Characterization of GaN grown on sapphire by laser-induced molecular beam epitaxy
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M. Gross and H. Schro¨der Institut fu¨r Technische Physik, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany (Received 19 October 1999; accepted 12 October 2000)
GaN grown on sapphire (␣–Al2O3) was characterized by laser-induced molecular beam epitaxy. Threading dislocations with Burgers vectors of 1⁄3〈1120〉, 1⁄3〈1123〉 and [0001] were observed with a predominance of the first type. Additionally, inversion domains with Ga-polarity existed with respect to the adjacent matrix, which was of N-polarity. The dislocation densities and coherence lengths were deduced from x-ray diffraction and found to be in accordance with those measured by transmission electron microscopy. Both displacement fringe contrast analysis and high-resolution transmission electron microscopy results indicated that the inversion domain boundaries had Ga–N bonds between domains and the adjacent matrix.
I. INTRODUCTION
GaN and related alloys have received intensive attention in recent years due to their interesting electric, optical, and acoustical properties. Most growth techniques are based on chemical vapor deposition (CVD),1–3 molecular beam epitaxy (MBE),4,5 vapor-phase epitaxy (VPE),6 and pulsed laser deposition (PLD).7,8 We use laser-induced molecular beam epitaxy (LIMBE)9–11 as the growth technique for epitaxial GaN films. This method is based on PLD. In contrast to the classic PLD configuration, we use a picosecond Nd:yttrium aluminum-garnet (YAG) laser system with a pulse rate around 2 kHz. The use of picosecond pulses instead of nanosecond pulses prevents the formation of particulates and droplets because the plasma pressure is lower, and the influence of thermocapillarity is reduced.12 Additionally, a pure Ga metal target in a nitrogen atmosphere is used. Pure metal targets provide clean growth conditions, unlike the usual nitride compound targets used in classic PLD. Threading dislocations with Burgers vectors of 1⁄3〈1120〉, 1⁄3〈1123〉 and [0001] (in the following referred to as a, a + c, and c-type dislocations, respectively) are the most common defects in GaN epilayers.2 In addition to the above defects, planar defects faceted along {1100} and {1210} planes are described in the literature as double positioning boundaries (DPBs),4 stacking mis-
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On leave of absence from Institute of Solid State Physics, Academia Sinica, Hefei 230031, People’s Republic of China. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 1, Jan 2001
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match boundaries (SMBs),13 or inversion domain boundaries (IDBs).14 –22 Only the {1100} boundaries have been observed in our GaN/sapphire samples. Two configurations for the atomic structure of these inversion domain boundaries (IDBs) have been reported. The first model consists of Ga–Ga and N–N bonds across the IDB. 23 The second one exhibits Ga–N bonds across the IDB.17,19,21 Basu et al.15 showed a c/8 displacement between basal planes of an inversion domain (ID) and the adjacent matrix. Wu et al.16 studied IDBs by diffraction contrast im
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