Microstructure of GaN Grown on (1120) Sapphire
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MICROSTRUCTURE OF GaN GROWN on (11 2 0) SAPPHIRE P. Ruterana*, A. E.Wickenden1, M. E. Twigg1, D.D. Koleske1, R. L. Henry1, O. Tottereau2 ESCTM , ISMRA, 6 Bd Maréchal Juin, 14050 Caen 1
Electronics Science and Technology Division, Naval Research Laboratory, Washington, D.C20375-5320 USA 2
CRHEA – CNRS, rue Bernard Gregory Sophia- Antipolis, Valbonne, 06560, France France
Abstract Most of the work done on GaN has taken into account layers grown on the (0001) sapphire. However one would expect the growth on (11 2 0) 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 layers and interfacial relationship. Inside the layers, the density of defects is comparable to that found conventionally in layers grown on top of (0001) sapphire. The growth mode is also mosaic with a grain size of a few microns. One interesting result is the interface structure, which differs from conventional growth where a flat or stepped interface is formed with a large distance between steps. In this case, the interface is found to be rough at the atomic scale so that this roughness has a random distribution. Moreover, the misfit dislocation spacing is 1nm which is only half the dislocation spacing found in GaN growth on (0001) sapphire.
Introduction High quality GaN growth is still a major research topic for the fabrication of commercial devices such as light emitting diodes [1], lasers [2] and high power electronic devices[3]. Until now, best results were obtained from layers grown on (0001) sapphire which exhibits large lattice and thermal misfits. Due to so important potential applications, much work is going on for new substrates with some success [4], bulk GaN [5], or self supported film by ELO [6]or Pendeo epitaxy[7]. For some time now, efforts have been going on in order to produce similar quality GaN layers on (11 2 0) sapphire [8]. This was done by optimizing the nucleation layer [9] and even by moving on to miscut substrate surface, leading to enhancement of electrical and layers with larger mosaic grains in the few micron range [10]. 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 numerous reports have shown that GaN grows along its six fold c axis on top of this a surface which exhibits a p2 symmetry [11]. The epitaxial relationships are (11 2 0)sap//(0001)GaN, [0001]sap//[ 1010 ]GaN, whereas [ 1010 ]sap//[11 2 0]GaN inside the best layers, but also a rotation of 90° has been reported [12]. Along these directions, the differences in lattice parameters are quite large which suggests a complex mechanism for lattice matching [13]. *
Author for correspondence: Tel 33 2 31 45 26 53 Fax 33 2 31 45 26 60 email [email protected]
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In this work, we have carried out structural analysis of the GaN layers and determined the interfacial relationships. As the growth takes place along the [0001] GaN direction, the thr
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