- Email: [email protected]
Study of step instability on Si surfaces
Thin Solid Films 424 (2003) 40–44
Study of step instability on Si surfaces Hiroki Minoda* Physics Department, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan Received 19 July 2002; received in revised form 10 September 2002; accepted 10 September 2002
Abstract Step instabilities on Si(1 1 1) vicinal surface and Si(1 1 1) vicinal surface induced by Au adsorption were observed by reflection electron microscopy. On the Si(0 0 1) vicinal surface faceting of (0 0 1) surface due to surface reconstruction takes place and surface steps are bunching. The kinetics of mass transport of Si depends on the substrate temperature and steps are impermeable for Si adatoms at lower temperature and they are permeable at higher temperature. On the Si(1 1 1) vicinal surface the permeability of steps depends on the heating current direction. The steps are permeable for step-up current heating and are impermeable for step-down current heating. Above a critical coverage of Au (f0.3 ML) steps are bunching irrespective of heating current direction and periodic array of extremely straight step bands is formed for the step-down current heating. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Faceting; Gold; Silicon; Surface morphology
1. Introduction Control of surface morphology of a semiconductor surface in nano-scale is one of the important technologies in the semiconductor industry. The most effective way for this aim is to use self-organization process. A vicinal surface of a low index solid surface with regular array of steps can break up into surfaces with different orientation (step bunching or faceting) by self-organization: thermodynamic or kinetic effect. Many researchers have extensively studied step instability on a Si surface including faceting and step bunching induced by thermodynamics or kinetic effect on Si surfaces. Metal adsorption on a Si surface changes the surface free energy and metal adsorption induces step bunching. The metal adsorption induced step bunching is an interesting phenomenon on vicinal surfaces induced by thermodynamic effect. Step instability induced by asymmetric diffusion of adatoms due to a d.c.-heating effect at high temperature is an interesting topic in the kinetic effects induced step instability. Especially, d.c.-heating induced step instability on a Si(1 1 1) vicinal surface is a very attractive topic because of its variety of step *Tel.: q81-3-5734-2481; fax: q81-3-5734-2079. E-mail address: [email protected] (H. Minoda).
configuration depending on the temperature and the heating current direction. Three times of transitions of step configuration between regular step configuration and step bunching exist above the structure phase transition temperature at 830 8C irrespective of the heating current direction and the transition takes place when heating current direction changes w1x. Recently we found a new type of step instability induced by d.c.heating on Si(1 1 1) and Si(0 0 1) vicinal surfaces w2x. That is called in-phase step wandering where surface steps of regular array wander in-phase. We have investigated step instability on a Si surface induced by the d.c.-heating effect and step bunching induced by metal adsorption by reflection electron microscopy (REM) and optical microscopy w2–15x. In the present paper our recent studies on dynamics of step instability induced by metal adsorption and d.c.-heating effect studied by REM were summarized w9,14x. 2. Experimental Observations of the changes of the surface morphology were performed by using an UHV electron microscope equipped with an evaporator for Au. Si samples were cut from wafers of Si(1 1 1) and Si(0 0 1) vicinal
0040-6090/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 9 0 4 - 5
H. Minoda / Thin Solid Films 424 (2003) 40–44
41
Fig. 1. A series of REM images showing changes of the morphology of the Si(0 0 1) 48 off vicinal surface during Au deposition at 870 8C. Numbers at the lower right in each image shows elapsed time from (a) was taken.
surfaces. The w1 1 2¯ x direction of the Si(1 1 1) and the w1 1¯ 0x direction of the Si(0 0 1) samples (the directions perpendicular to the steps) were chosen to be parallel to the longer side of the samples, respectively, and samples were heating by d.c.-current heating. Details of experimental procedures were described in the separated papers w9–11,14x. Observation was performed by REM and REM images are foreshortened by a factor of approximately 1y50 for a Si(1 1 1) surface and 1y40 for a Si(0 0 1) surface along the electron beam direction. Each single height step of the vicinal surface with large off-angle cannot be resolved under the present imaging condition and vicinal surfaces with regular array of steps are seen in homogeneous contrast. 3. Results and discussion 3.1. Au on a Si(0 0 1) vicinal surface Dynamic processes of changes of morphology of a Si(0 0 1) vicinal surface during Au deposition was observed by REM. Fig. 1 shows one of the examples of the changes in the morphology of a 48 off Si(0 0 1) vicinal surface during Au deposition. Electron beam was illuminated in the direction as indicated by an arrow in (a). The images are foreshortened in the electron beam direction as shown in a scale make in (a). The surface in (a) is in the homogeneous contrast due to regular configuration of the surface steps (an R surface) and each step is nearly parallel to the electron beam direction (the w1 1 0x direction). The numbers shown at the lower right in each panel is an elapsed time after (a). A dark area indicated by an arrow in (b) corresponds to a (0 0 1) terrace with the 5=3.2 reconstruction w8x and it grows larger in (c)–(h). The other (0 0 1) terraces seen in dark grow as indicated by arrows in (e) and (g). It
took approximately 200 s for nucleation of a 5=3.2 terrace after the start of the deposition and no morphology change was observed until Fig. 1a was taken. The dark area grows larger with time. The growth perpendicular to the steps is accompanied by step bunching around the (0 0 1) terrace seen in dark. The growth speed is very anisotropic and the growth speed of the (0 0 1) terrace parallel to the steps is very fast. Formation of one-dimensional band structure of the (0 0 1) terrace completes at the beginning of the growth of the terrace in (d), bunching processes (the growth of width W of a (0 0 1) terrace perpendicular to the steps) can be regard as one-dimensional process. The time evolution of the terrace width W was measured and it was fitted by using a function of elapsed time t in a form of WAt a, where an exponent factor a is a fitting parameter. The exponent factor a was evaluated under various deposition rates and the deposition rate dependence of a was not observed. The theoretical study, which discusses time evolution of width of an isolated terrace, was used to analyze these results w16x. The fact that deposition rate dependence of a does not exist shows that the factor a is governed by mass transport kinetics of Si. The mass transport kinetics of Au adsorbate does not affect the factor a. We found that a depends on the substrate temperature and the results are summarized in Fig. 2. Although all the data have experimental error as indicated in error bar in the figures, we can clearly see the temperature dependence of a. a is approximately 1y4 (the local mass exchange case) in the low temperature range at 820 8C and is approximately 1y2 (the non-local mass exchange case) in the high temperature range above 870 8C. It increases from 1y4 to 1y2 with increasing substrate temperature between these two temperature ranges. Consequently, we can say that mass transport kinetics
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3.2. Au on a Si(1 1 1) vicinal surface
Fig. 2. Plots showing temperature dependence of the exponential factor a. a depends on the substrate temperature is approximately 1y4 in the low temperature range and is 1y2 in the high temperature range showing that mass transport of Si adatoms depends on the temperature.
depends on the substrate temperature and the local mass exchange below 820 8C gradually change to the nonlocal mass exchange above 870 8C. This temperature dependence suggests the change of permeability of steps for Si adatoms and steps are impermeable in lower temperature region and are permeable in higher temperature region.
Fig. 3 reproduces REM images showing changes of morphologies of a 58 off Si(1 1 1) vicinal surface inclined toward w1 1 2¯ x direction. Fig. 3a was taken after step bunching by Au adsorption at 860 8C under stepup current heating. The electron beam direction was shown in a white arrow in (a). Bright and dark areas are step free and step band areas, respectively. Adsorption rate of Au was approximately 0.1 MLymin and step bunching started when approximately 0.3 ML of Au (a critical coverage) was adsorbed. When heating current direction was changed to the step-down direction surface morphology changes drastically as in (b). Wide step bands seen in (a) separated into narrower ones in (b) and the narrower step bands arrange regularly as in (c). Finally, the periodic array of step bands (periodic step density wave (PSDW)), which has not been observed before, was formed as in (d) and this can be said to be a new type of step instability. The PSDW pattern changes to the usual bunching pattern when the heating current direction changes to the stepup direction. The terrace width seen in bright and step bands seen in dark bands grow wider as in (e) and step bunching proceeds, and a coarsening of the step bands takes place and number of step bands decreases as in (f). Such a transition between usual bunching pattern and the PSDW pattern is reversible for a reverse of heating current direction. The transition between the usual step
Fig. 3. A series of REM images showing changes of morphologies of 58 off vicinal Si(1 1 1) surfaces at 860 8C. Heating current direction in (a), (e) and (f) was in the step-up direction and that in (b)–(d) was in the step-down direction. Deposition rate of Au was approximately 0.1 mlymin and step bunching started when approximately 0.3 ML of Au was deposited.
H. Minoda / Thin Solid Films 424 (2003) 40–44
43
Fig. 4. Time evolutions of the terrace width during Au deposition under step-up and step-down current heating conditions. Solid circles and open circles correspond to the case of step-down and step-up current heating, respectively.
bunching state and the PSDW state is caused by a change of heating current direction. Although steps were bunching, PSDW pattern was not observed when sample was heated by a.c.-current showing that step bunching is due to thermodynamic effect but electromigration of Si adatoms. Similar PSDW patterns were observed on the 1, 2, 5 and 78 off vicinal surfaces inclined toward the w1 1 2¯ x direction under step-down current heating condition, however, they were not observed on the 18 off vicinal surface inclined toward the w1¯ 1¯ 2x direction. Only the usual bunching pattern was observed on the vicinal surface inclined toward the direction irrespective of the heating current direction. This suggests that formation of the PSDW pattern is associated with step structure. The period of the PSDW pattern on the vicinal surface inclined toward the w1 1 2¯ x direction systematically depends on the off-angle and the substrate temperature, and the period reduces to 50 nm or less. Thus, these periodic patterns with small period could be used for a substrate for quantum wires or quantum dots. Details of the off-angle and temperature dependence of PSDW pattern will be described in elsewhere w15x. Time evolution of the (1 1 1) terrace width (W) between adjacent step bands along the w1 1 2¯ x direction was measured for both heating current directions and the results are presented in Fig. 4. One set of the plots presented by open circles is in the case of step bunching under the step-up heating condition, and the other set of the plots presented by solid circles is in the case of PSDW formation under the step-down heating condition. In both cases time evolutions of W were measured during Au deposition on the 58 off surface at 860 8C. In the former case W grows as t 1y2 and in the latter case it grows as t 1y4 as seen in Fig. 4. These time evolutions do not depend on deposition rate of Au, showing that
Fig. 5. A series of REM images showing transition of the surface morphology of a Si(1 1 1) vicinal surface with small off-angle during Au deposition under the step-down current heating.
an adsorption process of Au or mass transport kinetics of Au adatoms is not the rate determining process of step bunching. These time evolutions do not depend on either the substrate temperature or the off-angle of the vicinal surface. In contrast to the previous case of Au on a Si(0 0 1) vicinal surface, many facets are formed simultaneously in the present system. The theoretical study that discusses time evolution of characteristic size of the system in spinodal decomposition w17x, was used for analysis of the present case. The conclusion is the same as the case of the previous system and the power law factor 1y4 shows the conserved mass transport case and 1y2 shows the non-conserved mass transport case. Mass transport of Si adatoms is conserved or steps are impermeable for Si adatoms in the case of PSDW, and the mass transport of Si adatoms is non-conserved or steps are permeable for Si adatoms in the case of the usual step bunching. The permeability of steps for Si
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H. Minoda / Thin Solid Films 424 (2003) 40–44
adatoms depends on the heating current direction and steps are permeable for step-up current heating and are impermeable for step-down current heating. The reason for the change in the permeability of steps for the heating current direction is responsible for the change of the step structure depending on the heating current direction. The detailed discussion about the change in step structure depending on the heating current direction was described in the separated paper w14x. Transition of the surface morphology during Au deposition under step-down current heating was observed when a Si(1 1 1) vicinal surface with small off-angle was used. Fig. 5 show an example of the transition. A surface steps up to the bottom of the images and heating current direction is in the step-down direction as in (b). In (a) on a clean Si(1 1 1) vicinal surface steps are already bunched by electromigration of Si adatoms as in the broad bands in (a). The bunched steps separated into each step by Au deposition as in (b) and the steps are bunched again by further Au deposition as in (c). Thus, two times of the transitions of the surface morphology take place. The former transition from the bunching state into the R state was found previously and the reason of the transition was considered to be the change of the effective charge of Si adatoms at that time w13x. However, recent theoretical study showed that transition is induced by change of permeability of steps w18–20x and our recent study concludes that the former transition is due to the change of permeability of the steps for Si adatoms from the impermeable steps into the permeable steps w14x. Step bunching seen in (c) was due to the change of the surface free energy of the Si(1 1 1) surface and this step bunching is caused by thermodynamics as seen in Fig. 3. The mean step-step distance is too large and PSDW was not observed on the Si(1 1 1) vicinal surface with small off-angle. 4. Summary Recent REM studies of step instability on a Si(0 0 1) vicinal surface and that on a Si(1 1 1) vicinal surface induced by Au adsorption were presented. On a Si(0 0 1) vicinal surface faceting of (0 0 1) surface due to surface reconstruction takes place and steps are bunching by metal adsorption. The kinetics of mass transport of Si depends on the substrate temperature and steps are impermeable for Si adatoms at lower temperature and they change into the permeable ones at higher temper-
ature. On the Si(1 1 1) vicinal surface instability of steps depends on the heating current direction. Steps are permeable for step-up current heating and they are impermeable for step-down current heating. In addition to the permeability of steps pattern after bunching also depends on the heating current direction. Above a critical coverage of Au (f0.3 ML) steps are bunching irrespective of heating current direction and periodic array of extremely straight step bands are formed under the step-down current direction. Acknowledgments The author greatly thanks Prof. Katsumichi Yagi for the fruitful discussion. This work was supported by Grant-in-Aid of Ministry of Education, Science and Culture of Japan (No. 09NP1201). References w1x A.V. Latyshev, A.L. Aseev, A.B. Krasilnikov, S.I. Stenin, Surf. Sci. 213 (1989) 157. w2x M. Degawa, H. Nishimura, Y. Tanishiro, H. Minoda, K. Yagi, Jpn. J. Appl. Phys. 38 (1999) L308. w3x M. Degawa, H. Minoda, Y. Tanishiro, K. Yagi, J. Phys. Soc. Jpn 70 (2001) 1026. w4x M. Degawa, H. Minoda, Y. Tanishiro, K. Yagi, J. Phys.: Condens. Matter 11 (1999) L551. w5x H. Nishimura, H. Minoda, Y. Tanishiro, K. Yagi, Surf. Sci. 442 (1999) L1006. w6x M. Degawa, H. Minoda, Y. Tanishiro, K. Yagi, Phys. Rev. B63 (2001) 045309-1-8. w7x H. Minoda, I. Morishima, M. Degawa, Y. Tanishiro, K. Yagi, Surf. Sci. 493 (2001) 487. w8x H. Minoda, K. Yagi, F.-J. Meyer zu Heringdorf, A. Meire, D. Koeler, M. Horn von Hoegen, Phys. Rev. B59 (1999) 2363. w9x H. Minoda, K. Yagi, Phys. Rev. B60 (1999) 2715. w10x H. Minoda, T. Shimakura, K. Yagi, F.-J. Meyer zu Heringdorf, A. Meire, D. Koeler, M. Horn von Hoegen, Phys. Rev. B61 (2000) 5672. w11x H. Minoda, K. Yagi, Surf. Sci. 437 (1999) L761. w12x H. Minoda, Y. Takahashi, Y. Tanishiro, K. Yagi, Surf. Sci. 438 (1999) 68. w13x A.V. Latyshev, H. Minoda, Y. Tanishiro, K. Yagi, Appl. Surf. Sci. 130–132 (1998) 60. w14x H. Minoda, Phys. Rev. B64 (2001) 233305-1-4. w15x H. Minoda, unpublished. w16x W.W. Mullin, Philos. Mag. 6 (1961) 1313. w17x A.J. Bray, Physica A194 (1993) 41. w18x S. Stoyanov, V. Tonchev, Phys. Rev. B58 (1998) 1590. w19x M. Sato, M. Uwaha, Y. Saito, Phys. Rev. B62 (2000) 8452. w20x N. Suga, J. Kimbara, N.-J. Wu, H. Yasunaga, A. Natori, Jpn. J. Appl. Phys. 39 (2000) 4412.
Study of step instability on Si surfaces Hiroki Minoda* Physics Department, Tokyo Institute of Technology, Oh-okayama, Meguro, Tokyo 152-8551, Japan Received 19 July 2002; received in revised form 10 September 2002; accepted 10 September 2002
Abstract Step instabilities on Si(1 1 1) vicinal surface and Si(1 1 1) vicinal surface induced by Au adsorption were observed by reflection electron microscopy. On the Si(0 0 1) vicinal surface faceting of (0 0 1) surface due to surface reconstruction takes place and surface steps are bunching. The kinetics of mass transport of Si depends on the substrate temperature and steps are impermeable for Si adatoms at lower temperature and they are permeable at higher temperature. On the Si(1 1 1) vicinal surface the permeability of steps depends on the heating current direction. The steps are permeable for step-up current heating and are impermeable for step-down current heating. Above a critical coverage of Au (f0.3 ML) steps are bunching irrespective of heating current direction and periodic array of extremely straight step bands is formed for the step-down current heating. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electron microscopy; Faceting; Gold; Silicon; Surface morphology
1. Introduction Control of surface morphology of a semiconductor surface in nano-scale is one of the important technologies in the semiconductor industry. The most effective way for this aim is to use self-organization process. A vicinal surface of a low index solid surface with regular array of steps can break up into surfaces with different orientation (step bunching or faceting) by self-organization: thermodynamic or kinetic effect. Many researchers have extensively studied step instability on a Si surface including faceting and step bunching induced by thermodynamics or kinetic effect on Si surfaces. Metal adsorption on a Si surface changes the surface free energy and metal adsorption induces step bunching. The metal adsorption induced step bunching is an interesting phenomenon on vicinal surfaces induced by thermodynamic effect. Step instability induced by asymmetric diffusion of adatoms due to a d.c.-heating effect at high temperature is an interesting topic in the kinetic effects induced step instability. Especially, d.c.-heating induced step instability on a Si(1 1 1) vicinal surface is a very attractive topic because of its variety of step *Tel.: q81-3-5734-2481; fax: q81-3-5734-2079. E-mail address: [email protected] (H. Minoda).
configuration depending on the temperature and the heating current direction. Three times of transitions of step configuration between regular step configuration and step bunching exist above the structure phase transition temperature at 830 8C irrespective of the heating current direction and the transition takes place when heating current direction changes w1x. Recently we found a new type of step instability induced by d.c.heating on Si(1 1 1) and Si(0 0 1) vicinal surfaces w2x. That is called in-phase step wandering where surface steps of regular array wander in-phase. We have investigated step instability on a Si surface induced by the d.c.-heating effect and step bunching induced by metal adsorption by reflection electron microscopy (REM) and optical microscopy w2–15x. In the present paper our recent studies on dynamics of step instability induced by metal adsorption and d.c.-heating effect studied by REM were summarized w9,14x. 2. Experimental Observations of the changes of the surface morphology were performed by using an UHV electron microscope equipped with an evaporator for Au. Si samples were cut from wafers of Si(1 1 1) and Si(0 0 1) vicinal
0040-6090/03/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 9 0 4 - 5
H. Minoda / Thin Solid Films 424 (2003) 40–44
41
Fig. 1. A series of REM images showing changes of the morphology of the Si(0 0 1) 48 off vicinal surface during Au deposition at 870 8C. Numbers at the lower right in each image shows elapsed time from (a) was taken.
surfaces. The w1 1 2¯ x direction of the Si(1 1 1) and the w1 1¯ 0x direction of the Si(0 0 1) samples (the directions perpendicular to the steps) were chosen to be parallel to the longer side of the samples, respectively, and samples were heating by d.c.-current heating. Details of experimental procedures were described in the separated papers w9–11,14x. Observation was performed by REM and REM images are foreshortened by a factor of approximately 1y50 for a Si(1 1 1) surface and 1y40 for a Si(0 0 1) surface along the electron beam direction. Each single height step of the vicinal surface with large off-angle cannot be resolved under the present imaging condition and vicinal surfaces with regular array of steps are seen in homogeneous contrast. 3. Results and discussion 3.1. Au on a Si(0 0 1) vicinal surface Dynamic processes of changes of morphology of a Si(0 0 1) vicinal surface during Au deposition was observed by REM. Fig. 1 shows one of the examples of the changes in the morphology of a 48 off Si(0 0 1) vicinal surface during Au deposition. Electron beam was illuminated in the direction as indicated by an arrow in (a). The images are foreshortened in the electron beam direction as shown in a scale make in (a). The surface in (a) is in the homogeneous contrast due to regular configuration of the surface steps (an R surface) and each step is nearly parallel to the electron beam direction (the w1 1 0x direction). The numbers shown at the lower right in each panel is an elapsed time after (a). A dark area indicated by an arrow in (b) corresponds to a (0 0 1) terrace with the 5=3.2 reconstruction w8x and it grows larger in (c)–(h). The other (0 0 1) terraces seen in dark grow as indicated by arrows in (e) and (g). It
took approximately 200 s for nucleation of a 5=3.2 terrace after the start of the deposition and no morphology change was observed until Fig. 1a was taken. The dark area grows larger with time. The growth perpendicular to the steps is accompanied by step bunching around the (0 0 1) terrace seen in dark. The growth speed is very anisotropic and the growth speed of the (0 0 1) terrace parallel to the steps is very fast. Formation of one-dimensional band structure of the (0 0 1) terrace completes at the beginning of the growth of the terrace in (d), bunching processes (the growth of width W of a (0 0 1) terrace perpendicular to the steps) can be regard as one-dimensional process. The time evolution of the terrace width W was measured and it was fitted by using a function of elapsed time t in a form of WAt a, where an exponent factor a is a fitting parameter. The exponent factor a was evaluated under various deposition rates and the deposition rate dependence of a was not observed. The theoretical study, which discusses time evolution of width of an isolated terrace, was used to analyze these results w16x. The fact that deposition rate dependence of a does not exist shows that the factor a is governed by mass transport kinetics of Si. The mass transport kinetics of Au adsorbate does not affect the factor a. We found that a depends on the substrate temperature and the results are summarized in Fig. 2. Although all the data have experimental error as indicated in error bar in the figures, we can clearly see the temperature dependence of a. a is approximately 1y4 (the local mass exchange case) in the low temperature range at 820 8C and is approximately 1y2 (the non-local mass exchange case) in the high temperature range above 870 8C. It increases from 1y4 to 1y2 with increasing substrate temperature between these two temperature ranges. Consequently, we can say that mass transport kinetics
42
H. Minoda / Thin Solid Films 424 (2003) 40–44
3.2. Au on a Si(1 1 1) vicinal surface
Fig. 2. Plots showing temperature dependence of the exponential factor a. a depends on the substrate temperature is approximately 1y4 in the low temperature range and is 1y2 in the high temperature range showing that mass transport of Si adatoms depends on the temperature.
depends on the substrate temperature and the local mass exchange below 820 8C gradually change to the nonlocal mass exchange above 870 8C. This temperature dependence suggests the change of permeability of steps for Si adatoms and steps are impermeable in lower temperature region and are permeable in higher temperature region.
Fig. 3 reproduces REM images showing changes of morphologies of a 58 off Si(1 1 1) vicinal surface inclined toward w1 1 2¯ x direction. Fig. 3a was taken after step bunching by Au adsorption at 860 8C under stepup current heating. The electron beam direction was shown in a white arrow in (a). Bright and dark areas are step free and step band areas, respectively. Adsorption rate of Au was approximately 0.1 MLymin and step bunching started when approximately 0.3 ML of Au (a critical coverage) was adsorbed. When heating current direction was changed to the step-down direction surface morphology changes drastically as in (b). Wide step bands seen in (a) separated into narrower ones in (b) and the narrower step bands arrange regularly as in (c). Finally, the periodic array of step bands (periodic step density wave (PSDW)), which has not been observed before, was formed as in (d) and this can be said to be a new type of step instability. The PSDW pattern changes to the usual bunching pattern when the heating current direction changes to the stepup direction. The terrace width seen in bright and step bands seen in dark bands grow wider as in (e) and step bunching proceeds, and a coarsening of the step bands takes place and number of step bands decreases as in (f). Such a transition between usual bunching pattern and the PSDW pattern is reversible for a reverse of heating current direction. The transition between the usual step
Fig. 3. A series of REM images showing changes of morphologies of 58 off vicinal Si(1 1 1) surfaces at 860 8C. Heating current direction in (a), (e) and (f) was in the step-up direction and that in (b)–(d) was in the step-down direction. Deposition rate of Au was approximately 0.1 mlymin and step bunching started when approximately 0.3 ML of Au was deposited.
H. Minoda / Thin Solid Films 424 (2003) 40–44
43
Fig. 4. Time evolutions of the terrace width during Au deposition under step-up and step-down current heating conditions. Solid circles and open circles correspond to the case of step-down and step-up current heating, respectively.
bunching state and the PSDW state is caused by a change of heating current direction. Although steps were bunching, PSDW pattern was not observed when sample was heated by a.c.-current showing that step bunching is due to thermodynamic effect but electromigration of Si adatoms. Similar PSDW patterns were observed on the 1, 2, 5 and 78 off vicinal surfaces inclined toward the w1 1 2¯ x direction under step-down current heating condition, however, they were not observed on the 18 off vicinal surface inclined toward the w1¯ 1¯ 2x direction. Only the usual bunching pattern was observed on the vicinal surface inclined toward the direction irrespective of the heating current direction. This suggests that formation of the PSDW pattern is associated with step structure. The period of the PSDW pattern on the vicinal surface inclined toward the w1 1 2¯ x direction systematically depends on the off-angle and the substrate temperature, and the period reduces to 50 nm or less. Thus, these periodic patterns with small period could be used for a substrate for quantum wires or quantum dots. Details of the off-angle and temperature dependence of PSDW pattern will be described in elsewhere w15x. Time evolution of the (1 1 1) terrace width (W) between adjacent step bands along the w1 1 2¯ x direction was measured for both heating current directions and the results are presented in Fig. 4. One set of the plots presented by open circles is in the case of step bunching under the step-up heating condition, and the other set of the plots presented by solid circles is in the case of PSDW formation under the step-down heating condition. In both cases time evolutions of W were measured during Au deposition on the 58 off surface at 860 8C. In the former case W grows as t 1y2 and in the latter case it grows as t 1y4 as seen in Fig. 4. These time evolutions do not depend on deposition rate of Au, showing that
Fig. 5. A series of REM images showing transition of the surface morphology of a Si(1 1 1) vicinal surface with small off-angle during Au deposition under the step-down current heating.
an adsorption process of Au or mass transport kinetics of Au adatoms is not the rate determining process of step bunching. These time evolutions do not depend on either the substrate temperature or the off-angle of the vicinal surface. In contrast to the previous case of Au on a Si(0 0 1) vicinal surface, many facets are formed simultaneously in the present system. The theoretical study that discusses time evolution of characteristic size of the system in spinodal decomposition w17x, was used for analysis of the present case. The conclusion is the same as the case of the previous system and the power law factor 1y4 shows the conserved mass transport case and 1y2 shows the non-conserved mass transport case. Mass transport of Si adatoms is conserved or steps are impermeable for Si adatoms in the case of PSDW, and the mass transport of Si adatoms is non-conserved or steps are permeable for Si adatoms in the case of the usual step bunching. The permeability of steps for Si
44
H. Minoda / Thin Solid Films 424 (2003) 40–44
adatoms depends on the heating current direction and steps are permeable for step-up current heating and are impermeable for step-down current heating. The reason for the change in the permeability of steps for the heating current direction is responsible for the change of the step structure depending on the heating current direction. The detailed discussion about the change in step structure depending on the heating current direction was described in the separated paper w14x. Transition of the surface morphology during Au deposition under step-down current heating was observed when a Si(1 1 1) vicinal surface with small off-angle was used. Fig. 5 show an example of the transition. A surface steps up to the bottom of the images and heating current direction is in the step-down direction as in (b). In (a) on a clean Si(1 1 1) vicinal surface steps are already bunched by electromigration of Si adatoms as in the broad bands in (a). The bunched steps separated into each step by Au deposition as in (b) and the steps are bunched again by further Au deposition as in (c). Thus, two times of the transitions of the surface morphology take place. The former transition from the bunching state into the R state was found previously and the reason of the transition was considered to be the change of the effective charge of Si adatoms at that time w13x. However, recent theoretical study showed that transition is induced by change of permeability of steps w18–20x and our recent study concludes that the former transition is due to the change of permeability of the steps for Si adatoms from the impermeable steps into the permeable steps w14x. Step bunching seen in (c) was due to the change of the surface free energy of the Si(1 1 1) surface and this step bunching is caused by thermodynamics as seen in Fig. 3. The mean step-step distance is too large and PSDW was not observed on the Si(1 1 1) vicinal surface with small off-angle. 4. Summary Recent REM studies of step instability on a Si(0 0 1) vicinal surface and that on a Si(1 1 1) vicinal surface induced by Au adsorption were presented. On a Si(0 0 1) vicinal surface faceting of (0 0 1) surface due to surface reconstruction takes place and steps are bunching by metal adsorption. The kinetics of mass transport of Si depends on the substrate temperature and steps are impermeable for Si adatoms at lower temperature and they change into the permeable ones at higher temper-
ature. On the Si(1 1 1) vicinal surface instability of steps depends on the heating current direction. Steps are permeable for step-up current heating and they are impermeable for step-down current heating. In addition to the permeability of steps pattern after bunching also depends on the heating current direction. Above a critical coverage of Au (f0.3 ML) steps are bunching irrespective of heating current direction and periodic array of extremely straight step bands are formed under the step-down current direction. Acknowledgments The author greatly thanks Prof. Katsumichi Yagi for the fruitful discussion. This work was supported by Grant-in-Aid of Ministry of Education, Science and Culture of Japan (No. 09NP1201). References w1x A.V. Latyshev, A.L. Aseev, A.B. Krasilnikov, S.I. Stenin, Surf. Sci. 213 (1989) 157. w2x M. Degawa, H. Nishimura, Y. Tanishiro, H. Minoda, K. Yagi, Jpn. J. Appl. Phys. 38 (1999) L308. w3x M. Degawa, H. Minoda, Y. Tanishiro, K. Yagi, J. Phys. Soc. Jpn 70 (2001) 1026. w4x M. Degawa, H. Minoda, Y. Tanishiro, K. Yagi, J. Phys.: Condens. Matter 11 (1999) L551. w5x H. Nishimura, H. Minoda, Y. Tanishiro, K. Yagi, Surf. Sci. 442 (1999) L1006. w6x M. Degawa, H. Minoda, Y. Tanishiro, K. Yagi, Phys. Rev. B63 (2001) 045309-1-8. w7x H. Minoda, I. Morishima, M. Degawa, Y. Tanishiro, K. Yagi, Surf. Sci. 493 (2001) 487. w8x H. Minoda, K. Yagi, F.-J. Meyer zu Heringdorf, A. Meire, D. Koeler, M. Horn von Hoegen, Phys. Rev. B59 (1999) 2363. w9x H. Minoda, K. Yagi, Phys. Rev. B60 (1999) 2715. w10x H. Minoda, T. Shimakura, K. Yagi, F.-J. Meyer zu Heringdorf, A. Meire, D. Koeler, M. Horn von Hoegen, Phys. Rev. B61 (2000) 5672. w11x H. Minoda, K. Yagi, Surf. Sci. 437 (1999) L761. w12x H. Minoda, Y. Takahashi, Y. Tanishiro, K. Yagi, Surf. Sci. 438 (1999) 68. w13x A.V. Latyshev, H. Minoda, Y. Tanishiro, K. Yagi, Appl. Surf. Sci. 130–132 (1998) 60. w14x H. Minoda, Phys. Rev. B64 (2001) 233305-1-4. w15x H. Minoda, unpublished. w16x W.W. Mullin, Philos. Mag. 6 (1961) 1313. w17x A.J. Bray, Physica A194 (1993) 41. w18x S. Stoyanov, V. Tonchev, Phys. Rev. B58 (1998) 1590. w19x M. Sato, M. Uwaha, Y. Saito, Phys. Rev. B62 (2000) 8452. w20x N. Suga, J. Kimbara, N.-J. Wu, H. Yasunaga, A. Natori, Jpn. J. Appl. Phys. 39 (2000) 4412.