Description:FIELD OF THE INVENTION
[0001] The present disclosure relates to a technical field of semiconductor thin films and optoelectronic materials. More particularly, the present disclosure relates to an antimony doped indium sulfide thin film. Further, the present disclosure also relates to a method of preparation of an antimony doped indium sulfide thin film.

BACKGROUND OF THE INVENTION
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Photodetectors convert optical signals into electrical signals, allowing for the detection and measurement of light, are essential in applications, such as imaging systems, night-vision, artificial vision, optical communications, security, pollution detection and various sensing devices [Gong et al., Science, 2009, 1665-1667; Dong et al., Adv. Opt. Mater., 2014, 2, 549–554; Yoo et al., Adv. Mater., 2015, 27, 1712–1717]. Silicon is among the most widely studied materials for photodetectors due to its abundant availability, well-established fabrication processes and compatibility with existing electronic technologies [Green et al., Prog. Photovoltaics Res. Appl., 2021, 29, 3–15; Liu et al., Phys. Status Solidi Appl. Mater. Sci., 2021, 218, 1–16]. Gallium arsenide (GaAs) and indium gallium arsenide (InGaAs) have also gained attention for their application in broadband, high-performance and infrared photodetection applications [Shi and Nihtianov, IEEE Sens. J., 2012, 12, 2453–2459]. In addition to traditional semiconductors, metal oxides have emerged as promising materials for thin-film photodetectors. Metal oxides like zinc oxide (ZnO) and titanium dioxide (TiO₂) are especially well-studied due to their high electron mobility, wide bandgap and good sensitivity to UV light [Pandey et al., Sensors Actuators A Phys., 2023, 350, 114112]. Additionally, organic semiconductors, such as conjugated polymers and small molecules, have attracted significant interest as promising materials for thin-film photodetectors. The exploration of novel materials such as 2D semiconductors and quantum dots may open up new opportunities for high-performance photodetection [Guo et al., J. Mater. Chem. C., 2023, 12, 1233–1267]. Perovskites, also stand out in this field with their exceptional optoelectronic characteristics.
[0004] Thin film technology has gained prominence for fabricating high-performance photodetectors due to its flexibility, cost-effectiveness, and scalability. Semiconductors are widely used in thin films for their superior optoelectronic properties [Kumar et al., J. Alloys Compd., 2022, 892, 160801]. Key performance parameters include responsivity, detectivity, response time, and spectral range. Responsivity reflects sensitivity to light, while detectivity indicates the ability to detect weak signals. Response time is critical for high-speed applications, and spectral range defines the wavelengths over which the device operates, including visible, near-infrared, or ultraviolet regions [Haunsbhavi et al., Phys. Scr., 2022, 97, 055815].
[0005] Despite significant advancements in thin film photodetectors, several challenges remain unresolved. One major issue is the long-term stability of these devices, especially under harsh environmental conditions. Enhancing the stability of thin film materials and device structures is crucial for ensuring reliable operation over extended periods. Another challenge is the scalability of thin film photodetector fabrication. While thin film technology enables large-scale manufacturing, achieving consistent performance and reproducibility remains challenging. Integrating thin film photodetectors with other components, like light sources and waveguides, is also complex, as efficient light coupling is crucial. Advancements in fabrication techniques are needed for seamless integration into optoelectronic systems; therefore, future research should aim to address these challenges while enhancing the performance of photodetectors [Zhang et al., J. Mater. Chem. C., 2023, 11, 12453–12465].
[0006] Metal chalcogenides are emerging as promising materials for photodetectors, valued for their abundance, affordability, versatility, and excellent electrical properties. Among them, tin sulfide (SnS), bismuth sulfide (Bi2S3) and indium sulfide (In2S3) gained interest due to their favorable features for photodetection [Alagarasan et al., Phys. Scr., 2022, 97]. The performance of chalcogenide photodetectors can be controlled by doping with suitable dopant elements. It is reported that Ag doping enhances the performance of the SnS photodetectors [Alagarasan et al., Sensors Actuators A Phys., 2023, 349, 114065]. Ganesh et al. [Phys. Scr., 2023, 98, 1–10] explored the effect of Sn doping on Bi2S3 thin films for photodetector applications. An optimal performance of photodetector with a 2 wt% Sn doping concentration was achieved, resulting in a material well-suited for high-speed optoelectronic devices. Moreover, the preparation temperature also influences the optical properties of thin films, thereby affecting their photodetection properties [Barman et al., Superlattices Microstruct., 2019, 133, 106215].
[0007] Indium sulfide (In2S3) is one of the potential materials for the fabrication of UV photodetector due to its high photoconductivity, stability and suitable wide optical bandgap [Alagarasan et al., J. Photochem. Photobiol. A Chem., 2023, 444, 114941]. Depending on the formation temperature, Indium sulfide (In2S3) mainly exhibits three crystalline structures; α- In2S3 (cubic structure), β- In2S3 (spinel structure), and γ-In2S3 (layered structure) [Barreau, Sol. Energy., 2009, 83, 363–371; Pistor et al., Struct. Sci. Cryst. Eng. Mater., 2016, 72, 410–415]. Among these structures β-In2S3 is the most stable phase from room temperature up to 420 °C and above this temperature β- In2S3 structurally transforms to α- In2S3 which has stability up to 750 °C. γ- In2S3 is a stable trigonal crystal structure and is stable from 750 °C to the melting point (1090 °C) [Turan et al. Philos. Mag., 2012, 92, 1716–1726]. β- In2S3 possesses a high density of vacancies and its structure gives adequate interspace for doping and modifying its physical properties [Kaur et al., Mater. Sci. Eng. B., 2021, 264]. Among these, the β-In₂S₃ phase is the most stable and commonly used in optoelectronic applications.
[0008] Previously, doped In₂S₃ thin films have been investigated for their photodetection properties, with some reports highlighting their potential in UV photodetector applications. The effect of cerium doping on the photodetection efficiency of In₂S₃ thin films reveals that 2 wt% of doping significantly enhances the photo-sensing capabilities of the films [Gunavathy et al., Opt. Mater. (Amst)., 2023, 137, 113612]. Similarly, the report on terbium-doped In2S3 demonstrates an enhanced photoresponse for the film with 2 wt% dopant concentration [Alkallas et al., Phys. Scr., 2023, 98]. In another investigation on lanthanum-doped In2S3 films, the incorporation of a doping level of 3 w% significantly enhanced the surface morphology, crystallinity and optical properties, leading to improved photodetector performance parameters for UV detection [Alagarasan et al., J. Photochem. Photobiol. A Chem., 2023, 444, 114941]. It was also demonstrated that the optimal doping with 1 wt% bismuth was shown to significantly enhance the UV photodetection performance of In₂S₃ films by improving optical absorption, reducing the bandgap and enhancing photodetector parameters compared to other doping concentrations [Alagarasan et al., J. Photochem. Photobiol. A Chem., 2024, 454, 115697]. The photodetector properties of In2S3 thin films were also investigated with respect to substrate temperature [Kumar et al., J. Mater. Sci. Mater. Electron., 2019, 30, 17986–17998], thickness [Kumar et al., A Phys., 2019, 299, 111643] and molar concentration [Lavanya et al., Micro and Nanostructures., 2022, 169, 207337].
[0009] Thus, there is a need to develop a novel optoelectronic materials having improved responsivity, detectivity and external quantum efficiency (EQE).

OBJECTIVES OF THE INVENTION
[0010] An object of the present disclosure is to provide an antimony doped indium sulfide thin film.
[0011] Another objective of the present disclosure is to provide a method of preparation of an antimony doped indium sulfide thin film.
[0012] Still another objective of the present disclosure is to enhance the optoelectronic characteristics of the films, particularly for UV detection.
[0013] Yet another objective of the present disclosure is to develop novel antimony doped indium sulfide thin film having improved responsivity, detectivity and external quantum efficiency (EQE).

SUMMARY OF THE INVENTION
[0014] This summary is provided to introduce a selection of concepts in a simplified form that are further described below in Detailed Description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
[0015] An aspect of the present disclosure relates to an antimony doped indium sulfide thin film comprises: a) the thin film comprises indium sulfide (In2S3); and b) the thin film is doped with antimony (Sb) at a concentration ranging from 1 to 5 weight percent (wt%).
[0016] Another aspect of the present disclosure relates to an method of preparation of an antimony doped indium sulfide thin film comprising: a) dissolving 0.02 M indium precursor and 0.03 M thiourea in a solvent to obtain a solution; b) adding 1 to 5 wt % dopant in the solution with stirring to obtain a spray solution; and c) spraying the spray solution onto the heated glass substrate using a nebulizer spray setup to obtain an antimony doped indium sulfide thin film.
[0017] Other aspects of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learnt by the practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The accompanying drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0019] Figure 1 illustrates XRD patterns of undoped and Sb-doped (1-5 wt.%) In2S3 thin films.
[0020] Figure 2 illustrates FESEM images of undoped and Sb-doped (1 to 5 wt.%) In2S3 thin films.
[0021] Figure 3 illustrates EDS spectra of the undoped and 3 wt% Sb-doped In2S3 thin films.
[0022] Figure 4 illustrates Optical absorption spectra and Tauc plot of undoped and Sb-doped In2S3 thin films.
[0023] Figure 5 illustrates the photoluminescence emission spectra of undoped and Sb-doped In2S3 thin films.
[0024] Figure 6 illustrates Schematic diagram of photodetector device fabricated using Sb-doped In2S3 thin films.
[0025] Figure 7 illustrates Semi log current vs voltage (I-V) characteristics of undoped and Sb-doped In2S3 thin films in dark and light modes at ±5V bias. The irradiated light is with wavelength of 365 nm and a power density of 5 mW/cm².
[0026] Figure 8 illustrates the time-dependent photo-response characteristic of the undoped and Sb-doped In₂S₃ thin films, recorded under 365 nm illumination at varying light intensities ranging from 1 to 5 mW/cm².
[0027] Figure 9 illustrates variation of photocurrent at different illumination light intensities from 1 to 5 mW/ cm2, at an applied bias of 5 V for the undoped and Sb-doped In2S3 thin films measured using 365 nm laser source.
[0028] Figure 10 illustrates the time-dependent photo-response of 3 wt% Sb-doped In2S3 photodetector over 50 cycles at a UV light intensity of 5 mW/cm2.
[0029] Figure 11 illustrates the current under UV illumination intensity of 5 mW/cm2 measured for 3 wt% Sb-doped In2S3 PD, both immediately after fabrication and after a storage period of one month, demonstrates the long-term stability.

DETAILED DESCRIPTION OF THE INVENTION
[0030] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0031] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases, it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0032] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[0033] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[0034] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[0035] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range.
[0036] Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0037] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
[0038] Reference will now be made in detail to the exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings.
[0039] The following description provides different examples and embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus, if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.
[0040] All percentages, ratios, and proportions used herein are based on a weight basis unless otherwise specified.
[0041] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0042] The present disclosure provides a preparation and UV photodetection properties of Antimony (Sb) doped Indium Sulfide (In2S3) thin films. The nebulizer spray pyrolysis method is used to deposit the undoped and Sb-doped In2S3 thin films onto glass substrates at 350 °C. The X-ray diffraction (XRD) analysis verified that all deposited films exhibited the cubic phase β-In2S3 structure, with the highest crystallinity observed in the 3 wt% Sb-doped film. The morphological analysis showed densely packed, relatively larger grains in the 3 wt% Sb-doped In2S3 thin film. From UV-Vis investigation, it is observed that Sb doping enhanced optical absorption. The key parameters of the photodetectors are determined by Current-Voltage and Current-Time measurements. Among the fabricated photodetectors, the 3 wt% Sb-doped In2S3 thin film exhibited remarkable performance, with calculated Responsivity (R) of 1.020 A/W, Detectivity (D*) of 32.5 × 1010 Jones, and External Quantum Efficiency (EQE) of 346%. The photo-response investigation provided rise and fall times of 0.43 s and 0.26 s respectively, for the 3 wt% Sb-doped sample. Additionally, the stability test demonstrates the durability, reliability and suitability of the photodetectors for long-term applications. The remarkable enhancement in photodetector properties achieved through Sb doping highlights the exceptional quality and potential of the 3 wt% Sb-doped In2S3 thin films, setting a new standard for performance in advanced UV detection applications.
[0043] An embodiment of the present disclosure provides an antimony doped indium sulfide thin film comprises: a) the thin film comprises indium sulfide (In2S3); and b) the thin film is doped with antimony (Sb) at a concentration ranging from 1 to 5 weight percent (wt%) on a glass substrate.
[0044] In some embodiment, the antimony doped indium sulfide thin film has a cubic phase β-In₂S3 structure.
[0045] In some embodiment, the antimony doped indium sulfide thin film has an X-ray powder diffraction pattern (CuKα) comprising peaks at 2-theta of 14.50°, 27.86°, 33.76°, 44.1°, 48.30° and 70.63°.
[0046] In some embodiment, the antimony doped indium sulfide thin film has a crystallite size ranging from 35 to 55 nm.
[0047] In some embodiment, the antimony doped indium sulfide thin film has a responsivity of 1.02 AW-1, detectivity of 32.5x1010 Jones and external quantum efficiency (EQE) of 346 %.
[0048] An embodiment of the present disclosure provides a method of preparation of an antimony doped indium sulfide thin film comprising: a) dissolving 0.02 M indium chloride and 0.03 M thiourea in a solvent to obtain a solution; b) adding 1 to 5 wt % dopant in the solution with stirring to obtain a spray solution; and c) spraying the spray solution onto the heated glass substrate using a nebulizer spray setup to obtain an antimony doped indium sulfide thin film.
[0049] In some embodiment, the indium precursor is selected from a group comprising of indium chloride, indium fluoride, indium bromide, indium iodide, indium oxides, indium carboxylates and combination thereof. Preferably, the indium precursor is indium chloride.
[0050] In some embodiment, the solvent is selected from a group comprising of water, methanol, 1,4-butanediol, polyethylene glycol 600 and combination thereof. Preferably, the solvent is water.
[0051] In some embodiment, the dopant is selected from a group comprising of antimony nitrate, tin chloride, selenium chloride and combination thereof. Preferably, the dopant is antimony nitrate.
[0052] In some embodiment, the stirring in step b) is carried out at a speed ranging from 300 to 400 RPM for a time period ranging from 30 min to 2 hours. Preferably, the stirring speed is 350 RPM for a time period of 1 hour.
[0053] In some embodiment, the nebulizer spray setup includes air flow with a pressure ranging from 1 to 2 kg/cm² as the carrier gas to create fine aerosol droplets, the spray rate is set to 0.5 mL/min to 2 mL/min and the deposition time is maintained for a time period ranging from 5 to 10 minutes. Preferably, pressure is 1.5 kg/cm², the spray rate is 1 mL/min and the deposition time is 10 minutes.
[0054] In some embodiment, the heated glass substrate has a temperature in the range of 300 to 400 °C. Preferably, the temperature is 350 °C.
[0055] The undoped and Sb-doped thin films were deposited onto glass substrates at 350°C using the nebulizer spray pyrolysis (NSP) method. This technique ensures the formation of uniform, high-quality thin films, which are crucial for achieving reliable photodetector performance. Indium chloride (InCl₃), thiourea (NH₂-CSNH₂) and antimony nitrate (N₃O₉Sb) with 99.99% purity were used for the synthesis process.
[0056] X-ray diffraction (XRD) patterns of undoped and Sb-doped In₂S₃ thin films were recorded using a PANalytical X’Pert Pro diffractometer with a Cu Kα source (λ = 1.5405 Å) in the 2θ range of 10° to 90° to analyze their crystalline structure. Morphology and elemental composition were examined using a Field Emission Scanning Electron Microscope (FESEM, Aspreo S) and Energy Dispersive Spectroscopy (EDS, Oxford Instruments 50 mm²), respectively. The thickness of the films was measured using a stylus profilometer. Optical absorption spectra were recorded using a UV-VIS-NIR spectrometer (Perkin Elmer-Lambda 35) in the range of 300 nm to 900 nm, while photoluminescence (PL) spectra were obtained with a fluorescence spectrometer (PerkinElmer LS-55). The photo-sensing performance of the photodetectors was evaluated through current-voltage (I-V) measurements using a 365 nm laser as the light source and a Keithley 2450 source meter for photocurrent measurement.
[0057] The crystallite sizes in the prepared films were calculated using Scherrer’s formula, given by

……… (1)
where, β represents the corrected Full Width at Half Maximum (FWHM) of the diffraction peaks, while θ denotes the Bragg angle of diffraction. The dimensionless shape factor, represented by K, was set to 0.9. The microstrain values were calculated using the following equation.
……… (2)
[0058] The lattice constant (a) of the cubic unit cell and the unit cell volume (V) for the films were calculated using specific formulae,
……… (3)
V= a3 ……… (4)
[0059] The Tauc’s relation is given by
αhν = B[hν − Eg]n ………….. (5)
where, α represents the absorption coefficient, B is a proportionality constant, and n values of ½ and 2 correspond to direct and indirect allowed transitions, respectively.
[0060] Different parameters of photodetectors such as photoresponsivity (R), detectivity (D*), and external quantum efficiency (EQE) were calculated using the relations given below
………….. (6)
where, Iph represents the photocurrent, Pin denotes the incident light power and A is the surface area of the thin film detector.
………….. (7)
where, Idark represents the dark current and e is the electron charge.
………….. (8)
where, h is the Planck’s constant and λ represents the incident light’s wavelength.
[0061] While the foregoing describes various embodiments of the disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.
EXAMPLES
[0062] The present disclosure is further explained in the form of the following examples. However, it is to be understood that the examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope and spirit of the present invention.
Example 1
(i) Sb:In2S3 thin film preparation
[0063] 0.02 M InCl₃ and 0.03 M thiourea were dissolved in 10 mL of deionized water to prepare the spray solution. The doping concentration in the precursor solution was varied at levels of 0, 1, 2, 3, 4, and 5 wt%. The solution was magnetically stirred for one hour to ensure a homogeneous mixture, crucial for achieving uniform doping levels throughout the prepared films. During the nebulizer spray process, air flow with a pressure of 1.5 kg/cm² was employed as the carrier gas to create fine aerosol droplets. This approach enables a controlled and consistent deposition, which is crucial for obtaining the desired film thickness and uniformity. To obtain quality films, the spray rate was set to 1 mL/min, and the deposition time was maintained for 10 minutes. The nozzle was positioned 5 cm above the substrate to optimize droplet size and film coverage. The deposition chamber was kept at a temperature of 350 °C to ensure proper film crystallization.
(ii) Crystallite size and phase purity
[0064] Figure 1 presents the X-ray diffraction (XRD) patterns of pure and antimony (Sb)-doped indium sulfide (In2S3) thin films at varying doping levels. The XRD peaks are observed at 2θ angles of 14.50°, 27.86°, 33.76°, 44.1°, 48.30°, and 70.63°. These peaks, along with their respective d-spacing values, align well with the (111), (311), (400), (511), (440), and (800) diffraction planes of cubic phase In2S3. It is observed that, as the doping concentration increased from 0 to 3 wt%, the intensity of the strongest diffraction peak (400) significantly increased but decreased at higher doping levels. The observed rise in the (400) peak intensity suggests improved crystallinity and a reduction in strain within the films.
[0065] The calculated crystallite sizes (D), lattice constants, microstrain (ε) and cell volume values are presented in Table 1. From the table, it is evident that the film with 3 wt% Sb doping has the largest crystallite size of 52 nm and the lowest microstrain of 2.3×10-3. The observed increase in crystallite size with doping up to 3 wt% concentration is likely due to the substitutional doping of Sb³⁺ ions into the In³⁺ lattice sites. The substitution of Sb³⁺ ions for In³⁺ creates localized lattice distortions that facilitate grain growth, resulting in larger crystallites. Additionally, the enhanced nucleation and growth at this dopant level contribute to a more ordered crystalline structure, which improves the optoelectronic properties of the thin films, particularly for photodetection applications. The substitutional doping of Sb³⁺ for In³⁺ can replace the host atoms in the crystal lattice without introducing significant strain or distortion. This can reduce the likelihood of defect formation that would otherwise hinder crystallite growth. As a result, substitutional doping facilitates larger crystallites during thin film growth because fewer defects can mean improved crystal quality and larger grains. When the doping concentration increases from 3 wt% to 5 wt%, Sb³⁺ ions may either occupy interstitial sites within the In₂S₃ lattice or accumulate at grain boundaries, thereby hindering the oriented growth of grains. Beyond the doping concentration of 3 wt%, the structural properties of the films begin to degrade. The microstrain value decreases with Sb doping up to 3 wt%, after which they increase. This can be correlated with the crystallite size in the films, as a reduction in strain results in improved film quality.
Table 1. The structural parameters of Sb-doped In2S3 thin films obtained by XRD investigation.
Sb doping
Concentration (%) Crystallite
Size (nm) Strain
× 10-3 Lattice constant
a(Å) Cell
Volume (Å3)
0 39 3.05 10.663 1212.39
1 44 2.73 10.665 1213.05
2 48 2.51 10.665 1213.23
3 52 2.30 10.674 1216.20
4 40 2.99 10.662 1212.04
5 39 3.05 10.657 1210.62

[0066] The lattice constant (a) of the cubic unit cell and the unit cell volume (V) for the films are summarized in Table 1. The lattice constant of the cubic unit cell and the unit cell volume (V) showed a slight increase with Sb doping up to 3 wt% but decreased at higher doping levels of 4 wt% and 5 wt%. This observation is consistent with other materials where dopants with radii relatively close to that of the host are introduced. The incorporation of such dopants typically leads to weaker hybridization, attributed to the relaxation of anions surrounding the dopant cations. Consequently, this induces an antibonding expansion mechanism, resulting in an initial increase in the lattice parameter.
(iii) Surface morphology and Elemental composition
[0067] The effect of Sb-doping on the morphological features and grain structure of In₂S₃ thin films was examined using FESEM analysis, providing critical insights into the microstructural evolution with varying dopant concentrations. Figure 2 shows FESEM images of undoped and Sb-doped In₂S₃ films at various doping levels, revealing significant morphological changes due to the incorporation of Sb. In the undoped film, irregular and smaller grains dominate the surface, with a noticeable presence of an amorphous background that could contribute to its inferior optoelectronic property. Sb-doping leads to a noticeable change in the grain structure, resulting in larger and more uniformly distributed grains. The 3 wt% Sb-doped film exhibits densely packed, well-defined grains with enhanced surface coverage, suggesting an efficient crystallization process. At this particular concentration, the compact arrangement of the grains reduces the number of grain boundaries, which otherwise would act as scattering centers for charge carriers, and thereby could help in enhancing the optoelectronic performance of the film. As the doping concentration exceeds 3 wt%, the grains become less uniform and smaller. Beyond 3 wt%, grain size decreases, likely due to excess Sb interfering with crystallization, leading to an increase in defects. These observations align with XRD results, highlighting 3 wt% Sb-doping as optimal for balancing crystallinity and grain structure to enhance photodetector performance.
[0068] To ensure the accuracy of the doping process, Energy Dispersive X-ray Spectroscopy (EDS) was utilized to examine the elemental composition of the films. Figure 3 shows the EDS spectra confirming the presence of the elements Indium (In), Sulfur (S) and Antimony (Sb) in the undoped and 3 wt% Sb-doped films. For the 3 wt% Sb-doped film, the atomic percentages of In, S, and Sb are found to be 36.49%, 60.35% and 3.16% respectively. These values closely align with the level of doping concentration. The analysis thereby validates the chemical purity of the prepared films. A prominent X-ray peak in the spectra corresponds to Silicon (Si) from the glass substrate.
(iv) Optical properties of thin films
[0069] The UV-Vis absorption spectra for both undoped and Sb-doped In₂S₃ films were recorded across the 300 nm to 700 nm wavelength range, as shown in Figure 4a. The spectra show strong absorption in the UV region with a peak centered around 325 nm. The absorbance peak intensity increased as the Sb doping concentration was raised from 0 wt% to 3 wt%, but it decreased when the doping level was increased to 4 wt% and 5 wt%. At these higher doping levels, the optical absorbance was even lower than that of the undoped In₂S₃ film, indicating an optimal doping concentration of around 3 wt%. The increased optical absorption in the 3 wt% Sb-doped sample can be attributed to improved crystallinity and the introduction of localized impurity states within the bandgap. These impurity states could act as intermediate energy levels, allowing photons with energies lower than the original bandgap energy to be absorbed; thereby, enhancing the optical absorption. Furthermore, the improved crystallinity at the optimal doping level reduces defects and scattering centers, and contributes to more effective absorption of light. In contrast, at higher doping concentrations, the decrease in absorption could be attributed to increased disorder, which may hinder the effective contribution of impurity states to optical transitions. This indicates that excessive doping could lead to a disruption of the film's crystalline structure, counteracting the beneficial effects observed at lower concentrations.
[0070] With the known thickness t of thin films, and the absorbance value A, the absorption coefficient α was obtained by α=2.303 A/t. A thickness range of 400 nm to 700 nm has been reported suitable for PD applications. Within this range, a thickness of 600 nm is found particularly effective for Sb-doped In2S3 film for PD fabrication. The energy bandgaps of the films estimated through Tauc’s relation reveal a systematic modulation of the bandgap with doping. This bandgap engineering enabled by Sb incorporation, is crucial for tuning the films’ optical properties for applications in photodetectors and other optoelectronic devices.
[0071] Figure 4b shows the plot of photon energy (hν) versus (αhν)² for the estimation of the direct optical bandgap of films. The undoped In₂S₃ film shows a direct bandgap of 2.94 eV. For the Sb-doped films, the estimated optical bandgaps are 2.88 eV, 2.87 eV, 2.84 eV, 2.91 eV, and 2.90 eV for doping concentrations of 1 wt%, 2 wt%, 3 wt%, 4 wt%, and 5 wt%, respectively. The observed trend in bandgap variation reveals that the bandgap decreases with increasing Sb doping up to 3 wt%, reaching a minimum at this concentration. This decrease in bandgap could be attributed to the creation of localized states within the band structure due to doping. These localized states could provide additional channels for photon absorption resulting in bandgap narrowing. This bandgap narrowing allows optical absorption at lower photon energies; and therefore, increases the material’s sensitivity to UV light. However, as the doping level increases beyond 3 wt%, the bandgap begins to slightly increase. This increase in the bandgap at higher doping concentrations may result from increased strain and disorder within the crystal lattice. Similar trends were noted in In₂S₃ films doped with metal elements, where excessive doping disrupts the crystalline order and alters the electronic structure. These bandgap variations with Sb doping suggest an optimal doping concentration for tailoring the optical and electronic properties of In₂S₃ thin films, particularly for applications in UV photodetectors and optoelectronic devices, where precise control of the bandgap is crucial for performance optimization.
[0072] Photoluminescence (PL) measurements were conducted to explore the defect states and recombination mechanisms in both undoped and Sb-doped In₂S₃ thin films. Figure 5 illustrates the PL spectra of these films, obtained at room temperature in a wavelength range of 350 nm to 650 nm, using a 325 nm excitation source. The PL behavior of the films is significantly influenced by intrinsic defects such as vacancies and interstitials, which can act as trapping centers for charge carriers and could affect the emission characteristics of the films. The PL spectra exhibit two prominent emission peaks, one around 481 nm and another at 525 nm. The peak near 481 nm can be attributed to excitonic recombination, which occurs when photoinduced holes recombine with electrons occupying in the sulfur vacancies in defect states in the bandgap. The peak around 525 nm is linked to defect-related transitions, specifically transitions between indium (In) vacancies and sulfur (S) vacancies. Additionally, a weaker peak at 420 nm could be attributed to surface oxidation states. The emission intensity increases for films with 1 wt%, 2 wt% and 3 wt% Sb doping, reaching a maximum at 3 wt%, before decreasing at higher doping levels of 4 wt% and 5 wt%. The enhanced PL intensity observed in the 3 wt% Sb-doped In₂S₃ sample could be attributed to improved crystallinity and increased defect sites, leading to an increase in radiative recombination events. Moreover, a longer charge carrier lifetime due to reduced grain boundary defects also contributes to the enhanced intensity of PL spectra. The decline in PL intensity at higher doping concentrations is likely due to concentration quenching, where energy is mainly transferred non-radiatively between the activator (donor) and quenching (acceptor) sites. This behavior suggests an optimal doping level at 3 wt% for achieving enhanced photoluminescence properties in In₂S₃ thin films.
(v) Current-Voltage (I-V) characteristics and photoresponse
[0073] Figure 6 presents a schematic diagram of the Sb-doped In₂S₃ photodetector, accompanied by the configuration for I-V measurements. For the fabrication of photodetectors, an active area of 1 cm × 1 cm was defined by depositing silver paste over the surface of the thin film, forming the electrical contacts. This step is crucial for efficient charge collection and minimizing contact resistance, enabling precise photoresponse characterization.
[0074] Figure 7 shows the I-V characteristics of the photodetector films under both dark and illuminated conditions, with an external bias voltage of ±5 V. The photocurrent can be calculated by IP = IL – ID, where IL and ID are electric currents due to light irradiation of 365 nm with a power density of 5 mW/cm², and at dark conditions respectively. The selected light source has photon energy (3.4 eV) greater than the bandgap energy, consequently electron-hole pairs are generated in the material. All the films show a considerably higher light current than the dark current, confirming the sensitivity of photodetectors to UV light. The dark current is observed to be in the nanoampere range, while the light current increases by three orders of magnitude to reach the microampere range. In addition, no significant variation in dark current was noted following Sb-doping, indicating that the doping process did not adversely affect the basic electrical properties of the material. Among the prepared films, the 3 wt% Sb-doped film shows the maximum photocurrent. Even though the photocurrent values decrease at higher doping concentrations of 4 wt% and 5 wt%, they remained above those for the undoped thin film, indicating that even at suboptimal doping levels, Sb incorporation positively influences the photo-response of the material. The enhanced photocurrent of the doped films could be attributed to the improved crystallinity and absorption, plus the defect states introduced by Sb-doping. All these factors facilitate more efficient charge carrier generation and transport. The peak photocurrent of 51 × 10⁻⁶ A at a 5 V bias in 3 wt% Sb-doped film demonstrates a performance that is comparable to, and in some cases exceeds, the values reported on photodetectors based on In₂S₃.
[0075] Figure 8 illustrates the time-dependent photo-response of Sb-doped and undoped In₂S₃ thin film photodetectors, measured under incident light power densities varying from 1 to 5 mW/cm², with a constant bias of 5 V. The figure demonstrates that the photocurrent rapidly rises to its peak value upon exposure to UV light and swiftly returns to its baseline when the light is turned off, indicating a fast photo-response time. This quick response reflects the essential qualities such as high sensitivity and efficiency of the photodetectors, required for sub-second UV detection. Figure 9 shows that the photocurrent is directly proportional to the incident light intensity (1-5 mW/cm2) across all samples, suggesting an excellent response to varying light conditions and linearity of the fabricated photodetectors. The increase in photocurrent with an increase in incident light intensity could be attributed to the increase in the generation of photo-induced charge carriers. From the figure, it is also evident that the photocurrent does not saturate within the range of incident light intensity. All these characteristics are crucial for the practical deployment of Sb-doped In₂S₃ for photodetector applications.
[0076] The rise time (τr) and fall time (τf), often referred to as the response and recovery times, are crucial for determining how quickly a photodetector operates. The rise time measures how long it takes for the current to go from 10% to 90% of its maximum value, whereas the fall time represents the time required for the current to drop from 90% to 10% of its peak value. These times are essential for evaluating the speed at which a photodetector can respond to changes in light intensity. Among the prepared films, the film with 3 wt% Sb-doping exhibits optimal rise and fall times of 0.43 s and 0.26 s respectively, highlighting its enhanced light detection and recovery speeds. Table 2 provides the rise and fall times of all the films.
[0077] A cyclic test was conducted to evaluate the photo-response characteristics of the fabricated PDs, focusing on their stability and performance under repeated UV exposure. Figure 10 illustrates the photo-response of the 3 wt% Sb-doped In₂S₃ thin-film PD over 50 consecutive ON/OFF cycles, tested at an incident light power density of 5 mW/cm². Given the critical importance of long-term stability in PDs, a stability test was conducted specifically for the 3 wt% Sb-doped PD. The current under UV illumination was measured both immediately after fabrication and after a storage period of one month, as depicted in Figure 11. The results demonstrate that the photodetector maintained a stable light current with negligible variation, indicating no significant degradation over time. Furthermore, the absence of any current loss under repeated UV exposure highlights the material's durability. The consistent current output and rapid switching behavior observed across all cycles confirm the photodetector's robustness, reliability and suitability for long-term applications.
(vi) Parameters (R,D*and EQE) of photodetector
[0078] The calculated parameters for the photodetector films are summarized in Table 2. The data shows a significant enhancement in the calculated responsivity of the photodetectors upon Sb doping of In₂S₃ film. The responsivity gradually increased from 0.099 AW-1 to 1.020 AW-1 when the Sb-doping concentration increased from 0 wt% to 3 wt%. However, at higher doping levels, such as 4 wt% and 5 wt%, the responsivity declined. This suggests that while moderate Sb doping can substantially improve device performance, excessive doping introduces defects that act as recombination centers, diminishing the photodetector's efficiency. The responsivity value for the 3 wt% Sb-doped film is considerably higher than those reported in earlier studies on undoped and doped In₂S₃ thin film photodetectors, highlighting the benefits of Sb-doping for enhancing the photoresponse. This improvement can be attributed to enhanced crystallinity and the optimized defect states, which together promote efficient charge carrier generation and transport. In addition, the presence of large, uniformly distributed grains minimizes scattering and reflection losses of incident light, thereby boosting the photoresponsivity of the film. In contrast, at higher doping levels, the responsivity declines, due to excessive Sb incorporation disrupting the crystal lattice. This disruption leads to the creation of additional trap states, which reduce charge carrier mobility and degrade the device's performance.
[0079] The external quantum efficiency (EQE) exhibited a significant enhancement in Sb-doped In₂S₃ thin films. The EQE increased from 34% in the undoped In₂S₃ film to 346% in the 3 wt% Sb-doped film, representing a tenfold enhancement in photon-to-electron conversion efficiency. Such high EQE values are considerably greater than those reported in previous studies on undoped and doped In₂S₃ photodetectors. The remarkable increase in EQE demonstrates the effectiveness of Sb-doping in optimizing the optoelectronic properties of In₂S₃ thin films. Several factors contribute to this improvement. Primarily, the reduction in bandgap and increased optical absorption at the optimal 3 wt% doping level enabled the film to absorb more photons across a wider range of wavelengths, thereby generating more charge carriers. In addition, the enhanced crystalline property helped reduce crystal defects, minimizing energy losses due to non-radiative recombination and enabling more efficient charge separation and transport. These electronic and structural advantages enable a greater proportion of photogenerated carriers contributing to the photocurrent rather than being lost to defect states. Moreover, the introduction of Sb creates impurity states within the bandgap that act as traps, extending the lifetime of charge carriers by minimizing recombination. This longer carrier lifetime, combined with enhanced charge carrier mobility resulting from reduced lattice strain, improve the overall performance and efficiency of photodetector.
[0080] Detectivity (D*), an important parameter of a photodetector, indicates how well the device can detect weak optical signals. For the 3 wt% Sb-doped In₂S₃ film, the detectivity is nearly ten times higher than that of the undoped film, showing a significant improvement in its ability to sense low-intensity UV light. This remarkable increase in D* highlights the effectiveness of Sb doping in enhancing both the sensitivity and overall sensitivity of the photodetector. This substantial improvement in detectivity places the Sb-doped In₂S₃ thin film photodetectors among the most sensitive UV detectors currently in development, making them strong candidates for high-performance optoelectronic applications. With such promising detectivity, these Sb-doped devices have the potential to surpass traditional photodetectors in fields requiring precise UV sensing.
Table 2. The photodetector properties of the undoped and Sb-doped In2S3 thin films.
Sb doping
Concentration (%) Responsivity (AW−1) Detectivity (Jones) EQE
(%) Rise time (s) Fall time
(s)
0 0.099 4.20 ×1010 34 0.70 0.27
1 0.216 8.74×1010 73 0.28 0.67
2 0.274 9.50×1010 93 0.43 0.43
3 1.020 32.5×1010 346 0.43 0.26
4 0.336 19.3×1010 114 0.43 0.27
5 0.203 10.1×1010 69 0.93 0.27

[0081] Table 3 provides a detailed comparison of the photodetector performance from this study with previous research on doped In2S3 thin films. The data demonstrates that the key performance parameters R, D* and EQE achieved in this work are not only comparable to but, in many cases, surpass those reported in earlier studies. These improvements are especially evident in the 3 wt% Sb-doped In₂S₃ photodetector, where the optimized doping level significantly enhances the performance of the device. The improved performance at the optimal doping level suggests that carefully adjusting the doping level is crucial for achieving maximum device efficiency. The outcomes of this study emphasize the effectiveness of Sb doping in significantly enhancing the photodetector’s sensitivity to UV light.
Table 3. Comparison table of photodetector parameters for Sb-doped In2S3 thin films with previous reports on In2S3-based thin films.
Samples Responsivity (AW−1) Detectivity (Jones) EQE
(%) Rise
time (s) Fall
time (s) Reference
In2S3:Sb 3% 1.02 32.5×1010 346 0.43 0.26 This work
In2S3:Ce 2% 0.75 4.53×1010 243 0.3 0.4 Gunavathy et al., Opt. Mater. (Amst). 137 (2023) 113612
In2S3:La 3% 0.51 1.11×1011 118 0.3 0.3 Alagarasan et al., J. Photochem. Photobiol. A Chem. 444 (2023) 114941
In2S3:Bi 1% 0.65 15×1010 125 3.21 3.78 Alagarasan et al., J. Photochem. Photobiol. A Chem. 454 (2024) 115697
In2S3:Tb 2% 0.26 7.75×1010 60 2.9 3.6 Alkallas et al., Phys. Scr. 98 (2023)
In2S3:Cd 4% 0.21 1.84 ×1011 50 1.6 1.3 Rajeswari et al., J. Mater. Sci. Mater. Electron. 33 (2022) 19284–19296

[0082] The foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.

ADVANTAGES OF THE INVENTION
[0083] The antimony (Sb)-doped indium sulfide (In₂S₃) thin films were prepared by nebulizer spray pyrolysis (NSP) method.
[0084] The cubic phase of β-In₂S₃ was confirmed across all samples by XRD investigation, with the 3 wt% Sb-doped film displaying the highest crystallinity.
[0085] Morphological analysis reveals that the 3 wt% Sb-doped film has densely packed large grains, while UV-Vis studies show enhanced optical absorption. The 3 wt% Sb-doped In₂S₃ film demonstrated a superior photodetector performance, with a responsivity (R) of 1.020 A/W, detectivity (D*) of 32.5 × 10¹⁰ Jones, and external quantum efficiency (EQE) of 346%. Additionally, the photo-response analysis indicated rapid response times, with rise and fall times of 0.43 s and 0.26 s, respectively.
[0086] The stability test demonstrates the robustness, reliability and long-term applicability of the PDs.
[0087] The findings of the present invention establish Sb-doped In₂S₃ as a highly promising material for advanced UV detection technologies, offering a combination of efficiency, stability and superior optoelectronic properties for next-generation photodetector applications.
, Claims:1. An antimony doped indium sulfide thin film comprises:
a) the thin film comprises indium sulfide (In2S3); and
b) the thin film is doped with antimony (Sb) at a concentration ranging from 1 to 5 weight percent (wt%) on a glass substrate.
2. The antimony doped indium sulfide thin film as claimed in claim 1, wherein the antimony doped indium sulfide thin film has a cubic phase β-In₂S3 structure.
3. The antimony doped indium sulfide thin film as claimed in claim 1, wherein the antimony doped indium sulfide thin film has an X-ray powder diffraction pattern (CuKα) comprising peaks at 2-theta of 14.50°, 27.86°, 33.76°, 44.1°, 48.30° and 70.63°.
4. The antimony doped indium sulfide thin film as claimed in claim 1, wherein the antimony doped indium sulfide thin film has a crystallite size ranging from 35 to 55 nm.
5. A method of preparation of an antimony doped indium sulfide thin film comprising:
a) dissolving 0.02 M indium precursor and 0.03 M thiourea in a solvent to obtain a solution;
b) adding 1 to 5 wt % dopant in the solution with stirring to obtain a spray solution; and
c) spraying the spray solution onto the heated glass substrate using a nebulizer spray setup to obtain an antimony doped indium sulfide thin film.
6. The method as claimed in claim 5, wherein the indium precursor is selected from a group comprising of indium chloride, indium fluoride, indium bromide, indium iodide, indium oxides, indium carboxylates and combination thereof and the solvent is selected from a group comprising of water, methanol, 1,4-butanediol, polyethylene glycol 600 and combination thereof.
7. The method as claimed in claim 5, wherein the dopant is selected from a group comprising of antimony nitrate, tin chloride, selenium chloride and combination thereof.
8. The method as claimed in claim 5, wherein the stirring in step b) is carried out at a speed ranging from 300 to 400 RPM for a time period ranging from 30 min to 2 hours.
9. The method as claimed in claim 5, wherein the nebulizer spray setup includes air flow with a pressure ranging from 1 to 2 kg/cm² as the carrier gas to create fine aerosol droplets, the spray rate is set to 0.5 mL/min to 2 mL/min and the deposition time is maintained for a time period ranging from 5 to 10 minutes.
10. The method as claimed in claim 5, wherein the heated glass substrate has a temperature in the range of 300 to 400 °C.