The microstructure of GaN nucleation layers grown by MOCVD on (1120) sapphire versus pressure and temperature
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The microstructure of GaN nucleation layers grown by MOCVD on (11 2 0) sapphire versus pressure and temperature T. Wojtowicz, P. Ruterana∗, M.E. Twigg1, R.L. Henry1, D.D. Koleske1,2, A.E. Wickenden1,3 SIFCOM UMR 6176 CNRS, ENSICAEN, 6 Bd du Maréchal Juin, 14050 Caen Cedex, France 1 Electronics Science and Technology Division, NRL, Washington, DC 20375-5320 Abstract In this work, the evolution of morphology and defect structure in GaN nucleation layers on the a-plane of sapphire are investigated using TEM. The growth temperature and pressure were varied from 560 to 1100°C and from 20 to 600 torr, respectively. Whereas the highest growth temperature leads to a continuous layer, a 2D growth mode is not attained when the chamber pressure is varied from 20 to 600 torr at 1028°C. At the highest pressures (>300 torr), a large distribution is obtained for the island sizes. Introduction The growth of high quality GaN is a major research topic for the fabrication of commercial devices such as light emitting diodes [1], lasers [2] and high power electronic devices [3]. The best results were obtained from layers grown on (0001) sapphire, although they exhibit large lattice and thermal coefficient misfits. A 13.8% mismatch can be obtained using a-plane sapphire instead of 16.1% mismatch characteristic for c-plane sapphire growth. Due to the important potential applications, various growth processes have been investigated with some success [4, 5]. Laterally, the growth on (11 2 0) sapphire produces similar quality GaN layers [6]. This was done by optimising the nucleation layer [7, 8] and by moving to a miscut substrate surface, leading to enhancement of electrical properties in layers with larger grains (few micron range) [9]. If only the symmetry of the surfaces were considered, one would expect a different growth mode for GaN on such surfaces of sapphire. In fact, numerous reports have shown that in this case also, GaN grows along the c axis on top of the a sapphire surface which exhibits a p2 symmetry [10]. The epitaxial relationships are (11 2 0)sap//(0001)GaN and [1100] sap// [1100] GaN but also (11 2 0)sap//(0001)GaN and [1100] sap// [2110] GaN. In this work we analyse the effect of the growth pressure and temperature on the microstructure of the nucleation layers. Experimental The layers were grown by metal organic vapour phase deposition (MOCVD) in a vertical reactor consisting of an inductively-heated water cooled quartz tube. The sapphire substrate was annealed for 10 min minutes in H2 at high temperature (> 1080°C) before growth. Subsequently the substrate temperature was reduced to 1028°. The studied nucleation layers were grown for 30 min at pressures and temperatures varying from 20 to 600 torr, and 595°C ∗
Corresponding author: Tel. 33 2 31 45 26 53, Fax. 33 2 31 45 26 60, E-mail: [email protected] Present address: Sandia National Laboratories, Albuquerque, NM 87185 3 Present address : Army Research Laboratoy, Adelphi, MD 20783 2
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to 1100°C, respectively. All the other parameters, such as the o
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