Identification of a Cubic Phase in Epitaxial Layers of Predominantly Hexagonal GaN
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 44
Identification of a Cubic Phase in Epitaxial Layers of Predominantly Hexagonal GaN U. Strauss, H. Tews, H. Riechert, R. Averbeck, M. Schienle, B. Jobst Siemens AG, Corporate Research and Development D. Volm, T. Streibl, BK Meyer Physikdepartment, Technische Universitaet Muenchen W. W. Rühle Max-Planck-Institut für Festkörperforschung This article was received on June 3, 1996 and accepted on December 17, 1996.
Abstract Epitaxial layers of GaN on c-plane sapphire are analyzed by continuous-wave and time-resolved photoluminescence at 4K and by X-ray diffraction. Besides the well-known emissions from hexagonal GaN we observe luminescence bands at 3.279 and 3.15 to 3.21 eV which are identified as the transition of the donor bound exciton and the donor-acceptor pair recombination in cubic GaN, respectively. Measurements of the luminescence decay times are essential for the clarification of the emission processes. Due to the probing depth of about 200 nm in PL we find that the fraction of cubic phase typically decreases with layer thickness. In our best samples, however, we do not detect the cubic phase at all.
1. Introduction GaN, Inx Ga1-xN, and Aly Ga1-yN is suitable for visible light emitting diodes and short-wavelength lasers. [1] [2] [3] There are great efforts to grow epitaxial layers of high quality. For that, there is great interest for characterisation of these layers, for example the crystal perfection. Predominantly, the GaN epitaxial layers crystallise in the most stabile, hexagonal (wurtzite) structure, especially when grown on substrates with hexagonal symmetry such as sapphire or 6H-SiC. [4] The growth of cubic GaN was reported for cubic substrates, for example on GaAs, (111)-Si, and (001)-Si with β-SiC-coating. [5] [6] [7] [8] [9] [10] The presence of both crystal structures was observed in layers on layers on (100)-Si: the predomenantly hexagonal structure contains a cubic minor phase of GaN as detected by x-ray measurements. [9] In this work, we investigate GaN grown on sapphire (0001) substrates. We examine the presence of a cubic phase in our layers with dominant hexagonal structure. An often used method is an X-ray diffraction measurement (XRD), but XRD detects signals from the whole thickness’ of the samples and it is not clear, whether the cubic material is in the top layers or close to the buffer layers. Therefore, we additionally test the use of photoluminescence spectroscopy (PL), which is a method with high sensitivity to the top of the layers up to the penetration depth of about 200 nm. The identification of the luminescence peaks in GaN is difficult, because there exist many emission processes in GaN with nearly the same emission energies. In this work, the emission peaks of the cubic crystals are identified by time-resolved measurements. We present the structural analysis of the top layer of the GaN samples by using PL.
2. Experimental The investigated layers of GaN were grown by molecular beam epitaxy. A nitroge
