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Interface structure of GaN on sapphire (0 0 0 1) studied by transmission electron microscope
Journal of Crystal Growth 189/190 (1998) 295—300
Interface structure of GaN on sapphire (0 0 0 1) studied by transmission electron microscope Tsuyoshi Onitsuka!, Takahiro Maruyama!, Katsuhiro Akimoto!,*, Yoshio Bando" ! Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305, Japan " National Institute for Research in Inorganic Materials, Tsukuba, Ibaraki 305, Japan
Abstract Structural defects of GaN grown by plasma-assisted molecular beam epitaxy on sapphire (0 0 0 1) substrates have been studied by transmission electron microscope (TEM). Cubic GaN (c-GaN) islands, surrounded by hexagonal GaN (h-GaN), with the typical height and width of 6 and 50 nm, respectively, were observed in the vicinity of the substrate by selected area diffraction pattern and high-resolution image. The epitaxial relationship between c-GaN and h-GaN was determined as h-GaN (0 0 0 1)Ec-GaN (1 1 1), h-GaN(1 0!1 1)Ec-GaN (1 0 0) and h-GaN[1 1!2 0]Ec-GaN [1 1 0]. Because the boundary between c-GaN and h-GaN has high density of dislocations, the mixed cubic-hexagonal character near the substrate may play an important role in the relaxation of large misfit stress created by lattice mismatching between GaN and sapphire substrate. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 61.72.Nn; 68.55.!a; 81.05.Ea; 81.15.!z Keywords: Cubic GaN; Hexagonal GaN; TEM; Interface structure
1. Introduction GaN and related III—V nitrides are the focus of intensive research due to their potential for blue—violet light-emitting devices. However, epitaxial GaN layers used for device fabrication are usually highly faulted hexagonal crystals due to the lack of suitable substrate materials that are both lattice and thermally matched to these nitrides [1].
* Corresponding author. Tel.: #81 298 53 5274; fax: #81 298 55 7440; e-mail: [email protected].
Microstructural characterization of GaN by transmission electron microscopy (TEM) has been performed by many groups [2—9]. Threading dislocations are the most common line defects observed for films grown on sapphire (0 0 0 1) substrates. These dislocations are found to be originated at the substrate interface and caused by the inversion domain boundaries (IDB), double positioning boundaries (DPB) and stacking mismatch boundaries (SMB) [2,3]. These threading dislocations may contribute to a relaxation of misfit stress. We observed mixture of cubic and hexagonal GaN (c-GaN and h-GaN) only near the sapphire
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substrate. Though the mixed cubic-hexagonal material includes high density of dislocations, threading dislocations are not generated. In this paper, we report the observation of mixture of cubic-hexagonal material near the substrate, characterization of epitaxial relationship between c-GaN and hGaN, and discuss the role of the mixed phase from the view of the relaxation of misfit stress.
2. Experimental procedure The GaN films were grown on sapphire (0 0 0 1) substrates by radio frequency plasma-assisted molecular beam epitaxy. A general description of the system and the typical conditions used for nitride growth have been reported previously [10]. The substrates were chemically treated with 3 : 1 of H SO : H PO at 160°C and inserted into the 2 4 3 4 growth chamber. Prior to growth, the substrates were degassed at 800°C followed by nitridation of the substrates for 20 min. No buffer layer of AlN nor GaN were deposited at the beginning of the epitaxy. Total film thickness was 0.8 lm with the
growth rate of 0.4 lm/h and growth temperature was 700°C. Photoluminescence spectra of the samples measured at 77 K were dominated by the band edge emission and the intensity of deep level emission were about two orders of magnitude less than that of the band edge emission indicating high quality of crystals. TEM specimens were prepared in cross section, by first mechanically thinning to a thickness of about 10 lm, and then ion-milling to electron transparency with 4 keV Ar ions, for normal observation in the [1 1!2 0] projection. The samples were observed with a JEM-2000EX high-resolution electron microscope operated at 200 keV.
3. Results and discussion A typical low-magnification electron micrograph of a defective GaN film grown with RF plasma power of 150 W is shown in Fig. 1. The morphology is dominated by threading defects extending away from the substrate. The dislocation density was estimated to be about 1]109 cm~2. The
Fig. 1. Cross-sectional TEM micrograph of the GaN epilayer on the sapphire substrate.
T. Onitsuka et al. / Journal of Crystal Growth 189/190 (1998) 295–300
Fig. 2. (a) SAD pattern from both the film and the substrate. (b) The basic units of the diffraction pattern due to Al O , h2 3 and c-GaN are indicated and indexed.
corresponding [1 1!2 0] selected-area electron diffraction (SAD) pattern from both the film and substrate is shown in Fig. 2a. The SAD pattern shows that h-GaN grows epitaxially with the epitaxial relationship of h-GaN (0 0 0 1)EAl O (0 0 0 1) and 2 3 GaN [!1 2!1 0]EAl O [0 1!1 0] as shown in 2 3 Fig. 2b, in agreement with previous results [1]. In addition to hexagonal diffraction spots, extra diffraction spots are also observed as indicated by arrows in Fig. 2a. These 1 order spots can be inter3 preted as cubic (2 2 0) diffraction spots and indexed as shown in Fig. 2b. The mixture of c- and h-GaN phase was already observed [3,6], however, the distribution of each phases and orientation relationship between these two phases are not clear yet. The interplanar spacing evaluated from the extra spots was 2.5 A_ which is very close to that of c-GaN [1 1 1] interplanar spacing. Therefore, epitaxial
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relationship between h-GaN and c-GaN is h-GaN (0 0 0 1)Ec-GaN (1 1 1) and h-GaN [1 1!2 0]EcGaN [1 1 0]. Dark field image along the [1 1!2 0] with the extra 1 order spot is shown in Fig. 3. As can be seen 3 in the figure, the extra spots originate only from the interface region, that is, c-GaN was grown only near the substrate. The dimension of the c-GaN island is about 6 nm height with 20—50 nm wide and the distribution of islands seems to be roughly periodic. A high-resolution TEM image of an interface between c-GaN and h-GaN is shown in Fig. 4. The interface is sharp with no interfacial amorphous layer as indicated by solid line. In the c-GaN region, the FCC stacking sequence (ABCABC2) can easily be observed, and in the h-GaN region, hexagonal stacking sequence (ABAB2) can be observed. A highly enlarged image of the interface is shown in Fig. 5 for reference. Coming back again to Fig. 4, we can see that the interface parallel to the growth surface is flat, on the other hand, the interface perpendicular to the growth surface has zigzag structure. The zigzag structure has just 6lattice spacing period for the growth direction as shown in Fig. 4. This precise periodicity can be interpreted as follows. As mentioned above, the FCC stacking sequence is ABCABC2 and hexagonal stacking sequence is ABAB2, so the same stacking meets every 6-lattice spacing. As far as satisfying such a stacking sequences, dislocations extending to the growth direction would not be generated. In fact, no dislocation extending to the growth direction is observed at the interface of c-GaN and h-GaN. The interface between c-GaN and h-GaN consists of two kinds of facets, one is parallel to the growth surface, so the interface can be assigned as c-GaN (1 1 1) and h-GaN (0 0 0 1). The other interface is perpendicular to the growth surface which has an angle of about 30° from [0 0 0 1] direction. The atomic row indicated by a rod in h-GaN region in Fig. 4 makes an angle of 30° with [0 0 0 1] direction. The angle between h-GaN (0 0 0 1) and h-GaN (1 0!1 1) is 28°, so the interface between cand h-GaN for the hexagonal side may be (1 0!1 1). The atomic row indicated by a rod in c-GaN region makes an angle of 35° with [1 1 1] direction, which
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Fig. 3. Dark field image along the [1 1!2 0] with the extra 1 order spot as shown in Fig. 2. 3
Fig. 4. High-resolution TEM image of epitaxial GaN at the interface of c- and h-GaN. The growth direction is the upside of the photograph. The facets of c-GaN (1 0 0) and h-GaN (1 0!1 1) are indicated by rods.
Fig. 5. Highly enlarged cross-sectional TEM micrograph at the interface of c- and h-GaN showing the stacking sequences.
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is just the same as the angle between (1 0 0) and (1 1 1). So the interface for the cubic side may be (1 0 0). That is, the interface between cand h-GaN perpendicular to the growth surface can be assigned as c-GaN (1 0 0) and h-GaN (1!1 0 1). A highly enlarged image of the interface is shown in Fig. 6a and the structure model for the interface is shown in Fig. 6b. As shown in the model, the interface contains high density of dangling bonds or dislocations which extends not to the growth direction but to the in-plane of the GaN film. These dislocations may relax the misfit stress caused by the lattice mismatching between GaN and Al O 2 3 substrate. Though the IDB and SMB seem to be responsible for the threading dislocations which deteriorates crystal quality, the formation of cubic-hexagonal mixture at the initial growth stage may have a good effect on the crystal quality. The cause of the formation of the mixed phase only near the substrate is not known at present, however, it may be possible to decrease the dislocation density which extends to the growth direction by controlling the island size and density of c-GaN. The mixed phase is also observed near the substrate in GaN grown by metal organic vapor phase epitaxy (MOVPE) [6,11]. The effect of the mixed phase on relaxation of the stress and reducing the dislocation density may be similar between MBE and MOVPE grown GaN.
4. Summary Structural defect of GaN grown by plasma-assisted molecular beam epitaxy on sapphire (0 0 0 1) substrates have been studied by high-resolution TEM. The islands of c-GaN, surrounded by hGaN, with the typical height and width of 6 and 50 nm, respectively, were observed near the substrate by selected area diffraction pattern and high-resolution image. The epitaxial relationship between c-GaN and h-GaN was determined as h-GaN (0 0 0 1)Ec-GaN(1 1 1), h-GaN(1 0!1 1)EcGaN(1 0 0) and h-GaN[1 1!2 0]Ec-GaN[1 1 0]. Because the boundary between c- and h-GaN has high density of dislocations extending to in-plane of
Fig. 6. (a) Highly enlarged cross-sectional TEM micrograph at the interface of c- and h-GaN in the zigzag structure. (b) Schematic [1 1!2 0] projection of an interface of h-GaN(1 0!1 1) and c-GaN(1 0 0) showing a high density of dislocations.
the film, the mixed cubic-hexagonal character near the substrate may play an important role in relaxation of large misfit stress created by lattice mismatching between GaN and the substrate and also in decreasing dislocation density extending to the growth direction.
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References [1] H.Z. Xiao, N.E. Lee, R.C. Powell, Z. Ma, L.J. Chou, L.H. Allen, J.E. Green, A. Rockett, Appl. Phys. Lett. 76 (1994) 8195. [2] B.N. Sverdlov, G.A. Martin, H. Morkoc, D.J. Smith, Appl. Phys. Lett. 67 (1995) 2063. [3] D.J. Smith, D. Chandrasekhar, B. Sverdlov, A. Botchkarev, A. Salvador, H. Morkoc, Appl. Phys. Lett. 67 (1995) 1830. [4] L.T. Romano, J.E. Northrup, M.A. O’Keefe, Appl. Phys. Lett. 69 (1996) 2394. [5] F.A. Ponce, D. Cherns, W.T. Young, J.W. Steeds, Appl. Phys. Lett. 69 (1996) 770.
[6] X.H. Wu, D. Kapolnek, E.J. Tarsa, B. Heying, S. Keller, B.P. Keller, U.K. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 1371. [7] F.R. Chien, X.J. Ning, S. Stemmer, P. Pirouz, M.D. Bremser, R.F. Davis, Appl. Phys. Lett. 68 (1996) 2678. [8] Y. Xin, P.D. Brown, C.J. Humphreys, T.S. Cheng, C.T. Foxon, Appl. Phys. Lett. 70 (1997) 1308. [9] N. Grandjean, J. Massies, P. Vennegues, M. Laugt, M. Leroux, Appl. Phys. Lett. 70 (1997) 643. [10] S.H. Cho, K. Hata, T. Maruyama, K. Akimoto, J. Crystal Growth 173 (1997) 260. [11] X.H. Wu, L.M. Brown, D. Kapolnek, S. Keller, B. Keller, S.P. DenBaars, J.S. Speck, J. Appl. Phys. 80 (1996) 3228.
Interface structure of GaN on sapphire (0 0 0 1) studied by transmission electron microscope Tsuyoshi Onitsuka!, Takahiro Maruyama!, Katsuhiro Akimoto!,*, Yoshio Bando" ! Institute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305, Japan " National Institute for Research in Inorganic Materials, Tsukuba, Ibaraki 305, Japan
Abstract Structural defects of GaN grown by plasma-assisted molecular beam epitaxy on sapphire (0 0 0 1) substrates have been studied by transmission electron microscope (TEM). Cubic GaN (c-GaN) islands, surrounded by hexagonal GaN (h-GaN), with the typical height and width of 6 and 50 nm, respectively, were observed in the vicinity of the substrate by selected area diffraction pattern and high-resolution image. The epitaxial relationship between c-GaN and h-GaN was determined as h-GaN (0 0 0 1)Ec-GaN (1 1 1), h-GaN(1 0!1 1)Ec-GaN (1 0 0) and h-GaN[1 1!2 0]Ec-GaN [1 1 0]. Because the boundary between c-GaN and h-GaN has high density of dislocations, the mixed cubic-hexagonal character near the substrate may play an important role in the relaxation of large misfit stress created by lattice mismatching between GaN and sapphire substrate. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 61.72.Nn; 68.55.!a; 81.05.Ea; 81.15.!z Keywords: Cubic GaN; Hexagonal GaN; TEM; Interface structure
1. Introduction GaN and related III—V nitrides are the focus of intensive research due to their potential for blue—violet light-emitting devices. However, epitaxial GaN layers used for device fabrication are usually highly faulted hexagonal crystals due to the lack of suitable substrate materials that are both lattice and thermally matched to these nitrides [1].
* Corresponding author. Tel.: #81 298 53 5274; fax: #81 298 55 7440; e-mail: [email protected].
Microstructural characterization of GaN by transmission electron microscopy (TEM) has been performed by many groups [2—9]. Threading dislocations are the most common line defects observed for films grown on sapphire (0 0 0 1) substrates. These dislocations are found to be originated at the substrate interface and caused by the inversion domain boundaries (IDB), double positioning boundaries (DPB) and stacking mismatch boundaries (SMB) [2,3]. These threading dislocations may contribute to a relaxation of misfit stress. We observed mixture of cubic and hexagonal GaN (c-GaN and h-GaN) only near the sapphire
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 2 6 6 - 8
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substrate. Though the mixed cubic-hexagonal material includes high density of dislocations, threading dislocations are not generated. In this paper, we report the observation of mixture of cubic-hexagonal material near the substrate, characterization of epitaxial relationship between c-GaN and hGaN, and discuss the role of the mixed phase from the view of the relaxation of misfit stress.
2. Experimental procedure The GaN films were grown on sapphire (0 0 0 1) substrates by radio frequency plasma-assisted molecular beam epitaxy. A general description of the system and the typical conditions used for nitride growth have been reported previously [10]. The substrates were chemically treated with 3 : 1 of H SO : H PO at 160°C and inserted into the 2 4 3 4 growth chamber. Prior to growth, the substrates were degassed at 800°C followed by nitridation of the substrates for 20 min. No buffer layer of AlN nor GaN were deposited at the beginning of the epitaxy. Total film thickness was 0.8 lm with the
growth rate of 0.4 lm/h and growth temperature was 700°C. Photoluminescence spectra of the samples measured at 77 K were dominated by the band edge emission and the intensity of deep level emission were about two orders of magnitude less than that of the band edge emission indicating high quality of crystals. TEM specimens were prepared in cross section, by first mechanically thinning to a thickness of about 10 lm, and then ion-milling to electron transparency with 4 keV Ar ions, for normal observation in the [1 1!2 0] projection. The samples were observed with a JEM-2000EX high-resolution electron microscope operated at 200 keV.
3. Results and discussion A typical low-magnification electron micrograph of a defective GaN film grown with RF plasma power of 150 W is shown in Fig. 1. The morphology is dominated by threading defects extending away from the substrate. The dislocation density was estimated to be about 1]109 cm~2. The
Fig. 1. Cross-sectional TEM micrograph of the GaN epilayer on the sapphire substrate.
T. Onitsuka et al. / Journal of Crystal Growth 189/190 (1998) 295–300
Fig. 2. (a) SAD pattern from both the film and the substrate. (b) The basic units of the diffraction pattern due to Al O , h2 3 and c-GaN are indicated and indexed.
corresponding [1 1!2 0] selected-area electron diffraction (SAD) pattern from both the film and substrate is shown in Fig. 2a. The SAD pattern shows that h-GaN grows epitaxially with the epitaxial relationship of h-GaN (0 0 0 1)EAl O (0 0 0 1) and 2 3 GaN [!1 2!1 0]EAl O [0 1!1 0] as shown in 2 3 Fig. 2b, in agreement with previous results [1]. In addition to hexagonal diffraction spots, extra diffraction spots are also observed as indicated by arrows in Fig. 2a. These 1 order spots can be inter3 preted as cubic (2 2 0) diffraction spots and indexed as shown in Fig. 2b. The mixture of c- and h-GaN phase was already observed [3,6], however, the distribution of each phases and orientation relationship between these two phases are not clear yet. The interplanar spacing evaluated from the extra spots was 2.5 A_ which is very close to that of c-GaN [1 1 1] interplanar spacing. Therefore, epitaxial
297
relationship between h-GaN and c-GaN is h-GaN (0 0 0 1)Ec-GaN (1 1 1) and h-GaN [1 1!2 0]EcGaN [1 1 0]. Dark field image along the [1 1!2 0] with the extra 1 order spot is shown in Fig. 3. As can be seen 3 in the figure, the extra spots originate only from the interface region, that is, c-GaN was grown only near the substrate. The dimension of the c-GaN island is about 6 nm height with 20—50 nm wide and the distribution of islands seems to be roughly periodic. A high-resolution TEM image of an interface between c-GaN and h-GaN is shown in Fig. 4. The interface is sharp with no interfacial amorphous layer as indicated by solid line. In the c-GaN region, the FCC stacking sequence (ABCABC2) can easily be observed, and in the h-GaN region, hexagonal stacking sequence (ABAB2) can be observed. A highly enlarged image of the interface is shown in Fig. 5 for reference. Coming back again to Fig. 4, we can see that the interface parallel to the growth surface is flat, on the other hand, the interface perpendicular to the growth surface has zigzag structure. The zigzag structure has just 6lattice spacing period for the growth direction as shown in Fig. 4. This precise periodicity can be interpreted as follows. As mentioned above, the FCC stacking sequence is ABCABC2 and hexagonal stacking sequence is ABAB2, so the same stacking meets every 6-lattice spacing. As far as satisfying such a stacking sequences, dislocations extending to the growth direction would not be generated. In fact, no dislocation extending to the growth direction is observed at the interface of c-GaN and h-GaN. The interface between c-GaN and h-GaN consists of two kinds of facets, one is parallel to the growth surface, so the interface can be assigned as c-GaN (1 1 1) and h-GaN (0 0 0 1). The other interface is perpendicular to the growth surface which has an angle of about 30° from [0 0 0 1] direction. The atomic row indicated by a rod in h-GaN region in Fig. 4 makes an angle of 30° with [0 0 0 1] direction. The angle between h-GaN (0 0 0 1) and h-GaN (1 0!1 1) is 28°, so the interface between cand h-GaN for the hexagonal side may be (1 0!1 1). The atomic row indicated by a rod in c-GaN region makes an angle of 35° with [1 1 1] direction, which
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Fig. 3. Dark field image along the [1 1!2 0] with the extra 1 order spot as shown in Fig. 2. 3
Fig. 4. High-resolution TEM image of epitaxial GaN at the interface of c- and h-GaN. The growth direction is the upside of the photograph. The facets of c-GaN (1 0 0) and h-GaN (1 0!1 1) are indicated by rods.
Fig. 5. Highly enlarged cross-sectional TEM micrograph at the interface of c- and h-GaN showing the stacking sequences.
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is just the same as the angle between (1 0 0) and (1 1 1). So the interface for the cubic side may be (1 0 0). That is, the interface between cand h-GaN perpendicular to the growth surface can be assigned as c-GaN (1 0 0) and h-GaN (1!1 0 1). A highly enlarged image of the interface is shown in Fig. 6a and the structure model for the interface is shown in Fig. 6b. As shown in the model, the interface contains high density of dangling bonds or dislocations which extends not to the growth direction but to the in-plane of the GaN film. These dislocations may relax the misfit stress caused by the lattice mismatching between GaN and Al O 2 3 substrate. Though the IDB and SMB seem to be responsible for the threading dislocations which deteriorates crystal quality, the formation of cubic-hexagonal mixture at the initial growth stage may have a good effect on the crystal quality. The cause of the formation of the mixed phase only near the substrate is not known at present, however, it may be possible to decrease the dislocation density which extends to the growth direction by controlling the island size and density of c-GaN. The mixed phase is also observed near the substrate in GaN grown by metal organic vapor phase epitaxy (MOVPE) [6,11]. The effect of the mixed phase on relaxation of the stress and reducing the dislocation density may be similar between MBE and MOVPE grown GaN.
4. Summary Structural defect of GaN grown by plasma-assisted molecular beam epitaxy on sapphire (0 0 0 1) substrates have been studied by high-resolution TEM. The islands of c-GaN, surrounded by hGaN, with the typical height and width of 6 and 50 nm, respectively, were observed near the substrate by selected area diffraction pattern and high-resolution image. The epitaxial relationship between c-GaN and h-GaN was determined as h-GaN (0 0 0 1)Ec-GaN(1 1 1), h-GaN(1 0!1 1)EcGaN(1 0 0) and h-GaN[1 1!2 0]Ec-GaN[1 1 0]. Because the boundary between c- and h-GaN has high density of dislocations extending to in-plane of
Fig. 6. (a) Highly enlarged cross-sectional TEM micrograph at the interface of c- and h-GaN in the zigzag structure. (b) Schematic [1 1!2 0] projection of an interface of h-GaN(1 0!1 1) and c-GaN(1 0 0) showing a high density of dislocations.
the film, the mixed cubic-hexagonal character near the substrate may play an important role in relaxation of large misfit stress created by lattice mismatching between GaN and the substrate and also in decreasing dislocation density extending to the growth direction.
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References [1] H.Z. Xiao, N.E. Lee, R.C. Powell, Z. Ma, L.J. Chou, L.H. Allen, J.E. Green, A. Rockett, Appl. Phys. Lett. 76 (1994) 8195. [2] B.N. Sverdlov, G.A. Martin, H. Morkoc, D.J. Smith, Appl. Phys. Lett. 67 (1995) 2063. [3] D.J. Smith, D. Chandrasekhar, B. Sverdlov, A. Botchkarev, A. Salvador, H. Morkoc, Appl. Phys. Lett. 67 (1995) 1830. [4] L.T. Romano, J.E. Northrup, M.A. O’Keefe, Appl. Phys. Lett. 69 (1996) 2394. [5] F.A. Ponce, D. Cherns, W.T. Young, J.W. Steeds, Appl. Phys. Lett. 69 (1996) 770.
[6] X.H. Wu, D. Kapolnek, E.J. Tarsa, B. Heying, S. Keller, B.P. Keller, U.K. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 1371. [7] F.R. Chien, X.J. Ning, S. Stemmer, P. Pirouz, M.D. Bremser, R.F. Davis, Appl. Phys. Lett. 68 (1996) 2678. [8] Y. Xin, P.D. Brown, C.J. Humphreys, T.S. Cheng, C.T. Foxon, Appl. Phys. Lett. 70 (1997) 1308. [9] N. Grandjean, J. Massies, P. Vennegues, M. Laugt, M. Leroux, Appl. Phys. Lett. 70 (1997) 643. [10] S.H. Cho, K. Hata, T. Maruyama, K. Akimoto, J. Crystal Growth 173 (1997) 260. [11] X.H. Wu, L.M. Brown, D. Kapolnek, S. Keller, B. Keller, S.P. DenBaars, J.S. Speck, J. Appl. Phys. 80 (1996) 3228.