V ON ) for Device C1 is measured at -34.6V, and positively shifts to -3.33V and 10.8V in Devices D1 and E1, respectively. Decrease in I OFF , increase in turn ON voltage and high I ON /I OFF ratio are clear evidences of the improved switching property for the Device D1 and E1 (Table 5 ). The shift in the V ON is attributed to O 2 -rich thin films deposited at 300°C, as also confirmed by the high O 1s peak in the corresponding XPS spectrum (Fig. 7a-c). Sangwon Lee et al. [51] similarly claimed in their InGaZnO TFTs that shift in the turn-ON voltage is due to O 2 -rich layers. Table 5 lists the field effect mobility (µ FE ) and threshold voltage (V Th ) values of Devices C1, D1, are calculated from the respective transfer characteristics. In general, field effect mobility and threshold voltage are inversely related in a ZnO deposition [52]. Device C1 exhibits the highest V Th of 23.1V and lowest µ FE , whereas, Device D1 and E1 exhibits lower threshold voltage and higher field effect mobility as compared to Device C1. Our XPS data confirms the better incorporation of O 2 in the Devices D1 and E1, which explains the reduction in the threshold voltage and improvement in the field-effect mobility. In addition, our SEM images shows the presence of voids which may act as charge trapping regions affecting the threshold voltage of Device C1. Another parameter, which defines the TFT performance is Sub-threshold swing (SS) which shows a decreasing trend from RT to high temperature deposition. We propose the reason for high SS parameter to be nano-ampere range of leakage current in the O 2 -deficient Sample C1. As the deposition temperature is increased, O 2 -rich films in Devices D1 and E1are the plausible reason for lower leakage current leading to lower values of SS. Table 6 Electrical parameters of ZnO TFT with variation in active layer thickness Device V ON (V) V Th (V) SS (V/dec) I ON /I OFF ratio µ FE (cm 2 /V-sec) E1 (180 nm) 11.2 21.7 4.43 10 4 0.10 E2 (50nm) 10.4 23.29 2.8 10 5 1.1 Variation in ZnO layer thickness Table. 6 illustrates the variation of TFT parameters for 180 nm and 50 nm thick ZnO active layers. All other process parameters are unchanged except ZnO thickness. Device E2 exhibits lower leakage current compared to Device E1, which could be explained by channel thickness being less that the width of the depletion layer, which was calculated using the following relation. $${d}_{ch}<{W}_{dep}=\sqrt{\frac{{4\epsilon }_{0}{\epsilon }_{r}{\phi }_{b}}{q{N}_{e}} }$$ 8 Where, d ch – Channel thickness, W dep – Depletion width, ε r – relative permittivity of ZnO, φ b -for the potential gap between the Fermi level and intrinsic level, q – Electron charge, N e – Carrier concentration. Reducing the thickness of the channel layer has increased the I ON by 10-fold, leading to the improvement in the I ON /I OFF ratio from 10 4 to 10 5 . Consequently, the reduction in the sub-threshold slope is also observed in thinner Device E2. Conclusion In conclusion, ZnO thin films deposited at room temperature revealed low O 2 incorporation, high oxygen related defects, high defect induced carrier mobility, band-tail states at 1.53 eV and 2.8 eV. Deposition at 250°C and 300°C show significant improvement in the quality of the layers, attributed to the effective reduction of oxygen related defects with the reduction in the band-tail states. The improvement is attested by the increase in the crystallite size from XRD and SEM analyses, improvement in O 2 incorporation and reduction in O 2 vacancies from XPS results. Highest ZnO bulk mobility of 31.6 cm 2 /V-sec has been achieved for Sample deposited at 300°C. ZnO TFT fabricated by varying substrate temperature showed substantial improvement in leakage current, threshold voltage, sub-threshold swing and I ON /I OFF ratio. The process modification during the device fabrication of TFTs deposited at 300°C, ensured O 2 -rich surface through chemisorption, leading to the reduction in leakage current from the range of 10 − 10 A to 10 − 12 A and sub-threshold swing from 30V to 2.8V. Thus, the improvements in the TFT performance are governed by precise deposition process control towards the improvement in the crystalline quality and reduction in oxygen related defects. Hence, the combination of XRD, XPS, UV-Visible spectrophotometer and microscopy tools prove vital in analyzing TFT devices. Declarations Author Contribution Sasikala Muthusamy: Methodology, Software, Formal analysis, Investigation, Writing - Original Draft. Sudhakar Bharatan: Conceptualization, Writing - Review & Editing, Supervision. Sinthamani Sivaprakasam: Validation, Data Curation. Ranjithkumar Mohanam: Resources, Visualization. Acknowledgement This research work was carried out in the DST FIST sponsored Interdisciplinary Nano Research Centre, Sri Venkateswara College of Engineering, Sriperumbudur. 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Quevedo-Lopez, Rafael Ramirez- Bon (2016) Effect of depth of traps in ZnO polycrystalline thin films on ZnO-TFTs performance. Solid State Electron, 123:119-123. https://doi.org/10.1016/j.sse.2016.05.005 Additional Declarations No competing interests reported. Supplementary Files Supplementarymaterial.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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7","display":"","copyAsset":false,"role":"figure","size":157138,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea\u003c/strong\u003e XPS Spectrum of O 1s peak-Sample C1\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eb \u003c/strong\u003eXPS Spectrum of O 1s peak-Sample D1\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ec \u003c/strong\u003eXPS Spectrum of O 1s peak-Sample E1\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4599511/v1/f5d14022fc0cb3cb87fe8db8.jpg"},{"id":60601672,"identity":"c542cd7d-829b-4b7b-90e0-a85113034cce","added_by":"auto","created_at":"2024-07-18 16:06:28","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":205819,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) to (c) \u003c/strong\u003eDrain characteristics of TFT (a) Device C1 (b) Device D1 (c) Device E1\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4599511/v1/93f2cf65ab9161da176a144f.jpg"},{"id":60601651,"identity":"8577419b-c64f-43c6-aa3f-61aa80c1f9e5","added_by":"auto","created_at":"2024-07-18 16:06:26","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":243423,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003e(a) to (c) \u003c/strong\u003eTransfer characteristics of TFTs (a) Device C1 (b) Device D1 (c) Device E1\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4599511/v1/6419112f851107c38f0110a4.jpg"},{"id":60601607,"identity":"0b60f498-47bf-4874-8bff-6dd2b1504207","added_by":"auto","created_at":"2024-07-18 16:06:24","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":65422,"visible":true,"origin":"","legend":"\u003cp\u003eTransfer Characteristics of ZnO TFT with Variation in channel layer thickness\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-4599511/v1/a0b9212d44589104f84261a9.jpg"},{"id":60602363,"identity":"020c518d-3272-4e99-ba76-8b6cf6b6adc6","added_by":"auto","created_at":"2024-07-18 16:14:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1773723,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4599511/v1/7faea73c-e5f6-4235-a1a4-b78f9441ac77.pdf"},{"id":60601670,"identity":"e29f5cd8-5eba-4f1b-a4b1-0421561d2f2a","added_by":"auto","created_at":"2024-07-18 16:06:27","extension":"docx","order_by":4,"title":"","display":"","copyAsset":false,"role":"supplement","size":1636415,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarymaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-4599511/v1/a212523ff9dc45d36f572c1b.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eEffect of Deposition Temperature in RF Sputtered ZnO Thin Films on ZnO TFT Performance\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"Introduction","content":"\u003cp\u003eEnormous research efforts on metal oxide semiconductors have been carried out in the recent past, due to which, it has become a promising candidate in the field of solar cells [1\u0026ndash;4], thin film transistors [5\u0026ndash;8], photodetectors [9\u0026ndash;11], memory [12\u0026ndash;13], gas sensors [14\u0026ndash;17], bio sensors [18\u0026ndash;20] and CMOS circuits [21,22]. Key properties such as wide bandgap, high mobility, transparency, tunability and substrate compatibility play a vital role in deciding the applications of the metal oxide semiconductor material system. In most of the traditional electronic applications, Si based TFTs are widely used in pixel and peripheral driver circuits, because of its ease of integration with CMOS. The advancements in the field of IOT, AI and ML, requires the large memory and high bandwidth, which leads to the scaling down of devices and that would be possible only in 3D integration of devices. Even though Si technology has the advanced manufacturing processes, it suffers due to poor mobility, bias stress, threshold voltage instability and reliability issues. Hence, metal oxide based TFTs are the potential alternative to Si-based TFTs in the areas of flexible display, health care, environment and automotive applications. Process level advantage of oxide semiconductor is the ability to realize large area device properties on any substrate using a non-equilibrium deposition process such as sputtering.\u003c/p\u003e \u003cp\u003eAmong the various metal oxide semiconductor cations, Zn and Sn are considered non-toxic and abundantly available elements. ZnO with its wide band gap (3.37eV), low cost, excellent electrical and optical properties, makes it a promising candidate for numerous applications in the electronics field. Traditionally, ZnO thin films are deposited using a variety of processes, such as spin coating [23], spray analysis [24, 25], Pulsed laser deposition [26], Molecular beam epitaxy [27,28], Atomic layer deposition [29, 30] and RF sputtering [31,32,33]. Highest mobility of 300 cm\u003csup\u003e2\u003c/sup\u003e/V-sec, has been achieved [28] in Mg doped ZnO heterostructures using Molecular beam epitaxy system which require ultra-high vacuum, leading to high cost. However, RF sputtering method provides versatility, scalability, uniformity and high quality thin films in controlled environment even at low temperature, and has paved the way for large area and low cost device applications.\u003c/p\u003e \u003cp\u003eEven though the mobilities of RF sputtered ZnO thin films have reached the of range 70 cm\u003csup\u003e2\u003c/sup\u003e/V-sec, defects in the ZnO thin films play the crucial role in realizing high quality thin film transistor [34, 35] especially in flexible display, optoelectronic and gas sensing applications. Oxygen vacancies, interstitials, grain boundaries, dislocations, surface defects are reported to play a major role in forming n-type semiconductor [36, 37]. Among these, oxygen vacancies can have significant impact; they can act as shallow donors and/or deep acceptors depending on the charge state, which significantly affects the optical and electrical properties. Advanced characterization techniques like X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and UV-visible spectroscopy can be employed to understand and manipulate the defects in thin film optimization.\u003c/p\u003e \u003cp\u003eSingh S \u003cem\u003eet al.\u003c/em\u003e reported field effect mobility of 0.6134 cm\u003csup\u003e2\u003c/sup\u003e/V-sec and threshold voltage of 3.1V in their ZnO TFT using RF sputtering with SiO\u003csub\u003e2\u003c/sub\u003e as gate insulator [32]. Similarly, Jong Hoon Lee \u003cem\u003eet al.\u003c/em\u003e fabricated ZnO TFT with MgO as gate insulator and reported lower field-effect mobility of 0.0235 cm\u003csup\u003e2\u003c/sup\u003e/V-sec, I\u003csub\u003eON\u003c/sub\u003e/I\u003csub\u003eOFF\u003c/sub\u003e ratio of \u0026sim;10\u003csup\u003e5\u003c/sup\u003e, threshold voltage of 2.2V and SS value of 1.18 V/decade [33]. Brandon Walker \u003cem\u003eet al.\u003c/em\u003e compared the performance of ZnO TFT with various gate dielectric materials namely, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, HfO\u003csub\u003e2\u003c/sub\u003e and ZrO\u003csub\u003e2\u003c/sub\u003e, and achieved the highest on/off ratio of greater than 10\u003csup\u003e5\u003c/sup\u003e [38]. B. -S. Wang et al., achieved the mobility of 84.22 cm\u003csup\u003e2\u003c/sup\u003e/V-sec and I\u003csub\u003eON\u003c/sub\u003e/I\u003csub\u003eOFF\u003c/sub\u003e ratio of 3 \u0026times; 10 \u003csup\u003e6\u003c/sup\u003e by means of MgZnO/ZnO heterostructure TFT. Thus the presence of native defects in ZnO material system such as O\u003csub\u003e2\u003c/sub\u003e vacancies and interstitials, significantly affects the threshold voltage, which is crucial for the realization of a normally-off device (enhancement type transistor) [39].\u003c/p\u003e \u003cp\u003eHence, in this paper, the optimization of ZnO thin film is systematically carried out using RF sputtering technique with different argon: oxygen flow rate and different substrate temperature. Various properties of ZnO thin films are characterized by X-ray diffraction, XPS, Hall measurement, SEM imaging and UV-visible spectrophotometer. Based on these properties, ZnO TFTs were fabricated with different substrate temperatures and were electrically characterized. Various parameters such as threshold voltage, ON/OFF ratio, field effect mobility and subthreshold swing were determined for different W/L ratios of TFT.\u003c/p\u003e"},{"header":"Experimental Details","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eDeposition of ZnO Thin films\u003c/h2\u003e \u003cp\u003eZnO thin films are deposited using RF magnetron sputtering on p-Si substrate. Prior to ZnO deposition, substrates were cleaned using RCA1 and RCA2 methods to remove organic and metal contaminations, followed by HF dip to remove the native oxide. During ZnO deposition, target to substrate distance was maintained at 7.5 cm and chamber was evacuated to 5.5 x 10\u003csup\u003e\u0026minus;\u0026thinsp;6\u003c/sup\u003e mbar. In order to maintain the contamination free source material, the target was pre-sputtered for 10 mins prior with the shutter closed. In order to optimize the Ar/O\u003csub\u003e2\u003c/sub\u003e gas flow rate, a series of films are deposited at room temperature.\u003c/p\u003e \u003cp\u003eSamples A, B and C represents 180 nm thick ZnO films deposited at different Ar:O2 flow rates as described in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Sample A, B and C represents films deposited at room temperature followed by post-deposition annealing at 300⁰C in N\u003csub\u003e2\u003c/sub\u003e ambient, with Ar:O\u003csub\u003e2\u003c/sub\u003e flow rates of 20:0, 18:2, and 16:4, respectively. The gas flow rates are measured using the Aalborg mass flow controllers.\u003c/p\u003e \u003cp\u003eUpon completion of the flow rate optimization, deposition temperature optimization was carried out at room temperature (RT), 250\u0026deg;C and 300\u0026deg;C. Sample C, D and E represents 180 nm thick ZnO deposited at RT, 250\u0026deg;C and 300\u0026deg;C, respectively, with a constant Ar:O\u003csub\u003e2\u003c/sub\u003e flow rate ratio of 16:4. Deposition rates of ZnO films deposited at RT, 250\u0026deg;C and 300\u0026deg;C, are determined to be 3 nm/min, 6 nm/min and 7 nm/min, respectively, by step profile measurements using Bruker profilometer (Fig \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the ZnO thin film deposition temperature, Ar/O\u003csub\u003e2\u003c/sub\u003e flow rate and annealing temperatures.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDeposition parameters of ZnO thin films\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDeposition Temperature\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAr/O\u003csub\u003e2\u003c/sub\u003e flow rate\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAnnealing temperature\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e20/0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN2 ambient 300\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e18/2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN2 ambient 300\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16/4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eN2 ambient 300\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e250\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16/4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e300\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16/4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eGrazing Incidence (GI) θ/2θ X-ray diffraction was carried out on Samples A, B, C, D and E using Bruker/PAN Analytical X-ray Diffractometer. Based on the FWHM of (002) XRD peak, grain size, strain and dislocation density values are determined. Hall measurements were also carried out on the above samples at a magnetic field of 0.51T using Ecopia Van der Pauw HMS 3000 system.\u003c/p\u003e \u003cp\u003eSurface morphology of ZnO thin films was studied using Zeiss ULTRA 55 scanning electron microscopy (SEM) system on all the above samples. X-ray photoelectron spectroscopy (XPS) analysis of the ZnO thin films was performed using a Kratos Axis Ultra spectrometer, employing a monochromatic Al-Kα source. Absorption edge of all the above samples are derived from the transmission data measured in UV-1650PC Shimadzu spectrophotometer.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eDevice fabrication and characterization\u003c/h2\u003e \u003cp\u003eCommercially available SiO\u003csub\u003e2\u003c/sub\u003e/Si substrate with oxide thickness of 100 nm is used for thin film transistor device fabrication. With the optimized flow rate of 16:4 (Ar:O\u003csub\u003e2\u003c/sub\u003e) and 60W RF power, thin film transistors C1, D1 and E1 were fabricated with deposition temperatures as listed in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Post-deposition annealing was carried out only on Sample C1 at 300\u0026deg;C in N\u003csub\u003e2\u003c/sub\u003e ambient.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDeposition parameters and Hall performance of ZnO TFT\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eDeposition Temperature\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eAnnealing temperature and ambient\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAr/O\u003csub\u003e2\u003c/sub\u003eflow rate\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePost contact annealing\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eCarrier concentration (cm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRT\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e300\u0026deg;C in N\u003csub\u003e2\u003c/sub\u003e ambient\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e16/4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\" morerows=\"2\" rowspan=\"3\"\u003e \u003cp\u003e220\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e2.6\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\times\\)\u003c/span\u003e\u003c/span\u003e10\u003csup\u003e17\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e250\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\times\\)\u003c/span\u003e\u003c/span\u003e10\u003csup\u003e16\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e300\u0026deg;C\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e-\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1.6\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\times\\)\u003c/span\u003e\u003c/span\u003e10\u003csup\u003e17\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eMask for patterning the channel and electrode layer was written on the chrome glass using the mask writer (Fig S2). Exposure and pattering were carried on Karl SussMA6-BA6 mask aligner. Thin film transistor (TFT) fabrication was carried in two step lithographic process. In the first lithography process, 300\u0026micro;m x 300\u0026micro;m ZnO mesa structure was created by wet etching process. In the second lithography process, the standard lithography technique is used to pattern the source and drain electrodes. 100 nm Al metal was deposited at room temperature using thermal evaporation method, followed by lift-off process, to get the patterned Source/Drain. Finally, back side SiO\u003csub\u003e2\u003c/sub\u003e was selectively removed using HF dip, and blanket Al metal (100 nm) as gate electrode was deposited by thermal evaporation to get the final fabricated device (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). In order to improve the contact resistance, all the devices were annealed at 220\u0026deg;C in N\u003csub\u003e2\u003c/sub\u003e ambient for 10 mins.\u003c/p\u003e \u003cp\u003eElectrical properties of ZnO TFTs with Width to Length (W/L) ratio of 50\u0026micro;m/50\u0026micro;m were examined using I-V transfer and output characteristics. I-V characteristics were carried out on Summit 11000B-M (Cascade Microtech) precision 4-axis semi-automated Probe station platform.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results and discussion","content":"\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003eX-Ray Diffraction (XRD)\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e represents the grazing incident x-ray diffraction of Samples A, B and C, deposited at varying Ar/O\u003csub\u003e2\u003c/sub\u003e flow rates. Sample A showed high intensity (002) XRD peak at 34.47\u0026deg; and even higher (103) peak at 62\u0026deg;. As the O\u003csub\u003e2\u003c/sub\u003e ratio is increased, the overall intensity of ZnO (002) peak has decreased with the suppression of (103) ZnO peak in Samples B and C. The crystallite size, dislocation density and micro-strain where calculated using equations (\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Equ3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) and are listed in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$D=\\frac{0.9\\lambda }{\\beta Cos\\theta }$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\delta =\\frac{1}{{D}^{2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\epsilon =\\frac{\\beta }{4\\text{tan}\\theta }$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere λ is the wavelength of X-Ray (1.54Ȧ), θ the Bragg\u0026rsquo;s Diffraction angle and β is the full width of half maximum (FWHM) of the peak, δ is the dislocation density, D is the representation crystallite size.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eZnO Thin Films parameter extracted from XRD\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFWHM (β)\u003c/p\u003e \u003cp\u003e(radians)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2θ\u003c/p\u003e \u003cp\u003e(deg)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCrystallite size D (nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eDislocation density\u0026#120575;\u003c/p\u003e \u003cp\u003e( lines/m\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003elattice parameter spacing (nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eMicro Strain ε\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e34.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e14.596\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.0047\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.260\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.0008\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eB\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e34.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e11.876\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.0071\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.262\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.0009\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.76\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e34.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e10.940\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.0084\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.261\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.0010\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e34.31\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e13.858\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.0052\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.261\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.0008\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e34.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e11.385\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.0077\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.262\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.0010\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e compares the GIXRD of Samples C, D and E deposited at RT, 250\u0026deg;C and 300\u0026deg;C, respectively. By maintaining the flow rate at 16:4, Samples D and E exhibited notable improvement in (002) peak as compared to the Sample C, indicative of improvement in crystalline quality. All the samples except A sample exhibit preferred orientation along the (002) plane. Hence, all our TFT devices were grown at the optimized gas flow ratio of 16: 4 (Ar:O\u003csub\u003e2\u003c/sub\u003e) at room temperature, 250\u0026deg;C and 300\u0026deg;C. It may be noted that Sample D exhibits polycrystalline property with distinct peaks at (002), (101), (102), (110), (103) and (112).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e lists the crystalline size from the FWHM values of XRD spectrum. Sample A exhibited the lowest (002) XRD FWHM and highest crystalline size, whereas Sample C exhibited the highest FWHM and lowest crystalline size. Even though Sample A exhibits lowest strain in the microstructure, the presence of dominant (103) XRD peak suggests that the surface is affected by the non-equilibrium growth condition of RF sputtering. Similar (103) ZnO GIXRD peaks are reported by various groups [40]. At higher deposition temperatures of 250\u0026deg;C and 300\u0026deg;C in Samples D and E, the grain size increased to 13.85 nm and 11.38 nm, respectively. Additionally, reduction in dislocation density and micro strain has been observed in Samples D and E, suggesting overall improvement in the crystal quality.\u003c/p\u003e \u003cp\u003eFigure S3 shows the conventional x-ray diffraction spectra of Sample C, which exhibits dominant (002) ZnO peak at 34\u0026deg;, and a relatively low intensity (103) peak at 62\u0026deg;, indicative of preferential (002) crystal formation in the bulk layer. The intensity ratio of (002)/(103) peak has substantially increased from 3 (GIXRD) to 9.52 (Conventional XRD), indicative of c-axis orientation in the bulk layer. The sharp (002) peak in conventional XRD is the evidence for c-axis orientation, and relatively higher (103) peak in GIXRD is the evidence of surface re-structuring during the final stages of RF sputtering. Usually, (103) XRD peak are reported in the ZnO thin films prepared by electro-deposition [41] and sol-gel [42] processes. However, RF sputtering being a non-equilibrium growth technique, during the final stages of deposition, the re-orientation of atoms by diffusion may happen on the surface, which could be the reason for the presence of dominant (103) peak. Yunlan Wang \u003cem\u003eet al.\u003c/em\u003e [40] reported in their RF sputtered ZnO thin films that the appearance of (103) GIXRD orientation is due to native property of ZnO.\u003c/p\u003e \u003cp\u003eIt is observed that Sample A has the highest (103) peak intensity compared to any other sample. This could be attributed to the relatively low oxygen reactive species, and making the Zn atoms to move freely along the surface. As the O\u003csub\u003e2\u003c/sub\u003e flow rates are increased in Samples B, C, D and E, the influx of O\u003csub\u003e2\u003c/sub\u003e species suppressed the free movement or diffusion of Zn atoms leading to low intensity (103) peak. Hence, O\u003csub\u003e2\u003c/sub\u003e over-pressure during the sputtering process becomes vital in depositing smooth surface.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003eSEM\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea-e. show the SEM images of ZnO films A, B, C, D, and E respectively. Sample A deposited at an Ar/O\u003csub\u003e2\u003c/sub\u003e flow rate of 20/0 shows larger grain size with distinct contours. The presence of bigger grain crystalloids in SEM image corroborates with the calculated grain size of 14.59 nm for Sample A based on the XRD data. As the O\u003csub\u003e2\u003c/sub\u003e ambient was introduced at 2 sccm, the grain size started to decrease with highly dense surface morphology in Sample B. Both the samples exhibit granular and void free surface with high packing density. With further increase in O\u003csub\u003e2\u003c/sub\u003e flow rate to 4 sccm in Sample C, grain size has nominally remained the same as Sample B. However small voids between the grains started to appear as pointed in the Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. Both samples B and C, with higher O\u003csub\u003e2\u003c/sub\u003e content, distinct and isolated grains are formed and more structured morphology was observed.\u003c/p\u003e \u003cp\u003eThe voids disappear when the films are deposited at 250⁰C and 300⁰C as observed in Samples D and E, respectively. The conventional XRD (Fig.S3) shows dominant (002) XRD peak orienting along the c-axis, while the GIXRD spectra (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) shows multiple XRD peaks on the surface of Sample D matching the asymmetric contours in the corresponding SEM image (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eOptical Characterization\u003c/h2\u003e \u003cp\u003eThe Tauc plot (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) is obtained between (αhν)\u003csup\u003e2\u003c/sup\u003e and band gap(E\u003csub\u003eg\u003c/sub\u003e) to determine the absorption edge of the ZnO thin film Samples C, D and E. All the samples C, D and E show bulk-like absorption edge, attesting good quality ZnO thin films. Sample C deposited at RT exhibits absorption edge at 3.28 eV with comparatively low absorption. Whereas, Sample D deposited at 250\u0026deg;C shows a redshift in the bandgap of 3.25 eV with relatively higher absorption at higher energy levels attesting to possible absence of high energy band states above the conduction band minimum. Sample E deposited at 300\u0026deg;C shows further redshift to 3.15 eV with faster roll-off. Redshift in the absorption edge from 3.28 eV to 3.25 eV to 3.15 eV has been observed as the deposition temperature is changed from RT to 250\u0026deg;C and 300\u0026deg;C, respectively, which may be attributed increase in the grain size observed in SEM image and XRD calculation, leading to the increase in the tensile stress [43].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIntrinsically, ZnO thin films have both oxygen vacancies and Zn interstitials exhibiting n-type conductivity [44]. The observed absorption spectra of our ZnO thin films can be separated into the following transitions: (i) ZnO band-to-band transition; (ii) transitions to localized band states (widely known as Urbach tail) [45]. There are several reports on Urbach tail states in ZnO thin films, due to the structural disorders. Several groups have reported that shallow donors (oxygen vacancies) and deep acceptors (Zn interstitials) act as the crystal disorders. [46, 47].\u003c/p\u003e \u003cp\u003eIn order to delineate various sub-bands of our samples, natural log of the absorption edge and the bandgap was plotted [48]. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the plot between ln (α) and bandgap for samples C, D and E. Sample C exhibits Urbach tail bands at 2.8 eV and 1.53 eV, indicating the presence of localized defects. Our Room temperature hall data shows a high carrier concentration of 2.6 x 10\u003csup\u003e17\u003c/sup\u003ecm\u003csup\u003e\u0026minus;3\u003c/sup\u003e for Sample C (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), which could be attributed to the presence of defect-induced transport. All our hall measurements showed that ZnO thin films are n-type based on the hall coefficient. Sample C shows the higher mobility of 46.09 cm\u003csup\u003e2\u003c/sup\u003e/V-sec, whereas Sample D deposited at 250\u0026deg;C exhibited a hall bulk mobility of 20.43 cm\u003csup\u003e2\u003c/sup\u003e/V-sec, and Sample E1 exhibited an intermediate mobility of 31.63 cm\u003csup\u003e2\u003c/sup\u003e/V-sec.\u003c/p\u003e \u003cp\u003eAs shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, Sample D exhibits a weak band-tail at 2.42 eV and a strong tail state at 1.87 eV. Samples C and D shows more than one band-tail states, which could be attributed to possible presence of the combination of oxygen vacancies and Zn interstitials leading to deep donor and acceptor levels within the bandgap. Whereas, Sample E shows band-to-band absorption at 3.14 eV and a weak tail state at 2.47 eV representative of possible direct band transition and band-to-tail transition. Presence of only one transition shows that the sample E deposited at 300\u0026deg;C resulted in reduction of vacancy and interstitial related defects and the presence of shallow tail-band within the bandgap. The reduction in the tail states has led to the higher absorption in Sample D and even higher absorption in Sample E, attesting to the annihilation of defects. Yet, Samples D and E exhibits a slightly lower mobility as compared to defect induced transport in Sample C. Our SEM image shows the coalescing nature of ZnO denoting the wrinkled nature of the surface, which is also confirmed by the polycrystalline property from XRD spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The reason for the drop in the Hall mobility will be analyzed from the XPS results. Even though the Sample D exhibits a mobility of 20.43 cm\u003csup\u003e2\u003c/sup\u003e/V-sec, as compared to Sample C, our values are comparable to the other ZnO thin films in the literature [49]. Further improvement in the bulk mobility of Sample E has been observed, which is the highest among the other reports [50].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eXPS\u003c/h2\u003e \u003cp\u003eXPS analyses on the Samples C, D and E describes the origin of oxygen related defects and vacancies when the samples are deposited at RT, 250\u0026deg;C and 300\u0026deg;C, respectively (Figs.\u0026nbsp;7 (a-c)). The O 1s peaks for various films are shown in Figs.\u0026nbsp;7 (a-c). XPS analyses on the Samples C, D and E describes the origin of oxygen related defects and vacancies when the samples are deposited at RT, 250\u0026deg;C and 300\u0026deg;C, respectively. O 1s peak is resolved into 2 peaks at ~\u0026thinsp;530eV and ~\u0026thinsp;532eV, representative of O 1s peak and oxygen vacancies, respectively. Sample C exhibits narrower O 1s peak at 530.3eV, and broader O\u003csub\u003e2\u003c/sub\u003e vacancy peak at 531.7 eV. Since Sample C which was deposited at RT and undergo post-deposition annealing at 300\u0026deg;C, the strain in the thin film has remained at 0.001% (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), which explains the narrower O 1s peak. However, Sample C shows lower area (~\u0026thinsp;58.62%) under the O 1s curve, indicative of lower O\u003csub\u003e2\u003c/sub\u003e concentration, as compared to Samples D (~\u0026thinsp;61%) and E (~\u0026thinsp;70%), which was deposited at higher temperatures (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Sample E1 exhibits substantial increase in the area under the O 1s curve to as compared to Sample C1 Alternatively, Samples D and E exhibits relatively broader O 1s peak, and narrower oxygen vacancy peak. In particular, Sample D shows a broad O 1s peak with FHWM of 1.6 eV, suggesting the presence of O\u003csub\u003e2\u003c/sub\u003e complexes in addition to the O\u003csub\u003e2\u003c/sub\u003e vacancies, which also supports the drop in the Hall mobility.\u003c/p\u003e \u003cp\u003eOverall, our XPS data shows that depositing at higher temperatures 300\u0026deg;C assists in the better O\u003csub\u003e2\u003c/sub\u003e incorporation in the ZnO crystal. The voids in the SEM image (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec) and the presence of tail-bands at 2.8 eV and 1.53 eV in the tauc plot are clear evidences of high oxygen related defects in Sample C due to room temperature deposition. Additionally, annealing of Sample C at 300\u0026deg;C also did not help in the annihilation of defects. However, the percentage of O\u003csub\u003e2\u003c/sub\u003e vacancies in Samples D and E has reduced with a narrower O\u003csub\u003e2\u003c/sub\u003e vacancy peak resolved at 531.88 eV. Hence, based on the XPS analysis, it is clear that higher deposition temperatures not only enhances the O\u003csub\u003e2\u003c/sub\u003e incorporation, but also reduces the oxygen related defects.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eOxygen Vacancies obtained from XPS\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eO\u003csub\u003e2\u003c/sub\u003e vacancy\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e \u003cp\u003eO 1s peak\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBinding Energy (eV)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eArea\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eFWHM\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eBinding Energy (eV)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eFWHM\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eArea\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e531.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e41.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e530.354\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e58.62\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e532.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e39.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e530.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e60.9\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e531.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e30.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e529.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e69.74\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003eDevice Characterization\u003c/h2\u003e \u003cp\u003eThin film transistors were fabricated on 100 nm SiO\u003csub\u003e2\u003c/sub\u003e/Si substrates with ZnO channel layer. TFT Device C1 was fabricated at room temperature followed by 300\u0026deg;C post deposition annealing (similar to Sample C thin film). And Devices D1 and E1 represent TFTs deposited at 250\u0026deg;C and 300\u0026deg;C, respectively. All the TFTs exhibit n-type behavior as represented by the I\u003csub\u003eD\u003c/sub\u003e \u0026ndash; V\u003csub\u003eD\u003c/sub\u003e curves. The staggered bottom gate TFT structure has been fabricated. The drain current expressions of the enhancement mode TFT are:\u003c/p\u003e \u003cp\u003eIn Linear region\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$${I}_{D}=\\frac{W}{L}{C}_{ox}{\\mu }_{fe}\\left[\\left({V}_{GS}-{V}_{th}\\right){V}_{DS}-\\frac{1}{2}{V}_{DS}^{2}\\right] \\left(4\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eSaturation region\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$${I}_{D}=\\frac{W}{2L}{C}_{ox}{\\mu }_{fe}{\\left({V}_{GS}-{V}_{th}\\right)}^{2} \\left(5\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eField effect mobility:\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$${ \\mu }_{FE}=\\frac{{g}_{m}}{{C}_{ox}\\frac{W}{L}{V}_{DS}} \\left(6\\right)$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equd\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equd\" name=\"EquationSource\"\u003e\n$$SS=\\frac{{dV}_{GS}}{d\\left(\\text{log}{I}_{D}\\right)} \\left(7\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere W-Width of the channel, L-Length of the channel, C\u003csub\u003eox\u003c/sub\u003e-Capacitance per unit area.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eDrain Characteristics\u003c/h2\u003e \u003cp\u003eThe drain characteristics are measured for TFT devices with the same channel length and width of 50 \u0026micro;m. Gate-Source voltage (V\u003csub\u003eG\u003c/sub\u003e) is varied from 0 to 40V in steps of 5V. At V\u003csub\u003eG\u003c/sub\u003e of 40V, Device C1 reaches the highest drain current of 0.6\u0026micro;A, whereas, Device D1 and E1 reaches a maximum drain current of 0.11\u0026micro;A. The variation in the drain currents match well with the variation of bulk hall mobility on Samples C, D and E, as represented in Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. Since the thickness of these devices are 180 nm, bulk mobility dominates I\u003csub\u003eD\u003c/sub\u003e, where Sample C exhibits highest hall mobility of 46.09 cm\u003csup\u003e2\u003c/sup\u003e/V-sec corroborating with the high drain current of 0.6\u0026micro;A in Device C1 (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a)).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eTransfer Characteristics\u003c/h2\u003e \u003cp\u003eFigures\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e9\u003c/span\u003e (a-c) represents the transfer characteristics of the above TFT devices C1, D1 and E1 for W\u0026thinsp;=\u0026thinsp;50 \u0026micro;m and L\u0026thinsp;=\u0026thinsp;50 \u0026micro;m at V\u003csub\u003eD\u003c/sub\u003e = 5V. The gate voltage (V\u003csub\u003eG\u003c/sub\u003e) is swept from \u0026minus;\u0026thinsp;40V to +\u0026thinsp;40V. The effect of deposition temperature on the transfer curves shows that all the TFTs operate in the enhancement mode. Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e displays the electrical parameters of TFTs deposited at different temperatures. The I\u003csub\u003eoff\u003c/sub\u003e current for Device C1 is of the order of nanoampere (~\u0026thinsp;1.2x10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003eA), indicative of a leaky device. The presence of voids between the crystallites observed in the SEM image, smaller crystallite size (10.94 nm), and grain boundaries, could be the reason for nano ampere range leakage current in Sample C. Even though the bulk hall mobility is determined to be the highest in Sample C, we speculate the high carrier concentration of 2.6x10\u003csup\u003e17\u003c/sup\u003ecm\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e is predominantly defect induced transport. The defect is confirmed by the presence of two band-tail states in the absorption data (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.) at 1.53 eV and 2.8 eV. As the deposition temperature is increased to 250\u0026deg;C and 300\u0026deg;C in Devices D1 and E1, respectively, the leakage current substantially reduces to pico amperes of 9.96 x 10\u003csup\u003e\u0026minus;\u0026thinsp;12\u003c/sup\u003eA and 9.35 x 10\u003csup\u003e\u0026minus;\u0026thinsp;12\u003c/sup\u003eA, respectively.\u003c/p\u003e \u003cp\u003eWith all the device structural parameters remaining the same, only the deposition temperature is varied, we speculate the effect of surface modification due to O\u003csub\u003e2\u003c/sub\u003e over-pressure in Devices D and E may play a major role in the variation of the device parameters. It may be noted that the Devices D1 and E1 are subjected to oxygen over-pressure for ~\u0026thinsp;2 hours prior to breaking the vacuum. During the ramping down of the deposition temperature to RT, we speculate the surface layer is modified due to chemisorption of O\u003csub\u003e2\u003c/sub\u003e atoms. The effect of chemisorption is also confirmed in our GIXRD data, where the intensity of ZnO (103) peak is increased in Sample D and E, due to the O\u003csub\u003e2\u003c/sub\u003e over-pressure during the post-deposition ramp down. Our XPS analyses also support the theory of O\u003csub\u003e2\u003c/sub\u003e-rich layers in Samples D and E, where relatively high O 1s peaks are observed. Hence, the surface modification due to chemisorption is strongly believed to increase in the crystallite size (13.83 nm and 11.38 nm), reduce the grain boundaries and oxygen vacancies, which are combined indications of the reduced defects in high temperature deposited TFTs (D1 and E1).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eElectrical parameters of TFTs deposited at different temperatures\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eBulk Mobility (cm\u003csup\u003e2\u003c/sup\u003e/V-sec)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eI\u003csub\u003eON\u003c/sub\u003e (\u0026micro;A)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eV\u003csub\u003eON\u003c/sub\u003e (V)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eV\u003csub\u003eTh\u003c/sub\u003e (V)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eSS\u003c/p\u003e \u003cp\u003e(V/dec)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eI\u003csub\u003eON\u003c/sub\u003e/I\u003csub\u003eOFF\u003c/sub\u003e ratio\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003e\u0026micro;\u003csub\u003eFE\u003c/sub\u003e (cm\u003csup\u003e2\u003c/sup\u003e/V-sec)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e46.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.223\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-34. 6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e23.1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e10\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.062\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eD1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e-3.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e20.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e10\u003csup\u003e4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.64\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e31.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.127\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e10.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e21.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e4.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e10\u003csup\u003e4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe turn-ON voltage (V\u003csub\u003eON\u003c/sub\u003e) for Device C1 is measured at -34.6V, and positively shifts to -3.33V and 10.8V in Devices D1 and E1, respectively. Decrease in I\u003csub\u003eOFF\u003c/sub\u003e, increase in turn ON voltage and high I\u003csub\u003eON\u003c/sub\u003e/I\u003csub\u003eOFF\u003c/sub\u003e ratio are clear evidences of the improved switching property for the Device D1 and E1 (Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). The shift in the V\u003csub\u003eON\u003c/sub\u003e is attributed to O\u003csub\u003e2\u003c/sub\u003e-rich thin films deposited at 300\u0026deg;C, as also confirmed by the high O 1s peak in the corresponding XPS spectrum (Fig.\u0026nbsp;7a-c). Sangwon Lee \u003cem\u003eet al.\u003c/em\u003e [51] similarly claimed in their InGaZnO TFTs that shift in the turn-ON voltage is due to O\u003csub\u003e2\u003c/sub\u003e-rich layers.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e lists the field effect mobility (\u0026micro;\u003csub\u003eFE\u003c/sub\u003e) and threshold voltage (V\u003csub\u003eTh\u003c/sub\u003e) values of Devices C1, D1, are calculated from the respective transfer characteristics. In general, field effect mobility and threshold voltage are inversely related in a ZnO deposition [52]. Device C1 exhibits the highest V\u003csub\u003eTh\u003c/sub\u003e of 23.1V and lowest \u0026micro;\u003csub\u003eFE\u003c/sub\u003e, whereas, Device D1 and E1 exhibits lower threshold voltage and higher field effect mobility as compared to Device C1. Our XPS data confirms the better incorporation of O\u003csub\u003e2\u003c/sub\u003e in the Devices D1 and E1, which explains the reduction in the threshold voltage and improvement in the field-effect mobility. In addition, our SEM images shows the presence of voids which may act as charge trapping regions affecting the threshold voltage of Device C1.\u003c/p\u003e \u003cp\u003eAnother parameter, which defines the TFT performance is Sub-threshold swing (SS) which shows a decreasing trend from RT to high temperature deposition. We propose the reason for high SS parameter to be nano-ampere range of leakage current in the O\u003csub\u003e2\u003c/sub\u003e-deficient Sample C1. As the deposition temperature is increased, O\u003csub\u003e2\u003c/sub\u003e-rich films in Devices D1 and E1are the plausible reason for lower leakage current leading to lower values of SS.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eElectrical parameters of ZnO TFT with variation in active layer thickness\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eDevice\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eV\u003csub\u003eON\u003c/sub\u003e (V)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eV\u003csub\u003eTh\u003c/sub\u003e (V)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSS (V/dec)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eI\u003csub\u003eON\u003c/sub\u003e/I\u003csub\u003eOFF\u003c/sub\u003e ratio\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u0026micro;\u003csub\u003eFE\u003c/sub\u003e (cm\u003csup\u003e2\u003c/sup\u003e/V-sec)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE1 (180 nm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e11.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e21.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e4.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10\u003csup\u003e4\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.10\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eE2 (50nm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e23.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e2.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10\u003csup\u003e5\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eVariation in ZnO layer thickness\u003c/h2\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable. 6 illustrates the variation of TFT parameters for 180 nm and 50 nm thick ZnO active layers. All other process parameters are unchanged except ZnO thickness. Device E2 exhibits lower leakage current compared to Device E1, which could be explained by channel thickness being less that the width of the depletion layer, which was calculated using the following relation.\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$${d}_{ch}\u0026lt;{W}_{dep}=\\sqrt{\\frac{{4\\epsilon }_{0}{\\epsilon }_{r}{\\phi }_{b}}{q{N}_{e}} }$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e8\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere, d\u003csub\u003ech\u003c/sub\u003e \u0026ndash; Channel thickness, W\u003csub\u003edep\u003c/sub\u003e \u0026ndash; Depletion width, ε\u003csub\u003er\u003c/sub\u003e \u0026ndash; relative permittivity of ZnO, φ\u003csub\u003eb\u003c/sub\u003e -for the potential gap between the Fermi level and intrinsic level, q \u0026ndash; Electron charge, N\u003csub\u003ee\u003c/sub\u003e \u0026ndash; Carrier concentration.\u003c/p\u003e \u003cp\u003eReducing the thickness of the channel layer has increased the I\u003csub\u003eON\u003c/sub\u003e by 10-fold, leading to the improvement in the I\u003csub\u003eON\u003c/sub\u003e/I\u003csub\u003eOFF\u003c/sub\u003e ratio from 10\u003csup\u003e4\u003c/sup\u003e to 10\u003csup\u003e5\u003c/sup\u003e. Consequently, the reduction in the sub-threshold slope is also observed in thinner Device E2.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn conclusion, ZnO thin films deposited at room temperature revealed low O\u003csub\u003e2\u003c/sub\u003e incorporation, high oxygen related defects, high defect induced carrier mobility, band-tail states at 1.53 eV and 2.8 eV. Deposition at 250\u0026deg;C and 300\u0026deg;C show significant improvement in the quality of the layers, attributed to the effective reduction of oxygen related defects with the reduction in the band-tail states. The improvement is attested by the increase in the crystallite size from XRD and SEM analyses, improvement in O\u003csub\u003e2\u003c/sub\u003e incorporation and reduction in O\u003csub\u003e2\u003c/sub\u003e vacancies from XPS results. Highest ZnO bulk mobility of 31.6 cm\u003csup\u003e2\u003c/sup\u003e/V-sec has been achieved for Sample deposited at 300\u0026deg;C.\u003c/p\u003e \u003cp\u003eZnO TFT fabricated by varying substrate temperature showed substantial improvement in leakage current, threshold voltage, sub-threshold swing and I\u003csub\u003eON\u003c/sub\u003e/I\u003csub\u003eOFF\u003c/sub\u003e ratio. The process modification during the device fabrication of TFTs deposited at 300\u0026deg;C, ensured O\u003csub\u003e2\u003c/sub\u003e-rich surface through chemisorption, leading to the reduction in leakage current from the range of 10\u003csup\u003e\u0026minus;\u0026thinsp;10\u003c/sup\u003eA to 10\u003csup\u003e\u0026minus;\u0026thinsp;12\u003c/sup\u003eA and sub-threshold swing from 30V to 2.8V. Thus, the improvements in the TFT performance are governed by precise deposition process control towards the improvement in the crystalline quality and reduction in oxygen related defects. Hence, the combination of XRD, XPS, UV-Visible spectrophotometer and microscopy tools prove vital in analyzing TFT devices.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eSasikala Muthusamy: Methodology, Software, Formal analysis, Investigation, Writing - Original Draft. Sudhakar Bharatan: Conceptualization, Writing - Review \u0026amp; Editing, Supervision. Sinthamani Sivaprakasam: Validation, Data Curation. Ranjithkumar Mohanam: Resources, Visualization.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eThis research work was carried out in the DST FIST sponsored Interdisciplinary Nano Research Centre, Sri Venkateswara College of Engineering, Sriperumbudur. A portion of research work was performed using facilities at CeNSE, funded by Ministry of Electronics and Information Technology (MeitY), Govt. of India, and located at the Indian Institute of Science, Bengaluru.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eL. Shen, Z.Q. Ma, C. Shen, F. Li, Bo. He, F. Xu (2010) Studies on fabrication and characterization of a ZnO/p-Si-based solar cell. Superlattices Microstruct. 48(4):426-433. https://doi.org/10.1016/j.spmi.2010.08.007\u003c/li\u003e\n\u003cli\u003eT. Wong , S. Zhuk , S. Masudy-Panah and G. Dalapati (2016) Current status and future prospects of copper oxide heterojunction solar cells. 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Medina-Montes, Leonardo A. Baldenegro-Perez, Raul Sanchez-Zeferino, Lizeth Rojas-Blanco, Marcelino Becerril-Silva, Manuel A. Quevedo-Lopez, Rafael Ramirez- Bon (2016) Effect of depth of traps in ZnO polycrystalline thin films on ZnO-TFTs performance. Solid State Electron, 123:119-123. https://doi.org/10.1016/j.sse.2016.05.005\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"RF sputtering, ZnO, X-Ray diffraction, X-ray photoelectron Spectroscopy, Thin film transistor","lastPublishedDoi":"10.21203/rs.3.rs-4599511/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4599511/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eZnO thin films are deposited using RF magnetron sputtering by varying argon: oxygen gas flow rates and substrate temperatures. The structural and optical characterization of ZnO thin films are systematically carried out using X-ray diffraction (XRD), SEM, UV-visible spectroscopy and X-ray photoelectron spectroscopy (XPS). Dominant (002) Grazing incidence (GI) XRD peak on samples deposited at 300°C with Ar:O\u003csub\u003e2\u003c/sub\u003e (16:4) ratio suggest c-axis orientation both on the bulk and surface of ZnO thin film. Increase in the crystallite grain size were observed as the deposition temperature is increased from Room temperature (RT) to 300°C, leading to the reduction in grain boundaries. Absorption analyses show the reduction in band-tail states within the bandgap, supporting annihilation of defects, on the samples deposited at 250°C and 300°C. XPS spectra confirm the improved O\u003csub\u003e2\u003c/sub\u003e incorporation and reduction in oxygen vacancies in sample deposited at 300°C. Highest hall mobility of 46.09 cm\u003csup\u003e2\u003c/sup\u003e/V-sec has been observed on sample deposited at RT, and is dominated by defects. Whereas, films deposited at 250°C and 300°C exhibit Hall bulk mobilities of 20.43 cm\u003csup\u003e2\u003c/sup\u003e/V-sec and 31.63 cm\u003csup\u003e2\u003c/sup\u003e/V-sec, respectively. Further, bottom-gate ZnO thin film transistors (TFTs) are also fabricated on SiO\u003csub\u003e2\u003c/sub\u003e/p-Si substrate. Variation in substrate temperature showed performance enhancement in terms of leakage current, threshold voltage, sub-threshold swing and I\u003csub\u003eON\u003c/sub\u003e/I\u003csub\u003eOFF\u003c/sub\u003e ratio. Devices deposited at 300°C resulted in O\u003csub\u003e2\u003c/sub\u003e-rich surface through chemisorption, which led to the reduction in leakage current of upto 10\u003csup\u003e-12\u003c/sup\u003eA and 10-fold reduction in sub-threshold swing from 30V to 2.8V. Highest field-effect mobility of 1.1 cm\u003csup\u003e2\u003c/sup\u003e/V-sec has been achieved when the ZnO thickness in the TFT is reduced to 50 nm.\u003c/p\u003e","manuscriptTitle":"Effect of Deposition Temperature in RF Sputtered ZnO Thin Films on ZnO TFT Performance","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-07-18 16:06:17","doi":"10.21203/rs.3.rs-4599511/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b9662353-3593-4ec3-9d07-42b4ac48951f","owner":[],"postedDate":"July 18th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-07-18T16:06:22+00:00","versionOfRecord":[],"versionCreatedAt":"2024-07-18 16:06:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4599511","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4599511","identity":"rs-4599511","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}