Indium sulfide based metal-semiconductor-metal ultraviolet-visible photodetector

Sensors and Actuators A 299 (2019) 111643

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Indium sulfide based metal-semiconductor-metal ultraviolet-visible photodetector B. Hemanth Kumar, M.C. Santhosh Kumar ∗ Optoelectronic Materials and Devices Laboratory, Department of Physics, National Institute of Technology, Tiruchirappalli, Tamil Nadu, 620015, India

a r t i c l e

i n f o

Article history: Received 10 July 2019 Received in revised form 24 August 2019 Accepted 26 September 2019 Available online 26 September 2019 Keywords: Indium sulfide thin films Co-evaporation Williamson-Hall plot X-ray photoelectron spectroscopy Ultraviolet-visible photodetector

a b s t r a c t In recent years, the photodetectors gained much attention due to their wide range of applications in industry, military, space and biological fields. In this work, the metal-semiconductor-metal (MSM) photodetector was fabricated using In2 S3 thin films with Al interdigitated electrodes. The In2 S3 thin films were prepared by co-evaporation technique with various thicknesses in the range 130–700 nm at a constant substrate temperature of 350 ◦ C. The structural, morphological, compositional, optical and electrical properties of In2 S3 thin films were studied as a function of thickness. The energy band gap of films is found to be in the range 2.53–2.71 eV. I–V characteristics and photo response of photodetectors were recorded under UV and visible light illumination. The parameters of a photodetector such as photo sensitivity, responsivity and detectivity were calculated. The observed photo responsivity increases with increase of film thickness. The photo response of all photodetectors confirmed the stable and reproducible characteristics such as photo sensitivity, responsivity and detectivity. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Photodetector is the essential optoelectronic device used to detection of light. Photodetectors have potential applications in numerous fields that include flame detection, environmental security, military, medical, inter-satellite communication, space exploration and so on [1–3]. Hence the research and development of photodetectors are very important. Based on the operational wavelength range, photodetectors are categorized into two types: narrowband region photo response detectors and broadband region photo response detectors [4]. The broadband detectors with more spectral responsivity range from UV to IR are preferred in the field of optical communications, sensing, imaging and spectroscopy over narrowband detectors. The UV–vis photo detectors are suitable for the requirements of astronomical detection, wide spectral switches and memory storage [5]. Nowadays narrowband photodetectors are fabricated using various materials, that include NiO [6], SnO2 [7], ZnO [8] and Sb2 Te3 [9]. Recently, a few reports in literature show the possibility of broadband photodetectors based on MoS2 [10], SnS [4] and In2 Te3 [11]. Even though the fabrication of broadband photodetectors with low cost, non-toxic and with high performance is challenging and it is a hot topic for the

∗ Corresponding author. E-mail address: [email protected] (S.K. M.C.). https://doi.org/10.1016/j.sna.2019.111643 0924-4247/© 2019 Elsevier B.V. All rights reserved.

researchers. In recent years, metal-semiconductor-metal (MSM) photodetectors have become popular due to their simple structure, high photoconductive gain and lower capacitance [12,13]. Moreover, the indium sulfide turns to be of great importance in the development of photodetectors and photovoltaics because of its non-toxicity, photoconductive nature and stability [14,15]. In addition, indium sulfide has n-type conductivity, high transmission in the visible region and wide band gap that varies between 2.0 eV–3.7 eV depends on the preparation method [16–19]. In2 S3 is considered as a good alternative to replace the cadmium sulfide as a buffer layer in thin film solar cells. In2 S3 has three crystalline phases namely ˛, ˇ and , out of these phases, tetragonal ˇ − In2 S3 has wide applications due to its high stability [15,20]. In2 S3 is used in many applications namely gas sensors, photodetectors, photo electrochemical cells and solar cells [21,22]. In2 S3 thin films can be deposited in various chemical and physical methods namely chemical bath deposition [23], spray pyrolysis [24,25], atomic layer deposition [26], thermal evaporation [27], nebulized spray pyrolysis [28], RF- sputtering [29] and co-evaporation [30]. The chemical methods are simple and suitable for large area deposition but the disadvantage of chemical methods is that, the films contain compositional deficiency of In and S due to the large concentration of oxygen, which results in poor crystallinity [29]. Generally, for preparation of good quality and uniform films the physical methods are preferred. The physical properties of the films strongly depend

2

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

on the growth parameters, preparation method and thickness of the films [16,29]. In this study, we report the fabrication of MSM structured UV–vis photodetector based on indium sulfide thin films. We adopt co-evaporation technique for the preparation of indium sulfide thin films. The films were deposited on soda-lime glass substrate with various thicknesses in the range 130–700 nm at constant substrate temperature of 350 ◦ C. The variation in structural, morphological, compositional, optical and electrical properties was studied with different film thicknesses. The photodetectors were fabricated with different thickness of In2 S3 thin films using Al interdigitated electrodes. The fabricated photodetectors were characterized under UV and visible light. For all photodetectors, rise time, decay time, photo sensitivity, responsivity and detectivity are calculated and reported. 2. Experimental section Fig. 1. XRD patterns of In2 S3 films deposited at various thicknesses.

2.1. Preparation of In2 S3 thin films Indium Sulfide (In2 S3 ) thin films were deposited on soda-lime glass substrate in thermal evaporation unit by co-evaporation of Indium rod (Sigma-Aldrich, 99.99%) and Sulphur powder (SigmaAldrich) from molybdenum boat and glass crucible kept in tungsten wire basket respectively. The ultrasonically cleaned substrate was mounted on substrate holder and temperature was controlled by PID controller. The vacuum in the chamber was maintained at 1.2 × 10−5 torr. The vertical distance between substrate and boats was set 31 cm to avoid the heating of substrate via thermal radiation of boats. When both materials start evaporating, the shutter is opened to allow the material vapours to condense on the substrate. The films were deposited at different thicknesses in the range 130–700 nm with constant substrate temperature of 350 ◦ C. The SQM 160 quartz crystal thickness monitor was used to measure the thickness of the films which is placed just below the substrate holder. 2.2. Photodetector fabrication For the fabrication of metal-semiconductor-metal (MSM) photodetector, Aluminium (Al) interdigitated electrodes were used. The Al interdigitated electrodes were deposited on the prepared In2 S3 thin films using shadow metal mask. Al deposited by thermal evaporation method at room temperature. For this purpose, the Al wire used as raw material and it was evaporated from the helical tungsten wire boat. The chamber vacuum was 1.5 × 10−5 torr. The electrode pattern consists of two interdigitated electrodes, each having three fingers. 2.3. Characterization X-ray diffraction patterns of prepared films were recorded using RIGAKU smartlab GI-XRD. Raman scattering measurements were carried out using HORIBA JOBINYVON LabRAM HR800 system with laser wavelength of 532 nm at room temperature. The surface morphology of the films was recorded using BRUKER Dimension Icon Atomic Force Microscopy (AFM) and ZEISS Ultra55 GEMINI Field Emission Scanning Electron Microscope (FESEM). The composition of the films was found using Oxford instruments EDAX system. The X-ray photoelectron spectroscopy analysis was performed using Kratos Axis Ultra DLD system. The electrical properties were recorded by ECOPIA HMS-5000 Hall effect measurement system using Van der Paw configuration at room temperature. The optical transmittance measured in the wavelength range of 300–2500 nm using JASCO V-670 UV–vis-NIR spectrometer. The current-voltage (I–V) characteristics and photo response of all photodetectors were

recorded using Source Measure Unit (Keysight B2901A) under solar simulator AM 1.5 G spectrum of light intensity 100 mW/cm2 and Ultraviolet (UV) light of intensity 4 mW/cm2 . 3. Results and discussion 3.1. Structural analysis 3.1.1. X-ray diffraction The XRD patterns of Indium sulfide thin films deposited at various thickness are presented in Fig.1. The diffraction peaks observed at 2 values of 14.20◦ , 27.44◦ , 28.65◦ , 33.24◦ , 40.95◦ , 43.57◦ , 47.69◦ , 49.99◦ , 55.89◦ , 66.57◦ and 76.45◦ are belonging to the tetragonal ˇ In2 S3 phase (JCPDS: 73–1366). These diffraction peaks are attribute to (1 0 3), (1 0 9), (2 0 6), (0 0 12), (2 0 12), (3 0 9), (2 2 12), (2 1 15), (3 0 15), (4 1 15) and (4 3 15) planes respectively. From XRD studies, it is inferred that all the films are polycrystalline in nature. The film of thickness 130 nm has only six peaks whereas with the increase of film thickness additional peaks appeared. The appearance of additional peaks with increase of film thickness beyond 130 nm denotes the improvement crystallinity of the films. Ji et al. [29] reported similar trend in XRD patterns with variation of thickness for In2 S3 thin films. As the film thickness increased to 700 nm, the diffraction peaks become more intense, which indicates high crystalline nature of the films. Generally, the increase in film thickness increases the probability of crystallization [31]. Williamson-Hall (W-H) plot gives information about crystallite size and strain from the full width at half maxima (FWHM) of peaks [32]. The Williamson-Hall (W-H) plot of the films deposited at thickness range from 130 nm to 700 nm is presented in Fig.2. The average crystallite size (D) was given by Debye Scherrer’s formula [32] D=

k ˇ(hkl) cos

(1)

Where, k is the shape factor which is taken as 0.94, ␭ is the wavelength and its value is 1.54 A˙ for CuK ˛ of x-rays, ␤ is the FWHM and ␪ is the Bragg’s diffraction angle. The strain (ε) induced broadening in the films is due to crystal imperfection and distortion given by [33] ε=

ˇ(hkl) 4tan

(2)

The crystallite size and stain result in peak broadening and these are independent each other. The observed broadening is given by

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

3

Fig. 2. Williamson-Hall plots of In2 S3 thin films deposited at various thicknesses.

the sum of broadening due to crystallite size and broadening due to strain [34]. ˇ(hkl) = ˇ(crystallite size) + ˇ(strain)

(3)

From Eqs. (1) and (2) ˇ(hkl) =

k + 4εtan Dcos

(4)

By rearranging the above equation, we get ˇ(hkl) cos =

k + 4εsin D

(5)

Williamson-Hall (W-H) plot is drawn between 4sin along xaxis and ˇ(hkl) cos along the y-axis. The crystallite size and strain were calculated from the linear fit of the W-H plot. The slope of the graph gives the strain value and the crystallite size was calculated from the y-intercept [35]. The dislocation density (␦) values are computed using the Williamson and Smallman’s formula [36]. ı=

1 D2

(6)

The obtained values of crystallite size, strain and dislocation density are tabulated in Table 1. The crystallite size increases from 9 nm to 18 nm with the increase of film thickness from 130 nm to 700 nm. A similar change in crystallite size with increase of film thickness was observed for In2 S3 thin films [37], SnS thin films [38] and ZnO thin films [39]. The values of strain and dislocation density are proportion to the crystallite size and changes with increase of film thickness. 3.1.2. Raman spectroscopy The Raman spectra of In2 S3 thin films deposited at various thicknesses is shown in Fig.3 in the wavenumber range

Fig. 3. Raman spectra of In2 S3 thin films deposited at various thicknesses.

of 100–1000 cm−1 at room temperature. The Raman peaks are observed at wavenumber of 134 cm−1 , 174 cm−1 , 250 cm−1 , 306 cm−1 and 363 cm−1 belong to the ˇ -In2 S3 phase [40–43]. The film with lower thickness exhibits broad and weak Raman bands. With the increase of film thickness, the intensity of phonon modes increases. The Raman peaks observed at 250 cm−1 , 306 cm−1 and 363 cm−1 are correspond to A1g mode, A4g mode and A2g mode respectively [40]. The peak at 174 cm−1 corresponds to F2g mode [41]. The Raman bands at 250 cm−1 and 306 cm−1 are attributed to the vibrations of InS6 octahedral and InS4 tetrahedral respectively [29]. As thickness of the films increases from 130 nm to 700 nm, the peaks position and the band width remains invariant, which indicates no significant change of the chemical environment in the films [29]. There are no peaks related to other phases, which con-

4

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

Table 1 Calculated values of crystallite size, strain and dislocation density of co-evaporated In2 S3 films. Film thickness (nm)

Crystallite size (D) (nm)

Strain (␧) × 10−3

Dislocation density (ı) × 1016 (m−2 )

130 290 480 700

9 15 16 18

−3.49 −0.78 −0.32 0.23

1.23 0.44 0.39 0.30

Fig. 4. FESEM pictures of In2 S3 films deposited at various thicknesses a) 130 nm b) 290 nm c) 480 nm and d) 700 nm.

firms the formation of pure tetragonal ␤-In2 S3 phase thin films and is in agreement with the XRD analysis. 3.2. Morphological analysis The FESEM micrographs of the films deposited at thickness range 130–700 nm are shown in Fig.4. From the figures, it is observed that the spherical grains are uniformly distributed on the surface of the films. The average grain size was calculated using ImageJ software. The average grain size values of the films are tabulated in Table 2. With the increase of film thickness from 130 nm to 700 nm, the grain size increases from 40 nm to 118 nm. This result is in agreement with the variation in crystallite size obtained from XRD studies. The increase of grain size with increase of film thickness can be explained as follows, at the lower thickness the interaction between substrate and film will be more, which restrict the mobility of ad-atoms and hence the grains could not grow larger. Increase of film thickness results in increase of nucleation occurrence and as more number of ad-atoms are available for the growth process, causes the formation of larger grains [44]. The FESEM morphology is in good agreement with the previously reported studies for In2 S3 thin films [45–47]. Fig.5 depicts the AFM 2D micro graphs of films deposited at different thicknesses. The images were recorded in the area of

1 m × 1 m. From these micro graphs, it has interfered that the grains are distributed uniformly on the surface of the films and nearly in spherical in shape. The surface roughness and average grain size values of the films are tabulated in Table 2. It is observed that with the increase of film thickness, the average grain size and surface roughness of the films increases. This result is in agreement with the reported studies [31,38]. The average grain size increases from 54 nm to 81 nm with increase of film thickness from 130 nm to 700 nm. This variation is in good agreement with the FESEM results. The roughness of the films increases from 2.72 nm to 3.75 nm with increase of thickness from 130 nm to 700 nm. This increase in the roughness with film thickness could be attributed to the increase in the surface grain size [38,48]. At the initial stage of nucleation and growth, presence of many nucleation centers leads to the smaller crystallites. The lesser deposition time in the lower thickness films hinders the growth of the smaller crystallites, whereas, for higher thickness films, the deposition time is high so that these small crystallites grew in size. Due to the increase of grain size, the density of grain boundaries decreases which result in large variation in the height of the grains on the surface of film [38].

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

5

Table 2 Calculated values of grain size and roughness of the films. Film thickness (nm)

Average grain size (nm) FESEM

Average grain size (nm) AFM

Roughness (Ra)(nm) AFM

130 290 480 700

40 55 75 118

54 63 74 81

2.72 3.01 3.51 3.75

Fig. 5. AFM micrographs of In2 S3 films deposited at various thickness a) 130 nm b) 290 nm c) 480 nm and d) 700 nm. Table 3 Elemental composition of indium sulfide thin films. Film thickness (nm)

130 290 480 700

EDS

XPS In (at.%)

S (at.%)

S/In

In (at.%)

S (at.%)

S/In

46.45 43.03 42.18 41.57

53.55 56.97 57.82 58.43

1.15 1.32 1.37 1.40

45.32 42.43 41.23 40.72

54.68 57.57 58.77 59.28

1.20 1.35 1.42 1.45

3.3. Composition analysis XPS analysis was used to investigate the elemental composition of the films. The full range spectra of Indium sulfide thin films deposited at various thickness as shown in Fig.6(a). The spectra are corrected for the C1 s peak at 284.5 eV [49]. The constituent elements Indium and Sulfur are present in the spectra. Additionally, the C1s and O 1s peaks which attribute to the surface contamination by the atmosphere are having binding energy values of 284.5 eV and 532.1 eV respectively [50,51]. The Fig.6(b) shows the Indium

core level spectra. The In3d5/2 with binding energy 444.76 eV corresponds to non-oxide form and it corresponds to In2 S3 phase [52]. The binding energy of In3d3/2 is 452.34 eV and the separation between the In3d5/2 and In3d3/2 is 7.58 eV. These values are in good agreement with the reported values [50,53]. The Fig.6(c) shows the Sulfur core level spectra. The binding energies of the S2p3/2 and S2p1/2 are 161.48 eV and 162.64 eV respectively with a splitting of 1.16 eV. These values correspond to In2 S3 phase and are in good agreement with the reported values [54,55]. Quantitative analysis was performed for all the films to find the atomic con-

6

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

Fig. 6. XPS Spectra of the films deposited at various thicknesses a) Survey spectra, b) Indium core level spectra, c) Sulphur core level spectra.

Fig. 7. EDS spectrum of In2 S3 films deposited at various thickness a) 130 nm b) 290 nm c) 480 nm and d) 700 nm.

centration of elements. The atomic concentration of constituent elements is tabulated in Table 3, it is observed that the S/In ratio increases from 1.15 to 1.40 with increase of film thickness from 130 nm to 700 nm. These values are consistent with the reported values [45].

The EDS spectra of indium sulfide thin films deposited at various thicknesses in the range 130–700 nm is shown in Fig.7. The spectra confirmed the presence of constituent elements indium and sulfur. The elemental composition of the films tabulated in Table 3, it is observed that the S/In ratio increases from 1.20 to 1.45 with

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

7

Fig. 8. Transmission spectra of indium sulfide thin films.

Fig. 9. Tauc plot of indium sulfide thin films.

increase of film thickness from 130 nm to 700 nm. The EDS results also support the XPS results. These results are consistent with the reported values [29,45,56].

increase of film thickness. The energy band gap also follows the similar trend with increase of film thickness. Thus it can be inferred that the change in the energy band gap is dependent on the sulfur content in the films. Similar dependence of energy band gap on sulfur content has been reported by Sreejith et al. [57] for In2 S3 thin films. Similar increase of band gap with increase of film thickness was reported for SnO2 :F thin films [60], ZnO thin films [61] and ZnCoIn thin films [44]. Ji et al. [29] observed an increase of band gap with film thickness up to 380 nm and a further increase in thickness lead to decrease in band gap for In2 S3 thin films.

3.4. Optical studies The optical transmittance spectra of the indium sulfide thin films in the wavelength range from 300 nm to 2500 nm as a function of thickness is shown in the Fig.8. All the films show good transmittance in the visible and near IR region with a sharp absorption edge. The sharp decrease in the transmittance at the absorption edge can be attributed to the presence of direct optical transition and single phase of the films [38]. The transmittance spectra are strongly influenced by the film thickness. The absorption edge shifts from lower wavelength to higher wavelength region with increase of film thickness. The shift in the absorption edge may attribute to the change in the grain size [31]. The absorption coefficient is calculated using transmittance data by following relation [49] ˛=

ln( T1 )

(7)

t

Where, T is the transmittance and t is the thickness of the films. The energy band gap of all films was determined by Tauc relation [28] ˛h = A(h − Eg )

n

(8)

where, A is proportionality constant, ␣ is absorption coefficient, h is photon energy, and n is an integer with values of 1/2, 3/2, 2 and 3 for direct allowed, direct forbidden, indirect allowed and indirect forbidden transition respectively. The extrapolation of the linear region of graph (␣h)2 vs. h on x-axis gives the direct energy band gap values. Fig.9 shows the Tauc plot ((␣h)2 vs. h) of all films and the observed band gap values are 2.53 eV, 2.60 eV, 2.68 eV and 2.71 eV for the films deposited at thickness of 130 nm, 290 nm, 480 nm and 700 nm respectively. The observed energy band gap values are in consistent with the reported values for In2 S3 thin films [47,57,58]. The energy band gap increase from 2.53 eV to 2.71 eV with increase of film thickness from 130 nm to 700 nm. The variation in the optical band gap depends on the number of factors such as film crystallinity, grain size, carrier concentration, strain and film composition [44,59]. In this study the change in the band gap may be due to the variation in strain and variation in composition of the films with increase of film thickness, which is confirmed by XRD and compositional studies. The variation of strain with respect to film thickness is nominal. So, the variation in band gap could be attributed to the compositional changes. It is clear from composition analysis that the sulfur content in the films increases with

3.5. Electrical studies The electrical properties of In2 S3 thin films were measured using Hall Effect setup via Van der Paw technique. The variation of carrier concentration, resistivity and mobility with film thickness is listed in Table 4. From Table, it is clear that the carrier concentration and mobility of the films increase with the increase of film thickness. For all the films, n-type conductivity was observed. The resistivity of the films decreases from 1.43 × 102 to 1.23 × 101 with increase of film thickness from 170 nm to 700 nm. The decrease in resistivity with increase of thickness for In2 S3 films have been reported earlier [37]. The variation in resistivity depends on variation of grain size, lattice defects, grain boundary scattering and crystallite size [38]. In this study the decrease in the resistivity of the films with increase of thickness may be due to the increase in crystallite size, decrease in residual defects, which was confirmed by XRD studies. The increase in the grain size with film thickness, which was observed in AFM and FESEM studies also substantiate the decrease in resistivity. The conductivity or resistivity in semiconductor is mainly influenced by the grain boundaries. At lower thickness, the grain size is lower so that more grain boundaries are present, which restrict the flow of charges. The free movement of the charge carriers is aided by the decrease in grain boundaries when the thickness of the films is increased, thus leading to a reduction in resistivity. The observed resistivity is lower than the reported studies [41,62]. 3.6. Photodetector characteristics The MSM photodetector was fabricated by depositing Al contacts on indium sulfide thin films using a metal mask and tested under UV and visible light. The photodetectors were fabricated using indium sulfide thin films with different thickness and named as InS130, InS290, InS480 and InS700. The photograph and schematic diagram of fabricated photodetector are shown in Fig.10. The current-voltage (I–V) characteristics of all photodetector were measured using source measure unit under UV light and

8

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

Table 4 Electrical properties of indium sulfide thin films. Film thickness (nm) 130 290 480 700

Carrier concentration (cm−3 ) × 1016

Resistivity (cm)

Mobility (cm2 V−1 s−1 )

Carrier type

0.51 2.42 3.15 3.88

1.43 × 10 2.48 × 101 2.01 × 101 1.23 × 101

15.1 24.2 26.5 31.1

n n n n

2

Fig. 10. a) Photograph and b) Schematic diagram of MSM photodetector.

Fig. 11. I–V characteristics of photodetectors under UV light.

Fig. 12. I–V characteristics of photodetectors under visible light.

visible light in the voltage range of −5 V to + 5 V. Fig.11 shows the I–V curves of all photodetectors under dark and UV light of intensity 4 mW/cm2 and the Fig.12 shows the I–V curves of all photodetectors under dark and visible light of intensity 100 mW/cm2 . I–V curves of all photodetectors under dark, UV light and visible light show non-linear behaviour, which indicates the formation of a Schottky contact between metal-semiconductor interfaces. The Schottky contact is obtained for n-type semiconductor if the work function of metal (ϕM ) is greater than the work function of semiconductor (ϕS ), while ohmic contact is obtained if work function of metal (ϕM ) is less than the work function of semiconductor (ϕS ) [63,64]. For all photodetectors, compared to the dark current, the photo current increases under the illumination, indicating that all photodetectors are sensitive to the UV–vis light.

The photo response speed and the reproducibility of photodetector play a crucial role in determining the photodetector ability in technological applications. The photo response of all photodetectors was measured at a constant bias voltage of 5 V with ON/OFF switch cycle of 20 s under UV and visible light as shown in Fig.13 and Fig.14 respectively. The photo response of all photodetectors was recorded for four cycles. The ON/OFF states of photodetector can be seen in Fig.13(a). The dark current, light current and photo current of all photodetectors were tabulated in Table 5. The dark current of photodetectors increases with the increase of film thickness. This can be attributed to the decrease in resistivity of the films with increase of film thickness, confirmed by electrical studies. The light current and photo current values are in proportion with the dark current values. The increase in the photo current with increase of

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

Fig. 13. Photo response of photodetectors under UV light a) InS130 b) InS290 c) InS480 and d) InS700.

Fig. 14. Photo response of photodetectors under visible light a) InS130 b) InS290 c) InS480 and d) InS700.

9

10

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

Table 5 Photo detector parameters under UV and visible light. Light source

UV light

Photodetector ID (␮A) IL (␮A) Iph (␮A) IL /ID Sensitivity (%) Responsivity (mA/W) Detectivity × 107 (Jones)

InS130 3.17 9.42 6.25 4.52 197.16 0.930 119.66

Visible light InS290 16.15 23.93 7.78 2.17 48.17 1.157 65.24

InS480 45.64 58.43 12.79 1.68 28.02 1.903 64.52

film thickness can be attributed to the increase in mobility-lifetime product [65]. The measure of the change in the current of photodetectors upon illumination (Photo sensitivity) by UV and visible light are measured using following equation [66]. S (%) =

Iph Idark

× 100

(9)

Where, Iph is the photo current which is the difference between current under the light and dark. Idark is the current under dark. The values of photo sensitivity of all photodetectors under UV and visible light illumination are tabulated in Table 5. From table, it is observed that the photo sensitivity of the photodetectors decreases with the increase of film thickness. A similar trend in photo sensitivity with thickness was reported for CdS thin films [65]. Photo responsivity (R) of photo detector is the amount of photocurrent generated when the films are illuminated by a light source and it is given by [67] R=

Iph

(10)

A.P

Where, A is the illumination area and P is the intensity of light. The detectivity of photodetector was calculated by the relation [67]

D=

1 R.A ⁄2 1 (2 × e × Idark ) ⁄2

(11)

Where, R is the responsivity and e is the charge of electron. The values of responsivity and detectivity of all photodetectors under UV and visible light illumination are tabulated in Table 5. The photo responsivity and detectivity of photodetectors under illumination of UV light is approximately ten times more than that under the illumination of visible light. This can be attributed to creation of excess electron-hole pairs by incident UV light (365 nm) because the energy band gap of indium sulfide thin films (2.71 eV) is lower than the UV energy (h), therefore, it generates large number of electron-hole pairs. The photo responsivity increases with increase of indium sulfide thin film thickness. The increase in photo responsivity with increase of thickness is consistent with the reported studies for SnS thin films [38] and for ZnO:Al/Si hetero-structure [68]. Generally, the responsivity depends on Iph , area of illumination and intensity of light source. From Table 5, it is clear that Iph increases with increase of film thickness. In this work, the responsivity variation can be contributed to the increase of carrier concentration with increase of film thickness causing more light absorption. From electrical studies, it is noted that the increase of carrier concentration with increase of film thickness. The values of responsivity are higher than the CuSbS2 based photodetectors [69]. The rise and decay time of all photo detectors were calculated for one cycle of photo response measurements. The rise time of photodetectors InS130, InS290, InS480 and InS700 under UV and visible light are 1.50 s, 1.27 s, 3.35 s, 4.13 s and 0.85 s, 1.12 s, 1.30 s,

InS700 146.32 182.34 36.02 1.58 24.61 5.360 101.54

InS130 3.72 16.85 13.12 4.52 352.68 0.078 9.25

InS290 17.44 37.95 20.51 2.17 117.60 0.122 6.67

InS480 55.52 93.50 37.98 1.68 68.40 0.226 6.94

InS700 171.62 272.10 100.54 1.58 58.58 0.598 10.46

3.75 s respectively. To find the decay time of photodetectors, decay curve was fit to an exponential function of the form [70] I = I0 exp(−t ⁄ )

(12)

The calculated decay time of photodetectors InS130, InS290, InS480 and InS700 under UV and visible light are 1.48 s, 3.79 s, 4.19 s, 4.46 s and 1.05 s, 1.52 s, 2.52 s, 3.68 s respectively. 4. Conclusions In this study, the In2 S3 thin films were deposited successfully using co-evaporation method on soda-lime glass substrates. The films were deposited at various thicknesses in the range of 130–700 nm. The impact of film thickness on structural, morphological, compositional, optical and electrical properties was studied. The XRD analysis revealed an increase in crystallite size and a decrease in lattice defects with an increase of film thickness. XPS and EDS studies confirmed the presence of constituent elements in the films. The observed energy band gap of the films is in the range 2.53–2.71 eV. The resistivity of the films decreases from 1.42 × 102 to 1.23 × 101 with increase of film thickness from 170 nm to 700 nm. Finally, UV–vis MSM photodetector based on In2 S3 thin films fabricated successfully using Al interdigitated electrodes. The photodetectors were characterized under the illumination of UV–vis light. All photodetectors have stable photo response characteristics and highly reproducible parameters such as photo sensitivity, responsivity and detectivity. Acknowledgments The authors wish to acknowledge the INUP-IISc since this research (or a portion thereof) was performed using facilities at CeNSE, funded by Ministry of Electronics and Information Technology (MeitY), Govt. of India, and located at the Indian Institute of Science, Bengaluru. References [1] R. Anitha, R. Ramesh, R. Loganathan, D.S. Vavilapalli, K. Baskar, S. Singh, Large area ultraviolet photodetector on surface modified Si:GaN layers, Appl. Surf. Sci. 435 (2018) 1057–1064, http://dx.doi.org/10.1016/j.apsusc.2017.11.097. [2] G.H. He, H. Zhou, H. Shen, Y.J. Lu, H.Q. Wang, J.C. Zheng, B.H. Li, C.X. Shan, D.Z. Shen, Photodetectors for weak-signal detection fabricated from ZnO:(Li,N) films, Appl. Surf. Sci. 412 (2017) 554–558, http://dx.doi.org/10.1016/j.apsusc. 2017.03.295. [3] S. Abbas, M. Kumar, J. Kim, All metal oxide-based transparent and flexible photodetector, Mater. Sci. Semicond. Process. 88 (2018) 86–92, http://dx.doi. org/10.1016/j.mssp.2018.07.027. [4] M.S. Mahdi, K. Ibrahim, N.M. Ahmed, A. Hmood, F.I. Mustafa, S.A. Azzez, M. Bououdina, High performance and low-cost UV–Visible–NIR photodetector based on tin sulphide nanostructures, J. Alloys. Compd. 735 (2018) 2256–2262, http://dx.doi.org/10.1016/j.jallcom.2017.10.203. [5] B. Wang, S.P. Zhong, Z. Bin Zhang, Z.Q. Zheng, Y.P. Zhang, H. Zhang, Broadband photodetectors based on 2D group IV A metal chalcogenides semiconductors, Appl. Mater. Today. 15 (2019) 115–138, http://dx.doi.org/10.1016/j.apmt. 2018.12.010. [6] A.A. Ahmed, M. Devarajan, N. Afzal, Fabrication and characterization of high performance MSM UV photodetector based on NiO film, Sensors Actuators, A Phys. 262 (2017) 78–86, http://dx.doi.org/10.1016/j.sna.2017.05.028.

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643 [7] G. Marimuthu, K. Saravanakumar, K. Jeyadheepan, P.M. Razad, M. Jithin, V.R. Sreelakshmi, K. Mahalakshmi, Influence of twin boundaries on the photocurrent decay of nanobranch and dense-forest structured SnO 2 UV photodetectors, Superlattices Microstruct. 128 (2019) 181–198, http://dx.doi. org/10.1016/j.spmi.2019.01.032. [8] G. Li, J. Zhang, X. Hou, Temperature dependence of performance of ZnO-based metal-semiconductor- metal ultraviolet photodetectors, Sensors Actuators, A Phys. 209 (2014) 149–153, http://dx.doi.org/10.1016/j.sna.2014.01.029. [9] K. Zheng, L.B. Luo, T.F. Zhang, Y.H. Liu, Y.Q. Yu, R. Lu, H.L. Qiu, Z.J. Li, J.C.A. Huang, Optoelectronic characteristics of a near infrared light photodetector based on a topological insulator Sb 2 Te 3 film, J. Mater. Chem. C Mater. Opt. Electron. Devices 3 (2015) 9154–9160, http://dx.doi.org/10.1039/c5tc01772f. [10] Y.H. Zhou, H.N. An, C. Gao, Z.Q. Zheng, B. Wang, UV–Vis-NIR photodetector based on monolayer MoS2, Mater. Lett. 237 (2019) 298–302, http://dx.doi. org/10.1016/j.matlet.2018.11.112. [11] Z. Wang, M. Safdar, C. Jiang, J. He, High-performance UV-visible-NIR broad spectral photodetectors based on one-dimensional in 2 Te 3 nanostructures, Nano Lett. 12 (2012) 4715–4721, http://dx.doi.org/10.1021/nl302142g. [12] C.Y. Tsay, W.T. Hsu, Comparative studies on ultraviolet-light-derived photoresponse properties of ZnO, AZO, and GZO transparent semiconductor thin films, Materials (Basel). 10 (2017), http://dx.doi.org/10.3390/ ma10121379. [13] P. Fay, Photodetectors, Ref. Modul. Mater. Sci. Mater. Eng. 2 (2016) 1–16, http://dx.doi.org/10.1016/B978-0-12-803581-8.01813-0. ˜ M. Calixto-Rodriguez, S. Messina-Fernández, [14] S. Lugo-Loredo, Y. Pena-Méndez, A. Alvarez-Gallegos, A. Vázquez-Dimas, T. Hernández-García, Indium sulfide thin films as window layer in chemically deposited solar cells, Thin Solid Films 550 (2014) 110–113, http://dx.doi.org/10.1016/j.tsf.2013.10.115. [15] M.A. Mughal, R. Engelken, R. Sharma, Progress in indium (III) sulfide (In2 S3 ) buffer layer deposition techniques for CIS, CIGS, and CdTe-based thin film solar cells, Sol. Energy 120 (2015) 131–146, http://dx.doi.org/10.1016/j. solener.2015.07.028. [16] S.S. Wang, F.J. Shiou, C.C. Tsao, S.W. Huang, C.Y. Hsu, An evaluation of the deposition parameters for indium sulfide (in 2S3) thin films using the grey-based Taguchi method, Mater. Sci. Semicond. Proc. 16 (2013) 1879–1887, http://dx.doi.org/10.1016/j.mssp.2013.06.012. [17] S.P. Nehra, S. Chander, A. Sharma, M.S. Dhaka, Effect of thermal annealing on physical properties of vacuum evaporated In 2 S 3 buffer layer for eco-friendly photovoltaic applications, Mater. Sci. Semicond. Process. 40 (2015) 26–34, http://dx.doi.org/10.1016/j.mssp.2015.06.049. [18] T. Sall, B. Marí Soucase, M. Mollar, B. Hartitti, M. Fahoume, Chemical spray pyrolysis of ␤-In2 S3 thin films deposited at different temperatures, J. Phys. Chem. Solids 76 (2015) 100–104, http://dx.doi.org/10.1016/j.jpcs.2014.08. 007. [19] A.O. Juma, Stoichiometry and local bond configuration of In2S3:Cl thin films by Rutherford backscattering spectrometry, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms. 385 (2016) 84–88, http://dx.doi.org/ 10.1016/j.nimb.2016.09.005. [20] H. Zhu, X. Wang, W. Yang, F. Yang, X. Yang, Indium sulfide microflowers: fabrication and optical properties, Mater. Res. Bull. 44 (2009) 2033–2039, http://dx.doi.org/10.1016/j.materresbull.2009.05.023. [21] R. Souissi, N. Bouguila, A. Labidi, Ethanol sensing properties of sprayed B-In2S3 thin films, Sensors Actuators, B Chem. 261 (2018) 522–530, http://dx. doi.org/10.1016/j.snb.2018.01.175. [22] R.A. Ismail, N.F. Habubi, M.M. Abbod, Preparation of high-sensitivity in 2 S 3 /Si heterojunction photodetector by chemical spray pyrolysis, Opt. Quantum Electron. 48 (2016) 1–14, http://dx.doi.org/10.1007/s11082-016-0725-5. [23] Y. Ben Salem, M. Kilani, N. Kamoun, Effect of deposition runs on the physical properties of In2S3chemically synthesized for photocatalytic application, Results Phys. 10 (2018) 706–713, http://dx.doi.org/10.1016/j.rinp.2018.02. 078. [24] V.G. Rajeshmon, N. Poornima, C. Sudha Kartha, K.P. Vijayakumar, Modification of the optoelectronic properties of sprayed in 2S3 thin films by indium diffusion for application as buffer layer in CZTS based solar cell, J. Alloys. Compd. 553 (2013) 239–244, http://dx.doi.org/10.1016/j.jallcom.2012.11.106. [25] E. Kärber, K. Otto, A. Katerski, A. Mere, M. Krunks, Raman spectroscopic study of In2S3 films prepared by spray pyrolysis, Mater. Sci. Semicond. Proc. 25 (2014) 137–142, http://dx.doi.org/10.1016/j.mssp.2013.10.007. [26] N. Naghavi, R. Henriquez, V. Laptev, D. Lincot, Growth studies and characterisation of in 2 S 3 thin films deposited by atomic layer deposition (ALD), Appl. Surf. Sci. 222 (2004) 65–73, http://dx.doi.org/10.1016/j.apsusc. 2003.08.011. [27] S. Rasool, G. Phaneendra Reddy, K.T. Ramakrishna Reddy, M. Tivanov, V.F. Gremenok, Effect of substrate temperature on structural and optical properties of In2S3 thin films grown by thermal evaporation, Mater. Today Proc. 4 (2017) 12491–12495, http://dx.doi.org/10.1016/j.matpr.2017.10.049. [28] J. Raj Mohamed, L. Amalraj, Effect of precursor concentration on physical properties of nebulized spray deposited in 2 S 3 thin films, J. Asian Ceram. Soc. 4 (2016) 357–366, http://dx.doi.org/10.1016/j.jascer.2016.07.002. [29] Y. Ji, Y. Ou, Z. Yu, Y. Yan, D. Wang, C. Yan, L. Liu, Y. Zhang, Y. Zhao, Effect of film thickness on physical properties of RF sputtered in 2 S 3 layers, Surf. Coatings Technol. 276 (2015) 587–594, http://dx.doi.org/10.1016/j.surfcoat.2015.06. 011. [30] C. Laurencic, L. Arzel, F. couzinié-Devy, N. Barreau, Investigation of Cu(in,Ga)Se2/In2S3 diffuse interface by Raman scattering, Thin Solid Films 519 (2011) 7553–7555, http://dx.doi.org/10.1016/j.tsf.2010.12.089.

11

[31] N. Revathi, P. Prathap, K.T.R. Reddy, Thickness dependent physical properties of close space evaporated In2S3 films, Solid State Sci. 11 (2009) 1288–1296, http://dx.doi.org/10.1016/j.solidstatesciences.2009.04.019. [32] M.P. Deshpande, N. Garg, S.V. Bhatt, P. Sakariya, S.H. Chaki, Characterization of CdSe thin films deposited by chemical bath solutions containing triethanolamine, Mater. Sci. Semicond. Proc. 16 (2013) 915–922, http://dx. doi.org/10.1016/j.mssp.2013.01.019. [33] V. Mote, Y. Purushotham, B. Dole, Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles, J. Theor. Appl. Phys. 6 (2012) 2–9, http://dx.doi.org/10.1186/2251-7235-6-6. [34] R. Suriakarthick, V. Nirmal Kumar, T.S. Shyju, R. Gopalakrishnan, Effect of substrate temperature on copper antimony sulphide thin films from thermal evaporation, J. Alloys. Compd. 651 (2015) 423–433, http://dx.doi.org/10.1016/ j.jallcom.2015.08.061. [35] R. Suriakarthick, V. Nirmal Kumar, R. Indirajith, T.S. Shyju, R. Gopalakrishnan, Photochemically deposited and post annealed copper indium disulphide thin films, Superlattices Microstruct. 75 (2014) 667–679, http://dx.doi.org/10. 1016/j.spmi.2014.08.012. [36] R. Suriakarthick, V. Nirmal Kumar, T.S. Shyju, R. Gopalakrishnan, Investigation on post annealed copper sulfide thin films from photochemical deposition technique, Mater. Sci. Semicond. Proc. 26 (2014) 155–161, http://dx.doi.org/ 10.1016/j.mssp.2014.04.024. [37] P.M.R. Kumar, T.T. John, C.S. Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, Effects of thickness and post deposition annealing on the properties of evaporated in 2 S 3 thin films, J. Mater. Sci. 41 (2006) 5519–5525, http://dx. doi.org/10.1007/s10853-006-0307-1. [38] T.S. Reddy, M.C.S. Kumar, Co-evaporated SnS thin films for visible light photodetector applications, RSC Adv. 6 (2016) 95680–95692, http://dx.doi. org/10.1039/c6ra20129f. [39] S.S. Lin, J.L. Huang, Effect of thickness on the structural and optical properties of ZnO films by r.f. Magnetron sputtering, Surf. Coatings Technol. 185 (2004) 222–227, http://dx.doi.org/10.1016/j.surfcoat.2003.11.014. [40] J. Rousset, F. Donsanti, P. Genevée, G. Renou, D. Lincot, High efficiency cadmium free Cu(In,Ga)Se2 thin film solar cells terminated by an electrodeposited front contact, Sol. Energy Mater. Sol. Cells 95 (2011) 1544–1549, http://dx.doi.org/10.1016/j.solmat.2010.12.009. [41] M. Liu, Z. Li, J. Ji, M. Dou, F. Wang, Properties of nanostructured pure ␤-In 2 S 3 thin films prepared by sulfurization-assisted electrodeposition, J. Mater. Sci. Mater. Electron. 28 (2017) 5044–5052, http://dx.doi.org/10.1007/s10854016-6161-2. [42] K. Kambas, J. Spyridelis, M. Balkanski, Far infrared and raman optical study of ␣- and ␤-In2S3 compounds, Phys. Status Solidi 105 (1981) 291–296, http:// dx.doi.org/10.1002/pssb.2221050132, doi 10.1002%2Fpssb.2221050132. [43] P.E. Rodríguez-Hernández, K.E. Nieto-Zepeda, A. Guillén-Cervantes, J. Santoyo-Salazar, J. Santos-Cruz, J.S. Arias-Cerón, M. de la L Olvera, O. Zelaya-Ángel, G. Contreras-Puente, F. de Moure-Flores, Structural and optical properties of In2S3thin films grown by chemical bath deposition on pet flexible substrates, Chalcogenide Lett. 14 (2017) 331–335. [44] H. Slimi, A. Barhoumi, N. Waldhoff, B. Duponchel, C. Poupin, R. Cousin, G. Leroy, S. Guermazi, Thickness effects on physical and electrical properties of Zn0.97Co0.02In0.01O thin films grown by magnetron sputtering RF, Superlattices Microstruct. 120 (2018) 670–689, http://dx.doi.org/10.1016/j. spmi.2018.05.061. [45] K. Otto, A. Katerski, A. Mere, O. Volobujeva, M. Krunks, Spray pyrolysis deposition of indium sulphide thin films, Thin Solid Films 519 (2011) 3055–3060, http://dx.doi.org/10.1016/j.tsf.2010.12.027. [46] T. Todorov, J. Carda, P. Escribano, A. Grimm, J. Klaer, R. Klenk, Electro deposited In2S3 buffer layers for CuInS2 solar cells, Sol. Energy Mater. Sol. Cells 92 (2008) 1274–1278, http://dx.doi.org/10.1016/j.solmat.2008.04.022. ´ [47] S. Rasool, K. Saritha, K.T.R. Reddy, K.R. Reddy, L. Bychto, A. Patryn, M. Malinski, M.S. Tivanov, V.F. Gremenok, Optical properties of thermally evaporated In2S3 thin films measured using photoacoustic spectroscopy, Mater. Sci. Semicond. Process. 72 (2017) 4–8, http://dx.doi.org/10.1016/j.mssp.2017.09.009. [48] M. Wu, C. Zhang, S. Yu, L. Li, Thickness dependence of microstructure, dielectric and leakage properties of BaSn0.15Ti0.85O3 thin films, Ceram. Int. 44 (2018) 11466–11471, http://dx.doi.org/10.1016/j.ceramint.2018.03.208. [49] E. Jose, M.C.S. Kumar, Room-temperature Wide-range Luminescence and Structural, Optical, and Electrical Properties of SILAR Deposited Cu-Zn-S Nano-structured Thin Films, 2016, http://dx.doi.org/10.1117/12.2236883, 992917. [50] M. Mathew, M. Gopinath, C.S. Kartha, K.P. Vijayakumar, Y. Kashiwaba, T. Abe, Tin doping in spray pyrolysed indium sulfide thin films for solar cell applications, Sol. Energy 84 (2010) 888–897, http://dx.doi.org/10.1016/j. solener.2010.01.030. [51] L. Bhira, H. Essaidi, S. Belgacem, G. Couturier, J. Salardenne, N. Barreaux, J.C. Bernede, Structural and photoelectrical properties of sprayed ␤-In 2 S 3 thin films, Phys. Status Solidi Appl. Res. 181 (2000) 427–435, http://dx.doi.org/10. 1002/1521-396X(200010)181:2<427::AID-PSSA427>3.0.CO;2-P. [52] S.S. Tulenin, E.V. Maraeva, L.N. Maskaeva, V.F. Markov, Study of chemical bath deposited In2S3 thin films, Asian J. Chem. 29 (2017) 995–998, http://dx.doi. org/10.14233/ajchem.2017.20385. [53] T.T. John, C.S. Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, Preparation of indium sulfide thin films by spray pyrolysis using a new precursor indium nitrate, Appl. Surf. Sci. 252 (2005) 1360–1367, http://dx.doi.org/10.1016/j. apsusc.2005.02.093.

12

H.K. B. and S.K. M.C. / Sensors and Actuators A 299 (2019) 111643

[54] X. Feng, Y. Chen, M. Wang, L. Guo, Hydrothermal synthesis of pyramid-like In2S3 film for efficient photoelectrochemical hydrogen generation, Int. J. Hydrogen Energy 42 (2017) 15085–15095, http://dx.doi.org/10.1016/j. ijhydene.2017.04.283. [55] A. Omelianovych, J.H. Kim, L. Liudmila, B.T. Ahn, Effect of post annealing on the characteristics of In2S3 buffer layer grown by chemical bath deposition on a CIGS substrate, Curr. Appl. Phys. 15 (2015) 1641–1649, http://dx.doi.org/ 10.1016/j.cap.2015.08.019. [56] A. Akkari, C. Guasch, M. Castagne, N. Kamoun-Turki, Optical study of zinc blend SnS and cubic In2S3:Al thin films prepared by chemical bath deposition, J. Mater. Sci. 46 (2011) 6285–6292, http://dx.doi.org/10.1007/ s10853-011-5626-1. [57] S. Karthikeyan, A.E. Hill, R.D. Pilkington, Low temperature pulsed direct current magnetron sputtering technique for single phase ␤-In 2 S 3 buffer layers for solar cell applications, Appl. Surf. Sci. 418 (2017) 199–206, http:// dx.doi.org/10.1016/j.apsusc.2017.01.147. [58] D.H. Hwang, S. Cho, K.N. Hui, Y.G. Son, Effect of sputtering power on structural and optical properties of radio frequency-sputtered In2 S3 thin films, J. Nanosci. Nanotechnol. 14 (2014) 8978–8981, http://dx.doi.org/10.1166/jnn. 2014.10079. [59] T.S. Reddy, M.C.S. Kumar, Effect of substrate temperature on the physical properties of co-evaporated Sn2S3 thin films, Ceram. Int. 42 (2016) 12262–12269, http://dx.doi.org/10.1016/j.ceramint.2016.04.172. [60] B. Benhaoua, S. Abbas, A. Rahal, A. Benhaoua, M.S. Aida, Effect of film thickness on the structural, optical and electrical properties of SnO 2 : F thin films prepared by spray ultrasonic for solar cells applications, Superlattices Microstruct. 83 (2015) 78–88, http://dx.doi.org/10.1016/j.spmi.2015.03.017. [61] T. Prasada Rao, M.C. Santhoshkumar, Effect of thickness on structural, optical and electrical properties of nanostructured ZnO thin films by spray pyrolysis, Appl. Surf. Sci. 255 (2009) 4579–4584, http://dx.doi.org/10.1016/j.apsusc. 2008.11.079. [62] N. Revathi, P. Prathap, Y.P.V. Subbaiah, K.T.R. Reddy, Substrate temperature dependent physical properties of In2S 3 films, J. Phys. D Appl. Phys. 41 (2008), http://dx.doi.org/10.1088/0022-3727/41/15/155404. [63] E.H. Rhoderick, Metal-semiconductor contacts, IEE proc, I-Solid-State Electron Devices 129 (1) (1982). [64] R.F. Pierret, Semiconductor Device Fundamentals, Pearson Education, India, 1996. [65] J.P. Enríquez, X. Mathew, Influence of the thickness on structural, optical and electrical properties of chemical bath deposited CdS thin films, Sol. Energy

[66]

[67]

[68]

[69]

[70]

Mater. Sol. Cells 76 (2003) 313–322, http://dx.doi.org/10.1016/S09270248(02)00283-0. M.S. Mahdi, K. Ibrahim, A. Hmood, N.M. Ahmed, S.A. Azzez, F.I. Mustafa, A highly sensitive flexible SnS thin film photodetector in the ultraviolet to near infrared prepared by chemical bath deposition, RSC Adv. 6 (2016) 114980–114988, http://dx.doi.org/10.1039/c6ra24491b. K.S. Gour, O.P. Singh, B. Bhattacharyya, R. Parmar, S. Husale, T.D. Senguttuvan, V.N. Singh, Enhanced photoresponse of Cu 2 ZnSn(S, Se) 4 based photodetector in visible range, J. Alloys. Compd. 694 (2017) 119–123, http:// dx.doi.org/10.1016/j.jallcom.2016.09.299. T. Basu, M. Kumar, T. Som, Thickness-controlled photoresponsivity of ZnO:Al/Si heterostructures: role of junction barrier height, Mater. Lett. 135 (2014) 188–190, http://dx.doi.org/10.1016/j.matlet.2014.07.171. V. Vinayakumar, S. Shaji, D. Avellaneda, J.A. Aguilar-Martínez, B. Krishnan, Copper antimony sulfide thin films for visible to near infrared photodetector applications, RSC Adv. 8 (2018) 31055–31065, http://dx.doi.org/10.1039/ C8RA05662E. M.C. Santhosh Kumar, B. Pradeep, Formation and properties of AgInSe2 thin films by co-evaporation, User Model. Useradapt. Interact. 72 (2004) 369–378, http://dx.doi.org/10.1016/j.vacuum.2003.09.008.

Biographies B Hemanth Kumar received M.Sc Physics degree from the Department of Physics, National Institute of Technology, Tiruchirappalli, India in 2016. He is currently in the Ph.D program at Department of Physics, National Institute of Technology, Tiruchirappalli, India. His research interests include Thin film solar cells and Photodetectors. M C Santhosh Kumar is working as Associate Professor in Department of Physics, National Institute of Technology, Tiruchirappalli, India. He has received his Ph.D from Cochin University of Science and Technology (CUSAT), Kochi, India in 2003 in the field of semiconductor thin films. He has more than16 years of teaching experience in UG and PG level. He was a visiting researcher at Korea Advanced Institute of Science and Technology (KAIST), South Korea. His current research interests are in Optoelectronic materials, thin film solar cells and nanomaterials.