# Preparation and Characterization of CoevaporatedCd1−xZnxS Alloy Thin Films

Authors: Wei Li; Jiayi Yang; Zhen Sun; Lianghuan Feng; Jingquan Zhang; Lili Wu
Journal: International Journal of Photoenergy
 (2011)

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## Abstract

Cd1-xZnxS thin films have been prepared by the vacuum coevaporation method. The structural, compositional, and optical properties of Cd1-xZnxS thin films have been investigated using X-ray diffraction, X-ray fluorescence, and optical transmittance spectra. As-deposited Cd1-xZnxS thin films are polycrystalline and show the cubic structure for x=1 and hexagonal one for x<1 with the highly preferential orientation. The composition of Cd1-xZnxS thin films determined from Vegard's law and quartz thickness monitors agrees with that determined from the X-ray fluorescence spectra. Optical absorption edge of optical transmittance for Cd1-xZnxS thin films shows a blue shift with the increase of the zinc content. The band gap for Cd1-xZnxS thin films can be tuned nonlinearly with x from about 2.38 eV for CdS to 3.74 eV for ZnS. A novel structure for CuInS2-based solar cells with a Cd0.4Zn0.6S layer is proposed in this paper.


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## Body

## 1. Introduction

ZnS-based II-VI materials can form ternary alloys, such asCd1-xZnxS, with a direct fundamental band-gap range from 2.4 to 3.7 eV at room temperature. The energy position of the conduction band is changed primarily by varying the Zn content. Therefore, these materials have potential applications for efficient electron-confined structures, photovoltaic heterojunction, and electro-optical devices.In photovoltaic system, the replacement ofCdS with the higher band-gap Cd1-xZnxS has led to a decrease in window absorption losses and has resulted in an increase in the short-circuit current [1]. This Cd1-xZnxS ternary compound is also a useful buffer material for fabrication of p-n junctions without lattice mismatch [2].Cd1-xZnxS thin films have been prepared by different techniques, which include evaporation [2–5], chemical bath deposition [1, 6, 7], and organic vapor deposition [8]. However, very little information is available on the ternary Cd1-xZnxS alloy films with the desired composition prepared by the vacuum evaporation technique. Cd1-xZnxS thin films, to our best knowledge, have been prepared by vacuum evaporation involved with three methods, one of which is the thin films being prepared by using a mechanically alloyed mixture of CdS and ZnS [3]. It is obvious that a homogenous mixture of Cd1-xZnxS is difficult to obtain. So Kumar et al. [4] synthesized the fine powders of Cd1-xZnxS by solid-state reaction and then deposited Cd1-xZnxS thin films. Considering the large difference in the vapor pressures of CdS and ZnS, Torres and Gordillo [5] prepared Cd1-xZnxS thin films by evaporation, varying the diameter of the openings of the coaxial chambers which contain the CdS and ZnS precursors. However, homogenous Cd1-xZnxS thin films with desired content are not readily deposited by the evaporation mentioned above. In this work, Cd1-xZnxS(0≤x≤1) thin films have been prepared by using a coevaporation of CdS and ZnS from two horizontal sources, which allows independent control of the temperature and consequently control of the partial vapor pressures of both precursors. The structural, compositional, optical properties have been investigated.


## 2. Experimental Details

Cd1-xZnxS thin films were deposited by the vacuum coevaporation technique, as described elsewhere [9], to a thickness of ~300 nm at room temperature. The vacuum system had a base pressure of 10-4 Pa and was partitioned into two parts, one for a CdS powder source (5 N purity) and the other for a ZnS powder source (5 N purity). The CdS and ZnS deposition rates were measured by separate LHC-2 quartz monitors. The deposition rate of ZnS was adjusted to obtain different Zn concentrations in CdS thin films, varying between 0 and 1. The thickness was confirmed precisely by a surface profilometer. X-ray fluorescence (XRF) spectrometry was subsequently used to determine the composition of each sample. The crystallographic phase and lattice constants of each film were obtained by X-ray diffraction (XRD). Measurements of the optical transmittance were made over the wavelength range 200~800 nm with a data interval of 0.5 nm. Transmittance measurements were also made with the beam at near-normal incidence to the sample to prevent reflected light being returned to the monochromator.


## 3. Results and Discussion

To accurately determine the composition of theCd1-xZnxS alloys, X-ray fluorescence measurements of Cd1-xZnxS thin films were also carried out (Figure 1). Table 1 shows composition of the Cd1-xZnxS thin films from XRF using the fundamental parameter (FP) method.Table 1
Composition ofCd1-xZnxS thin films.
No. 1No. 2No. 3No. 4No. 5From XRFCdS0.93Cd0.72Zn0.28SCd0.59Zn0.61SCd0.11Zn0.89SZnS1.12From Vegard’s lawCdSCd0.723Zn0.277SCd0.42Zn0.58SCd0.1Zn0.9SZnSFrom monitorsCdSCd0.7Zn0.3SCd0.4Zn0.6SCd0.1Zn0.9SZnSTypical XRF ofCd1-xZnxS(x=0.61) thin films. (a) Cd-Kα,  (b) S-Kα, and (c) Zn-Kα.
(a)(b)(c)Figure2 demonstrates X-ray diffraction spectra of Cd1-xZnxS(0⩽x⩽1) thin films obtained by using Cu Kα radiation, where only one diffraction peak with a 2θ value between 26.38 and 28.370  is present. The absence of diffraction peaks associated with CdS and ZnS suggests that those thin films are single-phase materials. The presence of sharp peaks in XRD patterns demonstrates the polycrystalline nature of these Cd1-xZnxS(0⩽x⩽1) thin films, which have a strong preferential orientation. From XRD patterns, one can see that the diffraction angle (2θ) shifts towards a slightly higher angle with increasing x (Zn content) in the thin films. This is due to the substitution of Zn2+  for Cd2+, in which different ionic radius of Cd2+(0.095nm) and Zn2+(0.074nm) [10] leads to the increase of lattice constants of Cd1-xZnxS thin films. Figure 2 also shows that the intensity of XRD peaks decreases at higher concentration of Zn. The structure of Cd1-xZnxS thin films varies continuously from being cubic zincblende material with a preferred orientation of (111) plane at the ZnS end to being hexagonal wurtzite with a preferential orientation of (002) plane when zinc content decreased.Figure 2
XRD patterns of as-deposited polycrystallineCd1-xZnxS thin films.Since the lattice constant ofCd1-xZnxS thin films follows Vegard’s law, the composition of Cd1-xZnxS thin films can be determined by(1)xCd1-xZnxS=aCd1-xZnxS-aCdSaZnS-aCdS×100%,
where aCd1-xZnxS,  aCdS, and aZnS are, respectively, the lattice constants of Cd1-xZnxS,CdS, and ZnS. From the d values of the X-ray diffraction spectra shown in Figure 2 the lattice constants can be calculated as follows:(2)d=a(4/3)(h2+hk+k2)+(a/c)2l2.Therefore, from (1) and (2), one can obtain the composition of Cd1-xZnxS thin films as shown in Table 1. It can be seen that there is a good agreement among the three sets of x values. As for the low angle diffraction line the variation of the diffraction angle with the composition is small and the determined film composition is not so accurate. The error is estimated to be about 0.05 in x. The precision suggests that we can control the composition of Cd1-xZnxS thin films, which are prepared by coevaporation of CdS and ZnS. In addition, the comparison of calculated d values from the diffraction spectra (Figure 2) with the standard d* values from the JCPDS XRD spectra data for Cd1-xZnxS is shown in Table 2.Table 2
XRD results forCd1-xZnxS thin films.
Cd1-xZnxS2θdd*(hkl)JCPDSx=026.4423.36213.367(002)65–3414x=0.2826.9933.29723.290(002)40–0836x=0.6127.6133.22373.214a(002)—x=0.8928.3473.14803.142a(002)—x=128.5333.12573.124(111)65–1691aCalculated from Vegard’s law.The optical transmittance spectra of theCd1-xZnxS films with different x values were obtained, and steep absorption edges were observed, demonstrating that the thin films are homogeneous (Figure 3). The optical transmittance of the Cd1-xZnxS films is typically 60% at wavelengths beyond the absorption edge. It can be seen that Cd1-xZnxS films show the blue shift in the absorption edge with the increase of the zinc content.Figure 3
UV-visible transmission spectra ofCd1-xZnxS films.From the transmittance curves of Figure3, the absorption coefficient α of the thin films is estimated. The absorption coefficient α with the energy of the photon hυ could be expressed by the following equation: (3)αhυ∝(hυ-Eg)m,
where Eg is the optical band gap of the thin films, and the exponent m may take values, 1,2,3,1/2,3/2, depending on the electronic transitions in k-space. A linear fit is achieved for the Cd1-xZnxS films with m=1/2, indicating that this material is a direct gap semiconductor. So (αhυ)2 is plotted as a function of hυ for various zinc content (not shown there). The optical band energy gap can be determined from the intercept of the extrapolation of the straight portion to the hυ axis.The band-gap dependence on composition is shown in Figure4. It can be seen that the band gap varies with Zn content in a nonlinear way. This dependence can be determined by fitting the band-gap values to a parabolic form on the method of least square [11] and described as  (4)Eg(x)=2.38+0.69x+0.67x2.Figure 4
The variation of energy gap of theCd1-xZnxS as a function of x.It gives the bowing parameter (b) of 0.67 eV, which is much smaller than that reported on another Cd1-xZnxS alloy system in thin films by Borse et al. [6] (b=0.85eV) and that in single crystal by Muhamad [12] (b=0.82eV), but agrees well with that in thin films by Yamaguchi et al. [1] (b=0.724 eV). In the present case of Cd1-xZnxS thin films, Eg values vary from 2.38 to 3.74 eV for x=0 to 1 in a nonlinear way. This change in the band gap of CdS by addition of Zn shows formation of a continuous series of solid solutions. Therefore, the band energy gap of Cd1-xZnxS can be adjusted in the range of the binary band gaps. And the energy position of the conduction band, that is, electron affinity χ, can be also adjusted by varying the zinc content [2].For the photovoltaic solar cells, CuInS2 has a desirable direct band gap of 1.5 eV, which is also well-matched with the solar spectrum. Efficiency of 11.4% for CuInS2-based solar cells has been successfully made [13]. However, further improvements on performance of CuInS2-based solar cells have been restricted by the conventional structure due to the lattice mismatch between ZnO and CuInS2. Therefore, an approach to overcome this problem is to introduce an effective buffer layer, Cd1-xZnxS, in CuInS2-based solar cells.Furthermore, according to Anderson’s model, the band alignments withχabsorber⩽χbuffer⩽χwindow are desired to prevent the formation of a conduction band spike in the CuInS2-based solar cells. For Cu-rich films, electron affinity for CuInS2 thin films is about 4.1 eV (Eg~1.53 eV) [14, 15], and for ZnO thin films is about 4.2 eV (Eg~3.2 eV), so the electron affinity of 4.1 eV for Cd1-xZnxS thin films (i.e., x=0.6) [2] is taken into account. Based on these mentioned above, we propose a modified structure of CuInS2-based solar cells. The energy band diagram of CuInS2-based solar cells is shown in Figure 5. Cd0.4Zn0.6S thin films (Eg~3.03 eV, from (4)) provide lattice constant matching to the absorber and reduce the interface state density between the p- and n-type materials, meanwhile, the conduction band and valence band spikes are eliminated too.Figure 5
Flat band diagram of aCuInS2/ZnO heterojunction with a Cd0.4Zn0.6S buffer layer.


## 4. Conclusions

Cd1-xZnxS thin films with the required composition have been successfully deposited on glass substrates by the vacuum coevaporation method. The thin films are found to be cubic at the ZnS end and hexagonal for x<1, which show the highly preferred orientation. The measurements of optical transmittance for Cd1-xZnxS films demonstrate the blue shift in the absorption edge, and the optical band gap increases from 2.38 to 3.74 eV in a nonlinear way as composition varies from x=0 to 1. And the energy position of the conduction band can be tuned by varying the zinc content. Therefore, a buffer layer Cd1-xZnxS for x=0.6 is introduced in the CuInS2-based solar cells to eliminate band spikes and provide lattice matching to the absorber.



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*Source: 969214-2011-11-21.xml*