Preparation of InSe Thin Films by Thermal Evaporation Method and Their Characterization : Structural , Optical , and Thermoelectrical Properties

+e indium selenium (InSe) bilayer thin films of various thickness ratios, InxSe(1-x) (x� 0.25, 0.50, 0.75), were deposited on a glass substrate keeping overall the same thickness of 2500 Á̊ using thermal evaporation method under high vacuum atmosphere. Electrical, optical, and structural properties of these bilayer thin films have been compared before and after thermal annealing at different temperatures.+e structural andmorphological characterization was done using XRD and SEM, respectively.+e optical bandgap of these thin films has been calculated by Tauc’s relation that varies within the range of 1.99 to 2.05 eV. A simple low-cost thermoelectrical power measurement setup is designed which can measure the Seebeck coefficient “S” in the vacuum with temperature variation. +e setup temperature variation is up to 70°C. +is setup contains a Peltier device TEC1-12715 which is kept between two copper plates that act as a reference metal. Also, in the present work, the thermoelectric power of indium selenide (InSe) and aluminum selenide (AlSe) bilayer thin films prepared and annealed in the same way is calculated. +e thermoelectric power has beenmeasured by estimating the Seebeck coefficient for InSe and AlSe bilayer thin films. It was observed that the Seebeck coefficient is negative for InSe and AlSe thin films.


Introduction
Semiconductor compounds formed from group elements II-VI and III-V are extensively used in modern technology [1].eir bandgap lies between 1.0 and 2.0 eV [2].ey are abundantly available on earth, and they are also both chemically and electrochemically stable in either acid or alkaline condition [3].Among these materials, selenium is very useful in making optical devices [4][5][6].In the present work, InSe bilayer thin films are studied.InSe is used because of its application in optoelectronic devices [7], solar cells [8], and solid-state batteries [9].e structural, electronic, and electrical properties of InSe were studied by various researchers [10][11][12][13].
e thermoelectric (TE) phenomena can provide the direct conversion of applied temperature gradient into electricity (the Seebeck effect) or electricity into temperature difference (the Peltier effect). is solid-state technology can be applied in a variety of applications [14], and the most well known is as a thermocouple for temperature measurement [15].Nowadays, as the continuous fossil fuel supply decreases and world energy demand increases, TE materials have drawn renewed interest due to their potential to provide a sustainable supply of energy.Compared with other conventional electric generators, the reliability and simplicity of thermoelectricity enable niche applications even though many conventional processes are more efficient.
e Seebeck coefficient (S) measurement as a function of time is one of the significant methods to analyze electronic properties of solids.
e S can be used to determine the concentration of majority carrier, type of semiconductor, Fermi level position, and so on [16][17][18][19].In the present work, a setup is designed to calculate the Seebeck coefficient in the vacuum with variations in the temperature from 300 to 310 K. e setup is very compact, low power, and easy to operate.
In this paper, structural, optical, and thermoelectrical properties of InSe bilayer thin lms, having di erent thickness ratios, have been studied.

Experimental
2.1.Materials and Methods.In x Se 1-x bilayer thin lms of the same thickness (2500 Å ´) and di erent thickness ratios (x 0.25, 0.50, and 0.75) were deposited by the thermal evaporation method on a clean glass substrate under a vacuum of 10 −6 mbar.e structural, electrical, and optical measurements of the lms were carried on to investigate the e ect of annealing temperature and thickness ratio.Prepared bilayer lms were also annealed at substrate temperatures of 70 °C and 100 °C.e thin lms were prepared using the thermal evaporation coating unit shown in Figure 1.
e prepared bilayered thin lm and its schematic diagram are shown in Figure 2.
ermoelectric power measurement of composite selenium thin lms has been carried out in the temperature range of 300-310 K by taking pure metallic copper (Cu) as a reference metal.e thermoelectric module TEC1-12715 was used for temperature di erence in the two-end sample.
e module was sandwiched between two copper plates.e experimental setup used for sample testing is shown in Figure 3. e chamber was pumped rst by using a rotary  2 Journal of Nanotechnology pump to a rough vacuum of 10 −3 Torr.Only one end of the sample was heated leading to the creation of a hot junction and a cold junction to create a temperature gradient on the sample.e thermocouple used for the measurement of the temperature gradient (dT) is of K type.e sample under investigation was kept on copper plates, and the wires of the module were connected to a constant power supply to provide the voltage to the thermoelectric module in order to maintain the temperature di erence.A multimeter was used to measure the voltage di erence across the lm.e Seebeck coe cient was measured using the following relation: where dV is the voltage di erence across the lm and dT is the temperature gradient.

X-Ray Di raction Study.
To identify the structure of the lm, the X-ray di raction method is used.e XRD patterns of the as-deposited and annealed InSe thin lm are shown in Figure 4. e XRD spectrum exhibits the multiple characteristic peaks at 2θ 23.5 °, 29 °, 41 °, 45 °, and 52.4 °corresponding to the 004, 101, 204, 110, and 201 phases of indium and selenium.Furthermore, no additional peak was observed for the annealed sample as compared to the asdeposited thin lm, indicating that no new interfacial phase was formed after annealing.It has also been observed that only peak intensity is increased with an increase in the annealing temperature, which may be due to the small particle size and ordered distribution of the particles, hence resulting in an increase in the order of the crystalline nature of thin lms after heat treatment.

Scanning Electron Microscopy.
e SEM images of the InSe bilayer are shown in Figure 5. Figure 5(a) shows the image before annealing, and Figure 5(b) shows the images after annealing.Figure 5(a) shows that the as-deposited lm exhibits a smooth granular structure that appeared uniformly at the surface, and a stack-like structure also appeared due to stack layer deposition.After annealing, the surface morphology of thin lms is changed considerably, and it can be seen from Figure 5(b) that big cluster and uneven lumps have appeared in a spherical shape and are distributed on the entire surface.ese clusters may be formed due to the agglomeration of particles during heat treatment.According to these SEM images, we may conclude that the as-deposited thin lms have partially amorphous structure, while after annealing, the thin lms become polycrystalline in nature.

I-V Characteristics
. I-V characteristics of InSe lms were estimated using a two-terminal con guration by applying a voltage (≈10 V) to the sample and measuring the current through it using a Keithley 2450 sourcemeter.
e measurements were carried out at room temperature, and the variation in logarithm of conductivity with an inverse of temperature for di erent thickness ratios of InSe thin lms with ±2-3% error is shown in Figure 6.
It is cleared from gure that conductivity increases with an increase in temperature [20] which indicates the semiconducting behavior of InSe thin lms.In:Se is revealed that on increasing the indium concentration in InSe bilayer thin lms, the room temperature lm conductivity is increased from 2.13 × 10 −10 to 1.80 × 10 −7 (σ-cm) −1 in comparison with that of lower concentration of indium, as shown in Figure 7.It may be attributed to the interaction of metallic In with the Se thin lm layer, which leads to a decrease in the potential di erence at the interface of the InSe bilayer thin lm, which in turn is expected to form the crystalline structure of InSe bilayer thin lms; hence, the electrical resistance of composite lms decreases or the conductivity increases.Journal of Nanotechnology

Optical Properties.
In the high absorption region, absorption coefficient (α) can be represented according to Tauc [21] and Pankove [22], respectively, as where A is a constant depending on the transition probability, Eg is the optical bandgap, and n is the index that characterizes the optical absorption process and is theoretically equal to 2, 1/2, 3, and 3/2 for indirect allowed [23], direct allowed, indirect forbidden, and direct forbidden transition, respectively.e method for determining the value of bandgap Eg is obtained from the graph of (αh]) 1/n versus photon energy h].If an appropriate value of n is used to obtain a linear plot, the value of Eg will be given by the intercept on the h] axis.
e absorption edge of the InSe bilayer thin film is located in a visible range, that is, from 580 nm to 620 nm, as shown in Figure 8.It was observed that at a low concentration of indium absorption is low as the annealing temperature increases, while for high indium concentration, absorption increases with an increase in the annealing temperature that may be due to an increase in energy density because of higher concentration of indium which was further increased with the annealing temperature.6 Journal of Nanotechnology e bandgap of the prepared InSe bilayer thin lm was calculated using Tauc's relation and is shown in Figure 9. e bandgap value is summarized in Tables 1 and 2.

Optical Micrographs.
Optical micrographs of the InSe bilayer were taken with the help of a USB optical microscope and are shown in Figure 10, which shows that the InSe bilayer thin lm surface is smooth and no cracks are there.

ermoelectric Power.
e thermoelectromotive force, that is, the Seebeck coe cient of the prepared InSe and AlSe bilayer thin lms with di erent thickness ratios and annealing temperatures is shown in Figures 11-14.
As shown in Figures 11 and 12, the increase of conductivity with a decreasing Se ratio in composition indicates the semimetallic behavior.
Generally, the Seebeck coe cients increase with the increase in annealing temperature as shown in Figures 13 In:  is may be due to the better crystallinity and elimination of grain boundaries of the thin lms at higher annealing temperatures.
e Seebeck coe cient of the deposited lms has found to be negative, that is, electrons are predominant charge carriers which con rm the n-type behavior of indium selenide and aluminum selenide thin lms [24].

Conclusion
InSe bilayer thin lms with di erent thickness ratios have been prepared by the vacuum evaporation technique.e lms were uniform and had good adherence to the substrate.e XRD of InSe lms con rms the formation of InSe.SEM images show that the as-deposited lms are smooth, while   after annealing, the thin films become polycrystalline in nature.e bandgap energies of InSe thin films vary with the variation in thickness ratio and annealing temperature.e increase in conductivity with an increase in the temperature confirms the semiconducting behavior of the InSe films.Also, conductivity increases with an increase in the In : Se ratio in the films.e negative Seebeck coefficient confirms the n-type behavior of the deposited InSe and AlSe thin films.

Figure 3 :
Figure 3: Experimental setup for calculation of thermoelectric power.

Figure 7 :
Figure 7: Variation in electrical conductivity with In:Se ratio.

Table 2 :
Bandgap variation of InSe with different annealing temperatures.

Table 1 :
Bandgap variation of InSe with different thickness ratios.