Evolution of surface structures on Si(001) during hydrogen desorption

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Applied Surface Science 121/122

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Evolution of surface structures on Si(OO1)during hydrogen desorption Masamichi Yoshimura Toyota Technological

*, Kazuyuki Ueda

Institute, 2-12-l Hisakata, Tempaku-ku, Nagoya 4661,Japan

Received 5 November

1996; accepted 25 February

1997

Abstract We have examined atomistic processes of hydrogen desorption from the H/Si(OOl) surface by scanning tunneling microscopy @TM). At a low hydrogen dose, Si atoms are etched from the surface and the patched pattern of 2 X 1 and 1 X 2, where each step edge penetrates into neighboring steps, is observed. On the other hand, a quasi-stable c(4 X 4) surface reconstruction is observed for high doses of hydrogen before the complete recovery of the 2 X 1 structure. It is found that the formation of the c(4 X 4) structure needs a rather large density of vacancies and that the domains are pinned at the clusters and disordered areas on the surface. 0 1997 Elsevier Science B.V. PACS: 68.35.B~: 68.45.Da Keywords:

Scanning

tunneling

microscopy;

Etching; Silicon: Hydrogen:

1. Introduction The H/%(001) system has recently attracted considerable attention from various technological fields because hydrogen passivates the surface silicon bonds and alters the surface properties dramatically. For example, the wet process using HF solutions is very useful in Si technology in order to make the surface atomically flat and inert to the atmosphere [ll. The dry process using atomic hydrogen is used for the control of epitaxial growth, where the high-quality films can be grown on the hydrogen-terminated sur-

* Corresponding author. Tel.: +81-52-8091851; 8091853; e-mail: [email protected].

fax: +81-52.

0169.4332/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOl69-4332(97)00283-3

Surface defects; Surface diffusion

face. The thermal stability of the system is very important for technological applications, and a lot of work is reported on the thermal processes of the H/Si(OOl) surface. Gates et al. reported that SiH, desorbed at 375°C and higher silicon hydrides desorbed at 425°C from a H/Si(OOl) surface [2]. Thus, thermal annealing promotes the etching of Si atoms from the surface. It is noted that, at these temperature ranges, recovery processes of the Si surface simultaneously proceed. Kato et al. found that when the initial hydrogen coverage was too high to show the roughened 1 X 1 structure, then a new surface structure, a c(4 X 4) reconstruction, was observed after annealing around 600°C [3]. They suggested that the phase was a defect-related superstructure because hydrogen desorbed completely from the sur-

M. Yoshimura, K. Ueda /Applied Surface Science 121/122 (1997) 179-182

180

face at this temperature, which was supported by the STM studies [4-6]. However, the formation processes of the c(4 X 4) have not been clarified yet. In this paper, we pay attention to the desorption and recovery processes of the H/Si(001) by annealing. The evolution of the surface structure is examined by scanning tunneling microscopy (STM). The hydrogen dose dependency of the recovery process is presented.

3. Results

All experiments were performed in an UHV chamber with a base pressure of 5 X 10- ~ Torr. The STM used in this study is a commercial one from Unisoku. The sample is cut from an n-type Si(001) wafer of 0.008-0.02 f~. cm and mounted on the sample holder. It is degassed at 600°C overnight and finally flashed at 1150°C for several seconds in the low 10 - l ° Ton" range in order to get a clean surface. Atomic hydrogen was produced by the decomposition of molecular hydrogen with a 1500°C tungsten filament. The exposure is expressed in terms of the pressure of molecular hydrogen measured by an ion gauge (1 L = 1 X 10-6 Torr. s). The tungsten tip was electrochemically etched followed by electron beam heating in the UHV chamber before measurement. STM observations are performed at room temperature in a constant height mode.

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Fig. l(a) shows a STM current image of the Si(001) surface after 100 L exposure of hydrogen at room temperature. The dimer rows on the terraces and steps are clearly observed and small bright dots are considered to be etch products, Sill x species. When the surface is annealed at 630°C for 2 rain, the surface shows a patched pattern where each step seems to penetrate into the adjacent steps, as shown in Fig. l(b). This is due to etching of Si atoms from the surface by desorption of silicon hydrides. It is noted that 2 X 1 reconstruction covers the whole patched surface. Additionally 5 rain annealing promotes the recovery of the surface structure and the normal step and terrace structure is observed, as shown in Fig. l(c). The change in the surface structure observed here is similar to the result reported by Boland [7]. Fig. 2(a) shows a STM current image of Si(001) after 450 L exposure at the substrate temperature of 100°C. The 2 × 1 dimer structure is no longer observed on the surface. The etching reaction already occurs here and there on the terrace, the region of which is imaged as a flat hole, suggesting that the substrate temperature is somewhat raised by heat of the tungsten filament located near the sample. Etched holes are more clearly observed after annealing at 270°C for 5 min (Fig. 2(b)). The tip is relatively unstable during the measurement and it is rather

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Fig. 1. STM images of the Si(001) surface after 100 L exposure of hydrogen: (a) as exposed surface (V, = 2.3 v , 1 = 0.25 nA, 60 nm X 60 nm), (b) after annealing at 630°C for 2 min ( ~ = 1.8 V, 1 = 0.25 nA, 60 nm X 60 nm), (c) after annealing at 630°C for 7 min (V~ = - 2 . 0 V, 1 = 0.4 nA, 80 nm X 80 nm).

M. Yoshimura, K. Ueda / Applied Sur]ace Science 121 / 122 (1997) 179 182 difficult

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"2~ Fig. 2. STM images of the Si(001) surface after 450 L exposure at 100°C: (a) as exposed surface (V, = 2.5 V, I = 0.46 nA, 60 nm X 60 nm), (b) after annealing at 270°C for 5 min (V~ = 2.5 v , 1 = 0.21 hA, 40 nm X 40 rim), (c) after annealing at 350°C for 2 min (V, = 2.3 V, 1 = 0.22 nA, 60 nm X 60 nm), (d) after annealing at 430°C for 5 min (V, = 2.3 v, 1 = 0.21 nA, 44 nm X 44 nm), (e) after annealing at 650°C for 2 rain (V~ = - 2 . 4 1 V, 1 = 0.22 hA, 60 nm X 60 rim), (f) after annealing at 680°C for 10 rain (V, = 1.22 v , 1 = 0.21 hA, 80 nm × 80 nm).

182

M. Yoshimura, K. Ueda /Applied Surface Science 121 / 122 (1997) 179-182

After annealing at 430°C for 5 rain, the surface becomes more roughened, but a somewhat regular arrangement of small holes and clusters is visible (Fig. 2(d)). After annealing at 650°C for 5 min, the 2 X l-like structures with flat terraces are observed between the clusters (Fig. 2(e)). This is in contrast to the low exposure case of Fig. l(b, c) where the surface is completely recovered. After annealing at 680°C for 10 min, the flat terraces are spread with decrease in the density of clusters, indicative of the dissociation of clusters (Fig. 2(0). The atomic configuration is revealed to be different from the 2 × 1 dimer structure but shows the c(4 X 4) superstructure, as previously reported [4-6]. The c(4 X 4) domains consist of periodically arranged vacancies and are pinned by disorder areas or remained clusters. The c(4 × 4) domains disappear after annealing above 700°C and the 2 X 1 clean surface reappears. On the basis of the above observations, it is clear that the formation of the c(4 X 4) structure is driven by the surface vacancies produced by the etching reaction. A schematic of the desorption processes is shown in Fig. 3. When the initial hydrogen dose is low, the density of the vacancies is small. In this case, the vacancies can be easily wiped away at the step edges, resulting in the complicated patched pattern observed in Fig. l(b). On the other hand, with high doses of hydrogen, many vacancies remain on the surface after desorption. In addition, small clusters exist here and there on the surface. The clusters Low dose

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observed at 350-430°C are probably a mixture of Si clusters and the etched products, and those observed at 650°C are Si clusters [2]. In this case, the vacancies are difficult to move toward the step edges, in place, they are pinned by the clusters and built a stable structure in the limited region, i.e., a c(4 x 4) structure. We note that the formation of c(4 X 4) at 650-800°C was also reported during the processes of solid phase epitaxy (SPE) [8] or Si homoepitaxial growth [9,10]. These are also non-equilibrium, kinetic processes.

4. Conclusion We demonstrate a STM study of hydrogen desorption processes from the H/Si(001) surface by annealing. For small doses of hydrogen (100 L), the patched pattern is observed as a result of the migration of vacancies toward the step edges. On the other hand, a quasi-stable c(4 X 4) structure is observed for a large hydrogen dose (450 L). The domain of c(4 X 4) is not so large and surrounded by the Si clusters and disordered areas. It is found that the c(4 X 4) structure is formed during the recovery processes of the highly damaged surface including holes and hillocks.

References [1] P. Dumas, Y.T. Chabal, P. Jakob, Surf. Sci. 269/270 (1992) 867. [2] S.M. Gates. R.R. Kunz, C.M. Greenlief, Surf. Sci. 207 (1989) 364. [3] K. Kato, T. lde, T. Nishimori, T. Ichinokawa, Surf. Sci. 207 (1988) 177. [4] R.I.G. Uhrberg, J.E. Northrup, D.K. Biegelsen, R.D. Bringans, L.-E. Swartz, Phys. Rev. B 46 (1992) 10251. [5] T. Ide, T. Mizutani, Phys. Rev. B 45 (1992) 1447. [6] P. Moriarty, L. Koenders, G. Hughes, Phys. Rev. B 47 (1993) 15950. [7] J.J. Boland, Surf. Sci. 261 (1992) 17. [8] K. Uesugi, T. Yao, T. Sato, T. Sueyoshi, M. Iwatsuki, Appl. Phys. Lett. 62 (1993) 1600. [9] R.N. Thomas, M.H. Francombe, Appl. Phys. Lett. 11 (1967) 108. [10] T. Sakamoto, T. Takahashi, E. Suzuki, A. Shoji, H. Kawanami, Y. Komiya, Y. Tarui, Surf. Sci. 86 (1979) 102.