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Si(1 1 1) substrate using various buffer layers
Journal of Crystal Growth 224 (2001) 190–194
Characterization of GaN thin film growth on 3C–SiC/Si(1 1 1) substrate using various buffer layers C.I. Parka, J.H. Kanga, K.C. Kima, E.-K. Suha, K.Y. Lima,*, K.S. Nahmb a
Department of Semiconductor Science & Technology, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, South Korea b School of Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, South Korea Received 14 November 2000; accepted 25 January 2001 Communicated by J.B. Mullin
Abstract GaN thin films have been grown by MOCVD on GaN, AlN, and superlattice buffer layers predeposited on 3C–SiC/ Si(1 1 1) substrates. The growth of 3C–SiC intermediate layer was carried out by CVD on the Si(1 1 1) substrates using tetramethylsilane single source precursor. In the results of X-ray diffraction, GaN films grown with a superlattice buffer layer showed only c-oriented (0 0 0 2) plane of GaN. From the Raman spectra, the E2 high mode, agreed with the selection rule, was well observed in all GaN films. But, the A1(TO) and E1(TO) mode and the E1(TO) mode were additionally appeared in the GaN films grown without buffer layer and with GaN buffer layer, respectively. In the photoluminescence spectra at low temperature, the peaks associated with band edge emission and donor–accepter pair (D0A0) were observed in GaN films grown without buffer layer or with GaN buffer layer and AlN buffer layer. GaN films grown with superlattice buffer layer showed band edge and very weak D0A0 emission. The root mean square (RMS) roughness of the GaN film grown on superlattice buffer layer was only 4.21 A˚. The surface morphology and structural and optical qualities of GaN films were extremely improved using superlattice buffer layer. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.15.G; 81.05.Ea Keywords: A1. Characterization; A1. Crystallites; A3. Metalorganic chemical vapor deposition; B1. Nitrides; B2. Semiconducting silicon compounds
1. Introduction GaN and its related materials have received a great deal of attention due to their potential applications for the fabrication of blue- or ultra*Corresponding author. Tel.: +82-652-270-3443; fax: +82652-270-3585. E-mail address: [email protected] (K.Y. Lim).
violet-light emitting devices [1]. A key issue for the growth of GaN has been the lack of an ideal substrate, since GaN substrates are not readily available. Most GaN has been grown on sapphire [2]. The lattice mismatch between GaN and sapphire is large, partly resulting in the high defect density in the GaN films with possible deterioration of the optoelectronic properties of the GaN thin films. Consequently, SiC that is a wide-band-gap
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 8 5 6 - 9
C.I. Park et al. / Journal of Crystal Growth 224 (2001) 190–194
semiconductor with high mobility and relatively small lattice mismatch with GaN (in-plane lattice mismatch Dd/d=4%) [3] has attracted much attention as an alternative substrate. But SiC substrate is available only in small diameter and very expensive. So, the previous work has attempted to grow high quality GaN films on SOI substrates using SiC intermediate layer [4] and AlAs nucleation layer [5], stimulated by the unique merits of Si wafers such as low cost, high surface quality, large area wafer availability, high conductivity and well-established processing techniques. In this work, the high quality GaN films were grown on Si(1 1 1) substrates using the 3C–SiC intermediate layers by MOCVD and investigated the structural and optical properties of GaN films. The properties of the GaN films grown on SiC/ Si(1 1 1) with GaN, AlN, or superlattice buffer layer were compared with a buffer-free layer.
2. Experimental procedure The SiC films on Si(1 1 1) substrates were grown by MOCVD using the tetramethylsilane (TMS) as the single source precursor. The Si wafer was thermally cleaned at 11008C for 5 min in H2 ambient at 103 Torr. Then, the reactor temperature was increased to 12508C for the SiC deposition. During the SiC growth, the flow rate of TMS and H2 are 1 and 1000 sccm, respectively. After 1 h growth, the structural properties of the SiC films were investigated by X-ray diffraction (XRD). It was confirmed that the SiC film which had a 3C phase crystal structure was a single crystal. The GaN films were grown on 3C–SiC/Si(1 1 1) by MOCVD system using H2 carrier gas, trimethylgallium (TMG), trimethylaluminum (TMA) and ammonia (NH3) as Ga, Al and N precursors, respectively. Before the buffer layer growth, the 3C–SiC/Si(1 1 1) substrate was thermally treated at 11508C for 10 min in H2 flow. After the reactor temperature was cooled down to 5708C, a thin GaN buffer layer was grown for 150 s with TMG flow rate of 80 mmol/min and NH3 flow rate of 4000 sccm at 200 Torr. In the case of an AlN buffer, the reactor temperature ramp up to 11608C and then a thin AlN buffer layer was grown for
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5 min with TMA flow rate of 55 mmol/min and NH3 flow rate of 4000 sccm at the same reactor pressure. The thickness of GaN buffer and AlN buffer layer was calibrated to 200 A˚ for the growth time of them. In the case of superlattice buffer layer, a 32 A˚ GaN layer was grown at 8008C for 15 s by feeding TMG and NH3, and then a 20 A˚ AlN layer was grown on the GaN layer at the same temperature for 30 s by changing TMG for TMA. These works were done three times repeatedly to make about 200 A˚. To grow GaN epilayer, the reactor temperature was raised to 11608C under a flow of NH3 and H2 mixture. GaN epilayers were grown for 1 h with TMG flow rate of 120 mmmol/min and NH3 flow rate of 4000 sccm. After the growth, surface morphologies were studied using atomic force microscopy (AFM). To evaluate the structural properties, we carried out the measurement. Optical properties of the GaN films were studied with Raman spectroscopy at room temperature using a 514.5 nm wavelength of Ar+ laser and photoluminescence (PL) at various temperatures using a 325 nm wavelength of He-Cd laser.
3. Result and discussion The surface morphologies of GaN films were clearly revealed in the images of AFM as shown in Fig. 1. The root mean square (RMS) roughness of the GaN films grown with buffer-free layer, GaN buffer layer, AlN buffer layer, and superlattice buffer layer are 1020, 575, 38, and 4.21 A˚, respectively. The surface morphologies of Fig. 1(a) and (b) indicate that buffer layer is required to grow a high quality GaN on Si(1 1 1) substrate using SiC intermediate layer and a low temperature grown GaN buffer layer is not a good material to offer a smooth surface of GaN epilayer. However, we have found that the surface morphology was improved significantly in GaN film grown with AlN and superlattice buffer layers as shown in Fig. 1(c) and (d). As buffer layer was changed to superlattice, the surface morphology became smoother. The structural properties of GaN films were studied using XRD. In Fig. 2, the XRD spectra of GaN epilayers grown on
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Fig. 1. Atomic force microscopy (AFM) images of the surface morphology of GaN films grown on SiC/Si(1 1 1) substrate. (a) without buffer layer; (b) with 200 A˚ GaN buffer layer; (c) with 200 A˚ AlN buffer layer; and (d) with superlattice buffer layer which consisted of four periods of 32 A˚ GaN layer and 20 A˚ AlN layer.
SiC/Si(1 1 1) substrates exhibit strong peak at 2y=348 associated with the reflection of GaN(0 0 0 2) plane indicating a growth of wurtzite GaN film with dominant growth orientation along the c-axis. Fig. 2(d) shows that single crystalline GaN epilayers were grown on Si(1 1 1) substrate using a 3C–SiC intermediate layer. The GaN films grown with a superlattice buffer layer showed only c-oriented X-ray peak of GaN(0 0 0 2), and many other X-ray peaks related with other planes of GaN were not observed. As the buffer layer changed to a superlattice, the polycrystalline property of GaN films disappeared. Fig. 3 shows the room temperature Raman scattering spectra of GaN epilayer using various buffer layers. These spectra were taken in zðyuÞ z configuration with the z-direction along the c-axis of GaN. Obviously, the E2 at around 568 cm1 was observed in all these samples as we expected from the selection rule for the scattering configuration
employed. Fig. 3(a) and (b) show additionally the A1(TO) and the E1(TO) modes and the E1(TO) mode, respectively. But, the A1(TO) and the E1(TO) modes are forbidden by selection rules in this configuration [6]. The appearance of the A1(TO) and the E1(TO) modes were possibly related to the growth of polycrystal (a high defect density) and the surface roughness [7]. These results were well confirmed by the information obtained from the AFM and the XRD measurements. Finally, the GaN films grown on SiC/ Si(1 1 1) substrates were investigated by PL at various temperatures. Several emission bands are observed in all samples that were grown with various buffer layers. The GaN film that was grown with superlattice buffer layer is not observed in the yellow band. We observed at around 3.26 eV the donor–acceptor pairs (D0A0) recombination [8] and at around 3.47 eV the donor bound exciton (D0X) [9,10]. The ratio of D0X to D0A0
C.I. Park et al. / Journal of Crystal Growth 224 (2001) 190–194
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Fig. 4. The intensity ratio of donor bound exiton peak (D0X) to donor–acceptor pair (D0A0) peak as a kind of buffer layer. The conditions of buffer layers are specified in Fig. 1.
Fig. 2. Wide angular range X-ray diffraction (XRD) spectra of GaN films grown on SiC/Si(111) substrate. The conditions of buffer layers are specified in Fig. 1.
recombination can be taken as a measure of the quality of the samples. In the case of GaN epilayer using a superlattice buffer layer, as shown in Fig. 4, the ratio of the D0X peak intensity to the D0A0 emission intensity is higher than other case at low temperature (5 K) and the intensity ratio was 342 : 1.
4. Summary We have shown that the single crystalline, hexagonal GaN layers were successfully grown on Si(1 1 1) substrates using the SiC intermediate layer by MOCVD. Surface morphology, structural quality, and optical property were improved by introducing a superlattice buffer layer. The hexagonal GaN film grown with a superlattice buffer layer showed only GaN(0 0 0 2) plane in XRD spectrum and E2 high mode in Raman spectrum and had a RMS roughness of 4.21 A˚. The yellow luminescence, which corresponded to deep energy levels due to the imperfections of GaN, is not observed in the case of GaN epilayer using a superlattice buffer layer.
Acknowledgements Fig. 3. Room temperature Raman scattering spectra of GaN films grown on SiC/Si(1 1 1) substrate. The conditions of buffer layers are specified in Fig. 1.
This work was supported by Grant No. 1999-2114-008-5 from the interdisciplinary research
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C.I. Park et al. / Journal of Crystal Growth 224 (2001) 190–194
program of the Korea Science and Engineering Foundation.
References [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Jpn. J. Appl. Phys. 37 (1998) L1020. [2] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) L1705. [3] Y.-S. Park, J. Korean Phys. Soc. 34 (1999) S199. [4] A.J. Steckl, J. Devrajan, C. Tran, R.A. Stall, J. Electron. Mater. 26 (1997) 217.
[5] A. Strittmatter, A. Krost, M. Strabburg, V. Turck, D. Bimberg, J. Blasing, J. Shristen, Appl. Phys. Lett. 74 (1999) 1242. [6] H. Siegle, L. Eckey, A. Hoffmann, C. Thomsen, B.K. Meyer, D. Schikora, M. Hankeln, K. Lischka, Solid State Commun. 96 (1995) 943. [7] S. Miyoshi, K. Onabe, N. Ohkouchi, H. Yaguchi, R. Ito, S. Fukatsu, Y. Shiraki, J. Crystal Growth 124 (1992) 439. [8] M.R.H. Kahn, Y. Ohshita, N. Sawaki, I. Akasaki, Sol. State Commun. 57 (1986) 405. [9] R. Dingle, D.D. Sell, S.E. Stokowski, M. Ilegems, Phys. Rev. B 4 (1971) 1211. [10] L. Eckey, J.-Ch. Holst, P. Maxim, R. Heitz, A. Hoffmann, I. Broser, B.K. Meyer, C. Wetzel, E.N. Mokhov, P.G. Baranov, Appl. Phys. Lett. 68 (3) (1996) 415.
Characterization of GaN thin film growth on 3C–SiC/Si(1 1 1) substrate using various buffer layers C.I. Parka, J.H. Kanga, K.C. Kima, E.-K. Suha, K.Y. Lima,*, K.S. Nahmb a
Department of Semiconductor Science & Technology, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, South Korea b School of Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756, South Korea Received 14 November 2000; accepted 25 January 2001 Communicated by J.B. Mullin
Abstract GaN thin films have been grown by MOCVD on GaN, AlN, and superlattice buffer layers predeposited on 3C–SiC/ Si(1 1 1) substrates. The growth of 3C–SiC intermediate layer was carried out by CVD on the Si(1 1 1) substrates using tetramethylsilane single source precursor. In the results of X-ray diffraction, GaN films grown with a superlattice buffer layer showed only c-oriented (0 0 0 2) plane of GaN. From the Raman spectra, the E2 high mode, agreed with the selection rule, was well observed in all GaN films. But, the A1(TO) and E1(TO) mode and the E1(TO) mode were additionally appeared in the GaN films grown without buffer layer and with GaN buffer layer, respectively. In the photoluminescence spectra at low temperature, the peaks associated with band edge emission and donor–accepter pair (D0A0) were observed in GaN films grown without buffer layer or with GaN buffer layer and AlN buffer layer. GaN films grown with superlattice buffer layer showed band edge and very weak D0A0 emission. The root mean square (RMS) roughness of the GaN film grown on superlattice buffer layer was only 4.21 A˚. The surface morphology and structural and optical qualities of GaN films were extremely improved using superlattice buffer layer. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.15.G; 81.05.Ea Keywords: A1. Characterization; A1. Crystallites; A3. Metalorganic chemical vapor deposition; B1. Nitrides; B2. Semiconducting silicon compounds
1. Introduction GaN and its related materials have received a great deal of attention due to their potential applications for the fabrication of blue- or ultra*Corresponding author. Tel.: +82-652-270-3443; fax: +82652-270-3585. E-mail address: [email protected] (K.Y. Lim).
violet-light emitting devices [1]. A key issue for the growth of GaN has been the lack of an ideal substrate, since GaN substrates are not readily available. Most GaN has been grown on sapphire [2]. The lattice mismatch between GaN and sapphire is large, partly resulting in the high defect density in the GaN films with possible deterioration of the optoelectronic properties of the GaN thin films. Consequently, SiC that is a wide-band-gap
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 8 5 6 - 9
C.I. Park et al. / Journal of Crystal Growth 224 (2001) 190–194
semiconductor with high mobility and relatively small lattice mismatch with GaN (in-plane lattice mismatch Dd/d=4%) [3] has attracted much attention as an alternative substrate. But SiC substrate is available only in small diameter and very expensive. So, the previous work has attempted to grow high quality GaN films on SOI substrates using SiC intermediate layer [4] and AlAs nucleation layer [5], stimulated by the unique merits of Si wafers such as low cost, high surface quality, large area wafer availability, high conductivity and well-established processing techniques. In this work, the high quality GaN films were grown on Si(1 1 1) substrates using the 3C–SiC intermediate layers by MOCVD and investigated the structural and optical properties of GaN films. The properties of the GaN films grown on SiC/ Si(1 1 1) with GaN, AlN, or superlattice buffer layer were compared with a buffer-free layer.
2. Experimental procedure The SiC films on Si(1 1 1) substrates were grown by MOCVD using the tetramethylsilane (TMS) as the single source precursor. The Si wafer was thermally cleaned at 11008C for 5 min in H2 ambient at 103 Torr. Then, the reactor temperature was increased to 12508C for the SiC deposition. During the SiC growth, the flow rate of TMS and H2 are 1 and 1000 sccm, respectively. After 1 h growth, the structural properties of the SiC films were investigated by X-ray diffraction (XRD). It was confirmed that the SiC film which had a 3C phase crystal structure was a single crystal. The GaN films were grown on 3C–SiC/Si(1 1 1) by MOCVD system using H2 carrier gas, trimethylgallium (TMG), trimethylaluminum (TMA) and ammonia (NH3) as Ga, Al and N precursors, respectively. Before the buffer layer growth, the 3C–SiC/Si(1 1 1) substrate was thermally treated at 11508C for 10 min in H2 flow. After the reactor temperature was cooled down to 5708C, a thin GaN buffer layer was grown for 150 s with TMG flow rate of 80 mmol/min and NH3 flow rate of 4000 sccm at 200 Torr. In the case of an AlN buffer, the reactor temperature ramp up to 11608C and then a thin AlN buffer layer was grown for
191
5 min with TMA flow rate of 55 mmol/min and NH3 flow rate of 4000 sccm at the same reactor pressure. The thickness of GaN buffer and AlN buffer layer was calibrated to 200 A˚ for the growth time of them. In the case of superlattice buffer layer, a 32 A˚ GaN layer was grown at 8008C for 15 s by feeding TMG and NH3, and then a 20 A˚ AlN layer was grown on the GaN layer at the same temperature for 30 s by changing TMG for TMA. These works were done three times repeatedly to make about 200 A˚. To grow GaN epilayer, the reactor temperature was raised to 11608C under a flow of NH3 and H2 mixture. GaN epilayers were grown for 1 h with TMG flow rate of 120 mmmol/min and NH3 flow rate of 4000 sccm. After the growth, surface morphologies were studied using atomic force microscopy (AFM). To evaluate the structural properties, we carried out the measurement. Optical properties of the GaN films were studied with Raman spectroscopy at room temperature using a 514.5 nm wavelength of Ar+ laser and photoluminescence (PL) at various temperatures using a 325 nm wavelength of He-Cd laser.
3. Result and discussion The surface morphologies of GaN films were clearly revealed in the images of AFM as shown in Fig. 1. The root mean square (RMS) roughness of the GaN films grown with buffer-free layer, GaN buffer layer, AlN buffer layer, and superlattice buffer layer are 1020, 575, 38, and 4.21 A˚, respectively. The surface morphologies of Fig. 1(a) and (b) indicate that buffer layer is required to grow a high quality GaN on Si(1 1 1) substrate using SiC intermediate layer and a low temperature grown GaN buffer layer is not a good material to offer a smooth surface of GaN epilayer. However, we have found that the surface morphology was improved significantly in GaN film grown with AlN and superlattice buffer layers as shown in Fig. 1(c) and (d). As buffer layer was changed to superlattice, the surface morphology became smoother. The structural properties of GaN films were studied using XRD. In Fig. 2, the XRD spectra of GaN epilayers grown on
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Fig. 1. Atomic force microscopy (AFM) images of the surface morphology of GaN films grown on SiC/Si(1 1 1) substrate. (a) without buffer layer; (b) with 200 A˚ GaN buffer layer; (c) with 200 A˚ AlN buffer layer; and (d) with superlattice buffer layer which consisted of four periods of 32 A˚ GaN layer and 20 A˚ AlN layer.
SiC/Si(1 1 1) substrates exhibit strong peak at 2y=348 associated with the reflection of GaN(0 0 0 2) plane indicating a growth of wurtzite GaN film with dominant growth orientation along the c-axis. Fig. 2(d) shows that single crystalline GaN epilayers were grown on Si(1 1 1) substrate using a 3C–SiC intermediate layer. The GaN films grown with a superlattice buffer layer showed only c-oriented X-ray peak of GaN(0 0 0 2), and many other X-ray peaks related with other planes of GaN were not observed. As the buffer layer changed to a superlattice, the polycrystalline property of GaN films disappeared. Fig. 3 shows the room temperature Raman scattering spectra of GaN epilayer using various buffer layers. These spectra were taken in zðyuÞ z configuration with the z-direction along the c-axis of GaN. Obviously, the E2 at around 568 cm1 was observed in all these samples as we expected from the selection rule for the scattering configuration
employed. Fig. 3(a) and (b) show additionally the A1(TO) and the E1(TO) modes and the E1(TO) mode, respectively. But, the A1(TO) and the E1(TO) modes are forbidden by selection rules in this configuration [6]. The appearance of the A1(TO) and the E1(TO) modes were possibly related to the growth of polycrystal (a high defect density) and the surface roughness [7]. These results were well confirmed by the information obtained from the AFM and the XRD measurements. Finally, the GaN films grown on SiC/ Si(1 1 1) substrates were investigated by PL at various temperatures. Several emission bands are observed in all samples that were grown with various buffer layers. The GaN film that was grown with superlattice buffer layer is not observed in the yellow band. We observed at around 3.26 eV the donor–acceptor pairs (D0A0) recombination [8] and at around 3.47 eV the donor bound exciton (D0X) [9,10]. The ratio of D0X to D0A0
C.I. Park et al. / Journal of Crystal Growth 224 (2001) 190–194
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Fig. 4. The intensity ratio of donor bound exiton peak (D0X) to donor–acceptor pair (D0A0) peak as a kind of buffer layer. The conditions of buffer layers are specified in Fig. 1.
Fig. 2. Wide angular range X-ray diffraction (XRD) spectra of GaN films grown on SiC/Si(111) substrate. The conditions of buffer layers are specified in Fig. 1.
recombination can be taken as a measure of the quality of the samples. In the case of GaN epilayer using a superlattice buffer layer, as shown in Fig. 4, the ratio of the D0X peak intensity to the D0A0 emission intensity is higher than other case at low temperature (5 K) and the intensity ratio was 342 : 1.
4. Summary We have shown that the single crystalline, hexagonal GaN layers were successfully grown on Si(1 1 1) substrates using the SiC intermediate layer by MOCVD. Surface morphology, structural quality, and optical property were improved by introducing a superlattice buffer layer. The hexagonal GaN film grown with a superlattice buffer layer showed only GaN(0 0 0 2) plane in XRD spectrum and E2 high mode in Raman spectrum and had a RMS roughness of 4.21 A˚. The yellow luminescence, which corresponded to deep energy levels due to the imperfections of GaN, is not observed in the case of GaN epilayer using a superlattice buffer layer.
Acknowledgements Fig. 3. Room temperature Raman scattering spectra of GaN films grown on SiC/Si(1 1 1) substrate. The conditions of buffer layers are specified in Fig. 1.
This work was supported by Grant No. 1999-2114-008-5 from the interdisciplinary research
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C.I. Park et al. / Journal of Crystal Growth 224 (2001) 190–194
program of the Korea Science and Engineering Foundation.
References [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Jpn. J. Appl. Phys. 37 (1998) L1020. [2] S. Nakamura, Jpn. J. Appl. Phys. 30 (1991) L1705. [3] Y.-S. Park, J. Korean Phys. Soc. 34 (1999) S199. [4] A.J. Steckl, J. Devrajan, C. Tran, R.A. Stall, J. Electron. Mater. 26 (1997) 217.
[5] A. Strittmatter, A. Krost, M. Strabburg, V. Turck, D. Bimberg, J. Blasing, J. Shristen, Appl. Phys. Lett. 74 (1999) 1242. [6] H. Siegle, L. Eckey, A. Hoffmann, C. Thomsen, B.K. Meyer, D. Schikora, M. Hankeln, K. Lischka, Solid State Commun. 96 (1995) 943. [7] S. Miyoshi, K. Onabe, N. Ohkouchi, H. Yaguchi, R. Ito, S. Fukatsu, Y. Shiraki, J. Crystal Growth 124 (1992) 439. [8] M.R.H. Kahn, Y. Ohshita, N. Sawaki, I. Akasaki, Sol. State Commun. 57 (1986) 405. [9] R. Dingle, D.D. Sell, S.E. Stokowski, M. Ilegems, Phys. Rev. B 4 (1971) 1211. [10] L. Eckey, J.-Ch. Holst, P. Maxim, R. Heitz, A. Hoffmann, I. Broser, B.K. Meyer, C. Wetzel, E.N. Mokhov, P.G. Baranov, Appl. Phys. Lett. 68 (3) (1996) 415.