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Microstructures of GaN films grown by low pressure metal-organic vapor phase epitaxy on (011̄2) sapphire substrates
Journal of Crystal Growth 191 (1998) 641—645
Microstructures of GaN films grown by low pressure metalorganic vapor phase epitaxy on (0 1 11 2) sapphire substrates Lisen Cheng!,*, Ze Zhang!, Guoyi Zhang", Zhijian Yang" ! Beijing Laboratory of Electron Microscopy, Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, 100080 Beijing, People+s Republic of China " Laboratory of Mesoscopic Physics, Department of Physics, Peking University, 100871 Beijing, People+s Republic of China Received 23 December 1997; accepted 25 February 1998
Abstract Microstructures of GaN films grown by low pressure metalorganic vapor-phase epitaxy on (0 1 11 2) sapphire substrates were investigated using transmission electron microscopy. The crystallographic structure of the GaN buffer layer grown at 550°C was uniquely hexagonal. Grain boundaries and stacking faults in the as-grown buffer layer are much less than those in the buffer layer grown on (0 0 0 1) sapphire substrates. Defects in the as-grown epitaxial layers are predominantly edge type dislocations with Burgers vector b"1 S1 11 0 0T. No screw dislocations or domain boundaries 3 in the epilayer were observed in the as-grown samples. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 81.15.Gh; 81.05.Ea; 61.16.Bg Keywords: GaN; MOVPE; Microstructure; Dislocation
Because of its potential applications in the field of opto-electronics, GaN, especially in the form of thin films, has been studied extensively and systematically in the last few years [1]. GaN films are usually grown by metalorganic vapor-phase epitaxy (MOVPE) or molecular beam epitaxy (MBE) on sapphire substrates [2,3]. Gallium nitride has been deposited on many different sapphire orientations including a-plane (1 1 21 0), m-plane (1 0 11 0), r-plane (0 1 11 2) and c-plane (0 0 0 1) [4,5]. The
* Corresponding author. E-mail: [email protected].
crystallinity of the GaN films is closely related to the orientation of the sapphire substrates; typically the highest crystal quality is obtained on the (0 0 0 1) sapphire substrates. Therefore, most investigations on the GaN films, including physical and optical properties, growth techniques, microstructures and defects, and fabrication of light emitting diodes focused on the films grown on (0 0 0 1) basal plane sapphire [6—9]. However, it is difficult to cleave the GaN films grown on (0 0 0 1) plane sapphire which will limit, to certain extent, applications of the GaN films in the manufacture of laser diodes. Thus, other sapphire substrate orientations should be evaluated as some researchers’ results
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 3 8 3 - 2
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Lisen Cheng et al. / Journal of Crystal Growth 191 (1998) 641–645
indicated that the GaN films grown on (0 1 11 2) sapphire substrates have better electrical transportation and photoluminescence properties than those grown on (0 0 0 1) sapphire [2,10]. Distinctive surface morphologies of GaN films grown on the respective orientation sapphire substrates have been observed by scanning electron microscopy (SEM) [10]. This implies that the growth behavior, and therefore the microstructures, of GaN films grown on the respective orientation sapphire substrates should also be distinct. The electrical and photoluminescence properties of the GaN films grown on different orientation wafers of sapphire also suggest that the microstructures of the respective films are different [2]. Unfortunately, no detailed results about the microstructures of GaN films grown on (0 1 11 2) sapphire substrates have been reported so far. In this article, transmission electron microscopy (TEM) was utilized to investigate the microstructures of GaN films grown using the so-called two-step growth technique [11] on (0 1 11 2) sapphire substrates. In this investigation, GaN films were grown by using low pressure MOVPE on (0 1 11 2) sapphire substrates in a horizontal reactor. Pretreatment of the substrates and all the growth conditions were the same as those for growing GaN films on (0 0 0 1) basal plane sapphire [12]. Prior to the growth of the epitaxial layer of GaN at 1030°C, a thin GaN buffer layer with a thickness of about 25 nm was grown on the sapphire substrates at 550°C, then heated to 1030°C, and annealed at this temperature for 2 min before commencing deposition of GaN epitaxial layer. The average growth rate of the epitaxial GaN layer grown on (0 1 11 2) sapphire was measured to be 6 lm/h which is about 6 times that on (0 0 0 1) sapphire substrates. Thin foils of cross-sectional GaN films for TEM study were prepared by the standard technique. The as-grown films were first glued face to face and then ground and polished to the thickness of about 20 lm, and finally ion milled to the thickness which is transparent to the electron beam. During the ion milling process, the voltage as high as 5.5 kV and the ion beam current of about 1 mA were utilized to speed the thinning process. Diffraction contrast analysis was performed on a Phillips CM-12 transmission electron microscope with
the accelerating voltage of 120 kV and point to point resolution of 0.34 nm. High-resolution images were recorded by a JEOL-2010 high resolution electron microscope with the accelerating voltage of 200 kV and point to point resolution of 0.19 nm. As the GaN buffer layer is only 25 nm in thickness, large magnifications are necessary. High-resolution electron microscopy images were recorded in the vicinity of the interface between the GaN films and the sapphire substrates with the electron beam parallel to the [1 11 0 1] zone axis of GaN. A representative image, Fig. 1a, shows good quality epitaxy of GaN over the (0 1 11 2) sapphire surface. Scrutiny of the interface areas indicates that the crystallographic structure of the as-grown GaN buffer layer is uniquely hexagonal, which is different from the case of GaN films grown on (0 0 0 1) sapphire substrates, in which two kinds of GaN buffer layers were identified: a predominantly cubic, and a predominantly hexagonal structure [12,13]. Both kinds of the GaN buffer layers are mixtures of the cubic and hexagonal phase. In the as-grown specimen, no atomically abrupt interface between the GaN buffer layer and the epitaxial layer is observed, and in addition, the grain boundaries and stacking faults which are present with high density in the buffer layer over (0 0 0 1) sapphire substrates were rarely visualized. Amorphous regions in the buffer layer were sometimes observed in the specimen as indicated by the arrow head in Fig. 1a. To obtain the orientation relationship between the GaN films and the sapphire substrates, a composite selected area electron diffraction pattern was taken at the interface between the GaN film and the sapphire substrate with the electron beam parallel to the [0 1 11 1] azimuth of GaN and is presented in Fig. 1b. By indexing of the diffraction spots in this pattern, the orientation relationships can be determined as follows: (1 1 21 0) //(0 1 11 2) , G!N A-2O3 [1 11 0 0] //[1 1 21 0] , G!N A-2O3 [0 0 0 1] //[11 1 0 1] . G!N A-2O3 To further investigate the microstructures of the GaN epitaxial layer, especially dislocations and
Lisen Cheng et al. / Journal of Crystal Growth 191 (1998) 641–645
Fig. 1. (a) Cross-sectional high-resolution electron microscopy image near the interface between GaN film and sapphire substrate with the electron beam parallel to the [1 11 0 1] zone axis of GaN showing the hexagonal crystallographic structure of GaN buffer layer. (b) Composite selected area electron diffraction pattern which is superimposed by the diffraction pattern of [1 11 0 0] zone axis of GaN and that of the [1 1 21 0] zone axis of sapphire, respectively.
domain boundaries, a diffraction contrast technique was employed. A central dark field image of the as-grown film cross-section recorded near the [3 31 0 2] zone axis with the operative u"2 0 21 31 is shown in Fig. 2a. Surprisingly, a high density of
643
dislocations was obvious in this image (see the regions indicated by the small solid and large open arrow heads). Most dislocations were initiated from the interface between the GaN films and the sapphire substrates and extended up to the film surface. When the same area as that shown in Fig. 2a was imaged near the [2 11 11 0] zone axis with the operative u"0 0 0 2, most of the threading dislocations are out of contrast (see the same regions indicated by the solid and open arrow heads, respectively, in Fig. 2b as those in Fig. 2a). Further analyses indicate that a portion of the dislocations which were out of contrast in Fig. 2b can also be out of contrast under the excitation of u"1 1 21 0 when imaged near the [2 21 0 1] zone axis of GaN, but the others still remain in contrast (see the same areas indicated by the solid and open arrow heads in Fig. 2c as those in Fig. 2b). According to the contrast extinction criteria u ) b"0, the Burgers vector of the dislocations which can be out of contrast under the excitation of both u"0 0 0 2 and u"1 1 21 0 is determined to be parallel to the [1 11 0 0] direction of GaN. According to the crystallographic structure of wurtzite GaN, the possible magnitude of the Burgers vector should be DbD"1 [1 11 0 0]. 3 When the same region was imaged with the operative u"2 11 11 21 near the [2 21 0 3] zone axis, some dislocations which are in contrast while imaged with operative u"1 1 21 0 (see the area indicated by the open arrow head in Fig. 2c) but out of contrast while imaged with u"0 0 0 2 are out of contrast as shown in the area indicated by the open arrow head in Fig. 2d. Thus, the Burgers vector of these dislocations is deduced as b"1 [0 11 1 0]. Dislocations 3 with the Burgers vector b"1 [1 0 11 0] were also 3 observed in the same aforementioned region. No screw dislocations in the as-grown films were identified in this investigation. Therefore, we conclude that the dislocations formed in the as-grown GaN films are predominantly edge type dislocations with the Burgers vector b"1 S1 11 0 0T which 3 are different from those (with the most commonly observed Burgers vector b"1 S1 1 21 0T) formed in 3 the films grown on (0 0 0 1) sapphire substrates [14]. Another interesting feature of the as-grown films is that no domain boundaries which are usually visualized in the films grown on (0 0 0 1)
644
Lisen Cheng et al. / Journal of Crystal Growth 191 (1998) 641–645
Fig. 2. Central dark field images of a specific area in the as-grown film recorded with the operative (a) u"2 0 21 31 , (b) u"0 0 0 2, (c) u"1 1 21 0, and (d) u"2 11 11 21 , respectively.
wafer of sapphire [15] were observed in this experiment. In summary, the microstructures of GaN films grown on (0 1 11 2) sapphire substrates are different from those of the films grown on (0 0 0 1) sapphire substrates. The crystallographic structure of the as-grown GaN buffer layer is uniquely hexagonal. Both grain boundaries and stacking faults which are present frequently in the GaN buffer layer grown on (0 0 0 1) sapphire substrates are rarely observed in the as-grown buffer layer. No domain boundaries and atomically abrupt interface between GaN buffer layer and the epitaxial layer were viewed in the films grown on (0 1 11 2) sapphire substrates. The dislocations in the as-grown films are predominantly
edge type dislocations with the Burgers vector b"1 S1 11 0 0T. 3 References [1] I. Akasaki, H. Amano, International Symposium on Blue Laser and Light Emitting Diodes, Chiba University, Japan, 1996, pp. 11—16. [2] Y. Sasaki, S. Zembutsu, J. Appl. Phys. 61 (1987) 2533. [3] T.D. Moustakas, T. Lei, R.J. Molnar, Physica B 185 (1993) 36. [4] H. Amano, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 27 (1988) L1384. [5] K. Iwata, H. Asahi, K. Asami, R. Kuroiwa, Shun-ichi Gonda, Jpn. J. Appl. Phys. 36 (1997) L661. [6] A. Shintani, Y. Takano, S. Minagawa, M. Maki, J. Electrochem. Soc.: Solid-State Sci. Technol. 125 (1978) 2076.
Lisen Cheng et al. / Journal of Crystal Growth 191 (1998) 641–645 [7] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki, J. Crystal Growth 98 (1989) 209. [8] P.A. Grudowski, A.L. Holmes, C.J. Eiting, R.D. Dypuis, J. Electron. Mater. 26 (1997) 257. [9] Y. Xin, P.D. Brown, C.J. Humphreys, T.S. Cheng, C.T. Foxon, Appl. Phys. Lett. 70 (1997) 1308. [10] W.A. Melton, J.I. Pankove, J. Crystal Growth 178 (1997) 168. [11] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353.
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[12] L.S. Cheng, G.Y. Zhang, D.P. Yu, Z. Zhang, Appl. Phys. Lett. 70 (1997) 1408. [13] X.H. Wu, D. Kapolnek, E.J. Tarsa, B. Heying, S. Keller, B.P. Killer, U.K. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 1371. [14] X.J. Ning, F.R. Chien, P. Pirouz, J.W. Yang, M.A. Khan, J. Mater. Res. 11 (1996) 580. [15] L.T. Romano, J.E. Northrup, M.A. O’Keefe, Appl. Phys. Lett. 69 (1996) 239.
Microstructures of GaN films grown by low pressure metalorganic vapor phase epitaxy on (0 1 11 2) sapphire substrates Lisen Cheng!,*, Ze Zhang!, Guoyi Zhang", Zhijian Yang" ! Beijing Laboratory of Electron Microscopy, Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, 100080 Beijing, People+s Republic of China " Laboratory of Mesoscopic Physics, Department of Physics, Peking University, 100871 Beijing, People+s Republic of China Received 23 December 1997; accepted 25 February 1998
Abstract Microstructures of GaN films grown by low pressure metalorganic vapor-phase epitaxy on (0 1 11 2) sapphire substrates were investigated using transmission electron microscopy. The crystallographic structure of the GaN buffer layer grown at 550°C was uniquely hexagonal. Grain boundaries and stacking faults in the as-grown buffer layer are much less than those in the buffer layer grown on (0 0 0 1) sapphire substrates. Defects in the as-grown epitaxial layers are predominantly edge type dislocations with Burgers vector b"1 S1 11 0 0T. No screw dislocations or domain boundaries 3 in the epilayer were observed in the as-grown samples. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 81.15.Gh; 81.05.Ea; 61.16.Bg Keywords: GaN; MOVPE; Microstructure; Dislocation
Because of its potential applications in the field of opto-electronics, GaN, especially in the form of thin films, has been studied extensively and systematically in the last few years [1]. GaN films are usually grown by metalorganic vapor-phase epitaxy (MOVPE) or molecular beam epitaxy (MBE) on sapphire substrates [2,3]. Gallium nitride has been deposited on many different sapphire orientations including a-plane (1 1 21 0), m-plane (1 0 11 0), r-plane (0 1 11 2) and c-plane (0 0 0 1) [4,5]. The
* Corresponding author. E-mail: [email protected].
crystallinity of the GaN films is closely related to the orientation of the sapphire substrates; typically the highest crystal quality is obtained on the (0 0 0 1) sapphire substrates. Therefore, most investigations on the GaN films, including physical and optical properties, growth techniques, microstructures and defects, and fabrication of light emitting diodes focused on the films grown on (0 0 0 1) basal plane sapphire [6—9]. However, it is difficult to cleave the GaN films grown on (0 0 0 1) plane sapphire which will limit, to certain extent, applications of the GaN films in the manufacture of laser diodes. Thus, other sapphire substrate orientations should be evaluated as some researchers’ results
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 3 8 3 - 2
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Lisen Cheng et al. / Journal of Crystal Growth 191 (1998) 641–645
indicated that the GaN films grown on (0 1 11 2) sapphire substrates have better electrical transportation and photoluminescence properties than those grown on (0 0 0 1) sapphire [2,10]. Distinctive surface morphologies of GaN films grown on the respective orientation sapphire substrates have been observed by scanning electron microscopy (SEM) [10]. This implies that the growth behavior, and therefore the microstructures, of GaN films grown on the respective orientation sapphire substrates should also be distinct. The electrical and photoluminescence properties of the GaN films grown on different orientation wafers of sapphire also suggest that the microstructures of the respective films are different [2]. Unfortunately, no detailed results about the microstructures of GaN films grown on (0 1 11 2) sapphire substrates have been reported so far. In this article, transmission electron microscopy (TEM) was utilized to investigate the microstructures of GaN films grown using the so-called two-step growth technique [11] on (0 1 11 2) sapphire substrates. In this investigation, GaN films were grown by using low pressure MOVPE on (0 1 11 2) sapphire substrates in a horizontal reactor. Pretreatment of the substrates and all the growth conditions were the same as those for growing GaN films on (0 0 0 1) basal plane sapphire [12]. Prior to the growth of the epitaxial layer of GaN at 1030°C, a thin GaN buffer layer with a thickness of about 25 nm was grown on the sapphire substrates at 550°C, then heated to 1030°C, and annealed at this temperature for 2 min before commencing deposition of GaN epitaxial layer. The average growth rate of the epitaxial GaN layer grown on (0 1 11 2) sapphire was measured to be 6 lm/h which is about 6 times that on (0 0 0 1) sapphire substrates. Thin foils of cross-sectional GaN films for TEM study were prepared by the standard technique. The as-grown films were first glued face to face and then ground and polished to the thickness of about 20 lm, and finally ion milled to the thickness which is transparent to the electron beam. During the ion milling process, the voltage as high as 5.5 kV and the ion beam current of about 1 mA were utilized to speed the thinning process. Diffraction contrast analysis was performed on a Phillips CM-12 transmission electron microscope with
the accelerating voltage of 120 kV and point to point resolution of 0.34 nm. High-resolution images were recorded by a JEOL-2010 high resolution electron microscope with the accelerating voltage of 200 kV and point to point resolution of 0.19 nm. As the GaN buffer layer is only 25 nm in thickness, large magnifications are necessary. High-resolution electron microscopy images were recorded in the vicinity of the interface between the GaN films and the sapphire substrates with the electron beam parallel to the [1 11 0 1] zone axis of GaN. A representative image, Fig. 1a, shows good quality epitaxy of GaN over the (0 1 11 2) sapphire surface. Scrutiny of the interface areas indicates that the crystallographic structure of the as-grown GaN buffer layer is uniquely hexagonal, which is different from the case of GaN films grown on (0 0 0 1) sapphire substrates, in which two kinds of GaN buffer layers were identified: a predominantly cubic, and a predominantly hexagonal structure [12,13]. Both kinds of the GaN buffer layers are mixtures of the cubic and hexagonal phase. In the as-grown specimen, no atomically abrupt interface between the GaN buffer layer and the epitaxial layer is observed, and in addition, the grain boundaries and stacking faults which are present with high density in the buffer layer over (0 0 0 1) sapphire substrates were rarely visualized. Amorphous regions in the buffer layer were sometimes observed in the specimen as indicated by the arrow head in Fig. 1a. To obtain the orientation relationship between the GaN films and the sapphire substrates, a composite selected area electron diffraction pattern was taken at the interface between the GaN film and the sapphire substrate with the electron beam parallel to the [0 1 11 1] azimuth of GaN and is presented in Fig. 1b. By indexing of the diffraction spots in this pattern, the orientation relationships can be determined as follows: (1 1 21 0) //(0 1 11 2) , G!N A-2O3 [1 11 0 0] //[1 1 21 0] , G!N A-2O3 [0 0 0 1] //[11 1 0 1] . G!N A-2O3 To further investigate the microstructures of the GaN epitaxial layer, especially dislocations and
Lisen Cheng et al. / Journal of Crystal Growth 191 (1998) 641–645
Fig. 1. (a) Cross-sectional high-resolution electron microscopy image near the interface between GaN film and sapphire substrate with the electron beam parallel to the [1 11 0 1] zone axis of GaN showing the hexagonal crystallographic structure of GaN buffer layer. (b) Composite selected area electron diffraction pattern which is superimposed by the diffraction pattern of [1 11 0 0] zone axis of GaN and that of the [1 1 21 0] zone axis of sapphire, respectively.
domain boundaries, a diffraction contrast technique was employed. A central dark field image of the as-grown film cross-section recorded near the [3 31 0 2] zone axis with the operative u"2 0 21 31 is shown in Fig. 2a. Surprisingly, a high density of
643
dislocations was obvious in this image (see the regions indicated by the small solid and large open arrow heads). Most dislocations were initiated from the interface between the GaN films and the sapphire substrates and extended up to the film surface. When the same area as that shown in Fig. 2a was imaged near the [2 11 11 0] zone axis with the operative u"0 0 0 2, most of the threading dislocations are out of contrast (see the same regions indicated by the solid and open arrow heads, respectively, in Fig. 2b as those in Fig. 2a). Further analyses indicate that a portion of the dislocations which were out of contrast in Fig. 2b can also be out of contrast under the excitation of u"1 1 21 0 when imaged near the [2 21 0 1] zone axis of GaN, but the others still remain in contrast (see the same areas indicated by the solid and open arrow heads in Fig. 2c as those in Fig. 2b). According to the contrast extinction criteria u ) b"0, the Burgers vector of the dislocations which can be out of contrast under the excitation of both u"0 0 0 2 and u"1 1 21 0 is determined to be parallel to the [1 11 0 0] direction of GaN. According to the crystallographic structure of wurtzite GaN, the possible magnitude of the Burgers vector should be DbD"1 [1 11 0 0]. 3 When the same region was imaged with the operative u"2 11 11 21 near the [2 21 0 3] zone axis, some dislocations which are in contrast while imaged with operative u"1 1 21 0 (see the area indicated by the open arrow head in Fig. 2c) but out of contrast while imaged with u"0 0 0 2 are out of contrast as shown in the area indicated by the open arrow head in Fig. 2d. Thus, the Burgers vector of these dislocations is deduced as b"1 [0 11 1 0]. Dislocations 3 with the Burgers vector b"1 [1 0 11 0] were also 3 observed in the same aforementioned region. No screw dislocations in the as-grown films were identified in this investigation. Therefore, we conclude that the dislocations formed in the as-grown GaN films are predominantly edge type dislocations with the Burgers vector b"1 S1 11 0 0T which 3 are different from those (with the most commonly observed Burgers vector b"1 S1 1 21 0T) formed in 3 the films grown on (0 0 0 1) sapphire substrates [14]. Another interesting feature of the as-grown films is that no domain boundaries which are usually visualized in the films grown on (0 0 0 1)
644
Lisen Cheng et al. / Journal of Crystal Growth 191 (1998) 641–645
Fig. 2. Central dark field images of a specific area in the as-grown film recorded with the operative (a) u"2 0 21 31 , (b) u"0 0 0 2, (c) u"1 1 21 0, and (d) u"2 11 11 21 , respectively.
wafer of sapphire [15] were observed in this experiment. In summary, the microstructures of GaN films grown on (0 1 11 2) sapphire substrates are different from those of the films grown on (0 0 0 1) sapphire substrates. The crystallographic structure of the as-grown GaN buffer layer is uniquely hexagonal. Both grain boundaries and stacking faults which are present frequently in the GaN buffer layer grown on (0 0 0 1) sapphire substrates are rarely observed in the as-grown buffer layer. No domain boundaries and atomically abrupt interface between GaN buffer layer and the epitaxial layer were viewed in the films grown on (0 1 11 2) sapphire substrates. The dislocations in the as-grown films are predominantly
edge type dislocations with the Burgers vector b"1 S1 11 0 0T. 3 References [1] I. Akasaki, H. Amano, International Symposium on Blue Laser and Light Emitting Diodes, Chiba University, Japan, 1996, pp. 11—16. [2] Y. Sasaki, S. Zembutsu, J. Appl. Phys. 61 (1987) 2533. [3] T.D. Moustakas, T. Lei, R.J. Molnar, Physica B 185 (1993) 36. [4] H. Amano, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 27 (1988) L1384. [5] K. Iwata, H. Asahi, K. Asami, R. Kuroiwa, Shun-ichi Gonda, Jpn. J. Appl. Phys. 36 (1997) L661. [6] A. Shintani, Y. Takano, S. Minagawa, M. Maki, J. Electrochem. Soc.: Solid-State Sci. Technol. 125 (1978) 2076.
Lisen Cheng et al. / Journal of Crystal Growth 191 (1998) 641–645 [7] I. Akasaki, H. Amano, Y. Koide, K. Hiramatsu, N. Sawaki, J. Crystal Growth 98 (1989) 209. [8] P.A. Grudowski, A.L. Holmes, C.J. Eiting, R.D. Dypuis, J. Electron. Mater. 26 (1997) 257. [9] Y. Xin, P.D. Brown, C.J. Humphreys, T.S. Cheng, C.T. Foxon, Appl. Phys. Lett. 70 (1997) 1308. [10] W.A. Melton, J.I. Pankove, J. Crystal Growth 178 (1997) 168. [11] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Appl. Phys. Lett. 48 (1986) 353.
645
[12] L.S. Cheng, G.Y. Zhang, D.P. Yu, Z. Zhang, Appl. Phys. Lett. 70 (1997) 1408. [13] X.H. Wu, D. Kapolnek, E.J. Tarsa, B. Heying, S. Keller, B.P. Killer, U.K. Mishra, S.P. DenBaars, J.S. Speck, Appl. Phys. Lett. 68 (1996) 1371. [14] X.J. Ning, F.R. Chien, P. Pirouz, J.W. Yang, M.A. Khan, J. Mater. Res. 11 (1996) 580. [15] L.T. Romano, J.E. Northrup, M.A. O’Keefe, Appl. Phys. Lett. 69 (1996) 239.