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Orientation of CdTe epitaxial films on GaAs(100) grown by vacuum evaporation
Thin Solid Films, 203 (1991) 105-111
105
PREPARATION AND CHARACTERIZATION
O R I E N T A T I O N O F CdTe E P I T A X I A L FILMS ON GaAs(100) G R O W N BY V A C U U M E V A P O R A T I O N MAU-PHON HOUNG,SHEN-LIFU AND FENQ-LIN JENQ Department of Electrical Engineering, National Cheng Kung University, Tainan 70101 (Taiwan)
JIANN-RUEY CHEN Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30043 ( Taiwan )
(ReceivedNovember2, 1990;acceptedMarch 4, 1991)
The growth of(100)- and (111)-oriented CdTe epitaxial layers on (100)-oriented GaAs substrates were investigated. Ar ÷ plasma bombardment was used to remove the surface oxide layer, while preheating the substrate before evaporation was performed to deplete arsenic on the GaAs substrate surface. Results indicate that the CdTe(100) will grow on GaAs(100) with an oxide layer remaining on the surface. For the GaAs(100) substrate with the oxide layer removed by plasma bombardment, CdTe(100) will grow on the arsenic-depleted GaAs substrate, while CdTe(111) will grow on the GaAs substrate without arsenic depletion. A model is proposed that a tellurium-rich surface is formed on the arsenic-depleted GaAs surface through G a Te bonding on which the CdTe(100) will grow, whereas CdTe(111) will grow on a tellurium-poor surface. The photoluminescence investigation conforms to our proposed model.
1. INTRODUCTION The epitaxial growth of CdTe on GaAs substrates has been studied extensively because of its wide application in solar cells x and other optoelectronic devices 2-4. Various methods have been reported on the growth of CdTe epitaxial layers, including molecular beam epitaxy (MBE) 5, photoassisted molecular epitaxy 6, metal-organic chemical vapour deposition 7, laser-assisted deposition and annealing a and ion beam sputter deposition 9, with CdTe grown on GaAs with either of the following two epitaxial relations: (1) CdTe(lll)[IGaAs(100) and CdYe[211]llGaAs(VQQ)x°; (2) CdTe(100)tlGaAs(100 ) and CdTe[011]ll GaAs[011]11. Otsuka et a l ) 2 studied the interfaces between M BE-grown CdTe films and GaAs(100) substrates and suggested that the growth of CdTe(100) epitaxial layers was due to the presence of a thin oxide layer on GaAs(100) substrates. To observe the effect of the thin oxide layer on the orientation of CdTe epitaxial films, the thin oxide layer was generally removed by heating the GaAs substrate at about 580 °C. In this work, however, we study the growth of vacuum-evaporated CdTe epitaxial layers by introducing Ar ÷ plasma bombardment to remove the 0040-6090/91/$3.50
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oxide layer and substrate annealing at lower temperatures to deplete arsenic from the substrate surface before evaporation. X-ray diffraction analysis was used to observe the orientation of CdTe epitaxial films, while their relative coverages can be calculated from the corresponding X-ray intensities. The results will show that the orientations of CdTe films grown on GaAs substrates were not only affected by the thin oxide layer on the substrate surface but also effected by the stoichiometry of the GaAs substrate. Photoluminescence (PL) spectra were used to observe the quality of the epitaxial films. PL spectra were also used to identify the cadmium and tellurium vacancies, which confirms our proposed model that a tellurium-rich surface is formed on the arsenic-depleted GaAs surface through G a - T e bonding on which the CdTe(100) will grow, whereas CdTe(111) will grow on a tellurium-poor surface. 2.
EXPERIMENTAL PROCEDURES
Vacuum evaporation was used in this work to deposit CdTe onto GaAs(100) substrates. After standard cleaning procedures in organic solvent, the GaAs substrate and the polycrystalline CdTe target of 99.9999~o purity were loaded into the vacuum chamber. The growth temperature was kept at 255°C. The base pressure of the vacuum chamber is about 2 x 10-6 mbar, while the total pressure in the chamber was maintained at about 4 x 1 0 - 6 m b a r ~turing evaporation. Ar ÷ plasma bombardment was used to control quantitatively the amount of the surface oxide layer remaining on the GaAs(100) surface and to control the amount of arsenic depleted from the GaAs substrate. After evaporation, the grown CdTe films were examined by X-ray diffraction analysis to determine their film orientations, while the coverages of(111)- and (100)-oriented CdTe epitaxial films were calculated from the peak intensities of X-ray diffraction from CdTe(111) and CdTe(400) planes. PL spectra were used to observe the film qualities and to identify the cadmium and tellurium vacancies in the films. 3.
RESULTS AND DISCUSSION
In this work, the amount of the surface oxide layer remaining on the GaAs(100) surface was controlled by the Ar ÷ plasma bombardment. We show that the coverage of C d T e ( l l l ) and CdTe(100) grown on the GaAs(100) can be well controlled by varying the plasma power and/or plasma bombardment time. The coverages were calculated from the peak intensities of X-ray diffraction from CdTe(111) and CdTe(400) planes. If their intensities are represented by I(111) and /(400), the CdTe(111) coverage is expressed as I(111)/{/(111) + I(400)/0.06}, where 0.06 is the relative sensitivity factor of X-ray diffraction from CdTe(400) and CdTe(111). Although Ar ÷ ion bombardment will cause part of the GaAs surface to be in an amorphous state on which no epitaxial film will grow, all the significant results in this paper were obtained after the bombarded surface has been annealed and recrystallized. At low plasma power, the kinetic energy ofAr ÷ ions can only locally remove the oxide layer. CdTe(111) and CdTe(100) will coexist on the sample surface, with the coverage of CdTe(111) increasing with plasma power. Figure 1 shows the CdTe(111)
VACUUM-EVAPORATED EPITAXIAL C d T e ON G a A s ( 1 0 0 )
107
coverage v s . the Ar ÷ plasma power, where samples were bombarded by Ar ÷ plasma for 10 min, followed by annealing at 375 °C for 2 min. A slow increase in CdTe(111) coverage was observed with plasma power up to 1.4 kV. In this power region, the plasma power begins to remove the oxide layer locally and these local areas act as seeds to crystallize into CdTe(111) where gallium is not covered, while CdTe(100) will grow on the areas with oxides. The CdTe(111) grains grown on bare GaAs crystal increase abruptly with plasma power until it reaches about 2.3 kV, where the areas of the bare GaAs crystal are so large that full coverage with the CdTe(111) epitaxial layer was obtained by the combination of all these CdTe(111) grains. Full coverage with CdTe(ll 1) was maintained between 2.3 and 2.7 kV. If the plasma power is even higher, the C d T e ( l l l ) coverage suddenly drops to almost zero coverage at 3.2 kV. At this condition, the plasma power is so high that the GaAs substrate was severely transformed into amorphous state and could not be recrystallized by annealing at 375 °C for 2 min, while the bare amorphous GaAs could not function as the seed for CdTe(111) crystallization. It has been proposed that the oxide layer on the GaAs(100) substrate surface would adsorb tellurium atoms through the T e - O bonding, and CdTe(100) will grow on this tellurium-rich surface 13. However, CdTe(ll 1) will grow on the telluriumpoor GaAs surface where the oxide layer was removed. It is believed that the oxide layer can be removed by Ar ÷ plasma bombardment, and the amorphous layer formed by plasma bombardment can be recrystallized by annealing. Since the plasma bombardment is not directional as the ion sputtering is, the plasma bombards the substrate surface at random and locally removes the oxide layer on
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Fig. 1. CdTe(111) coverage v s . the argon plasma power, where samples were bombarded by Ar + plasma for 10 min, followed by annealing at 375 °C for 2 rain. Fig. 2. CdTe(111) coverage v s . the annealing time, where samples were bombarded by Ar + plasma at 2.3 kV for 10min, followed by annealing at 375 °C.
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the substrate surface. If the GaAs substrate is not gallium rich, the local areas where the oxide is removed will act as seeds for crystallization i n t o C d T e ( l l l ) , while CdTe(100) will grow on the areas with an oxide layer. In Fig. 1, we assume that the GaAs substrate was not gallium rich, i.e. no arsenic depletion occurred on the GaAs surface during the annealing procedure. The dependence of CdTe(111) coverage on annealing time is shown in Fig. 2, where samples were bombarded by an Ar + plasma at 2.3kV for I0 min followed by annealing at 375 °C. F r o m Fig. 1, we know that a plasma power of 2.3 kV will remove enough of the oxide layer for full coverage of C d T e ( l l l ) to grow if the b o m b a r d e d substrate had recrystallized. Figure 2 shows that, if the annealing time is longer than 15 s, the bombarded GaAs substrate will recrystallize completely and this will result in full coverage of CdTe(111). However, the CdTe(111) coverage decreased if the annealing time was longer than 2 min. We believe that the GaAs is depleted of arsenic which leaves dangling bonds at the gallium sites and provides the G a - T e bonding. However, CdTe(100) will grow on the tellurium-rich G a A s surface in a similar way to that resulting from tellurium adsorption by the oxide layer. Some test samples were prepared by evaporating CdTe on the substrate without any plasma b o m b a r d m e n t but preheated at 375 °C for various times to examine the effect of Ar ÷ plasma b o m b a r d m e n t and preheating on the substrate. The CdTe films grown on all these samples exhibit (100) orientation. This suggests that the surface oxide layer was removed by Ar + plasma b o m b a r d m e n t and not by preheating the GaAs substrate, while preheating the substrate before evaporation only recrystallized the amorphous layer and depleted arsenic from the GaAs surface. PL spectra were obtained at 17 K and PL spectra of samples listed in Table I are shown in Figs. 3 and 4. The PL spectra of CdTe epilayers display the exciton emission peak, the edge emission peak and multiple peaks on the defect band 14. We first discuss the PL spectra of the samples of Figs. 3, spectrum a, and 4, spectrum a, on which CdTe(100) was grown. The PL spectrum a of Fig. 3 exhibited the edge emission peak at 800nm, which arose from the undoped conduction d o n o r acceptor transition. The appearance of this peak provides evidence that a high quality CdTe epitaxial layer was obtained. However, around 866 nm, there was a broad band with multiple peaks which was determined to be a defect band and was contributed by the cadmium vacancy-interstitial complex transition and longitudinal optical (LO) phonon. In the PL spectrum a of Fig. 4, the edge emission peak TABLE I THE G R O W T H CONDITIONS A N D T H E FILM ORIENTATIONS OF T H E SAMPLES W H O S E SPECTRA ARE S H O W N IN FIGS.
Sample
1 2 3 4 5
PHOTOLUMINESCENCE
3 AND 4
PL spectrum
Fig. 3, spectrum a Fig. 3, spectrum b Fig. 4, spectrum a Fig. 4, spectrum b Fig. 4, spectrum c
Film orientation
CdTe(100) CdTe(111) CdTe(100) CdTe(111) CdTe(111)
Plasma power
Plasma time
Annealing temperature
Annealing time
(kV)
(min)
(°C)
(min)
. 375 375 375 375
0.5 2.0 1.0 0.5
. 2.3 -2.3 2.3
.
. 30 -10 10
VACUUM-EVAPORATED EPITAXIAL CdTe ON GaAs(100)
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was absent and,the peak intensity of cadmium vacancy-interstitial transition emission decreased while the width increased. These PL spectra showed that higher preheating temperatures would degrade the crystal quality of CdTe(100). CdTe(111) epitaxial films were grown on samples of Figs. 3, spectrum b, and 4, spectra b and c. Samples of Figs. 3, spectrum b, and 4, spectrum c, had the same growth conditions except that the plasma bombardment time for spectrum Fig. 3, b,
110
M.-P. HOUNG et al.
is longer. The PL spectrum b of Fig. 3 exhibits a dominant edge emission peak at 800nm. The extension of this peak either reveals the high quality of the CdTe epitaxial layer or implies increases in tellurium vacancies. Since around 866 nm there was a broad band with multiple peaks which was regarded as a defect band resulting from the cadmium vacancy-interstitial complex transition and LO phonon, the extension of the edge emission peak demonstrated the increase in tellurium vacancies. The PL spectrum c of Fig. 4 displayed a dominant emission peak at 780 nm which was contributed by the band exciton transition, while the 800 nm edge emission peak was absent. The intensity of band peak resulting from the cadmium vacancy-interstitial complex transition and LO phonon was decreased. The existence of the exciton emission peak indicates that the quality of the sample of Fig. 4, spectrum c, is better than that of the sample of Fig. 3, spectrum b. These results suggest that the quality of the CdTe(111) film degrades with increasing plasma bombardment time. The sample of Fig. 4, spectrum b, has the same growth condition as the sample of Fig. 4, spectrum c, except a longer annealing time. From the PL spectra b and c of Fig. 4, one finds that spectrum b of Fig. 4 exhibited an increase in the intensity of 800 nm edge emission peak, while the 860 nm cadmium vacancy peak was absent and the 780 nm exciton emission peak was decreased. From these PL spectra, one can conclude that an increase in the annealing time would increase the number of tellurium vacancies but degrade the film quality. These PL results also conform to our proposed model that C d T e ( l l l ) would be grown on tellurium-poor GaAs surfaces. It showed that the appearance of a tellurium vacancy was evident in the sample of Fig. 4, spectrum b, whose annealing time had been extended, and the quality was degraded by increasing preheating time. 4. CONCLUSIONS Epitaxial CdTe films can be grown on GaAs(100) by vacuum evaporation. By introducing Ar ÷ plasma bombardment to remove the oxide layer on the substrate surface, and by introducing substrate annealing at lower temperature to deplete arsenic from the substrate surface before evaporation, experimental results indicate that CdTe(100) will grow on GaAs(100) with an oxide layer remaining on the surface. For the GaAs(100) substrate with the oxide layer removed by plasma bombardment, CdTe(100) will grow on arsenic-depleted GaAs substrates while CdTe(111) will grow on the GaAs substrate without arsenic depletion. A model is proposed that a tellurium-rich surface is formed on the arsenic-depleted GaAs surface through G a Te bonding on which the CdTe(100) will grow, whereas CdTe(111) will grow on a tellurium-poor surface. The PL spectra conform to our proposed model. REFERENCES 1 N. Nakayama, H. Matsumoto, K. Yamaguchi, S. Ikegami and Y. Hioki, Jpn. J. Appl. Phys., 15 (1976) 2281. 2 D.L. Dreifus, R. M. Kolbas, K. A. Harris, R. N. Bicknell, R. L. Harper and J. F. Schetzina, Appl. Phys. Lett., 51 (1987) 931. 3 A.L. Fahrenbruch, J. Cryst. Growth, 39 (1977) 73.
VACUUM-EVAPORATED EPITAXIAL C d T e ON G a A s ( 1 0 0 )
4 5 6 7 8 9 10 11 12 13 14
i 11
C.C. Wang, M. Chu, S. H. Shin, W. E. Tennant, J. T. Cheung, M. Lanir, A. H. B. Vanderwyck, G. M. Williams, L. O Bubulac and R. J. Eisel, IEEE Trans. Electron Devices, 27 (1980) 154. R.N. Bicknell, R. W. Yanka, N. C. Giles, J. F. Schetzina, T. J. Magee, C. Leung and H. Kawayoshi, Appl. Phys. Lett., 44 (1984) 313. R.N. Bicknell, N. C. Giles and J. F. Schetzina, Appl. Phys. Lett., 49 (1986) 1095. P.L. Anderson, J. Vac. Sci. Technol. A, 4 (1986) 2162. J.T. Cheung, M. Khoshnevisan and T. Magee, Appl. Phys., 43 (1983) 462. S.V. Krishnaswamy, J. H. Rieger, N. J. Doyle and M. H. Francombe, J. Vac. Sci. Technol., A, 5 (1987) 2106. J.M. Ballingall, W. J. Takei and B. J. Feldman, Appl. Phys. Lett., 47 (1985) 599. E. Rauhala, J. Keinonen, K. Rakennus and M. Pessa, Appl. Phys. Lett., 51 (1987) 973. N. Otsuka, L. A. Kolodziejski, R. L. Gunshor, S. Datta, R. N. Bicknell and J. F. Schetzina, Appl. Phys. Lett., 46 (1985) 860. R.D. Feldman and R. F. Austin, Appl. Phys. Lett., 49 (1986) 954. K. Sugiyama, Thin Solid Films, 115 (1984) 97.
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PREPARATION AND CHARACTERIZATION
O R I E N T A T I O N O F CdTe E P I T A X I A L FILMS ON GaAs(100) G R O W N BY V A C U U M E V A P O R A T I O N MAU-PHON HOUNG,SHEN-LIFU AND FENQ-LIN JENQ Department of Electrical Engineering, National Cheng Kung University, Tainan 70101 (Taiwan)
JIANN-RUEY CHEN Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30043 ( Taiwan )
(ReceivedNovember2, 1990;acceptedMarch 4, 1991)
The growth of(100)- and (111)-oriented CdTe epitaxial layers on (100)-oriented GaAs substrates were investigated. Ar ÷ plasma bombardment was used to remove the surface oxide layer, while preheating the substrate before evaporation was performed to deplete arsenic on the GaAs substrate surface. Results indicate that the CdTe(100) will grow on GaAs(100) with an oxide layer remaining on the surface. For the GaAs(100) substrate with the oxide layer removed by plasma bombardment, CdTe(100) will grow on the arsenic-depleted GaAs substrate, while CdTe(111) will grow on the GaAs substrate without arsenic depletion. A model is proposed that a tellurium-rich surface is formed on the arsenic-depleted GaAs surface through G a Te bonding on which the CdTe(100) will grow, whereas CdTe(111) will grow on a tellurium-poor surface. The photoluminescence investigation conforms to our proposed model.
1. INTRODUCTION The epitaxial growth of CdTe on GaAs substrates has been studied extensively because of its wide application in solar cells x and other optoelectronic devices 2-4. Various methods have been reported on the growth of CdTe epitaxial layers, including molecular beam epitaxy (MBE) 5, photoassisted molecular epitaxy 6, metal-organic chemical vapour deposition 7, laser-assisted deposition and annealing a and ion beam sputter deposition 9, with CdTe grown on GaAs with either of the following two epitaxial relations: (1) CdTe(lll)[IGaAs(100) and CdYe[211]llGaAs(VQQ)x°; (2) CdTe(100)tlGaAs(100 ) and CdTe[011]ll GaAs[011]11. Otsuka et a l ) 2 studied the interfaces between M BE-grown CdTe films and GaAs(100) substrates and suggested that the growth of CdTe(100) epitaxial layers was due to the presence of a thin oxide layer on GaAs(100) substrates. To observe the effect of the thin oxide layer on the orientation of CdTe epitaxial films, the thin oxide layer was generally removed by heating the GaAs substrate at about 580 °C. In this work, however, we study the growth of vacuum-evaporated CdTe epitaxial layers by introducing Ar ÷ plasma bombardment to remove the 0040-6090/91/$3.50
© ElsevierSequoia/Printedin The Netherlands
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oxide layer and substrate annealing at lower temperatures to deplete arsenic from the substrate surface before evaporation. X-ray diffraction analysis was used to observe the orientation of CdTe epitaxial films, while their relative coverages can be calculated from the corresponding X-ray intensities. The results will show that the orientations of CdTe films grown on GaAs substrates were not only affected by the thin oxide layer on the substrate surface but also effected by the stoichiometry of the GaAs substrate. Photoluminescence (PL) spectra were used to observe the quality of the epitaxial films. PL spectra were also used to identify the cadmium and tellurium vacancies, which confirms our proposed model that a tellurium-rich surface is formed on the arsenic-depleted GaAs surface through G a - T e bonding on which the CdTe(100) will grow, whereas CdTe(111) will grow on a tellurium-poor surface. 2.
EXPERIMENTAL PROCEDURES
Vacuum evaporation was used in this work to deposit CdTe onto GaAs(100) substrates. After standard cleaning procedures in organic solvent, the GaAs substrate and the polycrystalline CdTe target of 99.9999~o purity were loaded into the vacuum chamber. The growth temperature was kept at 255°C. The base pressure of the vacuum chamber is about 2 x 10-6 mbar, while the total pressure in the chamber was maintained at about 4 x 1 0 - 6 m b a r ~turing evaporation. Ar ÷ plasma bombardment was used to control quantitatively the amount of the surface oxide layer remaining on the GaAs(100) surface and to control the amount of arsenic depleted from the GaAs substrate. After evaporation, the grown CdTe films were examined by X-ray diffraction analysis to determine their film orientations, while the coverages of(111)- and (100)-oriented CdTe epitaxial films were calculated from the peak intensities of X-ray diffraction from CdTe(111) and CdTe(400) planes. PL spectra were used to observe the film qualities and to identify the cadmium and tellurium vacancies in the films. 3.
RESULTS AND DISCUSSION
In this work, the amount of the surface oxide layer remaining on the GaAs(100) surface was controlled by the Ar ÷ plasma bombardment. We show that the coverage of C d T e ( l l l ) and CdTe(100) grown on the GaAs(100) can be well controlled by varying the plasma power and/or plasma bombardment time. The coverages were calculated from the peak intensities of X-ray diffraction from CdTe(111) and CdTe(400) planes. If their intensities are represented by I(111) and /(400), the CdTe(111) coverage is expressed as I(111)/{/(111) + I(400)/0.06}, where 0.06 is the relative sensitivity factor of X-ray diffraction from CdTe(400) and CdTe(111). Although Ar ÷ ion bombardment will cause part of the GaAs surface to be in an amorphous state on which no epitaxial film will grow, all the significant results in this paper were obtained after the bombarded surface has been annealed and recrystallized. At low plasma power, the kinetic energy ofAr ÷ ions can only locally remove the oxide layer. CdTe(111) and CdTe(100) will coexist on the sample surface, with the coverage of CdTe(111) increasing with plasma power. Figure 1 shows the CdTe(111)
VACUUM-EVAPORATED EPITAXIAL C d T e ON G a A s ( 1 0 0 )
107
coverage v s . the Ar ÷ plasma power, where samples were bombarded by Ar ÷ plasma for 10 min, followed by annealing at 375 °C for 2 min. A slow increase in CdTe(111) coverage was observed with plasma power up to 1.4 kV. In this power region, the plasma power begins to remove the oxide layer locally and these local areas act as seeds to crystallize into CdTe(111) where gallium is not covered, while CdTe(100) will grow on the areas with oxides. The CdTe(111) grains grown on bare GaAs crystal increase abruptly with plasma power until it reaches about 2.3 kV, where the areas of the bare GaAs crystal are so large that full coverage with the CdTe(111) epitaxial layer was obtained by the combination of all these CdTe(111) grains. Full coverage with CdTe(ll 1) was maintained between 2.3 and 2.7 kV. If the plasma power is even higher, the C d T e ( l l l ) coverage suddenly drops to almost zero coverage at 3.2 kV. At this condition, the plasma power is so high that the GaAs substrate was severely transformed into amorphous state and could not be recrystallized by annealing at 375 °C for 2 min, while the bare amorphous GaAs could not function as the seed for CdTe(111) crystallization. It has been proposed that the oxide layer on the GaAs(100) substrate surface would adsorb tellurium atoms through the T e - O bonding, and CdTe(100) will grow on this tellurium-rich surface 13. However, CdTe(ll 1) will grow on the telluriumpoor GaAs surface where the oxide layer was removed. It is believed that the oxide layer can be removed by Ar ÷ plasma bombardment, and the amorphous layer formed by plasma bombardment can be recrystallized by annealing. Since the plasma bombardment is not directional as the ion sputtering is, the plasma bombards the substrate surface at random and locally removes the oxide layer on
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Fig. 1. CdTe(111) coverage v s . the argon plasma power, where samples were bombarded by Ar + plasma for 10 min, followed by annealing at 375 °C for 2 rain. Fig. 2. CdTe(111) coverage v s . the annealing time, where samples were bombarded by Ar + plasma at 2.3 kV for 10min, followed by annealing at 375 °C.
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the substrate surface. If the GaAs substrate is not gallium rich, the local areas where the oxide is removed will act as seeds for crystallization i n t o C d T e ( l l l ) , while CdTe(100) will grow on the areas with an oxide layer. In Fig. 1, we assume that the GaAs substrate was not gallium rich, i.e. no arsenic depletion occurred on the GaAs surface during the annealing procedure. The dependence of CdTe(111) coverage on annealing time is shown in Fig. 2, where samples were bombarded by an Ar + plasma at 2.3kV for I0 min followed by annealing at 375 °C. F r o m Fig. 1, we know that a plasma power of 2.3 kV will remove enough of the oxide layer for full coverage of C d T e ( l l l ) to grow if the b o m b a r d e d substrate had recrystallized. Figure 2 shows that, if the annealing time is longer than 15 s, the bombarded GaAs substrate will recrystallize completely and this will result in full coverage of CdTe(111). However, the CdTe(111) coverage decreased if the annealing time was longer than 2 min. We believe that the GaAs is depleted of arsenic which leaves dangling bonds at the gallium sites and provides the G a - T e bonding. However, CdTe(100) will grow on the tellurium-rich G a A s surface in a similar way to that resulting from tellurium adsorption by the oxide layer. Some test samples were prepared by evaporating CdTe on the substrate without any plasma b o m b a r d m e n t but preheated at 375 °C for various times to examine the effect of Ar ÷ plasma b o m b a r d m e n t and preheating on the substrate. The CdTe films grown on all these samples exhibit (100) orientation. This suggests that the surface oxide layer was removed by Ar + plasma b o m b a r d m e n t and not by preheating the GaAs substrate, while preheating the substrate before evaporation only recrystallized the amorphous layer and depleted arsenic from the GaAs surface. PL spectra were obtained at 17 K and PL spectra of samples listed in Table I are shown in Figs. 3 and 4. The PL spectra of CdTe epilayers display the exciton emission peak, the edge emission peak and multiple peaks on the defect band 14. We first discuss the PL spectra of the samples of Figs. 3, spectrum a, and 4, spectrum a, on which CdTe(100) was grown. The PL spectrum a of Fig. 3 exhibited the edge emission peak at 800nm, which arose from the undoped conduction d o n o r acceptor transition. The appearance of this peak provides evidence that a high quality CdTe epitaxial layer was obtained. However, around 866 nm, there was a broad band with multiple peaks which was determined to be a defect band and was contributed by the cadmium vacancy-interstitial complex transition and longitudinal optical (LO) phonon. In the PL spectrum a of Fig. 4, the edge emission peak TABLE I THE G R O W T H CONDITIONS A N D T H E FILM ORIENTATIONS OF T H E SAMPLES W H O S E SPECTRA ARE S H O W N IN FIGS.
Sample
1 2 3 4 5
PHOTOLUMINESCENCE
3 AND 4
PL spectrum
Fig. 3, spectrum a Fig. 3, spectrum b Fig. 4, spectrum a Fig. 4, spectrum b Fig. 4, spectrum c
Film orientation
CdTe(100) CdTe(111) CdTe(100) CdTe(111) CdTe(111)
Plasma power
Plasma time
Annealing temperature
Annealing time
(kV)
(min)
(°C)
(min)
. 375 375 375 375
0.5 2.0 1.0 0.5
. 2.3 -2.3 2.3
.
. 30 -10 10
VACUUM-EVAPORATED EPITAXIAL CdTe ON GaAs(100)
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Fig. 3. PL spectra of samples 1 and 2 of Table I.
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was absent and,the peak intensity of cadmium vacancy-interstitial transition emission decreased while the width increased. These PL spectra showed that higher preheating temperatures would degrade the crystal quality of CdTe(100). CdTe(111) epitaxial films were grown on samples of Figs. 3, spectrum b, and 4, spectra b and c. Samples of Figs. 3, spectrum b, and 4, spectrum c, had the same growth conditions except that the plasma bombardment time for spectrum Fig. 3, b,
110
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is longer. The PL spectrum b of Fig. 3 exhibits a dominant edge emission peak at 800nm. The extension of this peak either reveals the high quality of the CdTe epitaxial layer or implies increases in tellurium vacancies. Since around 866 nm there was a broad band with multiple peaks which was regarded as a defect band resulting from the cadmium vacancy-interstitial complex transition and LO phonon, the extension of the edge emission peak demonstrated the increase in tellurium vacancies. The PL spectrum c of Fig. 4 displayed a dominant emission peak at 780 nm which was contributed by the band exciton transition, while the 800 nm edge emission peak was absent. The intensity of band peak resulting from the cadmium vacancy-interstitial complex transition and LO phonon was decreased. The existence of the exciton emission peak indicates that the quality of the sample of Fig. 4, spectrum c, is better than that of the sample of Fig. 3, spectrum b. These results suggest that the quality of the CdTe(111) film degrades with increasing plasma bombardment time. The sample of Fig. 4, spectrum b, has the same growth condition as the sample of Fig. 4, spectrum c, except a longer annealing time. From the PL spectra b and c of Fig. 4, one finds that spectrum b of Fig. 4 exhibited an increase in the intensity of 800 nm edge emission peak, while the 860 nm cadmium vacancy peak was absent and the 780 nm exciton emission peak was decreased. From these PL spectra, one can conclude that an increase in the annealing time would increase the number of tellurium vacancies but degrade the film quality. These PL results also conform to our proposed model that C d T e ( l l l ) would be grown on tellurium-poor GaAs surfaces. It showed that the appearance of a tellurium vacancy was evident in the sample of Fig. 4, spectrum b, whose annealing time had been extended, and the quality was degraded by increasing preheating time. 4. CONCLUSIONS Epitaxial CdTe films can be grown on GaAs(100) by vacuum evaporation. By introducing Ar ÷ plasma bombardment to remove the oxide layer on the substrate surface, and by introducing substrate annealing at lower temperature to deplete arsenic from the substrate surface before evaporation, experimental results indicate that CdTe(100) will grow on GaAs(100) with an oxide layer remaining on the surface. For the GaAs(100) substrate with the oxide layer removed by plasma bombardment, CdTe(100) will grow on arsenic-depleted GaAs substrates while CdTe(111) will grow on the GaAs substrate without arsenic depletion. A model is proposed that a tellurium-rich surface is formed on the arsenic-depleted GaAs surface through G a Te bonding on which the CdTe(100) will grow, whereas CdTe(111) will grow on a tellurium-poor surface. The PL spectra conform to our proposed model. REFERENCES 1 N. Nakayama, H. Matsumoto, K. Yamaguchi, S. Ikegami and Y. Hioki, Jpn. J. Appl. Phys., 15 (1976) 2281. 2 D.L. Dreifus, R. M. Kolbas, K. A. Harris, R. N. Bicknell, R. L. Harper and J. F. Schetzina, Appl. Phys. Lett., 51 (1987) 931. 3 A.L. Fahrenbruch, J. Cryst. Growth, 39 (1977) 73.
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