Deep level related yellow luminescence in p-type GaN grown by MBE on (0001) sapphire
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ABSTRACT Yellow luminescence (YL) has been studied in GaN:Mg doped with Mg concentrations ranging from 1019 to 1021 cm-3 by spectral CL (T=5K) and TEM and explained by suggesting that a different mechanism could be responsible for the YL in p-type GaN with respect to that acting in n-type GaN. Transitions at 2.2, 2.8, 3.27, 3.21, and 3.44 eV were found. In addition to the wurtzite phase, TEM showed a different amount of the cubic phase in the samples. Nano tubes with a density of 3x109 cm-2 were also observed by approaching the layer/substrate interface. Besides this, coherent inclusions were found with a diameter in the nm range and a volume fraction of about 1%. The 2.8 eV transition was correlated to a deep level at 600 meV below the conduction band (CB) due to MgGa-VN complexes. The 3.27 eV emission was ascribed to a shallow acceptor at about 170-190 meV above the valence band (VB) due to MgGa. The 2.2 eV yellow band, not present in low doped samples, increased by increasing the Mg concentration. It was ascribed to a transition between a deep donor level at 0.8-1.1 eV below the CB edge due to NGa and the shallow acceptor due to MgGa. This assumption was checked by studying the role of C in Mg compensation. CL spectra from a sample with high C content showed transitions between a C-related 200 meV shallow donor and a deep donor level at about 0.91.1 eV below the CB due to a NGa-VN complex. In our hypothesis this should induce a decrease of the integrated intensity in both the 2.2 and 2.8 eV bands, as actually shown by CL investigations.
INTRODUCTION The achievement of p-type epitaxial GaN by Mg doping is of interest for device applications. In addition, the nature of most of the basic luminescence recombination mechanisms in Nitride-based material systems and they
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dependence on material properties are still subject of discussion. Yellow luminescence in undoped or n-type GaN is supposed to be due to a transition between a shallow donor and a deep acceptor level located at about 1 eV above the VB [1,2] due to a complex defect involving VGa [3]. P-type GaN doped with Mg is supposed not to show YL due to the increase of the formation energy of Ga vacancies in semi-insulating or p-type GaN [3,4] and due to the compensation of VGa by Mg atoms [5-7]. Nevertheless, YL in Mg doped GaN has been observed, but only in n-type layers [8]. In this work a broad YL band is found in p-type Mg doped GaN layers grown by molecular beam epitaxy (MBE). The optical transitions and the structural properties of the layers are discussed on the basis of low temperature Spectral Cathodoluminescence (SCL) results in the Scanning Electron Microscope (SEM) and Transmission Electron Microscopy (TEM) respectively. Taking advantage of the special growth technique used, the results are discussed sugges
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