- Email: [email protected]
Initial domain structure of GaAs thin films grown on Si(001) substrates
surface science ELSEWIER
Applied Surface Science 117/118 (1997) 765-770
Initial domain structure of GaAs thin films grown on Si(OO1) substrates Tomoaki Kawamura a,*, Hisataka Takenaka b, Takayoshi Hayashi ‘, Masami Tachikawa d, Hidefumi Mori d a NTT Basic Research Laboratories, 3-1, Morinosato, Wakamiya, Atsugi, Kanagawa 243-01, Japan ’ NlTAduanced Technology Corporation, 3-9-11, Midori-cho, Musashino, Tokyo 180, Japan ’ NIT Integrated Information and Energy Systems Laboratories, 3-9-l I, Midori-cho, Musashino, Tokyo 180, Japan d NTT Opto-Electronics Laboratories, 3-l Morinosato, Wakamiya, Atsugi, Kanagawa 243-01, Japan
Abstract The initial domain structure of GaAs films grown on several Si(OO1) surfaces is investigated using X-ray standing waves. GaAs/Si(OOl) samples, 4 ML thick, grown on three different Si substrates were used: an epitaxial Si surface (ESS), a mechanochemically polished surface (MCP), and a mechanochemically polished surface with plasma cleaning (plasma MCP). The domain ratio ambiguousness due to the film thickness is avoided by observing independent Bragg reflections of Si substrates. The results of X-ray standing wave measurement reveal that all daAs films have double domain s&uctures at the initial stage, even though final domain structures are single. The ratio of the two domains was almost 1 : 1 on the MCP surface, 6 : 4 on the ESS surface, and 4.5 : 5.5 on the plasma MCP surface. The dominant GaAs domains on the ESS and plasma MCP surfaces were the same as those obtained on thicker GaAs films. This suggests there is a rapid decrease in the GaAs domain during the early stages of growth on an ESS and plasma MCP surfaces.
1. Introduction The epitaxial growth of III-V semiconductors on Si has attracted the interest of many researchers in recent years [l]. One of the fundamental problems concerning the epitaxy of polar semiconductors on (001) non-polar semiconductor substrates is the formation of antiphase boundaries and antiphase domains (APDs) [2]. The formation of antiphase free domains were previously thought to be due to the absence of single atomic steps at the Si surface [2]. Recent STM studies, however, contradict this view
* Corresponding [email protected].
author. Tel.: + 81-462-402035;
0169-4332/97/$17.00 Copyright PN SO169-4332(97)00133-5
e-mail: kawa-
[4]: they suggest double domain existence and annihilation during the growth process. Many techniques have been used to detect the presence of antiphase domains experimentally, including anisotropic etching [5], X-ray diffraction [6], and RHEED [2]. The use of anisotropic etching and X-ray diffraction is limited to GaAs layers thicker than 10 nm. The RHEED technique is only usable under UHV circumstances and is not valid for gasphase growth. The X-ray standing wave technique, which uses the interference effect of incident and diffracted Xrays, has been known as a powerful tool for determining surface and interface structures [7]. By observing the fluorescence yields from the epitaxial layers, one can get information about the position of
0 1997 Elsevier Science B.V. All rights reserved.
766
T. Kawamura et al/Applied
Surface Science 117/118
atoms at the interfaces and on the surfaces. However, there is still difficulty in determining the interface structures due to the thickness ambiguity of epitaxial layers. When X-ray standing waves are formed in the epitaxial layers, each atom senses the different X-ray intensity due to the different d-spacing between substrates and the epitaxial layers. This results in a different fluorescence yield profile from each atomic plane, giving the uncertainty in determining the interface structure and the domain ratio. This effect is more significant for large mismatch systems, such as GaAs/Si, which has a 4% lattice mismatch value. To solve this difficulty, we proposed the new method using the independent Bragg reflection for determining the domain ratio independently of epitaxial layer thickness [3]. In this work, we describe the principle of this new method and, using the method, we investigate different GaAs/Si interface structures made by gas-phase growth. One such structure is grown on an epitaxial silicon surface, and the others are grown on a mechanochemically polished silicon surface with and without plasma cleaning prior to the growth. Finally, we discuss the initial growth stage for these GaAs/Si systems.
(1997) 765-770
Here, e ia = x/l
XI,
Y=(di-ds)/ds7
(3)
where di and d, are the d-spacing value of the epitaxial layer and substrate crystals. This result shows that the thickness effect is included in the standing wave parameters, F and Z. If more than one site is occupied, the resulting Z and F in Eq. (1) are described as the summation of positions zi and probabilities cri of the individual sites [8]: Fe2”iZ
=fCgi
e*“iZi,
(4)
where Cgi = 1 and f is the ratio of ordered atoms to the total number of atoms. By determining ai from X-ray standing wave measurements, one can determine the domain ratio of GaAs films. Assuming that the Ga and As atoms occupy only two sites on a (111) reflecting plane, we can describe
2. Bases When X-rays are introduced into the crystal at the Bragg conditions, the incident and diffracted X-rays interfere, and make standing waves in the crystal and on the surface. By analyzing the fluorescence yield, If dependence on the Bragg reflection, from atoms in the X-ray fields one can obtain information about the atomic location, Z, normalized by the lattice spacing and the probability of occupation, F, of the atoms
b31. If= 1 + 1,y)* + 2Fa{
~.e-‘~~‘}, (1) where x is the complex reflectivity. The thickness effect of overlayers is included as follows [9]:
sin( rrNy) = 1 +]#+~F]x] X%{e-
IV sin( 7ry) 27ri(Z+t?r(N- I)y)+ ai
1.
(2)
Fig. 1. Domain structure of GaAs films on Si(OO1) substrate. In this figure, the As atoms are taken to connected with the Si atoms. In (a) and (b), the Si dimers are perpendicular and parallel to the step edge, respectively. It was not determined by X-ray standing waves whether the interface atoms between the GaAs and the Si are As or not.
T. Kawamura et al/Applied
positions and F,il
z 1,, and ~~7, and coherent based on Eq. (4):
F,,, c2”izl~l =f,,,a
c2?riz +fill(l
factors
Surface Science 117/ 118 (1997) 765-770
F, 1,
- a) e2ri(z+A), (5)
F _ e2rizlil =fii,(l Ill
_ c) e*niz +fiila
e*pi(z+A), (6)
where f, I, and fii, are the ratios of the number of ordered atoms to the total number of atoms for the (111) and (111) planes, respectively. Parameter (+ is the probability of atoms existing at site 1 (Fig. la and b); it corresponds to the domain ratio of the GaAs films. If the GaAs film has a completely single domain structure, (+ should be equal to 1.0 or 0.0. The z and A represent the normalized atomic position and the atomic distance between the two sites on the (111) plane, respectively; A is 0.25 for an Sic1 11) plane and 0.26 for a GaAs(ll1) plane with a 4% lattice mismatch. By considering the algebraic relations between these parameters, one can calculate fclllj, fcli1), (+, and z from A. Note that thickness ambiguity only affect the and z. This feature makes it value of fii~ t fi I I possible to decide the domain ratio, (+, independently of the thickness of the GaAs layer.
167
except for a conventional thermal Si-surface treatment prior to the GaAs growth. The interfaces of the GaAs/Si(lOO) samples were characterized using cross sectional high resolution transmission micrography. Flat GaAs islands about 10 ML thick and lo-20 nm in diameter were observed, suggesting island growth. Significantly, the lattice images between the Si and GaAs layers were maintained over most of the GaAs/Si interface, making it possible to analyze the interface structure using the X-ray standing waves. The X-ray standing wave was analyzed with a laboratory instruments. A symmetrical incident
300
E
400
500,
Channel number
1.5
; easured (Ga K) + Calculated
F10.26 Z=O.O6 -
1.0 1.5
0.5 3. Experimental GaAs/Si(lOO) samples were grown by vapour mixing epitaxy (VME) [ 10,111, which is based on hydride vapour phase epitaxy (hydride VPE). The substrate used in our experiments was an epitaxial Si(OO1) substrate, misaligned 2” toward the (110) direction: it had a 5-pm layer of homoepitaxial Si grown by atmospheric-pressure Si,H,Cl, chemical vapour phase deposition. The substrate was dipped in a 7% HF aqueous solution for 30 s for passivation by hydrogen termination [lo]. No thermal treatment was performed in the growth chamber before GaAs growth. After it was introduced into the growth chamber, GaCl-ASH, of 4-monolayer quantities was provided for GaAs growth. The growth temperature was 400°C. This procedure was also used to fabricate the GaAs/Si(OOl) samples on an MCP substrate,
1.0 1.0
-A
Si(ll1)-
0.5
0.5
0.0 -15
-10
-5
0
5
10
15
Angle (normalized) Fig.2. Typical fluorescence
yield profiles from Ga and As atoms of ESS GaAs/Si(OOl). The horizontal axis is normalized by the theoretical full width at half maximum. The solid lines with marks are the measured fluorescence yields of Ga and As K (Y; the dashed lines show the calculated yields. Coherent factor F and normalized position Z are as shown in the figure.
768
T. Kawamura et al./Applied
Surface Science 117/118
monochromator was used to increase the beam intensity. A molybdenum target was used to activate the Ga and As fluorescence yield. The secondary radiation, including the Ga and As Ka signals, was monitored with a pure Ge type solid state detector and accumulated into a multi-channel analyzer. The Ga and As fluorescence yield profiles were obtained after background subtraction from the fluorescence spectra. All instruments, except the X-ray source generator, were set in a thermo-stabilized room to avoid the drift of the Bragg peak due to thermal fluctuation. To increase the signal-to-noise ratio, both the Bragg reflection and the fluorescence yields were measured repeatedly. The repetition error was estimated to be less than 0.1 arcsecs from the Bragg reflection profiles. Fig. 2 shows typical fluorescence yields for both Ga and As K (Y. The inset shows the Ga and As Ka fluorescences. Due to the small Bragg angle (6.496”), the asymmetric effect of the samples needed to be evaluated [8]. By measuring the Bragg reflection profile dependence on the tilt angle of the samples, we obtained the asymmetry factor of the samples and used it in our analysis. Note that the intensity of As K (Y is 1S-3.0 times greater than that of Ga Ka for each sample. This suggests the possibilities of the ratio between Ga and As atoms being different from 1: 1, and the existence of an extra As atoms in the GaAs films.
4. Results and discussion By fitting the measured data to Eq. (11, we obtained normalized position Z and coherent factor F of the (111) and (111) planes. Table 1 summarizes coherent factor F and atomic position Z for each
(1997) 765-770
sample with R factors. An analysis of Table 1 reveals several points. (1) For ESS and plasma MCP GaAs/Si(OOl), the difference in position parameter Z between (111) and (lil) was smaller than 0.25, suggesting the existence of two domains. The dominant domain for ESS is Type B, while for plasma MCP it is Type A in Fig. la and b. If only a single domain exists, the difference in the position parameter is expected to be 0.25. (2) For MCP GaAs/Si(OOl), the Ga and As atoms had the same position parameter, showing that the domain ratio was almost 1: 1. (3) The As coherent factor was lower than that of Ga for all samples. This suggests the existence of extra amorphous As atoms in GaAs films, consistent with the results of our fluorescence yield analysis. As previously reported [lo], a single GaAs domain 100 nm thick can be obtained on ESS and plasma MCP surfaces. According to etch pit observations [5], the domain structures are Type B and A, respectively, as shown in Fig. 1. In contrast, only a double domain was obtained for GaAs films grown on MCP Si substrates [ll]. These results are qualitatively consistent with our X-ray standing wave results, except for the domain ratio during the initial growth stage. Fig. 3a and b show the dependence of cr on A for ESS and plasma MCP GaAs/Si(OOl). Assuming the interface models shown in Fig. 1, aoa and uAs represent the domain ratios of Type A and Type B, respectively. It should be noted that the X-ray standing wave technique determines the probability of each atom for both sites; it can not distinguish the first atoms directly connected with the Si atoms at the interface. Considering the vertical lattice expansion of GaAs
Table 1 Experimental results (F,
z) and estimated domain ratio D
%I,)
Z(lll)
Qilj
Z(lil)
(T
0.34 + 0.03 0.18 f 0.06 -
0.18 k 0.03 0.09 f 0.06 -
0.69 f 0.08 0.50 + 0.03 0.46 + 0.02
ESS
Ga
0.26 f 0.02
MCP
AS Ga
0.17 + 0.04 0.52 + 0.09
0.06 + 0.02 0.17 + 0.02 0.18 f 0.02
AS
0.28 f 0.07
0.16 + 0.02
Plasma MCP
Ga
0.53 + 0.18
0.19 * 0.02
0.55 + 0.15
0.16 f 0.03
As
0.04 + 0.02
0.15 f 0.08
0.23 f 0.9
0.21 + 0.03
T. Kawamura
et al./Applied
Surface Science 117/118
(1997) 765-770
169
have been obtained with a 100 nm thicknesses on ESS and plasma MCP Si, this can be explained by the rapid decline of non-dominant domains. Even with the atomic force microscopy observation [ 101, it is not clear whether the existence of both Ga-Si and As-Si bondings at the interface or single atomic steps are important in forming a double domain structure. For MCP GaAs/Si, in addition to the Ga and As connection with Si, the existence of single steps on the Si surface is expected, causing a completely double domain structure at the beginning of the growth. However, quantitative analysis of the domain ratio under several initial growth conditions is still necessary to understand the domain formation mechanism during initial growth.
1.0 ;a .g 0.8 "
0.2
5. Conclusions Difference between two sites(Delta)
B 1.0 -
-j+.* g n 2 g
e+... 0.4 -
0.2 -
00’
'0.0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 Difference between two sites(Delta)
Fig. 3. Probability (r and (b) plasma MCP consists of only Type dashed line is the site
dependence GaAs films A, croGa= 0 distance for
on site distance A for (a) ESS on Si(OO1). If the GaAs film and a,, = 1 are expected. The the Si-Si.
to horizontal compression [ 12,131, we limited the range of A in Eq. (5) and Eq. (6) to be from 0.26 to about 0.27. Row 7 in Table 1 shows the estimated domain ratio for each samples with the value A = 0.26. Remembering that single domain GaAs films
due
In conclusion, X-ray standing wave measurements of the initial GaAs domain structure on ESS, MCP and plasma MCP Si(OO1) surfaces showed a double domain for GaAs films on both surfaces. The domain ratio for MCP GaAs/Si(OOl) was equal to 0.5 : 0.5, suggesting a completely double domain structure. For ESS and plasma MCP GaAs/Si(OOl), although single domain GaAs films were obtained, the initial domain structure was double with a ratio of about 0.7 : 0.3 and 0.45 : 0.55, respectively; the dominant domain was the same as that finally obtained. Considering the previous results for GaAs films on both Si substrates, this can be explained by the rapid decrease in the initially existing recessive domain of the GaAs films during subsequent film growth. It should be noted that the reason the GaAs double domain structures are formed in the initial stage is not clear; further investigation of different substrates and growth conditions is thus required.
References 111Heteroepitaxy
on Silicon, NRS Symp. Proc: (a) Vol. 67, edited by J.C.C. Fan and J.M. Poate (1986); (b) Vol. 91, edited by J.C.C. Fan, J.M. Phillips and B.Y. Tsaur (1987); (c) Vol. 116, edited by H.K. Choi, R. Hull, H. Ishikawa and R.J. Nemanich (MRS, Pittsburgh, PA, 1988). 121S.F. Fang, K. Adomi, S. Iyer, H. Morkoq, H. Zabel, C. Choi, N. Otsuka, J. Appl. Phys. 68 (1990) R31. 131 T. Kawamura, H. Takenaka, T. Hayashi, M. Tachikawa, H. Mori, Appl. Phys. Lett. 68 (1996) 1969.
170
T. Kawamura et al. /Applied
Surface Science 117/118
[4] R.M. Tromp, R.J. Harmers, J.E. Demuth, Phys. Rev. Lett. 59 (1987) 1691. [5] K. Fujita, Y. Shiba, T. Yamamoto, J. Cryst. Growth 99 (1990) 341. [6] D.A. Neuman, H. Zabel, R. Fischer, H. Morko9, J. Appl. Phys. 61 (1987) 1023. [7] T. Kawamura, H. Takenaka, M. Uneta, M. Oshima, Y. Fukuda, Y. Ohmachi, K. Izumi, T. Ishikawa, S. Kikuta, Z.W. Zhang, Jpn. J. Appl. Phys. 32 (1993) 622. [8] S. Lagomarsino, F. Scarinci, C. C&mini, Phys. Rev. B 46 (1992) 13631.
(1997) 765-770
[9] E. Vlieg, A.E.M.J. Fischer, J.F. van der Veen, B.N. Dev, G. Materlik, Surf. Sci. 178 (1986) 36. [lo] H. Mori, M. Ogasawara, M. Yamamoto, M. Tachikawa, Appl. Phys. Lett. 51 (1987) 1245. [l l] M. Tachikawa, H. Mori, M. Sugo, Y. Itoh, Jpn. J. Appl. Phys. 32 (1993) L1252. [12] D.A. Neumann, H. Zabel, J. Appl. Phys. 61 (1987) 1023. (131 W. Stolz, F.E.G. Guuimaraes, K. Ploog, J. Appl. Phys. 63 (1988) 492.
Applied Surface Science 117/118 (1997) 765-770
Initial domain structure of GaAs thin films grown on Si(OO1) substrates Tomoaki Kawamura a,*, Hisataka Takenaka b, Takayoshi Hayashi ‘, Masami Tachikawa d, Hidefumi Mori d a NTT Basic Research Laboratories, 3-1, Morinosato, Wakamiya, Atsugi, Kanagawa 243-01, Japan ’ NlTAduanced Technology Corporation, 3-9-11, Midori-cho, Musashino, Tokyo 180, Japan ’ NIT Integrated Information and Energy Systems Laboratories, 3-9-l I, Midori-cho, Musashino, Tokyo 180, Japan d NTT Opto-Electronics Laboratories, 3-l Morinosato, Wakamiya, Atsugi, Kanagawa 243-01, Japan
Abstract The initial domain structure of GaAs films grown on several Si(OO1) surfaces is investigated using X-ray standing waves. GaAs/Si(OOl) samples, 4 ML thick, grown on three different Si substrates were used: an epitaxial Si surface (ESS), a mechanochemically polished surface (MCP), and a mechanochemically polished surface with plasma cleaning (plasma MCP). The domain ratio ambiguousness due to the film thickness is avoided by observing independent Bragg reflections of Si substrates. The results of X-ray standing wave measurement reveal that all daAs films have double domain s&uctures at the initial stage, even though final domain structures are single. The ratio of the two domains was almost 1 : 1 on the MCP surface, 6 : 4 on the ESS surface, and 4.5 : 5.5 on the plasma MCP surface. The dominant GaAs domains on the ESS and plasma MCP surfaces were the same as those obtained on thicker GaAs films. This suggests there is a rapid decrease in the GaAs domain during the early stages of growth on an ESS and plasma MCP surfaces.
1. Introduction The epitaxial growth of III-V semiconductors on Si has attracted the interest of many researchers in recent years [l]. One of the fundamental problems concerning the epitaxy of polar semiconductors on (001) non-polar semiconductor substrates is the formation of antiphase boundaries and antiphase domains (APDs) [2]. The formation of antiphase free domains were previously thought to be due to the absence of single atomic steps at the Si surface [2]. Recent STM studies, however, contradict this view
* Corresponding [email protected].
author. Tel.: + 81-462-402035;
0169-4332/97/$17.00 Copyright PN SO169-4332(97)00133-5
e-mail: kawa-
[4]: they suggest double domain existence and annihilation during the growth process. Many techniques have been used to detect the presence of antiphase domains experimentally, including anisotropic etching [5], X-ray diffraction [6], and RHEED [2]. The use of anisotropic etching and X-ray diffraction is limited to GaAs layers thicker than 10 nm. The RHEED technique is only usable under UHV circumstances and is not valid for gasphase growth. The X-ray standing wave technique, which uses the interference effect of incident and diffracted Xrays, has been known as a powerful tool for determining surface and interface structures [7]. By observing the fluorescence yields from the epitaxial layers, one can get information about the position of
0 1997 Elsevier Science B.V. All rights reserved.
766
T. Kawamura et al/Applied
Surface Science 117/118
atoms at the interfaces and on the surfaces. However, there is still difficulty in determining the interface structures due to the thickness ambiguity of epitaxial layers. When X-ray standing waves are formed in the epitaxial layers, each atom senses the different X-ray intensity due to the different d-spacing between substrates and the epitaxial layers. This results in a different fluorescence yield profile from each atomic plane, giving the uncertainty in determining the interface structure and the domain ratio. This effect is more significant for large mismatch systems, such as GaAs/Si, which has a 4% lattice mismatch value. To solve this difficulty, we proposed the new method using the independent Bragg reflection for determining the domain ratio independently of epitaxial layer thickness [3]. In this work, we describe the principle of this new method and, using the method, we investigate different GaAs/Si interface structures made by gas-phase growth. One such structure is grown on an epitaxial silicon surface, and the others are grown on a mechanochemically polished silicon surface with and without plasma cleaning prior to the growth. Finally, we discuss the initial growth stage for these GaAs/Si systems.
(1997) 765-770
Here, e ia = x/l
XI,
Y=(di-ds)/ds7
(3)
where di and d, are the d-spacing value of the epitaxial layer and substrate crystals. This result shows that the thickness effect is included in the standing wave parameters, F and Z. If more than one site is occupied, the resulting Z and F in Eq. (1) are described as the summation of positions zi and probabilities cri of the individual sites [8]: Fe2”iZ
=fCgi
e*“iZi,
(4)
where Cgi = 1 and f is the ratio of ordered atoms to the total number of atoms. By determining ai from X-ray standing wave measurements, one can determine the domain ratio of GaAs films. Assuming that the Ga and As atoms occupy only two sites on a (111) reflecting plane, we can describe
2. Bases When X-rays are introduced into the crystal at the Bragg conditions, the incident and diffracted X-rays interfere, and make standing waves in the crystal and on the surface. By analyzing the fluorescence yield, If dependence on the Bragg reflection, from atoms in the X-ray fields one can obtain information about the atomic location, Z, normalized by the lattice spacing and the probability of occupation, F, of the atoms
b31. If= 1 + 1,y)* + 2Fa{
~.e-‘~~‘}, (1) where x is the complex reflectivity. The thickness effect of overlayers is included as follows [9]:
sin( rrNy) = 1 +]#+~F]x] X%{e-
IV sin( 7ry) 27ri(Z+t?r(N- I)y)+ ai
1.
(2)
Fig. 1. Domain structure of GaAs films on Si(OO1) substrate. In this figure, the As atoms are taken to connected with the Si atoms. In (a) and (b), the Si dimers are perpendicular and parallel to the step edge, respectively. It was not determined by X-ray standing waves whether the interface atoms between the GaAs and the Si are As or not.
T. Kawamura et al/Applied
positions and F,il
z 1,, and ~~7, and coherent based on Eq. (4):
F,,, c2”izl~l =f,,,a
c2?riz +fill(l
factors
Surface Science 117/ 118 (1997) 765-770
F, 1,
- a) e2ri(z+A), (5)
F _ e2rizlil =fii,(l Ill
_ c) e*niz +fiila
e*pi(z+A), (6)
where f, I, and fii, are the ratios of the number of ordered atoms to the total number of atoms for the (111) and (111) planes, respectively. Parameter (+ is the probability of atoms existing at site 1 (Fig. la and b); it corresponds to the domain ratio of the GaAs films. If the GaAs film has a completely single domain structure, (+ should be equal to 1.0 or 0.0. The z and A represent the normalized atomic position and the atomic distance between the two sites on the (111) plane, respectively; A is 0.25 for an Sic1 11) plane and 0.26 for a GaAs(ll1) plane with a 4% lattice mismatch. By considering the algebraic relations between these parameters, one can calculate fclllj, fcli1), (+, and z from A. Note that thickness ambiguity only affect the and z. This feature makes it value of fii~ t fi I I possible to decide the domain ratio, (+, independently of the thickness of the GaAs layer.
167
except for a conventional thermal Si-surface treatment prior to the GaAs growth. The interfaces of the GaAs/Si(lOO) samples were characterized using cross sectional high resolution transmission micrography. Flat GaAs islands about 10 ML thick and lo-20 nm in diameter were observed, suggesting island growth. Significantly, the lattice images between the Si and GaAs layers were maintained over most of the GaAs/Si interface, making it possible to analyze the interface structure using the X-ray standing waves. The X-ray standing wave was analyzed with a laboratory instruments. A symmetrical incident
300
E
400
500,
Channel number
1.5
; easured (Ga K) + Calculated
F10.26 Z=O.O6 -
1.0 1.5
0.5 3. Experimental GaAs/Si(lOO) samples were grown by vapour mixing epitaxy (VME) [ 10,111, which is based on hydride vapour phase epitaxy (hydride VPE). The substrate used in our experiments was an epitaxial Si(OO1) substrate, misaligned 2” toward the (110) direction: it had a 5-pm layer of homoepitaxial Si grown by atmospheric-pressure Si,H,Cl, chemical vapour phase deposition. The substrate was dipped in a 7% HF aqueous solution for 30 s for passivation by hydrogen termination [lo]. No thermal treatment was performed in the growth chamber before GaAs growth. After it was introduced into the growth chamber, GaCl-ASH, of 4-monolayer quantities was provided for GaAs growth. The growth temperature was 400°C. This procedure was also used to fabricate the GaAs/Si(OOl) samples on an MCP substrate,
1.0 1.0
-A
Si(ll1)-
0.5
0.5
0.0 -15
-10
-5
0
5
10
15
Angle (normalized) Fig.2. Typical fluorescence
yield profiles from Ga and As atoms of ESS GaAs/Si(OOl). The horizontal axis is normalized by the theoretical full width at half maximum. The solid lines with marks are the measured fluorescence yields of Ga and As K (Y; the dashed lines show the calculated yields. Coherent factor F and normalized position Z are as shown in the figure.
768
T. Kawamura et al./Applied
Surface Science 117/118
monochromator was used to increase the beam intensity. A molybdenum target was used to activate the Ga and As fluorescence yield. The secondary radiation, including the Ga and As Ka signals, was monitored with a pure Ge type solid state detector and accumulated into a multi-channel analyzer. The Ga and As fluorescence yield profiles were obtained after background subtraction from the fluorescence spectra. All instruments, except the X-ray source generator, were set in a thermo-stabilized room to avoid the drift of the Bragg peak due to thermal fluctuation. To increase the signal-to-noise ratio, both the Bragg reflection and the fluorescence yields were measured repeatedly. The repetition error was estimated to be less than 0.1 arcsecs from the Bragg reflection profiles. Fig. 2 shows typical fluorescence yields for both Ga and As K (Y. The inset shows the Ga and As Ka fluorescences. Due to the small Bragg angle (6.496”), the asymmetric effect of the samples needed to be evaluated [8]. By measuring the Bragg reflection profile dependence on the tilt angle of the samples, we obtained the asymmetry factor of the samples and used it in our analysis. Note that the intensity of As K (Y is 1S-3.0 times greater than that of Ga Ka for each sample. This suggests the possibilities of the ratio between Ga and As atoms being different from 1: 1, and the existence of an extra As atoms in the GaAs films.
4. Results and discussion By fitting the measured data to Eq. (11, we obtained normalized position Z and coherent factor F of the (111) and (111) planes. Table 1 summarizes coherent factor F and atomic position Z for each
(1997) 765-770
sample with R factors. An analysis of Table 1 reveals several points. (1) For ESS and plasma MCP GaAs/Si(OOl), the difference in position parameter Z between (111) and (lil) was smaller than 0.25, suggesting the existence of two domains. The dominant domain for ESS is Type B, while for plasma MCP it is Type A in Fig. la and b. If only a single domain exists, the difference in the position parameter is expected to be 0.25. (2) For MCP GaAs/Si(OOl), the Ga and As atoms had the same position parameter, showing that the domain ratio was almost 1: 1. (3) The As coherent factor was lower than that of Ga for all samples. This suggests the existence of extra amorphous As atoms in GaAs films, consistent with the results of our fluorescence yield analysis. As previously reported [lo], a single GaAs domain 100 nm thick can be obtained on ESS and plasma MCP surfaces. According to etch pit observations [5], the domain structures are Type B and A, respectively, as shown in Fig. 1. In contrast, only a double domain was obtained for GaAs films grown on MCP Si substrates [ll]. These results are qualitatively consistent with our X-ray standing wave results, except for the domain ratio during the initial growth stage. Fig. 3a and b show the dependence of cr on A for ESS and plasma MCP GaAs/Si(OOl). Assuming the interface models shown in Fig. 1, aoa and uAs represent the domain ratios of Type A and Type B, respectively. It should be noted that the X-ray standing wave technique determines the probability of each atom for both sites; it can not distinguish the first atoms directly connected with the Si atoms at the interface. Considering the vertical lattice expansion of GaAs
Table 1 Experimental results (F,
z) and estimated domain ratio D
%I,)
Z(lll)
Qilj
Z(lil)
(T
0.34 + 0.03 0.18 f 0.06 -
0.18 k 0.03 0.09 f 0.06 -
0.69 f 0.08 0.50 + 0.03 0.46 + 0.02
ESS
Ga
0.26 f 0.02
MCP
AS Ga
0.17 + 0.04 0.52 + 0.09
0.06 + 0.02 0.17 + 0.02 0.18 f 0.02
AS
0.28 f 0.07
0.16 + 0.02
Plasma MCP
Ga
0.53 + 0.18
0.19 * 0.02
0.55 + 0.15
0.16 f 0.03
As
0.04 + 0.02
0.15 f 0.08
0.23 f 0.9
0.21 + 0.03
T. Kawamura
et al./Applied
Surface Science 117/118
(1997) 765-770
169
have been obtained with a 100 nm thicknesses on ESS and plasma MCP Si, this can be explained by the rapid decline of non-dominant domains. Even with the atomic force microscopy observation [ 101, it is not clear whether the existence of both Ga-Si and As-Si bondings at the interface or single atomic steps are important in forming a double domain structure. For MCP GaAs/Si, in addition to the Ga and As connection with Si, the existence of single steps on the Si surface is expected, causing a completely double domain structure at the beginning of the growth. However, quantitative analysis of the domain ratio under several initial growth conditions is still necessary to understand the domain formation mechanism during initial growth.
1.0 ;a .g 0.8 "
0.2
5. Conclusions Difference between two sites(Delta)
B 1.0 -
-j+.* g n 2 g
e+... 0.4 -
0.2 -
00’
'0.0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 Difference between two sites(Delta)
Fig. 3. Probability (r and (b) plasma MCP consists of only Type dashed line is the site
dependence GaAs films A, croGa= 0 distance for
on site distance A for (a) ESS on Si(OO1). If the GaAs film and a,, = 1 are expected. The the Si-Si.
to horizontal compression [ 12,131, we limited the range of A in Eq. (5) and Eq. (6) to be from 0.26 to about 0.27. Row 7 in Table 1 shows the estimated domain ratio for each samples with the value A = 0.26. Remembering that single domain GaAs films
due
In conclusion, X-ray standing wave measurements of the initial GaAs domain structure on ESS, MCP and plasma MCP Si(OO1) surfaces showed a double domain for GaAs films on both surfaces. The domain ratio for MCP GaAs/Si(OOl) was equal to 0.5 : 0.5, suggesting a completely double domain structure. For ESS and plasma MCP GaAs/Si(OOl), although single domain GaAs films were obtained, the initial domain structure was double with a ratio of about 0.7 : 0.3 and 0.45 : 0.55, respectively; the dominant domain was the same as that finally obtained. Considering the previous results for GaAs films on both Si substrates, this can be explained by the rapid decrease in the initially existing recessive domain of the GaAs films during subsequent film growth. It should be noted that the reason the GaAs double domain structures are formed in the initial stage is not clear; further investigation of different substrates and growth conditions is thus required.
References 111Heteroepitaxy
on Silicon, NRS Symp. Proc: (a) Vol. 67, edited by J.C.C. Fan and J.M. Poate (1986); (b) Vol. 91, edited by J.C.C. Fan, J.M. Phillips and B.Y. Tsaur (1987); (c) Vol. 116, edited by H.K. Choi, R. Hull, H. Ishikawa and R.J. Nemanich (MRS, Pittsburgh, PA, 1988). 121S.F. Fang, K. Adomi, S. Iyer, H. Morkoq, H. Zabel, C. Choi, N. Otsuka, J. Appl. Phys. 68 (1990) R31. 131 T. Kawamura, H. Takenaka, T. Hayashi, M. Tachikawa, H. Mori, Appl. Phys. Lett. 68 (1996) 1969.
170
T. Kawamura et al. /Applied
Surface Science 117/118
[4] R.M. Tromp, R.J. Harmers, J.E. Demuth, Phys. Rev. Lett. 59 (1987) 1691. [5] K. Fujita, Y. Shiba, T. Yamamoto, J. Cryst. Growth 99 (1990) 341. [6] D.A. Neuman, H. Zabel, R. Fischer, H. Morko9, J. Appl. Phys. 61 (1987) 1023. [7] T. Kawamura, H. Takenaka, M. Uneta, M. Oshima, Y. Fukuda, Y. Ohmachi, K. Izumi, T. Ishikawa, S. Kikuta, Z.W. Zhang, Jpn. J. Appl. Phys. 32 (1993) 622. [8] S. Lagomarsino, F. Scarinci, C. C&mini, Phys. Rev. B 46 (1992) 13631.
(1997) 765-770
[9] E. Vlieg, A.E.M.J. Fischer, J.F. van der Veen, B.N. Dev, G. Materlik, Surf. Sci. 178 (1986) 36. [lo] H. Mori, M. Ogasawara, M. Yamamoto, M. Tachikawa, Appl. Phys. Lett. 51 (1987) 1245. [l l] M. Tachikawa, H. Mori, M. Sugo, Y. Itoh, Jpn. J. Appl. Phys. 32 (1993) L1252. [12] D.A. Neumann, H. Zabel, J. Appl. Phys. 61 (1987) 1023. (131 W. Stolz, F.E.G. Guuimaraes, K. Ploog, J. Appl. Phys. 63 (1988) 492.