Synthesis of ZnSe thin films by solution-processed spin coating method for photonic integration applications

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Alamin Hossain Pappu, Bipanko Kumar Mondal, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5703824/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 26 May, 2025 Read the published version in Journal of Materials Science: Materials in Engineering → Version 1 posted You are reading this latest preprint version Abstract This investigation introduces a novel method for the fabrication of ZnSe thin films on glass substrates through the spin coating technique which employs thiol-amine co-solvents. The thiol-amine co-solvent system efficiently dissolves several metal and metal chalcogenide precursors, facilitating cost-effective, and low-temperature solution-based deposition compatible with flexible substrates. The synthesized ZnSe thin films underwent air annealing at temperatures between 250 and 350°C, thereby improving their structural and optical characteristics. The polycrystalline nature of ZnSe was elucidated via X-ray diffraction (XRD) analysis while scanning electron microscopy (SEM) assured the rise of surface smoothness and uniformity with annealing temperature. Energy dispersive spectroscopy (EDS) analysis indicated near-stoichiometric ZnSe composition and Fourier-transform infrared (FTIR) spectroscopy identified Zn-Se stretching vibrations in the 960–1120 cm − 1 range. The optical data demonstrated high transmittance with an optical bandgap of 3.32–3.85 eV. Furthermore, optical data of ZnSe were embarked for computation of Ge-on-ZnSe waveguide with SiO 2 cladding for long wave infra-red (LWIR) light. The waveguide showed a remarkable power confinement factor (PCF) of ~ 0.99 with nearly 1 dB/cm loss at a laser wavelength of 8 µm. These outputs are highly optimistic for the fabrication of solution-processed ZnSe for LWIR photonic integration. ZnSe thiol-amine spin coating bandgap waveguide photonics Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction In recent years, the II-VI metal chalcogenide families have garnered significant interest in the realm of optical devices and photonics. II-VI metal chalcogenides, comprising group II elements (e.g., Zn, Cd, Hg) and group VI elements (S, Se, Te), are esteemed for their exceptional physical, chemical, and electrical properties. These materials demonstrate distinct physical, chemical, and electrical properties, rendering them highly important for several applications across diverse sectors [ 1 ]. For instance, the nanometer-scale ZnS and CdSe chalcogenides display a typical fluorescence, making them suitable for biological imaging and diagnostics. In addition, CdTe and Cu(In,Ga)Se 2 are widely employed in thin-film solar technologies owing to their elevated absorption coefficients and effective light-to-electricity conversion. Furthermore, II-VI chalcogenides exhibit a broad spectrum of adjustable bandgap energies, spanning from the visible to the infrared range. This feature is essential for optoelectronic devices such as LEDs, laser diodes, and photonics [ 2 – 3 ]. Specifically, Zinc Selenide (ZnSe) stands as a significant entity within the II-VI group of semiconductors, distinguished by its remarkable properties. ZnSe exhibits a broad direct bandgap of 2.7 eV at ambient temperature with n-type conductivity [ 4 ]. Moreover, it has a substantial binding energy of 21 MeV, a strong luminous efficiency, a significant refractive index, and good stability [ 5 ]. ZnSe also has a low absorption coefficient and high transparency in the visible spectrum attributed to its wider bandgap. This property has positioned it as an excellent choice for use as a window or buffer layer in photovoltaic applications [ 6 ]. CdS is another II-VI compound widely used as a window or buffer layer in solar cells. However, CdS degrades efficiency due to its toxic nature, higher surface roughness, and low transmittance compared to ZnSe [ 7 ]. As a window or buffer layer for thin-film solar cells, ZnSe can be used as a substitute for the hazardous CdS. ZnSe has recently demonstrated its potential as a window layer for various absorbers, such as Sb 2 Se 3 , FeS 2 , Cu 2 SnS 3 , and ultra-thin Si wafer-based devices [ 7 – 10 ]. However, ZnSe has some limitations such as lower long-term strength and higher production costs compared to CdS, which can limit its use in large-scale applications [ 11 – 12 ]. Despite these limitations, ZnSe's unique properties have made it valuable in various applications in the field of optoelectronics. For instance, ZnSe could be used in blue-green laser diodes (LDs), white light-emitting diodes (LEDs) [ 13 ], optically controlled switches [ 14 ], remote sensing applications [ 15 ], photo electro-chemical devices [ 16 ], thin-film transistors [ 17 ], photodetectors [ 18 ], and photonic integrated circuits (PICs) [ 19 ]. ZnSe thin films have been fabricated using various techniques, including vacuum thermal evaporation [ 20 – 21 ], chemical bath deposition [ 22 – 23 ], chemical vapor deposition [ 24 ], pulsed laser deposition [ 25 ], electro-deposition [ 26 – 27 ], sol-gel technique [ 28 ], spray pyrolysis [ 29 ], RF magnetron sputtering [ 30 ], and electron-beam evaporation [ 31 ]. However, out of all these deposition techniques, spin coating is the most economical and straightforward way for forming consistent thin films. The spin coating method is user-friendly and cost-effective and does not require advanced tools. However, it faces challenges like non-uniformity and reproducibility due to environmental sensitivities. Scalability is limited by substrate size and batch processing constraints. Recently, a basic thiol-amine co-solvent solution has shown solubility for chalcogenide compounds and the V 2 VI 3 a family of chemicals (V = As, Bi, Sb; VI = Te, Se, S) [ 32 – 33 ]. Thin films of ZnTe, CdS, CdTe, and different phases of indium selenide have been successfully dissolved and synthesized utilizing this method [ 34 – 37 ]. However, there is limited literature describing the process of making ZnSe films by dissolving ZnSe powder directly in a thiol-amine co-solvent solution. Because of their exceptional capacity to dissolve a variety of bulk binary chalcogenides, thiol-amine solvent combinations are frequently employed in the solution processing of binary chalcogenide thin films [ 38 – 39 ]. Moreover, they enable low-temperature solution-based deposition methods which are more cost-effective and compatible with flexible substrates compared to traditional high-vacuum methods [ 40 ]. These solvent mixtures enhance the spin coating process of metal chalcogenide thin films and further promote the application of bulk binary chalcogenides [ 38 – 39 ]. ZnSe is widely used in integrated in integrated photonics because of its high bandgap and transparency across visible to mid infrared wavelength. It is mainly appreciated for its small optical losses and high refractive index, constructing it ideal for applications like waveguides. In addition, germanium (Ge)-based longwave infrared (LWIR) waveguide has been simulated on a ZnSe substrate with physical properties of the synthesized ZnSe which demonstrate low-loss properties [ 19 ]. It has already been reported that LWIR ranging from 6 µm to 14 µm has great potential in photonic sensing for molecular fingerprints, environmental monitoring and quality control of productions [ 19 , 41 – 43 ]. But, the designed waveguide doesn’t include cladding. The cladding in rectangular waveguides improves performance by providing little optical loss, extraordinary refractive index contrast, appropriate mode guidance, and better nonlinear performance [ 44 – 45 ]. However, the advancement of LWIR photonic integrated circuits (PICs) stays limited owing to availability of low-loss materials. ZnSe consisting of heavy elements and lower refractive index than Ge in the LWIR has a potential due to its extraordinary low-onset frequency for multiphonon absorption and amazing low losses in the LWIR [ 19 , 46 – 47 ]. In this study, we used a solution of ethylene diamine and 1,2-ethanedithiol as mixed solvents to produce ZnSe thin films using the spin coating deposition technique. To explore the potential of these fabricated ZnSe films, they were characterized using various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, energy-dispersive X-ray spectroscopy (EDS), and UV-visible spectroscopy. These analyses allowed us to examine the crystalline phase, surface morphology, local bonding, elemental composition, and optical properties such as transmittance. By analyzing the transmittance data, the bandgap of ZnSe were estimated at different annealing temperatures. Additionally, the study explored the application of ZnSe in germanium-based waveguides with emphasized the importance of SiO 2 cladding in Ge/ZnSe-based photonics. The designed waveguide shows a good PCF and low-loss at the LWIR wavelength. The computed results are optimistic for the application of solution-processed ZnSe in PICs. 2. Details of the experimental procedure 2.1 Materials and the making of the ZnSe precursor For making ZnSe precursor solution, ZnSe power (99.9999%), Ethylene-diamine and 1,2-ethanedithiol were acquired from Sigma Aldrich. Initially, a mixture of ethylenediamine and 1,2-ethanedithiol was prepared at a volume proportion of 10:1, followed by the dissolution of 1 wt% ZnSe powder in the resulting co-solvents. Then the mixer was stirred in a magnetic stirrer at 50 ˚C temperature at a speed of 350 rpm. After five hours, ZnSe power was completely dissolved within the co-solvents. The final solution was yellow in colour. The schematic of solution preparation is shown in Fig. 1 (a). 2.2 Deposition of ZnSe thin films The ZnSe thin layer was deposited onto a glass substrate (1×1 inch) through the spin coating method utilizing the prepared solution of ZnSe dissolved in thiol-amine. At first, the glass substrates underwent a thorough cleansing process utilizing piranha solution to eliminate any contamination present. The ZnSe precursor solution was applied to a pristine glass substrate using spin coating at a velocity of 1000 rpm for a duration of 30 seconds, incorporating a 10-second ramp-up to 500 rpm. In order to eliminate any residual solvents, the thin films underwent a pre-annealing process for a duration of 10 minutes at a temperature of 100°C. Finally, these films were post-annealed in a carbolite oven for 10 min at 250°C, 300°C and 350°C for crystallization. 2.3 Characterization of ZnSe thin films The Dektek 150 Stylus Profiler was used to measure the thickness of fabricated ZnSe thin films. The obtained average thickness of the ZnSe thin films was ~ 290 nm. The surface properties of ZnSe thin films was explored using scanning electron microscope (SEM) (Model: ZEOL, JSM-IT800). The elemental analysis was conducted utilizing an energy-dispersive x-ray spectroscopy (EDS) integrated with the SEM system. The crystal nature of ZnSe films was characterized by X-ray diffraction (XRD) (Model: PANalytical Empyrean) using the CuKα (wavelength, λ = 1.540598 Å) radiation. To understand the chemical composition and structure of materials in ZnSe films, FTIR spectra were captured within the 300–4000 cm − 1 wavelength range. Finally, the optical study was carried out by an Ultraviolet-Visible spectrophotometer (UV-1900i PC, Shimadzu Corporation, Japan) in the wavelength range of 300–1100 nm. 2.4 Photonic waveguide simulation The Ge-on-ZnSe LWIR waveguide was designed and simulated with COMSOL Multiphysics 6.2 software. The Ge core was considered on the ZnSe substrate for the waveguide. The SiO 2 cladding was considered for the simulation. The dimension of the Ge core was 4 µm×8 µm for the 8 µm Laser [ 48 ]. 3. Results and discussion 3.1 XRD study of ZnSe thin films The XRD patterns of the synthesized ZnSe thin films annealed at 250 ºC, 300 ºC and 350 ºC is shown in the Fig. 2. It is observed from the figure that five peaks appear at 2θ = 27.31º, 45.32º, 53.57º, 72.95º and 83.35º for all conditions of annealing temperatures of 250 ºC, 300 ºC and 350 ºC. These diffraction peaks correspond to the (111), (220), (311), (331), and (422) planes, respectively according to JCPDS card file No. 37-1463 [49–50]. Additionally, another peak is observed at 2θ = 65.79° for 250°C which can also be indexed as (400) plane according to the similar JCPDS card. These peaks confirmed the cubic crystal structure of ZnSe thin films with the lattice parameters of a = b = c = 5.668 Å and space group of F-43m. It is also noted that a very small peak appears at ~ 90° which may be attributed to impurities in the films. However, we observe that the peak intensity increases with the annealing temperature. That indicates the crystallinity of the synthesized thin films improved with annealing temperature. The figure clearly shows that the (111) plane has a significantly higher intensity than the others. Therefore, the growth of crystallites has been more concentrated in this direction. The XRD study ensures the polycrystalline nature of ZnSe thin films. In order to understand more about the crystallographic qualities, the XRD investigation yields structural features including full width at half maximum (FWHM), crystal size (D), density of dislocation (δ), and strain (ε). The crystallite size (D) can be calculated using the Debye-Scherrer equation [51]: $$\:D=\:\frac{0.94\lambda\:}{\beta\:cos\theta\:}$$ 1 In this context, λ denotes the X-ray's wavelength measured in nanometers, β indicates the FWHM expressed in radians, and θ represents the Bragg angle corresponding to the specific XRD intensity. In order to determine the dislocations (δ), which represent the number of dislocation lines per unit area in the crystal, the following formula is crucial [52]: $$\:{\delta\:}=\frac{1}{{D}^{2}}$$ 2 The following relation is used to determine strain (ε) based on the plot's slope (βcosθ vs sinθ). [53]: $$\:\beta\:=\frac{\lambda\:}{Dcos\theta\:}-\epsilon\:tan\theta\:$$ 3 The calculated structural information is shown in Table 1. The table ensures that the FWHM decreases comparatively with rising the annealing temperatures indicating the improvement in the crystal structure. As a result, the table illustrates how the annealing temperatures cause the crystallite sizes to rise. Table 1 An overview of the crystallographic information of ZnSe thin films. Temperature of Annealing (ºC) hkl planes Full width at half maximum, β (rad × 10 − 3 ) Size of Crystallites, D(nm) Density of dislocation, δ(lin/m 2 ) × 10 14 Strain, ε (lin − 2 .m − 4 ) ×10 − 3 (111) 3.864 38.60 6.71 2.58 (220) 6.011 26.10 14.68 21.98 250 (311) 5.152 31.51 10.07 5.34 (400) 2.576 67.01 2.23 2.08 (331) 4.294 41.88 5.70 3.06 (422) 4.363 44.48 5.06 8.53 (111) 1.288 115.71 0.75 0.68 (220) 2.147 73.14 1.87 6.54 300 (311) 3.005 54.08 3.42 3.53 (331) 1.717 104.66 0.91 1.26 (422) 4.490 41.82 5.72 9.03 (111) 2.147 69.47 2.07 1.32 (220) 3.005 52.24 3.66 9.28 350 (311) 5.152 31.51 10.07 5.26 (331) 5.152 34.98 8.17 2.89 (422) 3.260 57.57 3.02 6.94 3.2 The SEM study of ZnSe films Figure 3 illustrates the SEM micrographs of the fabricated ZnSe thin films. The SEM images demonstrate a dispersed pattern of grains of varying sizes throughout the films. The SEM morphology also indicates shallow ditches which decrease with the increase of annealing temperature. It is evident from the figures that the surface roughness of the synthesized films decreases with annealing temperature which is consistent with the reported works [36–37]. However, few largest grains are observed in the ZnSe layer annealed at 300°C as illustrated in the figure. In addition, small grains with different shapes are noticed at 350°C temperature. Comparing ZnSe thin films annealed at 350°C to those annealed at 250°C and 300°C, the former show superior uniformity and smoothness of the film surface. 3.3 Analysis of the EDS data of ZnSe thin films The chemical makeup of the produced ZnSe films has been assessed using the EDS analysis. To find a precise result, the measurement has been performed at different spots. The atomic and weight (%) of Zn and Se atoms in the ZnSe compound are shown in Table 2. The analysis of the deposited films revealed no presence of other elements except Zn and Se. This could point to the formation of superior ZnSe thin films. However, EDS analysis ensures the near stoichiometric behavior of ZnSe thin films fabricated in this route. Previous research has also found that ZnSe thin films have a similar near stoichiometric elemental composition using the chemical bath deposition technique [54]. Table 2 The elemental composition of ZnSe film as analyzed through EDS after undergoing annealing at different temperatures. Annealing temperature (°C) Element Weight (%) Atom (%) Stoichiometric ratio [Zn/Se] 250 Zn 41.42 46.05 0.853 Se 58.58 53.95 300 Zn 41.60 46.23 0.859 Se 58.40 53.77 350 Zn 41.18 45.80 0.845 Se 58.82 54.20 3.4 The FTIR study of ZnSe thin films The Fourier-transform infrared spectroscopy (FTIR) provides detailed information about the molecular composition and structure of materials. The FTIR spectra of ZnSe thin films in the 400–4000 cm − 1 region at various temperatures are displayed in Fig. 4. The peak intensity at 960–1120 cm − 1 indicates Zn-Se stretching vibration for the annealing conditions of 250–350°C [55–56]. However, the peak broadening might arise from the overlapping of several vibrational modes present in the same spectral area. For example, the C-H stretching vibration bond at 1074 cm − 1 may appear from the co-solvents [57]. As a result, there may be an overlapping between the Zn-Se stretching and C-H stretching bonds. Moreover, the O-H bond of water moisture has been found for all cases at the wavenumber of 3626 cm − 1 [55]. In addition, the stretching vibration connection between C and O is caused by the peak intensity at 2157 cm − 1 , which might be found in the air environment [56]. 3.5 Optical Study Figure 5 displays the optical transmittance of spin-coated ZnSe thin films for three distinct annealing temperatures in the photon wavelength range of 310–1085 nm. The graph shows that when the annealing temperature rises, the transmittance rises as well. At 350 ºC temperature, the ZnSe film exhibits maximum transmittance (above 90%) compared to the others. This outcome arises from elevated annealing temperature which enhances surface smoothness and crystallinity which are also ensured by the XRD and SEM studies [36]. Thus, ZnSe films indicate excellent transmittance throughout the visible region which makes it a superior candidate in photovoltaic applications as a window or buffer layer. The following formula has been used to calculate the absorption coefficient, α from the transmittance spectra [36–37]: $$\:\alpha\:=\frac{\text{l}\text{n}\left(\frac{100}{T}\right)}{d}$$ 4 Where, T is the transmittance and d is the thickness of thin film. The Tauc equation has a relationship with the absorption coefficient (α) and photon energy (hv) as follows: $$\:{\left(\alpha\:hv\right)}^{2}=A(hv-{E}_{g})$$ 5 In this context, E g represents the optical bandgap, while A denotes the constant. The energy bandgap \(\:\left({E}_{g}=hv\right)\) is obtained when the linear section of the plot of \(\:{\left(\alpha\:hv\right)}^{2}\:\)vs light energy (hv) is extrapolated to a value of zero. The Tauc plots of deposited ZnSe thin films are represented in Fig. 6 for different annealing temperatures. The direct bandgaps of the ZnSe thin films are found to be 3.32–3.85 eV for 250–350 ºC, respectively. It is observed that the bandgap increases with the annealing temperature. The rise in optical bandgap with annealing temperatures could be due to the decrease in disorder and defects within the films [36]. Nevertheless, a comparable bandgap of ZnSe thin films has been identified in the preceding works [55]. 3.6 Ge waveguide on ZnSe substrate The purpose of the waveguide is to transmit the electromagnetic wave through the guide path. Ge waveguide is designed to transmit the electromagnetic wave where ZnSe is used as a substrate on which the Ge core is built and SiO 2 is used as a cladding on the Ge core as shown in Fig. 7(a). The refractive index of the ZnSe has been found from the extrapolated refractive index data calculated from the transmittance data using formulas stated in other work [58]. The value of the refractive index of ZnSe is 1.73 which is much lower than that of Ge (4.0) which ensures a perfect substrate for waveguide [19]. Figures 7(b) and 7(c) display the confined mode inside Ge core on the ZnSe substrate for TM and TE mode and the corresponding value of the effective index for TM and TE are 3.852 and 3.877, respectively. The effective refractive index with transmission wavelength of Ge-on-ZnSe hybrid waveguide both for TE and TE modes is shown in Fig. 8. The effective index of the photonic structure decreases with an increase in wavelength, particularly at longer wavelengths for both TM and TE modes as shown in the figure. This behavior occurs because the effective refractive index is inversely proportional to wavelength [59]. Additionally, it may happen due to the effective refractive index typically decreases as the wavelength increases. This is because, at longer wavelengths, the wave tends to spread out more, and the interaction between the wave and the waveguide's core decreases leading to a lower effective refractive index. This effect can be understood through modal dispersion: as the wavelength increases, the wave starts to leak more into the cladding material, reducing the confinement in the core which effectively lowers the refractive index associated with the mode [60–61]. For the TE mode, the effective index varies from 3.99 to 3.42, while for the TM mode, it ranges from 3.99 to 3.58. For the TM mode, the electric field has a component along the direction of propagation, which interacts with the material's permittivity. For longer wavelengths, this interaction reduces, contributing to the decrease in the effective refractive index. For the TE mode, the electric field is purely transverse, and similar reasoning applies: longer wavelengths result in more field spreading, reducing confinement in the core and decreasing the effective refractive index [61–63]. This also indicates that the effective index for TM mode is higher than that of TE mode at longer wavelengths, although both TM and TE modes have the same effective index of 3.99 at shorter wavelengths. This discrepancy may arise because the TM mode is much more sensitive to inaccuracies in the vertical layers-such as thickness, angle, and compound anisotropy-compared to the horizontal layers [19, 64]. A similar behavior has been observed in previously published articles [59, 65]. The differences between TE and TM modes can significantly affect specific applications. TE mode is often preferred in optical communication systems because they exhibit lower attenuation and better PCF. In contrast, TM mode can be beneficial in applications that require strong interactions with the waveguide material, such as spectroscopy, light scattering, or wide-area lighting in photonic devices [66]. The power confinement factor (PCF) is defined as the ratio of the electromagnetic fields within the core region of a waveguide to the electromagnetic fields throughout the entire waveguide structure. In simpler terms, it indicates how effectively the electromagnetic fields are confined within the germanium (Ge) core as depicted in Fig. 9. The value of PCF decreases slightly with increasing wavelength, dropping from 99.99–97.45% for the transverse electric (TE) mode and from 99.98–95.56% for the transverse magnetic (TM) mode in the Ge-ZnSe-SiO 2 system. The PCF in a waveguide is influenced by several factors, particularly the refractive index. When the refractive index changes, the transverse electromagnetic modes can become perturbed, causing them to extend from the Ge core into the SiO 2 cladding. Consequently, variations in the PCF are observed within the core of the waveguide. As illustrated in Fig. 8, the effective refractive index decreases with increasing wavelength, which contributes to variations in the PCF [67]. Figure 10 illustrates the loss in a Ge-based waveguide that utilizes ZnSe and SiO 2 as the substrate and cladding materials. In waveguide design, loss refers to the attenuation or reduction of optical signal power as it travels through the waveguide. Minimizing these losses is essential for achieving efficient waveguide performance. For the TE and TM modes in the Ge-on-ZnSe waveguide, loss decreases with increasing wavelength. This indicates that the Ge-ZnSe-SiO 2 waveguide structure is more efficient at longer wavelengths. However, the loss exhibits a zigzag behavior as the wavelength increases especially TM mode. A similar zigzag behavior in loss relative to wavelength is also observed in GaN waveguides [68]. 4. Conclusion In this study, ZnSe thin films have been successfully synthesized using a solution process with a spin coating method incorporating thiol-amine co-solvents. These films are air-annealed at temperatures between 250°C and 350°C after being produced onto glass substrates. The XRD study reveals peaks at 2θ of 27.31º, 45.32º, 53.57º, 65.79°, 72.95º, and 83.35º with (111), (220), (311), (400), (331) and (422) planes, respectively. The XRD study exhibits the polycrystalline nature of ZnSe thin films. The SEM images illustrate that the surface roughness of the synthesized films decreases with annealing temperature. In comparison to ZnSe thin films annealed at 250°C and 300°C, those annealed at 350°C show superior surface uniformity and smoothness. FTIR spectroscopy confirms the effective synthesis of ZnSe as evidenced by the distinctive peak in the wavenumber of 960–1120 cm − 1 in all prepared samples. The optical study ensures the superior transmittance and wide optical bandgap of ZnSe films. The transmittance and optical bandgap increases with annealing temperatures and the bandgap ranging in 3.32–3.85 eV. Additionally, the application of ZnSe as a substrate in Ge waveguide with SiO 2 cladding exhibits remarkable PCF (0.99%) and low loss (~ 1.0 dB/cm) at the LWIR wavelength of 8 µm. These findings indicate potential applications of solution-processed ZnSe in the LWIR photonic integrations and optical devices because of its simplicity, low cost, uncomplicated apparatus used and environment friendliness. Declarations Acknowledgments The authors highly appreciate the Bangladesh Council of Scientific and Industrial Research (BCSIR) Dhaka, Bangladesh, for the providing facilities during XRD and SEM studies. The FTIR analysis was performed at the Central Science Laboratory, University of Rajshahi. In addition, the authors are indebted to the Plasma Science and Technology Lab (PSTL), University of Rajshahi, Bangladesh for the help with the optical study. Data Availability The data used to support the findings of this study are available from the corresponding author upon request. Conflicts of Interest The authors have no conflicts of interest. Funding: One of the authors, Tanzina Rahman, receives a grant (Grant number: 2075/5/52/RU/FoE-22/2023-2024) from the Faculty of Engineering, University of Rajshahi, Bangladesh. Declaration of generative AI and AI-assisted technologies: The authors did not embark on AI or AI-assisted technologies in any step in preparing this manuscript. CRediT Author Contribution Statement Tanzina Rahman : data curation, formal analysis, investigation, writing- original draft; Md. Alamin Hossain Pappu : formal analysis, validation, visualization, writing- original draft; Bipanko Kumar Mondal : formal analysis, validation, visualization, writing- original draft; Syeda Samiha Nushin : formal analysis, validation, visualization, writing- original draft; Jaker Hossain : conceptualization, formal analysis, visualization, supervision, validation, writing- original draft, review and editing. * Corresponding author: Email: [email protected] (Jaker Hossain) References J. Wang, M. Isshiki, Wide-Bandgap II–VI Semiconductors: Growth and Properties. In: Kasap, S., Capper, P. (eds) Springer Handbook of Electronic and Photonic Materials. Springer Handbooks. Springer, Boston, MA (2006). https://doi.org/10.1007/978-0-387-29185-7_16 S. Ali, F.K. Butt, J. Ahmad, Z.U. Rehman, S. Ullah, M. Firdous, S.U. Rehman, Z. 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Zhao, Low loss GaN waveguides at the visible spectral wavelengths for integrated photonics applications, optics express 25(25) (2017) 31758. Cite Share Download PDF Status: Published Journal Publication published 26 May, 2025 Read the published version in Journal of Materials Science: Materials in Engineering → 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. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5703824","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":444213009,"identity":"ea283801-4670-46e1-959d-7add8a38dcb4","order_by":0,"name":"Tanzina Rahman","email":"","orcid":"","institution":"University of Rajshahi","correspondingAuthor":false,"prefix":"","firstName":"Tanzina","middleName":"","lastName":"Rahman","suffix":""},{"id":444213010,"identity":"89f9653a-ac4b-4bdb-a9d5-6fdf255cc02f","order_by":1,"name":"Md. Alamin Hossain Pappu","email":"","orcid":"","institution":"University of Rajshahi","correspondingAuthor":false,"prefix":"","firstName":"Md.","middleName":"Alamin Hossain","lastName":"Pappu","suffix":""},{"id":444213011,"identity":"724b21b6-1e1a-4d65-9ca3-58a596840dbf","order_by":2,"name":"Bipanko Kumar Mondal","email":"","orcid":"","institution":"Begum Rokeya University","correspondingAuthor":false,"prefix":"","firstName":"Bipanko","middleName":"Kumar","lastName":"Mondal","suffix":""},{"id":444213012,"identity":"2e90f118-6088-43df-a285-40eaba1a188f","order_by":3,"name":"Syeda Samiha Nushin","email":"","orcid":"","institution":"University of Rajshahi","correspondingAuthor":false,"prefix":"","firstName":"Syeda","middleName":"Samiha","lastName":"Nushin","suffix":""},{"id":444213013,"identity":"21db98e1-63b4-4ae7-9e39-169591173fa8","order_by":4,"name":"Jaker 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06:19:52","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5703824/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5703824/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s40712-025-00299-4","type":"published","date":"2025-05-26T15:57:53+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":80914174,"identity":"2fbb698a-3f03-4c97-a315-6ba027c34568","added_by":"auto","created_at":"2025-04-18 17:11:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":799842,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The phases involved in the preparation of a ZnSe solution, and (b) the schematic representation depicting the procedures for cleaning the substrates and synthesis of ZnSe thin films utilizing the spin coating technique.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/a6cd7881ce05259fe366fd3b.png"},{"id":80913630,"identity":"dcff3156-84e6-4036-a3e9-192b70a1178c","added_by":"auto","created_at":"2025-04-18 17:03:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":343741,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of ZnSe thin films annealed at different temperatures.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/666534caf18dcf039e31e419.png"},{"id":80913632,"identity":"1453d265-9f85-4da9-a6ec-3d706e897419","added_by":"auto","created_at":"2025-04-18 17:03:13","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2108095,"visible":true,"origin":"","legend":"\u003cp\u003eThe scanning electron microscopy (SEM) study of ZnSe thin film annealed at different temperatures.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/12f2af39aa551f9b24d845e0.png"},{"id":80913636,"identity":"53a1809a-3215-4ca0-a2fa-19928e77f4ae","added_by":"auto","created_at":"2025-04-18 17:03:13","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":532805,"visible":true,"origin":"","legend":"\u003cp\u003eThe ZnSe films' FTIR spectra subjected to annealing at various conditions.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/16aa86b8388d1bd53f85340b.png"},{"id":80914366,"identity":"2fb290f9-e8dd-4246-832b-7f1438160b95","added_by":"auto","created_at":"2025-04-18 17:19:13","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":686822,"visible":true,"origin":"","legend":"\u003cp\u003eTransmittance spectra of ZnSe thin film at different temperatures of annealing.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/22d4caed4f53a02bbca54afe.png"},{"id":80913642,"identity":"2c25cf37-79de-4ea8-b551-3a1575fbf63d","added_by":"auto","created_at":"2025-04-18 17:03:13","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":487352,"visible":true,"origin":"","legend":"\u003cp\u003eThe bandgap of ZnSe films subjected to annealing at various conditions.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/f227cbae8b0216b85968d50e.png"},{"id":80913639,"identity":"589c9920-94f3-420d-94b9-8a85e916f445","added_by":"auto","created_at":"2025-04-18 17:03:13","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":577968,"visible":true,"origin":"","legend":"\u003cp\u003e(a) The schematic of the Ge-on-ZnSe waveguide with SiO\u003csub\u003e2\u003c/sub\u003e cladding, and Mode intensity profiles for (b) TE, and (c) TM polarizations simulated with COMSOL.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/9b37e5615fce9ee25bb61177.png"},{"id":80914168,"identity":"56988b5a-df73-4c84-a9b5-80fdcdc110ef","added_by":"auto","created_at":"2025-04-18 17:11:13","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":578830,"visible":true,"origin":"","legend":"\u003cp\u003eThe change in the effective refractive index of the Ge-on-ZnSe waveguide with transmission wavelength in TE and TM modes.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/0b34a94ada2a3fd3f73ff012.png"},{"id":80913635,"identity":"eff03307-7abf-433d-b7e2-53e5d0ba36b4","added_by":"auto","created_at":"2025-04-18 17:03:13","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":152232,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of power confinement factor (PCF) with transmission wavelength in TE and TE modes.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/6784013a72adf26d4933fb88.png"},{"id":80913641,"identity":"1d3a809b-c54e-438d-b157-3d3ee784f292","added_by":"auto","created_at":"2025-04-18 17:03:13","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1110305,"visible":true,"origin":"","legend":"\u003cp\u003eThe propagation loss of light in the Ge waveguide with the variation of transmission wavelength in the TE and TM modes.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/4dbb86e98c1faca72f50435e.png"},{"id":83783088,"identity":"87ff9ea8-cdea-4063-b407-dffeb2360ab5","added_by":"auto","created_at":"2025-06-02 16:10:43","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8851447,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5703824/v1/4b6d8486-0c7c-489b-ad0f-073ab5f1741c.pdf"}],"financialInterests":"","formattedTitle":"Synthesis of ZnSe thin films by solution-processed spin coating method for photonic integration applications","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eIn recent years, the II-VI metal chalcogenide families have garnered significant interest in the realm of optical devices and photonics. II-VI metal chalcogenides, comprising group II elements (e.g., Zn, Cd, Hg) and group VI elements (S, Se, Te), are esteemed for their exceptional physical, chemical, and electrical properties. These materials demonstrate distinct physical, chemical, and electrical properties, rendering them highly important for several applications across diverse sectors [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. For instance, the nanometer-scale ZnS and CdSe chalcogenides display a typical fluorescence, making them suitable for biological imaging and diagnostics. In addition, CdTe and Cu(In,Ga)Se\u003csub\u003e2\u003c/sub\u003e are widely employed in thin-film solar technologies owing to their elevated absorption coefficients and effective light-to-electricity conversion. Furthermore, II-VI chalcogenides exhibit a broad spectrum of adjustable bandgap energies, spanning from the visible to the infrared range. This feature is essential for optoelectronic devices such as LEDs, laser diodes, and photonics [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSpecifically, Zinc Selenide (ZnSe) stands as a significant entity within the II-VI group of semiconductors, distinguished by its remarkable properties. ZnSe exhibits a broad direct bandgap of 2.7 eV at ambient temperature with n-type conductivity [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Moreover, it has a substantial binding energy of 21 MeV, a strong luminous efficiency, a significant refractive index, and good stability [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. ZnSe also has a low absorption coefficient and high transparency in the visible spectrum attributed to its wider bandgap. This property has positioned it as an excellent choice for use as a window or buffer layer in photovoltaic applications [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. CdS is another II-VI compound widely used as a window or buffer layer in solar cells. However, CdS degrades efficiency due to its toxic nature, higher surface roughness, and low transmittance compared to ZnSe [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. As a window or buffer layer for thin-film solar cells, ZnSe can be used as a substitute for the hazardous CdS. ZnSe has recently demonstrated its potential as a window layer for various absorbers, such as Sb\u003csub\u003e2\u003c/sub\u003eSe\u003csub\u003e3\u003c/sub\u003e, FeS\u003csub\u003e2\u003c/sub\u003e, Cu\u003csub\u003e2\u003c/sub\u003eSnS\u003csub\u003e3\u003c/sub\u003e, and ultra-thin Si wafer-based devices [\u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. However, ZnSe has some limitations such as lower long-term strength and higher production costs compared to CdS, which can limit its use in large-scale applications [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Despite these limitations, ZnSe's unique properties have made it valuable in various applications in the field of optoelectronics. For instance, ZnSe could be used in blue-green laser diodes (LDs), white light-emitting diodes (LEDs) [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], optically controlled switches [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e], remote sensing applications [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], photo electro-chemical devices [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], thin-film transistors [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e], photodetectors [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e], and photonic integrated circuits (PICs) [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eZnSe thin films have been fabricated using various techniques, including vacuum thermal evaporation [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e], chemical bath deposition [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], chemical vapor deposition [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e], pulsed laser deposition [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], electro-deposition [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e], sol-gel technique [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e], spray pyrolysis [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e], RF magnetron sputtering [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e], and electron-beam evaporation [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. However, out of all these deposition techniques, spin coating is the most economical and straightforward way for forming consistent thin films. The spin coating method is user-friendly and cost-effective and does not require advanced tools. However, it faces challenges like non-uniformity and reproducibility due to environmental sensitivities. Scalability is limited by substrate size and batch processing constraints.\u003c/p\u003e \u003cp\u003eRecently, a basic thiol-amine co-solvent solution has shown solubility for chalcogenide compounds and the V\u003csub\u003e2\u003c/sub\u003eVI\u003csub\u003e3\u003c/sub\u003e a family of chemicals (V\u0026thinsp;=\u0026thinsp;As, Bi, Sb; VI\u0026thinsp;=\u0026thinsp;Te, Se, S) [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Thin films of ZnTe, CdS, CdTe, and different phases of indium selenide have been successfully dissolved and synthesized utilizing this method [\u003cspan additionalcitationids=\"CR35 CR36\" citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. However, there is limited literature describing the process of making ZnSe films by dissolving ZnSe powder directly in a thiol-amine co-solvent solution. Because of their exceptional capacity to dissolve a variety of bulk binary chalcogenides, thiol-amine solvent combinations are frequently employed in the solution processing of binary chalcogenide thin films [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Moreover, they enable low-temperature solution-based deposition methods which are more cost-effective and compatible with flexible substrates compared to traditional high-vacuum methods [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. These solvent mixtures enhance the spin coating process of metal chalcogenide thin films and further promote the application of bulk binary chalcogenides [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eZnSe is widely used in integrated in integrated photonics because of its high bandgap and transparency across visible to mid infrared wavelength. It is mainly appreciated for its small optical losses and high refractive index, constructing it ideal for applications like waveguides. In addition, germanium (Ge)-based longwave infrared (LWIR) waveguide has been simulated on a ZnSe substrate with physical properties of the synthesized ZnSe which demonstrate low-loss properties [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. It has already been reported that LWIR ranging from 6 \u0026micro;m to 14 \u0026micro;m has great potential in photonic sensing for molecular fingerprints, environmental monitoring and quality control of productions [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan additionalcitationids=\"CR42\" citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e]. But, the designed waveguide doesn\u0026rsquo;t include cladding. The cladding in rectangular waveguides improves performance by providing little optical loss, extraordinary refractive index contrast, appropriate mode guidance, and better nonlinear performance [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. However, the advancement of LWIR photonic integrated circuits (PICs) stays limited owing to availability of low-loss materials. ZnSe consisting of heavy elements and lower refractive index than Ge in the LWIR has a potential due to its extraordinary low-onset frequency for multiphonon absorption and amazing low losses in the LWIR [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn this study, we used a solution of ethylene diamine and 1,2-ethanedithiol as mixed solvents to produce ZnSe thin films using the spin coating deposition technique. To explore the potential of these fabricated ZnSe films, they were characterized using various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy, energy-dispersive X-ray spectroscopy (EDS), and UV-visible spectroscopy. These analyses allowed us to examine the crystalline phase, surface morphology, local bonding, elemental composition, and optical properties such as transmittance. By analyzing the transmittance data, the bandgap of ZnSe were estimated at different annealing temperatures. Additionally, the study explored the application of ZnSe in germanium-based waveguides with emphasized the importance of SiO\u003csub\u003e2\u003c/sub\u003e cladding in Ge/ZnSe-based photonics. The designed waveguide shows a good PCF and low-loss at the LWIR wavelength. The computed results are optimistic for the application of solution-processed ZnSe in PICs.\u003c/p\u003e"},{"header":"2. Details of the experimental procedure","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e\u003cb\u003e2.1 Materials and the making of the ZnSe precursor\u003c/b\u003e\u003c/h2\u003e \u003cp\u003eFor making ZnSe precursor solution, ZnSe power (99.9999%), Ethylene-diamine and 1,2-ethanedithiol were acquired from Sigma Aldrich. Initially, a mixture of ethylenediamine and 1,2-ethanedithiol was prepared at a volume proportion of 10:1, followed by the dissolution of 1 wt% ZnSe powder in the resulting co-solvents. Then the mixer was stirred in a magnetic stirrer at 50 ˚C temperature at a speed of 350 rpm. After five hours, ZnSe power was completely dissolved within the co-solvents. The final solution was yellow in colour. The schematic of solution preparation is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e(a).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Deposition of ZnSe thin films\u003c/h2\u003e \u003cp\u003eThe ZnSe thin layer was deposited onto a glass substrate (1\u0026times;1 inch) through the spin coating method utilizing the prepared solution of ZnSe dissolved in thiol-amine. At first, the glass substrates underwent a thorough cleansing process utilizing piranha solution to eliminate any contamination present. The ZnSe precursor solution was applied to a pristine glass substrate using spin coating at a velocity of 1000 rpm for a duration of 30 seconds, incorporating a 10-second ramp-up to 500 rpm. In order to eliminate any residual solvents, the thin films underwent a pre-annealing process for a duration of 10 minutes at a temperature of 100\u0026deg;C. Finally, these films were post-annealed in a carbolite oven for 10 min at 250\u0026deg;C, 300\u0026deg;C and 350\u0026deg;C for crystallization.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Characterization of ZnSe thin films\u003c/h2\u003e \u003cp\u003eThe Dektek 150 Stylus Profiler was used to measure the thickness of fabricated ZnSe thin films. The obtained average thickness of the ZnSe thin films was ~\u0026thinsp;290 nm. The surface properties of ZnSe thin films was explored using scanning electron microscope (SEM) (Model: ZEOL, JSM-IT800). The elemental analysis was conducted utilizing an energy-dispersive x-ray spectroscopy (EDS) integrated with the SEM system. The crystal nature of ZnSe films was characterized by X-ray diffraction (XRD) (Model: PANalytical Empyrean) using the CuKα (wavelength, λ\u0026thinsp;=\u0026thinsp;1.540598 \u0026Aring;) radiation. To understand the chemical composition and structure of materials in ZnSe films, FTIR spectra were captured within the 300\u0026ndash;4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e wavelength range. Finally, the optical study was carried out by an Ultraviolet-Visible spectrophotometer (UV-1900i PC, Shimadzu Corporation, Japan) in the wavelength range of 300\u0026ndash;1100 nm.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Photonic waveguide simulation\u003c/h2\u003e \u003cp\u003eThe Ge-on-ZnSe LWIR waveguide was designed and simulated with COMSOL Multiphysics 6.2 software. The Ge core was considered on the ZnSe substrate for the waveguide. The SiO\u003csub\u003e2\u003c/sub\u003e cladding was considered for the simulation. The dimension of the Ge core was 4 \u0026micro;m\u0026times;8 \u0026micro;m for the 8 \u0026micro;m Laser [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec8\"\u003e\n \u003ch2\u003e3.1 XRD study of ZnSe thin films\u003c/h2\u003e\n \u003cp\u003eThe XRD patterns of the synthesized ZnSe thin films annealed at 250 ºC, 300 ºC and 350 ºC is shown in the Fig. 2. It is observed from the figure that five peaks appear at 2θ = 27.31º, 45.32º, 53.57º, 72.95º and 83.35º for all conditions of annealing temperatures of 250 ºC, 300 ºC and 350 ºC. These diffraction peaks correspond to the (111), (220), (311), (331), and (422) planes, respectively according to JCPDS card file No. 37-1463 [49–50]. Additionally, another peak is observed at 2θ = 65.79° for 250°C which can also be indexed as (400) plane according to the similar JCPDS card. These peaks confirmed the cubic crystal structure of ZnSe thin films with the lattice parameters of a = b = c = 5.668 Å and space group of F-43m. It is also noted that a very small peak appears at ~ 90° which may be attributed to impurities in the films. However, we observe that the peak intensity increases with the annealing temperature. That indicates the crystallinity of the synthesized thin films improved with annealing temperature. The figure clearly shows that the (111) plane has a significantly higher intensity than the others. Therefore, the growth of crystallites has been more concentrated in this direction. The XRD study ensures the polycrystalline nature of ZnSe thin films.\u003c/p\u003e\n \u003cp\u003eIn order to understand more about the crystallographic qualities, the XRD investigation yields structural features including full width at half maximum (FWHM), crystal size (D), density of dislocation (δ), and strain (ε). The crystallite size (D) can be calculated using the Debye-Scherrer equation [51]:\u003c/p\u003e\n \u003cdiv id=\"Equ1\"\u003e\n \u003cdiv id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$\\:D=\\:\\frac{0.94\\lambda\\:}{\\beta\\:cos\\theta\\:}$$\u003c/div\u003e\n \u003cdiv\u003e1\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eIn this context, λ denotes the X-ray's wavelength measured in nanometers, β indicates the FWHM expressed in radians, and θ represents the Bragg angle corresponding to the specific XRD intensity.\u003c/p\u003e\n \u003cp\u003eIn order to determine the dislocations (δ), which represent the number of dislocation lines per unit area in the crystal, the following formula is crucial [52]:\u003c/p\u003e\n \u003cdiv id=\"Equ2\"\u003e\n \u003cdiv id=\"FileID_Equ2\" name=\"EquationSource\"\u003e$$\\:{\\delta\\:}=\\frac{1}{{D}^{2}}$$\u003c/div\u003e\n \u003cdiv\u003e2\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eThe following relation is used to determine strain (ε) based on the plot's slope (βcosθ vs sinθ). [53]:\u003c/p\u003e\n \u003cdiv id=\"Equ3\"\u003e\n \u003cdiv id=\"FileID_Equ3\" name=\"EquationSource\"\u003e$$\\:\\beta\\:=\\frac{\\lambda\\:}{Dcos\\theta\\:}-\\epsilon\\:tan\\theta\\:$$\u003c/div\u003e\n \u003cdiv\u003e3\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eThe calculated structural information is shown in Table 1. The table ensures that the FWHM decreases comparatively with rising the annealing temperatures indicating the improvement in the crystal structure. As a result, the table illustrates how the annealing temperatures cause the crystallite sizes to rise.\u003c/p\u003e\n \u003cdiv\u003e\n \u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 1\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eAn overview of the crystallographic information of ZnSe thin films.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTemperature of Annealing (ºC)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ehkl planes\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFull width at half maximum, β (rad × 10\u003csup\u003e− 3\u003c/sup\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSize of Crystallites, D(nm)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eDensity of dislocation, δ(lin/m\u003csup\u003e2\u003c/sup\u003e) × 10\u003csup\u003e14\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eStrain, ε (lin\u003csup\u003e− 2\u003c/sup\u003e.m\u003csup\u003e− 4\u003c/sup\u003e ) ×10\u003csup\u003e− 3\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(111)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.864\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e38.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.71\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.58\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(220)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.011\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e26.10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e14.68\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e21.98\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e250\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(311)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.152\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e31.51\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.34\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(400)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.576\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e67.01\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.23\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.08\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(331)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.294\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e41.88\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.70\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.06\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(422)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.363\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e44.48\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8.53\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(111)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.288\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e115.71\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.75\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.68\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(220)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.147\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e73.14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.87\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.54\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e300\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(311)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.005\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e54.08\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.53\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(331)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.717\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e104.66\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.91\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.26\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(422)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4.490\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e41.82\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.72\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e9.03\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(111)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.147\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e69.47\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.32\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(220)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.005\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e52.24\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.66\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e9.28\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e350\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(311)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.152\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e31.51\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10.07\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.26\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(331)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.152\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e34.98\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e8.17\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.89\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e(422)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.260\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e57.57\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.02\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6.94\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\"\u003e\n \u003ch2\u003e3.2 The SEM study of ZnSe films\u003c/h2\u003e\n \u003cp\u003eFigure 3 illustrates the SEM micrographs of the fabricated ZnSe thin films. The SEM images demonstrate a dispersed pattern of grains of varying sizes throughout the films. The SEM morphology also indicates shallow ditches which decrease with the increase of annealing temperature. It is evident from the figures that the surface roughness of the synthesized films decreases with annealing temperature which is consistent with the reported works [36–37]. However, few largest grains are observed in the ZnSe layer annealed at 300°C as illustrated in the figure. In addition, small grains with different shapes are noticed at 350°C temperature. Comparing ZnSe thin films annealed at 350°C to those annealed at 250°C and 300°C, the former show superior uniformity and smoothness of the film surface.\u003c/p\u003e\n \u003cdiv\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\"\u003e\n \u003ch2\u003e3.3 Analysis of the EDS data of ZnSe thin films\u003c/h2\u003e\n \u003cp\u003eThe chemical makeup of the produced ZnSe films has been assessed using the EDS analysis. To find a precise result, the measurement has been performed at different spots. The atomic and weight (%) of Zn and Se atoms in the ZnSe compound are shown in Table 2. The analysis of the deposited films revealed no presence of other elements except Zn and Se. This could point to the formation of superior ZnSe thin films. However, EDS analysis ensures the near stoichiometric behavior of ZnSe thin films fabricated in this route. Previous research has also found that ZnSe thin films have a similar near stoichiometric elemental composition using the chemical bath deposition technique [54].\u003c/p\u003e\n \u003cdiv\u003e\n \u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv\u003eTable 2\u003c/div\u003e\n \u003cdiv\u003e\n \u003cp\u003eThe elemental composition of ZnSe film as analyzed through EDS after undergoing annealing at different temperatures.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAnnealing temperature (°C)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eElement\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWeight (%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eAtom (%)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eStoichiometric ratio [Zn/Se]\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003e250\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eZn\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e41.42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e46.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" rowspan=\"2\"\u003e\n \u003cp\u003e0.853\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e58.58\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e53.95\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003e300\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eZn\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e41.60\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e46.23\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" rowspan=\"2\"\u003e\n \u003cp\u003e0.859\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e58.40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e53.77\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003e350\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eZn\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e41.18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e45.80\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" rowspan=\"2\"\u003e\n \u003cp\u003e0.845\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSe\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e58.82\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e54.20\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\"\u003e\n \u003ch2\u003e3.4 The FTIR study of ZnSe thin films\u003c/h2\u003e\n \u003cp\u003eThe Fourier-transform infrared spectroscopy (FTIR) provides detailed information about the molecular composition and structure of materials. The FTIR spectra of ZnSe thin films in the 400–4000 cm\u003csup\u003e− 1\u003c/sup\u003e region at various temperatures are displayed in Fig. 4. The peak intensity at 960–1120 cm\u003csup\u003e− 1\u003c/sup\u003e indicates Zn-Se stretching vibration for the annealing conditions of 250–350°C [55–56]. However, the peak broadening might arise from the overlapping of several vibrational modes present in the same spectral area. For example, the C-H stretching vibration bond at 1074 cm\u003csup\u003e− 1\u003c/sup\u003e may appear from the co-solvents [57]. As a result, there may be an overlapping between the Zn-Se stretching and C-H stretching bonds. Moreover, the O-H bond of water moisture has been found for all cases at the wavenumber of 3626 cm\u003csup\u003e− 1\u003c/sup\u003e [55]. In addition, the stretching vibration connection between C and O is caused by the peak intensity at 2157 cm\u003csup\u003e− 1\u003c/sup\u003e, which might be found in the air environment [56].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\"\u003e\n \u003ch2\u003e3.5 Optical Study\u003c/h2\u003e\n \u003cp\u003eFigure 5 displays the optical transmittance of spin-coated ZnSe thin films for three distinct annealing temperatures in the photon wavelength range of 310–1085 nm. The graph shows that when the annealing temperature rises, the transmittance rises as well. At 350 ºC temperature, the ZnSe film exhibits maximum transmittance (above 90%) compared to the others. This outcome arises from elevated annealing temperature which enhances surface smoothness and crystallinity which are also ensured by the XRD and SEM studies [36]. Thus, ZnSe films indicate excellent transmittance throughout the visible region which makes it a superior candidate in photovoltaic applications as a window or buffer layer.\u003c/p\u003e\n \u003cp\u003eThe following formula has been used to calculate the absorption coefficient, α from the transmittance spectra [36–37]:\u003c/p\u003e\n \u003cdiv id=\"Equ4\"\u003e\n \u003cdiv id=\"FileID_Equ4\" name=\"EquationSource\"\u003e$$\\:\\alpha\\:=\\frac{\\text{l}\\text{n}\\left(\\frac{100}{T}\\right)}{d}$$\u003c/div\u003e\n \u003cdiv\u003e4\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eWhere, T is the transmittance and d is the thickness of thin film. The Tauc equation has a relationship with the absorption coefficient (α) and photon energy (hv) as follows:\u003c/p\u003e\n \u003cdiv id=\"Equ5\"\u003e\n \u003cdiv id=\"FileID_Equ5\" name=\"EquationSource\"\u003e$$\\:{\\left(\\alpha\\:hv\\right)}^{2}=A(hv-{E}_{g})$$\u003c/div\u003e\n \u003cdiv\u003e5\u003c/div\u003e\n \u003c/div\u003e\n \u003cp\u003eIn this context, E\u003csub\u003eg\u003c/sub\u003e represents the optical bandgap, while A denotes the constant. The energy bandgap \\(\\:\\left({E}_{g}=hv\\right)\\) is obtained when the linear section of the plot of \\(\\:{\\left(\\alpha\\:hv\\right)}^{2}\\:\\)vs light energy (hv) is extrapolated to a value of zero. The Tauc plots of deposited ZnSe thin films are represented in Fig. 6 for different annealing temperatures. The direct bandgaps of the ZnSe thin films are found to be 3.32–3.85 eV for 250–350 ºC, respectively. It is observed that the bandgap increases with the annealing temperature. The rise in optical bandgap with annealing temperatures could be due to the decrease in disorder and defects within the films [36]. Nevertheless, a comparable bandgap of ZnSe thin films has been identified in the preceding works [55].\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\"\u003e\n \u003ch2\u003e3.6 Ge waveguide on ZnSe substrate\u003c/h2\u003e\n \u003cp\u003eThe purpose of the waveguide is to transmit the electromagnetic wave through the guide path. Ge waveguide is designed to transmit the electromagnetic wave where ZnSe is used as a substrate on which the Ge core is built and SiO\u003csub\u003e2\u003c/sub\u003e is used as a cladding on the Ge core as shown in Fig. 7(a). The refractive index of the ZnSe has been found from the extrapolated refractive index data calculated from the transmittance data using formulas stated in other work [58]. The value of the refractive index of ZnSe is 1.73 which is much lower than that of Ge (4.0) which ensures a perfect substrate for waveguide [19]. Figures\u0026nbsp;7(b) and 7(c) display the confined mode inside Ge core on the ZnSe substrate for TM and TE mode and the corresponding value of the effective index for TM and TE are 3.852 and 3.877, respectively.\u003c/p\u003e\n \u003cdiv\u003e\n \u003c/div\u003e\n \u003cp\u003eThe effective refractive index with transmission wavelength of Ge-on-ZnSe hybrid waveguide both for TE and TE modes is shown in Fig. 8. The effective index of the photonic structure decreases with an increase in wavelength, particularly at longer wavelengths for both TM and TE modes as shown in the figure. This behavior occurs because the effective refractive index is inversely proportional to wavelength [59]. Additionally, it may happen due to the effective refractive index typically decreases as the wavelength increases. This is because, at longer wavelengths, the wave tends to spread out more, and the interaction between the wave and the waveguide's core decreases leading to a lower effective refractive index. This effect can be understood through modal dispersion: as the wavelength increases, the wave starts to leak more into the cladding material, reducing the confinement in the core which effectively lowers the refractive index associated with the mode [60–61].\u003c/p\u003e\n \u003cp\u003eFor the TE mode, the effective index varies from 3.99 to 3.42, while for the TM mode, it ranges from 3.99 to 3.58. For the TM mode, the electric field has a component along the direction of propagation, which interacts with the material's permittivity. For longer wavelengths, this interaction reduces, contributing to the decrease in the effective refractive index.\u003c/p\u003e\n \u003cp\u003eFor the TE mode, the electric field is purely transverse, and similar reasoning applies: longer wavelengths result in more field spreading, reducing confinement in the core and decreasing the effective refractive index [61–63]. This also indicates that the effective index for TM mode is higher than that of TE mode at longer wavelengths, although both TM and TE modes have the same effective index of 3.99 at shorter wavelengths. This discrepancy may arise because the TM mode is much more sensitive to inaccuracies in the vertical layers-such as thickness, angle, and compound anisotropy-compared to the horizontal layers [19, 64]. A similar behavior has been observed in previously published articles [59, 65]. The differences between TE and TM modes can significantly affect specific applications. TE mode is often preferred in optical communication systems because they exhibit lower attenuation and better PCF. In contrast, TM mode can be beneficial in applications that require strong interactions with the waveguide material, such as spectroscopy, light scattering, or wide-area lighting in photonic devices [66].\u003c/p\u003e\n \u003cp\u003eThe power confinement factor (PCF) is defined as the ratio of the electromagnetic fields within the core region of a waveguide to the electromagnetic fields throughout the entire waveguide structure. In simpler terms, it indicates how effectively the electromagnetic fields are confined within the germanium (Ge) core as depicted in Fig. 9.\u003c/p\u003e\n \u003cp\u003eThe value of PCF decreases slightly with increasing wavelength, dropping from 99.99–97.45% for the transverse electric (TE) mode and from 99.98–95.56% for the transverse magnetic (TM) mode in the Ge-ZnSe-SiO\u003csub\u003e2\u003c/sub\u003e system. The PCF in a waveguide is influenced by several factors, particularly the refractive index. When the refractive index changes, the transverse electromagnetic modes can become perturbed, causing them to extend from the Ge core into the SiO\u003csub\u003e2\u003c/sub\u003e cladding. Consequently, variations in the PCF are observed within the core of the waveguide. As illustrated in Fig. 8, the effective refractive index decreases with increasing wavelength, which contributes to variations in the PCF [67].\u003c/p\u003e\n \u003cp\u003eFigure 10 illustrates the loss in a Ge-based waveguide that utilizes ZnSe and SiO\u003csub\u003e2\u003c/sub\u003e as the substrate and cladding materials. In waveguide design, loss refers to the attenuation or reduction of optical signal power as it travels through the waveguide. Minimizing these losses is essential for achieving efficient waveguide performance. For the TE and TM modes in the Ge-on-ZnSe waveguide, loss decreases with increasing wavelength. This indicates that the Ge-ZnSe-SiO\u003csub\u003e2\u003c/sub\u003e waveguide structure is more efficient at longer wavelengths. However, the loss exhibits a zigzag behavior as the wavelength increases especially TM mode. A similar zigzag behavior in loss relative to wavelength is also observed in GaN waveguides [68].\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this study, ZnSe thin films have been successfully synthesized using a solution process with a spin coating method incorporating thiol-amine co-solvents. These films are air-annealed at temperatures between 250\u0026deg;C and 350\u0026deg;C after being produced onto glass substrates. The XRD study reveals peaks at 2θ of 27.31\u0026ordm;, 45.32\u0026ordm;, 53.57\u0026ordm;, 65.79\u0026deg;, 72.95\u0026ordm;, and 83.35\u0026ordm; with (111), (220), (311), (400), (331) and (422) planes, respectively. The XRD study exhibits the polycrystalline nature of ZnSe thin films. The SEM images illustrate that the surface roughness of the synthesized films decreases with annealing temperature. In comparison to ZnSe thin films annealed at 250\u0026deg;C and 300\u0026deg;C, those annealed at 350\u0026deg;C show superior surface uniformity and smoothness. FTIR spectroscopy confirms the effective synthesis of ZnSe as evidenced by the distinctive peak in the wavenumber of 960\u0026ndash;1120 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in all prepared samples. The optical study ensures the superior transmittance and wide optical bandgap of ZnSe films. The transmittance and optical bandgap increases with annealing temperatures and the bandgap ranging in 3.32\u0026ndash;3.85 eV. Additionally, the application of ZnSe as a substrate in Ge waveguide with SiO\u003csub\u003e2\u003c/sub\u003e cladding exhibits remarkable PCF (0.99%) and low loss (~\u0026thinsp;1.0 dB/cm) at the LWIR wavelength of 8 \u0026micro;m. These findings indicate potential applications of solution-processed ZnSe in the LWIR photonic integrations and optical devices because of its simplicity, low cost, uncomplicated apparatus used and environment friendliness.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors highly appreciate the Bangladesh Council of Scientific and Industrial Research (BCSIR) Dhaka, Bangladesh, for the providing facilities during XRD and SEM studies. The FTIR analysis was performed at the Central Science Laboratory, University of Rajshahi. In addition, the authors are indebted to the Plasma Science and Technology Lab (PSTL), University of Rajshahi, Bangladesh for the help with the optical study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data used to support the findings of this study are available from the corresponding author upon request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of Interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e One of the authors, Tanzina Rahman, receives a grant (Grant number: 2075/5/52/RU/FoE-22/2023-2024) from the Faculty of Engineering, University of Rajshahi, Bangladesh.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of generative AI and AI-assisted technologies:\u0026nbsp;\u003c/strong\u003eThe authors did not embark on AI or AI-assisted technologies in any step in preparing this manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCRediT Author Contribution Statement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTanzina Rahman\u003c/strong\u003e: data curation, formal analysis, investigation, writing- original draft; \u003cstrong\u003eMd. Alamin Hossain Pappu\u003c/strong\u003e: formal analysis, validation, visualization, writing- original draft; \u003cstrong\u003eBipanko Kumar Mondal\u003c/strong\u003e: formal analysis, validation, visualization, writing- original draft; \u003cstrong\u003eSyeda Samiha Nushin\u003c/strong\u003e: formal analysis, validation, visualization, writing- original draft; \u003cstrong\u003eJaker Hossain\u003c/strong\u003e: conceptualization, formal analysis, visualization, supervision, validation, writing- original draft, review and editing.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u003csup\u003e*\u003c/sup\u003e\u003c/strong\u003e\u003cstrong\u003eCorresponding author:\u0026nbsp;\u003c/strong\u003eEmail: [email protected] (Jaker Hossain)\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eJ. Wang, M. Isshiki, Wide-Bandgap II\u0026ndash;VI Semiconductors: Growth and Properties. In: Kasap, S., Capper, P. 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Zhao, Low loss GaN waveguides at the visible spectral wavelengths for integrated photonics applications, optics express 25(25) (2017) 31758.\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":"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":"ZnSe, thiol-amine, spin coating, bandgap, waveguide, photonics","lastPublishedDoi":"10.21203/rs.3.rs-5703824/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5703824/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis investigation introduces a novel method for the fabrication of ZnSe thin films on glass substrates through the spin coating technique which employs thiol-amine co-solvents. The thiol-amine co-solvent system efficiently dissolves several metal and metal chalcogenide precursors, facilitating cost-effective, and low-temperature solution-based deposition compatible with flexible substrates. The synthesized ZnSe thin films underwent air annealing at temperatures between 250 and 350\u0026deg;C, thereby improving their structural and optical characteristics. The polycrystalline nature of ZnSe was elucidated via X-ray diffraction (XRD) analysis while scanning electron microscopy (SEM) assured the rise of surface smoothness and uniformity with annealing temperature. Energy dispersive spectroscopy (EDS) analysis indicated near-stoichiometric ZnSe composition and Fourier-transform infrared (FTIR) spectroscopy identified Zn-Se stretching vibrations in the 960\u0026ndash;1120 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e range. The optical data demonstrated high transmittance with an optical bandgap of 3.32\u0026ndash;3.85 eV. Furthermore, optical data of ZnSe were embarked for computation of Ge-on-ZnSe waveguide with SiO\u003csub\u003e2\u003c/sub\u003e cladding for long wave infra-red (LWIR) light. The waveguide showed a remarkable power confinement factor (PCF) of ~\u0026thinsp;0.99 with nearly 1 dB/cm loss at a laser wavelength of 8 \u0026micro;m. These outputs are highly optimistic for the fabrication of solution-processed ZnSe for LWIR photonic integration.\u003c/p\u003e","manuscriptTitle":"Synthesis of ZnSe thin films by solution-processed spin coating method for photonic integration applications","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-18 17:03:08","doi":"10.21203/rs.3.rs-5703824/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":"94a91ff4-64fa-4cb1-8046-8a1fc1359ebe","owner":[],"postedDate":"April 18th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-06-02T16:06:34+00:00","versionOfRecord":{"articleIdentity":"rs-5703824","link":"https://doi.org/10.1186/s40712-025-00299-4","journal":{"identity":"journal-of-materials-science-materials-in-engineering","isVorOnly":true,"title":"Journal of Materials Science: Materials in Engineering"},"publishedOn":"2025-05-26 15:57:53","publishedOnDateReadable":"May 26th, 2025"},"versionCreatedAt":"2025-04-18 17:03:08","video":"","vorDoi":"10.1186/s40712-025-00299-4","vorDoiUrl":"https://doi.org/10.1186/s40712-025-00299-4","workflowStages":[]},"version":"v1","identity":"rs-5703824","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5703824","identity":"rs-5703824","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}