Optical and energy properties of CdSe_1-xS_x thin films obtained by the method of high-frequency magnetron sputtering

1. Department of General Physics, Lviv Polytechnic National University, 12 Stepan Bandera St., Lviv, 79013, Ukraine
2. Faculty of Electronics and Computer Sciences, Koszalin University of Technology, ul. Śniadeckich 2, 75‑453 Koszalin, Poland
3. Faculty of Civil Engineering, Environmental and Geodetic Sciences, Koszalin University of Technology, ul. Śniadeckich 2, 75‑453 Koszalin, Poland

Abstract

CdSe_1-xS_x (x = 0, 0.3, 0.4, 0.6, and 1) thin films were deposited on a quartz and silicon substrate using high-frequency magnetron sputtering. X-ray diffraction analysis estimated that the CdSe_1-xS_x thin films are crystallized in a hexagonal structure [structure type – ZnO, space group P6₃ mc (No. 186)]. Spectral dependence of the optical transmittance between 300 and 1500 nm of the obtained thin films at room temperature was measured. Normalized integral optical transmittance, optical band gap, spin-orbit splitting, and the value of the bowing parameter of the CdSe_1-xS_x thin films are determined. The values of the optical band gaps for CdSe_1-xS_x thin films were estimated using the two methods (by Tauc plot and dT/dλ). Concentration dependences of the energy gaps connected with the leading optical transitions in CdSe_1-xS_x (Г₈ ^v–Г₆ ^c,Г₇ ^v–Г₆ ^c) and spin-orbit splitting are studied. It is shown that the concentration dependences of main optical transitions are quadratic. The principal explanation for this seems to be the Burstein–Moss effect, which is caused by the doping atoms' excess carriers (electrons and holes).

Introduction

One of the perspective materials used for application in solar cells are cadmium chalcogenides (CdTe, CdSe, and CdS) thin films [1–3]. Usually, solar cells are constructed in a ‘sandwich’ form [4]. Such forms have active elements as transparent front contact, light-absorbing layer, window layer, and back contact [4]. Usually, for the light-absorbing layer cadmium telluride is used and for the window layer – cadmium sulfide (CdS/CdTe heterojunctions) [3]. As is well known, CdTe was crystallized in a cubic crystal structure [5] and CdS in a hexagonal one [6]. Lattice incoherence between CdTe and CdS layers can cause a photocurrent occurrence, negatively affecting the work of the solar cell. The formation of CdTe-CdS solid solutions between these layers reduces the lattice incoherence, but a high defect density causes a loss of efficiency [7]. CdSe can be an alternative solution to the problems originating from the CdTe/CdS junction [8, 9]. Using the cadmium selenide in solar cells can enhance the short circuit current density (J_sc ) [10–14], but the open circuit voltage (VOC) and fill factor (FF) decrease [10–12]. Using a CdS/CdSe as a window layer can enhance J_sc and maintain VOC, thus improving device efficiency [10]. As a result, the optical and energy parameters of the CdSe_1-xS_x thin films still need to be studied.

Previously, the authors reported on the synthesis and crystal structure of the CdSe_1-xS_x thin film using the highfrequency (HF) magnetron sputtering method [15]. Here, the authors present the results of the experimental estimate of the leading optical properties of CdSe_1-xS_x thin films deposited by HF magnetron sputtering.

Details of experiment

CdSe_1-xS_x (x = 0, 0.3, 0.4, 0.6, and 1) thin films were deposited on quartz and silicon substrates (16 × 8 × 1.1 mm³ ) by the HF magnetron sputtering (13.6 МHz) using a VUP- 5M vacuum station (Selmi, Ukraine). Details of the thinfilm deposition condition and crystal structure were reported in [15] and some data from these measurements are listed in the Supplementary file. The spectral dependence of the optical transmittance of the obtained samples in the visible and near-infrared regions is studied at room temperature using a Shimadzu UV-3600 spectrometer.

Results and discussion

The spectral behaviour of the optical transmission of the CdSe_1-xS_x thin-film-substrate (quartz) system is shown in Fig. 1(a). A sharp increase in the transmission coefficient in the range of wavelengths from ~ 480 nm (x = 1) to ~ 760 nm (x = 0) can be observed. This growth is characteristic of the edge of fundamental absorption. Additionally, the optical spectra are highly influenced by the film structure, which is, in turn, shaped by the preparation method, film thickness, and deposition conditions. The transmission spectra of the thin films display periodic peaks due to interference effects. This highlights the high structural quality of the thin films.

The normalized integrated transmission was calculated by (1):

\( T_{\mathrm{aver}}=\frac{1}{b-a} \int_a^b T \cdot d \lambda \) (1)

where Taver is the average transmission [see Fig. 1(b)]. The concentration dependence of the integral transmission value is described by a parabolic dependence with an upward deviation from the linear one.

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Various methods were employed to determine the optical band gaps of the semiconductor films. One is the Tauc method [see Fig. 2(a)]. This method is based on a dependence of (α · hv)² = f (hv) [16]. Additionally, the optical band gap energy can be determined from the maximum position in the transmittance first derivative (dT/dλ). The estimated band gap values of the studied CdTe_1-xSe_x thin films obtained by the two above-mentioned methods show a good correlation (see Table 1).

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From Fig. 2(a), the value of the direct zone-zone optical transition ( E_g dir) was determined, and it is denoted as: Г₈ ^v–Г₆^c ( E_g) [17, 18]. It should be noted that the spectra of the absorption coefficient in Tauc coordinates [see Fig. 2(a)] show a break in linear behaviour at energies higher than E_g. This can be attributed to another optical transition, which corresponds to Г₇ ^v–Г6 с ( E_g2) [17, 18]. To confirm this assumption, the method of the first derivative of optical transmission spectra was used [see Fig. 2(b)]. From the example shown in Fig. 2(b), an intense peak that corresponds to the E_g, and a second one, less intense one that corresponds to the optical E_g2 can be observed. From the established optical transitions E_g and E_g2, it is easy to establish the amount of spin-orbit splitting (E_SO = E_g2 − E_g). The values of the optical band gaps and spin-orbit splitting are listed in Table 1.

Correlation analysis of the value of spin-orbit splitting with known literature data for a single crystal (see Table 2) indicates a decrease in the E_SO value when going from a single crystal sample (3D) to a thin film (2D). It can be assumed that this effect is associated with a decrease in the dispersion of energy levels in the thin film [18].

The concentration dependences of the optical transitions E_g, E_g2, and the value of E_SO are presented in Fig. 3. These dependences are not linear, being rather described by quadratic functions [see the legends of Fig. 3 and (2)]. Deviation from linear dependence is a well-known phenomenon commonly observed in ternary alloy thin films. The main reason is the difference in lattice constants between the “parent” and their substitute atoms [17, 18].

\( E(x)=x \cdot E_{\mathrm{CdS}}+(1-x) \cdot E_{\mathrm{CdS}}-\delta \cdot x \cdot(1-x) \) (2)

where δ is the bowing parameter. This bowing parameter can be decomposed into components [17, 18, 21]:

1. The component is related to the volume deformation effect, which is responsible for the change in the band gap of individual components of CdSe and CdS in a solid solution ( δ_VD).
2. The component is related to the redistribution of charge between different bonds in the disordered solid solution (δ_CE).
3. The component describes the change in band gap upon relaxation of the positions of ions in the lattice of the solid solution (δ_SR).
4. The component describes the change in band gap upon relaxation of the positions of ions in the lattice of the solid solution (δ_SR).

\( \delta=\delta_{\mathrm{VD}}+\delta_{\mathrm{CE}}+\delta_{\mathrm{SR}} \) (3)

The value of the bowing parameter and its comparative analysis are given in Table 3. Attention should be paid to the sign of the bowing parameter, which is negative (δ < 0). Similar behaviour of the bowing parameter was obtained for other solid solutions [22, 23]. The negative value of the bowing parameter means that the electron, excited from the valence band to the conduction band, should obtain a higher energy value than the band gap value. The manifestation of the Burstein can explain this phenomenon – Moss effect [24] which is associated with an excess of carriers (electrons and holes), which may originate from structural defects. These excess carriers lead to an increase in the band gap E_g.

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For a more detailed analysis of the nature of deviation of the optical transitions E_g, E_g2 from a linear dependence, the components of the bowing parameter should be determined. According to the method described in [21, 26], the parameters δ_VD, δ_CE, and δ_SR can be determined by (4)– (6), respectively:

\( \delta_{\mathrm{VD}}=\frac{E_{\mathrm{CdS}}^*-E_{\mathrm{CdS}}}{1-x}+\frac{E_{\mathrm{CdSe}}^*-E_{\mathrm{CdSe}}}{x} \) (4)

\( \\delta_{\mathrm{CE}}=\frac{E_{\mathrm{CdS}}}{1-x}+\frac{E_{\mathrm{CdSe}^2}}{x}-\frac{E_{\mathrm{CdSe}_1-x \mathrm{~S}_x}}{x(1-x)} \) (5)

\( \delta_{\mathrm{SR}}=\frac{E_{\mathrm{CdS}_{\mathrm{e}-x} \mathrm{~S}_x}-E_{\mathrm{CdS}_{\mathrm{e}_1-x} \mathrm{~S}_x}}{x(1-x)} \) (6)

In (4)–(6), the top symbol (*) means the equilibrium energy of the alloy with the composition index x (the related data are indicated in the legends of Fig. 3). The determined values of the components of bowing parameter are given in Table 4.

Having obtained the components of the bowing parameter (see Table 4), it can be observed that a minor contribution to the bowing parameter is made by the component associated with the deflection due to the volume deformation effect ( δ_VD). On the other hand, for CdSe_0.7S_0.3 and CdSe_0.6S_0.4 (x < 0.5) thin films, the determined contribution to the bowing parameter is the deflection coefficient which describes the change in the band gap upon relaxation of the ion positions in the solid solution lattice (δ_SR). However, in the CdSe0.4S0.6 thin film (x > 0.5), the bowing parameter is associated with the charge redistribution between different bonds in the disordered solid solution.

CdSe_1-xS_x thin films have attracted significant interest in recent years [27–31]. Based on the information in [27–31], the authors can assume that the presented research results may have applications in photovoltaics. Specifically, these solid solutions of CdSe_1-xS_x can form at the interface between the optical “window” (CdS or CdSe_1-xS_x [27]) and the absorbing layer (CdTe or CdSe_1-xTe_x [27]). Reference [32] reports the highest efficiencies of the (CdSe, CdTe)-junction solar cells at 26.55 % and 23.69 %, respectively. To achieve higher solar cell efficiency, one can choose compounds ( CdSe_1-xS_x, CdSe_1-xTe_x)-crystalline alloy junction solar cells instead of (CdSe, CdTe)-junction solar cells [27]. In particular, Reference [27] demonstrates that the maximal efficiencies (34.375 % (33.72 %)) were obtained in CdSe_1-xS_x, CdSe_1-xTe_x-crystalline alloy junction solar cells at 300 K.

Conclusions

Optical spectra of the CdSe_1-xS_x thin films deposited by HF magnetron sputtering were studied, as well as quartz and silicon were used as substrates for deposition films. The composition ratios of the thin films were established using the X-ray fluorescence spectroscopy (XRF) method (CdSe_0.7Se_0.3, CdSe_0.6Se_0.3, and CdTe_0.4S_0.36). X-ray diffraction (XRD) analysis estimated that the CdSe_1-xS_x thin films are crystallized in a hexagonal structure. The spectral dependence of the optical transmittance between 300 and 1500 nm of the CdSe_0.7Se_0.3, CdSe_0.6Se_0.3 and CdTe_0.4S_0.36 thin films at room temperature was measured.

The energy dependence of the optical absorption in the (α · hv)² = f(hv) coordinate demonstrates the presence of the fundamental absorption edge with the band gap of a direct type for all studies of thin films. The values of the optical band gaps for CdSe_1-xS_x (x = 0 − 1) thin films were also estimated using the dT/dλ. Both methods of the band gap estimation indicate good correlation and show an increase of the band gap with increasing of sulphur content in the thin films studied. An increasing character of the band gap dependence on the sulphur content E_g(x) was found in the optical studies. The value of spin-orbital splitting for CdSe_1-xS_x thin films was calculated based on the main optical energy transitions. A small deviation from the Vegard’s law with bowing has been observed for the main optical transitions (Г₈ ^v–Г₆ ^c,Г₇ ^v–Г₆ ^c) and spin-orbital splitting. This deviation is mainly due to the relaxation of the ion positions in the solid solution lattice (δ_SR) for x < 0.5, and to the charge redistribution between different bonds in the disordered solid solution (δ_CE) for x > 0.5.

Acknowledgements

This research was supported by the National research foundation of Ukraine (project no. 2022.01/0163).

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