Fabrication of ᵞ-In₂Se₃-Based Photodetector Using RF Magnetron Sputtering and Investigations of Its Temperature-Dependent Properties

Metal chalcogenide indium selenide (In2Se3) is attracting increasing research interest for photodetector applications due to its excellent photoresponse and superior stability under ambient conditions. However, the temperature-dependent performance of In2Se3-based photodetectors has rarely been reported. Here, <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>-In2Se3 thin films were prepared at various deposition pressures using the RF magnetron sputtering for photodetector applications. The formation of single-phase <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>-In2Se3 films has been confirmed by the X-ray diffraction (XRD) and Raman analyses. Binding energies and elemental composition of <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>-In2Se3 films were examined by XPS analysis. Field emission scanning electron microscopy (FE-SEM) images show that the prepared <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>-In2Se3 films were crack- and pore-free, dense, compact, smooth, and have small grains. The optical energy bandgap decreases from 2.2 to 1.7 eV with an increase in deposition pressure. Then, the photoresponse of <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>-In2Se3-based photodetectors was investigated. The photodetector fabricated with <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>-In2Se3 at 5 Pa on an ITO-coated interdigital electrode (IDE) exhibited excellent photoresponsivity (<inline-formula> <tex-math notation="LaTeX">$2.82~\mu \text{A}$ </tex-math></inline-formula>/W) and detectivity (<inline-formula> <tex-math notation="LaTeX">$7.06\times 10^{{7}}$ </tex-math></inline-formula> Jones) with a fast rise time of 0.26 s and a decay time of 0.32 s. Finally, the temperature-dependent photoresponse of the photodetector fabricated with <inline-formula> <tex-math notation="LaTeX">$\gamma $ </tex-math></inline-formula>-In2Se3 at 5 Pa is meticulously investigated. We found that the photodetector properties of a photodetector critically depend on the operating temperature.


I. INTRODUCTION
P HOTODETECTORS convert light energy into an electrical signal. They play significant roles in many fields, such as target tracking, spectral analysis, flame detections, missile warning system devices, missile plume detection, defense applications, biosensors, optical communications, and so forth [1], [2], [3], [4], [5], [6], [7]. In an optical communication system, a photodetector should satisfy basic requirements, such as excellent selectivity, reduced dark noise, high stability, and speed [8]. There are many types of photodetectors, such as p-n junction, phototransitive, avalanche, photoconductive, p-in junction, and metal-semiconductor-metal (MSM) photodetectors. The MSM-based photodetector offers various advantages, such as low dark current, high sensitivity, and ease of fabrication [9], [10]. Based on interdigitated back-to-back contact, MSM photodetector exhibits low capacitance, high operational speed, and also high sensitivity [11], [12]. The main drawback of MSM photodetector is its low responsivity, which occurs due to blockage of incoming light by the interdigitated finger metal electrodes. This problem can be resolved by the This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ rear illumination of the fabricated electrode, but it creates critical problems in optical lithography alignment, device processing, and chip packaging. Moreover, due to the complicated alignment procedures, the ease of fabrication is lost [13]. The use of transparent conducting electrodes, such as indium-tinoxide (ITO) or cadmium-tin-oxide (CTO), is another way to circumvent the above problems and also increase the responsivity of the MSM photodetector. Wohlmuth et al. [14] fabricated InGaAs/InAlAs MSM photodetector with Ti-Au, CTO, and ITO electrodes. The responsivity of the CTO and ITO MSM photodetector was higher than that of the Ti-Au MSM photodetector. Yadav et al. [15] fabricated GaN MSM photodetector with Au, Pd, and ITO contacts, and according to them, ITO is the best-suited electrode material for the fabrication of MSM photodetectors with high detectivity, sensitivity, and low dark current. Seo et al. [16] reported responsivity of ITO-MSMs is twice that of conventional Ti/Au-MSMs under normal operational bias conditions. ITO-MSMs exhibited a linear optical response over a wider bias range compared to Ti/Au-MSMs. Wuu et al. [17] fabricated GaN MSM photodetector on ITO and Pt conducting electrodes, and reported that ITO/GaN has higher responsivity than Pt/GaN. Therefore, using transparent conductors, such as ITO, in place of metal contact in MSM photodetector is an optimum proposition [18], [19], [20].
The next-generation photodetectors must be ultrasensitive and should operate at room temperature. 2-D metal chalcogenide materials show strong light-matter interactions [21] and a wide spectral response range [22], [23]. Among metal chalcogenides, indium selenide (In 2 Se 3 ) exhibits high response and superior stability under ambient conditions. It is an III-VI group layered chalcogenide material and has received a lot of attention in the field of solar cells [24], [25], [26], optoelectronic devices [27], [28], [29], [30], [31], [32], and phase change memory [33]. In 2 Se 3 shows a tunable thickness-dependent optical bandgap ranging from 1.45 to 2.8 eV, making it a promising candidate in photodetection [34]. It has a high absorption coefficient in the visible range and efficiently generates electron-hole pairs on photoexcitation [35], [36]. In 2 Se 3 exists in multiple crystalline phases, which include α, β, γ , δ, and κ-phases [37]. The material has been prepared by different chemical and vacuum techniques. The chemical route method [38], electrodeposition [39], and hot injection method [40] are some of the prominent chemical techniques employed for the synthesis of In 2 Se 3 . The vacuum methods include chemical vapor deposition [41], evaporation [42], MOCVD [43], molecular beam epitaxy [44], and sputtering method [45]. Each deposition method has its advantages and disadvantages. Among these methods, RF sputtering provides higher film density, largearea production, deposition at low temperatures, and better adhesion.
The properties of material prepared by RF sputtering are dependent on the process parameters, such as deposition time, sputtering power, substrate temperature, deposition pressure, and target-to-substrate distance. The deposition pressure plays a vital role in the growth of thin films. Liang et al. [46] reported a strong relationship between the growth orientation of films and deposition pressure. They deposited CdS/Sb 2 Se 3 films and found fewer defects in Sb 2 Se 3 film deposited at 8 Pa. Kulkarni et al. [47] had deposited CdTe films. The highest electrical conductivity and carrier concentration were observed for 1-Pa deposition pressure. Li et al. [48] deposited Ba 0.65 Sr 0.35 TiO 3 (BST) films and observed significant changes in film properties with variations in deposition pressure. It was deduced that the orientation of the deposited thin film could be tailored by adjusting the sputtering pressure. The deposition pressure plays a vital role in the growth of In 2 Se 3 films because their electrical conductivity and optoelectronic properties strongly depend on crystallinity. The photodetector properties, such as photosensitivity, photoresponsivity, photodetectivity, rise time, and decay time depend on the number of interdigital electrode (IDE) pairs present on the substrate and the distance between them. For example, Zhang et al. [49] reported that the increase in IDE pairs at a given bias improves the responsivity of the MgZnO photodetector. Zhao et al. [50] fabricated IDEs by the varying gap between two electrodes (5-300 µm). They have reported that the gap between two electrodes should be minimum to get a fast charge transfer, good repeatability, negligible hysteresis, quick response, and recovery time.
With this motivation, by using RF sputtering, we initiated the synthesis and study of the effect of deposition pressure on γ -In 2 Se 3 film properties for photodetector applications. High-quality, compact, and uniform γ -In 2 Se 3 films have been prepared at a deposition pressure of 3, 4, 5, and 6 Pa. The photoresponse of γ -In 2 Se 3 -based photodetectors fabricated at various deposition pressures on ITO-coated IDEs was studied. We found that γ -In 2 Se 3 -based photodetector fabricated at 5 Pa shows stable photoswitching behavior with excellent photoresponsivity (2.82 µA/W) and detectivity (7.06 × 10 7 Jones) with a fast rise and a decay time of 0.26 and 0.32 s, respectively. Finally, the temperature-dependent photoresponse of γ -In 2 Se 3 -based photodetector fabricated at a deposition pressure of 5 Pa is investigated. The temperature-dependent photoresponse is an important aspect of a photodetector, as an optoelectronic performance of a device is dependent on temperature and also on ambient conditions of it. Manik et al. [51] fabricated a green silicon photodiode and observed a reduction in photocurrent from 12 to 9 nA with a decrease in temperature from 353 to 213 K. Tak et al. [52] fabricated β-Ga 2 O 3 -based photodetector and reported the effect of electron-phonon interaction and its influence on the photoresponse of a photodetector. The photocurrent to dark current ratio value decreases from 7137 (25 • C) to 2.3 (250 • C) with an increase in temperature. Chen et al. [53] reported that 4H-SiC MSM photodetector gives maximum responsivity of 120 mA/W and quantum efficiency of 51.82% at a higher temperature of 800 K. To the best of our knowledge, this is the first report where a detailed investigation of the effect of temperature on the photoresponse of γ -In 2 Se 3 -based photodetector has been carried out. We found that the photodetector's sensitivity, responsivity, detectivity, and rise and decay time properties depend on the operating temperature.

II. EXPERIMENTAL A. Fabrication of Interdigital Electrodes
The schematics of the fabrication of the ITO-coated IDE substrate are shown in Fig. 1. The IDE pattern was first printed on glossy paper using LaserJet Pro M202 dw printer. Then, the pattern was transferred onto a cleaned ITO-coated glass substrate by heat treatment. Then, the IDE mask was coated on the ITO substrate. The Zn dust was spread over the substrate, and dilute HCl was added dropwise over the substrate. In the reaction between Zn dust and HCL, the uncovered ITO is etched out from the substrate. The covered ITO gives conducting IDE finger pattern. The ITO-coated IDE substrate was then cleaned with acetone, IPA, and distilled water. The area of fabricated IDE was 1.0 cm 2 with three IDE pairs. The separation between the two electrodes was ∼500 µm.

B. Deposition of γ -In 2 Se 3 Films
The ITO-coated IDE was ultrasonically cleaned in acetone, IPA, and distilled water solution, respectively, while the corning glass substrates were cleaned with piranha solution. Finally, IDE and glass substrates were rinsed with distilled water and dried. After loading the substrates on the holder, ∼2 × 10 −6 Torr pressure was attained in a sputter chamber by employing a rotary and turbomolecular pump. A 4-inch In 2 Se 3 target (99.99%, Vin Korola, USA) was used to deposit In 2 Se 3 films. The deposition substrate temperature, RF power, and sputtering time were kept constant, and the deposition pressure was varied between 3 and 6 Pa by controlling the Ar gas flow rate using a mass flow controller. Table I shows the values of process parameters used in the deposition process.
The deposited In 2 Se 3 films were then annealed in a tubular vacuum furnace at 300 • C for 1 h. Next, films were allowed to cool to room temperature naturally and taken out for characterization.

C. Material Characterization
To confirm the phase formation and crystal structure of the prepared In 2 Se 3 films, an X-ray diffractometer (D8 Advance; Bruker AXS) and Raman spectroscopy (Renishaw InVia Raman microscope, excitation wavelength: 532 nm) were used. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific, K-Alpha+, U.K.) was explored to investigate elemental composition. The surface morphology was examined and analyzed by field emission scanning electron microscopy (FE-SEM) using Nova NanoSEM 450. To estimate the optical band gap of In 2 Se 3 , the transmittance spectrum was recorded in the standard range of 1100-200 nm using JASCO, V-670 UVvisible-NIR spectrophotometer. Keithley 2450 source meter was used for the electrical measurement of γ -In 2 Se 3 films. The photoresponse of γ -In 2 Se 3 films was performed under dark and light conditions. For white light illumination (100 mWcm −2 ), Class ABA Solar Simulator ORIEL Sol 2A 94022A was used.

A. Deposition Rate and Growth of γ -In 2 Se 3 Films
The thickness of as-prepared γ -In 2 Se 3 films at different deposition pressures was measured by cross-sectional FE-SEM analysis. The deposition rate was then calculated using the film's thickness and deposition time ratio. Fig. 2 shows the variation of deposition rate as a function of deposition pressure of γ -In 2 Se 3 film deposited by RF magnetron sputtering. As expected, the deposition rate increases with an increase in the deposition pressure. For example, it increases from 23.5 to 79.4 nm/min when deposition pressure increases from 3 to 6 Pa. The estimated thickness and deposition rate values at various deposition pressures are listed in Table II. According to Wu et al. [54], the relation between the thin film deposition rate (D) and the target etching rate (E) is given by where F is constant. The relationship between the etching rate of the target (E) and the ion current density (J + ) on the target surface is expressed as where S is the sputtering yield, ρ is the density, and M is the atomic mass of the target. S and J + depend on the deposition gas pressure, the target-to-substrate distance, the substrate temperature, the sputtering power, and the target type.
In the gas discharge theory, the current density can be related to the cathode sheath thickness (d c ) and Townsend's third ionization coefficient (h) by [55] where ε 0 is the vacuum dielectric constant, µ i is the ion mobility, and V c is the cathode sheath. For a given gas, according to the gas dynamics, the average free path of the molecule (λ) and pressure (P) can be expressed as [54] where k is Boltzmann's constant, P is the pressure, T is the temperature, and d is the diameter of the gas molecule. At constant T and d, λ decreases as the deposition pressure increases. As a result, the collision probability between the gas molecule and sputtered atom becomes more prominent,  and Townsend's third ionization coefficient (h) increases. Consequently, there is a reduction in the thickness of the cathode sheath, which increases the ion current density. As a result, the deposition rate increases with an increase in the deposition pressure.

B. X-Ray Diffraction (XRD) Analysis
The XRD pattern of In 2 Se 3 films with variation in deposition pressure is shown in Fig. 3  The observed diffraction peaks are closely related to the hexagonal γ -In 2 Se 3 . The intensity of the most prominent peak (110) plane increases with an increase in the deposition pressure, indicating improvement in the crystallization. The difference in the peak intensity at 25.02 • (110) and 27.58 • (006) in the four γ -In 2 Se 3 samples is mainly related to the preferential crystallographic orientation. At a deposition pressure of 3 Pa, In 2 Se 3 preferentially grows along the (006) plane. However, as the deposition pressure increased to 5 Pa, the diffractions of the (110) plane became dominant.
The lattice parameters of hexagonal γ -In 2 Se 3 were calculated using the following equation: The calculated values of the lattice constants are a = b = 7.11 Å and c = 19.40 Å. The lattice values match well with the previously reported data [56]. The crystallite size (d X−ray ) of γ -In 2 Se 3 films was calculated using Debye-Scherrer's equation [57] d X−ray = 0.9λ βcosθ (6) where β is the full-width at half-maximum (FWHM), θ indicates Bragg's diffraction angle, and λ is the wavelength of the X-ray source. The average crystallite size for γ -In 2 Se 3 films deposited at 3-, 4-, 5-, and 6-Pa deposition pressures was ∼30.9, ∼42.8, ∼43.1, and ∼46.2 nm, respectively.

C. Raman Spectroscopy Analysis
The number of layers, phase transition, defects, and strain in the material can be studied using Raman spectroscopy [56]. Fig. 4 shows the Raman spectra of γ -In 2 Se 3 films at 3-, 4-, 5-, and 6-Pa deposition pressures by RF sputtering. The Raman spectra of In 2 Se 3 exhibit modes at ∼150 and ∼248 cm −1 , which is related to the γ -phase of In 2 Se 3 [58], [59], [60], [61], [62]. The intensity of the mode located at ∼150 cm −1 increases, and the peak becomes sharp with an increase in deposition pressure. An increase in intensity indicates an improvement in the crystallinity of In 2 Se 3 films. By increasing the deposition pressure, more and more gas atoms are available for ionization, increasing the number of ions moving toward the target and, thus, increasing the sputtering yield and improving the film's crystallinity. Thus, the formation of a single phase γ -In 2 Se 3 was confirmed by the Raman scattering measurements. These results are consistent with XRD analysis.

D. X-Ray Photoelectron Spectroscopy Analysis
The electronic structure and chemical properties of the γ -In 2 Se 3 film have been analyzed qualitatively using highresolution XPS. XPS spectra were calibrated with C 1s peak (284.6 eV) as reference. Fig. 5(a) illustrates the survey and high-resolution XPS spectra of the γ -In 2 Se 3 film deposited at a pressure of 5 Pa. As seen, the peaks corresponding to indium [In(3d)], selenium [Se(3d)], carbon [C(1s)], and oxygen [O(1s)] appear in the XPS spectra. Fig. 5(b) and (c) are the narrow scan XPS spectra for 3d-In and 3d-Se elements, respectively. Fig. 5(b) shows the In-3d core level, which splits into two peaks at ∼443.62 and ∼451.17 eV attributed to the In-3d 5/2 and In-3d 3/2 levels. The energy difference between In-3d 3/2 and In-3d 5/2 levels was estimated at around 7.60 eV. The well-resolved and intense peak located at ∼55.3 eV corresponds to the Se 3d [see Fig. 5(c)]. These results match with the previously reported data [63], [64], [65]. Thus, XPS results indicate the formation of pure γ -In 2 Se 3 under prevailing experimental conditions. Fig. 6(a)-(d) shows surface morphologies of γ -In 2 Se 3 thin films characterized by FE-SEM. All films are crack-free, porefree, dense, compact, smooth, and have small grains. The kinetics of atomic arrangement can affect the film crystallinity during deposition. For a high crystallinity film, the deposited atoms need sufficient time to grow in a thermodynamically stable site before the deposition of the next layer of atoms [66].

E. Morphological Studies
The γ -In 2 Se 3 film deposited at 3 Pa displayed a smooth and compact surface morphology. For γ -In 2 Se 3 film deposited at 4 Pa, the grains grew larger with increased deposition pressure. The γ -In 2 Se 3 films deposited at 5 Pa show uniform grain growth, but, above 5 Pa, irregular and nonuniform grain growth was observed. The increased deposition pressure at 6 Pa destroys the uniform grain growth distribution.

F. UV-Visible Spectroscopy Analysis
The optical properties of as-synthesized γ -In 2 Se 3 films were studied using UV-Vis spectroscopy in the 200-1100-nm range. Fig. 7(a) displays the absorbance spectra of γ -In 2 Se 3 thin films prepared at various deposition pressures. The wide absorbance is observed from 230 to 680 nm (UV-visible region). The absorbance spectra show that the absorbance increases with an increase in the deposition pressure. The high absorbance over a wide spectral range enables an improved carrier photogeneration mechanism [67]. A red shift in the absorption edge has been observed with an increase in the deposition pressure, which may be due to an increase in crystallite size and film thickness. Fig. 7(b) displays the transmittance spectra of γ -In 2 Se 3 thin films prepared at various deposition pressures. All γ -In 2 Se 3 thin films showed good transmittance in the visible region. The relation of optical bandgap (E g ), photon energy (hν), and absorption coefficient (α) are given by Tauc's relation [68] (αhν) n = A hν − E g where A is constant. From the transmittance spectra of the films, the absorption coefficient (α) can be calculated with the formula [69] where T is the transmittance and d is the thickness of the film. Fig. 7(c) shows Tauc's plot for bandgap calculation of γ -In 2 Se 3 films. The calculated optical bandgap values for γ -In 2 Se 3 films deposited at 3, 4, 5, and 6 Pa were 2.24, 2.16, 1.87, and 1.76 eV, respectively, and are summarized in Table II.

G. Photodetector Properties
A photodetector is an energy conversion device that transforms light energy into electrical energy. The deposited γ -In 2 Se 3 layer on ITO-coated IDE (active area 1 cm 2 ) was used for photodetector fabrication. The electrical measurements of γ -In 2 Se 3 thin films prepared at different deposition pressure were carried out at room temperature. The photoresponse was measured under alternating light and dark conditions at a bias voltage of 0.5 V. Fig. 8 shows the I − V plot of γ -In 2 Se 3 samples in the voltage sweep of −5 to 5 V. The observed photocurrent at a bias voltage of 5 V increases from 1.11, 3.27, 3.98, and 5.27 µA, with an increase in deposition pressure. The fabricated photodetectors were irradiated with the white light of intensity 100 mW/cm 2 at room temperature under normal atmospheric conditions. The irradiation source  was switched ON and OFF periodically at 10-s intervals. The bias voltage between the two electrodes was kept constant at 0.5 V. Fig. 9 shows the time-resolved photoresponse characteristics of γ -In 2 Se 3 films deposited at various deposition pressures by RF magnetron sputtering. In each curve, the photoresponse displayed three transient regimes: the first is the sharp rise, the second is the steady state, and the third is the sharp decay. These results indicate that all γ -In 2 Se 3 -based photodetectors exhibited excellent photoresponse properties. For example, at room temperature under normal atmospheric conditions, the γ -In 2 Se 3 -based photodetector fabricated at 3-Pa deposition pressure and the dark current was ∼5.08 nA.
However, the photocurrent approaches ∼98.07 nA under white light irradiation with an ON/OFF ratio of 19.43.  increase in deposition pressure. The improved crystallinity and density of the film lead to a lower resistance path for photogenerated electrons. However, for γ -In 2 Se 3 prepared deposition pressure of 6 Pa, the uniform grain growth distribution gets destroyed, which hampers the photocurrent. The XRD, Raman, and FE-SEM analysis support this. Interestingly, after multiple irradiations ON/OFF cycles, the photocurrent still responded similar to the light, demonstrating the photodetectors' high robustness and good reproducibility. The photodetector performance was measured by several parameters, such as photosensitivity and photoresponsivity. The photoresponsivity is calculated by the formula [45], [70] where P λ is the intensity of incident light (100 mW/cm 2 ), I = I Photo − I Dark is the change in photocurrent due to incident light on the effective photosensing area, and A is the active area of the film (1 cm 2 ). The photoresponsivity of the In 2 Se 3 photodetector increases from 0.9 to 2.35 µA/W with an increase in deposition pressure.
The photosensitivity (ξ ) of the photodetector is expressed as [71], [72] The values of photosensitivity (ξ ) of γ -In 2 Se 3 films decrease from ∼11.03 to ∼1.91 with an increase in deposition pressure. The decline in photosensitivity may be due to the increase in dark current with an increase in deposition pressure. Photodetectivity (D * ) is a figure of merit of the photodetector, defined as the signal-to-noise ratio. It is measured in terms of Jones and calculated using [73] where e is the electron charge and J Dark is the dark current density. The values of photodetectivity of γ -In 2 Se 3 -based photodetectors are shown in Table III. The photodetectivity increases from ∼1.76 × 10 7 to ∼7.06 × 10 7 Jones when deposition pressure increases from 3 to 5 Pa. However, for deposition pressure of 6 Pa, it decreases to ∼1.22 × 10 7 Jones. The dark current density limits the detector noise. The photodetector requires high photoresponse and low dark current density to achieve high detectivity.
The photodetector's rise time (τ Rise ) is the time required for the output signal to rise from 10% to 90% level of the maximum value when a steady input is instantaneously applied. Consequently, decay time (τ Decay ) is the time required for the output signal to drop from 90% to 10% of the maximum value when a steady input is instantaneously removed [74]. To determine τ Rise and τ Decay of γ -In 2 Se 3 -based photodetectors, we have considered an enlarged view of a single cycle of time-resolved photoresponse characteristics shown in Fig. 10. Fig. 10 shows the dynamic rise and decay data for one cycle of γ -In 2 Se 3 -based photodetectors fabricated at different deposition pressures. The calculated values of the rise and decay time for γ -In 2 Se 3 films deposited at different deposition pressures are listed in Table III. As seen, the photodetector fabricated using γ -In 2 Se 3 film deposited at 5-Pa deposition pressure shows excellent photoresponse with fast rise time and decay time of ∼0.26 and ∼0.32 s, respectively, with high photoresponsivity (∼2.82 µA/W), photosensitivity (∼56.58), and photodetectivity (∼7.06 × 10 7 Jones).
The rise and decay times of some recently reported metal chalcogenide-based photodetectors fabricated using different methods are compared with this work in Table IV. Thus, the effect of the deposition pressure on the photosensing behavior of γ -In 2 Se 3 thin film is thoroughly investigated. The film deposited at 5-Pa deposition pressure shows better photoresponsivity, photosensitivity, fast rise, and decay time.

H. Temperature-Dependent Photoresponse Properties
The performance of the photodetector is sensitive to environmental temperature. However, to the best of our knowledge, the temperature-dependent performance of γ -In 2 Se 3 -based photodetector has still not been carried out. In this work, the photodetector fabricated with γ -In 2 Se 3 film prepared at 5-Pa deposition pressure shows excellent photoresponse. Therefore temperature-dependent photoresponse investigation of photodetector fabricated with γ -In 2 Se 3 film prepared at 5-Pa deposition pressure is carried out. All measurements under light and dark conditions were carried out at different temperatures using a Keithley 2450 source meter with a bias voltage of 0.5 V in a vacuum. The temperature-dependent photoresponse was performed in the temperature range of −90 • C to +90 • C. Fig. 11 shows the typical temperature-dependent I-V curves of a photodetector fabricated with γ -In 2 Se 3 film prepared at 5 Pa by the RF magnetron sputtering. The photocurrent  is observed to increase with increasing applied voltage at elevated temperatures. When the applied voltage increases, the corresponding electric field, which is required to drift the charge carriers, is also increased. As a result, the photocurrent increases with increasing applied voltage at elevated temperatures. Furthermore, the photocurrent increases linearly with an increase in temperature. The enhancement in photocurrent with an increase in temperature can be attributed to the rise in the number of charge carriers when the γ -In 2 Se 3 photodetector is exposed to white light (100 mW/cm 2 ). Fig. 12 shows the temperature-dependent photoresponse of γ -In 2 Se 3 photodetector fabricated with γ -In 2 Se 3 film prepared at 5-Pa deposition pressure. As seen, the current increases with an increase in temperature. As the temperature increases, the thermal excitation causes electrons to move from the valance band to the conduction band. Consequently, carrier concentration increases with thermal excitation, increasing the conductivity of γ -In 2 Se 3 film. The mobility of electrons and holes is influenced by the carrier and lattice type of scattering mechanism. At lower temperatures (T = 0 K), a slow-moving carrier (electron) will likely be scattered more strongly by interacting with a charged ion. As temperature increases from 0 to 300 K, the kinetic energy and velocity of the electron increase, and the carrier scattering decreases. Hence, mobility in carrier scattering increases with temperature. However, lattice scattering dominates at higher temperatures (T > 300 • C). As temperature increases, a carrier moving through the crystal is scattered by lattice vibrations. The frequency of such scattering events increases with an increase in temperature [82], [83]. The mobility is given by where V D is the drift velocity and E is the electric field due to the applied potential difference. The relation between V D and relaxation time (time between two collisions, τ ) is [82], [83] where e and m are the charge and mass of the electron; thus, V D is proportional to E. However, this relation holds only up to a specific electric field. Therefore, V D is proportional to τ . As temperature increases, the time between two collisions reduces. Thus, drift velocity decreases with an increase in temperature. Hence, mobility decreases with an increase in temperature.
The electric conductivity (σ ) and relaxation time (τ ) are related by where n is the free electron density.
With an increase in temperature, the relaxation time decreases, and the electrical conductivity decreases effectively. At the same time, the relaxation time decreases with an increase in temperature, which reduces the electrical conductivity. However, the density of free electrons is a dominant factor compared to relaxation time. As a result, the electrical conductivity increases with an increase in temperature. Fig. 13 shows the variation in dark and photocurrent as a function of the temperature of γ -In 2 Se 3 photodetector fabricated with γ -In 2 Se 3 film prepared at 5 Pa  by the RF magnetron sputtering. The thermally generated charge carriers increase by increasing temperature from −90 • C to 90 • C.
As a result, both photocurrent and dark current increase with an increase in temperature. However, the rate of photocurrent increase is higher than the dark current. Fig. 14 shows the temperature-dependent photosensitivity of γ -In 2 Se 3 photodetector fabricated with γ -In 2 Se 3 film prepared at 5 Pa by the RF magnetron sputtering. It is observed that photosensitivity increases with an increase in temperature from −90 • C to 30 • C and then decreases.
Photosensitivity mainly depends on photo and dark currents. At high temperatures, the increase in dark current reduces photosensitivity, which mainly depends on photocurrent and dark current. At high temperatures, the increase in dark current reduces photosensitivity. The maximum photosensitivity ∼138 was observed at room temperature (30 • C). Fig. 15 shows the temperature-dependent photoresponsivity and photodetectivity of γ -In 2 Se 3 photodetector fabricated with γ -In 2 Se 3 film prepared at 5 Pa by the RF magnetron sputtering. It is observed that the photoresponsivity increases continuously with an increase in temperature. However, the photodetectivity rises with the temperature only up to 30 • C, but, at higher temperatures, photodetectivity decreases. The maximum photodetectivity ∼3.24 × 10 7 Jones was observed  Fig. 15. Temperature-dependent photoresponsivity and photodetectivity of γ-In2Se3 photodetector fabricated at a deposition pressure of 5 Pa.
at 30 • C. Table V shows the comparison of impact of temperature variations on the sensor performance with earlier reported results.

IV. CONCLUSION
The γ -In 2 Se 3 thin films were successfully synthesized using RF magnetron sputtering. The formation of hexagonal γ -In 2 Se 3 was confirmed by XRD. The average crystallite size increases from 30.9 to 46.2 nm with an increase in deposition pressure from 3 to 6 Pa. The γ -phase of In 2 Se 3 was confirmed by Raman spectroscopy. XPS confirmed the presence of In and Se elements in the prepared film. FE-SEM images show that the prepared γ In 2 Se 3 films are crackfree, pore-free, dense, compact, smooth, and small grains. The thickness and deposition rate of γ -In 2 Se 3 films increase from 465 to 1589 nm and 23.2 to 79.4 nm/min, respectively, with an increase in deposition pressure from 3 to 6 Pa. The reduction in bandgap from 2.24 to 1.76 eV with an increase in deposition pressure was observed from UV-visible spectroscopy analysis. The γ -In 2 Se 3 film deposited on ITO-coated IDE substrate at 5-Pa deposition pressure exhibited excellent photoresponsivity (2.82 µA/W) and detectivity (7.06 × 10 7 Jones) with a fast rise and a decay time of 0.26 and 0.32 s, respectively. The temperature-dependent photoresponse of γ -In 2 Se 3 thin film is thoroughly investigated. At higher temperatures, due to thermal excitation, the photocurrent increases with an increase in temperature. The maximum sensitivity was observed at room temperature. The photoresponsivity increases from 0.5 to 4.52 µA/W with an increase in temperature from −90 • C to +90 • C. Vidhika Sharma is a Senior Research Associate in the Department of Science and Technology (DST)-Indo-French Centre for the Promotion of Advanced Research (CEFIPRA) Project. She has a research interest in the area of renewable energy and is currently involved in research pertaining to the photoelectrochemical (PEC) production of hydrogen by the solar energyinduced splitting of water using nanostructured semiconductors. She has worked for more than ten years on PEC splitting of water using a variety of nanostructured semiconductors.