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Maskless selective area molecular beam epitaxy of semiconductors and metals using atomic step networks on silicon
Journal of Crystal Growth 201/202 (1999) 604}609
Maskless selective area molecular beam epitaxy of semiconductors and metals using atomic step networks on silicon Paul Finnie*, Yoshikazu Homma NTT Basic Research Laboratories, Atsugi-Shi, Kanagawa 243-0198, Japan
Abstract Step-patterned substrates of vicinal Si(1 1 1) were used for the selective growth of a variety of materials, both semiconductor (GaAs and Ge) and metal (Au and Ag). Step patterning was performed by etching substrates photolithographically, and annealing them. Subsequent growth by molecular beam epitaxy was observed by in situ ultra-high vacuum scanning electron microscopy. Materials grew preferentially on the prepared step bands either by a desorption mechanism or by a di!usion mechanism, depending on the temperature. The growth of silver was di!erent, although the initial monolayer was also selective. This growth technique can be applied to a small region or an entire wafer. Growth areas can be several microns wide, or, in principle, can be of nanometer scale 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.!z; 81.15.Gh; 68.35.Bs; 68.55.!a Keywords: Selective area epitaxy; Step patterning; Patterned growth; Molecular beam epitaxy
1. Introduction Among the various techniques for selective area epitaxy (SAE) [1], probably the most successful uses a mask of low sticking coe$cient. In Practice, this is more straightforward with gas sources [2], but it is also feasible by molecular beam epitaxy (MBE) [3]. Unfortunately, the mask and the associated processing complicates surface preparation and can introduce defects and impurities. However,
* Corresponding author. Tel.: #81-462-40-4361; fax: #81462-40-4718; e-mail: "[email protected].
a di!erent approach to selective area growth is possible . The use of a mask can be avoided altogether by using atomic step networks as templates for selective area growth. This approach is demonstrated for a variety of materials, below.
2. Experimental details The selective growth technique relies on a substrate that has been `step-patterneda. That is, the substrate is engineered to have step-free areas (which are therefore exceptionally #at), and bands where the step density is higher (and therefore,
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 4 2 0 - 1
P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
605
Fig. 1. (a) Schematic diagram of substrate. The substrate was vicinal Si(1 1 1). The surface normal, n, was tilted 1.53 toward the [1 1 2 ] direction. A grid pattern of holes was etched photolithographically, each hole having diameter, D, of about 1 lm. The center-to-center hole spacing was about "ve times larger. The grid was slightly rotated (53) from the [ 1 1 0] direction, which ran parallel to the average step direction. Various grid sizes were used. (b) Perspective view of substrate. The substrate is viewed after heating, from the same angle as the schematic. Three regions are seen: top, middle and bottom. At the bottom, remnants of the original circular crater can be seen. The top pattern had much smaller spacing, and is almost completely eroded away. In the middle a step network pattern is seen. There are many ultra-#at terraces, lacking atomic steps altogether. The alternating ribbon of light and dark is a large step band. Small step bands connect the large ones by traversing the #at terraces. The marker represents horizontal distances accurately.
where the vicinal tilt angle is larger). The process of creating a step-patterned substrate is brie#y outlined below. A detailed description can be found in Refs. [5}7]. A schematic of the initial substrate is shown in Fig. 1(a). The substrate was vicinal Si(1 1 1) into which an array of holes was etched photolithographically. Substrates were oxidized in H SO and H O (4 : 1), after which the sample was loaded into the UHV chamber. Samples were resistively heated through tantalum foil contacts on the sample holder. The sample was annealed to 6003C for several hours. Finally, heating to &12003C caused sublimation, and the surface eroded into the desired structure. Fig. 1(b) shows a scanning electron microscope (SEM) image of an actual substrate after preparation. The surface consists of a network of step
bands which are separated by step-free terraces. It will be shown in the following section that, under certain conditions, growth is more likely on the step bands than on the #at terraces. After this preparation, various materials were deposited by MBE. The elemental Au and Ag sources were droplet shaped, &1 mm in radius, and held in resistively heated tungsten wire baskets. Elemental Ge and Ga, as well as As , were supplied by conventional K-cells. A constant heating current was used for Au and Ag, while other sources were held at a "xed temperature, as measured by thermocouple. For Ge, Au and Ag the #ux was too low to be measured directly with the #ux monitor and one chamber pressures remained in the 10\ range throughout the growth. For GaAs growth, chamber pressures were higher (up to the 10\ Torr range) due to the slower pumping of arsenic.
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P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
3. Results and discussion 3.1. Germanium Germanium grows on silicon in the Stranski} Krastanov mode with a wetting layer thickness of &3 monolayers (MLs) [8]. Micrographs taken in situ after 30 min growth are shown in Fig. 2. In Fig. 2(a), grown at 4003C, no islands can be seen. However, at 5003C [Fig. 2(b)], di!usion is activated and small, #at islands were seen everywhere on the sample. Even at this temperature there was some evidence of a reduced number of islands on the #at step-free craters. At higher temperatures, such as 5503C, as in Fig. 2(c), islands are no longer seen anywhere except on the step bands. The islands are larger and rounded. Finally, at still higher temperatures [Fig. 2(d), 6003C] the Ge desorbs considerably, and only a few large islands are seen around the step bands. The interpretation of the result of Ge deposition is slightly complicated by the relatively thick wetting layer, but otherwise it is a good representative of the more general case. At low temperatures, growth is uniform across the substrate. As temper-
atures are increased, growth occurs preferentially on step patterns, "rst because of di!usion on the #at areas, and ultimately because of desorption from the #at areas. 3.2. Gallium Arsenide Near perfect selective growth of GaAs on steppatterned substrates was described in another paper [4]. Preferential desorption caused selectivity at high temperatures. At lower temperatures, selectivity was obtained by preferential migration from the terrace. The low-temperature mode should be important for nanofabrication since it occurs for low #uxes and when structures are smaller than the di!usion length. Unlike the other materials, GaAs is a binary compound, and therefore #ux ratios (As/Ga) can be varied. Fig. 3 shows a series in the arsenic #ux of the otherwise identical growths on nearly identical patterns. The beam equivalent pressure of arsenic increases from Fig. 3(a)}(e). In (a) barely resolvable islands cover the entire #at surface. As the arsenic pressure is increased, a denuded region near the step bands can be seen. There are fewer, larger
Fig. 2. Germanium deposition for various temperatures. Germanium deposited on patterned substrates at temperatures: (a) 4003C, (b) 5003C, (c) 5503C and (d) 6003C. The deposition was of 30 min in duration for each case, with a "xed cell temperature of 12603C. Substrates are viewed from an approximately 603 angle, so the marker measures horizontal distances only.
P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
607
length is reduced. The island shape changes } islands appear larger in height. A simple model of the change is as follows. Ideally, the surface is completely and evenly arsenic terminated and a long di!usion length for the gallium adatoms results. But, if the arsenic pressure is reduced, the termination is incomplete, gallium migration may be stopped, and growth nucleated when adatoms reach a relatively uncovered area. The fact that the denuded zone shrinks for increasing pressures beyond (d) can be explained if the steady-state arsenic coverage begins to exceed a complete ML. The extra adatoms above the completed layer can then act as nucleation sites. The model seems reasonable since these growth conditions cause steady-state arsenic coverages of very close to one ML [8,9]. 3.3. Gold
Fig. 3. As pressure dependence of GaAs growth. GaAs was deposited with a substrate temperature of 3603C, nominal gallium #ux 0.3 ML/min, and growth time 10 min. The beam equivalent arsenic pressures were (a) 2.5;10\ Torr (b) 3.7;10\ Torr (c) 8.5;10\ Torr (d) 2.1;10\ Torr and (e) 3.0;10\ Torr. Samples are viewed at an angle. The marker measures horizontal distances correctly.
islands and they are more localized. In (c) distinct, relatively large islands are seen, as well as clearly denuded areas near the step bands. Di!usion is the mechanism of selectivity. Material is migrating from the #at surfaces to the step bands. In the "rst four images the di!usion length increases with arsenic pressure. But, in Fig. 3(e), although arsenic pressure is increased, the denuded
Gold grows on Si(1 1 1) in a Stranski}Krastanov mode [8], and otherwise behaves similarly to the preceding materials. At low temperatures, small islands form all over the substrate, as seen in Fig. 4(a), for which a substrate temperature of 3503C was used. For growth with higher substrate temperature the islands are larger, and more widely spaced, since di!usion is activated [Fig. 4(b)]. At 5003C, as in Fig. 4(c) there is no growth on the #at surfaces. At still higher temperatures [Fig. 4(d)] the islands are only on the step bands. Di!usion is the main mechanism of selectivity. The increase in temperature activates the migration of the terraces. 3.4. Silver Silver behaves di!erently from other materials. Silver grows in a Stranski}Krastanov mode, with a wetting layer of only one ML or less [8]. Once this wetting layer was complete, island formation was observed, however, the islands were widely spaced at tens of microns apart. Electromigration caused islands to form only along one side of the trenches that were spaced roughly about every 100 lm, running perpendicularly to the heating current. These faceted islands grew continuously, becoming roughly square or hexagonal, depending on the temperature.
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P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
Fig. 4. Gold deposition for various temperatures. Gold deposited on patterned substrates at temperature (a) 3503C (b) 4003C (c) 5003C and (d) 6003C for &1 min with constant current resistive heating from a tungsten wire. Samples are viewed at an angle. The marker measures horizontal distances correctly.
Fig. 5. Silver deposition for various temperatures. Silver deposited on patterned substrates of temperature (a) 3503C (b) 4503C (c) 5503C and (d) 6503C. Silver was deposited for &4 min, using a constant current tungsten wire heater. The circular craters are ultra-#at terraces. Samples are viewed at a 603 angle. The marker is accurate for the horizontal direction.
P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
These large islands were so widely spaced that the patterning was irrelevant. There was no contrast change observed elsewhere, so all or most of the deposited material may have migrated to the islands. Because of the excessively long migration length, except perhaps for large patterns, step-patterned selective growth is not possible for more than one ML of silver. Selective growth was observed for less than an ML of silver deposition. This is shown, for various temperatures, in Fig. 5. At 3503C [Fig. 5(a)], barely resolvable islands are seen on the #at surfaces. For higher temperatures, these islands become larger, as seen in Fig. 5(b) and (c) for 4503C and 5503C, respectively. In Fig. 5(d) only one central island is seen. This indicates that the temperature activation of di!usion is a mechanism for the selectivity of the submonolayer silver. At the highest temperature, the lighter area covered less of the substrate and some desorption has occurred. There was no apparent enhancement of growth on the step bands. In fact, there seems to be a barrier to di!usion at the edge of the step, as can be seen by the light ring of silver on the #at surface, at the outside edge of the circle. Thin arcs of silver, seen in Fig. 5(c) and (d), could become the basis of a technique to fabricate wires with nanometer-size cross sections.
4. Conclusion Selective area MBE growth on step-patterned substrates has been demonstrated for Ge, GaAs
609
and Au. A di!erent kind of selectivity is seen for submonolayer Ag. The mechanism is preferential di!usion from #at surfaces, or preferential desorption from #at surfaces, depending on the temperature. Step patterning allows for maskless SAE, and may enable nanostructure fabrication.
Acknowledgements The authors thank T. Ogino for helpful discussion.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
W.T. Tsang, Semicond. Sci. Technol. 8 (1993) 1016. H. Heinecke, M. Wachter, Appl. Surf. Sci. 113/114 (1997) 1. A. Okamoto, Semicond. Sci. Technol. 8 (1993) 1011. P. Finnie, Y. Homma, J. Appl. Phys. 72 (1998) 827. T. Ogino, H. Hibino, Y. Homma, Appl. Surf. Sci. 117/118 (1997) 642. Y. Homma, H. Hibino, T. Ogino, N. Aizawa, Phys. Rev. B 55 (1997) 10237. Y. Homma, N. Aizawa, T. Ogino, Jpn. J. Appl. Phys. 35 (1996) L241. V.G. Lifshits, A.A. Saranin, A.V. Zotov, Surface Phases on Silicon, Wiley, Singapore, 1994. M. Copel, R.M. Tromp, U.K. Kohler, Phys. Rev. B. 37 (1988) 10756.
Maskless selective area molecular beam epitaxy of semiconductors and metals using atomic step networks on silicon Paul Finnie*, Yoshikazu Homma NTT Basic Research Laboratories, Atsugi-Shi, Kanagawa 243-0198, Japan
Abstract Step-patterned substrates of vicinal Si(1 1 1) were used for the selective growth of a variety of materials, both semiconductor (GaAs and Ge) and metal (Au and Ag). Step patterning was performed by etching substrates photolithographically, and annealing them. Subsequent growth by molecular beam epitaxy was observed by in situ ultra-high vacuum scanning electron microscopy. Materials grew preferentially on the prepared step bands either by a desorption mechanism or by a di!usion mechanism, depending on the temperature. The growth of silver was di!erent, although the initial monolayer was also selective. This growth technique can be applied to a small region or an entire wafer. Growth areas can be several microns wide, or, in principle, can be of nanometer scale 1999 Elsevier Science B.V. All rights reserved. PACS: 81.15.!z; 81.15.Gh; 68.35.Bs; 68.55.!a Keywords: Selective area epitaxy; Step patterning; Patterned growth; Molecular beam epitaxy
1. Introduction Among the various techniques for selective area epitaxy (SAE) [1], probably the most successful uses a mask of low sticking coe$cient. In Practice, this is more straightforward with gas sources [2], but it is also feasible by molecular beam epitaxy (MBE) [3]. Unfortunately, the mask and the associated processing complicates surface preparation and can introduce defects and impurities. However,
* Corresponding author. Tel.: #81-462-40-4361; fax: #81462-40-4718; e-mail: "[email protected].
a di!erent approach to selective area growth is possible . The use of a mask can be avoided altogether by using atomic step networks as templates for selective area growth. This approach is demonstrated for a variety of materials, below.
2. Experimental details The selective growth technique relies on a substrate that has been `step-patterneda. That is, the substrate is engineered to have step-free areas (which are therefore exceptionally #at), and bands where the step density is higher (and therefore,
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 4 2 0 - 1
P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
605
Fig. 1. (a) Schematic diagram of substrate. The substrate was vicinal Si(1 1 1). The surface normal, n, was tilted 1.53 toward the [1 1 2 ] direction. A grid pattern of holes was etched photolithographically, each hole having diameter, D, of about 1 lm. The center-to-center hole spacing was about "ve times larger. The grid was slightly rotated (53) from the [ 1 1 0] direction, which ran parallel to the average step direction. Various grid sizes were used. (b) Perspective view of substrate. The substrate is viewed after heating, from the same angle as the schematic. Three regions are seen: top, middle and bottom. At the bottom, remnants of the original circular crater can be seen. The top pattern had much smaller spacing, and is almost completely eroded away. In the middle a step network pattern is seen. There are many ultra-#at terraces, lacking atomic steps altogether. The alternating ribbon of light and dark is a large step band. Small step bands connect the large ones by traversing the #at terraces. The marker represents horizontal distances accurately.
where the vicinal tilt angle is larger). The process of creating a step-patterned substrate is brie#y outlined below. A detailed description can be found in Refs. [5}7]. A schematic of the initial substrate is shown in Fig. 1(a). The substrate was vicinal Si(1 1 1) into which an array of holes was etched photolithographically. Substrates were oxidized in H SO and H O (4 : 1), after which the sample was loaded into the UHV chamber. Samples were resistively heated through tantalum foil contacts on the sample holder. The sample was annealed to 6003C for several hours. Finally, heating to &12003C caused sublimation, and the surface eroded into the desired structure. Fig. 1(b) shows a scanning electron microscope (SEM) image of an actual substrate after preparation. The surface consists of a network of step
bands which are separated by step-free terraces. It will be shown in the following section that, under certain conditions, growth is more likely on the step bands than on the #at terraces. After this preparation, various materials were deposited by MBE. The elemental Au and Ag sources were droplet shaped, &1 mm in radius, and held in resistively heated tungsten wire baskets. Elemental Ge and Ga, as well as As , were supplied by conventional K-cells. A constant heating current was used for Au and Ag, while other sources were held at a "xed temperature, as measured by thermocouple. For Ge, Au and Ag the #ux was too low to be measured directly with the #ux monitor and one chamber pressures remained in the 10\ range throughout the growth. For GaAs growth, chamber pressures were higher (up to the 10\ Torr range) due to the slower pumping of arsenic.
606
P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
3. Results and discussion 3.1. Germanium Germanium grows on silicon in the Stranski} Krastanov mode with a wetting layer thickness of &3 monolayers (MLs) [8]. Micrographs taken in situ after 30 min growth are shown in Fig. 2. In Fig. 2(a), grown at 4003C, no islands can be seen. However, at 5003C [Fig. 2(b)], di!usion is activated and small, #at islands were seen everywhere on the sample. Even at this temperature there was some evidence of a reduced number of islands on the #at step-free craters. At higher temperatures, such as 5503C, as in Fig. 2(c), islands are no longer seen anywhere except on the step bands. The islands are larger and rounded. Finally, at still higher temperatures [Fig. 2(d), 6003C] the Ge desorbs considerably, and only a few large islands are seen around the step bands. The interpretation of the result of Ge deposition is slightly complicated by the relatively thick wetting layer, but otherwise it is a good representative of the more general case. At low temperatures, growth is uniform across the substrate. As temper-
atures are increased, growth occurs preferentially on step patterns, "rst because of di!usion on the #at areas, and ultimately because of desorption from the #at areas. 3.2. Gallium Arsenide Near perfect selective growth of GaAs on steppatterned substrates was described in another paper [4]. Preferential desorption caused selectivity at high temperatures. At lower temperatures, selectivity was obtained by preferential migration from the terrace. The low-temperature mode should be important for nanofabrication since it occurs for low #uxes and when structures are smaller than the di!usion length. Unlike the other materials, GaAs is a binary compound, and therefore #ux ratios (As/Ga) can be varied. Fig. 3 shows a series in the arsenic #ux of the otherwise identical growths on nearly identical patterns. The beam equivalent pressure of arsenic increases from Fig. 3(a)}(e). In (a) barely resolvable islands cover the entire #at surface. As the arsenic pressure is increased, a denuded region near the step bands can be seen. There are fewer, larger
Fig. 2. Germanium deposition for various temperatures. Germanium deposited on patterned substrates at temperatures: (a) 4003C, (b) 5003C, (c) 5503C and (d) 6003C. The deposition was of 30 min in duration for each case, with a "xed cell temperature of 12603C. Substrates are viewed from an approximately 603 angle, so the marker measures horizontal distances only.
P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
607
length is reduced. The island shape changes } islands appear larger in height. A simple model of the change is as follows. Ideally, the surface is completely and evenly arsenic terminated and a long di!usion length for the gallium adatoms results. But, if the arsenic pressure is reduced, the termination is incomplete, gallium migration may be stopped, and growth nucleated when adatoms reach a relatively uncovered area. The fact that the denuded zone shrinks for increasing pressures beyond (d) can be explained if the steady-state arsenic coverage begins to exceed a complete ML. The extra adatoms above the completed layer can then act as nucleation sites. The model seems reasonable since these growth conditions cause steady-state arsenic coverages of very close to one ML [8,9]. 3.3. Gold
Fig. 3. As pressure dependence of GaAs growth. GaAs was deposited with a substrate temperature of 3603C, nominal gallium #ux 0.3 ML/min, and growth time 10 min. The beam equivalent arsenic pressures were (a) 2.5;10\ Torr (b) 3.7;10\ Torr (c) 8.5;10\ Torr (d) 2.1;10\ Torr and (e) 3.0;10\ Torr. Samples are viewed at an angle. The marker measures horizontal distances correctly.
islands and they are more localized. In (c) distinct, relatively large islands are seen, as well as clearly denuded areas near the step bands. Di!usion is the mechanism of selectivity. Material is migrating from the #at surfaces to the step bands. In the "rst four images the di!usion length increases with arsenic pressure. But, in Fig. 3(e), although arsenic pressure is increased, the denuded
Gold grows on Si(1 1 1) in a Stranski}Krastanov mode [8], and otherwise behaves similarly to the preceding materials. At low temperatures, small islands form all over the substrate, as seen in Fig. 4(a), for which a substrate temperature of 3503C was used. For growth with higher substrate temperature the islands are larger, and more widely spaced, since di!usion is activated [Fig. 4(b)]. At 5003C, as in Fig. 4(c) there is no growth on the #at surfaces. At still higher temperatures [Fig. 4(d)] the islands are only on the step bands. Di!usion is the main mechanism of selectivity. The increase in temperature activates the migration of the terraces. 3.4. Silver Silver behaves di!erently from other materials. Silver grows in a Stranski}Krastanov mode, with a wetting layer of only one ML or less [8]. Once this wetting layer was complete, island formation was observed, however, the islands were widely spaced at tens of microns apart. Electromigration caused islands to form only along one side of the trenches that were spaced roughly about every 100 lm, running perpendicularly to the heating current. These faceted islands grew continuously, becoming roughly square or hexagonal, depending on the temperature.
608
P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
Fig. 4. Gold deposition for various temperatures. Gold deposited on patterned substrates at temperature (a) 3503C (b) 4003C (c) 5003C and (d) 6003C for &1 min with constant current resistive heating from a tungsten wire. Samples are viewed at an angle. The marker measures horizontal distances correctly.
Fig. 5. Silver deposition for various temperatures. Silver deposited on patterned substrates of temperature (a) 3503C (b) 4503C (c) 5503C and (d) 6503C. Silver was deposited for &4 min, using a constant current tungsten wire heater. The circular craters are ultra-#at terraces. Samples are viewed at a 603 angle. The marker is accurate for the horizontal direction.
P. Finnie, Y. Homma / Journal of Crystal Growth 201/202 (1999) 604}609
These large islands were so widely spaced that the patterning was irrelevant. There was no contrast change observed elsewhere, so all or most of the deposited material may have migrated to the islands. Because of the excessively long migration length, except perhaps for large patterns, step-patterned selective growth is not possible for more than one ML of silver. Selective growth was observed for less than an ML of silver deposition. This is shown, for various temperatures, in Fig. 5. At 3503C [Fig. 5(a)], barely resolvable islands are seen on the #at surfaces. For higher temperatures, these islands become larger, as seen in Fig. 5(b) and (c) for 4503C and 5503C, respectively. In Fig. 5(d) only one central island is seen. This indicates that the temperature activation of di!usion is a mechanism for the selectivity of the submonolayer silver. At the highest temperature, the lighter area covered less of the substrate and some desorption has occurred. There was no apparent enhancement of growth on the step bands. In fact, there seems to be a barrier to di!usion at the edge of the step, as can be seen by the light ring of silver on the #at surface, at the outside edge of the circle. Thin arcs of silver, seen in Fig. 5(c) and (d), could become the basis of a technique to fabricate wires with nanometer-size cross sections.
4. Conclusion Selective area MBE growth on step-patterned substrates has been demonstrated for Ge, GaAs
609
and Au. A di!erent kind of selectivity is seen for submonolayer Ag. The mechanism is preferential di!usion from #at surfaces, or preferential desorption from #at surfaces, depending on the temperature. Step patterning allows for maskless SAE, and may enable nanostructure fabrication.
Acknowledgements The authors thank T. Ogino for helpful discussion.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
W.T. Tsang, Semicond. Sci. Technol. 8 (1993) 1016. H. Heinecke, M. Wachter, Appl. Surf. Sci. 113/114 (1997) 1. A. Okamoto, Semicond. Sci. Technol. 8 (1993) 1011. P. Finnie, Y. Homma, J. Appl. Phys. 72 (1998) 827. T. Ogino, H. Hibino, Y. Homma, Appl. Surf. Sci. 117/118 (1997) 642. Y. Homma, H. Hibino, T. Ogino, N. Aizawa, Phys. Rev. B 55 (1997) 10237. Y. Homma, N. Aizawa, T. Ogino, Jpn. J. Appl. Phys. 35 (1996) L241. V.G. Lifshits, A.A. Saranin, A.V. Zotov, Surface Phases on Silicon, Wiley, Singapore, 1994. M. Copel, R.M. Tromp, U.K. Kohler, Phys. Rev. B. 37 (1988) 10756.