- Email: [email protected]
Surface morphology studies on sublimation grown GaN by atomic force microscopy
Journal of Crystal Growth 200 (1999) 348—352
Surface morphology studies on sublimation grown GaN by atomic force microscopy R.S. Qhalid Fareed *, S. Tottori, K. Nishino, S. Sakai Satellite Venture Business Laboratory, The University of Tokushima, 2-1 Minami josanjima, Tokushima 770-8506, Japan Department of Electrical and Electronic Engineering, The University of Tokushima, 2-1 Minami josanjima, Tokushima 770-8506, Japan Received 16 December 1998 Communicated by M. Schieber
Abstract Bulk single crystals and selectively grown gallium nitride (GaN) have been obtained using the sublimation technique. Crystals of size about 2—3 mm in length and 0.8—1.0 mm in width have been grown successfully. Atomic force microscopy has been employed to analyze the surface morphology of the as-grown samples to understand the growth mechanism. In free standing bulk GaN single crystals, two-dimensional growth is dominated by step growth mechanism. However, in the selective growth of GaN by sublimation, spiral growth originating from screw dislocation dominates. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: GaN; Single crystal; Selective growth; Sublimation; AFM
1. Introduction Gallium nitride (GaN) is known to be one of the most promising wide band gap semiconductor materials for opto-electronic devices operating in the blue spectrum e.g. highly effective blue light emitting diodes and short wavelength laser diodes for high optical storage [1—4]. One of the main problems in fabricating high-quality GaN layers is the lack of lattice matched substrates. Hence, it is a challenging task to grow GaN epitaxial layers with low defect density and flat surface morphology suitable for device structures. Bulk GaN crystals
* Corresponding author. E-mail: [email protected].
are the most suitable substrates for the growth of strain-free epilayers suitable for high-quality nitride-based laser diodes and devices. However, it is very difficult to grow these crystals due to their high melting temperature, low sublimation/decomposition temperature and very high nitrogen vapor pressures at moderate temperatures. Despite these difficulties, attempts to grow GaN by different techniques like high-pressure solution [5], vapor phase [6], thermal gradient solution growth [7], ammonolysis of gallium sub-oxide [8] have been reported. In recent years, large size GaN single crystals have been grown from Ga solution in the form of platelets with the high-pressure and high-temperature technique [9,10]. Yamane et al. [11] have reported the growth of GaN single crystals using Na flux with sizes of about 1 mm
0022-0248/99/$ — see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 4 3 3 - X
R.S. Qhalid Fareed et al. / Journal of Crystal Growth 200 (1999) 348—352
platelets. However, it is difficult to grow large quantites of GaN crystals using this technique. Sublimation method is one of the most simple methods for the growth of GaN crystals and does not require high-pressure and high-temperature. Using this method, there is a possibility of growing large size GaN single crystals on a large scale with better crystal quality. The growth of bulk GaN single crystals by the sublimation technique has been reported [12—15]. Due to nonavailability of large size single crystals, studies on their structural and surface properties have not yet been carried out in detail so far. Nowak et al. [16] have reported the surface morphologies on the as-grown and annealed bulk GaN crystals grown by the high-pressure and high-temperature method. Structural studies carried out on the platelet bulk GaN crystals have also been reported [17]. It is essential to understand the morphology and growth mechanism of these crystals as these crystals are intended for homo-epitaxial growth for fabricating device structures. In this paper, we present for the first time, the surface morphology of bulk GaN crystals and selective GaN epitaxial layer grown using the sublimation technique. In the present study, atomic force microscopy (AFM) has been applied to analyze the surface morphology of the as-grown samples.
2. Experimental procedure Free-standing bulk single crystals of GaN have been grown in a vertical quartz tube reactor with RF heating. The system used in the growth is schematically shown in Fig. 1. The source powder was synthesized by heating metallic gallium (Ga) in a quartz tube in NH atmosphere at 1000°C for 3 h. The source powder, quartz substrate holder and the substrate were placed in a carbon crucible with inductively coupled RF heating system in nitrogen ambient at atmospheric pressure. The flow rate of NH was 30—50 sccm. The crystals were grown by the self-nucleation method for a growth time of about 2 h in the temperature range 1000—1050°C. In one growth run, it is possible to get about 20—30 crystals of different sizes ranging from 0.5 to 3 mm in length. Due to spontaneous nucleation in initial
349
Fig. 1. Schematic diagram of the system used in the sublimation growth.
stages of growth, many micro-crystals were also found in the remaining source powder. Depending on the flow direction of NH and distribution of the source powder, the size and quality of the crystal differ remarkably. Gallium nitride has been selectively grown by the sublimation method on MOCVD GaN. The sample preparation was as follows: A 1.2 lm thick GaN epilayer was grown by MOCVD on a (0 0 0 1) sapphire substrate, using low-temperature GaN buffer layer. Using E-beam evaporation, 50 nm Si thin mask was coated on the epitaxial layers. The patterning of the mask was carried out by the conventional photolithography technique combined with wet etching. Square-shaped openings of size 500;500 lm were formed on the substrate. Growth was carried out at 1000—1030°C for about 2 h. About 60—70 lm thick GaN epitaxial layers were grown in the window area. Surface morphology of the as-grown bulk GaN single crystals and selectively grown epi-layers have been observed with an atomic force microscope (SII Seiko Instrument, Model SPA 300) using the noncontact mode.
3. Results and discussion Fig. 2 shows one of the free-standing single crystals grown by the sublimation method. The morphology of the crystal growing without the
350
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Fig. 2. One of the bulk GaN single crystals grown by the sublimation method.
intentional seeding of crystallization depends on the growth temperature, quality and distribution of the source in the crucible and distance between the source and the substrate. It is found that spontaneous nucleation growth occurred rapidly as the temperature is raised, leading to the formation of many microcrystallites. Hexagonal crystallites with size ranging from 2 to 3 mm in length and about 0.8 to 1.0 mm in diameter have been grown. The crystals are well faceted with c-axis along the length of the crystal. The longest axis was along (0 0 0 1) direction. X-ray diffraction studies have been carried out to know the quality of the crystal. The full-width at half-maximum (FWHM) are 48 arcsec along (0 0 0 2), 32 arcsec for (1 0 1 1) and 38 arcsec for (1 1 0 2) directions. The dislocation density of the bulk crystal measured using transmission electron microscopy (TEM) is found to be less than 10— cm [12,13]. Fig. 3a and Fig. 3b shows the AFM image of the as-grown bulk GaN crystal and the cross-sectional profile of the surface, respectively. Scanning was done on the [1 1 2 0] plane of the crystal. The surface of the crystal is almost atomically flat with clear step growth along the (0 0 0 1) direction. The average width of smaller and larger steps are about 60—70 and 110—120 nm, respectively. The height of the steps varies from 1 to 6 As , i.e in the range of atomic scale growth. The RMS roughness is found to be 1.434 As . The crystal is found to grow with steps of mono-molecular height in its surface and growth takes place by the advance of these steps
Fig. 3. (a) 2;2 lm area AFM image of GaN single crystals and (b) surface spectrum of the AFM image of a GaN crystal.
forming new molecular layers. The rate of the advancement of these steps depends on the structure and the direction of the growth. It is also observed that at the step edge, Ga micro-precipitates are oriented in a regular manner. Similar micro-precipitates of Ga have also been reported in the high-pressure grown bulk GaN [17]. This uniform distribution at the step edge also improves the stability of the growth face of the crystal and gives rise to an atomically flat surface leading to smooth morphology. Fig. 4 shows the AFM image of another bulk single crystal grown at the edge of the crucible under the same growth conditions. The surface of the crystals is microscopically flat with steps both parallel and perpendicular to the growth axis. In these crystals, regular steps of larger width can be observed along the growth axis, i.e. c-axis along
R.S. Qhalid Fareed et al. / Journal of Crystal Growth 200 (1999) 348—352
351
Fig. 5. Optical microscopic image of screw dislocations on selectively grown GaN. Fig. 4. AFM image of a bulk GaN single crystal grown at the edge of the crucible.
(0 0 0 1). The step height is about 2—3 nm and width is about 0.3—0.5 lm. Steps are also observed perpendicular to the c-axis, i.e. along (1 1 2 0) at irregular intervals. In this crystal, a twin step is also observed as shown in the figure. The formation of these steps mainly depends on the growth conditions. In the GaN crystals grown by the highpressure technique [17], flat and rough surface related to the crystal polarity along the c-axis are observed. These two different types of surface morphology have not been observed in sublimation grown crystals and studies to identify the crystal polarity are in progress. Fig. 5 shows the optical microscopic image of the screw dislocation observed on one of the edges of the windows in the selectively grown GaN deposited on the MOCVD-GaN by the sublimation method. The thickness of the epilayer is about 50—70 lm. A monolayer or sub-monolayer step originates from the point where screw dislocation line intersects the surface of the bulk plane. This step is constrained to terminate at the dislocation emergence point and winds up into a spiral during the growth process. According to BCF theory, the step width j, is given by j"20o , where ‘o ’ is the radius of the critical 2D nucleus [18]. In this selectively grown GaN, two-dimen-
Fig. 6. 20;20 lm area AFM image of a spiral growth on selectively grown GaN.
sional layer-by-layer growth originates from the screw dislocation. It is found that one plane is smooth whereas the other plane is little rough. In Fig. 6, AFM image of a spiral growth observed in the selective growth of GaN on MOCVD-GaN is shown. The presence of monoatomic steps on the layer surfaces, often in the form of large diameter (tens of lm) spirals suggests twodimensional growth on screw dislocations originating from the threading dislocation present in the substrate. The average step height is about 40—42 nm and width of each step is about 1 lm.
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The step due to a dislocation winds into a spiral in such a way that a single screw dislocation sends out successive turn of steps. This successive turning of steps forms pyramids on the crystal surface. If the rate of advancement of the step is independent of its orientation, the growing spiral forms a low cone but it will form a pyramid when the rate of advance depends on orientation. In the selective growth of GaN, the dislocations present in the MOCVD grown GaN have a component of displacement vector normal to the crystal face (0 0 0 1) at which they emerge. These are screw dislocations. Therefore, spiral growth is commonly observed in the selectively grown GaN epitaxial layers by sublimation.
4. Conclusions In summary, GaN single crystal growth and selective area epitaxy have been carried out by the sublimation method. Hexagonal crystallites with size ranging from 2 to 3 mm in length and about 0.8 to 1.0 mm in diameter have been grown. In the sublimation method, the surface morphology of the crystals depends on the growth conditions such as source material, temperature gradient and position of crystal nucleation. Surface analysis by AFM shows that the two-dimensional growth is dominated by step growth mechanism in bulk GaN crystals. However, in the selective growth of GaN, spiral growth mechanism originating from the dislocation present in the substrate is dominant. The present understanding is essential in homoepitaxial growth for good performance of the device structures.
References [1] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [2] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoko, Y. Sugimoto, Jpn. J. Appl. Phys. Part 2 35 (1996) 217. [3] I. Akasaki, S. Sota, H. Sakai, T. Tanaka, K. Koike, H. Amano, Electron Lett. 32 (1996) 1105. [4] R.P. Vaudo, I.K. Goepfert, T.D. Moustakas, D.M. Geyea, T.J. Frey, K. Meehan, J. Appl. Phys. 79 (1996) 2779. [5] R. Madar, G. Jacob, J. Hallais, R. Fruchart, J. Crystal Growth 31 (1975) 197. [6] E. Ejder, J. Crystal Growth 22 (1974) 44. [7] R.A. Logan, C.D. Thurmond, J. Electrochem. Soc. 119 (1972) 1727. [8] T.L. Chu, K. Ito, R.K. Smeltzer, S.C. Chu, J. Electrochem. Soc. 121 (1974) 159. [9] S. Porowski, I. Grzegory, J. Crystal Growth 178 (1997) 174. [10] I. Grzegory, J. Jun, M. Bockowski, ST. Krukowski, M. Wroblewski, B. Lucznik, S. Porowski, J. Phys. Chem. Solids 56 (1995) 639. [11] H. Yamane, M. Shimada, T. Sekiguchi, F.J. DiSalvo, J. Crystal Growth 186 (1998) 8. [12] S. Kurai, K. Nishino, S. Sakai, Jpn. J. Appl. Phys. 36 (1997) L184. [13] Y. Naoi, M. Kobatake, S. Kurai, K. Nishino, H. Sato, M. Nozaki, S. Sakai, Y. Shintani, J. Crystal Growth 190 (1997) 163. [14] C.M. Balkas, Z. Sitar, T. Zheleva, L. Bergman, I.K. Shmagin, J.F. Muth, R.M. Kolbas, R. Nemanich, R.F. Davis, Mater. Res. Soc. Symp. Proc., Vol. 449, p. 41. [15] I. Shmagin, J.F. Muth, J.H. Lee, R.M. Kolbas, C.M. Balkas, Z. Sitar, R.F. Davis, Appl. Phys. Lett. 71 (1997) 455. [16] G. Nowak, S. Krukowski, I. Grzegory, S. Porowski, J.M. Baranowski, K. Pakula, J. Zak, MRS Int. J. Nitride Semicond. Res. 1 (1996) 17. [17] Z. Liliental-Weber, C. Kisielowski, X. Liu, L. Schloss, U. Washburn, E.R. Weber, I. Grzegory, M. Bockowski, J. Jun, T. Suski, S. Porowski, Proceedings of the Workshop on GaN and Related Materials, Nagoya, 1995, p. 167. [18] W.K. Burton, N. Cabrera, F.C. Frank, Philos. Trans. Roy. Soc. A. 243 (1951) 300.
Surface morphology studies on sublimation grown GaN by atomic force microscopy R.S. Qhalid Fareed *, S. Tottori, K. Nishino, S. Sakai Satellite Venture Business Laboratory, The University of Tokushima, 2-1 Minami josanjima, Tokushima 770-8506, Japan Department of Electrical and Electronic Engineering, The University of Tokushima, 2-1 Minami josanjima, Tokushima 770-8506, Japan Received 16 December 1998 Communicated by M. Schieber
Abstract Bulk single crystals and selectively grown gallium nitride (GaN) have been obtained using the sublimation technique. Crystals of size about 2—3 mm in length and 0.8—1.0 mm in width have been grown successfully. Atomic force microscopy has been employed to analyze the surface morphology of the as-grown samples to understand the growth mechanism. In free standing bulk GaN single crystals, two-dimensional growth is dominated by step growth mechanism. However, in the selective growth of GaN by sublimation, spiral growth originating from screw dislocation dominates. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: GaN; Single crystal; Selective growth; Sublimation; AFM
1. Introduction Gallium nitride (GaN) is known to be one of the most promising wide band gap semiconductor materials for opto-electronic devices operating in the blue spectrum e.g. highly effective blue light emitting diodes and short wavelength laser diodes for high optical storage [1—4]. One of the main problems in fabricating high-quality GaN layers is the lack of lattice matched substrates. Hence, it is a challenging task to grow GaN epitaxial layers with low defect density and flat surface morphology suitable for device structures. Bulk GaN crystals
* Corresponding author. E-mail: [email protected].
are the most suitable substrates for the growth of strain-free epilayers suitable for high-quality nitride-based laser diodes and devices. However, it is very difficult to grow these crystals due to their high melting temperature, low sublimation/decomposition temperature and very high nitrogen vapor pressures at moderate temperatures. Despite these difficulties, attempts to grow GaN by different techniques like high-pressure solution [5], vapor phase [6], thermal gradient solution growth [7], ammonolysis of gallium sub-oxide [8] have been reported. In recent years, large size GaN single crystals have been grown from Ga solution in the form of platelets with the high-pressure and high-temperature technique [9,10]. Yamane et al. [11] have reported the growth of GaN single crystals using Na flux with sizes of about 1 mm
0022-0248/99/$ — see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 4 3 3 - X
R.S. Qhalid Fareed et al. / Journal of Crystal Growth 200 (1999) 348—352
platelets. However, it is difficult to grow large quantites of GaN crystals using this technique. Sublimation method is one of the most simple methods for the growth of GaN crystals and does not require high-pressure and high-temperature. Using this method, there is a possibility of growing large size GaN single crystals on a large scale with better crystal quality. The growth of bulk GaN single crystals by the sublimation technique has been reported [12—15]. Due to nonavailability of large size single crystals, studies on their structural and surface properties have not yet been carried out in detail so far. Nowak et al. [16] have reported the surface morphologies on the as-grown and annealed bulk GaN crystals grown by the high-pressure and high-temperature method. Structural studies carried out on the platelet bulk GaN crystals have also been reported [17]. It is essential to understand the morphology and growth mechanism of these crystals as these crystals are intended for homo-epitaxial growth for fabricating device structures. In this paper, we present for the first time, the surface morphology of bulk GaN crystals and selective GaN epitaxial layer grown using the sublimation technique. In the present study, atomic force microscopy (AFM) has been applied to analyze the surface morphology of the as-grown samples.
2. Experimental procedure Free-standing bulk single crystals of GaN have been grown in a vertical quartz tube reactor with RF heating. The system used in the growth is schematically shown in Fig. 1. The source powder was synthesized by heating metallic gallium (Ga) in a quartz tube in NH atmosphere at 1000°C for 3 h. The source powder, quartz substrate holder and the substrate were placed in a carbon crucible with inductively coupled RF heating system in nitrogen ambient at atmospheric pressure. The flow rate of NH was 30—50 sccm. The crystals were grown by the self-nucleation method for a growth time of about 2 h in the temperature range 1000—1050°C. In one growth run, it is possible to get about 20—30 crystals of different sizes ranging from 0.5 to 3 mm in length. Due to spontaneous nucleation in initial
349
Fig. 1. Schematic diagram of the system used in the sublimation growth.
stages of growth, many micro-crystals were also found in the remaining source powder. Depending on the flow direction of NH and distribution of the source powder, the size and quality of the crystal differ remarkably. Gallium nitride has been selectively grown by the sublimation method on MOCVD GaN. The sample preparation was as follows: A 1.2 lm thick GaN epilayer was grown by MOCVD on a (0 0 0 1) sapphire substrate, using low-temperature GaN buffer layer. Using E-beam evaporation, 50 nm Si thin mask was coated on the epitaxial layers. The patterning of the mask was carried out by the conventional photolithography technique combined with wet etching. Square-shaped openings of size 500;500 lm were formed on the substrate. Growth was carried out at 1000—1030°C for about 2 h. About 60—70 lm thick GaN epitaxial layers were grown in the window area. Surface morphology of the as-grown bulk GaN single crystals and selectively grown epi-layers have been observed with an atomic force microscope (SII Seiko Instrument, Model SPA 300) using the noncontact mode.
3. Results and discussion Fig. 2 shows one of the free-standing single crystals grown by the sublimation method. The morphology of the crystal growing without the
350
R.S. Qhalid Fareed et al. / Journal of Crystal Growth 200 (1999) 348—352
Fig. 2. One of the bulk GaN single crystals grown by the sublimation method.
intentional seeding of crystallization depends on the growth temperature, quality and distribution of the source in the crucible and distance between the source and the substrate. It is found that spontaneous nucleation growth occurred rapidly as the temperature is raised, leading to the formation of many microcrystallites. Hexagonal crystallites with size ranging from 2 to 3 mm in length and about 0.8 to 1.0 mm in diameter have been grown. The crystals are well faceted with c-axis along the length of the crystal. The longest axis was along (0 0 0 1) direction. X-ray diffraction studies have been carried out to know the quality of the crystal. The full-width at half-maximum (FWHM) are 48 arcsec along (0 0 0 2), 32 arcsec for (1 0 1 1) and 38 arcsec for (1 1 0 2) directions. The dislocation density of the bulk crystal measured using transmission electron microscopy (TEM) is found to be less than 10— cm [12,13]. Fig. 3a and Fig. 3b shows the AFM image of the as-grown bulk GaN crystal and the cross-sectional profile of the surface, respectively. Scanning was done on the [1 1 2 0] plane of the crystal. The surface of the crystal is almost atomically flat with clear step growth along the (0 0 0 1) direction. The average width of smaller and larger steps are about 60—70 and 110—120 nm, respectively. The height of the steps varies from 1 to 6 As , i.e in the range of atomic scale growth. The RMS roughness is found to be 1.434 As . The crystal is found to grow with steps of mono-molecular height in its surface and growth takes place by the advance of these steps
Fig. 3. (a) 2;2 lm area AFM image of GaN single crystals and (b) surface spectrum of the AFM image of a GaN crystal.
forming new molecular layers. The rate of the advancement of these steps depends on the structure and the direction of the growth. It is also observed that at the step edge, Ga micro-precipitates are oriented in a regular manner. Similar micro-precipitates of Ga have also been reported in the high-pressure grown bulk GaN [17]. This uniform distribution at the step edge also improves the stability of the growth face of the crystal and gives rise to an atomically flat surface leading to smooth morphology. Fig. 4 shows the AFM image of another bulk single crystal grown at the edge of the crucible under the same growth conditions. The surface of the crystals is microscopically flat with steps both parallel and perpendicular to the growth axis. In these crystals, regular steps of larger width can be observed along the growth axis, i.e. c-axis along
R.S. Qhalid Fareed et al. / Journal of Crystal Growth 200 (1999) 348—352
351
Fig. 5. Optical microscopic image of screw dislocations on selectively grown GaN. Fig. 4. AFM image of a bulk GaN single crystal grown at the edge of the crucible.
(0 0 0 1). The step height is about 2—3 nm and width is about 0.3—0.5 lm. Steps are also observed perpendicular to the c-axis, i.e. along (1 1 2 0) at irregular intervals. In this crystal, a twin step is also observed as shown in the figure. The formation of these steps mainly depends on the growth conditions. In the GaN crystals grown by the highpressure technique [17], flat and rough surface related to the crystal polarity along the c-axis are observed. These two different types of surface morphology have not been observed in sublimation grown crystals and studies to identify the crystal polarity are in progress. Fig. 5 shows the optical microscopic image of the screw dislocation observed on one of the edges of the windows in the selectively grown GaN deposited on the MOCVD-GaN by the sublimation method. The thickness of the epilayer is about 50—70 lm. A monolayer or sub-monolayer step originates from the point where screw dislocation line intersects the surface of the bulk plane. This step is constrained to terminate at the dislocation emergence point and winds up into a spiral during the growth process. According to BCF theory, the step width j, is given by j"20o , where ‘o ’ is the radius of the critical 2D nucleus [18]. In this selectively grown GaN, two-dimen-
Fig. 6. 20;20 lm area AFM image of a spiral growth on selectively grown GaN.
sional layer-by-layer growth originates from the screw dislocation. It is found that one plane is smooth whereas the other plane is little rough. In Fig. 6, AFM image of a spiral growth observed in the selective growth of GaN on MOCVD-GaN is shown. The presence of monoatomic steps on the layer surfaces, often in the form of large diameter (tens of lm) spirals suggests twodimensional growth on screw dislocations originating from the threading dislocation present in the substrate. The average step height is about 40—42 nm and width of each step is about 1 lm.
352
R.S. Qhalid Fareed et al. / Journal of Crystal Growth 200 (1999) 348—352
The step due to a dislocation winds into a spiral in such a way that a single screw dislocation sends out successive turn of steps. This successive turning of steps forms pyramids on the crystal surface. If the rate of advancement of the step is independent of its orientation, the growing spiral forms a low cone but it will form a pyramid when the rate of advance depends on orientation. In the selective growth of GaN, the dislocations present in the MOCVD grown GaN have a component of displacement vector normal to the crystal face (0 0 0 1) at which they emerge. These are screw dislocations. Therefore, spiral growth is commonly observed in the selectively grown GaN epitaxial layers by sublimation.
4. Conclusions In summary, GaN single crystal growth and selective area epitaxy have been carried out by the sublimation method. Hexagonal crystallites with size ranging from 2 to 3 mm in length and about 0.8 to 1.0 mm in diameter have been grown. In the sublimation method, the surface morphology of the crystals depends on the growth conditions such as source material, temperature gradient and position of crystal nucleation. Surface analysis by AFM shows that the two-dimensional growth is dominated by step growth mechanism in bulk GaN crystals. However, in the selective growth of GaN, spiral growth mechanism originating from the dislocation present in the substrate is dominant. The present understanding is essential in homoepitaxial growth for good performance of the device structures.
References [1] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L797. [2] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoko, Y. Sugimoto, Jpn. J. Appl. Phys. Part 2 35 (1996) 217. [3] I. Akasaki, S. Sota, H. Sakai, T. Tanaka, K. Koike, H. Amano, Electron Lett. 32 (1996) 1105. [4] R.P. Vaudo, I.K. Goepfert, T.D. Moustakas, D.M. Geyea, T.J. Frey, K. Meehan, J. Appl. Phys. 79 (1996) 2779. [5] R. Madar, G. Jacob, J. Hallais, R. Fruchart, J. Crystal Growth 31 (1975) 197. [6] E. Ejder, J. Crystal Growth 22 (1974) 44. [7] R.A. Logan, C.D. Thurmond, J. Electrochem. Soc. 119 (1972) 1727. [8] T.L. Chu, K. Ito, R.K. Smeltzer, S.C. Chu, J. Electrochem. Soc. 121 (1974) 159. [9] S. Porowski, I. Grzegory, J. Crystal Growth 178 (1997) 174. [10] I. Grzegory, J. Jun, M. Bockowski, ST. Krukowski, M. Wroblewski, B. Lucznik, S. Porowski, J. Phys. Chem. Solids 56 (1995) 639. [11] H. Yamane, M. Shimada, T. Sekiguchi, F.J. DiSalvo, J. Crystal Growth 186 (1998) 8. [12] S. Kurai, K. Nishino, S. Sakai, Jpn. J. Appl. Phys. 36 (1997) L184. [13] Y. Naoi, M. Kobatake, S. Kurai, K. Nishino, H. Sato, M. Nozaki, S. Sakai, Y. Shintani, J. Crystal Growth 190 (1997) 163. [14] C.M. Balkas, Z. Sitar, T. Zheleva, L. Bergman, I.K. Shmagin, J.F. Muth, R.M. Kolbas, R. Nemanich, R.F. Davis, Mater. Res. Soc. Symp. Proc., Vol. 449, p. 41. [15] I. Shmagin, J.F. Muth, J.H. Lee, R.M. Kolbas, C.M. Balkas, Z. Sitar, R.F. Davis, Appl. Phys. Lett. 71 (1997) 455. [16] G. Nowak, S. Krukowski, I. Grzegory, S. Porowski, J.M. Baranowski, K. Pakula, J. Zak, MRS Int. J. Nitride Semicond. Res. 1 (1996) 17. [17] Z. Liliental-Weber, C. Kisielowski, X. Liu, L. Schloss, U. Washburn, E.R. Weber, I. Grzegory, M. Bockowski, J. Jun, T. Suski, S. Porowski, Proceedings of the Workshop on GaN and Related Materials, Nagoya, 1995, p. 167. [18] W.K. Burton, N. Cabrera, F.C. Frank, Philos. Trans. Roy. Soc. A. 243 (1951) 300.