Investigation Of Some Optical And Electrical Properties Of InSe Thin Film, a Window Layer for Photovoltaic Cell Growth on Glass/GaSe Substrate by M-CBD Method

The InSe thin film was successfully grown on a glass/GaSe substrate using the modified chemical bath deposition ( M CBD) method and a group was annealed one hour at 80 °C in atmospheric conditions . The energy band gap ( 𝐸𝐸𝐸𝐸 ) values of the as - deposited and annealed films were defined 1,89 eV and 1,15 eV, respectively. The transmitance values (%) of the as - deposited and annealed films were asigned %0.65, %4.46 and %11,02, %10,35 at 400 and 700 nm, respectively. The I- V characteristics of the device at different temperatures was examined, the resistivity decreased and the conductivity increased with the increasing temperature . The activation energy in low temperature region was defined 0,05 eV, while it was defined 0.6 eV in high temperature region .


Introduction
InxSey compound can be formed different atomic combination and phases like InSe, In6Se7 In4Se3,In2Se3, In9Se11, etc. have been confirmed as attractiving inorganic semiconductors during past few decades (Igasaki and Fujiwara 1996, Gopal, Viswanathan et al. 2004. Indium selenite (InxSey) belong to A III -B VI group contains a layered structure similar to the GaS and GaSe compounds. This structure is bounded in two dimensions by covalent bonds and by Van der Waals forces in three dimensions (Choi and Yu 2003). Indium selenite, which can exhibit n-type, has direct band gap (Pathan, Kulkarni et al. 2005). They are of particular interest to researchers because of their remarkable optical and photophysical properties that are promising for capacitors, solar cells, field effect transistors, photovoltaics and a variety of nanoelectronic applications (Mudd, Svatek et al. 2013, Han, Chen et al. 2014, Yuksek, Yaglioglu et al. 2014, Ho and Chu 2015, Boukhvalov, Gurbulak et al. 2017, Politano, Campi et al. 2017). It has been widely researched for its application especially in photovoltaic device fabrication (Gopal, Viswanathan et al. 2005). InxSey has been used either as a windows layer (El-Sayed 2003) in CIS/InxSey as well as an absorber layer (Julien, Chevy et al. 1990) in InxSey/CdS solar cell. There are number of deposition techniques such as flash evaporation (Julien, Benramdane et al. 1990), molecular beam epitaxy , chemical vapor deposition (Jin- Ho Park, Mohammad Afzaal et al. 2003), van der walls epitaxy (Lang, Klein et al. 1995) and vacuum evaporation (Gopal, Viswanathan et al. 2005) for preparation of InxSey thin films. However, the modified chemical bath deposition (M-CBD) method is an simple, low temperature and inexpensive technique that could fabricate films with high quality for device applications such as heterojunction devices, switching etc. (Lokhande and Pawar 1989).
In this study, we have coated the InSe semiconductor on glass/GaSe thin film substrate for M-CBD method and examined its optical and electrical properties. It is aimed to synthesize glass/GaSe/InSe heterojunction at low cost, examine their optical and electrical properties, and as a result, to present them as an alternative material that can be used in the construction of opto-electronic circuit elements.

Material and Methods
InSe thin films were deposited at room temperature on glass/GaSe(hexagonal) thin film substrate of 3x3 cm dimensions by M-CBD method. For cationic solutions, acidic (pH≈3) In2(SO4)3 solutions of 0.07 M concentrations were used while for anionic ones, basic (pH≈12) Na2SeSO3 solution of 0.05 M was used.
The glass/GaSe substrates were 60 times immersed in In2(SO4)3 precursor solution for 30 seconds, in distilled water for 70 seconds, in Na2SeSO3 precursor solution for 10 seconds and distilled water for 70 seconds respectively. The two of the fabricated films were also annealed at 80 °C temperatures in air atmosphere for one hour.
Optical properties were investigated by Perkin-Elmer Lambda 25 UV-Vis spectrophotometer.
The I-V measurements were carried out with Keithley 6486 pico-amperometer and Pasco Scientific SF-9585 A power source using two probe technique in which silver metal was used for contacts.

Optical Properties
The thickness of InSe thin film growth on glass/GaSe substrate in 60 steps was estimated to be 90 nm.
The optical absorbance and transmittance spectra against wavelength in the range 400 -1100 nm at room temperature for glass/GaSe/InSe film is shown in Figure 1a and c, respectively. The graph of the absorption coefficient (αa) versus photon energy is given in Figure 1b. The highest value of αa is 9,3x10 6 at 2,26 eV. As seen in Figure 1 a, the maximum absorption peak is 547 nm in the UV region and the absorption decreases with increasing wavelength. This behavior is a characteristic feature of existence of states with free carries that localized in the energy band gap (ÜNAL, İZGİ et al. 2020). Figure 1 c also plotted wavelength versus percent transmitance. Glass/GaSe/InSe thin film is highly transmitance at wavelengths greater than 700 nm but the transmittance value at 400 and 700 nm is %0.65, %4.46, respectively. These values are very low and mean that the glass/GaSe/InSe thin film is a good absorbent material at the visible region boundaries.
The basic absorption spectrum is one of the most used methods to determine the energy band gap ( ). The relation between absorption coefficient and energy band gap is given (Tauc 1968) where is the absorption coefficient and ℎ is the energy of absorbance light. The value of n is ½ for direct allowed transitions for InSe (Gopal, Viswanathan et al. 2005) materials. This method uses the plot of ( ℎ ) 1 versus hυ (Figure 1 d).
is determined by extrapolating the straight line portion of this plot to the energy axis. The intercept on energy axis gives the value of , and have yielded 1.89 eV. It is seen that the result obtained is lower than the energy band gaps of metal oxide semiconductor films (Dugan, Koç et al. 2020, Yetim, Aslan et al. 2020). Optical absorption and transmittance plotted against the 400-1100 nm wavelength of glass/GaSe/InSe(annealed) thin film are given in Figure 2a and c, respectively. The absorption coefficient (αa) and ( ℎ ) plotted against photon energy are given in Figure 2b and d, respectively.
The highest value of αa is 3,5x10 6 at 1,49 eV. As seen in Figure 2a, the highest absorption was seen at 837 nm. With the effect of annealing, the highest absorption peak shifted from the visible to the infrared region. This caused the forbidden energy band gap to decrease to 1.15 eV, as seen in Figure   2d. In Figure 2c

Electrical Properties
The I-V characteristics of the as-deposited and annealed films were examined under dark and yellow light intensity 100 w/m 2 . Figure 3a shows the schematic representation of the device produced, shows the graphs of the junction resistance Rj ( = ) versus voltage.
In Figure 4a, it is seen that both as-deposited and annealed films are sensitive to light and their electrical conductivity increases under light. It is also seen that the photoconductivity of the asdeposited film is higher than that of the annealed film. The reason for this can be shown, as seen in the optical analysis, the shift of the maximum absorption peak from the visible to the infrared region, the decrease in the maximum absorption coefficient and the increase in the transmitance in the visible region. It is also seen that there are three different regions in Figure. 4a. The first region suggests a trap controlled space charge limited. The second region is the region of superquadratic behavior caused by low concentration charge carriers. In the second region, the trap filling mechanism is dominant. In the 3rd region, as the application voltage increases, all trap levels in the band gap of the semiconductor layer are filled. This suggests that most of traps are filled and contribution of free carrier to electric filed becomes appreciable (Kumar, Jain et al. 2003, Yakuphanoglu 2008, Chaudhary, Kumar et al. 2019.   Figure 5a and b shows I-V and Rj-V graphs against temperature change, respectively. With the increase in temperature, the electrical conductivity increased with the decrease of Rj. In Figure 6a and b, graphs of resistivity and conductivity versus temperature are given at a constant potential, respectively. While the resistivity was determined as 0.7x10 9 (ohm.cm) at 285 °K, it was determined as 0.1x10 9 (ohm.cm) at 407 °K. The conductivity was determined as 1,3x10 -9 (ohm.cm) -1 at 285 °K and 6,6x10 -9 (ohm.cm) -1 at 407 °K. This is one of the biggest properties that distinguishes semiconductors from metals.  ,Also activation energies for as-deposited film were calculated within 285-407 °K temperature range and corresponding logσ-10 3 /T plots were recorded. The calculations were made according to following equation (Kobbi andKesri 2004, Yilmaz 2008): The logσ-10 3 /T curve depicted in Figure 7 in this temperature range represent that the activation energy in low temperature region (285-342 °K) is 0,05 eV, it is 0,23 eV in middle temperature region

Conclusions and Recommendations
The InSe thin film was successfully grown on a glass/GaSe substrate using the M-CBD method and a group was annealed. While the energy band gap of the as-deposited film was 1.89eV, the energy band gap of the annealed film was determined as 1.15 eV. The maximum absorption region shifted from the visible to the infrared region by annealing. The as-deposited device is ideal for optoelectronic applications in the visible region, while the annealed film is ideal for applications used in the infrared region. In addition, with annealing while the maximum absorption coefficient αa decreased, optical transmittance and electrical conductivity increased.
The I-V characterization of the device at different temperatures was examined, the resistivity decreased and the conductivity increased with the increase of the temperature. The activation energy in low temperature region is 0,05 eV, while it is 0.6 eV in high temperature region.