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Structural analysis of Si(111) surfaces during homoepitaxial growth
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Surface Science North-Holland
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surface science
298 (1993) 284-292
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Structural A. Ichimiya
analysis of Si( 111) surfaces during homoepitaxial “, H. Nakahara
‘, T. Hashizume
’ and
growth
T. Sakurai
” Department of Applied Physics, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan h Institute of Materials Research, Tohoku UniL~ersity,Katahira, Aoba-ky Sendai 980, Japan Received
11 July 1993; accepted
for publication
25 August
1993
Atomic structures of SK1111 surfaces during silicon growth are investigated by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). For silicon deposition on the substrate at room temperature, it is concluded that backbonds of adatoms of Takayanagi’s dimer-adatom-stacking-fault (DAS) structure are broken at an initial stage of the deposition. The adsorbed atoms are nucleated randomly on the surface, and coalescence of the growth nuclei occurs with few nuclei crossing the dimer rows of the DAS structure. For further deposition, the dimers and the stacking faults remain at the interface between the substrate and the growth amorphous layer. At the substrate temperature of about 3OO”C, rocking curves of RHEED intensities from the surface during growth are very different from the curve before growth. The structure of the growing surface layer is determined as a pyramidal cluster-type structure by analysis with RHEED dynamical calculations. At this temperature, characteristic features of a cluster structure are found on the surface at an initial stage of the growth. Since crystal nuclei smaller than half the cluster segment are scarcely observed in the STM images, it is concluded that the cluster is the smallest unit of nucleation of homoepitaxial growth. At substrate temperatures from 400 to 600°C a mixed phase of 5 X 5 and 7 X 7 structures, which is the DAS structure, is observed by RHEED and STM. It is discussed that the formation of metastable structures, such as the pyramidal cluster-type structure and the 5 X 5 one, promotes successive epitaxial growth accompanied with stacking-fault dissolution at the dimer-stacking-fault framework
1. Introduction Since fast electrons are scattered dominantly in a forward direction by atoms, dynamic diffraction of high-energy electrons in a crystal mainly occurs in a forward direction. This means that the fast electrons interact approximately with a projected crystal potential which is an averaged crystal potential in the direction of the’incident electron beam. Using this feature of high-energy electrons, reflection high-energy electron diffraction (RHEED) intensities are calculated by a dynamical theory with diffracted beams in a zeroth Laue zone [1], because the Fourier coefficients of the crystal potential in the zeroth Laue zone are the same as those of the projected one in the direction of the incident beam. When a crystal orientation for the incident beam is chosen at several degrees off from a certain crystal axis, the pro0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
jected potential is approximately one-dimensional in a direction perpendicular to the surface. At this condition, electrons are diffracted mainly by lattice planes parallel to the surface. This diffraction condition is named the one-beam condition [2,3], because the main diffraction beam is simply the specular one. A rocking curve at this condition is a function of surface normal components of atomic positions, but hardly depends on lateral components of them. Surface normal components of atomic positions of surface layers are determined by dynamical calculation analysis of onebeam rocking curves with short computation times. It is easy to determine surface structures during adsorption processes, epitaxial growth and phase transition processes by the one-beam RHEED analysis [4-61. Homoepitaxial growth on a Si(11117 X 7 surface requires successive dissolution of stacking-
B.V. All rights reserved
A. Ichimiya et al. / Si(lll)
fault regions during growth, because it is considered that the surface intends to form the dimeradatom-stacking-fault(DAS) structure [7]. In experiments on homoepitaxial growth of Si(lll), the RHEED intensity oscillation periods are irregular at the initial stage of the deposition called incubation region [6,8-121. Altsinger et al. 1131 showed by intensity analysis of LEED that double height islands nucleate at this region, and have suggested that such double-height nucleation stimulates destruction of stacking-fault layers in the 7 x 7 structures. Such islands were also observed by STM experiments [14]. During homoepitaxial growth on Si(ll1) with RHEED intensity oscillations, the 5 x 5 structure is observed with the 7 X 7 one [6]. By a microprobe RHEED system, Ichikawa and Doi [lo] observed the 5 x 5 structure only at a wide terrace region while the structure disappeared by annealing at 700°C. Since step-flow growth takes place with the 7 X 7 structure, it seems that the 5 x 5 structure promotes destruction of the stacking fault in the terrace region during growth [6]. In the present paper we show the results of a structural analysis of the Si(ll1) surface during growth by RHEED and field-ion STM (FI-STM) [153 at various substrate temperatures. Comparing the above results, we discuss the surface structures during growth and the growth mechanism of silicon on Si(111)7 X 7.
285
during homoepitaxial growth
direct current at about 1200°C. The deposition rate was about 0.1 monolayer (ML)/min. During silicon deposition, the substrate was heated by direct current. The substrate temperature was measured by an optical pyrometer [6]. The RHEED apparatus was reported on elsewhere [6]; the electron gun was operated at 10 kV. The RHEED intensity oscillation curve and rocking curves were measured by the same system described earlier [6]. The RHEED intensities were measured at about 7.5” off from the [llz] direction of the incident azimuth of the one-beam condition for Si(ll1) [3].
3. Silicon deposition at room temperature In the RHEED pattern during deposition of silicon, the 7 x 7 pattern changed by the DAS structure into another 7 x 7 one called 67 x 7 1161.The 67 x 7 pattern faded out into a diffuse background, except for specular reflection upon further deposition. Fig. 1 shows RHEED intensities versus deposition time. The (OO)-rod intensity was taken under the one-beam condition [3] with
2. Experimental Mirror-polished n-type silicon wafers were used as substrates. For RHEED experiments, a Pdoped 5 R . cm Si(ll1) wafer was cut into 3 x 10 X 0.3 mm3 samples. For a FI-STM experiment, a P-doped 3 R. cm SXlll) wafer was cut into 4 x 19 X 0.3 mm3 samples. Prior to introducing the specimen into an ultrahigh-vacuum (UHV) chamber, only washing with methylalcohol was administrated for sample cleaning. Clean silicon surfaces were obtained by resistively heating above 1200°C in a UHV chamber. An evaporation source in the RHEED chamber was heated by electron bombardment [6]. In the FI-STM chamber, the silicon evaporation source was heated by
Coverage
(bilayer)
Fig. 1. Changes of RHEED intensity during silicon at room temperature; (a) for (00) rod and (b) for rod. Curve (a) was measured under the one-beam with a glancing angle of 1.1” and curve (b) for [llz] with a glancing angle of 3.8”.
deposition (3/7, 3/7) condition incidence
286
A. lchimiya d al. / Si(lll)
a glancing angle of 1.1”; the (3/7, 3/7) rod one was measured for the [112] incidence at a glancing angle of 3.8”. Just after the deposition, the (3/7, 3/7) rod intensity decreased rapidly and almost disappeared at 0.25 BL (BL: bilayer) deposition. Since (3/7, 3/7) spots are mainly due to the periodici~ of the adatoms of the DAS structure, disappearance of the spots indicates disarrangement of the adatoms. The coverage of 0.25 BL, at which the (3/7, 3/7) spot intensity is quickly damped, corresponds to twice the adatom density of the DAS structure. This means that two adsorbed silicon atoms break one adatom bond at the initia1 stage. Fig. 2a shows rocking curves during silicon deposition at room temperature for various coverages of silicon overlayer. The rocking curves were analyzed by one-beam RHEED dynamical calculations [31 with the parameters shown in fig. 3. For the calculations we assumed that the fraction of the DAS region was proportional to the coverage of the adatoms. The surface normal components of the atomic positions of the DAS region were taken the same as the values obtained for the clean surface 123.For the 67 X 7 region, it was assumed that Iayers 0 and 1 in fig. 3 included dimers and vacancies. The dimer positions were taken the same as the posi-
6( Fig. 2. Rocking curves during the deposition at room temperature for various deposition coverages, 6, shown in the figure; (a) experim~ntai, (b) calculated ones. Parameters used in the calculations are shown in table 1 and fig. 3.
during homoepitaxial growth
Fig. 3. [Oli] side view of geometric arrangements for the calculations in fig. 2b. d and B are parameters of layer positions and layer coverages, respectively. The parameters d,, d,, f?,, O2 and 0s correspond to those in table 1. Layers 0 and 1 are fied at the bulk position. Positions of dimers, adatoms and atoms of the DAS structure regions are taken from the clean 7 x 7 DAS structure 121.
tions of the DAS structure [2]. In the 67 X 7 region, the positions of layers 0 and 1 were taken at the bulk values referred to the 57 x 7 structure of hydrogen-adsorbed Si(ll1) 2171.For the calculations the parameters shown in fig. 3, 6,, 19,, 8,, d, and d,,were changed widely for each deposition coverage. The calculated rocking curves with the best fit shown in fig. 2b were obtained for the values of table 1. The above result is consistent with the results of rocking curve analysis during growth. From this analysis of silicon deposition shown it is concluded that backbonds of adatoms are broken by adsorbed atoms and randomly distributed pyramidal clusters are formed subsequently on the dimer-stalking-fault framework, as shown fig. 3. The values of d and the layer coverages during growth, obtained by rocking curve analysis, are shown in table 1. In STM images of a silicon surface after growing on Si(lllf7 X 7 at room temperature, it is observed in fig. 4a that growth nuclei of silicon atoms distribute randomly on the 7 X 7 DAS unit cell, mostly on faulted half parts, in the initial stage as less than 0.025 BL deposition. Upon further deposition the nucleated clusters cover the whole surface and do not cover the dimer rows, because lines of the dimer rows of the DAS structure are cIearly seen in fig. 4b. Therefore, it is concluded that coalescence of the nuclei does not occur crossing over the dimer rows at room temperature. These RHEED and STM results suggest that silicon atoms adsorb randomly at adatom sites
A. Ichimiya et al. / Si(lli)
Table I Parameters for calculated shown in fig. 2b
rocking curves with the best fit
Coverages (BL)
Positions CW)
Total %
Adatom %,
Layer 2
Layer 3
Layer 2
Layer 3
$2
$3
0.00 0.04 0.12 0.25 0.45 0.55 0.65
0.12 0.10 0.06 0.03 0.00 0.00 0.00
0.00
0.00
0.06
0.00
0.18
0.00
0.28 0.43 0.43 0.43
0.06 0.14 0.24 0.34
d, _ 3.1 3.1 3.1 3.3 3.3 3.3
d, _ 4.0 3.7 3.7 3.7
Errors were estimated to be about +O.l a for positions and about f 0.05 BL for coverages.
and induce no destruction of the dimer-stacking-fault framework at room temperature. Upon further deposition, an amorphous silicon layer is formed on the 67 x 7 structure because the rocking curve profile changes periodically with damping intensity oscillation. This means that stacking fault and dimers remain at the interface between the crystal and the amorphous silicon layer. These results are consistent with those of X-ray diffrac-
during homoepitaxial
growth
287
tion and cross-sectional transmission electron microscopy (TEM). VaIues for d and the Iayer coverage (statistical occupancies for each layer) for the RHEED analysis are compared with the X-ray diffraction results obtained by Robinson et al. 1183. The values of the atomic positions are in very good agreement with each other. The values of coverage B for the Iayers d, and d, are, however, very large, in comparison with those of the X-ray results. Since the one-beam RHEED analysis is insensitive to lateral displacements of atoms, it is considered that the coverage values obtained by the present anaIysis include the density of the disordered atoms.
4. Homoepitaxial growth at higher temperature At substrate temperatures higher than about 300°C the RHEED intensity oscillation is observed during homoepitaxial growth on a Si(lll)7 x 7 surface, as shown in fig. 5. For the first 5 BL deposition at temperatures up to 500°C the oscillation amplitude is irregular, but after that the oscillation becomes stable and periodic. At
Fig. 4. STM images of a growing silicon surface on Siflll) 7 X 7 at room temperature; (a) initial stage of silicon deposition of less than 0.025 BL, (b) about 0.5 BL deposition.
A. fchimiya et al. / Si(lll)
5
0
growth
ary at 300°C. At 4Oo”C, very few nuclei on the 7 X 7 unit cells were observed on terraces with large triangular islands grown on an upper terrace from a step edge, as shown in fig. 6b. It is suggested that the stacking-fault region is easily dissolved to the normal stacking structure by adsorption at step edges on upper terraces. At the incubation region of the RHEED intensity oscii-
(C) -(d)
0
during homoepitaxial
5
Deposition tine (min) Fig. 5. RHEED intensity oscillation during epitaxial growth of silicon on Sic1 11); (a) at 3OO”C, (b) at 4OO”C, (c) at 600°C and (d) at 700°C.
the periodic region, the profiles of the rocking curves changed periodically with coverage 1111. Therefore, it has been concluded that defects and stacking faults remain scarcely in the epitaxial layers. In RHEED patterns during oscillations, 5 x 5 spots were observed with the 7 x 7 ones 161. At low temperatures up to 4OO”C,the 5 X 5 and 7 x 7 spots are diffuse and streaky. At higher temperatures above 5OO”C, the 5 x 5 spots were clearly observable in the patterns. Therefore, the size of the 5 X 5 and the 7 x 7 domains increases with increasing temperatures [6,191. When the RHEED intensity oscillation disappeared at about 7OO”C, only 7 x 7 spots were observed in the RHEED pattern. It is understood that RHEED oscillation is observed when an epitaxial layer-by-layer growth takes place with 5 X 5 nucleations on terraces, and that intensity oscillation is scarcely observed when the growth proceeds by 7 X 7 growth from step edges as step-flow mode [6,101. From STM images at about 4OO”C, shown in fig. 6a, 5 X 5 domains and islands nucleated at domain boundaries of 5 X 5 and 7 X 7 on a terrace. In STM images it has been observed that 5 x 5 islands are nucleated at defects and domain boundaries. Fig. 9e shows a large island of 5 X 5 DAS structure formed on the 2/7 phase bound-
Fig. 6. STM images of silicon growth at 400°C; (a) mixing phase of the 5 x5 and the 7X7 structures, and (b) triangular islands growing at step edges on upper terraces.
A. Ichimiya et al, / Si(ll1)
iation, KGhler et al. 1141 observed double-height islands of the DAS structure which were suggested by LEED e~eriments [13]. It is considered that nucleation on the initial perfect 7 x 7 DAS surface is very hard and few nucleation islands distribute on the surface in this temperature region. Since upon further deposition nucleation occurs from a step edge of an upper terrace of the island, it seems that double-height islands grow easily at the incubation region. Therefore, If;lany small 5 X 5 and 7 x 7 domains of about 100 A width, which are formed by nucleations at domain boundaries and at the upper terrace edges, distribute on terraces after the incubation period. Stable RHEED intensity oscillation takes place due to layer-by-layer growth with nucleation at the domain boundaries formed on terraces. At substrate temperatures higher than 5OO”C, step-flow growth becomes dominant and 5 x 5 islands are scarcely observed in the STM images, although the 5 x 5 structure is observed in the RHEED pattern at substrate temperatures up to 600°C. It is concluded that the 7 x 7 regions grow with step-flow mode rather than 2D-nucleation growth, while the 5 X 5 domains grow by 2Dnucleations.
Glancing
angle (deg)
Fig. 7. RHEED intensity rocking curves at the one-beam condition during homoepitaxial growth; (a) at 3OO”C,(b) at 4Oo”C, (c) at 6OO”C,(d) at 700°C and (e) the clean surface at room temperature.
during homoepitaxial growth
289
Fig. 8. [Oli] side view of the pyramidal cluster model. Atomic positions shown by broken lines are a!ded for the0 67X7 structure. The values of d are d, = 6.1 A, d, = 5.43 A, d, = 3.135 A, d, = 2.34 i and d, = 2.25 A.
At the ma~mum intensi~ of the oscillations, one-beam rocking curves at higher than 400°C are very similar to the curve from the Si(11117 X 7 DAS structure, as shown in fig. 7. Therefore, it is concluded that the 5 x 5 and 7 x 7 structures during growth in this temperature region are nearly the same as the DAS structure. At a substrate temperature of about 3Oo”C, the onebeam rocking curve is very different from those of the DAS structure as shown in fig. 7. The rocking curve is similar to that from the 67 X 7 structure 16,121. Since the RHEED intensity oscillation is very stable in this temperature region, as shown in fig. 5, it seems that there are no defects remaining in the growing layers. This suggests that the stacking-fault regions are dissolved into the normal stacking with stimulation by further deposition of atoms because of the existence of metastable structures such as the pyramidal cluster-type structure, shown in fig. 8. Atomic positions of the structure are determined by dynamical diffraction analysis of the one-beam rocking curve in fig. 7a. In STM images at a substrate temperature of 3OO”C, characteristic features of a cluster structure were observed with various island structures at the incubation region, as shown in fig. 9a. Figs. 9b and 9c show enlarged STM images of the cluster for empty and filled states, respectively. This structure of the nucleation cluster at a substrate temperature of about 300°C was first observed by STM [20], and scarcely observed at room temperature and at higher temperatures. Most of the clusters formed on a unit cell of the
DAs st1xi ctur e were symmetric with respect to the dime:r t-OM1. The half segment of the symmetric clustc:r wa: ; also observed in fig. 9. The height
of the cluster was estimated to be about l/3 of the step height of the DAS structure. Therefore, the structure is not of the DAS type, but a
Fig. 9. STM images of silicon growth at 3OO”C,(a) initial stage of growth, (b) a growth nucleus (empty state), (c) a growth nucleus (filled state), (d) a 5 x 5 island nucleated at a terrace, (e) a large 5 X 5 island on a defect and (f) an image upon further deposition.
A. Ichimiya et al. / Si(lll)
structure including dimers and/or milk-stool-ape ciusters. Crystal nuclei smaller than half the cluster segment were not observed in the STM images. It is concluded that the cluster is the smallest unit of nucleation of homoepitaxial growth on Si(ll1) at 300°C. Since this cluster structure is scarcely observed at higher temperatures, it is suggested that the cluster structure is a metastable phase of nucleation. In fig. 9d, small islands of the 5 x 5 structure with only one or two unit cells are observed. Large islands of the 5 X 5 structure are only located on defects such as antiphase domain boundaries, as shown in fig. 9e. Upon further deposition of more than 2 BL, the structure image becomes a very complicated one with triangular islands different from the DAS image, while isIands with DAS structure were formed at the initial growth stage. Since the dimer rows are clearly seen in fig. 9f, the dimer-stacking-fault framework of the 7 x 7 structure remains on the growth surface. Since the surface structure determined by RHEED analysis is a pyramidal cluster-type one, it seems that the complicated STM image corresponds to the pyramidal cluster-like structure. From above, results of RHEED and STM at a substrate temperature of 3OO”C, show that very small 5 x 5 islands are nucleated on terraces, but large islands on defects. Therefore, many small domains are formed on the growing surface in the initiaI stage and, subsequently, change into the pyramidal clustertype structure during further deposition. During homoepitaxial growth on Si(111)7 x 7, reconst~ction of the stacking-fault layer into the normal stacking by deposition of sihcon atoms is assisted via breaking backbonds of adatoms and dimer bonds by coexistence of the 5 x 5 and 7 x 7 phases. Homoepitaxial growth on the 7 x 7 surface takes place at lower temperatures than the temperature of the phase transition, while the reconstruction into the normal stacking, such as phase transition from the 7 X 7 to “1 x l”, occurs at a high temperature of about 830°C for the clean 7 X 7 surface [5]. The stacking-fault layer is reconstructed by the transition from the 67 x 7 structure to a \15 x fi-one by adsorption of sev-
during homoepitaxial growth
291
eral metals at about 400°C. In this case it is suggested that the 67 x 7 structure by metal adsorption promotes dissolution of the stacking fault [21]. Therefore, it is concluded that the pyramidal cluster-type or the nucleated cluster-type structure promotes epitaxial growth as a precursor state at an intermediate stage at low substrate temperatures, and that adatom bond breaking makes a trigger of the dissolution of the stacking-fault layer leading to epitaxial growth at higher temperatures. Successive epitaxial growth at low temperatures is promoted by instabili~ of the metastable structures.
Acknowledgements This work was carried out under the collaboration program of the Institute of Materials Research, Tohoku University, and under the support of a Grant-in-Aid to the Scientific Research on Priority Areas by the Japanese Ministry of Education, Science and Culture (Nos. 03243219 and 04227216).
References 111A. Ichimiya, Surf. Sci. 235 (1990) 75. 121A. Ichimiya, Surf. Sci. 192 (1987) I.893. [31 A. Ichimiya, Structure of Surfaces III (1990) 162. [4] S. Kohmoto, S. Mizuno and A. Ichimiya, Appl. Surf. Sci. 41/42 (1989) 107. [S] S. Kohmoto and A. Ichimiya, Surf. Sci. 223 (1989) 400. [61 H. Nakahara and A. Ichimiya, Surf. Sci. 241 (1991) 124. [7] K. Takayanagi, Y. Tanishiro, S. Takahashi and M. Takahashi, Surf. Sci. 164 (1985) 367. 181 T. Sakamoto, N.J. Kawai, T. Nakagawa, K. Ohta, T. Kojima and G. Hashiguchi, Surf. Sci. 174 (1986) 651. [91 J. Aarts and P.K. Larsen, Surf. Sci. 188 (1987) 391. HOI M. Ichikawa and T. Doi, in: Reflection High Energy Electron Diffraction and Reflection Electron Imaging of Surfaces, Eds. P.K. Larsen and P.3. Dobson (Plenum, New York, 1988) p. 343. 1111H. Nakahara and A. Ichimiya, J. Cryst. Growth 95 (1989) 472. WI H. Nakahara and A. Ichimiya, Surf. Sci. 242 (1991) 162. 1131 R. Altsinger, H. Busch, M. Horn and M. Henzler, Surf. Sci. 200 (1988) 235. I141 U. Kiihler, J.E. Demuth and R.J. Hamers, J. Vat. Sci. Technol. A 7 (1989) 2860.
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A. Ichimiya et al. / Si(lll)
[151 T. Sakurai, T. Hashizume, I. Kamiya, Y. Hasegawa, N. Sano, H. Pikering and A. Sakai, Prog. Surf. Sci. 33 (1990) [161 k. Daimon and S. Ino Surf. Sci 164 (19851320. [17] A. Ichimiya and S. M&no, Surf. Sci. 191 (1987) L765. [18] I.K. Robinson, W.K. Waskiewicz and R.T. Tung, Phys. Rev. Lett. 57 (1986) 2714.
during homoepitaxial
growth
[19] M. Horn von Hoegen, J. Falta and M. Henzler, Thin Solid Films 183 (1989) 213. [ZO] A. Ichimiya, T. Hashizume, K. Ishiyama, K. Motai and T. Sakurai, Ultramicroscopy 42-44 (1992) 910. [Zl] A. Ichimiya, Mater. Res. Sot. Symp. Proc. 208 (1991) 3.
Surface Science North-Holland
.:.:...:.y
. . . . ,.,
.....,....,.,..h ,.,:, ,:)~.:’
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4:;::i:i:;:;j:::> .... &.”
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surface science
298 (1993) 284-292
.:~::ii:j:~::+.:.:. . .. .,:.,. .,,,,,,,,,., ,/ .....:::j:::i::~j,i,i,::i~~~.:::..: : ,.... “I”....‘.’ “““~“‘:‘.‘.‘.:...:.:.:..::.:.:.:...~~::::~ ~“““~““‘.‘.‘.~.‘.‘........,.,.,, .,,,., ,,,,,, ,,.::: ,.,:(,:::: ““:‘“::::::::::ij::::j::~ ,.:.:::,,:,:,: ~.:.,. .,.,., .,::::::_:. :
Structural A. Ichimiya
analysis of Si( 111) surfaces during homoepitaxial “, H. Nakahara
‘, T. Hashizume
’ and
growth
T. Sakurai
” Department of Applied Physics, School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan h Institute of Materials Research, Tohoku UniL~ersity,Katahira, Aoba-ky Sendai 980, Japan Received
11 July 1993; accepted
for publication
25 August
1993
Atomic structures of SK1111 surfaces during silicon growth are investigated by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). For silicon deposition on the substrate at room temperature, it is concluded that backbonds of adatoms of Takayanagi’s dimer-adatom-stacking-fault (DAS) structure are broken at an initial stage of the deposition. The adsorbed atoms are nucleated randomly on the surface, and coalescence of the growth nuclei occurs with few nuclei crossing the dimer rows of the DAS structure. For further deposition, the dimers and the stacking faults remain at the interface between the substrate and the growth amorphous layer. At the substrate temperature of about 3OO”C, rocking curves of RHEED intensities from the surface during growth are very different from the curve before growth. The structure of the growing surface layer is determined as a pyramidal cluster-type structure by analysis with RHEED dynamical calculations. At this temperature, characteristic features of a cluster structure are found on the surface at an initial stage of the growth. Since crystal nuclei smaller than half the cluster segment are scarcely observed in the STM images, it is concluded that the cluster is the smallest unit of nucleation of homoepitaxial growth. At substrate temperatures from 400 to 600°C a mixed phase of 5 X 5 and 7 X 7 structures, which is the DAS structure, is observed by RHEED and STM. It is discussed that the formation of metastable structures, such as the pyramidal cluster-type structure and the 5 X 5 one, promotes successive epitaxial growth accompanied with stacking-fault dissolution at the dimer-stacking-fault framework
1. Introduction Since fast electrons are scattered dominantly in a forward direction by atoms, dynamic diffraction of high-energy electrons in a crystal mainly occurs in a forward direction. This means that the fast electrons interact approximately with a projected crystal potential which is an averaged crystal potential in the direction of the’incident electron beam. Using this feature of high-energy electrons, reflection high-energy electron diffraction (RHEED) intensities are calculated by a dynamical theory with diffracted beams in a zeroth Laue zone [1], because the Fourier coefficients of the crystal potential in the zeroth Laue zone are the same as those of the projected one in the direction of the incident beam. When a crystal orientation for the incident beam is chosen at several degrees off from a certain crystal axis, the pro0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
jected potential is approximately one-dimensional in a direction perpendicular to the surface. At this condition, electrons are diffracted mainly by lattice planes parallel to the surface. This diffraction condition is named the one-beam condition [2,3], because the main diffraction beam is simply the specular one. A rocking curve at this condition is a function of surface normal components of atomic positions, but hardly depends on lateral components of them. Surface normal components of atomic positions of surface layers are determined by dynamical calculation analysis of onebeam rocking curves with short computation times. It is easy to determine surface structures during adsorption processes, epitaxial growth and phase transition processes by the one-beam RHEED analysis [4-61. Homoepitaxial growth on a Si(11117 X 7 surface requires successive dissolution of stacking-
B.V. All rights reserved
A. Ichimiya et al. / Si(lll)
fault regions during growth, because it is considered that the surface intends to form the dimeradatom-stacking-fault(DAS) structure [7]. In experiments on homoepitaxial growth of Si(lll), the RHEED intensity oscillation periods are irregular at the initial stage of the deposition called incubation region [6,8-121. Altsinger et al. 1131 showed by intensity analysis of LEED that double height islands nucleate at this region, and have suggested that such double-height nucleation stimulates destruction of stacking-fault layers in the 7 x 7 structures. Such islands were also observed by STM experiments [14]. During homoepitaxial growth on Si(ll1) with RHEED intensity oscillations, the 5 x 5 structure is observed with the 7 X 7 one [6]. By a microprobe RHEED system, Ichikawa and Doi [lo] observed the 5 x 5 structure only at a wide terrace region while the structure disappeared by annealing at 700°C. Since step-flow growth takes place with the 7 X 7 structure, it seems that the 5 x 5 structure promotes destruction of the stacking fault in the terrace region during growth [6]. In the present paper we show the results of a structural analysis of the Si(ll1) surface during growth by RHEED and field-ion STM (FI-STM) [153 at various substrate temperatures. Comparing the above results, we discuss the surface structures during growth and the growth mechanism of silicon on Si(111)7 X 7.
285
during homoepitaxial growth
direct current at about 1200°C. The deposition rate was about 0.1 monolayer (ML)/min. During silicon deposition, the substrate was heated by direct current. The substrate temperature was measured by an optical pyrometer [6]. The RHEED apparatus was reported on elsewhere [6]; the electron gun was operated at 10 kV. The RHEED intensity oscillation curve and rocking curves were measured by the same system described earlier [6]. The RHEED intensities were measured at about 7.5” off from the [llz] direction of the incident azimuth of the one-beam condition for Si(ll1) [3].
3. Silicon deposition at room temperature In the RHEED pattern during deposition of silicon, the 7 x 7 pattern changed by the DAS structure into another 7 x 7 one called 67 x 7 1161.The 67 x 7 pattern faded out into a diffuse background, except for specular reflection upon further deposition. Fig. 1 shows RHEED intensities versus deposition time. The (OO)-rod intensity was taken under the one-beam condition [3] with
2. Experimental Mirror-polished n-type silicon wafers were used as substrates. For RHEED experiments, a Pdoped 5 R . cm Si(ll1) wafer was cut into 3 x 10 X 0.3 mm3 samples. For a FI-STM experiment, a P-doped 3 R. cm SXlll) wafer was cut into 4 x 19 X 0.3 mm3 samples. Prior to introducing the specimen into an ultrahigh-vacuum (UHV) chamber, only washing with methylalcohol was administrated for sample cleaning. Clean silicon surfaces were obtained by resistively heating above 1200°C in a UHV chamber. An evaporation source in the RHEED chamber was heated by electron bombardment [6]. In the FI-STM chamber, the silicon evaporation source was heated by
Coverage
(bilayer)
Fig. 1. Changes of RHEED intensity during silicon at room temperature; (a) for (00) rod and (b) for rod. Curve (a) was measured under the one-beam with a glancing angle of 1.1” and curve (b) for [llz] with a glancing angle of 3.8”.
deposition (3/7, 3/7) condition incidence
286
A. lchimiya d al. / Si(lll)
a glancing angle of 1.1”; the (3/7, 3/7) rod one was measured for the [112] incidence at a glancing angle of 3.8”. Just after the deposition, the (3/7, 3/7) rod intensity decreased rapidly and almost disappeared at 0.25 BL (BL: bilayer) deposition. Since (3/7, 3/7) spots are mainly due to the periodici~ of the adatoms of the DAS structure, disappearance of the spots indicates disarrangement of the adatoms. The coverage of 0.25 BL, at which the (3/7, 3/7) spot intensity is quickly damped, corresponds to twice the adatom density of the DAS structure. This means that two adsorbed silicon atoms break one adatom bond at the initia1 stage. Fig. 2a shows rocking curves during silicon deposition at room temperature for various coverages of silicon overlayer. The rocking curves were analyzed by one-beam RHEED dynamical calculations [31 with the parameters shown in fig. 3. For the calculations we assumed that the fraction of the DAS region was proportional to the coverage of the adatoms. The surface normal components of the atomic positions of the DAS region were taken the same as the values obtained for the clean surface 123.For the 67 X 7 region, it was assumed that Iayers 0 and 1 in fig. 3 included dimers and vacancies. The dimer positions were taken the same as the posi-
6( Fig. 2. Rocking curves during the deposition at room temperature for various deposition coverages, 6, shown in the figure; (a) experim~ntai, (b) calculated ones. Parameters used in the calculations are shown in table 1 and fig. 3.
during homoepitaxial growth
Fig. 3. [Oli] side view of geometric arrangements for the calculations in fig. 2b. d and B are parameters of layer positions and layer coverages, respectively. The parameters d,, d,, f?,, O2 and 0s correspond to those in table 1. Layers 0 and 1 are fied at the bulk position. Positions of dimers, adatoms and atoms of the DAS structure regions are taken from the clean 7 x 7 DAS structure 121.
tions of the DAS structure [2]. In the 67 X 7 region, the positions of layers 0 and 1 were taken at the bulk values referred to the 57 x 7 structure of hydrogen-adsorbed Si(ll1) 2171.For the calculations the parameters shown in fig. 3, 6,, 19,, 8,, d, and d,,were changed widely for each deposition coverage. The calculated rocking curves with the best fit shown in fig. 2b were obtained for the values of table 1. The above result is consistent with the results of rocking curve analysis during growth. From this analysis of silicon deposition shown it is concluded that backbonds of adatoms are broken by adsorbed atoms and randomly distributed pyramidal clusters are formed subsequently on the dimer-stalking-fault framework, as shown fig. 3. The values of d and the layer coverages during growth, obtained by rocking curve analysis, are shown in table 1. In STM images of a silicon surface after growing on Si(lllf7 X 7 at room temperature, it is observed in fig. 4a that growth nuclei of silicon atoms distribute randomly on the 7 X 7 DAS unit cell, mostly on faulted half parts, in the initial stage as less than 0.025 BL deposition. Upon further deposition the nucleated clusters cover the whole surface and do not cover the dimer rows, because lines of the dimer rows of the DAS structure are cIearly seen in fig. 4b. Therefore, it is concluded that coalescence of the nuclei does not occur crossing over the dimer rows at room temperature. These RHEED and STM results suggest that silicon atoms adsorb randomly at adatom sites
A. Ichimiya et al. / Si(lli)
Table I Parameters for calculated shown in fig. 2b
rocking curves with the best fit
Coverages (BL)
Positions CW)
Total %
Adatom %,
Layer 2
Layer 3
Layer 2
Layer 3
$2
$3
0.00 0.04 0.12 0.25 0.45 0.55 0.65
0.12 0.10 0.06 0.03 0.00 0.00 0.00
0.00
0.00
0.06
0.00
0.18
0.00
0.28 0.43 0.43 0.43
0.06 0.14 0.24 0.34
d, _ 3.1 3.1 3.1 3.3 3.3 3.3
d, _ 4.0 3.7 3.7 3.7
Errors were estimated to be about +O.l a for positions and about f 0.05 BL for coverages.
and induce no destruction of the dimer-stacking-fault framework at room temperature. Upon further deposition, an amorphous silicon layer is formed on the 67 x 7 structure because the rocking curve profile changes periodically with damping intensity oscillation. This means that stacking fault and dimers remain at the interface between the crystal and the amorphous silicon layer. These results are consistent with those of X-ray diffrac-
during homoepitaxial
growth
287
tion and cross-sectional transmission electron microscopy (TEM). VaIues for d and the Iayer coverage (statistical occupancies for each layer) for the RHEED analysis are compared with the X-ray diffraction results obtained by Robinson et al. 1183. The values of the atomic positions are in very good agreement with each other. The values of coverage B for the Iayers d, and d, are, however, very large, in comparison with those of the X-ray results. Since the one-beam RHEED analysis is insensitive to lateral displacements of atoms, it is considered that the coverage values obtained by the present anaIysis include the density of the disordered atoms.
4. Homoepitaxial growth at higher temperature At substrate temperatures higher than about 300°C the RHEED intensity oscillation is observed during homoepitaxial growth on a Si(lll)7 x 7 surface, as shown in fig. 5. For the first 5 BL deposition at temperatures up to 500°C the oscillation amplitude is irregular, but after that the oscillation becomes stable and periodic. At
Fig. 4. STM images of a growing silicon surface on Siflll) 7 X 7 at room temperature; (a) initial stage of silicon deposition of less than 0.025 BL, (b) about 0.5 BL deposition.
A. fchimiya et al. / Si(lll)
5
0
growth
ary at 300°C. At 4Oo”C, very few nuclei on the 7 X 7 unit cells were observed on terraces with large triangular islands grown on an upper terrace from a step edge, as shown in fig. 6b. It is suggested that the stacking-fault region is easily dissolved to the normal stacking structure by adsorption at step edges on upper terraces. At the incubation region of the RHEED intensity oscii-
(C) -(d)
0
during homoepitaxial
5
Deposition tine (min) Fig. 5. RHEED intensity oscillation during epitaxial growth of silicon on Sic1 11); (a) at 3OO”C, (b) at 4OO”C, (c) at 600°C and (d) at 700°C.
the periodic region, the profiles of the rocking curves changed periodically with coverage 1111. Therefore, it has been concluded that defects and stacking faults remain scarcely in the epitaxial layers. In RHEED patterns during oscillations, 5 x 5 spots were observed with the 7 x 7 ones 161. At low temperatures up to 4OO”C,the 5 X 5 and 7 x 7 spots are diffuse and streaky. At higher temperatures above 5OO”C, the 5 x 5 spots were clearly observable in the patterns. Therefore, the size of the 5 X 5 and the 7 x 7 domains increases with increasing temperatures [6,191. When the RHEED intensity oscillation disappeared at about 7OO”C, only 7 x 7 spots were observed in the RHEED pattern. It is understood that RHEED oscillation is observed when an epitaxial layer-by-layer growth takes place with 5 X 5 nucleations on terraces, and that intensity oscillation is scarcely observed when the growth proceeds by 7 X 7 growth from step edges as step-flow mode [6,101. From STM images at about 4OO”C, shown in fig. 6a, 5 X 5 domains and islands nucleated at domain boundaries of 5 X 5 and 7 X 7 on a terrace. In STM images it has been observed that 5 x 5 islands are nucleated at defects and domain boundaries. Fig. 9e shows a large island of 5 X 5 DAS structure formed on the 2/7 phase bound-
Fig. 6. STM images of silicon growth at 400°C; (a) mixing phase of the 5 x5 and the 7X7 structures, and (b) triangular islands growing at step edges on upper terraces.
A. Ichimiya et al, / Si(ll1)
iation, KGhler et al. 1141 observed double-height islands of the DAS structure which were suggested by LEED e~eriments [13]. It is considered that nucleation on the initial perfect 7 x 7 DAS surface is very hard and few nucleation islands distribute on the surface in this temperature region. Since upon further deposition nucleation occurs from a step edge of an upper terrace of the island, it seems that double-height islands grow easily at the incubation region. Therefore, If;lany small 5 X 5 and 7 x 7 domains of about 100 A width, which are formed by nucleations at domain boundaries and at the upper terrace edges, distribute on terraces after the incubation period. Stable RHEED intensity oscillation takes place due to layer-by-layer growth with nucleation at the domain boundaries formed on terraces. At substrate temperatures higher than 5OO”C, step-flow growth becomes dominant and 5 x 5 islands are scarcely observed in the STM images, although the 5 x 5 structure is observed in the RHEED pattern at substrate temperatures up to 600°C. It is concluded that the 7 x 7 regions grow with step-flow mode rather than 2D-nucleation growth, while the 5 X 5 domains grow by 2Dnucleations.
Glancing
angle (deg)
Fig. 7. RHEED intensity rocking curves at the one-beam condition during homoepitaxial growth; (a) at 3OO”C,(b) at 4Oo”C, (c) at 6OO”C,(d) at 700°C and (e) the clean surface at room temperature.
during homoepitaxial growth
289
Fig. 8. [Oli] side view of the pyramidal cluster model. Atomic positions shown by broken lines are a!ded for the0 67X7 structure. The values of d are d, = 6.1 A, d, = 5.43 A, d, = 3.135 A, d, = 2.34 i and d, = 2.25 A.
At the ma~mum intensi~ of the oscillations, one-beam rocking curves at higher than 400°C are very similar to the curve from the Si(11117 X 7 DAS structure, as shown in fig. 7. Therefore, it is concluded that the 5 x 5 and 7 x 7 structures during growth in this temperature region are nearly the same as the DAS structure. At a substrate temperature of about 3Oo”C, the onebeam rocking curve is very different from those of the DAS structure as shown in fig. 7. The rocking curve is similar to that from the 67 X 7 structure 16,121. Since the RHEED intensity oscillation is very stable in this temperature region, as shown in fig. 5, it seems that there are no defects remaining in the growing layers. This suggests that the stacking-fault regions are dissolved into the normal stacking with stimulation by further deposition of atoms because of the existence of metastable structures such as the pyramidal cluster-type structure, shown in fig. 8. Atomic positions of the structure are determined by dynamical diffraction analysis of the one-beam rocking curve in fig. 7a. In STM images at a substrate temperature of 3OO”C, characteristic features of a cluster structure were observed with various island structures at the incubation region, as shown in fig. 9a. Figs. 9b and 9c show enlarged STM images of the cluster for empty and filled states, respectively. This structure of the nucleation cluster at a substrate temperature of about 300°C was first observed by STM [20], and scarcely observed at room temperature and at higher temperatures. Most of the clusters formed on a unit cell of the
DAs st1xi ctur e were symmetric with respect to the dime:r t-OM1. The half segment of the symmetric clustc:r wa: ; also observed in fig. 9. The height
of the cluster was estimated to be about l/3 of the step height of the DAS structure. Therefore, the structure is not of the DAS type, but a
Fig. 9. STM images of silicon growth at 3OO”C,(a) initial stage of growth, (b) a growth nucleus (empty state), (c) a growth nucleus (filled state), (d) a 5 x 5 island nucleated at a terrace, (e) a large 5 X 5 island on a defect and (f) an image upon further deposition.
A. Ichimiya et al. / Si(lll)
structure including dimers and/or milk-stool-ape ciusters. Crystal nuclei smaller than half the cluster segment were not observed in the STM images. It is concluded that the cluster is the smallest unit of nucleation of homoepitaxial growth on Si(ll1) at 300°C. Since this cluster structure is scarcely observed at higher temperatures, it is suggested that the cluster structure is a metastable phase of nucleation. In fig. 9d, small islands of the 5 x 5 structure with only one or two unit cells are observed. Large islands of the 5 X 5 structure are only located on defects such as antiphase domain boundaries, as shown in fig. 9e. Upon further deposition of more than 2 BL, the structure image becomes a very complicated one with triangular islands different from the DAS image, while isIands with DAS structure were formed at the initial growth stage. Since the dimer rows are clearly seen in fig. 9f, the dimer-stacking-fault framework of the 7 x 7 structure remains on the growth surface. Since the surface structure determined by RHEED analysis is a pyramidal cluster-type one, it seems that the complicated STM image corresponds to the pyramidal cluster-like structure. From above, results of RHEED and STM at a substrate temperature of 3OO”C, show that very small 5 x 5 islands are nucleated on terraces, but large islands on defects. Therefore, many small domains are formed on the growing surface in the initiaI stage and, subsequently, change into the pyramidal clustertype structure during further deposition. During homoepitaxial growth on Si(111)7 x 7, reconst~ction of the stacking-fault layer into the normal stacking by deposition of sihcon atoms is assisted via breaking backbonds of adatoms and dimer bonds by coexistence of the 5 x 5 and 7 x 7 phases. Homoepitaxial growth on the 7 x 7 surface takes place at lower temperatures than the temperature of the phase transition, while the reconstruction into the normal stacking, such as phase transition from the 7 X 7 to “1 x l”, occurs at a high temperature of about 830°C for the clean 7 X 7 surface [5]. The stacking-fault layer is reconstructed by the transition from the 67 x 7 structure to a \15 x fi-one by adsorption of sev-
during homoepitaxial growth
291
eral metals at about 400°C. In this case it is suggested that the 67 x 7 structure by metal adsorption promotes dissolution of the stacking fault [21]. Therefore, it is concluded that the pyramidal cluster-type or the nucleated cluster-type structure promotes epitaxial growth as a precursor state at an intermediate stage at low substrate temperatures, and that adatom bond breaking makes a trigger of the dissolution of the stacking-fault layer leading to epitaxial growth at higher temperatures. Successive epitaxial growth at low temperatures is promoted by instabili~ of the metastable structures.
Acknowledgements This work was carried out under the collaboration program of the Institute of Materials Research, Tohoku University, and under the support of a Grant-in-Aid to the Scientific Research on Priority Areas by the Japanese Ministry of Education, Science and Culture (Nos. 03243219 and 04227216).
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[151 T. Sakurai, T. Hashizume, I. Kamiya, Y. Hasegawa, N. Sano, H. Pikering and A. Sakai, Prog. Surf. Sci. 33 (1990) [161 k. Daimon and S. Ino Surf. Sci 164 (19851320. [17] A. Ichimiya and S. M&no, Surf. Sci. 191 (1987) L765. [18] I.K. Robinson, W.K. Waskiewicz and R.T. Tung, Phys. Rev. Lett. 57 (1986) 2714.
during homoepitaxial
growth
[19] M. Horn von Hoegen, J. Falta and M. Henzler, Thin Solid Films 183 (1989) 213. [ZO] A. Ichimiya, T. Hashizume, K. Ishiyama, K. Motai and T. Sakurai, Ultramicroscopy 42-44 (1992) 910. [Zl] A. Ichimiya, Mater. Res. Sot. Symp. Proc. 208 (1991) 3.