Growth of GaAs on Si by MOVCD

Journal of Crystal Growth 68 (1984) 21—26 North-Holland, Amsterdam

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GROWTH OF GaAs ON Si BY MOVCD Masahiro AKIYAMA, Yoshihiro KAWARADA and Katsuzo KAMINISHI Research Laboratory, OKI Electric Industry, Co. Ltd., 550-5 Higashiasakawa, 1-lachioji, Tokyo, Japan

GaAs layers with a mirror surface were grown on (100)-oriented Si substrate by MOCVD. The combination of thin GaAs layers grown at low temperatures and GaAs/GaAlAs alternating layers at conventional growth temperatures were found to be effective as a buffer layer. The top GaAs layer showed a relatively high PL intensity although the peak shifted to 834 nm due to the tensile stress arising from the difference in thermal expansion coefficient between GaAs and Si. The growth conditions and the construction of the buffer layer to enable the growth of a good GaAs layer were studied.

1. Introduction MOCVD has been successfully developed as the technique to grow various 111—V and Il—VI materials and has become one of the most important methods for the growth of compound semiconductors. As the substrates for growth, compound materials have been mainly used. The epitaxial growth on the substrate of group IV materials has, on the other hand, not yet been so widely studied. For the growth of compound semiconductors on Si or Ge substrates, there are problems not only with the lattice mismatch between the grown layer and the substrate but also with electric polarities of the substrate surfaces. For example, although GaAs has a lattice constant close to Ge, the GaAs layer grown directly on the (100)-oriented Ge surface shows a milky surface because of the antiphase domain structure [1,2]. On the other hand, it was reported that GaAs on (111)-oriented Ge substrates showed a single domain structure [2]. In our previous work, we have showed that a single domain layer with a mirror surface can be grown on a (100)-oriented Ge substrate by introducing alternating layers of GaAs/ GaAlAs as a buffer layer between GaAs and the Ge substrate [3]. GaAs wafers grown on large area Si substrates would be, if they are realized, very useful for economical high efficiency solar cells, LED display panels and so on. However, it is more difficult to

grow a GaAs epitaxial layer on a Si substrate than on a Ge substrate because this combination has a large lattice mismatch of about 4%. Therefore, an effective buffer layer must be introduced between the GaAs layer and the Si substrate. A Ge layer has been used as the buffer layer [4—7].Using this buffer layer, a GaAs solar cell on a Si substrate was successfully fabricated [5]. If the buffer layer can be constructed with GaAs itself or with other Ill—V materials, however, there is a merit that the heteroepitaxial growth can be easily performed in one growth run. After the experiments using various approaches, we found that the combination of a thin GaAs layer deposited at relatively low temperatures of 400—600°C and thin GaAs/GaA1As alternating layers grown at the conventional growth temperatures of about 7000 C was a very effective buffer layer. By introducing this buffer layer, a subsequent GaAs layer with a mirror surface was obtamed on a Si substrate. In this paper, the experimental results of the MOCVD growth of GaAs on (100)-oriented Si substrates with this buffer layer are reported.

2. Experimental A low pressure system with a vertical and water cooled reactor containing a graphite pedestal was used for the growth. TMG (Ga(CH3)3), TMA

0022-0248/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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Growth of GaAs on Si by MOCVD

(A1(CH3)3), and AsH3 were used as source materials. The carrier gas was H2 and the total flow rate was kept at 1500 SCCM and at a pressure of 100 Torr. Partial pressures of TMG, TMA and AsH~ for the growth of the buffer layer were 8 x 10- 2 2.4>< 10-2 and 4 Torr, respectively. When the top layer was grown, the partial pressure of TMG was increased to 1.6 X 10—1 Torr. The growth rates for GaAs were about 1000 and 2000 A/mm for the above partial pressures of TMG, respectively. (100)-oriented Si wafers were used as the substrate without back-sealing. Each wafer was degreased and the surface oxide layer was removed by HF dipping just before loading into the reactor. No further surface treatments were performed in the reactor, Growth temperature (the temperature measured by the thermocouple set in the graphite pedestal) was changed in the range of 400—730 and 650—730°Cfor the buffer layer and the top GaAs layer, respectively. The typical sequence for the substrate temperatures are schematically shown in fig. 1. The typical thickness of the top GaAs layer was 3 ~tm. The morphology of the grown sample was observed with a Nomarski microscope. To characterize the top GaAs layer, photoluminescence spectra at 77 K were observed for some samples. Reflection high energy electron diffraction (RHEED) measurement was also performed to study the effects and the crystallinity of the buffer layer. The carrier density and the mobility of the top GaAs layer were measured by the

conventional Van der Pauw method. To observe the domain properties, the surface of the top GaAs layer was slightly etched by molten KOH.

3. Results and discussion 3.!. Growth conditions of the buffer layer

GaAs layers deposited directly on (100)-onented Si substrate with no buffer layers showed a milky surface as shown in fig. 2, where the deposited layer was constructed with many small crystals. When the composite buffer layer. consisting of a thin GaAs layer deposited at relatively low temperatures and GaAs/GaAIAs alternating layers grown at conventional growth temperatures, was introduced between a GaAs layer and a Si substrate, the top GaAs layer showed good morphology; a photograph is shown in fig. 3. Fig. 4 is the photograph of GaAs layer grown on a Si substrate of 2 inch diameter. The surface showed almost a mirror surface. However, small pits haying a density of about 10~cm 2 still remained. The growth temperature and the thickness of the first GaAs layer were important factors for the crystallinity of the top GaAs layer. When these conditions were optimized, the top layer showed a relatively good morphology without GaAs/ GaAlAs layers. However, the growth condition

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TIME Fig. 1. Typical sequence of substrate temperatures in the experiments.

10pm Fig. 2. Photograph of the surface of a GaAs layer deposited directly on a (100)-oriented Si substrate.

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Growth of GaAs on Si by MOCVD

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layer was deposited at the conventional growth temperature of about 700 °C, no effects were observed. On the other hand, when the temperatures were too low, the deposition did not occur. (b) The thickness of the first layer must be less than about 2000 A. When the thickness was more than 2000 A, the top layer did not show a good morphology. However, when it was as thin as 200 A, the improvement of the morphology of the top GaAs layer was observed. (c) GaAs/ GaAlAs layers (the thickness of each layer was fixed at 500 A in the experiments) shows

10pm Fig. 3. Photograph of the surface of the GaAs layer on a (100)-oriented Si substrate with the composite buffer layer consisting of a thin GaAs layer deposited at 500°C and GaAs/GaAlAs alternating layers grown at 700 °C.

was very critical and the reproducibility was poor. By adding the GaAs/GaA1As layers to the buffer layer, the growth condition was less critical and the reproducibility was improved for growing the top GaAs layer with a good morphology. To obtain an optimum buffer layer, experiments were done by varying the growth condition and the construction of the buffer layer. The results obtained were as follows: (a) The first layer must be deposited at low temperatures in the range of 400—600°C. When the

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Fig. 4. Photograph of the GaAs layer on a (100)-oriented Si substrate of 2 inch diameter,

a good effect when grown alternately about five times. When the number of the layers was small, the effect was not so clear. The effect did not increase as the number increased when the layers were repeated more than five or six times. (d) Suitable growth temperatures of GaAs/ GaAlAs and the top GaAs layers were in the range of 680—710°C. When the temperature was too low, the top layer showed a milky surface. On the other hand, pit density on the surface increased when the temperature was too high. 3.2. RHEED measurements

To clarify the effect of each layer in the buffer layer, the crystallinity was observed by RHEED method. Fig. 5 shows the RHEED patterns. The photograph (a) shows the pattern of the first GaAs layer deposited 400 A at 450°C,(b) is that of the first layer deposited at the same condition and annealed at 700°Cfor 5 mm and (c) is the pattern of the GaAs/ GaAlAs alternating layers grown on the first GaAs layer at 700 °C. The thin GaAs layer deposited at 450°C exhibited a twinned crystal pattern. However, after annealing at 700 °C for 5 mm the layer showed a single crystal pattern without twins. The GaAs/GaA1As layers on the annealed first GaAs layer showed a streak pattern. These results show that the twin structure disappeared by rearrangement of the atoms in the first layer during the annealing and GaAs/GaAlAs layers improved the surface flatness. When GaAs is grown at low temperatures, the migration of the atoms on the growing surface is small and the layer is thought to be constructed with many small domains. Therefore, it will be relatively easy to

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Growth of GaAs on Si by MOGUl)

rearrange the atoms in the first layer by annealing. On the other hand, When the layer is deposited at high temperatures, each domain size is too large to be reconstructed by rearrangement of the atoms. When the layer is thick enough, it is also difficult

to rearrange under the influence of the substrate orientation even if the layer is deposited at low temperatures. For this reason the first layer had to be thin and to be deposited at low temperatures.

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3.3. Domain properties

To study the domain properties of the GaAs layer on a (100)-oriented Si substrate, some samples with good morphologies were slightly etched by molten KOH. The photograph of the etched surface is shown in fig. 6. In spite of the mirror like surface, an antiphase domain structure still remained. Referring to the growth of a GaAs layer on the (100)-ortented Ge substrate as mentioned before, it is expected that a single domain GaAs layer can be grown on a Si substrate by further optimizing the growth structure of the GaAs/ GaAlAs layers which were not optimized in this experiments. 3.4. Electrical and optical properties We also studied the electrical and optical properties of the topGaAs layer. An undoped top

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Fig. 5. RHEED patterns of the buffer layer on a (100)-oriented Si substrate: (a) shows that of the first GaAs layer deposited 400 A at 4500 C; (b) is that of the first layer after annealing at 700 0 C for 5 mm; (c) is that of the GaAs/GaA]As alternating

Fig. 6. Photograph of the top GaAs layer on a (100)-oriented Si substrate with the buffer layer after slight etching by molten

layers on the annealed firsi GaAs layer.

KOH.

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Growth of GaAs on Si by MOCVD

GaAs layer showed n type conduction whose carrier density and the mobility were about I X 1016 cm3 and 2000 cm2 V~ s1 or less at room temperature, respectively. The homoepitaxially grown GaAs layer at the same growth conditions showed a highly pure layer with a carrier density of less than 3. Therefore, the relatively 10i5 cm high carrier density would be due to autodoping from the Si substrate which was not backsealed. The low mobility would be due to the scattering at the antiphase domain boundaries. Fig. 7 shows the PL spectra of the top GaAs layer on the Si substrate and homoepitaxially grown GaAs layer observed at 77 K. Both samples had the same carrier density of I x 1016 cm3. The wavelength at the peak intensity of the layer on a Si substrate shifted to 834 nm. The thermal expansion coefficient of GaAs is about twice that of Si. Therefore, the tensile stress to the GaAs layer on Si will be strong. The shift of the peak is thought to be due to the tensile stress. However, the PL intensity was as high as 40% of that of the homoepitaxially grown layer. This relatively high PL intensity shows the GaAs layer on a Si substrate has fairly good crystallinity. The top GaAs layer did not show any cracking as long as the thickness was less than 3 ~.tm.

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4. Summary It was found that a GaAs epitaxial layer with a good morphology could be obtained on a (100)oriented Si substrate by introducing a buffer layer consisting of only GaAs and GaAlAs. The buffer layer was constructed with a thin GaAs layer deposited at low temperatures of 400—600°Cand GaAs/GaAlAs alternating layers grown on it at the conventional growth temperature of about 700°C. The first layer deposited at low temperatures included many twins. However, if the thickness was less than about 2000 A, the atoms in the layer were rearranged and the twin structure disappeared by annealing at relatively low temperatures of 700°C. The role of the GaAs/GaAlAs alternating layers was the improvement of the surface flatness of the buffer layer. In spite of the strong tensile stress and because of the difference in the thermal expansion between GaAs and Si, the grown layer did not show any cracking when the thickness was less than about 3 ~.tm and exhibited a relatively high PL intensity although the peak shifted to 834 nm. The carrier density of the top GaAs layer with no intentional doping was about 1 x 3 due to the auto10t6 cmThe mobility of the doping from the Si substrate. top GaAs layer was about 2000 cm2 V or less due to the antiphase domain structure still remained. The growth of GaAs on a Si substrate with this

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buffer layer is one of the most convenient ways of obtaining a large area and economical GaAs wafer. A

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Acknowledgements This work was performed under the manage-

ment of the R&D Association for Future Electron Devices as a part oft he R&D Project of Basic Technology for Future Industries sponsored by the

on Si

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Agency of Industrial Science and Technology, MITI. 75°

WAVELENGTH (nm) Fig. 7. PL spectra of the top GaAs layer on a Si substrate and the homoepitaxially grown GaAs layer.

References [1] CA. Chang, J. Appi. Phys. 53 (1982) 1253. [2] K. Morizane, J. Crystal Growth 38 (1977) 249.

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Growth of GaAs on Si by MOCVD

[3] M. Akiyama, Y. Kawarada and K. Kaminishi, in: Extended Abstracts 15th Conf. on Solid State Devices and Materials, Tokyo, 1983, p. 293.

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Davis, IEEE Electron Device Letters EDL-2 (1981) 169. (6] B.-Y. Tsaur, R.W. McClelland, i.C.C. Fan, R.P. Gale, J.P. Salerno, BA. Vojak and CO. Bozler, AppI. Phys. Letters 4

(1982) 41. [7] Y. Ohmachi. T. Nishioka and Y. Shinoda. Electron. Letters

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