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Characteristics of the CdZnS thin film doped by indium diffusion
Thin Solid Films 416 (2002) 184–189
Characteristics of the CdZnS thin film doped by indium diffusion Jae-Hyeong Leea,*, Woo-Chang Songa, Kea-Joon Yangb, Yeong-Sik Yooc a
School of Electrical and Computer Engineering, Sungkyunkwan University, 300 chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea b Department of Electronic Engineering, Chungju National University, 123 Komdan-ri, Iryu-myon, Chungju-shi, Chungbuk 380-702, South Korea c Department of Electricity, Yeojoo Institute of Technology, 454-5 Kyo-ri, Yeojoo-up, Yeojoo-gun, Kyonggi-do 469-705, South Korea Received 12 July 2001; received in revised form 4 June 2002; accepted 15 July 2002
Abstract Effects of the thickness of indium films and the annealing temperature on structural, optical and electrical properties of chemically deposited CdZnS films were investigated. The diffusion process of evaporated indium was carried out by heating the sample in air at 150–550 8C for 1 h. X-Ray diffraction patterns of CdZnS films indicate that the minimum thickness and annealing temperature for the formation of an In2 O3 layer, which acts as a barrier preventing the out-diffusion of indium and gives a high optical transmittance, are 20 nm and 350 8C, respectively. As the thickness of indium film and the annealing temperature increase, the conductivity of CdZnS films improves and the lowest resistivity of 0.3 V-cm is attained for CdZnS films with a 40 nm indium coating and annealed at 450 8C. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cadmium zinc sulfide; Indium doping; Heat treatment; Diffusion; Solar cells
1. Introduction Cadmimum zinc sulfide (CdZnS) thin films have been widely used as a wide band-gap window material in heterojunction solar cells w1–5x and in photoconductive devices w6x. In solar cell systems, where CdS films have been demonstrated to be effective, the replacement of CdS with the higher band gap ternary CdZnS has led to a decrease in window absorption losses, and has resulted in an increase in the short circuit current in the solar cell w7x. This CdZnS ternary compound is also potentially useful as a window material for the fabrication of p–n junctions without lattice mismatch in the devices based on quaternary materials like CuInxGa1yxSe2 w8x or CuIn(SzSe1yz)2 w9x. However, the resistivity of CdZnS films increases rapidly with the composition of zinc. It is evident that the composition dependence of resistivity is a basic property of the CdZnS solid–solution system and is not appreciably altered by the method of preparation. This high resistiv*Corresponding author. E-mail address: [email protected] (J.-H. Lee).
ity of CdZnS films limited their utilization as a window material in most heterojunction devices. The resistivity can be reduced appreciably on incorporation of donors by: (i) doping the films with indium; andyor (ii) annealing the films in H2 atmosphere. It has been the endeavor of researchers in the field of polycrystalline solar cells to improve the conductivity of CdZnS thin films so as to use it directly as a window layer in heterojunction solar cells w10–13x. We propose a simple method to n-type dope CdZnS thin films by thermal annealing in air of InyCdZnS bilayers deposited on glass substrates. Several attempts to dope chemically deposited CdS thin films from an evaporated thin indium film on its surface under annealing have been reported w14,15x. However, we are not aware of any further work on the application of this technique to the CdZnS films. In the present work, thin indium films (10–40 nm in thickness) were evaporated on the surface of chemically deposited CdZnS films in order to dope by thermal diffusion. Effects of the thickness of indium films and the annealing temperature on structural, optical and electrical properties of CdZnS films were investigated. The resulting CdZnS thin films were found to retain the
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beneficial optical and electrical properties for application as a window material in solar cells. 2. Experimental details The CdZnS thin films were chemically deposited onto Corning 7059 glass. The deposition was carried out with 0.015 M cadmium acetate, 0.01 M zinc acetate, 0.05 M thiourea, 0.6 M ammonia and 0.1 M ammonium acetate. All the solutions were prepared using analytical grade reagents. All the reactants were mixed after heating zinc acetate to 75 8C, then CdZnS thin films were prepared from the mixed solution whose temperature was kept at 75 8C. The reaction vessel containing the substrates and the deposition mixture was vigorously stirred electrically during deposition. After 40 min, the coated substrates were removed and washed with deionized water to remove loosely bound CdZnS powder and finally cleaned ultrasonically. To enable optical measurement, the CdZnS film on one side of the substrate was removed by cotton swabs dipped in diluted HCl. The thickness of the film measured by the surface profiler was approximately 300 nm. The as-deposited CdZnS thin films were yellow-orange in color, uniform, highly adherent and specularly reflective. The diffraction pattern of the films exhibited peaks corresponding to the hexagonal phase. A thin film of indium was deposited on the CdZnS film by evaporation of indium of 99.999% purity in a vacuum of ;10y6 torr. The CdZnS films were not heated during deposition. Indium films of thickness varying from 10 to 40 nm were deposited. Heat treatment of the CdZnSyIn films was done in air at selected temperatures in the range of 150–550 8C for 1 h. X-Ray diffraction (XRD) measurements were performed to study the preferential orientation and crystallinity of the film by using a Philips PW 1800 ˚ CuKa radiation. The diffractometer with 1.5418 A surface morphology and grain size of films were determined by scanning electron microscopy (SEM). The optical properties of the films were measured at normal incidence in the wavelength range from 300 to 900 nm with a double-beam spectrophotometer. The electrical resistivity was measured through silver paint electrodes by using a Keithley electrometer in the dark and under illumination of 100 mWycm2. 3. Results and discussion Fig. 1 shows the XRD patterns of vacuum-deposited indium film and the films annealed at 150, 250 8C in air for 1 h. As-deposited indium film on the glass substrate exhibits a very strong diffraction peak at 2us 32.948, corresponding to the (101) reflection of tetragonal phase, and very weak peaks at 2us36.248, 69.048, corresponding to the (002), (202) planes, respectively. This indicates that as-grown indium film has the tetrag-
Fig. 1. XRD patterns of vacuum-evaporated indium films with thickness of 40 nm and annealed at 150 and 250 8C.
onal structure with the preferred orientation of the (101) plane. One of the effects of the thermal annealing is the oxidation of indium; this can be seen in the XRD patterns of the samples annealed at 150 and 250 8C. In these patterns a diffraction peak corresponding to the (222) crystalline planes of the cubic phase of In2O3 appears at approximately 30.68 and the intensity of the peak increases with annealing temperature. Meanwhile, the (101) diffraction peak of indium decreases markedly after annealing at 150 8C and almost disappears after annealing at 250 8C. These results can be explained in terms of the oxidation of the indium layer surface exposed to the air during the annealing. An external layer of In2O3 is formed during the annealing, which grows from the indium layer with increasing the annealing temperature. The indium films deposited directly over the glass slides were found to disappear by reevaporation when annealed at temperatures )250 8C. As-deposited indium film consists of dense layer of crystallites having a grain size of approximately 0.15 mm, as seen in Fig. 2a. After annealing at 250 8C the grain size of the film slightly decreases to 0.1 mm, but no appreciable changes in microstructure are observed. The XRD patterns of CdZnS thin films with indium coating (40 nm) through various stages of heat treatments are given in Fig. 3. CdZnS films annealed at 150 8C have a diffraction peak from the (101) plane of indium, indicating the presence of metallic indium on the surface of the CdZnS film. In addition, the very weak CdZnS peak is observable because of the masking
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the hexagonal phase perpendicular to the plane of the glass substrate. In addition, the peak corresponding to CdO are also observed, indicating partial conversion of CdZnS to CdO. The heating at 550 8C leads to the reduction of XRD peak heights of CdZnS, possibly due to the loss of the material by evaporation. The increase of CdO peaks in the film at the expense of CdZnS peaks height is notable. In Fig. 4 SEM surface images from the CdZnS films with indium coating (40 nm) and annealed at 150 (top) and 550 8C (bottom) are shown. A gradual change is observed in the surface morphology from the sample annealed at 150 8C, which displays the metallic indium without oxidation, to the sample annealed at 550 8C. At high annealing temperature, the surface is completely oxidized and then the image corresponds to the surface morphologies of In2O3 and CdO. Fig. 5 shows the XRD patterns of CdZnS films with various indium thicknesses and annealed at 450 8C for 1 h in air. For CdZnS films with 10 nm indium film, the oxidation of Cd begins at this temperature as manifested by the presence of three diffraction peaks at approximately 338, 388 and 558, corresponding to the (111), (200) and (220) planes of the cubic phase of CdO. Note that no peaks corresponding to metallic indium or its oxide are shown in XRD patterns. This implies that the thickness of 10 nm is too thin to form an In2O3 layer or diffuse indium into CdZnS films. In the case of CdZnS film with indium coating of 20 nm, the dominant feature is the presence of In2O3 peaks at Fig. 2. SEM micrographs of (a) as-deposited indium films and (b) the films annealed at 250 8C in air for 1 h (b), indium thickness ;40 nm.
by the indium film. When the sample is annealed at 250 8C in air, the peak corresponding to the (222) plane of In2O3 appears in the XRD spectra along with the peak corresponding to indium, indicating the beginning of oxidation of the top layer of the indium film. This also indicates the trapping of a layer of the indium film between the underlying CdZnS thin film and a top layer of In2O3. The XRD pattern of the film heated at 350 8C shows an increase in the peak heights of In2O3. However, the peaks corresponding to indium are absent. This suggests that the amount of indium decreases with annealing temperature due to the partial conversion into the oxide and also due to the partial diffusion into the CdZnS layer. As the annealing temperature increases up to 450 8C, the (002) peak of CdZnS appears very pronounced in this spectrum, indicating the increase of the preferred orientation toward this direction. Similar results were also observed by other investigators. George et al. w14x reported that the recrystallization of CdS in the presence of diffused indium involves a strong preferential orientation of the crystallites with the c-axis of
Fig. 3. X-Ray diffraction spectra of CdZnS films coated with 40-nmthick indium films and heated at various temperatures for 1 h in air.
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the annealing temperature. This result has also been observed by other authors for CdS films w15,16x. The growth of the CdZnS grain, the relaxation of the CdZnS lattice constant or the composition changes in the samples as a consequence of the thermal annealing could produce the red shift of the CdZnS absorption edge. A temperature at 550 8C caused the optical transmittance to decrease, while the absorption edge almost disappeared. This behavior might be due to a dissociation or re-evaporation process as a consequence of such a high temperature. For CdZnS films with 40 nm indium coating, the metallic indium on the surface reduces the optical transmittance. The figure shows that partially opaque CdZnS film transforms to transparent films when annealed at temperature above 350 8C. This indicates the partial conversion of metallic indium film to transparent In2O3 film and diffusion into the CdZnS film. In the case of the CdZnS film with a 40 nm indium coating, heat treatment at 350 8C is necessary for such a transformation, whereas for CdZnS film with a 20 nm indium film, heat treatment at 250 8C is sufficient for the same. As the annealing temperature further increases, the optical transmittance improves in the whole measurement range. It can also be seen that the films with 40 nm indium have higher transmittance than those of films with 10 nm indium coating when annealed at the same temperature. This may be partially due to antireflection of incident light on the In2O3 layer. The optical band gap values were estimated from the transmittance curves of Fig. 6 using the method reported earlier w17,18x. As seen in Fig. 7, there is variation in the optical band gap among the different heat-treated Fig. 4. SEM micrographs of CdZnS films coated with 40-nm-thick indium films and heated at (a) 150 8C and (b) 550 8C for 1 h in air.
2us30.388 indicating pyrolytic decomposition of indium. As the thickness of the indium film increases, some CdO peaks disappear or become very weak, which suggests the inhibition of oxidation of CdZnS at this temperature by the In2O3 films. By comparing the relative intensities of the CdO peaks in the three cases, one can see that the formation of In2O3 on the CdZnS film continues to inhibit the conversion of CdZnS to CdO. Fig. 6 shows two sets of transmittance spectra of CdZnS films with indium films of 10 and 40 nm. The spectra were recorded after heat treatment at various temperatures for 1 h in air. The optical transmittance of the films is typically 70% at wavelengths beyond the absorption edge. It can be seen that the CdZnS films with 10 nm indium show high optical transmittance in spite of the evaporated indium coating. This may be due to re-evaporation andyor low thickness of the indium. Another interesting feature in these spectra is the red shift in the absorption edge of CdZnS films with
Fig. 5. XRD patterns of CdZnS films annealed at 450 8C with various indium film thickness.
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Fig. 7. Dependence of the optical band gap of CdZnS films on the indium film thickness and the annealing temperature.
nm indium coating, heating of the sample at 250–550 8C causes the change in the dark resistivity to various extents. This arises from the cumulative effect of grain size enhancement and oxygen incorporation at grain boundaries. Significant increase in the electrical conductivity is observed in samples annealed at temperatures of 450 8C. This is due to the partial conversion of CdZnS to CdO as seen from Fig. 5. For CdZnS with indium thickness of 40 nm, the films heated at temperatures above 350 8C show increased conductivity probably due to the combined effect of indium diffusion and partial conversion to CdO. As the thickness of the indium layer increases, the dark resistivity of CdZnS films annealed at the same temperature highly decreases. The increase in conductivity may be due to the diffusion Fig. 6. Two sets of optical transmittance spectra of CdS thin films coated with 10-nm-thick (top set) and 40-nm-thick (bottom set) indium films.
samples depending on the thickness of the indium film and the temperature of heat treatment. In the case of the CdZnS films with 10 nm indium coating, the band gap slightly decreases with the annealing temperature. This reduction in the band gap arises from the grain size growth and composition changes taking place in the samples. The values of the optical band gap of the samples annealed at temperatures higher than 350 8C increase with indium thickness. Note that the optical band at 2.94 eV observed in the case of the CdZnS film with 40 nm indium coating and annealed at 550 8C, is larger than those of all other samples. Fig. 8 shows the dark resistivity of CdZnS film with various thicknesses of indium as a function of the annealing temperature. The as-prepared CdZnS film (without indium coating) shows a high resistivity of 2.4=105 V-cm. In the case of CdZnS films with 10
Fig. 8. Dependence of the dark resistivity of CdZnS films on the indium film thickness and the annealing temperature.
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films depends on the thickness of indium film and the annealing temperature. The optical band gap and transmission spectra also show changes depending on the process parameters. The formation of an external In2O3 layer due to a partial oxidation of the In layer protects the CdZnS film against oxidation and allows the diffusion of In into CdZnS. Such films would be advantageous for application in solar cells because they offer a wider optical band gap ()2.5 eV) compared with that of CdS films, and a low resistivity. An immediate application of the CdZnS film obtained by the present method may be in the fabrication of heterojunction solar cell structures. Further work needs to be directed toward this effort. References Fig. 9. Photosensitivity, (IphotoyIdark)yIdark, of chemically deposited CdZnS thin films plotted as a function of the annealing temperature of films with different indium thickness.
of indium. The lowest value of the dark resistivity of 0.25 V-cm is obtained for the CdZnS film coated with 40 nm indium and annealed at 450 8C for 1 h in air. The photosensitivity (S) given in Fig. 9 is defined as Ss(IphotoyIdark)yIdark and is plotted against annealing temperature for different indium thickness. The important point to note in Fig. 9 is that the annealing of CdZnS films with 10 nm indium coating leads to a deterioration of the photoconductivity as well as of photosensitivity due to oxygen chemisorption at the grain boundaries which would drastically reduce the charge carrier mobility w19x. In sharp contrast, there is a considerable increase in the dark conductivity of CdZnS with 40 nm indium coating. The photosensitivity is lost completely in these films, which is characteristic of extrinsic photoconductors. Loss of photosensitivity upon conversion of the sample to conducting type is in agreement with the earlier observation w17x. 4. Conclusions In this work we presented a technique employing thermal diffusion of vacuum-evaporated indium resulting in CdZnS thin films with high conductivity (;3.3 Vy1 cmy1). The electrical conductivity of such CdZnS
w1x K.T.R. Reddy, P.J. Reddy, J. Phys. D 25 (1992) 1345. w2x H.S. Kim, H.B. Im, Thin Solid Films 214 (1992) 207. w3x Y.K. Jun, H.B. Im, J. Electrochem. Soc.: Electrochem. Sci. Technol. 135 (1988) 1658. w4x B.M. Basol, J. Appl. Phys. 55 (1984) 601. w5x K.W. Mitchell, A.L. Fahrenbruch, R.H. Bube, J. Appl. Phys. 48 (1977) 4365. w6x J. Torres, G. Gordillo, Thin Solid Films 207 (1992) 231. w7x T. Yamaguchi, Y. Yamamoto, T. Tanaka, Y. Demizu, A. Yoshida, Thin Solid Films 281y282 (1996) 375. w8x T. Yamaguchi, J. Matsufusa, A. Yoshida, Jpn. J. Appl. Phys. 3 (1992) L703. w9x T. Walter, M. Ruckh, K.O. Velthaus, H.W. Schock, Proceedings of the 11th EC Photovoltaic Solar Energy Conference, Montreux, 1992, p. 124. w10x T.A. Chynoweth, R.H. Bube, J. Appl. Phys. 51 (1980) 1844. w11x J.C. Joshi, B.K. Sarkar, P. Kumar, Thin Solid Films 88 (1982) 89. w12x G.K. Padam, G.L. Maholtra, S.U.M. Rao, J. Appl. Phys. 63 (1988) 770. w13x T.L. Chu, S.S. Chu, J. Britt, C. Ferekides, C.Q. Wu, J. Appl. Phys. 70 (1991) 2688. w14x P.J. George, A. Sanchez, ´ P.K. Nair, L. Huang, J. Cryst. Growth 158 (1996) 53. w15x S.J. Castillo, A. Mendoza-Galvan, ´ R. Ramırez-Bon, ´ F.J. Espi´ M. Sotelo-Lerma, J. Gonzalez-Hernandez, ´ ´ noza-Beltran, G. ´ Martınez, Thin Solid Films 373 (2000) 10. w16x P.J. George, A. Snchez, P.K. Nair, M.T.S. Nair, Appl. Phys. Lett. 66 (1995) 3624. w17x M.T.S. Nair, P.K. Nair, R.A. Zingaro, E.A. Meyers, J. Appl. Phys. 75 (1994) 1557. w18x G. Hodes, A.A. Yaron, F. Decker, P. Motisuke, Phys. Rev. B 36 (1987) 4215. w19x J.W. Orton, B.J. Goldsmith, J.A. Chapman, M.J. Powell, J. Appl. Phys. 53 (1982) 1602.
Characteristics of the CdZnS thin film doped by indium diffusion Jae-Hyeong Leea,*, Woo-Chang Songa, Kea-Joon Yangb, Yeong-Sik Yooc a
School of Electrical and Computer Engineering, Sungkyunkwan University, 300 chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea b Department of Electronic Engineering, Chungju National University, 123 Komdan-ri, Iryu-myon, Chungju-shi, Chungbuk 380-702, South Korea c Department of Electricity, Yeojoo Institute of Technology, 454-5 Kyo-ri, Yeojoo-up, Yeojoo-gun, Kyonggi-do 469-705, South Korea Received 12 July 2001; received in revised form 4 June 2002; accepted 15 July 2002
Abstract Effects of the thickness of indium films and the annealing temperature on structural, optical and electrical properties of chemically deposited CdZnS films were investigated. The diffusion process of evaporated indium was carried out by heating the sample in air at 150–550 8C for 1 h. X-Ray diffraction patterns of CdZnS films indicate that the minimum thickness and annealing temperature for the formation of an In2 O3 layer, which acts as a barrier preventing the out-diffusion of indium and gives a high optical transmittance, are 20 nm and 350 8C, respectively. As the thickness of indium film and the annealing temperature increase, the conductivity of CdZnS films improves and the lowest resistivity of 0.3 V-cm is attained for CdZnS films with a 40 nm indium coating and annealed at 450 8C. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cadmium zinc sulfide; Indium doping; Heat treatment; Diffusion; Solar cells
1. Introduction Cadmimum zinc sulfide (CdZnS) thin films have been widely used as a wide band-gap window material in heterojunction solar cells w1–5x and in photoconductive devices w6x. In solar cell systems, where CdS films have been demonstrated to be effective, the replacement of CdS with the higher band gap ternary CdZnS has led to a decrease in window absorption losses, and has resulted in an increase in the short circuit current in the solar cell w7x. This CdZnS ternary compound is also potentially useful as a window material for the fabrication of p–n junctions without lattice mismatch in the devices based on quaternary materials like CuInxGa1yxSe2 w8x or CuIn(SzSe1yz)2 w9x. However, the resistivity of CdZnS films increases rapidly with the composition of zinc. It is evident that the composition dependence of resistivity is a basic property of the CdZnS solid–solution system and is not appreciably altered by the method of preparation. This high resistiv*Corresponding author. E-mail address: [email protected] (J.-H. Lee).
ity of CdZnS films limited their utilization as a window material in most heterojunction devices. The resistivity can be reduced appreciably on incorporation of donors by: (i) doping the films with indium; andyor (ii) annealing the films in H2 atmosphere. It has been the endeavor of researchers in the field of polycrystalline solar cells to improve the conductivity of CdZnS thin films so as to use it directly as a window layer in heterojunction solar cells w10–13x. We propose a simple method to n-type dope CdZnS thin films by thermal annealing in air of InyCdZnS bilayers deposited on glass substrates. Several attempts to dope chemically deposited CdS thin films from an evaporated thin indium film on its surface under annealing have been reported w14,15x. However, we are not aware of any further work on the application of this technique to the CdZnS films. In the present work, thin indium films (10–40 nm in thickness) were evaporated on the surface of chemically deposited CdZnS films in order to dope by thermal diffusion. Effects of the thickness of indium films and the annealing temperature on structural, optical and electrical properties of CdZnS films were investigated. The resulting CdZnS thin films were found to retain the
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beneficial optical and electrical properties for application as a window material in solar cells. 2. Experimental details The CdZnS thin films were chemically deposited onto Corning 7059 glass. The deposition was carried out with 0.015 M cadmium acetate, 0.01 M zinc acetate, 0.05 M thiourea, 0.6 M ammonia and 0.1 M ammonium acetate. All the solutions were prepared using analytical grade reagents. All the reactants were mixed after heating zinc acetate to 75 8C, then CdZnS thin films were prepared from the mixed solution whose temperature was kept at 75 8C. The reaction vessel containing the substrates and the deposition mixture was vigorously stirred electrically during deposition. After 40 min, the coated substrates were removed and washed with deionized water to remove loosely bound CdZnS powder and finally cleaned ultrasonically. To enable optical measurement, the CdZnS film on one side of the substrate was removed by cotton swabs dipped in diluted HCl. The thickness of the film measured by the surface profiler was approximately 300 nm. The as-deposited CdZnS thin films were yellow-orange in color, uniform, highly adherent and specularly reflective. The diffraction pattern of the films exhibited peaks corresponding to the hexagonal phase. A thin film of indium was deposited on the CdZnS film by evaporation of indium of 99.999% purity in a vacuum of ;10y6 torr. The CdZnS films were not heated during deposition. Indium films of thickness varying from 10 to 40 nm were deposited. Heat treatment of the CdZnSyIn films was done in air at selected temperatures in the range of 150–550 8C for 1 h. X-Ray diffraction (XRD) measurements were performed to study the preferential orientation and crystallinity of the film by using a Philips PW 1800 ˚ CuKa radiation. The diffractometer with 1.5418 A surface morphology and grain size of films were determined by scanning electron microscopy (SEM). The optical properties of the films were measured at normal incidence in the wavelength range from 300 to 900 nm with a double-beam spectrophotometer. The electrical resistivity was measured through silver paint electrodes by using a Keithley electrometer in the dark and under illumination of 100 mWycm2. 3. Results and discussion Fig. 1 shows the XRD patterns of vacuum-deposited indium film and the films annealed at 150, 250 8C in air for 1 h. As-deposited indium film on the glass substrate exhibits a very strong diffraction peak at 2us 32.948, corresponding to the (101) reflection of tetragonal phase, and very weak peaks at 2us36.248, 69.048, corresponding to the (002), (202) planes, respectively. This indicates that as-grown indium film has the tetrag-
Fig. 1. XRD patterns of vacuum-evaporated indium films with thickness of 40 nm and annealed at 150 and 250 8C.
onal structure with the preferred orientation of the (101) plane. One of the effects of the thermal annealing is the oxidation of indium; this can be seen in the XRD patterns of the samples annealed at 150 and 250 8C. In these patterns a diffraction peak corresponding to the (222) crystalline planes of the cubic phase of In2O3 appears at approximately 30.68 and the intensity of the peak increases with annealing temperature. Meanwhile, the (101) diffraction peak of indium decreases markedly after annealing at 150 8C and almost disappears after annealing at 250 8C. These results can be explained in terms of the oxidation of the indium layer surface exposed to the air during the annealing. An external layer of In2O3 is formed during the annealing, which grows from the indium layer with increasing the annealing temperature. The indium films deposited directly over the glass slides were found to disappear by reevaporation when annealed at temperatures )250 8C. As-deposited indium film consists of dense layer of crystallites having a grain size of approximately 0.15 mm, as seen in Fig. 2a. After annealing at 250 8C the grain size of the film slightly decreases to 0.1 mm, but no appreciable changes in microstructure are observed. The XRD patterns of CdZnS thin films with indium coating (40 nm) through various stages of heat treatments are given in Fig. 3. CdZnS films annealed at 150 8C have a diffraction peak from the (101) plane of indium, indicating the presence of metallic indium on the surface of the CdZnS film. In addition, the very weak CdZnS peak is observable because of the masking
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the hexagonal phase perpendicular to the plane of the glass substrate. In addition, the peak corresponding to CdO are also observed, indicating partial conversion of CdZnS to CdO. The heating at 550 8C leads to the reduction of XRD peak heights of CdZnS, possibly due to the loss of the material by evaporation. The increase of CdO peaks in the film at the expense of CdZnS peaks height is notable. In Fig. 4 SEM surface images from the CdZnS films with indium coating (40 nm) and annealed at 150 (top) and 550 8C (bottom) are shown. A gradual change is observed in the surface morphology from the sample annealed at 150 8C, which displays the metallic indium without oxidation, to the sample annealed at 550 8C. At high annealing temperature, the surface is completely oxidized and then the image corresponds to the surface morphologies of In2O3 and CdO. Fig. 5 shows the XRD patterns of CdZnS films with various indium thicknesses and annealed at 450 8C for 1 h in air. For CdZnS films with 10 nm indium film, the oxidation of Cd begins at this temperature as manifested by the presence of three diffraction peaks at approximately 338, 388 and 558, corresponding to the (111), (200) and (220) planes of the cubic phase of CdO. Note that no peaks corresponding to metallic indium or its oxide are shown in XRD patterns. This implies that the thickness of 10 nm is too thin to form an In2O3 layer or diffuse indium into CdZnS films. In the case of CdZnS film with indium coating of 20 nm, the dominant feature is the presence of In2O3 peaks at Fig. 2. SEM micrographs of (a) as-deposited indium films and (b) the films annealed at 250 8C in air for 1 h (b), indium thickness ;40 nm.
by the indium film. When the sample is annealed at 250 8C in air, the peak corresponding to the (222) plane of In2O3 appears in the XRD spectra along with the peak corresponding to indium, indicating the beginning of oxidation of the top layer of the indium film. This also indicates the trapping of a layer of the indium film between the underlying CdZnS thin film and a top layer of In2O3. The XRD pattern of the film heated at 350 8C shows an increase in the peak heights of In2O3. However, the peaks corresponding to indium are absent. This suggests that the amount of indium decreases with annealing temperature due to the partial conversion into the oxide and also due to the partial diffusion into the CdZnS layer. As the annealing temperature increases up to 450 8C, the (002) peak of CdZnS appears very pronounced in this spectrum, indicating the increase of the preferred orientation toward this direction. Similar results were also observed by other investigators. George et al. w14x reported that the recrystallization of CdS in the presence of diffused indium involves a strong preferential orientation of the crystallites with the c-axis of
Fig. 3. X-Ray diffraction spectra of CdZnS films coated with 40-nmthick indium films and heated at various temperatures for 1 h in air.
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the annealing temperature. This result has also been observed by other authors for CdS films w15,16x. The growth of the CdZnS grain, the relaxation of the CdZnS lattice constant or the composition changes in the samples as a consequence of the thermal annealing could produce the red shift of the CdZnS absorption edge. A temperature at 550 8C caused the optical transmittance to decrease, while the absorption edge almost disappeared. This behavior might be due to a dissociation or re-evaporation process as a consequence of such a high temperature. For CdZnS films with 40 nm indium coating, the metallic indium on the surface reduces the optical transmittance. The figure shows that partially opaque CdZnS film transforms to transparent films when annealed at temperature above 350 8C. This indicates the partial conversion of metallic indium film to transparent In2O3 film and diffusion into the CdZnS film. In the case of the CdZnS film with a 40 nm indium coating, heat treatment at 350 8C is necessary for such a transformation, whereas for CdZnS film with a 20 nm indium film, heat treatment at 250 8C is sufficient for the same. As the annealing temperature further increases, the optical transmittance improves in the whole measurement range. It can also be seen that the films with 40 nm indium have higher transmittance than those of films with 10 nm indium coating when annealed at the same temperature. This may be partially due to antireflection of incident light on the In2O3 layer. The optical band gap values were estimated from the transmittance curves of Fig. 6 using the method reported earlier w17,18x. As seen in Fig. 7, there is variation in the optical band gap among the different heat-treated Fig. 4. SEM micrographs of CdZnS films coated with 40-nm-thick indium films and heated at (a) 150 8C and (b) 550 8C for 1 h in air.
2us30.388 indicating pyrolytic decomposition of indium. As the thickness of the indium film increases, some CdO peaks disappear or become very weak, which suggests the inhibition of oxidation of CdZnS at this temperature by the In2O3 films. By comparing the relative intensities of the CdO peaks in the three cases, one can see that the formation of In2O3 on the CdZnS film continues to inhibit the conversion of CdZnS to CdO. Fig. 6 shows two sets of transmittance spectra of CdZnS films with indium films of 10 and 40 nm. The spectra were recorded after heat treatment at various temperatures for 1 h in air. The optical transmittance of the films is typically 70% at wavelengths beyond the absorption edge. It can be seen that the CdZnS films with 10 nm indium show high optical transmittance in spite of the evaporated indium coating. This may be due to re-evaporation andyor low thickness of the indium. Another interesting feature in these spectra is the red shift in the absorption edge of CdZnS films with
Fig. 5. XRD patterns of CdZnS films annealed at 450 8C with various indium film thickness.
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Fig. 7. Dependence of the optical band gap of CdZnS films on the indium film thickness and the annealing temperature.
nm indium coating, heating of the sample at 250–550 8C causes the change in the dark resistivity to various extents. This arises from the cumulative effect of grain size enhancement and oxygen incorporation at grain boundaries. Significant increase in the electrical conductivity is observed in samples annealed at temperatures of 450 8C. This is due to the partial conversion of CdZnS to CdO as seen from Fig. 5. For CdZnS with indium thickness of 40 nm, the films heated at temperatures above 350 8C show increased conductivity probably due to the combined effect of indium diffusion and partial conversion to CdO. As the thickness of the indium layer increases, the dark resistivity of CdZnS films annealed at the same temperature highly decreases. The increase in conductivity may be due to the diffusion Fig. 6. Two sets of optical transmittance spectra of CdS thin films coated with 10-nm-thick (top set) and 40-nm-thick (bottom set) indium films.
samples depending on the thickness of the indium film and the temperature of heat treatment. In the case of the CdZnS films with 10 nm indium coating, the band gap slightly decreases with the annealing temperature. This reduction in the band gap arises from the grain size growth and composition changes taking place in the samples. The values of the optical band gap of the samples annealed at temperatures higher than 350 8C increase with indium thickness. Note that the optical band at 2.94 eV observed in the case of the CdZnS film with 40 nm indium coating and annealed at 550 8C, is larger than those of all other samples. Fig. 8 shows the dark resistivity of CdZnS film with various thicknesses of indium as a function of the annealing temperature. The as-prepared CdZnS film (without indium coating) shows a high resistivity of 2.4=105 V-cm. In the case of CdZnS films with 10
Fig. 8. Dependence of the dark resistivity of CdZnS films on the indium film thickness and the annealing temperature.
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films depends on the thickness of indium film and the annealing temperature. The optical band gap and transmission spectra also show changes depending on the process parameters. The formation of an external In2O3 layer due to a partial oxidation of the In layer protects the CdZnS film against oxidation and allows the diffusion of In into CdZnS. Such films would be advantageous for application in solar cells because they offer a wider optical band gap ()2.5 eV) compared with that of CdS films, and a low resistivity. An immediate application of the CdZnS film obtained by the present method may be in the fabrication of heterojunction solar cell structures. Further work needs to be directed toward this effort. References Fig. 9. Photosensitivity, (IphotoyIdark)yIdark, of chemically deposited CdZnS thin films plotted as a function of the annealing temperature of films with different indium thickness.
of indium. The lowest value of the dark resistivity of 0.25 V-cm is obtained for the CdZnS film coated with 40 nm indium and annealed at 450 8C for 1 h in air. The photosensitivity (S) given in Fig. 9 is defined as Ss(IphotoyIdark)yIdark and is plotted against annealing temperature for different indium thickness. The important point to note in Fig. 9 is that the annealing of CdZnS films with 10 nm indium coating leads to a deterioration of the photoconductivity as well as of photosensitivity due to oxygen chemisorption at the grain boundaries which would drastically reduce the charge carrier mobility w19x. In sharp contrast, there is a considerable increase in the dark conductivity of CdZnS with 40 nm indium coating. The photosensitivity is lost completely in these films, which is characteristic of extrinsic photoconductors. Loss of photosensitivity upon conversion of the sample to conducting type is in agreement with the earlier observation w17x. 4. Conclusions In this work we presented a technique employing thermal diffusion of vacuum-evaporated indium resulting in CdZnS thin films with high conductivity (;3.3 Vy1 cmy1). The electrical conductivity of such CdZnS
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