Luminescence of a New Material: GaN Grown on NdGaO 3
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ABSTRACT It is shown that heteroepitaxial GaN layers grown on NdGaO 3, in spite of a very high conductivity (- 10 T'•cm"') have very efficient luminescence properties. It is shown that a high electrical conductivity is caused by contamination of GaN layers with oxygen. Efficient emission due to donor bound excitons (at hv = 3.475 eV), free excitons and free electron - hole recombination have been identified. The total PL emission in the exciton region exceeds the intensity from the homoepitaxial GaN layers. It is argued that a high oxygen concentration eliminates nonradiative channels connected with point defects, leading to efficient radiative recombination.
INTRODUCTION Up to now, GaN layers are grown mainly on sapphire or SiC substrates. Homoepitaxial layers are rare, mainly due to the difficulty to grow bulk GaN single crystals. Since there is a large mismatch of lattice parameters between GaN and sapphire or SiC, the density of dislocations in these types of GaN is very high. Therefore new substrate materials are searched. Neodymium gallate NdG-aO 3 (NGO) has been previously tried as a substrate for growing high temperature
superconductors YBa 2 Cu 307.x [1]. It has a lattice constant similar to GaN and is tolerant on high temperature treatment, so we have chosen it as a new candidate as a substrate for growth of GaN. There is no straight correlation between high optical quality of devices made on GaN and good crystal structure of the layers, so it is very important to compare rather optical properties than structural properties of the gallium nitride layers. In this work we present photo-luminescence (PL) and time resolved photo-luminescence (TRPL) measurements of an MOCVD layer of GaN grown on a NdGaO 3 substrate. The PL intensity of GaN on NGO has been compared with homoepitaxial GaN layers grown on GaN substrates. SAMPLES AND EXPERIMENTAL TECHNIQUES Neodymium gallate NdGaO 3 (NGO) is an orthorhombic crystal. Its lattice constants are equal to: a = 0.54276 nm, b = 0.54979 nim and c = 0.77078 rnm [1]. However, due to a small difference between the a and b lattice constants, its crystal structure can be described as pseudocubic (perovskite-like) with lattice constants a, = 0.3863 nm and c, = 0.3854 nm [1, 2]. The gallium atoms in NdGaO3 form layers in (111) planes of the pseudocubic lattice, which corresponds to the (011) planes of the orthorhombic lattice. On the other hand the a-GaN has hexagonal structure with lattice constants: a = 0.31868 nm, c = 0.51538 un. The gallium atoms in GaN form layers in (0001) planes. The configuration of gallium atoms in the CaN the (0001) plane and 725 Mat. Res. Soc. Symp. Proc. Vol. 482 01998 Materials Research Society
in NdGaO 3 the (011) orthorhombic plane is shown in Fig. 1. The distance between gallium atoms (in the [1 1 2 0], [2 1 1 0] or [2 T- 1 0] directions) s = a•/3 = 0.55 197 nm is nearly equal to corresponding distances in the NGO crystal. The mismatches (SGaN - SNGO)/SNGO are 1.8% and 1.2% for [1 0 0) and [1 2 2] directions (in the NGO orthorhombic lattice),
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