Scholarly article on topic ' Preparation and Characterization of Coevaporated     Cd   1−x     Zn  x  S   Alloy Thin Films '

Preparation and Characterization of Coevaporated Cd 1−x Zn x S Alloy Thin Films Academic research paper on "Nano-technology"

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Academic research paper on topic " Preparation and Characterization of Coevaporated Cd 1−x Zn x S Alloy Thin Films "

Hindawi Publishing Corporation International Journal of Photoenergy Volume 2011, Article ID 969214, 5 pages doi:10.1155/2011/969214

Research Article

Preparation and Characterization of Coevaporated Cdi-xZnxS Alloy Thin Films

Wei Li, Jiayi Yang, Zhen Sun, Lianghuan Feng, Jingquan Zhang, and Lili Wu

College of Materials Science and Engineering, Sichuan University, Chengdu 610064, China Correspondence should be addressed to Wei Li, waylee2000@sohu.com Received 30 April 2011; Accepted 30 August 2011 Academic Editor: Leonardo Palmisano

Copyright © 2011 Wei Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

1. Introduction

ZnS-based II-VI materials can form ternary alloys, such as Cd1-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 of CdS 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 solidstate 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 coevap-oration 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 was partitioned into two

12 108-

20 (deg) (a)

1.6 -1.4 -1.2 1 0.8 0.6 0.4 0.2 0

7 6 )5

3 2 1 0

107 108 109 110 111 112 113 114 115

20 (deg)

20 (deg) (c)

Figure 1: Typical XRF ofCdi_xZnxS (x = 0.61) thin films. (a) Cd-Ka, (b) S-Ka, and (c) Zn-Ka.

parts, one for a CdS powder source (5N purity) and the other for a ZnS powder source (5N 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 the Cd1_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.

Figure 2 demonstrates X-ray diffraction spectra of Cd1_xZnxS (0 ^ x ^ 1) thin films obtained by using Cu Ka radiation, where only one diffraction peak with a 29 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 (29) shifts towards a slightly higher angle with increasing x (Zn content) in the thin films. This is due to the substitution of Zn2+ for Cd , in which different ionic radius of Cd2+ (0.095 nm) 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 ofZn. The structure of Cd1_xZnxS thin films varies continuously from being cubic zincblende material with a preferred

_No. 1

From XRF CdS0.93

From Vegard's law CdS

From monitors CdS

Table 1: Composition of Cd1-xZnxS thin films.

_No. 2_No. 3

Cd0.72Zn0.28S Cd0.59Zn0.6i S

Cd0.723Zn0.277S Cd0.42Zn0.58S

Cd0.7Zn0.3S Cd0.4Zn0.eS

No. 4_No. 5

Cd0.11Zn0.89S ZnSi.12

Cdo.1Zno.9S ZnS

Cd01Zn09S ZnS

10 20 30 40

20 (deg)

Figure 2: XRD patterns of as-deposited polycrystalline Cd1-xZnxS thin films.

200 300 400 500 600 700 800 Wavelength (nm)

Figure 3: UV-visible transmission spectra of Cd1-xZnxS films.

orientation of (111) plane at the ZnS end to being hexagonal wurtzite with a preferential orientation of (002) plane when zinc content decreased.

Since the lattice constant of Cd1-xZnxS thin films follows Vegard's law, the composition of Cd1 -xZnxS thin films can be determined by

aCd1-x Znx S — aCdS -.mno/

xCd1-xZnxS = —---- X 100%, (1)

aZnS - aCdS

where aCd1-xZnxs, «CdS, 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:

(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.

The optical transmittance spectra of the Cd1-xZnx S 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.

From the transmittance curves of Figure 3, the absorption coefficient a of the thin films is estimated. The absorption coefficient a with the energy of the photon hv could be expressed by the following equation:

ahv o (hv - E,

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 fc-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 (ahv)2 is plotted as a function of hv 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 hv axis.

The band-gap dependence on composition is shown in Figure 4. 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

Eg (x) = 2.38 + 0.69x + 0.67x2.

It gives the bowing parameter (b) of 0.67 eV, which is much smaller than that reported on another Cdi_xZnxS alloy

Table 2: XRD results for Cd1-xZnxS thin films.

Cd1-xZnxS 26 d d* (hkl) JCPDS

x = 0 26.442 3.3621 3.367 (002) 65-3414

x = 0.28 26.993 3.2972 3.290 (002) 40-0836

x = 0.61 27.613 3.2237 3.214a (002) —

x = 0.89 28.347 3.1480 3.142a (002) —

x = 1 28.533 3.1257 3.124 (111) 65-1691

" Calculated from Vegard's law.

3.8 -3.63.43.2 -

3 2.8 2.6 2.4 2.2

0.4 0.6

Figure 4: The variation of energy gap of the Cd1-xZnxS as a function of x.

system in thin films by Borse et al. [6] (b = 0.85 eV) and that in single crystal by Muhamad [12] (b = 0.82 eV), 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 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 with Xabsorber < Xbuffer < Xwindow 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

1.53 eV

3.03 eV

■ Evac

4.2 eV

3.2 eV

-EC ■ Ef

CuInS2

Figure 5: Flat band diagram of a CuInS2/ZnO heterojunction with a Cdo.4Zn0 6S buffer layer.

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.

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.

Acknowledgments

Financial support from the National Basic Research Program of China (Grant no. 2011CBA007008), National Natural Science Foundation of China (Grant no. 61076058), and the Science and Technology Program of Sichuan Province, China (Grant no. 2008GZ0027) is gratefully acknowledged.

The authors thank Ms. Huiqin Sun, Analytical and Testing Center, Sichuan University, for optical transmittance spectra measurements.

References

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