Fabrication and characterization of nanostructured Cadmium Oxide thin films doped with Indium by sol-gel spin-coating for CdO (n)/Si (p) heterojunction photodiode applications

Fabrication and characterization of nanostructured Cadmium Oxide thin films doped with Indium by sol-gel spin-coating for CdO (n)/Si (p) heterojunction photodiode applications | 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 Fabrication and characterization of nanostructured Cadmium Oxide thin films doped with Indium by sol-gel spin-coating for CdO (n)/Si (p) heterojunction photodiode applications M. Jlassi, I. Ben Miled, I. Sta, M. Zouaoui This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6444485/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 21 Jul, 2025 Read the published version in Silicon → Version 1 posted 10 You are reading this latest preprint version Abstract In this study, nanostructured thin films based on cadmium oxide doped with Indium CdO: In were fabricated by sol-gel spin-coating technique on p-type monocrystalline silicon c-Si (p) for the integration in n-p heterojunction photodiode applications. A comprehensive analysis for structural, morphological, compositional, optical and electrical properties of the surface of undoped CdO and CdO:In films on silicon substrates is conducted. Upon In doping, the cubic structure of polycrystalline CdO film and the preferential (111) orientation are maintained. In the uniformly distributed nanostructures network, the average grain size of CdO/Si is decreased from 35 nm to 23 nm for CdO:In/Si. Smooth heterointerfaces with good adhesion of CdO-based thin films to the Si substrate imply suitable heterojunction quality and favorize good surface passivation and electrical transport. Based on chemical composition and electronic states analysis, In dopants uniform distribution and incorporation by In 3+ ion substitution in quasi-stoichiometric CdO thin films are confirmed. The anti-reflection role of undoped CdO and doped CdO: In is validated through lower optical reflectance compared to bare Si substrate, especially in visible range. The electrical current-voltage I-V characteristics in dark and under illumination conditions are employed to determine the main diode parameters of different Cd (n)/ c-Si (p) heterojunction structures. A clear rectifying diode behavior with asymmetrical and non-linear dependency is obtained for CdO/Si and CdO:In heterojunctions. Compared to undoped CdO thin films, doping of CdO with In leads to higher ideality factor and reverse saturation current, but lower potential barrier and series resistance. Higher photogenerated current at Si region with more light sensitivity is obtained CdO:In/Si diode owing to better transparency and wider bandgap than undoped CdO film.Owing to lower conduction band offset at CdO:In/Si heterojunction, the charge carrier transport is improved for CdO:In compared to CdO/Si diode. Our results demonstrate the effective integration of nanostructured undoped and In-doped CdO thin films in CdO/Si n-p heterojunction photodiode applications. Metal oxide thin films n-p heterojunction Sol–gel method Nanostructures Photodiode optoelectronic properties Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Metal-oxide-semiconductors are extensively employed in various optoelectronic applications owing to high optical transmission and excellent electrical conductivity [ 1 ]. Among transparent conductive oxides, cadmium oxide (CdO) thin films have acquired considerable scientific progress and technological development [ 2 ], [ 3 ], [ 4 ], [ 5 ]. It is well known that CdO is an n-type semiconductor with a direct band gap energy ranging from 2.2 to 3.5 eV, featuring simple crystal structure, high conductivity and suitability for p–njunction applications [ 4 ], [ 5 ], [ 6 ], [ 7 ]. For the fabrication of high quality CdO layers, several physical and chemical methods either in vacuum deposition equipment or in solution-based coating setups have been used, such as spray pyrolysis[ 8 ], molecular beam epitaxy [ 9 ], magnetronsputtering [ 10 ],pulsed laser deposition [ 11 ] and sol–gel coating [ 4 ], [ 12 ]. To overcome the resistivity limitations of CdO semiconductor due to native defects of oxygen vacancies and cadmium interstitials, doping with different metal nanomaterials has been applied to further improve the optoelectronic properties [ 2 ], [ 3 ], [ 13 ], [ 14 ], [ 15 ], [ 16 ]. Recently, we have developed undoped CdO and Indium-doped CdO (CdO:In) thin films with good optical transparency and acceptable electrical conductivity prepared by simple and cost-effective sol–gel spin-coating at low temperature [ 4 ], [ 5 ]. In this work, we report the fabrication and characterization integration of nanostructured CdO and CdO:In thin films by sol-gel spin-coating technique on p-type monocrystalline silicon c-Si (p) for the integration in n-p heterojunction photodiode applications. Along this work, the structural, morphological, compositional, optical and electrical properties of the surface of undoped CdO and CdO: In films on silicon substrates are investigated. The preservation of cubic structure, the preferential (111) orientation and the reduction of grains size after the In doping of CdO films are presented. The uniform distribution and the incorporation of In dopants by In 3+ ion substitution mechanism are confirmed. Both undoped CdO and doped CdO:In possess anti-reflection capabilities inducing lower optical reflectance of underlying Si substrate. The characteristic diode parameters of different Cd (n) / c-Si (p) hetero-junction structures are determined through I-V measurements of rectifying behavior in dark and under illumination conditions. Mainly, higher photocurrent are obtained for CdO:In/Si compared to CdO/Si due to lower potential barrier and series resistance. Overall, our work indicates the suitability of CdO-based thin films for CdO/Si n-p heterojunction photodiode applications. 2. Experimental details In this study, the following heterojunction structures Al / CdO (n) / Si (p) / Al, and Al / CdO:In (n) / Si (p) / Al were fabricated for sol-gel spin coating, the used solution was prepared using 0.3M cadmium acetate dihydrate [C 4 H 6 CdO 4 .2H 2 O] together with 2- Metoxyethanol and monoethanolamine (MEA). As solvent and stabilizer, 2- Metoxyethanol and MEA were used, respectively. To obtain a homogeneous coating solution, a stirring step for 2 hours was done during the preparation. The CdO were deposited on 600 µm-thick p-type monocrystalline silicon substrate (1–10 Ωcm resistivity). Before using the silicon substrates, a first step of solvent cleaning with acetone, ethanol and deionized water successively in an ultrasonic bath was applied. Then, native oxide was removed from c-Si surface by HF: H 2 O (1:10) solution, followed by DI water rinsing. The coating solution was applied onto c-Si substrate surface followed by a rotation at 3500 rpm. After the spin coating, the films were dried at 250°C for 10 min to evaporate the organic compounds. For In doped CdO film preparation, Indium (III) acetate was added to Cd acetate to form the desired mixture for spin coating process. This coating/drying process was repeated for four sequences followed by a thermal annealing at 450°C in air for one hour [ 4 ]. The thickness of the CdO films was determined to be 211 nm using profilometry technique. Aluminum metal films were evaporated on the back of Si wafer to form the back ohmic contacts. A grid of Al with 100 nm thickness was used as front contacts. Then, the heterojunction structures underwent a thermal annealing at 570°C for 3 min in N 2 atmosphere. The structural, morphological, compositional, optical and electrical properties of the Al / CdO (n) / Si (p) / Al and Al / In: CdO (n) / Si (p) / Al structures were studied. To investigate the crystallographic structure of the films, X-ray diffraction (XRD) was performed using X-ray diffractometer (Bruker D8 advance) with Cu Kα ( \(\:{\lambda\:}_{\text{C}\text{u}\text{K}{\alpha\:}}\) = 1.5418 Å) radiation, for 2θ values in the range of 20–70°. The measurements of the optical transmittance were carried out in the wavelength range of 200–1000 nm using a UV–Vis–NIR (Lambda 950) spectrophotometer. The surface morphology of the CdO-based thin films as well as CdO/Si and CdO:In heterostructures are investigated by were examined using the scanning electron microscopy (SEM) (Hitachi S4800 operated at 2 kV for secondary electron imaging. The deposited films were examined using energy-dispersive X-ray (EDX) spectroscopy for the analysis of material compositions. The XPS measurements were carried out using a Thermo Scientific SPECS PHOIBOS 100 DLD Under ultra-high vacuum (better than 1 × 10 −9 mbar). Furthermore, electrical properties were characterized with current–voltage (I–V) measurements using a Keithley 2400setup. While, DC power supply and Keithley electrometer were employed for dark I-V measurements, tungsten–halogen lamp light was utilized illuminated I-V. All the mentioned measurements were conducted at room temperature. 3. Results and discussions 3.1. Structural properties of CdO-based films on Si substrate The X- ray diffraction patterns of the undoped and In-doped CdO thin films deposited on glass substrate are shown in Fig. 1. The patterns of CdO-based films shows well-defined diffraction peaks at 2θ = 33.05°, 38.41°, 55.41°, 65.9° and 69.24° assigned to (111), (200), (220), (311) and (222) planes, respectively. The diffraction peaks of these layers are indexed and compared with standard bulk CdO pattern from the JCPDS data [JCPDS data card No. 75-0594]. It is confirmed that all the films were grown in cubic structure. The lattice parameter for the undoped and In-doped thin films were found to be 4.67, 4.71 respectively. This change of lattice parameter after doping is accompanied with a shift of (111) and (200) peaks towards lower angles. In fact, the ionic radius size of In 3+ (0.80Å) are different than that of Cd 2+ (0.97Å) [15], [16], [17], [18] . Therefore, the substitution of Cd 2+ by In 3+ ions induces lattice expansion causing such peak shift. Importantly, this is a clear indication that doping In 3+ ions were successfully incorporated into the CdO matrix. The preferential orientation (111) of undoped CdO on Si substrate is preserved for doped CdO:In. A closer look at XRD patterns reveals that the peak intensities of different planes, especially dominant (111) and (200), decrease for doped-CdO films relative to undoped CdO counterpart. This peak drop indicates a crystallinity degradation of doped-CdO films after the incorporation of In ions that may lead to prevention of CdO nanocrystals aggregation [19]. 3.2. Morphological properties of CdO-based films on Si substrate Fig. 2 shows the surface morphology and the cross-section images of the undoped CdOand CdO:In thin films deposited on Si substrate. The surfaces of different polycrystalline films consist of uniformly distributed nanostructured grains with irregular rounded or faceted shapes, along with several small aggregations and agglomerations. In the cross-section images, the regions of undoped and doped CdO films are differentiated from Si substrate based on contrast. It is clearly observed that different CdO/Si heterointerfaces are smooth ensuring good adhesion of thin films to the substrate and suitable heterojunction quality for surface passivation and electrical transport. The grain sizes distribution and the average size for undoped CdO and CdO:In thin films on Si wafer substrate are determined from the corresponding SEM images and plotted as histograms in Fig. 3. The average particle size of CdO/Si is decreased from 35 nm to 23 nm CdO:In/Si. By introducing In doping, the average grain size decreases and the surface uniformity improves compared to undoped CdO layer. These morphological changes can be attributed to the incorporation of In dopants into expanded CdO lattice by ionic substitution and in the interstitial sites [5], [20]. 3.3. Compositional properties of CdO-based films on Si substrate Fig. 4 shows the EDX spectra and the elemental colored maps of CdO/Si and CdO:In/Si structures. It can be deduced that CdO films are characterized by nearly a stoichiometric ratio of Cd and O with oxygen deficiency. Interestingly, In dopant elements are clearly incorporated and uniformly distributed over the film surfaces. The chemical composition and electronic states of undoped and doped CdO films on Si are investigated using XPS analysis. The wide survey and the narrow scan spectra of Cd, O, In and Ag elements are shown in Fig 5 (a) (b). The binding energy of Cd 3d 5/2 and Cd 3d 3/2 correspond to 406 eV and 412 eV, indicating Cd 2+ states for all undoped and doped CdO film structures [21]. The binding energy corresponding to the peak O1s is located at 531 eV confirming Cd-O bonds, and O 2- oxidation statefor all CdO based films. For CdO:In, a less intense binding energy peaks at 444.5 eV and 452.03 eV are attributed to the In 3d 5/2 and In 3d 3/2 respectively with In 3+ oxidation state [14], [22]. These XPS results confirm clearly the incorporation of In doping elements into CdO:In. 3.4. Optical propertiesof CdO-based films on Si substrate Fig. 6 shows the reflectance spectra of CdO (n) / Si (p) andCdO:In (n) / Si (p) structures. It is obvious that all undoped and doped CdO films lead to quite low reflectance than bare Si substrate over the full UV-VIS-NIR spectrum, especially for wavelengths shorter than 800 nm. This is a clear indication for the role of CdO-based film as anti-reflection coatings on Si wafer. However, the reflectance of doped CdO:In/Si sample is higher than undoped CdO/Si counterpart, in particular for wavelengths shorter than 500 nm. This can be attributed to the contribution of free carrier absorption due to the increase of conductivity with higher charge carrier density [22]. 3.5. Electrical characterization of different CdO (n) / c-Si (p) diodes In this section, we perform a detailed analysis of the current-voltage (I-V) characteristics in dark and under illumination conditions to determine the main diode parameters including reverse saturation current, threshold voltage, barrier height, series resistance and diode ideality factor of different CdO (n)/ c-Si (p) heterojunction structures. Beside the I-V characteristics, the above-mentioned electrical parameters can be also calculated using different analytical approach including Cheung's and Norde's methods (of supplementary information) [23]. A schematic illustration of Al/ CdO (n)/ c-Si (p)/ Al architecture is presented in Fig. 7. Table 1: Electrical parameters of CdO (n)/c-Si (p) and CdO: In (n)/c-Si (p) heterojunctions determined using several methods. Sample Threshold voltage (V) Is (A) Ideality factor n ɸ B (eV) ɸ B Norde (eV) Rs dV/d(Ln I) –I (Ω) Rs H(I) –I (Ω) CdO/Si 0.15 4 2.12 0.69 0.74 1300 1387 CdO: In/Si 0.11 2.84 2.99 0.65 0.71 1020 985 The current-voltage ( I-V ) characteristics in voltage range from -1V to +1V of different CdO (n)/ c-Si (p) heterojunction diodes under dark condition are shown in Fig. 8 (a) (b). For the fabricated CdO (n)/c-Si (p) and CdO:In (n)/c-Si (p) diodes, a clear rectifying behavior with asymmetrical and non-linear dependency is observed. Logarithmic plots of the I-V characteristics at relatively high forward voltage bias between 0.4 and 1V are shown in Figure S1. Table 1 gathers the values of main diode parameters in dark including reverse saturation current I 0 , threshold voltage, barrier height ϕ b , series resistance R s and diode ideality factor n of different Cd (n)/ c-Si (p) heterojunction structures. The values of ideality factors n in dark condition are 2.12 and 2.99 for CdO (n)/c-Si (p) and CdO: In (n)/c-Si (p) diodes. Regarding the dark saturation current I 0 , the obtained values from I-V characteristics are (2.36 × 10 - 8 and 1.12 × 10 -8 ,) for CdO (n)/c-Si (p) and CdO: In (n)/c-Si (p) heterojunctions, respectively. It is well known that such n values greater than 2 imply a dominant leakage current over tunneling currents and generation-recombination mechanisms [24]. This non-ideality behavior of different CdO (n) /c-Si (p) heterojunction diodes can be attributed to a synergy of phenomena such as thickness anomalies of CdO films, interfacial charge non-uniformity and bias-dependent barrier heights [25]. Moreover, as can be noticed in SEM pictures of Fig. 2, the disparity in the effective contact area between the CdO multi-shape nanoparticles composing different CdO films and Si substrate and the heterointerface corrugations can induce a change in the current density, trapping interface states as well as large distribution of barrier inhomogeneities [26]. It is noticed that the ideality factor n is lower for undoped CdO (n)/c-Si (p) compared to CdO: In (n)/c-Si (p). Such difference can be assumed to several detrimental effects of doping at film bulk and at both heterointerface with semiconductor Si CdO/c-Si and with metal contact Al/CdO. This includes aggravated defects film bulk and heterointerfaces, polarization-dependent barrier height, non-uniformity of the interfacial charges and the existence of interfacial thin layer of native oxide at the heterointerfaces of CdO films with semiconductor Si CdO/c-Si and with metal contact Al/CdO. For the barrier height ϕ b , parameter, the determined values from I-V characteristics in dark are 0.69 eV and 0.65 eV for CdO (n)/c-Si (p) and CdO:In (n)/c-Si (p) diodes, respectively. Using Norde’s method, the estimated barrier height values are 0.74 eV and 0.71 eV for CdO (n)/c-Si (p) and CdO:In (n)/c-Si (p) heterojunctions, respectively. The change in barrier height of CdO/Si after doping with In can be ascribed to the influence on the space charge region and induced potential fluctuations at the heterointerface [24], [25]. This implies that the current flow in the diode is more preferential through lower barriers in the potential distribution. Another important electrical parameter is the series resistance, depending on nature and quality of contacts. The experimental values are estimated using Cheung’s method from the linear and the slope parts of the curves (dV / dLn (I)) versus I, as shown in figure S3. The values of series resistance R s drops from 1300 Ω for CdO (n)/c-Si (p) to 1020 Ω for CdO:In (n)/c-Si (p). Using the H (I)- I method (figure S4) , the series resistance values are 1387 Ω and 985 Ω for CdO (n)/c-Si (p) and CdO:In (n)/c-Si (p) diodes, respectively.It is noteworthy that the determined results from both methods are consistent. This decrease in series resistance due to In doping of CdO can be ascribed to several mechanisms like the change in free charge carriers, minimization of trapping defect sites in grain boundaries of polycrystalline CdO films and at both semiconductor Si CdO/c-Si and with metal contact Al/CdO heterointerfaces as well as the alteration of work function and forward leakage path to minority charge carriers [27]. The current -voltage characteristics of Al/CdO/Si/Al and Al/CdO:In/Si/Al diodes under illumination is shown in Fig. 9. It is observed that both forward and reverse bias currents increase with light intensity for different CdO/Si and CdO:In diodes. Such photocurrent rise can be attributed electron-hole pairs generation in space charge zone at the heterojunctions between CdO thin films and Si. The change for both CdO-based diodes is more pronounced for reverse bias in contrast to forward bias, implying closer ideal behavior under illumination [28]. As CdO:In layer enable better transparency than CdO film due to wider band gap, higher photogenerated current at Si region with more light sensitivity is expected for CdO:In/Si diode. In order to elucidate the charge transfer mechanisms in both CdO (n)/Si (p) heterojunction diodes, the corresponding energy band diagram is depicted in Fig. 10. The corresponding physical parameters are presented in table S1 of the supplementary information. It is noticed that the conduction band offset ΔE C is lower than valance band discontinuity ΔE V . This means energetic barrier for electrons is much lower than for holes, implying higher injection of electrons from CdO (n) to Si (p) compared to opposite mechanism of holes injection from Si (p) to CdO (n).Under forward bias, lower potential barrier enables better injection of electrons into p-Si, thus, leading to a rapid increase of forward current. Moreover, the injection of electrons into Si (p) induces a widening in depletion region at CdO (n) side and a delay of holes injection, resulting in higher recombination saturation current [29]. Owing to minimal potential barrier height at heterojunction between CdO:In and Si, the resistance is lower for CdO:In compared to CdO/Si diode. As the charge carriers transport is harmfully affected by the trapping sites and charge recombination centers existing at the heterointerfaces and grain boundaries of CdO-based films, higher conductivity with better crystallinity, mobility and charge density is desired for prolonged lifetime, long diffusion length and improved photogenerated carrier collection in photodiodes [30]. Overall, the above-mentioned electrical characteristics point out the good performance of CdO/Si n-p heterojunction and the benefits of In doping of CdO in CdO:In/Si photodiode. Further improvement can be considered in the optimization of process fabrication and post-coating treatments for enhanced CdO-based bulk films quality and better heterointerface with Si substrate. To this end, these obtained optoelectronic results suggest that different n-p CdO/p-Si heterojunction diodes are suitable for photodetector fabrication in optoelectronic applications. Conclusion In this work, nanostructured undoped and In-doped cadmium oxide using sol-gel spin-coating technique are successfully integrated in CdO/Si n-p heterojunction structures. The structural, morphological, compositional, optical and electrical properties of the surface of undoped CdO and CdO: In films on silicon substrates are investigated. It is revealed that undoped and In-doped CdO films on Si substrates are characterized by cubic structure with preferential (111) orientation. Based on XRD results, the incorporation of In dopants preserves the crystallinity of CdO film in spite of possible slight peak shifts, lattice expansion and prevention nanocrystals aggregation. The in-depth microscopic investigation unveils undoped and doped CdO surfaces of different polycrystalline films with uniformly distributed nanostructured grains. After doping, the average particle size of CdO/Si is decreased from 35 nm to 23 nm for CdO:In/Si. The inspection of different CdO/Si heterostructures shows smooth heterointerfaces with good adhesion of CdO-based thin films to the Si substrate for suitable heterojunction quality favorizing good surface passivation and electrical transport. By introducing In dopants, the average grain size decreases and the surface uniformity improves compared to undoped CdO layer. These morphological changes can be attributed to the incorporation of In dopants into expanded CdO lattice by ionic substitution and in the interstitial sites. According to the chemical composition and electronic states analysis of undoped and In-doped CdO films on Si using XPS and EDX analysis, the incorporation and the uniform distribution of In dopants with In 3+ electronic state over the nearly stoichiometric CdO surface is confirmed. The anti-reflection role of undoped CdO and doped CdO:In is validated through lower reflectance compared to bare Si substrate, especially for wavelengths shorter than 800 nm. The electrical current-voltage I-V characteristics in dark and under illumination conditions are employed to determine the main diode parameters including reverse saturation current, barrier height, series resistance and diode ideality factor of different CdO (n)/ c-Si (p) heterojunction structures. A clear rectifying diode behavior with asymmetrical and non-linear dependency is obtained for CdO/Si and CdO:In heterojunctions. Compared to undoped CdO thin films, doping of CdO with In leads to higher ideality factor and reverse saturation current due to a synergy of physical phenomena like thickness anomalies of CdO films, non-uniformity of interfacial charge, existence of interfacial thin layer of native oxide and bias-dependent barrier heights trapping interface states as well as large distribution of barrier inhomogeneities and fluctuations at film bulk and at both heterointerfaces with semiconductor Si CdO/Si and with metal contact Al/CdO In contrast, doping of CdO with In enables lower potential barrier and series resistance. This can be assigned to several mechanisms like the change in free charge carriers , minimization of trapping defect sites in grain boundaries of polycrystalline CdO films and at both semiconductor Si CdO/c-Si and with metal contact Al/CdO heterointerfaces as well as the alteration of work function and forward leakage path to minority charge carriers. For illuminated I-V analysis, higher photogenerated current at Si region with more light sensitivity is obtainedCdO: In /Si diode owing to better transparency and wider bandgap than undoped CdO film. Based on energy band diagram, lower conduction band offset at heterojunction between CdO:In and Si, the charge carrier transport is improved for CdO:In compared to CdO/Si diode owing to prolonged lifetime, long diffusion length and improved photogenerated carrier collection in photodiode. Our findings point out the efficient integration of fabricated n-p CdO/p-Si structures for heterojunction photodiode applications. Declarations Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Funding Statement This work did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors Author Contribution M. 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Moholkar et al. , “Temperature dependent structural, luminescent and XPS studies of CdO:Ga thin films deposited by spray pyrolysis,” J Alloys Compd , vol. 506, no. 2, pp. 794–799, Sep. 2010, doi: 10.1016/j.jallcom.2010.07.072. C. Dantus, D. Timpu, D. Luca, and F. Iacomi, “UV irradiation influence on the structural and optical properties of CdO thin films,” The European Physical Journal Applied Physics , vol. 55, no. 1, p. 10301, Jul. 2011, doi: 10.1051/epjap/2011110055. S. K. Cheung and N. W. Cheung, “Extraction of Schottky diode parameters from forward current-voltage characteristics,” Appl Phys Lett , vol. 49, no. 2, pp. 85–87, Jul. 1986, doi: 10.1063/1.97359. S. Steingrube, O. Breitenstein, K. Ramspeck, S. Glunz, A. Schenk, and P. P. Altermatt, “Explanation of commonly observed shunt currents in c-Si solar cells by means of recombination statistics beyond the Shockley-Read-Hall approximation,” J Appl Phys , vol. 110, no. 1, Jul. 2011, doi: 10.1063/1.3607310. I. L. P. Raj et al. , “A comprehensive study on effect of annealing on structural, morphological and optical properties of CdO and photodetection of heterojunction n-CdO/p-Si diode,” Optik (Stuttg) , vol. 241, p. 166406, Sep. 2021, doi: 10.1016/j.ijleo.2021.166406. Y. Zhang and J. Hao, “Metal-ion doped luminescent thin films for optoelectronic applications,” J Mater Chem C Mater , vol. 1, no. 36, p. 5607, 2013, doi: 10.1039/c3tc31024h. W. Mönch, “Electronic Properties of Semiconductor Interfaces,” in Springer Handbook of Electronic and Photonic Materials , Boston, MA: Springer US, 2006, pp. 147–160. doi: 10.1007/978-0-387-29185-7_8. O. J. Sandberg, S. Dahlström, M. Nyman, S. Wilken, D. Scheunemann, and R. Österbacka, “Impact of a Doping-Induced Space-Charge Region on the Collection of Photogenerated Charge Carriers in Thin-Film Solar Cells Based on Low-Mobility Semiconductors,” Phys Rev Appl , vol. 12, no. 3, p. 034008, Sep. 2019, doi: 10.1103/PhysRevApplied.12.034008. F. Yakuphanoglu, M. Caglar, Y. Caglar, and S. Ilican, “Electrical characterization of nanocluster n-CdO/p-Si heterojunction diode,” J Alloys Compd , vol. 506, no. 1, pp. 188–193, Sep. 2010, doi: 10.1016/j.jallcom.2010.06.174. S. Sebastian, P. Diana, V. Ganesh, D. Nagaraju, and P. V. Raja Shekar, “Synthesis and characterization of Fe-doped CdO nanoparticles via a coprecipitation method: application as a promising photodetector,” Applied Physics A , vol. 131, no. 3, p. 160, Mar. 2025, doi: 10.1007/s00339-025-08270-y. Additional Declarations No competing interests reported. Supplementary Files Supplementaryinformation.docx Cite Share Download PDF Status: Published Journal Publication published 21 Jul, 2025 Read the published version in Silicon → Version 1 posted Editorial decision: Revision requested 10 Jun, 2025 Reviews received at journal 24 May, 2025 Reviews received at journal 20 May, 2025 Reviewers agreed at journal 17 May, 2025 Reviewers agreed at journal 16 May, 2025 Reviewers agreed at journal 16 May, 2025 Reviewers invited by journal 15 May, 2025 Editor assigned by journal 09 May, 2025 Submission checks completed at journal 09 May, 2025 First submitted to journal 14 Apr, 2025 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. 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Jlassi","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABFElEQVRIie3PMUvDQBQH8AeBdIme45VIv4FwErgOET/LlYBZLuAkGaR0ahfd8z2EzpGAXQ7nAzsUAnXpEB1EKIjv2orTJavg/TnuveH94D0AF5c/mcB8nndsirgGIAC+6Ukn2Y0JBtCf7AnWdrIfAySs7CBns/vHd53HPb+3qJsVW44edLJeNTnQocVw9ZyEUqWeH0hOBVuP5vpqeF4ooKelhWjJwmxa4S3Sx1uqiGvBw6MpjKllMSTRNvtCQl7rxpCoSD8MoS2Eh9kECW6FrxowKnk7UYrH8glvoZvdLQOqNjf9QlE7WdxFL/I2TghJ67fPfBmQWTqnTX5hJT9JDrUEOBGm6QIAl7+ElJ3TLi4uLv8r305GVkFEr13SAAAAAElFTkSuQmCC","orcid":"","institution":"University of Manouba","correspondingAuthor":true,"prefix":"","firstName":"M.","middleName":"","lastName":"Jlassi","suffix":""},{"id":458130474,"identity":"14939fe9-95e0-4596-8505-1cf263010490","order_by":1,"name":"I. Ben Miled","email":"","orcid":"","institution":"Borj-Cédria Technopole","correspondingAuthor":false,"prefix":"","firstName":"I.","middleName":"Ben","lastName":"Miled","suffix":""},{"id":458130475,"identity":"674e4a52-8e46-485d-86bc-9c48c92de1c4","order_by":2,"name":"I. Sta","email":"","orcid":"","institution":"Borj-Cédria Technopole","correspondingAuthor":false,"prefix":"","firstName":"I.","middleName":"","lastName":"Sta","suffix":""},{"id":458130476,"identity":"6c19d618-6501-40ac-bc3e-5d7ef2e1f3f1","order_by":3,"name":"M. Zouaoui","email":"","orcid":"","institution":"University of Carthage","correspondingAuthor":false,"prefix":"","firstName":"M.","middleName":"","lastName":"Zouaoui","suffix":""}],"badges":[],"createdAt":"2025-04-14 09:23:24","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6444485/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6444485/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s12633-025-03391-8","type":"published","date":"2025-07-21T15:57:24+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":83138394,"identity":"88b1f414-ebef-4baa-be38-de14e2c51e23","added_by":"auto","created_at":"2025-05-20 11:47:40","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":26274,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eX-ray diffraction spectra of undoped and doped CdO thin films\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figures1.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/673b4266bc51ed3f37195a9d.png"},{"id":83138396,"identity":"352b5a66-d8fe-4437-aa3e-d28dba2478bb","added_by":"auto","created_at":"2025-05-20 11:47:40","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1133923,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eSEM images of surface morphology and corresponding cross-sections for (a,b) undoped CdO/Si (c,d) In-CdO/Si heterojunction structures\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figures2.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/9c3796d95ca6a49dc8f26b43.png"},{"id":83139067,"identity":"9ee37abc-93f2-490a-91af-7f629c02573c","added_by":"auto","created_at":"2025-05-20 11:55:40","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":14937,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eHistogram of grain size distribution and average grain size for the surface of the following structures (a) CdO/Si; (b) CdO: In/Si\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figures3.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/7a12b4503f18a09615272d30.png"},{"id":83138405,"identity":"3b3f5f38-7b14-4dbb-9383-5cf51c806c80","added_by":"auto","created_at":"2025-05-20 11:47:40","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1685388,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eEDX spectra (left) and elemental colored maps (right) for the surface of the following structures (a) CdO/Si ; (b) CdO:In/Si\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figures4.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/3a6b2839c3119733b872e2d6.png"},{"id":83139069,"identity":"2831f052-c5e3-4cd8-8404-571a6621be2d","added_by":"auto","created_at":"2025-05-20 11:55:40","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":125209,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ea) \u003c/strong\u003e\u003cem\u003eA typical X-ray photoelectron spectrum for undoped n-CdO/p-Si heterojunction\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eb) \u003c/strong\u003e\u003cem\u003eA typical X-ray photoelectron spectrum for 3 wt. % n-In: CdO/p-Si films\u003c/em\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/80d7c010cd72f0b3780d5951.png"},{"id":83138402,"identity":"5945f002-47a1-45f5-a61c-d2e45f8ddfd7","added_by":"auto","created_at":"2025-05-20 11:47:40","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":24611,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eThe reflectance spectra of the CdO and In:CdO films deposited on Si substrate\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figures7.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/fc2cbe694c81119f259b7706.png"},{"id":83139070,"identity":"c79482a2-870f-4237-a78d-e913f5de2c42","added_by":"auto","created_at":"2025-05-20 11:55:40","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":15240,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eArchitecture of \u003c/em\u003eAl/ CdO (n)/ c-Si (p)/ Al\u003cem\u003e structures\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figures8.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/989d7bf6cfa1b1668dd13ce2.png"},{"id":83139068,"identity":"3fdec4aa-58d6-4cac-8210-b0d71c16c526","added_by":"auto","created_at":"2025-05-20 11:55:40","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":30270,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003e\u003cstrong\u003ea)\u003c/strong\u003e\u003c/em\u003e\u003cem\u003e I-V Characteristic of n-CdO/p-Si heterojunction under dark and under illumination.\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cem\u003e\u003cstrong\u003eb) \u003c/strong\u003e\u003c/em\u003e\u003cem\u003eI-V Characteristic of n-(In:CdO)/p-Si heterojunction under dark and under illumination.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figures9.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/69ae1448ed91d7f3f4e98a50.png"},{"id":83138401,"identity":"062b2715-ee92-4abd-9a6e-db97bc2bed2f","added_by":"auto","created_at":"2025-05-20 11:47:40","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":25197,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eI-V Characteristic of n-(In:CdO)/p-Si heterojunction under dark and illumination.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figures10.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/92acc3b1d1453243fe52ebba.png"},{"id":83139467,"identity":"03b8a5ce-5fa7-4ff6-9c3d-8d8db2ee66ae","added_by":"auto","created_at":"2025-05-20 12:03:40","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":163017,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eEnergy-band diagram for n-CdO/p-Si heterojunction under zero bias\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figures11.png","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/77dec9ba59189755ea60d6e3.png"},{"id":87756848,"identity":"bea832b1-4bda-4143-8d9e-a397e570cc81","added_by":"auto","created_at":"2025-07-28 16:09:40","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4580266,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/087d116f-38da-443f-9c8b-a85533414607.pdf"},{"id":83138404,"identity":"3a5ba398-20a2-4e26-8df5-0343ab619f1e","added_by":"auto","created_at":"2025-05-20 11:47:40","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":170261,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-6444485/v1/11ec727f7cde660a7fde8e5e.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003e\u003cstrong\u003eFabrication and characterization of nanostructured Cadmium Oxide thin films doped with Indium by sol-gel spin-coating for CdO (n)/Si (p) heterojunction photodiode applications\u003c/strong\u003e\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eMetal-oxide-semiconductors are extensively employed in various optoelectronic applications owing to high optical transmission and excellent electrical conductivity [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Among transparent conductive oxides, cadmium oxide (CdO) thin films have acquired considerable scientific progress and technological development [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. It is well known that CdO is an n-type semiconductor with a direct band gap energy ranging from 2.2 to 3.5 eV, featuring simple crystal structure, high conductivity and suitability for p\u0026ndash;njunction applications [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFor the fabrication of high quality CdO layers, several physical and chemical methods either in vacuum deposition equipment or in solution-based coating setups have been used, such as spray pyrolysis[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e], molecular beam epitaxy [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], magnetronsputtering [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e],pulsed laser deposition [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] and sol\u0026ndash;gel coating [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. To overcome the resistivity limitations of CdO semiconductor due to native defects of oxygen vacancies and cadmium interstitials, doping with different metal nanomaterials has been applied to further improve the optoelectronic properties [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. Recently, we have developed undoped CdO and Indium-doped CdO (CdO:In) thin films with good optical transparency and acceptable electrical conductivity prepared by simple and cost-effective sol\u0026ndash;gel spin-coating at low temperature [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this work, we report the fabrication and characterization integration of nanostructured CdO and CdO:In thin films by sol-gel spin-coating technique on p-type monocrystalline silicon c-Si (p) for the integration in n-p heterojunction photodiode applications. Along this work, the structural, morphological, compositional, optical and electrical properties of the surface of undoped CdO and CdO: In films on silicon substrates are investigated. The preservation of cubic structure, the preferential (111) orientation and the reduction of grains size after the In doping of CdO films are presented. The uniform distribution and the incorporation of In dopants by In\u003csup\u003e3+\u003c/sup\u003e ion substitution mechanism are confirmed. Both undoped CdO and doped CdO:In possess anti-reflection capabilities inducing lower optical reflectance of underlying Si substrate. The characteristic diode parameters of different Cd (n) / c-Si (p) hetero-junction structures are determined through I-V measurements of rectifying behavior in dark and under illumination conditions. Mainly, higher photocurrent are obtained for CdO:In/Si compared to CdO/Si due to lower potential barrier and series resistance. Overall, our work indicates the suitability of CdO-based thin films for CdO/Si n-p heterojunction photodiode applications.\u003c/p\u003e"},{"header":"2. Experimental details","content":"\u003cp\u003eIn this study, the following heterojunction structures Al / CdO (n) / Si (p) / Al, and Al / CdO:In (n) / Si (p) / Al were fabricated for sol-gel spin coating, the used solution was prepared using 0.3M cadmium acetate dihydrate [C\u003csub\u003e4\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003eCdO\u003csub\u003e4\u003c/sub\u003e.2H\u003csub\u003e2\u003c/sub\u003eO] together with 2- Metoxyethanol and monoethanolamine (MEA). As solvent and stabilizer, 2- Metoxyethanol and MEA were used, respectively. To obtain a homogeneous coating solution, a stirring step for 2 hours was done during the preparation. The CdO were deposited on 600 \u0026micro;m-thick p-type monocrystalline silicon\u0026thinsp;\u0026lt;\u0026thinsp;100\u0026thinsp;\u0026gt;\u0026thinsp;substrate (1\u0026ndash;10 Ωcm resistivity). Before using the silicon substrates, a first step of solvent cleaning with acetone, ethanol and deionized water successively in an ultrasonic bath was applied. Then, native oxide was removed from c-Si surface by HF: H\u003csub\u003e2\u003c/sub\u003eO (1:10) solution, followed by DI water rinsing. The coating solution was applied onto c-Si substrate surface followed by a rotation at 3500 rpm. After the spin coating, the films were dried at 250\u0026deg;C for 10 min to evaporate the organic compounds. For In doped CdO film preparation, Indium (III) acetate was added to Cd acetate to form the desired mixture for spin coating process. This coating/drying process was repeated for four sequences followed by a thermal annealing at 450\u0026deg;C in air for one hour [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe thickness of the CdO films was determined to be 211 nm using profilometry technique. Aluminum metal films were evaporated on the back of Si wafer to form the back ohmic contacts. A grid of Al with 100 nm thickness was used as front contacts. Then, the heterojunction structures underwent a thermal annealing at 570\u0026deg;C for 3 min in N\u003csub\u003e2\u003c/sub\u003e atmosphere.\u003c/p\u003e \u003cp\u003eThe structural, morphological, compositional, optical and electrical properties of the Al / CdO (n) / Si (p) / Al and Al / In: CdO (n) / Si (p) / Al structures were studied. To investigate the crystallographic structure of the films, X-ray diffraction (XRD) was performed using X-ray diffractometer (Bruker D8 advance) with Cu Kα (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\lambda\\:}_{\\text{C}\\text{u}\\text{K}{\\alpha\\:}}\\)\u003c/span\u003e\u003c/span\u003e= 1.5418 \u0026Aring;) radiation, for 2θ values in the range of 20\u0026ndash;70\u0026deg;. The measurements of the optical transmittance were carried out in the wavelength range of 200\u0026ndash;1000 nm using a UV\u0026ndash;Vis\u0026ndash;NIR (Lambda 950) spectrophotometer. The surface morphology of the CdO-based thin films as well as CdO/Si and CdO:In heterostructures are investigated by were examined using the scanning electron microscopy (SEM) (Hitachi S4800 operated at 2 kV for secondary electron imaging. The deposited films were examined using energy-dispersive X-ray (EDX) spectroscopy for the analysis of material compositions. The XPS measurements were carried out using a Thermo Scientific SPECS PHOIBOS 100 DLD Under ultra-high vacuum (better than 1 \u0026times; 10\u003csup\u003e\u0026minus;9\u003c/sup\u003e mbar). Furthermore, electrical properties were characterized with current\u0026ndash;voltage (I\u0026ndash;V) measurements using a Keithley 2400setup. While, DC power supply and Keithley electrometer were employed for dark I-V measurements, tungsten\u0026ndash;halogen lamp light was utilized illuminated I-V. All the mentioned measurements were conducted at room temperature.\u003c/p\u003e"},{"header":"3. Results and discussions","content":"\u003cp\u003e\u003cstrong\u003e3.1. Structural properties of CdO-based films on Si substrate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe X- ray diffraction patterns of the undoped and In-doped CdO thin films deposited on glass substrate are shown in Fig. 1. The patterns of CdO-based films shows well-defined diffraction peaks at 2\u0026theta; = 33.05\u0026deg;, 38.41\u0026deg;, 55.41\u0026deg;, 65.9\u0026deg; and 69.24\u0026deg; assigned to (111), (200), (220), (311) and (222) planes, respectively.\u0026nbsp;The diffraction peaks of these layers are indexed and compared with standard bulk CdO pattern from the JCPDS data [JCPDS data card No. 75-0594]. It is confirmed that all the films were grown in cubic structure. The lattice parameter for the undoped and In-doped thin films were found to be 4.67, 4.71 respectively. This change of lattice parameter after doping is accompanied with a shift of (111) and (200) peaks towards lower angles. In fact, the ionic radius size of In\u003csup\u003e3+\u003c/sup\u003e (0.80\u0026Aring;) are different than that of Cd\u003csup\u003e2+\u003c/sup\u003e (0.97\u0026Aring;) \u003cspan class=\"Policepardfaut1\"\u003e\u003cspan lang=\"EN-US\"\u003e[15], [16], [17], [18]\u003c/span\u003e\u003c/span\u003e. Therefore, the substitution of Cd\u003csup\u003e2+\u003c/sup\u003e by In\u003csup\u003e3+\u003c/sup\u003eions induces lattice expansion causing such peak shift. Importantly, this is a clear indication that doping In\u003csup\u003e3+\u003c/sup\u003e ions were successfully incorporated into the CdO matrix.\u003c/p\u003e\n\u003cp\u003eThe preferential orientation (111) of undoped CdO on Si substrate is preserved for doped CdO:In. A closer look at XRD patterns reveals that the peak intensities of different planes, especially dominant (111) and (200), decrease for doped-CdO films relative to undoped CdO counterpart. This peak drop indicates a crystallinity degradation of doped-CdO films after the incorporation of In ions that may lead to prevention of CdO nanocrystals aggregation [19].\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.2. Morphological properties of CdO-based films on Si substrate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFig. 2 shows the surface morphology and the cross-section images of the undoped CdOand CdO:In thin films deposited on Si substrate. The surfaces of different polycrystalline films consist of uniformly distributed nanostructured grains with irregular rounded or faceted shapes, along with several small aggregations and agglomerations.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn the cross-section images, the regions of undoped and doped CdO films are differentiated from Si substrate based on contrast. It is clearly observed that different CdO/Si heterointerfaces are smooth ensuring good adhesion of thin films to the substrate and suitable heterojunction quality for surface passivation and electrical transport.\u003c/p\u003e\n\u003cp\u003eThe grain sizes distribution and the average size for undoped CdO and CdO:In thin films on Si wafer substrate are determined from the corresponding SEM images and plotted as histograms in Fig. 3. The average particle size of CdO/Si is decreased from 35 nm to 23 nm CdO:In/Si.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBy introducing In doping, the average grain size decreases and the surface uniformity improves compared to undoped CdO layer. These morphological changes can be attributed to the incorporation of In dopants into expanded CdO lattice by ionic substitution and in the interstitial sites [5], [20]. \u0026nbsp;\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.3. Compositional properties of CdO-based films on Si substrate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFig. 4 shows the EDX spectra and the elemental colored maps of CdO/Si and CdO:In/Si structures. It can be deduced that CdO films are characterized by nearly a stoichiometric ratio of Cd and O with oxygen deficiency. Interestingly, In dopant elements are clearly incorporated and uniformly distributed over the film surfaces.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe chemical composition and electronic states of undoped and doped CdO films on Si are investigated using XPS analysis. The wide survey and the narrow scan spectra of Cd, O, In and Ag elements are shown in Fig 5 (a) (b).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe binding energy of Cd 3d\u003csub\u003e5/2\u0026nbsp;\u003c/sub\u003eand Cd 3d\u003csub\u003e3/2\u0026nbsp;\u003c/sub\u003ecorrespond to 406 eV and 412 eV, indicating Cd\u003csup\u003e2+\u003c/sup\u003e states for all undoped and doped CdO film structures [21]. The binding energy corresponding to the peak O1s is located at 531 eV confirming Cd-O bonds, and O\u003csup\u003e2-\u003c/sup\u003e oxidation statefor all CdO based films. For CdO:In, a less intense binding energy peaks at 444.5 eV and 452.03 eV are attributed to the In 3d\u003csub\u003e5/2\u0026nbsp;\u003c/sub\u003eand In 3d\u003csub\u003e3/2\u0026nbsp;\u003c/sub\u003erespectively with In\u003csup\u003e3+\u003c/sup\u003eoxidation state [14], [22]. These XPS results confirm clearly the incorporation of In doping elements into CdO:In.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.4. Optical propertiesof CdO-based films on Si substrate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFig. 6 shows the reflectance spectra of CdO (n) / Si (p) andCdO:In (n) / Si (p) structures. It is obvious that all undoped and doped CdO films lead to quite low reflectance than bare Si substrate over the full UV-VIS-NIR spectrum, especially for wavelengths shorter than 800 nm. This is a clear indication for the role of CdO-based film as anti-reflection coatings on Si wafer. However, the reflectance of doped CdO:In/Si sample is higher than undoped CdO/Si counterpart, in particular for wavelengths shorter than 500 nm. This can be attributed to the contribution of free carrier absorption due to the increase of conductivity with higher charge carrier density [22].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e3.5. Electrical characterization of different CdO (n) / c-Si (p) diodes\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn this section, we perform a detailed analysis of the current-voltage (I-V) characteristics in dark and under illumination conditions to determine the main diode parameters including reverse saturation current, threshold voltage, barrier height, series resistance and diode ideality factor of different CdO (n)/ c-Si (p) heterojunction structures. Beside the I-V characteristics, the above-mentioned electrical parameters can be also calculated using different analytical approach including Cheung\u0026apos;s and Norde\u0026apos;s methods (of supplementary information) [23]. A schematic illustration of Al/ CdO (n)/ c-Si (p)/ Al architecture is presented in Fig. 7.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003cem\u003eTable 1:\u0026nbsp;\u003c/em\u003e\u003c/strong\u003e\u003cem\u003eElectrical parameters of CdO (n)/c-Si (p) and CdO: In (n)/c-Si (p) heterojunctions determined using several methods.\u003c/em\u003e\u003c/p\u003e\n\u003cdiv align=\"\"\u003e\n \u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"658\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 118px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eSample\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 90px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eThreshold voltage (V)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eIs (A)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 83px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eIdeality factor\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003en\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eɸ\u003csub\u003eB\u003c/sub\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e(eV)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 65px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eɸ\u003csub\u003eB\u003c/sub\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eNorde (eV)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 115px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eRs\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003edV/d(Ln I) \u0026ndash;I\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e(Ω)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 68px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eRs\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003eH(I) \u0026ndash;I\u003c/strong\u003e\u003c/p\u003e\n \u003cp\u003e\u003cstrong\u003e(Ω)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 118px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCdO/Si\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 90px;\"\u003e\n \u003cp\u003e0.15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 83px;\"\u003e\n \u003cp\u003e2.12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e0.69\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 65px;\"\u003e\n \u003cp\u003e0.74\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 115px;\"\u003e\n \u003cp\u003e1300\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 68px;\"\u003e\n \u003cp\u003e1387\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 118px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCdO: In/Si\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 90px;\"\u003e\n \u003cp\u003e0.11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 56px;\"\u003e\n \u003cp\u003e2.84\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 83px;\"\u003e\n \u003cp\u003e2.99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 64px;\"\u003e\n \u003cp\u003e0.65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 65px;\"\u003e\n \u003cp\u003e0.71\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 115px;\"\u003e\n \u003cp\u003e1020\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 68px;\"\u003e\n \u003cp\u003e985\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eThe current-voltage (\u003cem\u003eI-V\u003c/em\u003e) characteristics in voltage range from -1V to +1V of different CdO (n)/ c-Si (p) heterojunction diodes under dark condition are shown in Fig. 8 (a) (b). For the fabricated CdO (n)/c-Si (p) and CdO:In (n)/c-Si (p) diodes, a clear rectifying behavior with asymmetrical and non-linear dependency is observed. Logarithmic plots of the \u003cem\u003eI-V\u0026nbsp;\u003c/em\u003echaracteristics at relatively high forward voltage bias between 0.4 and 1V are shown in Figure S1.\u003c/p\u003e\n\u003cp\u003eTable 1 gathers the values of main diode parameters in dark including reverse saturation current I\u003csub\u003e0\u003c/sub\u003e, threshold voltage, barrier height\u0026nbsp;ϕ\u003csub\u003eb\u003c/sub\u003e, series resistance R\u003csub\u003es\u003c/sub\u003e and diode ideality factor n of different Cd (n)/ c-Si (p) heterojunction structures.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe values of ideality factors n in dark condition are 2.12 and 2.99 for CdO (n)/c-Si (p) and CdO: In (n)/c-Si (p) diodes. Regarding the dark saturation current I\u003csub\u003e0\u003c/sub\u003e, the obtained values from I-V\u0026nbsp;characteristics\u0026nbsp;are\u0026nbsp;(2.36 \u0026times; 10\u003csup\u003e- 8\u0026nbsp;\u003c/sup\u003eand 1.12 \u0026times; 10\u003csup\u003e-8\u003c/sup\u003e,) for CdO (n)/c-Si (p) and CdO: In (n)/c-Si (p) heterojunctions, respectively.\u003c/p\u003e\n\u003cp\u003eIt is well known that such n values greater than 2 imply a dominant leakage current over tunneling currents and generation-recombination mechanisms [24]. This non-ideality behavior of different CdO (n) /c-Si (p) heterojunction diodes can be attributed to a synergy of phenomena such as thickness anomalies of CdO films, interfacial charge non-uniformity and bias-dependent barrier heights [25]. Moreover, as can be noticed in SEM pictures of Fig. 2, the disparity in the effective contact area between the CdO multi-shape nanoparticles composing different CdO films and Si substrate and the heterointerface corrugations can induce a change in the current density, trapping interface states as well as large distribution of barrier inhomogeneities [26]. It is noticed that the ideality factor n is lower for undoped CdO (n)/c-Si (p) compared to CdO: In (n)/c-Si (p). Such difference can be assumed to several detrimental effects of doping at film bulk and at both heterointerface with semiconductor Si CdO/c-Si and with metal contact Al/CdO. This includes aggravated defects film bulk and heterointerfaces, polarization-dependent barrier height, non-uniformity of the interfacial charges and the existence of interfacial thin layer of native oxide at the heterointerfaces of CdO films with semiconductor Si CdO/c-Si and with metal contact Al/CdO.\u003c/p\u003e\n\u003cp\u003eFor the barrier height\u0026nbsp;ϕ\u003csub\u003eb\u003c/sub\u003e, parameter, the determined values from I-V characteristics in dark are 0.69 eV and 0.65 eV for CdO (n)/c-Si (p) and CdO:In (n)/c-Si (p) diodes, respectively. Using\u0026nbsp;Norde\u0026rsquo;s method, the estimated barrier height values are 0.74 eV and 0.71 eV for CdO (n)/c-Si (p) and CdO:In (n)/c-Si (p) heterojunctions, respectively. The change in barrier height of CdO/Si after doping with In can be ascribed to the influence on the space charge region and induced potential fluctuations at the heterointerface [24], [25]. This implies that the current flow in the diode is more preferential through lower barriers in the potential distribution.\u003c/p\u003e\n\u003cp\u003eAnother important electrical parameter is the series resistance, depending on nature and quality of contacts. The experimental values are estimated using Cheung\u0026rsquo;s method from the linear and the slope parts of the curves (dV / dLn (I)) versus I, as shown in figure S3. The values of series resistance R\u003csub\u003es\u003c/sub\u003e drops from 1300 Ω for CdO (n)/c-Si (p) to 1020 Ω for CdO:In (n)/c-Si (p). Using the H (I)- I method (figure S4) , the series resistance values are 1387 Ω and 985 Ω for CdO (n)/c-Si (p) and CdO:In (n)/c-Si (p) diodes, respectively.It is noteworthy that the determined results from both methods are consistent. This decrease in series resistance due to In doping of CdO can be ascribed to several mechanisms like the change in free charge carriers, minimization of trapping defect sites in grain boundaries of polycrystalline CdO films and at both semiconductor Si CdO/c-Si and with metal contact Al/CdO heterointerfaces as well as the alteration of work function and forward leakage path to minority charge carriers [27].\u003c/p\u003e\n\u003cp\u003eThe current -voltage characteristics of Al/CdO/Si/Al and Al/CdO:In/Si/Al diodes under illumination is shown in Fig. 9. It is observed that both forward and reverse bias currents increase with light intensity for different CdO/Si and CdO:In diodes. Such photocurrent rise can be attributed electron-hole pairs generation in space charge zone at the heterojunctions between CdO thin films and Si. The change for both CdO-based diodes is more pronounced for reverse bias in contrast to forward bias, implying closer ideal behavior under illumination [28]. As CdO:In layer enable better transparency than CdO film due to wider band gap, higher photogenerated current at Si region with more light sensitivity is expected for CdO:In/Si diode.\u003c/p\u003e\n\u003cp\u003eIn order to elucidate the charge transfer mechanisms in both CdO (n)/Si (p) heterojunction diodes, the corresponding energy band diagram is depicted in Fig. 10. The corresponding physical parameters are presented in table S1 of the supplementary information. It is noticed that the conduction band offset \u0026Delta;E\u003csub\u003eC\u003c/sub\u003e is lower than valance band discontinuity \u0026Delta;E\u003csub\u003eV\u003c/sub\u003e. This means energetic barrier for electrons is much lower than for holes, implying higher injection of electrons from CdO (n) to Si (p) compared to opposite mechanism of holes injection from Si (p) to CdO (n).Under forward bias, lower potential barrier enables better injection of electrons into p-Si, thus, leading to a rapid increase of forward current. Moreover, the injection of electrons into Si (p) induces a widening in depletion region at CdO (n) side and a delay of holes injection, resulting in higher recombination saturation current [29]. Owing to minimal potential barrier height at heterojunction between CdO:In and Si, the resistance is lower for CdO:In compared to CdO/Si diode.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAs the charge carriers transport is harmfully affected by the trapping sites and charge recombination centers existing at the heterointerfaces and grain boundaries of CdO-based films, higher conductivity with better crystallinity, mobility and charge density is desired for prolonged lifetime, long diffusion length and improved photogenerated carrier collection in photodiodes [30].\u003c/p\u003e\n\u003cp\u003eOverall, the above-mentioned electrical characteristics point out the good performance of CdO/Si n-p heterojunction and the benefits of In doping of CdO in CdO:In/Si photodiode.\u003c/p\u003e\n\u003cp\u003eFurther improvement can be considered in the optimization of process fabrication and post-coating treatments for enhanced CdO-based bulk films quality and better heterointerface with Si substrate. To this end, these obtained optoelectronic results suggest that different n-p CdO/p-Si heterojunction diodes are suitable for photodetector fabrication in optoelectronic applications.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eIn this work, nanostructured undoped and In-doped cadmium oxide using sol-gel spin-coating technique are successfully integrated in CdO/Si n-p heterojunction structures. The structural, morphological, compositional, optical and electrical properties of the surface of undoped CdO and CdO: In films on silicon substrates are investigated. It is revealed that undoped and In-doped CdO films on Si substrates are characterized by cubic structure with preferential (111) orientation. Based on XRD results, the incorporation of In dopants preserves the crystallinity of CdO film in spite of possible slight peak shifts, lattice expansion and prevention nanocrystals aggregation. The in-depth microscopic investigation unveils undoped and doped CdO surfaces of different polycrystalline films with uniformly distributed nanostructured grains. After doping, the average particle size of CdO/Si is decreased from 35 nm to 23 nm for CdO:In/Si. The inspection of different CdO/Si heterostructures shows smooth heterointerfaces with good adhesion of CdO-based thin films to the Si substrate for suitable heterojunction quality favorizing good surface passivation and electrical transport. By introducing In dopants, the average grain size decreases and the surface uniformity improves compared to undoped CdO layer. These morphological changes can be attributed to the incorporation of In dopants into expanded CdO lattice by ionic substitution and in the interstitial sites.\u003c/p\u003e\n\u003cp\u003eAccording to the chemical composition and electronic states analysis of undoped and In-doped CdO films on Si using XPS and EDX analysis, the incorporation and the uniform distribution of In dopants with In\u003csup\u003e3+\u003c/sup\u003e electronic state over the nearly stoichiometric CdO surface is confirmed. The anti-reflection role of undoped CdO and doped CdO:In is validated through lower reflectance compared to bare Si substrate, especially for wavelengths shorter than 800 nm. The electrical current-voltage I-V characteristics in dark and under illumination conditions are employed to determine the main diode parameters including reverse saturation current, barrier height, series resistance and diode ideality factor of different CdO (n)/ c-Si (p) heterojunction structures. A clear rectifying diode behavior with asymmetrical and non-linear dependency is obtained for CdO/Si and CdO:In heterojunctions. Compared to undoped CdO thin films, doping of CdO with In leads to higher ideality factor and reverse saturation current due to a synergy of physical phenomena like thickness anomalies of CdO films, non-uniformity of interfacial charge, existence of interfacial thin layer of native oxide and bias-dependent barrier heights trapping interface states as well as large distribution of barrier inhomogeneities and fluctuations at film bulk and at both heterointerfaces with semiconductor Si CdO/Si and with metal contact Al/CdO In contrast, doping of CdO with In enables lower potential barrier and series resistance. This can be assigned to several mechanisms like the change in free charge carriers , minimization of trapping defect sites in grain boundaries of polycrystalline CdO films and at both semiconductor Si CdO/c-Si and with metal contact Al/CdO heterointerfaces as well as the alteration of work function and forward leakage path to minority charge carriers. For illuminated I-V analysis, higher photogenerated current at Si region with more light sensitivity is obtainedCdO: In /Si diode owing to better transparency and wider bandgap than undoped CdO film. Based on energy band diagram, lower conduction band offset at heterojunction between CdO:In and Si, the charge carrier transport is improved for CdO:In compared to CdO/Si diode owing to prolonged lifetime, long diffusion length and improved photogenerated carrier collection in photodiode. Our findings point out the efficient integration of fabricated n-p CdO/p-Si structures for heterojunction photodiode applications.\u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eDeclaration of interests\u003c/h2\u003e \u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding Statement\u003c/h2\u003e \u003cp\u003eThis work did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eM. Jlassi, I. Ben Miled, M. Zouaoui wrote the main manuscript text and M. Jlassi,I. Ben Miled, I. Sta prepared figures. All authors reviewed the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eM. A. Habeeb, R. S. A. Hamza, I. 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M\u0026ouml;nch, \u0026ldquo;Electronic Properties of Semiconductor Interfaces,\u0026rdquo; in \u003cem\u003eSpringer Handbook of Electronic and Photonic Materials\u003c/em\u003e, Boston, MA: Springer US, 2006, pp. 147\u0026ndash;160. doi: 10.1007/978-0-387-29185-7_8.\u003c/li\u003e\n\u003cli\u003eO. J. Sandberg, S. Dahlstr\u0026ouml;m, M. Nyman, S. Wilken, D. Scheunemann, and R. \u0026Ouml;sterbacka, \u0026ldquo;Impact of a Doping-Induced Space-Charge Region on the Collection of Photogenerated Charge Carriers in Thin-Film Solar Cells Based on Low-Mobility Semiconductors,\u0026rdquo; \u003cem\u003ePhys Rev Appl\u003c/em\u003e, vol. 12, no. 3, p. 034008, Sep. 2019, doi: 10.1103/PhysRevApplied.12.034008.\u003c/li\u003e\n\u003cli\u003eF. Yakuphanoglu, M. Caglar, Y. Caglar, and S. 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Raja Shekar, \u0026ldquo;Synthesis and characterization of Fe-doped CdO nanoparticles via a coprecipitation method: application as a promising photodetector,\u0026rdquo; \u003cem\u003eApplied Physics A\u003c/em\u003e, vol. 131, no. 3, p. 160, Mar. 2025, doi: 10.1007/s00339-025-08270-y.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"silicon","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scon","sideBox":"Learn more about [Silicon](https://www.springer.com/journal/12633)","snPcode":"12633","submissionUrl":"https://submission.nature.com/new-submission/12633/3","title":"Silicon","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Metal oxide, thin films, n-p heterojunction, Sol–gel method, Nanostructures, Photodiode, optoelectronic properties","lastPublishedDoi":"10.21203/rs.3.rs-6444485/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6444485/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this study, nanostructured thin films based on cadmium oxide doped with Indium CdO: In were fabricated by sol-gel spin-coating technique on p-type monocrystalline silicon c-Si (p) for the integration in n-p heterojunction photodiode applications. A comprehensive analysis for structural, morphological, compositional, optical and electrical properties of the surface of undoped CdO and CdO:In films on silicon substrates is conducted. Upon In doping, the cubic structure of polycrystalline CdO film and the preferential (111) orientation are maintained. In the uniformly distributed nanostructures network, the average grain size of CdO/Si is decreased from 35 nm to 23 nm for CdO:In/Si. Smooth heterointerfaces with good adhesion of CdO-based thin films to the Si substrate imply suitable heterojunction quality and favorize good surface passivation and electrical transport. Based on chemical composition and electronic states analysis, In dopants uniform distribution and incorporation by In\u003csup\u003e3+\u003c/sup\u003eion substitution in quasi-stoichiometric CdO thin films are confirmed. The anti-reflection role of undoped CdO and doped CdO: In is validated through lower optical reflectance compared to bare Si substrate, especially in visible range. The electrical current-voltage I-V characteristics in dark and under illumination conditions are employed to determine the main diode parameters of different Cd (n)/ c-Si (p) heterojunction structures. A clear rectifying diode behavior with asymmetrical and non-linear dependency is obtained for CdO/Si and CdO:In heterojunctions. Compared to undoped CdO thin films, doping of CdO with In leads to higher ideality factor and reverse saturation current, but lower potential barrier and series resistance. Higher photogenerated current at Si region with more light sensitivity is obtained CdO:In/Si diode owing to better transparency and wider bandgap than undoped CdO film.Owing to lower conduction band offset at CdO:In/Si heterojunction, the charge carrier transport is improved for CdO:In compared to CdO/Si diode.\u003c/p\u003e \u003cp\u003eOur results demonstrate the effective integration of nanostructured undoped and In-doped CdO thin films in CdO/Si n-p heterojunction photodiode applications.\u003c/p\u003e","manuscriptTitle":"Fabrication and characterization of nanostructured Cadmium Oxide thin films doped with Indium by sol-gel spin-coating for CdO (n)/Si (p) heterojunction photodiode applications","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-05-20 11:47:35","doi":"10.21203/rs.3.rs-6444485/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-06-11T02:38:49+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-05-24T21:46:49+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-05-20T20:42:19+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"61270937044865728261945726059240057254","date":"2025-05-18T03:20:28+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"251247506314813505525540434067607008342","date":"2025-05-16T17:20:37+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"251392411823748820755817132708690508851","date":"2025-05-16T17:19:04+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-05-16T03:05:35+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-05-09T08:35:29+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-05-09T08:30:30+00:00","index":"","fulltext":""},{"type":"submitted","content":"Silicon","date":"2025-04-14T09:17:06+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"silicon","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scon","sideBox":"Learn more about [Silicon](https://www.springer.com/journal/12633)","snPcode":"12633","submissionUrl":"https://submission.nature.com/new-submission/12633/3","title":"Silicon","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"41c5d3bd-4b2f-4c12-895e-a56bcd48998a","owner":[],"postedDate":"May 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-07-28T16:05:25+00:00","versionOfRecord":{"articleIdentity":"rs-6444485","link":"https://doi.org/10.1007/s12633-025-03391-8","journal":{"identity":"silicon","isVorOnly":false,"title":"Silicon"},"publishedOn":"2025-07-21 15:57:24","publishedOnDateReadable":"July 21st, 2025"},"versionCreatedAt":"2025-05-20 11:47:35","video":"","vorDoi":"10.1007/s12633-025-03391-8","vorDoiUrl":"https://doi.org/10.1007/s12633-025-03391-8","workflowStages":[]},"version":"v1","identity":"rs-6444485","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6444485","identity":"rs-6444485","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}