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Surface chemistry during metalorganic molecular beam epitaxy studied by pulsed molecular beam scattering
j. . . . . . . .
CRYIITAL OROWTH
Journal of Crystal Growth 175/176 (1997) 1178-1185
ELSEVIER
Surface chemistry during metalorganic molecular beam epitaxy studied by pulsed molecular beam scattering Masahiro
S a s a k i a'*, S e i k o h Y o s h i d a b
alnstitute of Applied Physics, University of Tsukuba, Tennoudai, Tsukuba, Ibaraki 305, Japan b The Furukawa Electric Co. Ltd., Yokohama R&D Laboratories, Okano, Nishi-ku, Yokohama 220, Japan
Abstract The surface chemistry during metalorganic molecular beam epitaxy (MOMBE) is discussed on the basis of the time-of-flight (time-of-arrival) distributions of trimethylgallium (TMG) molecules scattered from structure-controlled GaAs(1 0 0), GaAs(1 1 0) and GaAs(1 1 1)B surfaces as well as oxidized GaAs surfaces. The scattering (adsorption and desorption) of TMG from GaAs surfaces, the initial stage of the surface chemistry, is interpreted within the framework of precursor-mediated chemisorption, where the precursor state is deepened by the charge distribution of the relaxed or reconstructed GaAs surface and the dissociative chemisorption is suppressed on the surface with the highly stabilized structure. The growth suppression on oxidized GaAs surface is due to the absence of the deep precursor states, implying the mechanism of selective area growth. The growth suppression on an As-excess GaAs(1 1 1)B surface, by which the GaAs lateral growth is realized on a grooved GaAs(1 1 1)B substrate, is explained by neutral As trimers which geometrically hinder the TMG trapping in the precursor state. It is also found that the TMG scattering varies with the surface preparation method even if electron diffraction shows similar reconstructions, which suggests that TMG molecules diffuse over the wide area during trapping in the precursor state.
1. Introduction It is well known that the growth rate of metalorganic molecular beam epitaxy (MOMBE) drastically varies with the growth condition. By utilizing this behavior of M O M B E , we can three-dimensionally control the epitaxial growth. For example,
*Corresponding author. Fax: + 81 298 53 5205; e-mail: [email protected].
GaAs can be selectively grown on window areas opened on mask materials, such as oxides and nitrides, where M O M B E growth is highly suppressed [1-3]. Atomic layer epitaxy (ALE) is realized by alternately supplying source gases, since the M O M B E growth is limited to one atomic monolayer under single source gas supply [4, 5]. Furthermore, using the behavior that the growth rate is very low on the GaAs(1 1 1)B surface under a high As flux [6, 7], we can laterally grow GaAs on a grooved GaAs(1 1 1)B substrate [8]. However, the mechanism governing the drastically varying
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)0087 1 -8
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) 1178-1185
MOMBE growth rate has not been well understood so far. In order to clarify the mechanism, the growth rate as a function of growth condition has been investigated in detail [9-12]. Several analytical methods such as reflection difference spectroscopy [13], surface photo-absorption measurement [14], grazing incidence X-ray diffraction [ 15], have been applied. Mass spectrometry of desorbed species enable us to in situ observe the surface chemistry during MOMBE [16-19]. These studies implied the mechanism in ALE. Furthermore, it is demonstrated that the GaAs growth selectivity in selective area growth is attributed to the difference in the decomposition rate of such Ga source gases as trimethylgallium (TMG) and triethylgallium (TEG) from the mass spectrometric analysis of the desorbed species [2]. The decomposition suppression of source gases on the As-rich GaAs(1 1 1)B surface was also found by mass spectrometry [6], which is against the expectation that the As-containing species promote T M G decomposition as is known in the vapor-phase reaction [20]. Also on the GaAs(1 0 0) surface, we found that the surface reaction of TMG is not influenced by the As density on surface but by the surface structure; TMG decomposition is suppressed on a GaAs(1 0 0)-(2 x 4) surface with a specific As coverage, which has a highly stabilized structure [21]. As mentioned above, great effort has been done to understand the mechanism of the surface-reaction. However, within static behaviors of MOMBE growth, the mechanism of the surface-dependent reaction has not been clear. In order to extract the essential factors in the surface-dependent reaction, the information beyond the static behaviors has been required. Pulsed molecular beam scattering is a powerful technique to observe the dynamical behavior of the surface reaction, especially of the adsorption and desorption of gases [22]. Recently, we demonstrate that the surface residence time and the energy relaxation of TMG during scattering can be directly evaluated from the time-of-arrival distributions of TMG scattered from well-defined GaAs surfaces and that the surface scattering of TMG is interpreted within the framework of precursor-mediated chemisorption mechanism [23-25].
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In this paper, we discuss the surface chemistry during MOMBE on the basis of the results of the pulsed TMG beam scattering from the well-defined GaAs surfaces which carefully prepared by epitaxial growth and subsequent treatments, focusing on the mechanism of selective area growth and lateral growth of GaAs. Furthermore, we examine the TMG scatterings from GaAs surfaces with similar reconstructions which are prepared by different methods.
2. Experimental procedure The experiments were carried out in a multichamber system comprising an MOMBE growth facility, a pulsed molecular beam scattering chamber, and a surface treatment chamber, which have already been described elsewhere [23-25]. We generated pulsed TMG beams using a rotating chopper with two slits of 2 mm in width and a synchronized high-speed pulsed valve with a 0.5 mm nozzle. The pulse shape of incident TMG beams was well reproduced by the velocity distribution of translationally shifted Maxwell-Boltzmann with a translational shift velocity of 457 m/s and a velocity spread of 103 m/s. Using a cryoshrouded quadrupole mass spectrometer, we measured the time-of-arrival (time-of-flight) distributions of TMG molecules scattered from GaAs surfaces prepared in the MOMBE growth facility and the surface treatment chamber. GaAs surface has variously reconstructed or relaxed structures, depending on the surface stoichiometry as well as the surface orientation. Furthermore, surface properties are known to vary with the surface preparation method even if the reconstructions are similar [26]. In this study, we compared the TMG scatterings from differently prepared GaAs surfaces, containing epitaxially prepared GaAs(1 0 0), GaAs(1 1 0), and GaAs(1 1 1)B surfaces with controlled structures as well as Asdesorption-prepared GaAs surfaces. We also examined the scattering from the oxidized GaAs surface, where T M G decomposition is known to be suppressed. The oxidized GaAs surfaces were prepared by oxygen exposure under no light irradiation.
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) 1178-1185
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3. Results and discussion
3.1. Scattering of TMG from oxidized GaAs surfaces First, we examined the scattering of TMG molecules from oxidized GaAs surfaces in order to study the mechanism of selective area growth of GaAs. The time-of-arrival distributions of TMG molecules scattered from oxidized GaAs surfaces were found to be broader than those expected from the velocity distribution of the incident TMG beams. This broadening is due to the relaxation of translational energy of TMG molecules during scattering. From the analysis of the time-of-arrival distributions, we can evaluate the relaxation of the translational energy of TMG molecules. These time-ofarrival distributions were well reproduced by the convolution of the velocity distributions of incident and scattered TMG, where the velocity distribution of scattered TMG molecules were assumed to be of shifted Maxwell-Boltzmann, while the velocity distribution of incident TMG molecules are directly measured [24].
From the curve-fitting of the time-of-arrival distributions, we extracted two parameters in the velocity distribution of scattered TMG molecules, velocity spread and shift velocity. Fig. 1 shows these parameters as functions of the surface temperature. If there is no energy relaxation during scattering, the velocity distribution of the scattered TMG molecules would be equal to that of the incident TMG molecules. On the other hand, if TMG scattering occurs under thermal equilibrium at the surface temperature, the velocity spread would be equal to the solid line and the shift velocity would be zero. It is noticed from Fig. 1 that the parameters extracted from time-of-arrival distributions trace the values corresponding to those under thermal equilibrium. According to the mass spectrometric study of desorbed species, TMG molecules do not decompose on this oxidized GaAs surface [27]. Therefore, it is considered that TMG molecules are not decomposed even when TMG molecules obtain thermal energy from surface during scattering under thermal equilibrium. It should be noted that TMG
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TEMPERATURE (K)
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TEMPERATURE (K)
Fig. 1. (a) Velocity spread and (b) shift velocity in velocity distribution of TMG scattered from oxidized GaAs surface as functions of the surface temperature. These values were obtained by curve-fitting of time-of-arrival distributions of scattered TMG to the convolution formula of velocity distributions of incident and scattered TMG. The velocity distributions are assumed to be of shifted Maxwell-Boltzmann. If there is no energy relaxation during scattering, the velocity distribution of the scattered TMG molecules would be equal to that of the incident TMG molecules; that is, velocity spread and shift velocity would be 457 and 103 m/s, respectively. On the other hand, if TMG molecules are completely thermalized by the substrate surface, the velocity spread would be equal to the solid line ( ~ ) and the shift velocity would be zero.
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) I 178-1185
molecules are efficiently decomposed on the GaAs surface at the same temperature. The result suggests that the difference in the TMG decomposition rate is not attributed to the difference in the amount of the energy transfer from GaAs surface to TMG molecules during scattering.
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3.2. Scattering of TMG from GaAs surfaces 1
According to the TMG decomposition rate on GaAs(1 0 0) surface as a function of surface As coverage, the TMG decomposition is anomalously suppressed on a specific GaAs(1 0 0)-(2 x 4) surface (0As = 0.75), which is reproducibly obtained by a controlled cooling process after GaAs epitaxy [21]. Since the structure of this surface is known to be highly stabilized, we presume that TMG decomposition is suppressed when the structure stability is sufficiently high. In the case of the TMG scattering from the specific GaAs(1 0 0)-(2 x 4) surface, the time-of-arrival distribution was found to be much broader and cannot be interpreted only by the translational energy relaxation. This distribution was well reproduced by adding a component for scattering with a trapping during scattering for such a long period that we can directly observe within the time resolution of the system we used. Fig. 2 shows the result of the curve-fitting of the time-of-arrival distribution of TMG to the sum of the components for scatterings with and without surface residence. This indicates that there exists a precursor state, where TMG molecules are temporarily trapped in the initial stage of scattering. From the temperature dependence of the reciprocal surface residence time (TMG escape rate from the precursor state to vacuum), the depth of the precursor state is estimated to be 0.85 eV [28]. Although TMG molecules are dominantly trapped in the deep precursor state, almost all the TMG molecules are desorbed without a permanent sticking (the sticking probability is nearly zero). This behavior was also observed on the GaAs(1 1 0) and GaAs(1 1 1)B-(x/~ x x / ~ ) surfaces. It has been established that the GaAs(1 1 0) surface is stabilized [29]. From the temperature programmed desorption study, the stability of the GaAs(1 1 1)B-(x/~ x x//]-9) structure is considered
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2 3 TIME (ms)
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Fig. 2. The result of the curve-fitting of the time-of-arrival distribution of TMG scattered from the TMG-decompositionsuppressed GaAs(1 0 0)-(2 × 4) surface at 273°C to the sum of the components of scatterings with and without a surface residence during scattering, where the relaxation of translational energy during scattering was taken into account. The obtained result shows that 77% of TMG molecules are desorbed after trapping for a surface residence time of about 0.9 ms in the precursor state without dissociative chemisorption.
to be high [25]. Therefore, we conclude that this behavior is common for the scattering of TMG from the highly stabilized GaAs surfaces. From the temperature dependence of the surface residence time during scattering, the depths of the precursor states for GaAs(1 1 0) and GaAs(1 1 1)B(xfl~ x x / ~ ) surfaces are estimated to be 0.32 and 0.35 eV, respectively [30]. In order to explain the depth of precursor states, we simply examine the charge distribution of the relaxed or reconstructed GaAs surface structures. The electrons of dangling states in Ga are completely moved to As to maintain a semiconducting surface [31]. Therefore, in the buckled picture of the GaAs(1 1 0) surface [29], the topmost Ga-As pair generates a sufficiently large electrostatic dipole moment outside the crystal, which can polarize TMG molecules coming near to the surface, resulting in the TMG trapping in the deep precursor states observed here [25]. In the case of the GaAs(1 1 1)B-(x/~ x x / ~ ) surface [32], the electrostatic behavior is considered to be similar to that for the (1 1 0) surface. The deeper precursor state for the GaAs(1 0 0)-(2 × 4) surface is explained by the higher density of the Ga-As pairs in the proposed (2 × 4) structure [33]. Therefore, the surface electrostatic behavior explains well the depth of the precursor state [30].
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) 1178-1185
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In the case of the scattering from less stabilized GaAs surfaces such as GaAs(1 0 0)-c(4 × 4), GaAs(1 0 0)-(1 × 6), and GaAs(1 1 1)B-(1 x 1) surfaces, where high T M G decomposition rate is observed [6,21], the time-of-arrival distributions were observed to have no component for the scattering with a long surface residence, as shown in Fig. 3. The small amount of scattered T M G corresponds to the large amount of T M G dissociatively chemisorbed 1-25]. We consider that there exists a precursor state also on the GaAs surface with less stability, since the surfaces have As-Ga pairs generating the large electric dipole, and that T M G trapped in the precursor state is efficiently decomposed, providing such a high T M G decomposition rate as about 0.5. From the comparison of the scatterings from clean GaAs and oxidized GaAs surfaces, it is concluded that the T M G decomposition suppression on the oxide is attributed to the lack of the precursor state [24].
T M G molecules are scattered without trapping in a deep precursor state although the energy relaxation occurs. This feature is interpreted by the reconstructed surface structure. The GaAs(1 1 1)B-(2×2) surface consists of As-trimers on As-terminated GaAs(1 1 1)B surface 1-32]. According to the simple estimation of electron distribution similar to that for the above-mentioned GaAs surfaces, the topmost As trimers on As-terminated surface are neutral 1,30]. The As-Ga pairs in the second and third atomic layers, where electrons are biased, certainly generate large electric dipole moments. However, the space between As trimers where T M G molecules can approach is small compared with the T M G molecular size. We consider that T M G trapping is hindered geometrically by such neutral adatoms as topmost As-trimers on the As surface. This mechanism is consistent with the results on the super-Asrich GaAs(1 0 0)-c(4 × 4) that T M G sticking probability decrease as the As coverage further increases 1,,25].
3.3. TMG decomposition suppression by excess As T M G decomposition is highly suppressed on the As-excess GaAs(1 1 1)B surface with (2 x 2) reconstruction, resulting in the lateral growth of GaAs on a grooved GaAs(1 1 1)B substrate by controlling As flux I-8]. We examined pulsed T M G beam scattering also on the GaAs(1 1 1)B-(2 x 2) surface. The T M G scattering was found to be very similar to that from the oxidized GaAs surfaces; that is, all the A
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Fig. 3. The time-of-arrival distribution of TMG scattered from a reactive GaAs(1 0 0)-(1 × 6) surface at 273°C, which is well reproduced by the single component of scattering without surface residence, where translational energy relaxation during scattering is taken into account. The smaller signal corresponds to the higher sticking probability of TMG.
3.4. Surface preparation dependence of the TMG scattering The above-mentioned GaAs(1 0 0)-(2 x 4) surface, where T M G decomposition is suppressed, was prepared by controlling As flux during the cooling process after GaAs epitaxial growth (referred to as epitaxially prepared surface). A GaAs(1 0 0) surface showing (2 × 4) reconstruction can also be obtained by another method such as a process containing As desorption from an As-covered GaAs surface (referred to as As-desorption-prepared surface). It has been reported that the work function of the epitaxially prepared GaAs(100)-(2x4) surface is largely different from that of the As-desorption prepared GaAs(1 0 0)-(2 x 4) surface [26]. This demonstrates that these surfaces are not the same. Here, we examine the scatterings of pulsed T M G beam from these surfaces. Fig. 4 shows the temperature-programmed desorption (TPD) spectrum of As2 from an As-rich GaAs(1 0 0)-c(4 x 4) surface, in which observed reconstructions are also given. The ramp rate is set at 0.1 °C/s. It is considered that the bottom of the deep valley at 480°C in Fig. 4 corresponds to the
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) 1178-1185 100
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Fig. 4. Temperature programmed desorption spectrum of As 2 from an As-excess GaAs(1 0 0)-c(4 x 4) surface. The ramp rate is 0.1°C/s. The reconstructions observed by R H E E D during the temperature programmed desorption measurement are given.
GaAs(100) surface with the highest stability among the As-desorption-prepared (2 x 4)-reconstructed surfaces. For TMG scattering experiments, we prepared GaAs(100) surfaces by heating the As-rich GaAs(10 0)-c(4 x 4) surface up to different treatment temperatures ( T t ' s ) at a ramp rate of 0.1°C/s and quenching it. The time-of-arrival distributions of TMG molecules scattered from these surfaces were analyzed by curve-fitting to the sum of the components for scatterings with and without surface residence during scattering in the same manner as in the cases of the Fig. 2. Fig. 5 shows the ratio (Rd) of the amount of TMG molecules in the component for scattering with a long surface residence to the total amount of scattered TMG as a function of T t. The Rd value was evaluated by integrating the fitted time-ofarrival distribution. The dashed line in Fig. 5 indicates the Rd value for the scattering from the epitaxially prepared GaAs(1 0 0)-(2 x 4) surface (Fig. 2). We found in Fig. 5 that Rd's for the As-desorption-prepared surfaces are much smaller than that for the epitaxially prepared surface. This result indicates that TMG surface chemistry varies with the surface preparation even if the reconstructions observed by low-energy electron diffraction (LEED) or reflection high-energy electron diffraction (RHEED) are similar. The component for the scattering with the surface residence corresponds to the
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Fig. 5. The ratio (Rd) of the amount of T M G molecules in the component for the scattering with the surface residence time to the total amount of T M G scattered from As-desorption prepared GaAs(1 0 0) surfaces as a function of the treatment temperature. These amounts were evaluated by integrating the fitted distributions; R d = SFd(t) dt/SF(t) dt, where Fd(t ) and F(t) are the fitted time-of-arrival distribution for the component for the scattering with surface residence and the fitted total time-ofarrival distribution, respectively. The dashed line indicates the R d value for the scattering from the epitaxially prepared GaAs(1 0 0)-(2 × 4) surface (Fig. 2).
TMG molecules which survive dissociative chemisorption in the precursor states for a long period. The dissociative chemisorption on GaAs surface is considered to occur at the surface sites with less stabilized structure (referred to as active sites). Therefore, high Rd indicates a low density of active sites. This results suggests that the density of active sites on As-desorption-prepared surface is higher than that on the epitaxially prepared surface. As can be seen in Fig. 5, Rd is relatively high when the surface shows (2 x4) reconstruction. However, we cannot observe a tendency that Rd become larger at the treatment temperature corresponding to the valley bottom in the TPD spectrum. Here the difference in As desorption rate in the TPD spectrum suggests the difference in the stability of the surface. This discrepancy is explained by taking into account the surface diffusion, which enables TMG efficiently to reach to active sites even if the number of the active site is very small. This behavior is consistent with the As coverage dependence of the TMG decomposition rate [21]. The GaAs surface during MOMBE growth is considered to be different from the static surfaces
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M. SasakL S. Yoshida /Journal of Crystal Growth 175/176 (1997) 1178 1185
we examined in this study, even if the surface reconstructions are observed to be similar. During growth, surface atoms are not necessarily in the most stable sites but moving dynamically. Then, the density of active sites is probably higher. Therefore, it is expected that MOMBE growth is not so strongly suppressed by the surface stability. In fact, it was observed that the As-coverage dependence of MOMBE growth rate on GaAs(1 0 0) is not so drastic as that of TMG scattering in this study [-10], although the growth rate decrease due to the high surface stability, which is well explained by the mechanism proposed in this study, is clearly observed.
4. Summary We measured the time-of-arrival distributions of TMG scattered from variously reconstructed or relaxed GaAs(1 0 0), GaAs(1 1 0) and GaAs(1 1 1)B surfaces as well as oxidized GaAs surface. The initial stage of the TMG surface chemistry on GaAs is interpreted within the framework of precursormediated chemisorption. We discussed the TMG decomposition suppression in the cases of the selective area growth and the lateral growth of GaAs. The growth suppression on oxidized GaAs surfaces during the selective area epitaxy of GaAs is due to the absence of the deep precursor states, rather than smaller energy that TMG obtains during scattering from substrate surface. On the other hand, the growth suppression on the As-excess GaAs(1 1 1)B surface during the lateral growth of GaAs is originated from neutral As-trimers geometrically hindering TMG trapping. It was also found that TMG scattering was so sensitive that it varies with the surface preparation method even if the diffraction patterns are similar. This suggests that TMG diffuse over wide area during trapping in the precursor states.
Acknowledgements This work was carried out at Optoelectronics Technology Research Laboratory (OTL). The authors would like to thank Dr. Y. Katayama,
Dr. C. Yamada, Dr. M. Yamada, Dr. I. Hayashi and Dr. M. Tamura for their fruitful discussion and continuous encouragement.
References [1] E. Tokumitsu, Y. Kudou, M. Konagai and K. Takahashi, J. Appl. Phys. 55 (1983) 3163. [2] Y. Hiratani, Y. Ohki, Y. Sugimoto, K. Akita, M. Taneya and H. Hidaka, Jpn. J. Appl. Phys. 29 (1990) L1360. [-3] S. Yoshida and M. Sasaki, in: Proc 19th Int. Syrup. GaAs and Related Compounds, Karuizawa, Japan, 1992, Eds. T. Ikegami, H. Hasegawa, and Y. Takeda, Inst. Phys. Conf. Ser. No. 129 (1993) p. 49. [4] J. Nishizawa, H. Abe and T. Kurabayashi, J. Electrochem. Soc. 132 (1985) 1197. [5] A. Watanabe, T. Isu, M. Hata, T. Kamijo and Y. Katayama, Jpn. J. Appl. Phys. 28 (1989) LI080. [6] Y. Ohki, Y. Hiratani and M. Sasaki, Appl. Phys. Lett. 59 (1991) 2538. [7] Y. Ohki and Y. Hiratani, Appl. Organometal. Chem. 5 (1991) 277. [8] Y. Nomura, Y. Morishita, S. Goto and Y. Katayama, J. Electron. Mater. 23 (1994) 97. [9] T.H. Chiu, J.E. Cunningham and A. Robertson, J. Crystal Growth 95 (1989) 136. [10] T. Isu, M. Hata and A. Watanabe, J. Crystal Growth 105 (1990) 209. [11] T. Kaneko, H. Asahi and S. Gonda, J. Crystal Growth 120 (1992) 39. [12] T. Kaneko, O. Naji, T.S. Jones and B.A. Joyce, J. Crystal Growth 127 (1993) 1059. [13] D.E. Aspnes, I. Kamiya, H. Tanaka and R. Bhat, J. Vac. Sci. Technol. B 10 (1992) 1725. [14] N. Kobayashi and Y. Horikoshi, Jpn. J. Appl. Phys. 30 (1991) L319. [15] D.W. Kisker, G.B. Stephensen, P.H. Fuoss, F.J. Lamelas, S. Brennan and P. Imperatori, J. Crystal Growth 124 (1992) 1. [16] J. Nishizawa, T. Kurabayashi, H. Abe and A. Nozoe, Surf. Sci. 185 (1987) 249. [17] J.R. Creighton, K. Lykke, V.A. Shamamian and B.D. Kay, Appl. Phys. Lett. 57 (1990) 279. [18] J.R. Creighton, Surf. Sci. 234 (1990) 287. [19] M.L. Yu, J. Appl. Phys. 73 (1993) 716. [20] M. Mashita, Jpn. J. Appl. Phys. 29 (1990) 813. [21] M. Sasaki and S. Yoshida, J. Vac. Sci. Technol. B 10 (1992) 1720. [22] M. Asscher and G.A. Somorjai, in: Atomic and Molecular Beam Methods, Vol. 2, Ed. G. Scole (Oxford University Press, New York, 1992) p. 488. [23] M. Sasaki and S. Yoshida, Jpn. J. Appl. Phys. 33 (1994) L884. [24] M. Sasaki and S. Yoshida, Surf. Sci. 315 (1994) L964. [25] M. Sasaki and S. Yoshida, Surf. Sci. 356 (1996) 233.
M Sasaki, S. Yoshida/Journal of Crystal Growth 175/176 (1997) 1178-1185 [26] R. Duszak, C.J. Palmstrom, L.T. Florez, Y.-N. Yang and J.H. Weaver, J. Vac. Sci. Technol. B 10 (1992) 1891. [27] Y. Hiratani and Y. Ohki, unpublished. [28] M. Sasaki and S. Yoshida, Jpn. J. Appl. Phys. 34 (1995) 1113. [29] C.B. Duke, J. Vac. Sci. Technol. A 10 (1992) 2032.
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[30] M. Sasaki and S. Yoshida, Surf. Sci. 357/358 (1996) 863. [31] D.J. Chadi, J. Vac. Sci. Technol. A 5 (1987) 834. [32] D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.-E. Swartz, Phys. Rev. Lett. 65 (1990) 452. [33] M.D. Pashley, Phys. Rev. B 40 (1989) 10481.
CRYIITAL OROWTH
Journal of Crystal Growth 175/176 (1997) 1178-1185
ELSEVIER
Surface chemistry during metalorganic molecular beam epitaxy studied by pulsed molecular beam scattering Masahiro
S a s a k i a'*, S e i k o h Y o s h i d a b
alnstitute of Applied Physics, University of Tsukuba, Tennoudai, Tsukuba, Ibaraki 305, Japan b The Furukawa Electric Co. Ltd., Yokohama R&D Laboratories, Okano, Nishi-ku, Yokohama 220, Japan
Abstract The surface chemistry during metalorganic molecular beam epitaxy (MOMBE) is discussed on the basis of the time-of-flight (time-of-arrival) distributions of trimethylgallium (TMG) molecules scattered from structure-controlled GaAs(1 0 0), GaAs(1 1 0) and GaAs(1 1 1)B surfaces as well as oxidized GaAs surfaces. The scattering (adsorption and desorption) of TMG from GaAs surfaces, the initial stage of the surface chemistry, is interpreted within the framework of precursor-mediated chemisorption, where the precursor state is deepened by the charge distribution of the relaxed or reconstructed GaAs surface and the dissociative chemisorption is suppressed on the surface with the highly stabilized structure. The growth suppression on oxidized GaAs surface is due to the absence of the deep precursor states, implying the mechanism of selective area growth. The growth suppression on an As-excess GaAs(1 1 1)B surface, by which the GaAs lateral growth is realized on a grooved GaAs(1 1 1)B substrate, is explained by neutral As trimers which geometrically hinder the TMG trapping in the precursor state. It is also found that the TMG scattering varies with the surface preparation method even if electron diffraction shows similar reconstructions, which suggests that TMG molecules diffuse over the wide area during trapping in the precursor state.
1. Introduction It is well known that the growth rate of metalorganic molecular beam epitaxy (MOMBE) drastically varies with the growth condition. By utilizing this behavior of M O M B E , we can three-dimensionally control the epitaxial growth. For example,
*Corresponding author. Fax: + 81 298 53 5205; e-mail: [email protected].
GaAs can be selectively grown on window areas opened on mask materials, such as oxides and nitrides, where M O M B E growth is highly suppressed [1-3]. Atomic layer epitaxy (ALE) is realized by alternately supplying source gases, since the M O M B E growth is limited to one atomic monolayer under single source gas supply [4, 5]. Furthermore, using the behavior that the growth rate is very low on the GaAs(1 1 1)B surface under a high As flux [6, 7], we can laterally grow GaAs on a grooved GaAs(1 1 1)B substrate [8]. However, the mechanism governing the drastically varying
0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)0087 1 -8
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) 1178-1185
MOMBE growth rate has not been well understood so far. In order to clarify the mechanism, the growth rate as a function of growth condition has been investigated in detail [9-12]. Several analytical methods such as reflection difference spectroscopy [13], surface photo-absorption measurement [14], grazing incidence X-ray diffraction [ 15], have been applied. Mass spectrometry of desorbed species enable us to in situ observe the surface chemistry during MOMBE [16-19]. These studies implied the mechanism in ALE. Furthermore, it is demonstrated that the GaAs growth selectivity in selective area growth is attributed to the difference in the decomposition rate of such Ga source gases as trimethylgallium (TMG) and triethylgallium (TEG) from the mass spectrometric analysis of the desorbed species [2]. The decomposition suppression of source gases on the As-rich GaAs(1 1 1)B surface was also found by mass spectrometry [6], which is against the expectation that the As-containing species promote T M G decomposition as is known in the vapor-phase reaction [20]. Also on the GaAs(1 0 0) surface, we found that the surface reaction of TMG is not influenced by the As density on surface but by the surface structure; TMG decomposition is suppressed on a GaAs(1 0 0)-(2 x 4) surface with a specific As coverage, which has a highly stabilized structure [21]. As mentioned above, great effort has been done to understand the mechanism of the surface-reaction. However, within static behaviors of MOMBE growth, the mechanism of the surface-dependent reaction has not been clear. In order to extract the essential factors in the surface-dependent reaction, the information beyond the static behaviors has been required. Pulsed molecular beam scattering is a powerful technique to observe the dynamical behavior of the surface reaction, especially of the adsorption and desorption of gases [22]. Recently, we demonstrate that the surface residence time and the energy relaxation of TMG during scattering can be directly evaluated from the time-of-arrival distributions of TMG scattered from well-defined GaAs surfaces and that the surface scattering of TMG is interpreted within the framework of precursor-mediated chemisorption mechanism [23-25].
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In this paper, we discuss the surface chemistry during MOMBE on the basis of the results of the pulsed TMG beam scattering from the well-defined GaAs surfaces which carefully prepared by epitaxial growth and subsequent treatments, focusing on the mechanism of selective area growth and lateral growth of GaAs. Furthermore, we examine the TMG scatterings from GaAs surfaces with similar reconstructions which are prepared by different methods.
2. Experimental procedure The experiments were carried out in a multichamber system comprising an MOMBE growth facility, a pulsed molecular beam scattering chamber, and a surface treatment chamber, which have already been described elsewhere [23-25]. We generated pulsed TMG beams using a rotating chopper with two slits of 2 mm in width and a synchronized high-speed pulsed valve with a 0.5 mm nozzle. The pulse shape of incident TMG beams was well reproduced by the velocity distribution of translationally shifted Maxwell-Boltzmann with a translational shift velocity of 457 m/s and a velocity spread of 103 m/s. Using a cryoshrouded quadrupole mass spectrometer, we measured the time-of-arrival (time-of-flight) distributions of TMG molecules scattered from GaAs surfaces prepared in the MOMBE growth facility and the surface treatment chamber. GaAs surface has variously reconstructed or relaxed structures, depending on the surface stoichiometry as well as the surface orientation. Furthermore, surface properties are known to vary with the surface preparation method even if the reconstructions are similar [26]. In this study, we compared the TMG scatterings from differently prepared GaAs surfaces, containing epitaxially prepared GaAs(1 0 0), GaAs(1 1 0), and GaAs(1 1 1)B surfaces with controlled structures as well as Asdesorption-prepared GaAs surfaces. We also examined the scattering from the oxidized GaAs surface, where T M G decomposition is known to be suppressed. The oxidized GaAs surfaces were prepared by oxygen exposure under no light irradiation.
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) 1178-1185
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3. Results and discussion
3.1. Scattering of TMG from oxidized GaAs surfaces First, we examined the scattering of TMG molecules from oxidized GaAs surfaces in order to study the mechanism of selective area growth of GaAs. The time-of-arrival distributions of TMG molecules scattered from oxidized GaAs surfaces were found to be broader than those expected from the velocity distribution of the incident TMG beams. This broadening is due to the relaxation of translational energy of TMG molecules during scattering. From the analysis of the time-of-arrival distributions, we can evaluate the relaxation of the translational energy of TMG molecules. These time-ofarrival distributions were well reproduced by the convolution of the velocity distributions of incident and scattered TMG, where the velocity distribution of scattered TMG molecules were assumed to be of shifted Maxwell-Boltzmann, while the velocity distribution of incident TMG molecules are directly measured [24].
From the curve-fitting of the time-of-arrival distributions, we extracted two parameters in the velocity distribution of scattered TMG molecules, velocity spread and shift velocity. Fig. 1 shows these parameters as functions of the surface temperature. If there is no energy relaxation during scattering, the velocity distribution of the scattered TMG molecules would be equal to that of the incident TMG molecules. On the other hand, if TMG scattering occurs under thermal equilibrium at the surface temperature, the velocity spread would be equal to the solid line and the shift velocity would be zero. It is noticed from Fig. 1 that the parameters extracted from time-of-arrival distributions trace the values corresponding to those under thermal equilibrium. According to the mass spectrometric study of desorbed species, TMG molecules do not decompose on this oxidized GaAs surface [27]. Therefore, it is considered that TMG molecules are not decomposed even when TMG molecules obtain thermal energy from surface during scattering under thermal equilibrium. It should be noted that TMG
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TEMPERATURE (K)
Fig. 1. (a) Velocity spread and (b) shift velocity in velocity distribution of TMG scattered from oxidized GaAs surface as functions of the surface temperature. These values were obtained by curve-fitting of time-of-arrival distributions of scattered TMG to the convolution formula of velocity distributions of incident and scattered TMG. The velocity distributions are assumed to be of shifted Maxwell-Boltzmann. If there is no energy relaxation during scattering, the velocity distribution of the scattered TMG molecules would be equal to that of the incident TMG molecules; that is, velocity spread and shift velocity would be 457 and 103 m/s, respectively. On the other hand, if TMG molecules are completely thermalized by the substrate surface, the velocity spread would be equal to the solid line ( ~ ) and the shift velocity would be zero.
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) I 178-1185
molecules are efficiently decomposed on the GaAs surface at the same temperature. The result suggests that the difference in the TMG decomposition rate is not attributed to the difference in the amount of the energy transfer from GaAs surface to TMG molecules during scattering.
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3.2. Scattering of TMG from GaAs surfaces 1
According to the TMG decomposition rate on GaAs(1 0 0) surface as a function of surface As coverage, the TMG decomposition is anomalously suppressed on a specific GaAs(1 0 0)-(2 x 4) surface (0As = 0.75), which is reproducibly obtained by a controlled cooling process after GaAs epitaxy [21]. Since the structure of this surface is known to be highly stabilized, we presume that TMG decomposition is suppressed when the structure stability is sufficiently high. In the case of the TMG scattering from the specific GaAs(1 0 0)-(2 x 4) surface, the time-of-arrival distribution was found to be much broader and cannot be interpreted only by the translational energy relaxation. This distribution was well reproduced by adding a component for scattering with a trapping during scattering for such a long period that we can directly observe within the time resolution of the system we used. Fig. 2 shows the result of the curve-fitting of the time-of-arrival distribution of TMG to the sum of the components for scatterings with and without surface residence. This indicates that there exists a precursor state, where TMG molecules are temporarily trapped in the initial stage of scattering. From the temperature dependence of the reciprocal surface residence time (TMG escape rate from the precursor state to vacuum), the depth of the precursor state is estimated to be 0.85 eV [28]. Although TMG molecules are dominantly trapped in the deep precursor state, almost all the TMG molecules are desorbed without a permanent sticking (the sticking probability is nearly zero). This behavior was also observed on the GaAs(1 1 0) and GaAs(1 1 1)B-(x/~ x x / ~ ) surfaces. It has been established that the GaAs(1 1 0) surface is stabilized [29]. From the temperature programmed desorption study, the stability of the GaAs(1 1 1)B-(x/~ x x//]-9) structure is considered
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2 3 TIME (ms)
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Fig. 2. The result of the curve-fitting of the time-of-arrival distribution of TMG scattered from the TMG-decompositionsuppressed GaAs(1 0 0)-(2 × 4) surface at 273°C to the sum of the components of scatterings with and without a surface residence during scattering, where the relaxation of translational energy during scattering was taken into account. The obtained result shows that 77% of TMG molecules are desorbed after trapping for a surface residence time of about 0.9 ms in the precursor state without dissociative chemisorption.
to be high [25]. Therefore, we conclude that this behavior is common for the scattering of TMG from the highly stabilized GaAs surfaces. From the temperature dependence of the surface residence time during scattering, the depths of the precursor states for GaAs(1 1 0) and GaAs(1 1 1)B(xfl~ x x / ~ ) surfaces are estimated to be 0.32 and 0.35 eV, respectively [30]. In order to explain the depth of precursor states, we simply examine the charge distribution of the relaxed or reconstructed GaAs surface structures. The electrons of dangling states in Ga are completely moved to As to maintain a semiconducting surface [31]. Therefore, in the buckled picture of the GaAs(1 1 0) surface [29], the topmost Ga-As pair generates a sufficiently large electrostatic dipole moment outside the crystal, which can polarize TMG molecules coming near to the surface, resulting in the TMG trapping in the deep precursor states observed here [25]. In the case of the GaAs(1 1 1)B-(x/~ x x / ~ ) surface [32], the electrostatic behavior is considered to be similar to that for the (1 1 0) surface. The deeper precursor state for the GaAs(1 0 0)-(2 × 4) surface is explained by the higher density of the Ga-As pairs in the proposed (2 × 4) structure [33]. Therefore, the surface electrostatic behavior explains well the depth of the precursor state [30].
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) 1178-1185
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In the case of the scattering from less stabilized GaAs surfaces such as GaAs(1 0 0)-c(4 × 4), GaAs(1 0 0)-(1 × 6), and GaAs(1 1 1)B-(1 x 1) surfaces, where high T M G decomposition rate is observed [6,21], the time-of-arrival distributions were observed to have no component for the scattering with a long surface residence, as shown in Fig. 3. The small amount of scattered T M G corresponds to the large amount of T M G dissociatively chemisorbed 1-25]. We consider that there exists a precursor state also on the GaAs surface with less stability, since the surfaces have As-Ga pairs generating the large electric dipole, and that T M G trapped in the precursor state is efficiently decomposed, providing such a high T M G decomposition rate as about 0.5. From the comparison of the scatterings from clean GaAs and oxidized GaAs surfaces, it is concluded that the T M G decomposition suppression on the oxide is attributed to the lack of the precursor state [24].
T M G molecules are scattered without trapping in a deep precursor state although the energy relaxation occurs. This feature is interpreted by the reconstructed surface structure. The GaAs(1 1 1)B-(2×2) surface consists of As-trimers on As-terminated GaAs(1 1 1)B surface 1-32]. According to the simple estimation of electron distribution similar to that for the above-mentioned GaAs surfaces, the topmost As trimers on As-terminated surface are neutral 1,30]. The As-Ga pairs in the second and third atomic layers, where electrons are biased, certainly generate large electric dipole moments. However, the space between As trimers where T M G molecules can approach is small compared with the T M G molecular size. We consider that T M G trapping is hindered geometrically by such neutral adatoms as topmost As-trimers on the As surface. This mechanism is consistent with the results on the super-Asrich GaAs(1 0 0)-c(4 × 4) that T M G sticking probability decrease as the As coverage further increases 1,,25].
3.3. TMG decomposition suppression by excess As T M G decomposition is highly suppressed on the As-excess GaAs(1 1 1)B surface with (2 x 2) reconstruction, resulting in the lateral growth of GaAs on a grooved GaAs(1 1 1)B substrate by controlling As flux I-8]. We examined pulsed T M G beam scattering also on the GaAs(1 1 1)B-(2 x 2) surface. The T M G scattering was found to be very similar to that from the oxidized GaAs surfaces; that is, all the A
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Fig. 3. The time-of-arrival distribution of TMG scattered from a reactive GaAs(1 0 0)-(1 × 6) surface at 273°C, which is well reproduced by the single component of scattering without surface residence, where translational energy relaxation during scattering is taken into account. The smaller signal corresponds to the higher sticking probability of TMG.
3.4. Surface preparation dependence of the TMG scattering The above-mentioned GaAs(1 0 0)-(2 x 4) surface, where T M G decomposition is suppressed, was prepared by controlling As flux during the cooling process after GaAs epitaxial growth (referred to as epitaxially prepared surface). A GaAs(1 0 0) surface showing (2 × 4) reconstruction can also be obtained by another method such as a process containing As desorption from an As-covered GaAs surface (referred to as As-desorption-prepared surface). It has been reported that the work function of the epitaxially prepared GaAs(100)-(2x4) surface is largely different from that of the As-desorption prepared GaAs(1 0 0)-(2 x 4) surface [26]. This demonstrates that these surfaces are not the same. Here, we examine the scatterings of pulsed T M G beam from these surfaces. Fig. 4 shows the temperature-programmed desorption (TPD) spectrum of As2 from an As-rich GaAs(1 0 0)-c(4 x 4) surface, in which observed reconstructions are also given. The ramp rate is set at 0.1 °C/s. It is considered that the bottom of the deep valley at 480°C in Fig. 4 corresponds to the
M. Sasaki, S. Yoshida / Journal of Crystal Growth 175/176 (1997) 1178-1185 100
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Fig. 4. Temperature programmed desorption spectrum of As 2 from an As-excess GaAs(1 0 0)-c(4 x 4) surface. The ramp rate is 0.1°C/s. The reconstructions observed by R H E E D during the temperature programmed desorption measurement are given.
GaAs(100) surface with the highest stability among the As-desorption-prepared (2 x 4)-reconstructed surfaces. For TMG scattering experiments, we prepared GaAs(100) surfaces by heating the As-rich GaAs(10 0)-c(4 x 4) surface up to different treatment temperatures ( T t ' s ) at a ramp rate of 0.1°C/s and quenching it. The time-of-arrival distributions of TMG molecules scattered from these surfaces were analyzed by curve-fitting to the sum of the components for scatterings with and without surface residence during scattering in the same manner as in the cases of the Fig. 2. Fig. 5 shows the ratio (Rd) of the amount of TMG molecules in the component for scattering with a long surface residence to the total amount of scattered TMG as a function of T t. The Rd value was evaluated by integrating the fitted time-ofarrival distribution. The dashed line in Fig. 5 indicates the Rd value for the scattering from the epitaxially prepared GaAs(1 0 0)-(2 x 4) surface (Fig. 2). We found in Fig. 5 that Rd's for the As-desorption-prepared surfaces are much smaller than that for the epitaxially prepared surface. This result indicates that TMG surface chemistry varies with the surface preparation even if the reconstructions observed by low-energy electron diffraction (LEED) or reflection high-energy electron diffraction (RHEED) are similar. The component for the scattering with the surface residence corresponds to the
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Fig. 5. The ratio (Rd) of the amount of T M G molecules in the component for the scattering with the surface residence time to the total amount of T M G scattered from As-desorption prepared GaAs(1 0 0) surfaces as a function of the treatment temperature. These amounts were evaluated by integrating the fitted distributions; R d = SFd(t) dt/SF(t) dt, where Fd(t ) and F(t) are the fitted time-of-arrival distribution for the component for the scattering with surface residence and the fitted total time-ofarrival distribution, respectively. The dashed line indicates the R d value for the scattering from the epitaxially prepared GaAs(1 0 0)-(2 × 4) surface (Fig. 2).
TMG molecules which survive dissociative chemisorption in the precursor states for a long period. The dissociative chemisorption on GaAs surface is considered to occur at the surface sites with less stabilized structure (referred to as active sites). Therefore, high Rd indicates a low density of active sites. This results suggests that the density of active sites on As-desorption-prepared surface is higher than that on the epitaxially prepared surface. As can be seen in Fig. 5, Rd is relatively high when the surface shows (2 x4) reconstruction. However, we cannot observe a tendency that Rd become larger at the treatment temperature corresponding to the valley bottom in the TPD spectrum. Here the difference in As desorption rate in the TPD spectrum suggests the difference in the stability of the surface. This discrepancy is explained by taking into account the surface diffusion, which enables TMG efficiently to reach to active sites even if the number of the active site is very small. This behavior is consistent with the As coverage dependence of the TMG decomposition rate [21]. The GaAs surface during MOMBE growth is considered to be different from the static surfaces
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M. SasakL S. Yoshida /Journal of Crystal Growth 175/176 (1997) 1178 1185
we examined in this study, even if the surface reconstructions are observed to be similar. During growth, surface atoms are not necessarily in the most stable sites but moving dynamically. Then, the density of active sites is probably higher. Therefore, it is expected that MOMBE growth is not so strongly suppressed by the surface stability. In fact, it was observed that the As-coverage dependence of MOMBE growth rate on GaAs(1 0 0) is not so drastic as that of TMG scattering in this study [-10], although the growth rate decrease due to the high surface stability, which is well explained by the mechanism proposed in this study, is clearly observed.
4. Summary We measured the time-of-arrival distributions of TMG scattered from variously reconstructed or relaxed GaAs(1 0 0), GaAs(1 1 0) and GaAs(1 1 1)B surfaces as well as oxidized GaAs surface. The initial stage of the TMG surface chemistry on GaAs is interpreted within the framework of precursormediated chemisorption. We discussed the TMG decomposition suppression in the cases of the selective area growth and the lateral growth of GaAs. The growth suppression on oxidized GaAs surfaces during the selective area epitaxy of GaAs is due to the absence of the deep precursor states, rather than smaller energy that TMG obtains during scattering from substrate surface. On the other hand, the growth suppression on the As-excess GaAs(1 1 1)B surface during the lateral growth of GaAs is originated from neutral As-trimers geometrically hindering TMG trapping. It was also found that TMG scattering was so sensitive that it varies with the surface preparation method even if the diffraction patterns are similar. This suggests that TMG diffuse over wide area during trapping in the precursor states.
Acknowledgements This work was carried out at Optoelectronics Technology Research Laboratory (OTL). The authors would like to thank Dr. Y. Katayama,
Dr. C. Yamada, Dr. M. Yamada, Dr. I. Hayashi and Dr. M. Tamura for their fruitful discussion and continuous encouragement.
References [1] E. Tokumitsu, Y. Kudou, M. Konagai and K. Takahashi, J. Appl. Phys. 55 (1983) 3163. [2] Y. Hiratani, Y. Ohki, Y. Sugimoto, K. Akita, M. Taneya and H. Hidaka, Jpn. J. Appl. Phys. 29 (1990) L1360. [-3] S. Yoshida and M. Sasaki, in: Proc 19th Int. Syrup. GaAs and Related Compounds, Karuizawa, Japan, 1992, Eds. T. Ikegami, H. Hasegawa, and Y. Takeda, Inst. Phys. Conf. Ser. No. 129 (1993) p. 49. [4] J. Nishizawa, H. Abe and T. Kurabayashi, J. Electrochem. Soc. 132 (1985) 1197. [5] A. Watanabe, T. Isu, M. Hata, T. Kamijo and Y. Katayama, Jpn. J. Appl. Phys. 28 (1989) LI080. [6] Y. Ohki, Y. Hiratani and M. Sasaki, Appl. Phys. Lett. 59 (1991) 2538. [7] Y. Ohki and Y. Hiratani, Appl. Organometal. Chem. 5 (1991) 277. [8] Y. Nomura, Y. Morishita, S. Goto and Y. Katayama, J. Electron. Mater. 23 (1994) 97. [9] T.H. Chiu, J.E. Cunningham and A. Robertson, J. Crystal Growth 95 (1989) 136. [10] T. Isu, M. Hata and A. Watanabe, J. Crystal Growth 105 (1990) 209. [11] T. Kaneko, H. Asahi and S. Gonda, J. Crystal Growth 120 (1992) 39. [12] T. Kaneko, O. Naji, T.S. Jones and B.A. Joyce, J. Crystal Growth 127 (1993) 1059. [13] D.E. Aspnes, I. Kamiya, H. Tanaka and R. Bhat, J. Vac. Sci. Technol. B 10 (1992) 1725. [14] N. Kobayashi and Y. Horikoshi, Jpn. J. Appl. Phys. 30 (1991) L319. [15] D.W. Kisker, G.B. Stephensen, P.H. Fuoss, F.J. Lamelas, S. Brennan and P. Imperatori, J. Crystal Growth 124 (1992) 1. [16] J. Nishizawa, T. Kurabayashi, H. Abe and A. Nozoe, Surf. Sci. 185 (1987) 249. [17] J.R. Creighton, K. Lykke, V.A. Shamamian and B.D. Kay, Appl. Phys. Lett. 57 (1990) 279. [18] J.R. Creighton, Surf. Sci. 234 (1990) 287. [19] M.L. Yu, J. Appl. Phys. 73 (1993) 716. [20] M. Mashita, Jpn. J. Appl. Phys. 29 (1990) 813. [21] M. Sasaki and S. Yoshida, J. Vac. Sci. Technol. B 10 (1992) 1720. [22] M. Asscher and G.A. Somorjai, in: Atomic and Molecular Beam Methods, Vol. 2, Ed. G. Scole (Oxford University Press, New York, 1992) p. 488. [23] M. Sasaki and S. Yoshida, Jpn. J. Appl. Phys. 33 (1994) L884. [24] M. Sasaki and S. Yoshida, Surf. Sci. 315 (1994) L964. [25] M. Sasaki and S. Yoshida, Surf. Sci. 356 (1996) 233.
M Sasaki, S. Yoshida/Journal of Crystal Growth 175/176 (1997) 1178-1185 [26] R. Duszak, C.J. Palmstrom, L.T. Florez, Y.-N. Yang and J.H. Weaver, J. Vac. Sci. Technol. B 10 (1992) 1891. [27] Y. Hiratani and Y. Ohki, unpublished. [28] M. Sasaki and S. Yoshida, Jpn. J. Appl. Phys. 34 (1995) 1113. [29] C.B. Duke, J. Vac. Sci. Technol. A 10 (1992) 2032.
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[30] M. Sasaki and S. Yoshida, Surf. Sci. 357/358 (1996) 863. [31] D.J. Chadi, J. Vac. Sci. Technol. A 5 (1987) 834. [32] D.K. Biegelsen, R.D. Bringans, J.E. Northrup and L.-E. Swartz, Phys. Rev. Lett. 65 (1990) 452. [33] M.D. Pashley, Phys. Rev. B 40 (1989) 10481.