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Si
Thin Solid Films 316 Ž1998. 93]99
Total low temperature plasma process for epitaxial growth of compound semiconductors on Si: InSbrSi S. Yamauchi a,U , T. Hariu a , H. Ohbab , K. Sawamurac a
Department of Systems Engineering, Ibaraki Uni¨ersity, Nakanarusawa 4-12-1, Hitachi 316, Japan b Tohoku Electric Power Co., Ltd., Sendai 980, Japan c Oki Electric Industry Co. Ltd., Hachioji 193, Japan
Abstract Hydrogen plasma surface cleaning of Si at 6008C and the surface stabilization with arsenic-vapor at lower temperature were employed for heteroepitaxial growth of InSb on Si with large lattice mismatch of 19%. The passivated Si surface showed 1 = 1 structure and was stable even in air. InSb was successively grown in hydrogen plasma on the Si surface at 4508C with controlled SbrIn supply ratio. Initial buffer layer grown at 2108C with four average monolayers was found to be optimum with suppressed three-dimensional nucleation and strain with the longitudinal lattice mismatch around 12]13%. Hall mobility of 2 m m thick heteroepitaxial InSb film on Si successively grown by the total plasma process was 1.2= 10 4 cm2rVs at room temperature, compared to the lower value of 3 = 10 3 cm2rVs for the polycrystalline film. Q 1998 Elsevier Science S.A. Keywords: Epitaxial growth; InSb; Plasma process; Semiconductors
1. Introduction Heteroepitaxial growth of compound semiconductors on Si has been extensively investigated with a partial success in view of possible monolithic integration of compound semiconductor devices with Si IC. Low temperature processes have been pursued for fabricating advanced device structures. This approach reduces the effects of differential thermal expansion and chemical reaction between layers and substrates. The purpose of this paper is to describe the total low temperature process of epitaxial growth of InSb on Si, paying particular attention to low temperature cleaning and stabilization of the Si surface. Additionally, the optimization of the initial buffer layer in the two-step growth process is discussed. InSb on Si is an attractive systems for infrared OEIC, galvanomagnetic IC and high-speed IC appli-
U
Corresponding author. Tel.: q81 294 385199; fax: q81 294 385199; e-mail: [email protected] 0040-6090r98r$19.00 Q 1998 Elsevier Science S.A. All rights reserved PII S0040-6090Ž98.00396-4
cations, but is also a much more difficult system to work with as compared to GaAs on Si. The difficulty arises due to the larger lattice mismatch of 19%, making the epitaxial growth much more sensitive to the surface condition of Si. As a result, the grown layers tend to poly-crystallize easily. Although the present discussion is limited to the case of InSb on Si, the results will elucidate the role of the initial buffer layer and its optimization for heteroepitaxial growth with large mismatch such as GaAs on Si, Ge on Si, GaN on sapphire or SiC. These are certain to be intensively investigated to broaden the development of electronic material systems. 2. Experimental InSb thin films were grown on n-type Si substrates by plasma-assisted epitaxy w1x using hydrogen plasma. Si substrates were chemically etched by RCA method w2x, and then the surfaces were terminated by hydrogen in dilute HF solution. The use of hydrogenterminated Si as a substrate was also attempted, but it
S. Yamauchi et al. r Thin Solid Films 316 (1998) 93]99
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has not provided a robust substrate for InSb growth. This material is too sensitive to such surface structure as steps and is not always stable against residual oxygen at relatively low temperatures. Therefore, since As]Si bond is much stronger than H]Si bond, arsenic-terminated Si was prepared by exposure to arsenic vapor in hydrogen plasma during cooling, after a hydrogen plasma treatment at 6008C to remove native oxide w3x. Pure elemental In and Sb Ž99.9999%. were supplied by resistive heating through hydrogen plasma, which was excited by rf power at 13.56 MHz through inductive coupling. The details of the plasma-assisted epitaxy apparatus have been described elsewhere w4x. InSb epitaxial layer on the Si was grown by a two-step process. An initial InSb layer with a few monolayers was grown at relatively low temperatures 180]2708C, with the second active layer grown at 4508C. Detailed growth conditions are shown in Table 1. Surface structures and morphologies were examined by reflection high-energy electron diffraction ŽRHEED. and scanning electron microscope ŽSEM.. X-Ray diffraction ŽCo K a radiation. was used to determine the film structure. Hall mobility and carrier concentration were measured by the van der Pauw method under low magnetic field Ž2000 Gauss. at room temperature. 3. Results and discussion 3.1. Si surface cleaning and stabilization Si surface cleaning is a prerequisite for successive epitaxial growth on Si. Hydrogen-termination has been used for some kinds of heteroepitaxy on Si, however, the process has not given a sufficiently stabilized Si surface for the present InSbrSi heteroepitaxial growth. XRD patterns shown in Fig. 1 show non-epitaxial growth on hydrogen-terminated Ž111. and Ž100.Si. Epitaxial layers were in fact sometimes obtained, but not reproducibly. The polycrystalline growth is due to the high sensitivity of growth behavior on Si-surface structure and poor stability of Sisurface against residual oxygen at relatively low temperature. Fig. 2 shows RHEED patterns along the w011x-direction of hydrogen-terminated Ž100.Si surface
after annealing at 2708C in vacuum ŽFig. 2a. and in hydrogen plasma ŽFig. 2b.. The hydrogen-terminated surface structure which includes some steps or kinks on the annealed sample in vacuum was maintained. However, the structure was disordered after hydrogen plasma treatment. It can be concluded from these results that the hydrogen-terminated surface is not stable against hydrogen-plasma treatment, even at relatively low temperature, and the hydrogen-desorbed surface is easily oxidized. To avoid this, we employed hydrogen plasma treatment at 6008C to remove native oxide on the Si-surface, and then exposed the surface to arsenic vapor in the plasma during cooling. This was motivated by the fact that As]Si bond is much stronger than H]Si bond w5x. Fig. 3 shows RHEED patterns along w011x-direction of Ž100.Si-surface treated by hydrogen plasma at 6008C without ŽFig. 3a. and with ŽFig. 3b. exposure to arsenic vapor during cooling. These samples were exposed to air after treatment in the chamber. The RHEED pattern shows that, although the native oxide was removed from the surface treated at 6008C by hydrogen plasma, the surface without exposure to arsenic vapor was oxidized by residual oxygen in air ŽFig. 3a.. On the other hand, the surface exposed to arsenic vapor during cooling was so stable that even in air the RHEED pattern showed clear 1 = 1 structure of arsenic-terminated surface. RHEED patterns along w211x-direction of InSb films grown on Ž111.Si treated by these two different surface cleaning processes without ŽFig. 4a. and with ŽFig. 4b. arsenic vapor exposure are shown in Fig. 4. Epitaxial InSb growth was confirmed on the Si surface terminated with arsenic ŽFig. 4b., in contrast to poly-
Table 1 Detailed growth conditions for initial and active layers
Total pressure Growth temperature Induced rf power SbxrIn supply ratio Growth rate
Initial layer
Active layer
0.002 Torr 180]2708C 20]80 W 2]4 0.1]0.3 m mrh
0.02 Torr 4508C 40 W 17 0.7]0.8 m mrh
Fig. 1. XRD spectra of InSb films grown on Ža. Ž111.Si and Žb. Ž100.Si terminated by hydrogen.
S. Yamauchi et al. r Thin Solid Films 316 (1998) 93]99
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Fig. 2. RHEED patterns of hydrogen-terminated Ž100.Si surfaces along w011x azimuth after annealed at 2708C Ža. in vacuum and Žb. in hydrogen plasma.
crystalline InSb growth on the surface treated only by the hydrogen plasma ŽFig. 4a.. This was probably caused by partial oxidation of the Si-surface by residual oxygen. These results indicate that the arsenictermination process on Si is effective for the surface passivation against native oxide formation prior to the epitaxial growth. A probable model of hydrogen cleaning and arsenic passivation is schematically shown in Fig. 5. Surface terminating hydrogen can be desorbed at high temperature in vacuum, with the bare Si-surface being
partially oxidized by residual oxygen ŽFig. 5a.. The partially oxidized surface is cleaned by hydrogen plasma at 6008C, but if arsenic vapor is supplied to the surface at the same temperature, the active hydrogen not only removes the oxygen, but also the arsenic atoms from the surface ŽFig. 5b.. This was confirmed by non-epitaxial growth when the cleaned Si-surface was exposed to arsenic vapor at 6008C but not during cooling. In the cooling process, the hydrogen plasma continues to remove the oxygen at a lower temperature than 6008C, however, the arsenic-atoms
Fig. 3. RHEED patterns of Ž100.Si surface Ža. without As-termination and Žb. with As-termination along w011x azimuth. A 1 = 1 structure was maintained even in air.
Fig. 4. RHEED patterns along w211xazimuth of InSb films grown on Ž111.Si, Ža. cleaned only by hydrogen plasma at 6008C, and Žb. terminated by arsenic.
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Fig. 6. Variation of FWHM value of InSb films grown on Asterminated Ž111. and Ž100. Si with variable As-flux during the passivation process.
Fig. 5. Model for As-termination in As-contained hydrogen plasma during cooling after hydrogen plasma cleaning to remove native oxide.
are adsorbed onto the Si-surface up to one monolayer during cooling ŽFig. 5c.. Finally, the Si-surface will be passivated by arsenic atoms without oxygen ŽFig. 5d.. Arsenic-flux supplied for the passivation process should be optimized as shown in Fig. 6, where FWHM
Fig. 7. RHEED patterns along w211xazimuth and SEM images of initial InSb layers with Ža. 4ML, Žb. 8ML and Žc. 16ML grown at 2708C with a growth rate of 0.3 m mrh.
S. Yamauchi et al. r Thin Solid Films 316 (1998) 93]99
values of XRD peak patterns measured for InSb films grown on Ž111. and Ž100.Si are shown as a function of arsenic-cell temperature during the passivation process. In both cases on Ž111. and Ž100.Si, the value of FWHM is minimum at the same arsenic-cell temperature Ž2708C.. This indicates that the excess arsenic-flux which forms arsenic-layers thicker than a monolayer results in the deterioration of crystalline quality of InSb. 3.2. Optimization of initial buffer layer Bulk-InSb has a lattice constant about 19% larger than Si. This large lattice mismatch makes the epitaxial growth much more sensitive to the surface condition of partial oxidation on Si and also to such growth conditions as SbrIn supply ratio. Then three dimensional polycrystalline growth begins at quite an early stage of initial growth as shown in Fig. 7. This figure shows the RHEED patterns and SEM images of initial layers grown at 2708C with a growth rate of 0.3
97
m mrh. Ring-like patterns of polycrystalline InSb were observed at a growth stage of several monolayers and InSb grains grew larger with thickness. This tendency is more pronounced at higher temperatures. Thus, a very thin initial layer having a few monolayers should be used for initial layer in the two-step growth process. Detailed RHEED observations were employed to optimize the initial layer. Fig. 8 shows RHEED patterns of the initial layer grown at 1808C ŽFig. 8a., 2408C ŽFig. 8b. and 2708C ŽFig. 8c. with four monolayers on the Ž111.Si by lowering the growth rate down to 0.1 m mrh, and of active layers ŽFig. 8a9, b9 and c9. grown on the various initial layers. The three dimensional island growth was suppressed at relatively low temperature at 2408C, although the layer was grown as polycrystalline and amorphous at 2708C and at 1808C, respectively. The initial layers are not continuous ŽVolmer]Weber growth mode., and the size and density of islands depend on the growth temperature and growth rate. Larger grains with lower density grow epitaxially up to a few average monolayers but
Fig. 8. RHEED patterns along w211xazimuth of InSb initial layers and active layers.Initial layer grown at Ža. 1808C, Žb. 2408C and Žc. 2708C with growth rate of 0.1 m mrh. Active layers Ža9., Žb9. and Žc9. were grown on initial layers Ža., Žb. and Žc., respectively.
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Fig. 10. The effect of rf power applied for the initial layer growth at 2108C on the electronic properties of second active layers grown at 4508C. Fig. 9. The dependence of electronic properties of InSbrŽ111.Si films on the initial layer growth temperature for two different initial layer growth rate of 0.1 and 0.5 m mrh.
tend to polycrystallize at quite an early stage at higher temperature and lower growth rate. This is probably when the growing fronts of islands begin to collapse upon each other. With this kind of initial layer, the successive layer cannot be grown epitaxially ŽFig. 8c9.. When the initial layer was grown at too low temperature, it tends to be amorphous with the successive layer polycrystallized ŽFig. 8a9.. The film grown on the initial layer grown around 2408C has better crystal quality with less microtwins compared with that grown at the lower and higher temperatures ŽFig. 8b9 and c9.. The RHEED pattern also shows the initial layer was strained with longitudinal lattice mismatch around 12]13%, compared with 19% between bulk InSb and Si. This type of strained initial layer of a few monolayers with suppressed three dimensional island is useful to relax large lattice mismatch w6x. 3.3. Electronic property Epitaxial active layers of InSb were successively grown by the two-step plasma process on arsenic-passivated Si after hydrogen plasma cleaning. Because the electronic properties of InSb film grown on Si are more sensitive to the supply ratio than that grown on GaAs w7x, the supply flux ratio of SbrIn must be certainly optimized for this type of growth, as well as for initial layer growth. Fig. 9 shows Hall mobility and carrier concentration of 2 m m thick InSb films, which were grown at 4508C on the initial layer with 4ML, as a function of initial growth temperature. The epitaxial InSb layers showed n-type conductivity. The mobility of the epitaxial film was increased up to 1.2= 10 4 cm2rVs on the initial layer grown at 2108C with the low growth rate of 0.1 m mrh, in comparison to a lower value around 3 = 10 3 cm2rVs for the polycrys-
talline film directly grown on the Si without the initial layer. It is reasoned that the optimum growth condition for the initial buffer layer is determined by the competition between the number, the size and the quality of seeds. For lower temperature growth than the optimum, the surface of the initial layer is smooth and includes a large number of seeds; however, the quality of the seeds is poor as shown in Fig. 8a. In contrast, the quality of the seeds grown at higher temperature than optimum is improved but the surface is roughened by the larger size and smaller number of grains as shown in Fig. 8c. When the growth rate of the initial layer is increased up to 0.5 m mrh, the optimum shifts to a higher temperature of around 2408C. The mobilities showed that the quality of the active InSb layers is not dependent upon the growth rate of the initial layer for the higher growth temperatures, but is dependent on the rate at lower temperatures. This could result from the fact that the substrate surface is not a perfect mirror, but contains many kinks and steps which influence the nucleation of seeds. According to these results, the growth conditions for the initial layer should be optimized not only for temperature but also for the growth rate. It is worth mentioning here that the plasma should be optimized, because an increase in the rf power has a similar effect as growth temperature, shown in Fig. 10. This suggests that the temperature of the total process can be further lowered by higher excitation of the plasma. 4. Conclusions Total low temperature epitaxial growth of InSb on Si was performed by two-step plasma-assisted epitaxy. Prior to the growth, native oxide on Si-surface was removed by hydrogen plasma treatment at 6008C, and the surface stabilized with arsenic-containing hydrogen plasma during cooling. The arsenic-terminated Si
S. Yamauchi et al. r Thin Solid Films 316 (1998) 93]99
surface showed 1 = 1 structure and was stable, even in air. Detailed RHEED observations showed the initial layer of InSb tends to polycrystallize and form three dimensional islands within several monolayers. Optimum initial layers with about four monolayers were grown at low temperature, approximately 210]2408C, with lower growth rate. The initial layer was not continuous and was strained with longitudinal lattice mismatch around 12]13%, as compared with the large lattice mismatch of 19% between InSb and Si. The active InSb epitaxial layer was successively grown at 4508C on the initial layer with the mobility increased up to 1.2= 10 4 cm2rVs, whereas a value of 3 = 10 3 cm2rVs for polycrystalline InSb film was achieved. This type of strained initial layer of a few monolayers, grown at low temperature with suppressed three dimensional island formation, should be useful for other classes of heteroepitaxial growth with large lattice mismatch.
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Acknowledgement This work was partly supported by the Satellite Venture Business Laboratory project of Ibaraki University. References w1x K. Takenaka, T. Hariu, Y. Shibata, Jpn. J. Appl. Phys. 19 Ž1979. 765. w2x W. Kern, D.A. Puotinen, RCA Rev. 31 Ž1970. 187. w3x Q.Z. Gao, T. Hariu, S. Ono, Jpn. J. Appl. Phys. 26 Ž1987. L1576. w4x Y. Sato, K. Matsushita, T. Hariu, Y. Shibata, Appl. Phys. Lett. 44 Ž1984. 592. w5x R.D. Bringans, M.A. Olmstead, R.I.G. Uhrberg, R.Z. Bachrach, Appl. Phys. Lett. 51 Ž1987. 523. w6x M. Akiyama, Y. Kawarada, K. Kaminishi, Jpn. J. Appl. Phys. 23 Ž1984. L1843. w7x T. Hariu, K. Sawamura, F. Ito, H. Ohba, Proc. 23rd Int. Symp. Compound Semiconductors. Inst. Phys. Ser. 155 Ž1997. 331.
Total low temperature plasma process for epitaxial growth of compound semiconductors on Si: InSbrSi S. Yamauchi a,U , T. Hariu a , H. Ohbab , K. Sawamurac a
Department of Systems Engineering, Ibaraki Uni¨ersity, Nakanarusawa 4-12-1, Hitachi 316, Japan b Tohoku Electric Power Co., Ltd., Sendai 980, Japan c Oki Electric Industry Co. Ltd., Hachioji 193, Japan
Abstract Hydrogen plasma surface cleaning of Si at 6008C and the surface stabilization with arsenic-vapor at lower temperature were employed for heteroepitaxial growth of InSb on Si with large lattice mismatch of 19%. The passivated Si surface showed 1 = 1 structure and was stable even in air. InSb was successively grown in hydrogen plasma on the Si surface at 4508C with controlled SbrIn supply ratio. Initial buffer layer grown at 2108C with four average monolayers was found to be optimum with suppressed three-dimensional nucleation and strain with the longitudinal lattice mismatch around 12]13%. Hall mobility of 2 m m thick heteroepitaxial InSb film on Si successively grown by the total plasma process was 1.2= 10 4 cm2rVs at room temperature, compared to the lower value of 3 = 10 3 cm2rVs for the polycrystalline film. Q 1998 Elsevier Science S.A. Keywords: Epitaxial growth; InSb; Plasma process; Semiconductors
1. Introduction Heteroepitaxial growth of compound semiconductors on Si has been extensively investigated with a partial success in view of possible monolithic integration of compound semiconductor devices with Si IC. Low temperature processes have been pursued for fabricating advanced device structures. This approach reduces the effects of differential thermal expansion and chemical reaction between layers and substrates. The purpose of this paper is to describe the total low temperature process of epitaxial growth of InSb on Si, paying particular attention to low temperature cleaning and stabilization of the Si surface. Additionally, the optimization of the initial buffer layer in the two-step growth process is discussed. InSb on Si is an attractive systems for infrared OEIC, galvanomagnetic IC and high-speed IC appli-
U
Corresponding author. Tel.: q81 294 385199; fax: q81 294 385199; e-mail: [email protected] 0040-6090r98r$19.00 Q 1998 Elsevier Science S.A. All rights reserved PII S0040-6090Ž98.00396-4
cations, but is also a much more difficult system to work with as compared to GaAs on Si. The difficulty arises due to the larger lattice mismatch of 19%, making the epitaxial growth much more sensitive to the surface condition of Si. As a result, the grown layers tend to poly-crystallize easily. Although the present discussion is limited to the case of InSb on Si, the results will elucidate the role of the initial buffer layer and its optimization for heteroepitaxial growth with large mismatch such as GaAs on Si, Ge on Si, GaN on sapphire or SiC. These are certain to be intensively investigated to broaden the development of electronic material systems. 2. Experimental InSb thin films were grown on n-type Si substrates by plasma-assisted epitaxy w1x using hydrogen plasma. Si substrates were chemically etched by RCA method w2x, and then the surfaces were terminated by hydrogen in dilute HF solution. The use of hydrogenterminated Si as a substrate was also attempted, but it
S. Yamauchi et al. r Thin Solid Films 316 (1998) 93]99
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has not provided a robust substrate for InSb growth. This material is too sensitive to such surface structure as steps and is not always stable against residual oxygen at relatively low temperatures. Therefore, since As]Si bond is much stronger than H]Si bond, arsenic-terminated Si was prepared by exposure to arsenic vapor in hydrogen plasma during cooling, after a hydrogen plasma treatment at 6008C to remove native oxide w3x. Pure elemental In and Sb Ž99.9999%. were supplied by resistive heating through hydrogen plasma, which was excited by rf power at 13.56 MHz through inductive coupling. The details of the plasma-assisted epitaxy apparatus have been described elsewhere w4x. InSb epitaxial layer on the Si was grown by a two-step process. An initial InSb layer with a few monolayers was grown at relatively low temperatures 180]2708C, with the second active layer grown at 4508C. Detailed growth conditions are shown in Table 1. Surface structures and morphologies were examined by reflection high-energy electron diffraction ŽRHEED. and scanning electron microscope ŽSEM.. X-Ray diffraction ŽCo K a radiation. was used to determine the film structure. Hall mobility and carrier concentration were measured by the van der Pauw method under low magnetic field Ž2000 Gauss. at room temperature. 3. Results and discussion 3.1. Si surface cleaning and stabilization Si surface cleaning is a prerequisite for successive epitaxial growth on Si. Hydrogen-termination has been used for some kinds of heteroepitaxy on Si, however, the process has not given a sufficiently stabilized Si surface for the present InSbrSi heteroepitaxial growth. XRD patterns shown in Fig. 1 show non-epitaxial growth on hydrogen-terminated Ž111. and Ž100.Si. Epitaxial layers were in fact sometimes obtained, but not reproducibly. The polycrystalline growth is due to the high sensitivity of growth behavior on Si-surface structure and poor stability of Sisurface against residual oxygen at relatively low temperature. Fig. 2 shows RHEED patterns along the w011x-direction of hydrogen-terminated Ž100.Si surface
after annealing at 2708C in vacuum ŽFig. 2a. and in hydrogen plasma ŽFig. 2b.. The hydrogen-terminated surface structure which includes some steps or kinks on the annealed sample in vacuum was maintained. However, the structure was disordered after hydrogen plasma treatment. It can be concluded from these results that the hydrogen-terminated surface is not stable against hydrogen-plasma treatment, even at relatively low temperature, and the hydrogen-desorbed surface is easily oxidized. To avoid this, we employed hydrogen plasma treatment at 6008C to remove native oxide on the Si-surface, and then exposed the surface to arsenic vapor in the plasma during cooling. This was motivated by the fact that As]Si bond is much stronger than H]Si bond w5x. Fig. 3 shows RHEED patterns along w011x-direction of Ž100.Si-surface treated by hydrogen plasma at 6008C without ŽFig. 3a. and with ŽFig. 3b. exposure to arsenic vapor during cooling. These samples were exposed to air after treatment in the chamber. The RHEED pattern shows that, although the native oxide was removed from the surface treated at 6008C by hydrogen plasma, the surface without exposure to arsenic vapor was oxidized by residual oxygen in air ŽFig. 3a.. On the other hand, the surface exposed to arsenic vapor during cooling was so stable that even in air the RHEED pattern showed clear 1 = 1 structure of arsenic-terminated surface. RHEED patterns along w211x-direction of InSb films grown on Ž111.Si treated by these two different surface cleaning processes without ŽFig. 4a. and with ŽFig. 4b. arsenic vapor exposure are shown in Fig. 4. Epitaxial InSb growth was confirmed on the Si surface terminated with arsenic ŽFig. 4b., in contrast to poly-
Table 1 Detailed growth conditions for initial and active layers
Total pressure Growth temperature Induced rf power SbxrIn supply ratio Growth rate
Initial layer
Active layer
0.002 Torr 180]2708C 20]80 W 2]4 0.1]0.3 m mrh
0.02 Torr 4508C 40 W 17 0.7]0.8 m mrh
Fig. 1. XRD spectra of InSb films grown on Ža. Ž111.Si and Žb. Ž100.Si terminated by hydrogen.
S. Yamauchi et al. r Thin Solid Films 316 (1998) 93]99
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Fig. 2. RHEED patterns of hydrogen-terminated Ž100.Si surfaces along w011x azimuth after annealed at 2708C Ža. in vacuum and Žb. in hydrogen plasma.
crystalline InSb growth on the surface treated only by the hydrogen plasma ŽFig. 4a.. This was probably caused by partial oxidation of the Si-surface by residual oxygen. These results indicate that the arsenictermination process on Si is effective for the surface passivation against native oxide formation prior to the epitaxial growth. A probable model of hydrogen cleaning and arsenic passivation is schematically shown in Fig. 5. Surface terminating hydrogen can be desorbed at high temperature in vacuum, with the bare Si-surface being
partially oxidized by residual oxygen ŽFig. 5a.. The partially oxidized surface is cleaned by hydrogen plasma at 6008C, but if arsenic vapor is supplied to the surface at the same temperature, the active hydrogen not only removes the oxygen, but also the arsenic atoms from the surface ŽFig. 5b.. This was confirmed by non-epitaxial growth when the cleaned Si-surface was exposed to arsenic vapor at 6008C but not during cooling. In the cooling process, the hydrogen plasma continues to remove the oxygen at a lower temperature than 6008C, however, the arsenic-atoms
Fig. 3. RHEED patterns of Ž100.Si surface Ža. without As-termination and Žb. with As-termination along w011x azimuth. A 1 = 1 structure was maintained even in air.
Fig. 4. RHEED patterns along w211xazimuth of InSb films grown on Ž111.Si, Ža. cleaned only by hydrogen plasma at 6008C, and Žb. terminated by arsenic.
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Fig. 6. Variation of FWHM value of InSb films grown on Asterminated Ž111. and Ž100. Si with variable As-flux during the passivation process.
Fig. 5. Model for As-termination in As-contained hydrogen plasma during cooling after hydrogen plasma cleaning to remove native oxide.
are adsorbed onto the Si-surface up to one monolayer during cooling ŽFig. 5c.. Finally, the Si-surface will be passivated by arsenic atoms without oxygen ŽFig. 5d.. Arsenic-flux supplied for the passivation process should be optimized as shown in Fig. 6, where FWHM
Fig. 7. RHEED patterns along w211xazimuth and SEM images of initial InSb layers with Ža. 4ML, Žb. 8ML and Žc. 16ML grown at 2708C with a growth rate of 0.3 m mrh.
S. Yamauchi et al. r Thin Solid Films 316 (1998) 93]99
values of XRD peak patterns measured for InSb films grown on Ž111. and Ž100.Si are shown as a function of arsenic-cell temperature during the passivation process. In both cases on Ž111. and Ž100.Si, the value of FWHM is minimum at the same arsenic-cell temperature Ž2708C.. This indicates that the excess arsenic-flux which forms arsenic-layers thicker than a monolayer results in the deterioration of crystalline quality of InSb. 3.2. Optimization of initial buffer layer Bulk-InSb has a lattice constant about 19% larger than Si. This large lattice mismatch makes the epitaxial growth much more sensitive to the surface condition of partial oxidation on Si and also to such growth conditions as SbrIn supply ratio. Then three dimensional polycrystalline growth begins at quite an early stage of initial growth as shown in Fig. 7. This figure shows the RHEED patterns and SEM images of initial layers grown at 2708C with a growth rate of 0.3
97
m mrh. Ring-like patterns of polycrystalline InSb were observed at a growth stage of several monolayers and InSb grains grew larger with thickness. This tendency is more pronounced at higher temperatures. Thus, a very thin initial layer having a few monolayers should be used for initial layer in the two-step growth process. Detailed RHEED observations were employed to optimize the initial layer. Fig. 8 shows RHEED patterns of the initial layer grown at 1808C ŽFig. 8a., 2408C ŽFig. 8b. and 2708C ŽFig. 8c. with four monolayers on the Ž111.Si by lowering the growth rate down to 0.1 m mrh, and of active layers ŽFig. 8a9, b9 and c9. grown on the various initial layers. The three dimensional island growth was suppressed at relatively low temperature at 2408C, although the layer was grown as polycrystalline and amorphous at 2708C and at 1808C, respectively. The initial layers are not continuous ŽVolmer]Weber growth mode., and the size and density of islands depend on the growth temperature and growth rate. Larger grains with lower density grow epitaxially up to a few average monolayers but
Fig. 8. RHEED patterns along w211xazimuth of InSb initial layers and active layers.Initial layer grown at Ža. 1808C, Žb. 2408C and Žc. 2708C with growth rate of 0.1 m mrh. Active layers Ža9., Žb9. and Žc9. were grown on initial layers Ža., Žb. and Žc., respectively.
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Fig. 10. The effect of rf power applied for the initial layer growth at 2108C on the electronic properties of second active layers grown at 4508C. Fig. 9. The dependence of electronic properties of InSbrŽ111.Si films on the initial layer growth temperature for two different initial layer growth rate of 0.1 and 0.5 m mrh.
tend to polycrystallize at quite an early stage at higher temperature and lower growth rate. This is probably when the growing fronts of islands begin to collapse upon each other. With this kind of initial layer, the successive layer cannot be grown epitaxially ŽFig. 8c9.. When the initial layer was grown at too low temperature, it tends to be amorphous with the successive layer polycrystallized ŽFig. 8a9.. The film grown on the initial layer grown around 2408C has better crystal quality with less microtwins compared with that grown at the lower and higher temperatures ŽFig. 8b9 and c9.. The RHEED pattern also shows the initial layer was strained with longitudinal lattice mismatch around 12]13%, compared with 19% between bulk InSb and Si. This type of strained initial layer of a few monolayers with suppressed three dimensional island is useful to relax large lattice mismatch w6x. 3.3. Electronic property Epitaxial active layers of InSb were successively grown by the two-step plasma process on arsenic-passivated Si after hydrogen plasma cleaning. Because the electronic properties of InSb film grown on Si are more sensitive to the supply ratio than that grown on GaAs w7x, the supply flux ratio of SbrIn must be certainly optimized for this type of growth, as well as for initial layer growth. Fig. 9 shows Hall mobility and carrier concentration of 2 m m thick InSb films, which were grown at 4508C on the initial layer with 4ML, as a function of initial growth temperature. The epitaxial InSb layers showed n-type conductivity. The mobility of the epitaxial film was increased up to 1.2= 10 4 cm2rVs on the initial layer grown at 2108C with the low growth rate of 0.1 m mrh, in comparison to a lower value around 3 = 10 3 cm2rVs for the polycrys-
talline film directly grown on the Si without the initial layer. It is reasoned that the optimum growth condition for the initial buffer layer is determined by the competition between the number, the size and the quality of seeds. For lower temperature growth than the optimum, the surface of the initial layer is smooth and includes a large number of seeds; however, the quality of the seeds is poor as shown in Fig. 8a. In contrast, the quality of the seeds grown at higher temperature than optimum is improved but the surface is roughened by the larger size and smaller number of grains as shown in Fig. 8c. When the growth rate of the initial layer is increased up to 0.5 m mrh, the optimum shifts to a higher temperature of around 2408C. The mobilities showed that the quality of the active InSb layers is not dependent upon the growth rate of the initial layer for the higher growth temperatures, but is dependent on the rate at lower temperatures. This could result from the fact that the substrate surface is not a perfect mirror, but contains many kinks and steps which influence the nucleation of seeds. According to these results, the growth conditions for the initial layer should be optimized not only for temperature but also for the growth rate. It is worth mentioning here that the plasma should be optimized, because an increase in the rf power has a similar effect as growth temperature, shown in Fig. 10. This suggests that the temperature of the total process can be further lowered by higher excitation of the plasma. 4. Conclusions Total low temperature epitaxial growth of InSb on Si was performed by two-step plasma-assisted epitaxy. Prior to the growth, native oxide on Si-surface was removed by hydrogen plasma treatment at 6008C, and the surface stabilized with arsenic-containing hydrogen plasma during cooling. The arsenic-terminated Si
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surface showed 1 = 1 structure and was stable, even in air. Detailed RHEED observations showed the initial layer of InSb tends to polycrystallize and form three dimensional islands within several monolayers. Optimum initial layers with about four monolayers were grown at low temperature, approximately 210]2408C, with lower growth rate. The initial layer was not continuous and was strained with longitudinal lattice mismatch around 12]13%, as compared with the large lattice mismatch of 19% between InSb and Si. The active InSb epitaxial layer was successively grown at 4508C on the initial layer with the mobility increased up to 1.2= 10 4 cm2rVs, whereas a value of 3 = 10 3 cm2rVs for polycrystalline InSb film was achieved. This type of strained initial layer of a few monolayers, grown at low temperature with suppressed three dimensional island formation, should be useful for other classes of heteroepitaxial growth with large lattice mismatch.
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Acknowledgement This work was partly supported by the Satellite Venture Business Laboratory project of Ibaraki University. References w1x K. Takenaka, T. Hariu, Y. Shibata, Jpn. J. Appl. Phys. 19 Ž1979. 765. w2x W. Kern, D.A. Puotinen, RCA Rev. 31 Ž1970. 187. w3x Q.Z. Gao, T. Hariu, S. Ono, Jpn. J. Appl. Phys. 26 Ž1987. L1576. w4x Y. Sato, K. Matsushita, T. Hariu, Y. Shibata, Appl. Phys. Lett. 44 Ž1984. 592. w5x R.D. Bringans, M.A. Olmstead, R.I.G. Uhrberg, R.Z. Bachrach, Appl. Phys. Lett. 51 Ž1987. 523. w6x M. Akiyama, Y. Kawarada, K. Kaminishi, Jpn. J. Appl. Phys. 23 Ž1984. L1843. w7x T. Hariu, K. Sawamura, F. Ito, H. Ohba, Proc. 23rd Int. Symp. Compound Semiconductors. Inst. Phys. Ser. 155 Ž1997. 331.