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Growth process of twinned epitaxial layers on Si(111)3×3 -B and their thermal stability
Applied Surface Science 130–132 Ž1998. 41–46
Growth process of twinned epitaxial layers on Si ž111 /(3 = (3 -B and their thermal stability H. Hibino ) , K. Sumitomo, T. Ogino NTT Basic Research Laboratories, Atsugi, Kanagawa 243-01, Japan Received 5 September 1997; accepted 18 November 1997
Abstract We investigated the growth of twinned epitaxial Si layers on a SiŽ111.'3 = '3 -B surface. In the initial growth stages, untwinned bilayer-high ŽBL-high. and twinned 2-BL-high islands are nucleated, and the ratio of the number of Si atoms included in the twinned 2-BL-high islands to the number of the total deposited Si atoms increases as the surface B concentration increases. Preferred nucleation of Si islands occurs at domain boundaries of the '3 = '3 reconstruction. Moreover, BL-high islands rather than 2-BL-high islands nucleate there. Coalescence of 2-BL-high islands causes the domain boundary density on the first two bilayers to be much larger than that on the substrate. Therefore, after completion of the first twinned two bilayers, BL-high islands are formed predominantly. BL-high islands follow the stacking sequences of the twinned two bilayers. Thus, grown layers are totally twinned. We also investigated the thermal stability of twinned epitaxial layers. The temperature at which twinned epitaxial layers are transformed into untwinned layers strongly depends on the thickness. q 1998 Elsevier Science B.V. All rights reserved. PACS: 68.35.-p; 66.55.Bd; 61.72.Mm Keywords: Silicon; Boron; Epitaxial growth; Twin
B-induced '3 = '3 reconstruction plays a special role in SirSiŽ111. interface formation. It has been reported that the reconstruction is preserved even after amorphous-Si Ža-Si. deposition w1–3x, a twin boundary ŽTB. is formed at the interface between epitaxially grown Si layers and the substrate w4,5x, the two-dimensional B layer at the a-SirSiŽ111. interface is electrically active w6,7x, and Si grows in a layer-by-layer fashion with the unit of two bilayers ŽBL. w5,8x. These unique features are mostly at-
)
Corresponding author. Tel.: q81-462-40-3467; fax: q81462-40-4718; e-mail: [email protected]
tributed to the atomic structure of the B-induced '3 = '3 reconstruction; B atoms locate at S5 sites in the 2nd layer just below the Si T4 adatoms w9–11x. The preservation of the '3 = '3 reconstruction and the nearly complete activation of B at the aSirBrSiŽ111. interface are explained by the fact that B locates not at the outermost surface but at the subsurface. However, the reasons for the formation of the twinned epitaxial layers and for the 2-BLheight layer-by-layer growth are not clear. In this paper, we investigate the growth process of twinned epitaxial layers on SiŽ111.'3 = '3 -B. We demonstrate that twinned epitaxial layers are caused by twinned 2-BL-high island formation. We also clarify
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 0 2 2 - 1
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H. Hibino et al.r Applied Surface Science 130–132 (1998) 41–46
the roles of the domain boundaries of the '3 = '3 reconstruction in twinned epitaxial layer growth. Furthermore, we investigate thermal stability of twinned epitaxial layers. Our results on the growth process of the twinned epitaxial layers and their thermal stability show a possibility of introducing TBs into the grown layers repeatedly. We used two UHV systems. One was equipped with a scanning tunneling microscope ŽSTM. and the other included chambers equipped with molecularbeam epitaxy ŽMBE. and medium-energy ion scattering ŽMEIS. apparatuses. We investigated surface atomic structures using the STM and the atomic structures of epitaxially grown layers by MEIS. Si was grown at sample temperatures between 4008C and 7008C, but we mainly show results for 6008C. Sample temperatures were measured with an infrared pyrometer. In order to investigate surface morphologies on a wider scale than those obtained by STM, atomic force microscopy ŽAFM. images were taken in air after the samples were taken out of the vacuum. Highly B-doped SiŽ111. samples with resistivity of 0.001 V cm were used as substrates. The misorientation angle was about 0.38 and the steps ran nearly normal to ²110:. Assuming BL-high steps, the average terrace width is 60 nm. Because this value is too small to investigate island formation at 7008C, we formed terraces as wide as 1 m m through step bunching by annealing the sample with d.c. electric currents in the step-down direction Žfor ex-
ample, Ref. w12x.. The '3 = '3 reconstruction was prepared using the surface segregation of B during annealing. It has been reported that the surface B concentration u B depends on the temperature w13– 15x. In order to control u B , the samples were quenched after annealing at temperatures between 8508C and 12508C. Fig. 1 shows STM images of SiŽ111.'3 = '3 -B surfaces after Si growth at 6008C. The amount of grown Si was about 0.7 BL. The difference between Fig. 1a and b is the preparation of the substrate: quenching after annealing at 9008C for 10 min for Ža. and after annealing at 10508C for 1 min for Žb.. This caused a difference in u B . On the SiŽ111.'3 = '3 -B surface, all adatoms are Si, but some of the B atoms in the S 5 sites are replaced by Si w10,11x. Bright and dark adatoms in the STM images are Si adatoms above the S 5 Si and S 5 B atoms, respectively w10,11x. Therefore, u B was determined by counting the bright and dark adatoms in the STM images. u B on the substrates in Fig. 1a and b is 0.31 and 0.24 monolayers ŽML., respectively. We also measured the ratios of the u B values between the samples quenched after annealing at 9008C for 10 min and after annealing at 10508C for 1 min using secondary ion mass spectroscopy and Auger electron spectroscopy. These ratios were consistent with the ratio obtained by STM. In both Fig. 1a and b, Si islands were formed, and the '3 = '3 reconstruction can be seen on the islands. However, the islands
Fig. 1. STM images of SiŽ111.'3 = '3 -B surfaces where 0.7-BL Si was grown at 6008C. The amount of B in the S 5 site on the substrates in Ža. and Žb. was 0.31 and 0.24 ML, respectively. The scanning area was 65 = 65 nm2 . The sample bias and the tunneling current were q2 V and 0.1 nA. A BL island is indicated by BL in Ža., and 2BL islands are indicated by 2BL in Žb..
H. Hibino et al.r Applied Surface Science 130–132 (1998) 41–46
in Fig. 1b have a height of BL and 2-BL, but almost all of the islands in Fig. 1a are 2-BL high. The ratio of the number of Si atoms in the 2-BL islands to the number of the total deposited Si atoms depends on u B w16x. The larger u B is, the larger the ratio becomes, and, at the growth temperature of 6008C, islands grown on the substrate with u B s 0.32 ML are all 2-BL high and islands grown on the substrate with u B s 0.2 ML are all BL high. BL and 2-BL islands differ not only in height but also in their epitaxial relationship with the substrate. Comparison between the positions of the adatoms in the '3 = '3 reconstruction on the Si islands and the substrate using STM showed that the 2-BL islands are twinned with the substrate, but that the BL islands are not twinned w16x. It has been reported that the domain boundary of the 7 = 7 reconstruction on SiŽ111. influences various phenomena, such as epitaxial growth and adsorption w17–22x. During Si MBE, two-dimensional islands are preferentially nucleated at the domain boundaries of the 7 = 7 reconstruction w17x. How about at the domain boundaries of the B-induced '3 = '3 reconstruction? Fig. 2a shows an STM image of a SiŽ111. surface on which 0.7-BL Si was grown at 6008C. Fig. 2b shows a magnified image of the area indicated by the square in Fig. 2a. In Fig. 2b, troughs between adatoms on the substrate are indicated by straight lines. Shifts of the straight lines indicate that the '3 = '3 reconstruction on the substrate includes a domain boundary. The position of the domain boundary is indicated by lines in Fig. 2a,
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and one can see that islands preferentially nucleate at the domain boundary. Furthermore, on this sample, BL islands beside those nucleated at the domain boundaries were rarely observed. Islands nucleated at the domain boundary of the '3 = '3 reconstruction prefer to be BL high rather than 2-BL high. Twinned epitaxial Si layers grown on SiŽ111.'3 = '3 -B was first reported by Headrick et al. w4x. They showed that Si layers grown on a substrate covered with 0.33-ML B at 4008C are essentially twinned, but that smaller B coverage caused an increase in the ratio of the untwinned layers w4x. 2-BL-height layer-by-layer growth is also sensitive to the B coverage w5,8x. Our STM results show that at higher u B there are more 2-BL islands than BL islands, and that the 2-BL islands include TBs at the interfaces. This clearly indicates a direct relation between the formation of the twinned epitaxial layers and 2-BL-height layer-by-layer growth. However, if Si grows in the 2-BL-height layer-by-layer mode, twinned 2-BL islands should be formed on the first twinned 2-BL. Then the second 2-BL should be twinned with the first 2-BL, and the second 2-BL should have a normal epitaxial relationship with the substrate. In other words, 2-BL-height layer-by-layer growth does not cause uniform twinned layers. In reality, however, Headrick et al. w4x reported that film grown on a substrate covered with 0.33-ML B at 4008C was essentially twinned. Furthermore, in the reported cross-sectional transmission electron microscopy images of Si layers grown on SiŽ111.'3 = '3 -B, the grown layers were totally twinned w4,5x.
Fig. 2. STM images of a SiŽ111.'3 = '3 -B surface where 0.7-BL Si was grown at 6008C. The sample bias and the tunneling current were q2 V and 0.1 nA. Žb. is a magnified STM image of the area indicated by the square in Ža.. The scanning area of Ža. was 200= 200 nm2 .
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H. Hibino et al.r Applied Surface Science 130–132 (1998) 41–46
We, therefore, investigated the growth process of totally twinned layers. Fig. 3 shows STM images of Si layers just after the completion of the first 2-BL. In Fig. 3a, BL islands nucleate more than 2-BL islands. Fig. 3b is a magnified STM image of the area indicated by the square in Ža.. Troughs between adatoms are indicated by straight lines and show that the BL islands nucleate at the domain boundary. In Fig. 3a, domain boundaries are indicated by lines, and most of the islands nucleate at the domain boundaries. After completion of the first 2-BL, the coalescence of 2-BL islands makes the domain boundary density much higher than that on the substrate. Moreover, BL islands rather than 2-BL islands nucleate at the domain boundary. Thus, a growth mode transition from 2-BL islands to BL islands occurs after the completion of the first 2-BL. The growth mode transition was also confirmed by AFM. Fig. 4a and b show AFM images of SiŽ111.'3 = '3 -B surfaces after growth of 1-BL and 3-BL Si, respectively. In the figures, steps run between the arrowheads. After 1-BL Si growth, some islands are arranged continuously across the terraces. This is because islands preferentially nucleate at the domain boundaries of the '3 = '3 reconstruction. In Fig. 4a, almost all the islands except those formed at the domain boundaries are 2-BL high. After the completion of
the first 2-BL, however, BL islands nucleate more than 2-BL islands do. In Fig. 4b, the area covered with BL islands is about four times wider than that covered with 2-BL islands. BL islands on the first twinned 2-BL follows the twin relation. Therefore, grown layers are totally twinned. So far, we have neglected the effect of decrease in u B on the twinned epitaxial layer growth, since u B on the 2-BL islands formed at 6008C is as large as that on the substrate, as shown in Fig. 1. It should be noted, however, that, at lower growth temperatures, the transition from 2-BL to BL islands after the completion of the first 2-BL may be caused by the decrease in u B as well as the increase in the domain boundary density. Conversely, if we want to grow epitaxial layers twinned with the already-grown twinned layers, post-growth anneal is necessary to reduce the domain boundary density and to increase u B . We therefore investigated the thermal stability of twinned epitaxial layers grown on SiŽ111.'3 = '3 -B by MEIS. MEIS angle spectra from the samples on which 2-BL-thick and 4-BL-thick Si layers were grown at 6008C verified that twinned layers were epitaxially grown. Changes in MEIS angle spectra by post-growth anneal also showed that the 2-BL-thick twinned layers are transformed into untwinned layers even after annealing at 7008C, but that 4-BL-thick twinned layers are resistant against annealing at
Fig. 3. STM images of a SiŽ111.'3 = '3 -B surface where 2.1-BL Si was grown at 6008C. The sample bias and the tunneling current were q2 V and 0.05 nA. Žb. is a magnified STM image of the area indicated by the square in Ža.. Lines in Ža. show positions of domain boundaries of the '3 = '3 reconstruction. The scanning area of Ža. was 140 = 140 nm2 .
H. Hibino et al.r Applied Surface Science 130–132 (1998) 41–46
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Fig. 4. AFM images of SiŽ111.'3 = '3 -B surfaces where Ža. 1-BL Si and Žb. 3-BL Si were grown at 6008C, respectively. The scanning area was 1 = 1 m m2 . Steps run between the arrowheads.
9008C. The thermal stability of twinned epitaxial layers strongly depends on the thickness. To reduce the domain boundary density, post-growth anneal at higher temperatures is desirable. Therefore, postgrowth anneal temperature must be carefully selected to grow epitaxial layers twinned with the alreadygrown twinned layers. The reason twinned 2-BL islands are formed on SiŽ111.'3 = '3 -B has not been completely clarified yet. AFM images of the SiŽ111.'3 = '3 -B surface with u B f 1r3 ML show that almost all steps except those far apart due to step bunching are 2-BL high. This indicates that the step formation energy of a 2-BL-high step is smaller than that of two BL-high steps on a SiŽ111.'3 = '3 -B surface. This is consistent with the formation of islands with 2-BL height. We also suppose that the relaxation of a large surface tensile stress induced by the short B-Si bonds w9–11,15,23x is responsible for the TB formation. Islanding partially relaxes the surface tensile stress. 2-BL-high steps should be more favorable than BLhigh steps for the stress relaxation as well as the step energy. Additionally, we assume that twinned 2-BL islands relax the surface stress most efficiently. The energy cost of TBs w24x is proportional to the area of the twinned 2-BL islands. But the stress relaxation energy would be rather closely related to the length of the island edge, because stress is effectively relaxed near the periphery of the islands. Therefore, we conjecture that, in the initial growth stages, the small nucleus of the twinned 2-BL islands is stable.
We observed using STM that twinned 2-BL islands are formed at 7008C, which is the temperature at which 2-BL-thick twinned layers are transformed into untwinned layers. These findings support our conjecture that the small nucleus of twinned 2-BL islands is stable. Because, at lower growth temperatures, kinetics prevents twinned 2-BL islands from being transformed into untwinned islands during growth, twinned 2-BL islands grow in size. However, this scenario is only a possibility. Energy and strain calculations as well as more detailed experiments are required to clarify the mechanism of the twinned 2-BL island formation. In conclusion, we have investigated the growth process of Si twinned epitaxial layers on a SiŽ111.'3 = '3 -B surface. In the initial stages, untwinned BL islands and twinned 2-BL islands are formed. The ratio of 2-BL islands to BL islands increases with increasing u B . Preferred nucleation of BL islands rather than of 2-BL islands occurs at the domain boundaries of the '3 = '3 reconstruction. After the growth of the first twinned 2-BL, the growth mode changes from 2-BL islands to BL islands because the coalescence of the islands causes the domain boundary density on the first 2-BL to be much higher than that on the substrate. This leads to the growth of totally twinned layers. We have also investigated the transformation of twinned epitaxial layers into untwinned layers by post-growth anneal. Our results point to the possibility of forming superlattices of twinned and untwinned layers by precisely
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controlling the parameters of the growth and postgrowth anneal.
References w1x K. Akimoto, J. Mizuki, I. Hirosawa, T. Tatsumi, H. Hirayama, N. Aizaki, J. Matsui, Extended Abstracts of the 19th Conference on Solid State Devices and Materials, Business Center for Academic Societies, Tokyo, 1987, p. 463. w2x R.L. Headrick, L.C. Feldman, I.K. Robinson, Appl. Phys. Lett. 55 Ž1989. 442. w3x K. Akimoto, I. Hirosawa, T. Tatsumi, H. Hirayama, J. Mizuki, J. Matsui, Appl. Phys. Lett. 56 Ž1990. 1225. w4x R.L. Headrick, B.E. Weir, J. Bevk, B.S. Freer, D.J. Eaglesham, L.C. Feldman, Phys. Rev. Lett. 65 Ž1990. 1128. w5x Y. Kumagai, R. Mori, K. Ishimoto, K.-H. Park, F. Hasegawa, Jpn. J. Appl. Phys. 34 Ž1995. 4593. w6x R.L. Headrick, A.F.J. Levi, H.S. Luftman, J. Kovalchick, L.C. Feldman, Phys. Rev. B 43 Ž1991. 14711. w7x A.V. Zotov, F. Wittmann, J. Lechner, I. Eisele, S.V. Ryzhkov, V.G. Lifshits, Appl. Phys. Lett. 67 Ž1995. 611. w8x Y. Kumagai, R. Mori, K. Ishimoto, F. Hasegawa, Jpn. J. Appl. Phys. 33 Ž1994. L817. w9x R.L. Headrick, I.K. Robinson, E. Vlieg, L.C. Feldman, Phys. Rev. Lett. 63 Ž1989. 1253.
w10x P. Bedrossian, R.D. Meade, K. Mortensen, D.M. Chen, J.A. Golovchenko, D. Vanderbilt, Phys. Rev. Lett. 63 Ž1989. 1257. w11x I.-W. Lyo, E. Kaxiras, Ph. Avouris, Phys. Rev. Lett. 63 Ž1989. 1261. w12x Y. Homma, R.J. McClelland, H. Hibino, Jpn. J. Appl. Phys. 29 Ž1990. L2254. w13x E. de Fresart, K.L. Wang, S.S. Rhee, Appl. Phys. Lett. 53 ´ Ž1988. 48. w14x T.P. Roge, F. Thibaudau, Ph. Mathiez, Ph. Dumas, F. Salvan, J. Vac. Sci. Technol. B 12 Ž1994. 2040. w15x D.E. Jones, J.P. Pelz, Y. Hong, E. Bauer, I.S.T. Tsong, Phys. Rev. Lett. 77 Ž1996. 330. w16x H. Hibino, T. Ogino, submitted to Surf. Sci. w17x U. Kohler, J.E. Demuth, R.J. Hamers, J. Vac. Sci. Technol. ¨ A 7 Ž1989. 2860. w18x H. Hibino, N. Shimizu, Y. Shinoda, Vac. J. Sci. Technol. A 11 Ž1993. 2458. w19x H. Hibino, N. Shimizu, Y. Shinoda, T. Ogino, Mat. Res. Soc. Symp. Proc. 317 Ž1994. 41. w20x Y. Homma, H. Hibino, N. Aizawa, Surf. Sci. 324 Ž1995. L333. w21x H. Hibino, T. Ogino, Appl. Phys. Lett. 67 Ž1995. 915. w22x A.W. Denier van der Gon, R.M. Tromp, Phys. Rev. Lett. 69 Ž1992. 3519. w23x R.D. Meade, D. Vanderbilt, Phys. Rev. Lett. 63 Ž1989. 1404. w24x L.F. Mattheiss, J.R. Patel, Phys. Rev. B 23 Ž1981. 5384.
Growth process of twinned epitaxial layers on Si ž111 /(3 = (3 -B and their thermal stability H. Hibino ) , K. Sumitomo, T. Ogino NTT Basic Research Laboratories, Atsugi, Kanagawa 243-01, Japan Received 5 September 1997; accepted 18 November 1997
Abstract We investigated the growth of twinned epitaxial Si layers on a SiŽ111.'3 = '3 -B surface. In the initial growth stages, untwinned bilayer-high ŽBL-high. and twinned 2-BL-high islands are nucleated, and the ratio of the number of Si atoms included in the twinned 2-BL-high islands to the number of the total deposited Si atoms increases as the surface B concentration increases. Preferred nucleation of Si islands occurs at domain boundaries of the '3 = '3 reconstruction. Moreover, BL-high islands rather than 2-BL-high islands nucleate there. Coalescence of 2-BL-high islands causes the domain boundary density on the first two bilayers to be much larger than that on the substrate. Therefore, after completion of the first twinned two bilayers, BL-high islands are formed predominantly. BL-high islands follow the stacking sequences of the twinned two bilayers. Thus, grown layers are totally twinned. We also investigated the thermal stability of twinned epitaxial layers. The temperature at which twinned epitaxial layers are transformed into untwinned layers strongly depends on the thickness. q 1998 Elsevier Science B.V. All rights reserved. PACS: 68.35.-p; 66.55.Bd; 61.72.Mm Keywords: Silicon; Boron; Epitaxial growth; Twin
B-induced '3 = '3 reconstruction plays a special role in SirSiŽ111. interface formation. It has been reported that the reconstruction is preserved even after amorphous-Si Ža-Si. deposition w1–3x, a twin boundary ŽTB. is formed at the interface between epitaxially grown Si layers and the substrate w4,5x, the two-dimensional B layer at the a-SirSiŽ111. interface is electrically active w6,7x, and Si grows in a layer-by-layer fashion with the unit of two bilayers ŽBL. w5,8x. These unique features are mostly at-
)
Corresponding author. Tel.: q81-462-40-3467; fax: q81462-40-4718; e-mail: [email protected]
tributed to the atomic structure of the B-induced '3 = '3 reconstruction; B atoms locate at S5 sites in the 2nd layer just below the Si T4 adatoms w9–11x. The preservation of the '3 = '3 reconstruction and the nearly complete activation of B at the aSirBrSiŽ111. interface are explained by the fact that B locates not at the outermost surface but at the subsurface. However, the reasons for the formation of the twinned epitaxial layers and for the 2-BLheight layer-by-layer growth are not clear. In this paper, we investigate the growth process of twinned epitaxial layers on SiŽ111.'3 = '3 -B. We demonstrate that twinned epitaxial layers are caused by twinned 2-BL-high island formation. We also clarify
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 0 2 2 - 1
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H. Hibino et al.r Applied Surface Science 130–132 (1998) 41–46
the roles of the domain boundaries of the '3 = '3 reconstruction in twinned epitaxial layer growth. Furthermore, we investigate thermal stability of twinned epitaxial layers. Our results on the growth process of the twinned epitaxial layers and their thermal stability show a possibility of introducing TBs into the grown layers repeatedly. We used two UHV systems. One was equipped with a scanning tunneling microscope ŽSTM. and the other included chambers equipped with molecularbeam epitaxy ŽMBE. and medium-energy ion scattering ŽMEIS. apparatuses. We investigated surface atomic structures using the STM and the atomic structures of epitaxially grown layers by MEIS. Si was grown at sample temperatures between 4008C and 7008C, but we mainly show results for 6008C. Sample temperatures were measured with an infrared pyrometer. In order to investigate surface morphologies on a wider scale than those obtained by STM, atomic force microscopy ŽAFM. images were taken in air after the samples were taken out of the vacuum. Highly B-doped SiŽ111. samples with resistivity of 0.001 V cm were used as substrates. The misorientation angle was about 0.38 and the steps ran nearly normal to ²110:. Assuming BL-high steps, the average terrace width is 60 nm. Because this value is too small to investigate island formation at 7008C, we formed terraces as wide as 1 m m through step bunching by annealing the sample with d.c. electric currents in the step-down direction Žfor ex-
ample, Ref. w12x.. The '3 = '3 reconstruction was prepared using the surface segregation of B during annealing. It has been reported that the surface B concentration u B depends on the temperature w13– 15x. In order to control u B , the samples were quenched after annealing at temperatures between 8508C and 12508C. Fig. 1 shows STM images of SiŽ111.'3 = '3 -B surfaces after Si growth at 6008C. The amount of grown Si was about 0.7 BL. The difference between Fig. 1a and b is the preparation of the substrate: quenching after annealing at 9008C for 10 min for Ža. and after annealing at 10508C for 1 min for Žb.. This caused a difference in u B . On the SiŽ111.'3 = '3 -B surface, all adatoms are Si, but some of the B atoms in the S 5 sites are replaced by Si w10,11x. Bright and dark adatoms in the STM images are Si adatoms above the S 5 Si and S 5 B atoms, respectively w10,11x. Therefore, u B was determined by counting the bright and dark adatoms in the STM images. u B on the substrates in Fig. 1a and b is 0.31 and 0.24 monolayers ŽML., respectively. We also measured the ratios of the u B values between the samples quenched after annealing at 9008C for 10 min and after annealing at 10508C for 1 min using secondary ion mass spectroscopy and Auger electron spectroscopy. These ratios were consistent with the ratio obtained by STM. In both Fig. 1a and b, Si islands were formed, and the '3 = '3 reconstruction can be seen on the islands. However, the islands
Fig. 1. STM images of SiŽ111.'3 = '3 -B surfaces where 0.7-BL Si was grown at 6008C. The amount of B in the S 5 site on the substrates in Ža. and Žb. was 0.31 and 0.24 ML, respectively. The scanning area was 65 = 65 nm2 . The sample bias and the tunneling current were q2 V and 0.1 nA. A BL island is indicated by BL in Ža., and 2BL islands are indicated by 2BL in Žb..
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in Fig. 1b have a height of BL and 2-BL, but almost all of the islands in Fig. 1a are 2-BL high. The ratio of the number of Si atoms in the 2-BL islands to the number of the total deposited Si atoms depends on u B w16x. The larger u B is, the larger the ratio becomes, and, at the growth temperature of 6008C, islands grown on the substrate with u B s 0.32 ML are all 2-BL high and islands grown on the substrate with u B s 0.2 ML are all BL high. BL and 2-BL islands differ not only in height but also in their epitaxial relationship with the substrate. Comparison between the positions of the adatoms in the '3 = '3 reconstruction on the Si islands and the substrate using STM showed that the 2-BL islands are twinned with the substrate, but that the BL islands are not twinned w16x. It has been reported that the domain boundary of the 7 = 7 reconstruction on SiŽ111. influences various phenomena, such as epitaxial growth and adsorption w17–22x. During Si MBE, two-dimensional islands are preferentially nucleated at the domain boundaries of the 7 = 7 reconstruction w17x. How about at the domain boundaries of the B-induced '3 = '3 reconstruction? Fig. 2a shows an STM image of a SiŽ111. surface on which 0.7-BL Si was grown at 6008C. Fig. 2b shows a magnified image of the area indicated by the square in Fig. 2a. In Fig. 2b, troughs between adatoms on the substrate are indicated by straight lines. Shifts of the straight lines indicate that the '3 = '3 reconstruction on the substrate includes a domain boundary. The position of the domain boundary is indicated by lines in Fig. 2a,
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and one can see that islands preferentially nucleate at the domain boundary. Furthermore, on this sample, BL islands beside those nucleated at the domain boundaries were rarely observed. Islands nucleated at the domain boundary of the '3 = '3 reconstruction prefer to be BL high rather than 2-BL high. Twinned epitaxial Si layers grown on SiŽ111.'3 = '3 -B was first reported by Headrick et al. w4x. They showed that Si layers grown on a substrate covered with 0.33-ML B at 4008C are essentially twinned, but that smaller B coverage caused an increase in the ratio of the untwinned layers w4x. 2-BL-height layer-by-layer growth is also sensitive to the B coverage w5,8x. Our STM results show that at higher u B there are more 2-BL islands than BL islands, and that the 2-BL islands include TBs at the interfaces. This clearly indicates a direct relation between the formation of the twinned epitaxial layers and 2-BL-height layer-by-layer growth. However, if Si grows in the 2-BL-height layer-by-layer mode, twinned 2-BL islands should be formed on the first twinned 2-BL. Then the second 2-BL should be twinned with the first 2-BL, and the second 2-BL should have a normal epitaxial relationship with the substrate. In other words, 2-BL-height layer-by-layer growth does not cause uniform twinned layers. In reality, however, Headrick et al. w4x reported that film grown on a substrate covered with 0.33-ML B at 4008C was essentially twinned. Furthermore, in the reported cross-sectional transmission electron microscopy images of Si layers grown on SiŽ111.'3 = '3 -B, the grown layers were totally twinned w4,5x.
Fig. 2. STM images of a SiŽ111.'3 = '3 -B surface where 0.7-BL Si was grown at 6008C. The sample bias and the tunneling current were q2 V and 0.1 nA. Žb. is a magnified STM image of the area indicated by the square in Ža.. The scanning area of Ža. was 200= 200 nm2 .
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We, therefore, investigated the growth process of totally twinned layers. Fig. 3 shows STM images of Si layers just after the completion of the first 2-BL. In Fig. 3a, BL islands nucleate more than 2-BL islands. Fig. 3b is a magnified STM image of the area indicated by the square in Ža.. Troughs between adatoms are indicated by straight lines and show that the BL islands nucleate at the domain boundary. In Fig. 3a, domain boundaries are indicated by lines, and most of the islands nucleate at the domain boundaries. After completion of the first 2-BL, the coalescence of 2-BL islands makes the domain boundary density much higher than that on the substrate. Moreover, BL islands rather than 2-BL islands nucleate at the domain boundary. Thus, a growth mode transition from 2-BL islands to BL islands occurs after the completion of the first 2-BL. The growth mode transition was also confirmed by AFM. Fig. 4a and b show AFM images of SiŽ111.'3 = '3 -B surfaces after growth of 1-BL and 3-BL Si, respectively. In the figures, steps run between the arrowheads. After 1-BL Si growth, some islands are arranged continuously across the terraces. This is because islands preferentially nucleate at the domain boundaries of the '3 = '3 reconstruction. In Fig. 4a, almost all the islands except those formed at the domain boundaries are 2-BL high. After the completion of
the first 2-BL, however, BL islands nucleate more than 2-BL islands do. In Fig. 4b, the area covered with BL islands is about four times wider than that covered with 2-BL islands. BL islands on the first twinned 2-BL follows the twin relation. Therefore, grown layers are totally twinned. So far, we have neglected the effect of decrease in u B on the twinned epitaxial layer growth, since u B on the 2-BL islands formed at 6008C is as large as that on the substrate, as shown in Fig. 1. It should be noted, however, that, at lower growth temperatures, the transition from 2-BL to BL islands after the completion of the first 2-BL may be caused by the decrease in u B as well as the increase in the domain boundary density. Conversely, if we want to grow epitaxial layers twinned with the already-grown twinned layers, post-growth anneal is necessary to reduce the domain boundary density and to increase u B . We therefore investigated the thermal stability of twinned epitaxial layers grown on SiŽ111.'3 = '3 -B by MEIS. MEIS angle spectra from the samples on which 2-BL-thick and 4-BL-thick Si layers were grown at 6008C verified that twinned layers were epitaxially grown. Changes in MEIS angle spectra by post-growth anneal also showed that the 2-BL-thick twinned layers are transformed into untwinned layers even after annealing at 7008C, but that 4-BL-thick twinned layers are resistant against annealing at
Fig. 3. STM images of a SiŽ111.'3 = '3 -B surface where 2.1-BL Si was grown at 6008C. The sample bias and the tunneling current were q2 V and 0.05 nA. Žb. is a magnified STM image of the area indicated by the square in Ža.. Lines in Ža. show positions of domain boundaries of the '3 = '3 reconstruction. The scanning area of Ža. was 140 = 140 nm2 .
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Fig. 4. AFM images of SiŽ111.'3 = '3 -B surfaces where Ža. 1-BL Si and Žb. 3-BL Si were grown at 6008C, respectively. The scanning area was 1 = 1 m m2 . Steps run between the arrowheads.
9008C. The thermal stability of twinned epitaxial layers strongly depends on the thickness. To reduce the domain boundary density, post-growth anneal at higher temperatures is desirable. Therefore, postgrowth anneal temperature must be carefully selected to grow epitaxial layers twinned with the alreadygrown twinned layers. The reason twinned 2-BL islands are formed on SiŽ111.'3 = '3 -B has not been completely clarified yet. AFM images of the SiŽ111.'3 = '3 -B surface with u B f 1r3 ML show that almost all steps except those far apart due to step bunching are 2-BL high. This indicates that the step formation energy of a 2-BL-high step is smaller than that of two BL-high steps on a SiŽ111.'3 = '3 -B surface. This is consistent with the formation of islands with 2-BL height. We also suppose that the relaxation of a large surface tensile stress induced by the short B-Si bonds w9–11,15,23x is responsible for the TB formation. Islanding partially relaxes the surface tensile stress. 2-BL-high steps should be more favorable than BLhigh steps for the stress relaxation as well as the step energy. Additionally, we assume that twinned 2-BL islands relax the surface stress most efficiently. The energy cost of TBs w24x is proportional to the area of the twinned 2-BL islands. But the stress relaxation energy would be rather closely related to the length of the island edge, because stress is effectively relaxed near the periphery of the islands. Therefore, we conjecture that, in the initial growth stages, the small nucleus of the twinned 2-BL islands is stable.
We observed using STM that twinned 2-BL islands are formed at 7008C, which is the temperature at which 2-BL-thick twinned layers are transformed into untwinned layers. These findings support our conjecture that the small nucleus of twinned 2-BL islands is stable. Because, at lower growth temperatures, kinetics prevents twinned 2-BL islands from being transformed into untwinned islands during growth, twinned 2-BL islands grow in size. However, this scenario is only a possibility. Energy and strain calculations as well as more detailed experiments are required to clarify the mechanism of the twinned 2-BL island formation. In conclusion, we have investigated the growth process of Si twinned epitaxial layers on a SiŽ111.'3 = '3 -B surface. In the initial stages, untwinned BL islands and twinned 2-BL islands are formed. The ratio of 2-BL islands to BL islands increases with increasing u B . Preferred nucleation of BL islands rather than of 2-BL islands occurs at the domain boundaries of the '3 = '3 reconstruction. After the growth of the first twinned 2-BL, the growth mode changes from 2-BL islands to BL islands because the coalescence of the islands causes the domain boundary density on the first 2-BL to be much higher than that on the substrate. This leads to the growth of totally twinned layers. We have also investigated the transformation of twinned epitaxial layers into untwinned layers by post-growth anneal. Our results point to the possibility of forming superlattices of twinned and untwinned layers by precisely
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controlling the parameters of the growth and postgrowth anneal.
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