- Email: [email protected]
Epitaxial growth of GaAs on HF-treated Si substrates
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applii surface science Applied Surface Science lOiI/lOl (1996) 478-481
Epitaxial growth of GaAs on HF-treated Si substrates Yoko Uchida ‘,*, Junko Minemura ‘, Yoshiaki Yazawa ‘, Terunori Warabisako
b
Received 15 August 1995; accepted I1 December 1995
Abstract Introduction of a Ga layer on an HF-treated Si substrate and its effect on the generation and behavior of dislocations in the over-grown GaAs layers are studied. There are no phases except for the crystal phase and many dislocations are observed in the lattice image at the GaAs/Si interface, although an amorphous Ga layer on the hydrogen-terminated Si is observed by
RHEED before the growth of a GaAs layer. In spite of the high dislocation density at the GaAs/Si interface, the dislocation density at the GaAs surface is the same as that of conventional structures. This indicates that initial growth condition may control the formation of dislocations.
1. Introduction Heteroepitaxial growth of GaAs on Si has been attracting attention for its application to monolithic integration of GaAs and Si devices. However. so far we have not been able to obtain a good quality heterostructure because structural defects degrade electrical and optical properties of the heterostructure, in spite of many improvements in growth techniques. The structural defects are generated by the distinctions of lattice constant. thermal expansion and atomic electrical polarity between GaAs and Si. Recently several new approaches from other perspectives have been tried to avoid generation of dislocations as a surfactant epitaxy [ 1,2] or Van der Waals
* Corresponding author. Tel.: + 8 1-423-23 I 1I 1:fax: + e-mail: [email protected].
237714:
8 I-423.
epitaxy [3]. These approaches involve the idea that the generation of dislocations is suppressed by modifying the surface properties, which provokes the consequent growth mode transition. In fact, during growth of highly strained InGaAs layers on GaAs with a preadsorbed Te surfactant layer, two-dimensional (2D) layer-by-layer growth can be dominant instead of 3D island growth, even beyond the critical thickness [4]. The heteroepitaxy between layered materials can be realized without the generation of dislocations even though there is a large lattice mismatch [5]. We have approached the advanced method of the surface modification for GaAs/Si system. It is well known that HF-treatment for Si not only removes surface oxide but also produces chemically stable surfaces because the surface dangling bonds are terminated by hydrogen [6.7]. Therefore, the layer should grow incoherently on the HF-treated Si without the generation of dislocations. On the other hand.
0169.4332/96/$15.00 Copyright 6 1996 Elsrvier Science B.V. All rights reserved. PII SO 169.3332(96)00322-3
Y. Uchida et al. /Applied
Surface Science 100 / 101 C1YYh) 478-481
if a Ga layer is adopted as the initial layer of a GaAs layer, its surface can easily be transformed into a GaAs epitaxial layer by As effusion. We tried introducing a Ga layer on an HF-treated Si substrate to improve the quality of a GaAs layer on a Si substrate. In this study, we report the effects of introducing the Ga layer onto the HF-treated Si substrate on the generation and behavior of dislocations in the over-grown GaAs layer.
2. Experimental The samples were prepared by molecular-beam epitaxy (MBE) on HF-treated Si (100) substrates with 2” misorientation toward (0 11). The substrates were degreased in organic solvents and oxidized in an HCl : H,OZ : H,O = 1 : 1 : 3 mixture. The substrate was subsequently HF-treated to remove chemical oxide and to passivate the surface by hydrogen. The HF treatment was as follows: (11 the substrate was dipped in 5% HF solution for 30 s, (2) rinsed with de-ionized water for 10 s, and blown with dry N,. The HF-treated substrates were immediately loaded into an ultra-high vacuum growth chamber. In the sample with a Ga layer, the Ga was evaporated by Knudsen cell used for MBE, and metal coverage was estimated from the growth rate of GaAs. After the Ga layer was formed, the As cell temperature was heated to deposit As. Ten layers of Ga and As monolayers were alternately formed, except for an initial Ga layer, at a substrate temperature of 300°C. Then a 4 p.m thick GaAs layer was grown at 500°C. A conventionally fabricated sample was also prepared for comparison. This sample was annealed at high temperature to remove the surface oxide and formed by the two-step growth method [8]. Reflection high-energy electron diffraction (RHEED) method was used to monitor surface structures in the growth chamber. A scanning tunneling microscopy @TM) chamber was connected with the growth chamber so that we could observe a sample prepared in the growth chamber without exposing it to air. All STM images were taken at room temperature. We used cross-sectional transmission electron microscopy (TEM) to evaluate dislocations near the interface between GaAs and Si, and in the GaAs layer.
479
3. Results and discussion 3. I. Obsermtion
of initial growth stage
Fig. 1 shows RHEED patterns from the HF-treated Si (100) surfaces taken at [Ol l] incidence before Ga deposition, after Ga deposition and consequently after As deposition. After HF treatment, the RHEED patterns of the (1 X 1) surface structure are observed from the Si (100) substrate, as shown in Fig. l(a). Sakurai and Hangstrum reported that the surface atoms of Si were terminated with hydrogen and not reconstructed 191. The structure is maintained up to the substrate temperature of 400°C. Above 400°C. the RHEED patterns change in sequence into (2 X 11 structures to desorb hydrogen atoms that terminate Si surface atoms [lo, 111. A halo pattern simultaneously overlaps the RHEED pattern of the (1 X 1) surface structure after Ga deposition. For over ten monolayers (ML) of deposition, only the halo pattern is observed, as shown in Fig. l(b). According to reports on the behavior of Ga on Si (loo), Ga adsorption results in the formation of Ga-Ga dimers. and these dimers arrange themselves to form regions of (2 X 3) symmetry and other symmetries at low coverage below one monolayer [ 12- 141. However, except for the halo, no patterns appear after Ga deposi-
03 after Ga deposition
1 ML
IOML
Fig. I. RHEED patterns from the HF-treated Si (100) surfaces with [Ol l] incidence (a) before Ga deposition, (b) after Ga deposition of 1 ML and 10 ML, Cc) consequently after As deposition.
380
Y. Uchida et al./Applied
Suface Science 100 / 101 Cl9961 478-481
(4
mass of Si surface atoms enveloped in Ga. The substrate surface is not flat enough for Ga droplets to form, because the substrate is not annealed at high temperature. for instance over 800°C. 3.2. Evaluation
The structural defects in the GaAs layer on Si was observed by cross-sectional transmission electron microscopy (TEM). Fig. 3 shows the TEM micrographs of the GaAs/Si heterostructures. In the sample in Fig. 3(a), a Ga layer is introduced on the HF-treated Si substrate before the formation of a GaAs layer. On the other hand, the sample in Fig. 3(b) is fabricated by a conventional method with high temperature annealing before the growth of GaAs layer and two-step growth [s], and has no Ga inter-layer. More dislocations are generated at the interface between GaAs and Si in the sample with a Ga layer on the HF-treated Si than are generated in
I-+
1Onm
1 nm Fig. 2. STM images of the HF-treated layer of 10 ML.
of epitaxial layer
(4 Si (100) covered with a Ga
tion on the HF-treated Si (100). This result suggests that the Ga layer on the HF-treated Si (100) does not have an ordered phase, but instead has an amorphous phase. This pattern changes into a spotty pattern with As deposition or As atmosphere as shown in Fig. l(c). By the start of GaAs growth, the pattern changes to fine streaks. On the other hand, when As is deposited on HF-treated Si, the pattern does not change right after deposition but gradually becomes spotty, and twin spots or mesh patterns appear. These patterns remain after Ga deposition. The result reflects an amplitude of surface roughness, probably because of As precipitation. STM images are taken from a sample covered with 10 ML of Ga (Fig. 2). The surface of the sample is covered with clusters that have 5-10 nm diameters (Fig. 2(a)). A magnified image reveals that several particles with 1 nm diameters are enveloped in a thin film and form a cluster (Fig. 2(b)). We suppose that the clusters are not droplets of Ga but a
i,,
W
Fig. 3. Cross-sectional TEM micrographs of the GaAs/Si heterostructures (a) with a Ga inter-layer on the HF-treated Si substrate and (b) prepared by a conventional method.
Y. Uchida et al/Applied
Surface Science lOO/ 101 (1996) 478-481
the conventional sample. However, the rate of dislocations that climb towards the surface is not high. In fact, the dislocation density at the GaAs surface is the same as that of the conventional sample, which is 5 X lo7 cm-*. One reason is that the principal dislocations run parallel to the growth direction, not along the inclined plane; therefore, the dislocations have low mobility. Another reason is that the generation of dislocations is suppressed in the GaAs layer because a lot of dislocations are formed at the interface between GaAs and Si, and consequently a large relaxation generates in the GaAs layer. High resolution lattice images at the interface between GaAs and Si show ordered atomic layers, even in the sample with a Ga layer, though RHEED patterns indicate an amorphous phase as mentioned above. The reason is that hydrogen desorbs from Si surface atoms and rearrangement of bonds takes place during GaAs growth at 500°C. However, there are several differences in the images. More stacking faults along (1111, which is the inclined plane, are observed than in the conventional sample, and a few discrepancies of atomic rows are seen along the growth direction. These results show that the initial growth condition has a critical effect on the behavior of dislocations. In this study, dislocations unlikely to climb can be generated by introducing a Ga inter-layer, although the generation of dislocations is not inhibited.
4. Conclusion We introduced a Ga layer on an HF-treated Si (100) substrate to study its effect on the generation and behavior of dislocations in the over-grown GaAs layer. The Ga layer on the HF-treated Si substrate had an amorphous phase and covered the Si surface, forming clusters with 5-10 nm diameters. These appear to be several Si atoms enveloped in thin film.
481
After the growth of a GaAs over-layer, the amorphous phase was no longer observed, and a lot of dislocations were generated at the interface between GaAs and Si. However, the climb of dislocations towards the surface was suppressed. The initial growth conditions are important for controlling the formation of dislocations.
Acknowledgements This work has been partly supported by the New Energy and Industrial Technology Development Organization (NED01 as part of the ‘New Sunshine Program’ under the Ministry of International Trade and Industry.
References [I] M. Copel, M.C. Reuter, E. Kaxiras and R.M. Tromp, Phys. Rev. Lett. 63 (1989) 632. [2] .I. Massies, N. Grandjean and V.H. E&ens, Appl. Phys. Lett. 61 (1992) 99. [3] A. Koma, K. Ueno and K. Saiki, .I. Cryst. Growth 111 (1991) 1029. [4] N. Grandjean, J. Massies, C. Delamarre, L.P. Wang, A. Dubon and J.Y. Laval, Appl. Phys. Lett. 63 (1993) 66. [5] A. Koma, K. Sunouchi and T. Miyajima, J. Vat. Sci. Technol. B 3 (1985) 724. [6] M. Grundner and H. Jacob, Appl. Phys. A 39 (1986) 73. [7] Y.J. Chabal, G.S. Higashi, K. Raghavachari and V.A. Burrows, J. Vat. Sci. Technol. A 7 (1989) 2104. [8] Y. Uchida, Y. Yazawa and T. Warabisako, Appl. Phys. Lett. 67 (1995) 127. [9] T. Sakurai and H.D. Hangstrum, Phys. Rev. B 14 (1976) 1593. [lo] S.S. Iyer, M. Arienzo and E. de Fresart, Appl. Phys. Lett. 57 (1990) 893. [ 111 B.S. Meyerson, F.J. Himpsel and K.J. Uram, Appl. Phys. Lett. 57 (1990) 1034. [12] A.A. Baski, J. Nogami and CF. Quate, J. Vat. Sci. Technol. A 8 (1990) 245. [13] J. Nogami, S. Park and C.F. Quate, Appl. Phys. Lett. 53 (1988) 2086. [14] T. Sakamoto and H. Kawanami, Surf. Sci. 111 (1981) 177.
__ . .!!I3
ELSEVIER
..:. .\ .,.,:.i..:‘:::;.~:~~:ii’I-i:lj.f
:’ ::., :. .,
,‘y:..:::.‘:;:::;:: ‘_::::.?::;,‘,y
.:‘i,:,:,,:,: .’ _._)>
,.;,: ::::
,...
applii surface science Applied Surface Science lOiI/lOl (1996) 478-481
Epitaxial growth of GaAs on HF-treated Si substrates Yoko Uchida ‘,*, Junko Minemura ‘, Yoshiaki Yazawa ‘, Terunori Warabisako
b
Received 15 August 1995; accepted I1 December 1995
Abstract Introduction of a Ga layer on an HF-treated Si substrate and its effect on the generation and behavior of dislocations in the over-grown GaAs layers are studied. There are no phases except for the crystal phase and many dislocations are observed in the lattice image at the GaAs/Si interface, although an amorphous Ga layer on the hydrogen-terminated Si is observed by
RHEED before the growth of a GaAs layer. In spite of the high dislocation density at the GaAs/Si interface, the dislocation density at the GaAs surface is the same as that of conventional structures. This indicates that initial growth condition may control the formation of dislocations.
1. Introduction Heteroepitaxial growth of GaAs on Si has been attracting attention for its application to monolithic integration of GaAs and Si devices. However. so far we have not been able to obtain a good quality heterostructure because structural defects degrade electrical and optical properties of the heterostructure, in spite of many improvements in growth techniques. The structural defects are generated by the distinctions of lattice constant. thermal expansion and atomic electrical polarity between GaAs and Si. Recently several new approaches from other perspectives have been tried to avoid generation of dislocations as a surfactant epitaxy [ 1,2] or Van der Waals
* Corresponding author. Tel.: + 8 1-423-23 I 1I 1:fax: + e-mail: [email protected].
237714:
8 I-423.
epitaxy [3]. These approaches involve the idea that the generation of dislocations is suppressed by modifying the surface properties, which provokes the consequent growth mode transition. In fact, during growth of highly strained InGaAs layers on GaAs with a preadsorbed Te surfactant layer, two-dimensional (2D) layer-by-layer growth can be dominant instead of 3D island growth, even beyond the critical thickness [4]. The heteroepitaxy between layered materials can be realized without the generation of dislocations even though there is a large lattice mismatch [5]. We have approached the advanced method of the surface modification for GaAs/Si system. It is well known that HF-treatment for Si not only removes surface oxide but also produces chemically stable surfaces because the surface dangling bonds are terminated by hydrogen [6.7]. Therefore, the layer should grow incoherently on the HF-treated Si without the generation of dislocations. On the other hand.
0169.4332/96/$15.00 Copyright 6 1996 Elsrvier Science B.V. All rights reserved. PII SO 169.3332(96)00322-3
Y. Uchida et al. /Applied
Surface Science 100 / 101 C1YYh) 478-481
if a Ga layer is adopted as the initial layer of a GaAs layer, its surface can easily be transformed into a GaAs epitaxial layer by As effusion. We tried introducing a Ga layer on an HF-treated Si substrate to improve the quality of a GaAs layer on a Si substrate. In this study, we report the effects of introducing the Ga layer onto the HF-treated Si substrate on the generation and behavior of dislocations in the over-grown GaAs layer.
2. Experimental The samples were prepared by molecular-beam epitaxy (MBE) on HF-treated Si (100) substrates with 2” misorientation toward (0 11). The substrates were degreased in organic solvents and oxidized in an HCl : H,OZ : H,O = 1 : 1 : 3 mixture. The substrate was subsequently HF-treated to remove chemical oxide and to passivate the surface by hydrogen. The HF treatment was as follows: (11 the substrate was dipped in 5% HF solution for 30 s, (2) rinsed with de-ionized water for 10 s, and blown with dry N,. The HF-treated substrates were immediately loaded into an ultra-high vacuum growth chamber. In the sample with a Ga layer, the Ga was evaporated by Knudsen cell used for MBE, and metal coverage was estimated from the growth rate of GaAs. After the Ga layer was formed, the As cell temperature was heated to deposit As. Ten layers of Ga and As monolayers were alternately formed, except for an initial Ga layer, at a substrate temperature of 300°C. Then a 4 p.m thick GaAs layer was grown at 500°C. A conventionally fabricated sample was also prepared for comparison. This sample was annealed at high temperature to remove the surface oxide and formed by the two-step growth method [8]. Reflection high-energy electron diffraction (RHEED) method was used to monitor surface structures in the growth chamber. A scanning tunneling microscopy @TM) chamber was connected with the growth chamber so that we could observe a sample prepared in the growth chamber without exposing it to air. All STM images were taken at room temperature. We used cross-sectional transmission electron microscopy (TEM) to evaluate dislocations near the interface between GaAs and Si, and in the GaAs layer.
479
3. Results and discussion 3. I. Obsermtion
of initial growth stage
Fig. 1 shows RHEED patterns from the HF-treated Si (100) surfaces taken at [Ol l] incidence before Ga deposition, after Ga deposition and consequently after As deposition. After HF treatment, the RHEED patterns of the (1 X 1) surface structure are observed from the Si (100) substrate, as shown in Fig. l(a). Sakurai and Hangstrum reported that the surface atoms of Si were terminated with hydrogen and not reconstructed 191. The structure is maintained up to the substrate temperature of 400°C. Above 400°C. the RHEED patterns change in sequence into (2 X 11 structures to desorb hydrogen atoms that terminate Si surface atoms [lo, 111. A halo pattern simultaneously overlaps the RHEED pattern of the (1 X 1) surface structure after Ga deposition. For over ten monolayers (ML) of deposition, only the halo pattern is observed, as shown in Fig. l(b). According to reports on the behavior of Ga on Si (loo), Ga adsorption results in the formation of Ga-Ga dimers. and these dimers arrange themselves to form regions of (2 X 3) symmetry and other symmetries at low coverage below one monolayer [ 12- 141. However, except for the halo, no patterns appear after Ga deposi-
03 after Ga deposition
1 ML
IOML
Fig. I. RHEED patterns from the HF-treated Si (100) surfaces with [Ol l] incidence (a) before Ga deposition, (b) after Ga deposition of 1 ML and 10 ML, Cc) consequently after As deposition.
380
Y. Uchida et al./Applied
Suface Science 100 / 101 Cl9961 478-481
(4
mass of Si surface atoms enveloped in Ga. The substrate surface is not flat enough for Ga droplets to form, because the substrate is not annealed at high temperature. for instance over 800°C. 3.2. Evaluation
The structural defects in the GaAs layer on Si was observed by cross-sectional transmission electron microscopy (TEM). Fig. 3 shows the TEM micrographs of the GaAs/Si heterostructures. In the sample in Fig. 3(a), a Ga layer is introduced on the HF-treated Si substrate before the formation of a GaAs layer. On the other hand, the sample in Fig. 3(b) is fabricated by a conventional method with high temperature annealing before the growth of GaAs layer and two-step growth [s], and has no Ga inter-layer. More dislocations are generated at the interface between GaAs and Si in the sample with a Ga layer on the HF-treated Si than are generated in
I-+
1Onm
1 nm Fig. 2. STM images of the HF-treated layer of 10 ML.
of epitaxial layer
(4 Si (100) covered with a Ga
tion on the HF-treated Si (100). This result suggests that the Ga layer on the HF-treated Si (100) does not have an ordered phase, but instead has an amorphous phase. This pattern changes into a spotty pattern with As deposition or As atmosphere as shown in Fig. l(c). By the start of GaAs growth, the pattern changes to fine streaks. On the other hand, when As is deposited on HF-treated Si, the pattern does not change right after deposition but gradually becomes spotty, and twin spots or mesh patterns appear. These patterns remain after Ga deposition. The result reflects an amplitude of surface roughness, probably because of As precipitation. STM images are taken from a sample covered with 10 ML of Ga (Fig. 2). The surface of the sample is covered with clusters that have 5-10 nm diameters (Fig. 2(a)). A magnified image reveals that several particles with 1 nm diameters are enveloped in a thin film and form a cluster (Fig. 2(b)). We suppose that the clusters are not droplets of Ga but a
i,,
W
Fig. 3. Cross-sectional TEM micrographs of the GaAs/Si heterostructures (a) with a Ga inter-layer on the HF-treated Si substrate and (b) prepared by a conventional method.
Y. Uchida et al/Applied
Surface Science lOO/ 101 (1996) 478-481
the conventional sample. However, the rate of dislocations that climb towards the surface is not high. In fact, the dislocation density at the GaAs surface is the same as that of the conventional sample, which is 5 X lo7 cm-*. One reason is that the principal dislocations run parallel to the growth direction, not along the inclined plane; therefore, the dislocations have low mobility. Another reason is that the generation of dislocations is suppressed in the GaAs layer because a lot of dislocations are formed at the interface between GaAs and Si, and consequently a large relaxation generates in the GaAs layer. High resolution lattice images at the interface between GaAs and Si show ordered atomic layers, even in the sample with a Ga layer, though RHEED patterns indicate an amorphous phase as mentioned above. The reason is that hydrogen desorbs from Si surface atoms and rearrangement of bonds takes place during GaAs growth at 500°C. However, there are several differences in the images. More stacking faults along (1111, which is the inclined plane, are observed than in the conventional sample, and a few discrepancies of atomic rows are seen along the growth direction. These results show that the initial growth condition has a critical effect on the behavior of dislocations. In this study, dislocations unlikely to climb can be generated by introducing a Ga inter-layer, although the generation of dislocations is not inhibited.
4. Conclusion We introduced a Ga layer on an HF-treated Si (100) substrate to study its effect on the generation and behavior of dislocations in the over-grown GaAs layer. The Ga layer on the HF-treated Si substrate had an amorphous phase and covered the Si surface, forming clusters with 5-10 nm diameters. These appear to be several Si atoms enveloped in thin film.
481
After the growth of a GaAs over-layer, the amorphous phase was no longer observed, and a lot of dislocations were generated at the interface between GaAs and Si. However, the climb of dislocations towards the surface was suppressed. The initial growth conditions are important for controlling the formation of dislocations.
Acknowledgements This work has been partly supported by the New Energy and Industrial Technology Development Organization (NED01 as part of the ‘New Sunshine Program’ under the Ministry of International Trade and Industry.
References [I] M. Copel, M.C. Reuter, E. Kaxiras and R.M. Tromp, Phys. Rev. Lett. 63 (1989) 632. [2] .I. Massies, N. Grandjean and V.H. E&ens, Appl. Phys. Lett. 61 (1992) 99. [3] A. Koma, K. Ueno and K. Saiki, .I. Cryst. Growth 111 (1991) 1029. [4] N. Grandjean, J. Massies, C. Delamarre, L.P. Wang, A. Dubon and J.Y. Laval, Appl. Phys. Lett. 63 (1993) 66. [5] A. Koma, K. Sunouchi and T. Miyajima, J. Vat. Sci. Technol. B 3 (1985) 724. [6] M. Grundner and H. Jacob, Appl. Phys. A 39 (1986) 73. [7] Y.J. Chabal, G.S. Higashi, K. Raghavachari and V.A. Burrows, J. Vat. Sci. Technol. A 7 (1989) 2104. [8] Y. Uchida, Y. Yazawa and T. Warabisako, Appl. Phys. Lett. 67 (1995) 127. [9] T. Sakurai and H.D. Hangstrum, Phys. Rev. B 14 (1976) 1593. [lo] S.S. Iyer, M. Arienzo and E. de Fresart, Appl. Phys. Lett. 57 (1990) 893. [ 111 B.S. Meyerson, F.J. Himpsel and K.J. Uram, Appl. Phys. Lett. 57 (1990) 1034. [12] A.A. Baski, J. Nogami and CF. Quate, J. Vat. Sci. Technol. A 8 (1990) 245. [13] J. Nogami, S. Park and C.F. Quate, Appl. Phys. Lett. 53 (1988) 2086. [14] T. Sakamoto and H. Kawanami, Surf. Sci. 111 (1981) 177.