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Preparation of ZnSe films through chemical solution reduction process
Materials Research Bulletin, Vol. 34, Nos. 10/11, pp. 1637–1641, 1999 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter
PII S0025-5408(99)00187-7
PREPARATION OF ZnSe FILMS THROUGH CHEMICAL SOLUTION REDUCTION PROCESS
C. Wang, X.F. Qian, W.X. Zhang, X.M. Zhang, Y. Xie, and Y.T. Qian* Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, P.R. China (Refereed) (Received September 11, 1998; Accepted December 1, 1998)
ABSTRACT In a novel wet process, zinc selenide films were prepared on Teflon and polycrystalline ␣-Al2O3 substrates using zinc chloride, selenium, and aluminum foil as source materials. This method is simple and can be fulfilled easily. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectrum (XPS). © 2000 Elsevier Science Ltd
KEYWORDS: A. semiconductors, B. chemical synthesis INTRODUCTION As one of the wide-bandgap II–VI semiconductors, ZnSe has gained much interest for its technological application in blue laser diodes [1,2]. This has stimulated interest in its synthesis. The chemical formation of ZnSe in aqueous solution can be accomplished through the metathesis reactions between selenium ions such as Na2Se or H2Se, and zinc ions [3]. Because of the immediate reactions between these ions, however, they are not suitable for fabricating ZnSe films. Monocrystal layers of ZnSe on GaAs substrates are usually grown by molecular beam epitaxy (MBE) [4] or metalorganic vapor phase epitaxy (MOVBE) [5]. These methods can produce high-quality ZnSe films in a vacuum or a low-pressure atmosphere, but they are not suitable for the preparation of film on a substrate with a large surface area [6]. This limitation can be overcome by solution growth techniques, which can be performed at relatively low temperatures and are relatively inexpensive.
*To whom correspondence should be addressed. 1637
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Vol. 34, Nos. 10/11
Using sodium selenosulfate, CdSe and PbSe films have been deposited in an alkaline medium [7–9]. Pramanik and Biswas [10] extended this method and successfully deposited ZnSe thin film on a glass slide at 100°C in aqueous solution. In the present study, ZnSe films were deposited on Teflon and ␣-Al2O3 substrates using a solution growth technique. EXPERIMENTAL The Teflon substrate (2 cm ⫻ 2 cm ⫻ 0.3 cm) was treated with smooth polish paper (500 mesh), washed in 3.0 mol/L HCl and 1.0 mol/L HNO3 solution overnight, then cleaned ultrasonically in distilled water for 30 min and dried under infrared light. For the ␣-Al2O3 substrate (3 cm ⫻ 0.5 cm ⫻ 0.2 cm), a similar treatment was carried out, but without using polish paper. The substrates were immersed vertically into the solution. Zinc chloride, selenium, and aluminum were used as the basic source materials. The samples were obtained in 1.0 mol/L NaOH solution at 80°C and characterized by X-ray diffraction pattern (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectrum (XPS). All reactants were analytical grade and purchased from Shanghai Chemical Co. Ltd. After dissolving 0.80 g (about 6.0 ⫻ 10⫺3 mol) into 100 ml 1.0 mol/L NaOH aqueous solution, 0.24 g (about 3.0 ⫻ 10⫺3 mol) selenium powder and 0.27 g (about 1.0 ⫻ 10⫺2 mol) aluminum foil were added to the solution. This mixture was put into a Teflon-lined stainlesssteel autoclave. The autoclave was then heated at 80°C for a certain duration time. After cooling, the as-prepared films were cleaned ultrasonically in distilled water to remove loosely bonded precipitates, washed with distilled water and absolute ethanol, and dried in a vacuum at room temperature. The phase and crystallinity were characterized by X-ray diffraction pattern, which was carried out using a Rigaku D/max ␥A X-ray diffractometer with Cu K␣ ( ⫽ 1.5418 Å) incident radiation. The surface morphologies of the films were observed by scanning electron microscope on a Hitachi-600. The chemical composition was determined by elemental analysis, using a Perkin-Elmer 1100B atomic absorption spectrophotometer. X-ray photoelectron spectroscopy (VGESCALAB MKII X-ray photoelectron spectrometer using non-monochromatized Al K␣ radiation as the excitation source) was also carried out. RESULTS AND DISCUSSION XRD patterns of the films prepared at 80°C for 48 h (Fig. 1a and 1b) and the powder formed (Fig. 1c) in the same system were recorded. By comparing the obtained XRD patterns with the standard (JCPDS 5-522), it was found that cubic zinc selenide was the only crystalline phase formed in this process. The other peaks in 1(a) and 1(b) can be attributed to the diffraction of the two substrates used. There is no significant difference in the broadening of the diffraction peaks between the three patterns shown in Figure 1. This indicates that the substrates used may not promote growth of grains. The broadening of the diffraction peaks also indicates that the as-formed films are comprised of very small particles. Figure 2a and 2b show the surface morphology of the zinc selenide films deposited on Teflon and ␣-Al2O3 substrates at 80°C for 48 h. It can be seen that both films are dense, smooth, and homogeneous, without visible pores. The cleavage of film deposited on ␣-Al2O3 gives a thickness of 1.8 m (Fig. 2c).
Vol. 34, Nos. 10/11
ZINC SELENIDE FILMS
1639
FIG. 1 X-ray diffraction patterns of ZnSe films deposited on Teflon (a) and polycrystalline ␣-Al2O3 (b) substrates, and powder formed in the same system (c). 夹: the phase of ZnSe.
XPS results show the presence of Zn and Se from the nanocrystals and C and O of gaseous molecules adsorbed on the nanocrystals’ surface. The peak cores at 1022.20 eV and 54.10 eV correspond to Zn 2p3 and Se 3d, respectively. After identification, no peak could be assigned to elemental selenium present in the sample deposited at 80°C for 48 h. This indicates that the films were free of contamination from selenium. Peak areas of the Zn and Se cores were measured and used to calculate the Zn:Se ratio in the films. They gave a Zn:Se ratio of 1.02:1. This result was confirmed by elemental analysis, which gave a Zn:Se ratio of 1.01:1. In the present study, the formation of ZnSe could be described as
2⫺
3Se ⫹ 6OH ⫺ 3 2Se 2⫺ ⫹ SeO 32⫺ ⫹ 3H 2O
(1)
Se 2⫺ ⫹ Zn(OH) 42⫺ 3 ZnSe ⫹ 4OH ⫺
(2)
Selenium ions (Se ) are produced slowly from the disproportionation of elemental selenium, to satisfy the film deposition rate in the alkaline aqueous solution. Previous reports on preparing II–VI semiconductor films containing selenium in aqueous solutions concentrated mainly on the use of sodium selenosulfate as one of the source materials. The release of Se2⫺ ions was believed to be caused by the hydrolysis of sodium selenosulfate in an alkaline medium [7–9,11–13].
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FIG. 2 SEM micrographs of the films prepared on Teflon (a) and polycrystalline ␣-Al2O3 (b) substrates, and cleavage of films on polycrystalline ␣-Al2O3 (c). The addition of aluminum in this process is essential, otherwise the yield of ZnSe is very low and no film can be obtained. Aluminum reacts with OH– in the solution, to act as a 2– reducing agent, ensuring the full transformation of SeO2– ions: 3 to Se 2 Al ⫹ SeO 32⫺ ⫹ 3H 2O ⫹ 2OH ⫺ 3 2 Al(OH) 4⫺ ⫹ Se 2⫺
(3)
It was also observed that the elemental selenium could almost disperse homogeneously in the NaOH solution after heat treatment. This dispersion, accompanied by the dissolution of Zn2⫹ in basic solution, results in a ZnSe formation process similar to the one occurring in homogeneous solutions. The growth of zinc selenide films was studied at 80°C with variable OH⫺ concentrations, temperature, and growth duration time. If the concentration of OH⫺ was lower than 0.3 mol/L, growth was slow. The optimal concentration range of OH⫺ was about 0.5–1.0 mol/L; higher concentration was unnecessary. Although the chemical processes involved could be accelerated with the elevation of temperature, the temperature should be maintained below 125°C, since the appearance of ZnO at 125°C [14] contaminated the ZnSe films, decreasing their quality. A growth duration time of 48 h for a typical reaction could supply films with thicknesses of up 1.8 m. Thin films can be obtained, if necessary, by decreasing the amount of selenium added or the duration time. Decreasing the duration time from 48 to 12 h, the thickness of the films could be adjusted from 1.8 to 0.4 m. It should be noted here that the chemical processes could not be completed at 80°C, even with a duration time of 36 h. Fortunately, the presence of unreacted elemental selenium had no negative effect on the ZnSe films deposited on the Teflon and ␣-Al2O3 substrates. This was probably due to the low affinity of selenium to the substrates and the as-formed ZnSe. CONCLUSION Zinc selenide films (1.8 m thick) were prepared by the chemical solution reduction process. XRD patterns showed the films to be polycrystalline in nature. SEM micrographs indicated that the films were dense, smooth, and homogeneous. This method is simple and can be applied in the synthesis of other metal selenide films.
Vol. 34, Nos. 10/11
ZINC SELENIDE FILMS
1641
ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China and Huo Yingdong Foundation for Youth Teachers. REFERENCES 1. C.-H. Lee, Y.-D. Choi, G.-N. Jeon, S.-C. Yu, and S.-Y. Ko, J. Crystal Growth 167, 473 (1996). 2. F. Kadotsuji, H. Ohnishi, N. Kawabata, M. Kimura, A. Tanaka, and T. Sukegawa, J. Crystal Growth 155, 23 (1995). 3. N. Chestnoy, R. Hull, and L.E. Brus, J. Chem. Phys. 85, 2237 (1986). 4. N. Matsumura, K. Maemura, T. Mori, and J. Saraie, J. Crystal Growth 159, 85 (1996). 5. M.D. McCluskey, E.E. Haller, F.X. Zach, and E.D. Bourret-Courchesne, Appl. Phys. Lett. 68, 3476 (1996). 6. S. Deki and Y. Aoi, J. Mater. Res. 13, 883 (1998). 7. G.M. Fofanov and G.A. Kitaev, Russ. J. Inorg. Chem. 14, 322 (1969). 8. G.A. Kitaev and T.S. Terekhova, Russ. J. Inorg. Chem. 15, 25 (1970). 9. R.C. Kainthla, D.K. Pandya, and K.L. Chopra, J. Electrochem. Soc. 127, 277 (1980). 10. P. Pramanik and S. Biswas, J. Electrochem. Soc. 133, 350 (1986). 11. P.S. Mane and C.D. Lokhande, Thin Solid Films 304, 56 (1997). 12. G.S. Shahane, B. More, C.B. Botti, and L.P. Deshmukh, Mater. Chem. Phys. 47, 263 (1997). 13. S. Gorer, G. Hodes, Y. Sorek, and R. Reisfeld, Mater. Lett. 31, 209 (1997). 14. S.X. Yan and C.F. Wang, Normal Inorganic Chemistry, Peking University Press, Beijing (1994).
PII S0025-5408(99)00187-7
PREPARATION OF ZnSe FILMS THROUGH CHEMICAL SOLUTION REDUCTION PROCESS
C. Wang, X.F. Qian, W.X. Zhang, X.M. Zhang, Y. Xie, and Y.T. Qian* Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, P.R. China (Refereed) (Received September 11, 1998; Accepted December 1, 1998)
ABSTRACT In a novel wet process, zinc selenide films were prepared on Teflon and polycrystalline ␣-Al2O3 substrates using zinc chloride, selenium, and aluminum foil as source materials. This method is simple and can be fulfilled easily. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectrum (XPS). © 2000 Elsevier Science Ltd
KEYWORDS: A. semiconductors, B. chemical synthesis INTRODUCTION As one of the wide-bandgap II–VI semiconductors, ZnSe has gained much interest for its technological application in blue laser diodes [1,2]. This has stimulated interest in its synthesis. The chemical formation of ZnSe in aqueous solution can be accomplished through the metathesis reactions between selenium ions such as Na2Se or H2Se, and zinc ions [3]. Because of the immediate reactions between these ions, however, they are not suitable for fabricating ZnSe films. Monocrystal layers of ZnSe on GaAs substrates are usually grown by molecular beam epitaxy (MBE) [4] or metalorganic vapor phase epitaxy (MOVBE) [5]. These methods can produce high-quality ZnSe films in a vacuum or a low-pressure atmosphere, but they are not suitable for the preparation of film on a substrate with a large surface area [6]. This limitation can be overcome by solution growth techniques, which can be performed at relatively low temperatures and are relatively inexpensive.
*To whom correspondence should be addressed. 1637
1638
C. WANG et al.
Vol. 34, Nos. 10/11
Using sodium selenosulfate, CdSe and PbSe films have been deposited in an alkaline medium [7–9]. Pramanik and Biswas [10] extended this method and successfully deposited ZnSe thin film on a glass slide at 100°C in aqueous solution. In the present study, ZnSe films were deposited on Teflon and ␣-Al2O3 substrates using a solution growth technique. EXPERIMENTAL The Teflon substrate (2 cm ⫻ 2 cm ⫻ 0.3 cm) was treated with smooth polish paper (500 mesh), washed in 3.0 mol/L HCl and 1.0 mol/L HNO3 solution overnight, then cleaned ultrasonically in distilled water for 30 min and dried under infrared light. For the ␣-Al2O3 substrate (3 cm ⫻ 0.5 cm ⫻ 0.2 cm), a similar treatment was carried out, but without using polish paper. The substrates were immersed vertically into the solution. Zinc chloride, selenium, and aluminum were used as the basic source materials. The samples were obtained in 1.0 mol/L NaOH solution at 80°C and characterized by X-ray diffraction pattern (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectrum (XPS). All reactants were analytical grade and purchased from Shanghai Chemical Co. Ltd. After dissolving 0.80 g (about 6.0 ⫻ 10⫺3 mol) into 100 ml 1.0 mol/L NaOH aqueous solution, 0.24 g (about 3.0 ⫻ 10⫺3 mol) selenium powder and 0.27 g (about 1.0 ⫻ 10⫺2 mol) aluminum foil were added to the solution. This mixture was put into a Teflon-lined stainlesssteel autoclave. The autoclave was then heated at 80°C for a certain duration time. After cooling, the as-prepared films were cleaned ultrasonically in distilled water to remove loosely bonded precipitates, washed with distilled water and absolute ethanol, and dried in a vacuum at room temperature. The phase and crystallinity were characterized by X-ray diffraction pattern, which was carried out using a Rigaku D/max ␥A X-ray diffractometer with Cu K␣ ( ⫽ 1.5418 Å) incident radiation. The surface morphologies of the films were observed by scanning electron microscope on a Hitachi-600. The chemical composition was determined by elemental analysis, using a Perkin-Elmer 1100B atomic absorption spectrophotometer. X-ray photoelectron spectroscopy (VGESCALAB MKII X-ray photoelectron spectrometer using non-monochromatized Al K␣ radiation as the excitation source) was also carried out. RESULTS AND DISCUSSION XRD patterns of the films prepared at 80°C for 48 h (Fig. 1a and 1b) and the powder formed (Fig. 1c) in the same system were recorded. By comparing the obtained XRD patterns with the standard (JCPDS 5-522), it was found that cubic zinc selenide was the only crystalline phase formed in this process. The other peaks in 1(a) and 1(b) can be attributed to the diffraction of the two substrates used. There is no significant difference in the broadening of the diffraction peaks between the three patterns shown in Figure 1. This indicates that the substrates used may not promote growth of grains. The broadening of the diffraction peaks also indicates that the as-formed films are comprised of very small particles. Figure 2a and 2b show the surface morphology of the zinc selenide films deposited on Teflon and ␣-Al2O3 substrates at 80°C for 48 h. It can be seen that both films are dense, smooth, and homogeneous, without visible pores. The cleavage of film deposited on ␣-Al2O3 gives a thickness of 1.8 m (Fig. 2c).
Vol. 34, Nos. 10/11
ZINC SELENIDE FILMS
1639
FIG. 1 X-ray diffraction patterns of ZnSe films deposited on Teflon (a) and polycrystalline ␣-Al2O3 (b) substrates, and powder formed in the same system (c). 夹: the phase of ZnSe.
XPS results show the presence of Zn and Se from the nanocrystals and C and O of gaseous molecules adsorbed on the nanocrystals’ surface. The peak cores at 1022.20 eV and 54.10 eV correspond to Zn 2p3 and Se 3d, respectively. After identification, no peak could be assigned to elemental selenium present in the sample deposited at 80°C for 48 h. This indicates that the films were free of contamination from selenium. Peak areas of the Zn and Se cores were measured and used to calculate the Zn:Se ratio in the films. They gave a Zn:Se ratio of 1.02:1. This result was confirmed by elemental analysis, which gave a Zn:Se ratio of 1.01:1. In the present study, the formation of ZnSe could be described as
2⫺
3Se ⫹ 6OH ⫺ 3 2Se 2⫺ ⫹ SeO 32⫺ ⫹ 3H 2O
(1)
Se 2⫺ ⫹ Zn(OH) 42⫺ 3 ZnSe ⫹ 4OH ⫺
(2)
Selenium ions (Se ) are produced slowly from the disproportionation of elemental selenium, to satisfy the film deposition rate in the alkaline aqueous solution. Previous reports on preparing II–VI semiconductor films containing selenium in aqueous solutions concentrated mainly on the use of sodium selenosulfate as one of the source materials. The release of Se2⫺ ions was believed to be caused by the hydrolysis of sodium selenosulfate in an alkaline medium [7–9,11–13].
1640
C. WANG et al.
Vol. 34, Nos. 10/11
FIG. 2 SEM micrographs of the films prepared on Teflon (a) and polycrystalline ␣-Al2O3 (b) substrates, and cleavage of films on polycrystalline ␣-Al2O3 (c). The addition of aluminum in this process is essential, otherwise the yield of ZnSe is very low and no film can be obtained. Aluminum reacts with OH– in the solution, to act as a 2– reducing agent, ensuring the full transformation of SeO2– ions: 3 to Se 2 Al ⫹ SeO 32⫺ ⫹ 3H 2O ⫹ 2OH ⫺ 3 2 Al(OH) 4⫺ ⫹ Se 2⫺
(3)
It was also observed that the elemental selenium could almost disperse homogeneously in the NaOH solution after heat treatment. This dispersion, accompanied by the dissolution of Zn2⫹ in basic solution, results in a ZnSe formation process similar to the one occurring in homogeneous solutions. The growth of zinc selenide films was studied at 80°C with variable OH⫺ concentrations, temperature, and growth duration time. If the concentration of OH⫺ was lower than 0.3 mol/L, growth was slow. The optimal concentration range of OH⫺ was about 0.5–1.0 mol/L; higher concentration was unnecessary. Although the chemical processes involved could be accelerated with the elevation of temperature, the temperature should be maintained below 125°C, since the appearance of ZnO at 125°C [14] contaminated the ZnSe films, decreasing their quality. A growth duration time of 48 h for a typical reaction could supply films with thicknesses of up 1.8 m. Thin films can be obtained, if necessary, by decreasing the amount of selenium added or the duration time. Decreasing the duration time from 48 to 12 h, the thickness of the films could be adjusted from 1.8 to 0.4 m. It should be noted here that the chemical processes could not be completed at 80°C, even with a duration time of 36 h. Fortunately, the presence of unreacted elemental selenium had no negative effect on the ZnSe films deposited on the Teflon and ␣-Al2O3 substrates. This was probably due to the low affinity of selenium to the substrates and the as-formed ZnSe. CONCLUSION Zinc selenide films (1.8 m thick) were prepared by the chemical solution reduction process. XRD patterns showed the films to be polycrystalline in nature. SEM micrographs indicated that the films were dense, smooth, and homogeneous. This method is simple and can be applied in the synthesis of other metal selenide films.
Vol. 34, Nos. 10/11
ZINC SELENIDE FILMS
1641
ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China and Huo Yingdong Foundation for Youth Teachers. REFERENCES 1. C.-H. Lee, Y.-D. Choi, G.-N. Jeon, S.-C. Yu, and S.-Y. Ko, J. Crystal Growth 167, 473 (1996). 2. F. Kadotsuji, H. Ohnishi, N. Kawabata, M. Kimura, A. Tanaka, and T. Sukegawa, J. Crystal Growth 155, 23 (1995). 3. N. Chestnoy, R. Hull, and L.E. Brus, J. Chem. Phys. 85, 2237 (1986). 4. N. Matsumura, K. Maemura, T. Mori, and J. Saraie, J. Crystal Growth 159, 85 (1996). 5. M.D. McCluskey, E.E. Haller, F.X. Zach, and E.D. Bourret-Courchesne, Appl. Phys. Lett. 68, 3476 (1996). 6. S. Deki and Y. Aoi, J. Mater. Res. 13, 883 (1998). 7. G.M. Fofanov and G.A. Kitaev, Russ. J. Inorg. Chem. 14, 322 (1969). 8. G.A. Kitaev and T.S. Terekhova, Russ. J. Inorg. Chem. 15, 25 (1970). 9. R.C. Kainthla, D.K. Pandya, and K.L. Chopra, J. Electrochem. Soc. 127, 277 (1980). 10. P. Pramanik and S. Biswas, J. Electrochem. Soc. 133, 350 (1986). 11. P.S. Mane and C.D. Lokhande, Thin Solid Films 304, 56 (1997). 12. G.S. Shahane, B. More, C.B. Botti, and L.P. Deshmukh, Mater. Chem. Phys. 47, 263 (1997). 13. S. Gorer, G. Hodes, Y. Sorek, and R. Reisfeld, Mater. Lett. 31, 209 (1997). 14. S.X. Yan and C.F. Wang, Normal Inorganic Chemistry, Peking University Press, Beijing (1994).