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Publicly Available Published by De Gruyter July 11, 2019

Synthesis and Non-Destructive Characterization of Zinc Selenide Thin Films

  • Brijesh Kumar Yadav EMAIL logo , Pratima Singh and Dharmendra Kumar Pandey

Abstract

The present work encloses the deposition of three zinc selenide (ZnSe) thin films of thickness 175 nm, 243 nm, and 286 nm using thermal evaporation technique under a vacuum of 5 × 10−5 mbar. The deposited ZnSe thin films are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), surface profilometer, ultraviolet (UV)-visible (Vis)-near-infrared (NIR) spectrophotometer and Raman spectroscopic measurements. The structure and morphology measurements reveal that the deposited ZnSe material is nanocrystalline having a cubic structure whose crystallinity increases with an increase in film thickness/evaporation rate. The optical band gap estimated from the optical transmission spectra of the films is found to be 2.62 eV, 2.60 eV, and 2.57 eV, respectively, which decreases with an increase in film thickness. The estimation and polynomial curve fit analysis of refractive index, extinction coefficient, and dielectric constant indicates that these physical quantities are fifth-order polynomial function of wavelength. The obtained results are compared and analysed for justification and application of ZnSe thin films.

1 Introduction

Metal chalcogenide thin films attracted considerable attention as promising linear and nonlinear optical (NLO) materials due to their novel properties such as wide band gap, significant absorption coefficients, high chemical, thermal stability, and environment-friendly applications. The optical study of the ZnSe thin film is found to be of keen interest to researchers due to their wide applications in various optoelectronic devices such as blue-green light-emitting diodes, laser diodes, solar cells, thin-film transistors, etc. [1], [2], [3], [4], [5], [6], [7]. It is a crystalline semiconducting material and has a direct band gap of ∼2.70 eV at room temperature with high optical transparency. The ZnSe thin film is non-toxic in nature. Because of having a comparable conduction band with other window layer materials of solar cells, it transmits higher energy photons towards the absorber layer for the enhancement of light absorption [8], [9]. It also possesses a high refractive index and shows less optical absorptions in the visible (Vis) and infrared (IR) regions, which makes it also suitable for blue, green, and red light emitters, laser screens, ultrasound transducers, etc. It is a widely valuable material for optoelectronic applications at large scale in the photovoltaic industry as replacement of CdS window layers in the preparation of highly absorbed CdTe- and Cu(In, Ga)Se2-based thin-film solar cells for reducing environmental problems because ZnSe is a less toxic buffer layer material than CdS [8], [9], [10], [11]. The thin films can be prepared by different methods such as electrodeposition [12], spray pyrolysis [13], sputtering [14], atomic layer deposition [15], thermal evaporation under vacuum [16] methods, etc. Among these methods, the thermal evaporation method is an effective and costless method to prepare the thin films at room temperature. This method produces stable, ordered, defectless, and good bonding materials.

The structural and optical properties of the ZnSe thin films were investigated in detail by different researchers [17], [18], [19], [20]. Hankare and co-workers studied the structural, optical, and electrical properties of the ZnSe thin films and reported a comparison between the as-deposited and the annealed thin films [17]. They deduced that the thin films show good optical absorption and electrical conductivity with the n-type conduction mechanism. The effect of a reducing environment was studied on the chemically grown ZnSe thin films, and a slight change in the structural and optical properties [18] was found. However, the structural and optical properties of the thin film are also reported to be influenced by varying the concentrations of the Zn/Se contents [19]. Rusu and his co-workers reported that the values of roughness of the ZnSe thin films depend on thickness and post annealing temperature [20].

The effect of concentration on the thin-film thickness was studied in detail, but the effect of deposition time on the film thickness and its properties still needs to be investigated according to the knowledge of the author. In addition, several work were performed in the field of structural, morphological, and optical characterisation of ZnSe thin films; however, the wavelength and size-dependent refractive index, extinction coefficient, and dielectric constant of the thin film still needs to be investigated. Thus, the present work is focussed on the deposition and characterisation of three ZnSe thin films having different thicknesses. The structural and morphological characterisation of the thin films is carried out using X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive spectroscopy (EDS) measurements. The optical properties of these films are monitored using ultraviolet (UV)-Vis-near-infrared (NIR) transmission and Raman spectroscopic techniques. The optical transmittance spectra of the chosen thin films are tailored for the estimation of band gap, refractive index, extinction coefficient, and dielectric constant. The obtained structural and optical properties of the selected thin film are analysed under the variation of evaporation rate, film thickness, and wavelength of light for the enhancement of applicability and suitability of thin films in a new dimension.

2 Experimental Method

The zinc selenide thin films were deposited on a glass substrate by thermal evaporation technique under high vacuum at room temperature for time intervals 35, 50, and 60 s using a vacuum coating unit model 12A4D-T “HIND HIVAC” Co (P) Ltd, Bangaluru, Karnataka, India at IIT Kanpur (India). The glass substrate was cleaned rigorously with acetone prior to deposition and rinsed in distilled water. The zinc selenide powder purchased from Sigma-Aldrich Company, Bangaluru, Karnataka (India) (99.9 % pure), glass substrate, molybdenum boat, high vacuum 5 × 10−5 mbar, roughing/backing vacuum 5 × 10−2 mbar, primary current 6.0–6.5 A, and secondary current 110–120 A were the important parameters used in the deposition. A surface profilometer (Veeco Dektak 150, Plainview, NY, USA) apparatus was used to measure the thickness of the deposited films. The XRD measurements of the deposited thin films (S1, S2, and S3) were carried out with a Rigaku Ultima IV (Tokyo, Japan at IIT Roorkee, Uttarakhand, India) using Cu–Kα radiation (λ = 1.5405 Å). The XRD patterns were recorded in the 2θ angular range of 10–80° with a step size of 0.02°. The SEM (Carl Zeiss EVO 40, Cambridge, UK at Jawaharlal Nehru University, New Delhi, India) was used to record the surface morphology of the zinc selenide thin films. The UV–Vis–NIR double-beam spectrophotometer (Model Cary 500 make, Aligent Technology at DMSRDE-DRDO, Kanpur, India) was used to measure the quality of thin films for optical transmission capability in the spectral range of 200–1600 nm. Raman measurements were taken using Renishaw inVia Raman microscope at Delhi University, New Delhi, India in the range of 100–1200 cm−1.

3 Results and Discussion

3.1 Structural Characterization

3.1.1 Thickness Measurements

The measurements made on deposited thin films with a surface profilometer reveal that the films having deposition times of 35 s (S1), 50 s (S2), and 60 s (S3) have 175 nm, 243 nm, and 286 nm thickness, respectively. Thus, the deposition time governs the film thickness. As the source heating current melts the material, the pressure of the chamber and the distance between the source and substance are kept constant; therefore, the deposition rate becomes fixed. Hence, the thickness of the film is found to increase with enhancement in deposition time.

Figure 1: XRD patterns of the ZnSe thin films for samples S1, S2, and S3.
Figure 1:

XRD patterns of the ZnSe thin films for samples S1, S2, and S3.

Figure 2: SEM micrograph of the ZnSe thin films for sample S1 (left), S2 (right), and S3 (mid-centre).
Figure 2:

SEM micrograph of the ZnSe thin films for sample S1 (left), S2 (right), and S3 (mid-centre).

3.1.2 XRD Measurements

The XRD measurements are very important approaches to analyse the crystallite size and phase of the grown thin films. The recorded XRD pattern of the deposited thin films is shown in Figure 1, which is found to match well with ICDD JCPDS File no. 37-1463 [21]. The comparison of the recorded XRD pattern with the JCPDS file indicates that the phase of the thin films is of a Ctype cubic with lattice parameters a = 5.668 Å and space group F4¯3m (216). It also confirms that these ZnSe thin films are polycrystalline in nature. A slight shifting in the XRD peaks towards the 2θ angle side for the lattice plane (111), and the full width at half maxima (FWHM) of the XRD peaks is obtained to decrease with an increase in film thickness (inset in Fig. 1). The crystallite size (D) of the ZnSe thin films was calculated using the Debye–Scherrers’ formula from the FWHM (β) of the peaks expressed in radians:

(1)D=Kλβcosθ

where the constant K is equal to 0.94, λ is the X-ray wavelength, β is the FWHM, and θ is the diffraction angle. The values of the crystallite size are found to be 13 nm, 15 nm, and 17 nm for the samples S1, S2, and S3, respectively. This indicates that the crystallite size of the film increases when increasing the film thickness/deposition time [22].

3.1.3 SEM and EDS Measurements

The SEM technique was employed to study the surface morphology of the thin films. Figure 2 represents the SEM micrograph of the ZnSe thin films (S1, S2, and S3). This shows that the deposition time affects the surface morphology of the ZnSe thin films. The SEM micrographs of the film samples S1 and S2 indicate that particles in these thin films are spherical in shape having average particle sizes of 175 nm and 225 nm, respectively. Thus, the SEM micrograph reveals that particles of the ZnSe thin film are approximately spherical in shape, and their size increases with deposition time. Very few particles of the film of Sample S2 are not perfectly spherical. In addition, film sample S3, having greater thickness, is found to be made up of v-shaped/nano-flower-like structures. The change in shape of the particles from spherical to v-shaped flower-like structures with increase in deposition time/thickness is due to random coagulation of ZnSe atoms caused by enhancement of ZnSe concentration.

Figure 3: The EDS spectrum of the ZnSe thin film sample (S2).
Figure 3:

The EDS spectrum of the ZnSe thin film sample (S2).

Figure 4: Optical transmittance spectra of the ZnSe thin film samples S1, S2, and S3.
Figure 4:

Optical transmittance spectra of the ZnSe thin film samples S1, S2, and S3.

On increasing the deposition time, the nucleation of the ZnSe particle increases due to enhancement in the concentration of ZnSe. After the formation of stable ZnSe nuclei, crystal growth starts. Because of this reason, the average particle size increases with deposition time or film thickness in the initial stage, and later on, it becomes a v-shaped structure. The similar characteristics of particles in the ZnSe thin films with concentration were reported in literature [23]. Although the particles are distributed randomly, the film has a uniform structure. Thus, through the SEM photograph, it is realised that the deposited films are free from cracks and deformations. Figure 3 shows the EDS spectrum of the prepared thin-film sample (S2), which confirms the presence of Zn and Se elements in the film. The presence of the Au element in the spectrum is due to the Au coating made during the SEM measurement. The spectrum clearly demonstrates that the elements used during the preparation are completely present in the film. The spectrum also shows that the prepared thin film does not contain any additional impurity element.

Figure 5: Plots (αhν)2∼hν for the thin films S1, S2, and S3.
Figure 5:

Plots (αhν)2hν for the thin films S1, S2, and S3.

3.2 Optical Characterization

3.2.1 Optical Transmittance Measurements

The optical transmittance spectra of the ZnSe thin films S1, S2, and S3 are recorded with a UV-Vis spectrophotometer in transmittance mode in the range of 200–1600 nm and are shown in Figure 4. The transmission spectrum consists of a few numbers of distinct fringes ranging from mid-Vis to IR regions whose intensity varies between 75 % and 85 %. These fringes are due to interference caused by superposition of the waves originating from the air–film, film–substrate, and substrate–air interface medium. The re-distribution of energy due to interference causes maxima and minima in the spectrum of the transmitted region. The transmission spectrum also indicates that the transmittance of the thin film decreases upon increasing the thickness as the maximum for film S1 and minimum for film S3 are found. According to Fresnel’s theory of reflection/transmission of electromagnetic waves, the amplitude ratio of the transmitted and incident electromagnetic waves or transmittance of the medium decreases with increase in refractive index of the concerned medium. Thus, the reduction in transmittance reveals the enhancements in refractive index of ZnSe thin films. As the maxima in the transmittance spectrum are found at un-equidistant wavelengths, therefore, the energy bands of the ZnSe thin films will not be distributed uniformly.

We also calculated the optical band gap for the thin films using the Wood and Tauc formula [24]:

(2)α=B(hνEg)mhν

where Eg, hν, and B are the optical band gap, energy of radiation, and tailoring band constant, respectively. The constant m has a value of 1/2 for the direct band gap of the allowed transition [25]. We plotted the curves of (αhν)2hν for the deposited thin films (S1, S2, and S3), and the obtained curves are shown in Figure 5. The optical band gap for the thin-film samples S1, S2, and S3 are found to be 2.62 eV, 2.60 eV, and 2.57 eV, respectively. This indicates that the optical band gap decreases with an increase in the thickness/deposition time of the film. This decrease in band gap with increase in thickness can be explained by the crystallite size effect. Higher thickness clearly leads to an improvement in the crystallinity as well as grain size. When the film thickness increased, the concentration of the structural defect is reduced considerably due to the enlarged crystallite size. The similar result and characteristics of the band gap with thickness are also reported in the literature [8], [22]. Therefore, the optical band gap of the deposited ZnSe thin film will decrease with increase in crystallite size/thickness of the film.

Figure 6: Refractive index of ZnSe thin films S1, S2, and S3.
Figure 6:

Refractive index of ZnSe thin films S1, S2, and S3.

3.2.2 Refractive Index, Extinction Coefficient, and Dielectric Constant Calculations

The optical constants of the thin film are primary parameters for transmitting the light waves, such as the refractive index (n), extinction coefficient (k), and dielectric constant (ε). The refractive index is an important property of the thin film and signifies the amount of light transmitting through it. The spectral dependence of the refractive index (n) of the films was calculated using Swanepoel’s envelope method with the help of the following expression [26].

(3)n=[N+(N2S2)1/2]1/2;N=2S(TMTmTM.Tm)+(S2+1)2

Here, TM and Tm represent the maximum and minimum transmittance at a given wavelength, respectively, and S is the refractive index of the glass substrate. The minimum and maximum values of the refractive index of the film for a selected range of wavelength are found to be 2.22–2.29, 2.34–2.41, and 2.48–2.49 for the samples S1, S2 and S3, respectively. It is clear from Figure 6 that the refractive index increases with increasing the film thickness.

Figure 7: Extinction coefficient of ZnSe thin films S1, S2, and S3.
Figure 7:

Extinction coefficient of ZnSe thin films S1, S2, and S3.

The extinction coefficient (k) measures the absorption of light by the medium. Numerically, it is equal to αλ/4π. Here, α is the absorption coefficient and can be obtained with the help of transmittance as α=d1lnT1. Here, d is the thickness of the film [23]. The calculated values of k are shown in Figure 7. The minimum and maximum values of k for the films S1, S2, and S3 in the selected range of wavelength are 0.137–0.210, 0.111–0.123, and 0.047–0.117, respectively. From Figure 7, it is clear that the value of the extinction coefficient decreases with increasing film thickness. The polynomial curve fit analysis of the refractive index and extinction coefficient indicates that these physical quantities are fifth-order polynomial functions of the wavelength (nork=i=05Aiλi). Here, the value of coefficient Ai depends on the deposition time or film thickness. This reveals that the thin films do not follow the Cauchy’s relation between the refractive index and wavelength of the electromagnetic wave. In other words, we can also say that the thin film reduces to an isotropic medium if the thickness or deposition time is such that the powers of wavelength seize to two unique values 0 and −2.

Figure 8: Real dielectric constant of ZnSe thin films S1, S2, and S3.
Figure 8:

Real dielectric constant of ZnSe thin films S1, S2, and S3.

Figure 9: Imaginary dielectric constant ZnSe thin films S1, S2, and S3.
Figure 9:

Imaginary dielectric constant ZnSe thin films S1, S2, and S3.

The dielectric constant is a well-known fundamental intrinsic property of a material. The wave propagation vector is a complex number (n + ik) for an anisotropic medium or thin film. Therefore, the dielectric constant of the medium/thin film reduces to (n + ik)2 with a real value ε1=n2k2 and an imaginary value ε2=2nk [27]. Although the real part of the dielectric constant is much larger than the imaginary part, both enforce the property of slowing down the speed of light in the medium. The estimated real and imaginary parts of the dielectric constant are shown in Figures 8 and 9. The minimum and maximum values of the ε1 for the films S1, S2, and S3 in a selected range of wavelengths are found to be 4.94–5.23, 5.49–5.83, and 6.107–6.193, respectively, whereas, the minimum and maximum values of the ε2 for the films S1, S2, and S3 in a selected range of wavelengths are found to be 0.61–1.011, 0.524–0.598, and 0.235–0.585, respectively. These values are very close to the earlier reported values for the ZnSe thin film [26], [28], [29]. The slight variation in these values is due to a different thickness of the present synthesized ZnSe thin film. The polynomial curve fit analysis of real and imaginary dielectric constants indicates that the physical quantities ε1 and ε2 are also fifth-order polynomial functions of the wavelength.

3.2.3 Raman Measurements

Figure 10 depicts the Raman spectra of the deposited film samples S1, S2, and S3. The sample gives various bands centred at 206, 253, 500, 780, and 1120 cm−1. In the spectra, the band at 253 cm−1 is more intense compared to the other bands. This band is assigned to arise due to the longitudinal optical (LO) phonon frequency, which is in consistent with the earlier reported band of the bulk ZnSe material [29], [30]. It indicates the formation of polycrystalline thin films of the ZnSe material. It is clear from the inset figure that the intensity of all the bands increases regularly when increasing the thickness of the thin film.

Figure 10: Raman spectra of ZnSe thin films S1, S2, and S3.
Figure 10:

Raman spectra of ZnSe thin films S1, S2, and S3.

It is also clear from the inset that the FWHM of the band is similar for the thin films S1 and S2. However, it is slightly blue shifted in the case of thin film S3, which is due to a change in the local crystal structure of the thin film. As the thickness increases, the crystal structure of the film changes simultaneously. Kathalingam et al. also studied the effect of thickness on the Raman shift and reported a blue shift when increasing the thickness of the film [19]. There are three other modes of LO phonon frequency observed at 500, 780, and 1120 cm−1 with relatively weak intensity in the Raman spectra. The spectra also contain a weak band centred at 206 cm−1, and it is due to a transverse optical (TO) phonon frequency of the ZnSe film [31], [32].

4 Conclusions

The present structural and morphological study of the deposited ZnSe thin film by thermal evaporation technique, having thicknesses of 175 nm, 243 nm, and 286 nm, show the nano-crystalline nature of the films as the crystallite size is found to be 13, 15, and 17 nm, respectively. The crystallinity of the films is found to increase with an increase in the film thickness/evaporation rate. The SEM/SEM-EDS measurements justify the formation of spherical particles with a smooth surface containing Zn and Se elements in the present ZnSe thin film. The thickness of the thin film is found to increase with the deposition time/evaporation time. The optical study reveals that the optical band gap of the ZnSe thin film decreases upon increasing the thickness of the film. The estimation and polynomial curve fit analysis of the refractive index, extinction coefficient, and dielectric constant confirms that these physical quantities are fifth-order polynomial functions of the wavelength. Raman measurement reveals that the present ZnSe film is a polycrystalline thin film having a characteristic band at 253 cm−1. A slightly blue shift in the band is found with an increase in the thickness of the film. It is also found that the optical transmittance efficiency of the present synthesized ZnSe film is up to 85 %. Our results can be used for further investigation and application of the ZnSe thin films to open a new dimension in the field of optoelectronic industries.

Acknowledgements

The authors wish to acknowledge Prof. K. Srinivas, Department of Physics, University of Delhi, New Delhi, India, for providing the Raman measurement facility. The authors are also grateful to Dr. Gobardhan Lal, DMSRDE-DRDO Kanpur, India, for providing the UV-Vis-NIR measurement facility. The authors are also highly thankful to Dr. Amit Kumar Srivastav, Principal, D.A.-V. College, Kanpur, for providing the necessary facilities and encouragement during the course of work.

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Received: 2019-04-06
Accepted: 2019-06-04
Published Online: 2019-07-11
Published in Print: 2019-11-26

© 2019 Walter de Gruyter GmbH, Berlin/Boston

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