Effect of growth temperature on the microstructure of the nucleation layers of GaN grown by MOCVD on (1120) sapphire
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Effect of growth temperature on the microstructure of the nucleation layers of GaN grown by MOCVD on (11 2 0) sapphire T. Wojtowicz, P. Ruterana*, M. E. Twigg1, R. L. Henry1,D.D. Koleske1,2,A.E. Wickenden1,3 LERMAT, ISMRA, 6 Bd Maréchal Juin, 14050 Caen, France 1 Electronics Science and Technology Division, Naval Research Laboratory, Washington, D.C., 20375-5320 USA
Abstract Most of the work done on GaN has taken into account layers grown on the (0001) sapphire plane. However one would expect the growth on the (11 2 0) plane to lead to different structural defects. As has been shown, in one direction, the mismatch is rather small. In this work, we have carried out structural analysis of nucleation layers grown at temperatures ranging from 600°C to 1100°C. It is shown that for many of the structural parameters, such as the orientation relationships, the layer morphology and the nucleation mechanism critically depend on the growth temperature. At the lowest temperatures, the growth is completely three dimensional with a mixture of the two traditional orientation relationships, but the coalescence thickness is small. In a next step, the A orientation relationship predominates and the layer roughness tends to slightly decrease. This orientation is never perfect, and there is always 1.5° misorientation to the same direction in sapphire, whereas the B orientation is always perfect. At an intermediate temperature, island growth is predominant, whereas towards the high temperature end the B orientation becomes predominant. For the highest growth temperatures, the nucleated layers are completely flat and with the B orientation, although they contain a quite large number of defects such as inversion domains.
Introduction The growth of high quality GaN growth 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]. In the late eighties, one of the main breakthrough was the use of a low temperature nucleation layer to initiate the growth of the GaN active layers. Although the best results were obtained from layers grown on (0001) sapphire, which exhibits large lattice and thermal misfits. Due to the important potential applications, work is going on for other growth processes and new substrates with some success [4,5]. For some time now, efforts have been going on in order to produce similar quality GaN layers on (11 2 0) sapphire [6]. This was done by optimizing the nucleation layer [7] and even by moving on to a miscut substrate surface, leading to enhancement of electrical properties, and layers with larger mosaic grains in the few micron range [8]. Of course, if we consider only the symmetries of the surfaces, one would expect rather different growth of GaN on such surface of sapphire. The already *
Author for correspondence: Tel 33 2 31 45 26 53 Fax 33 2 31 45 26 60 email [email protected] Present address: Sandia National Laboratories, Albuquerque, NM 87185 3 Present address : Army Research Laboratoy, Adelphi, MD 20783
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