Shallow-impurity-related photoluminescence in homoepitaxial GaN
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Shallow-impurity-related photoluminescence in homoepitaxial GaN. V.Kirilyuka, M. Zielinskib, P.C.M. Christianena, A.R.A. Zaunera, J.L.Weyhera,c, P.R. Hagemana, and P.K. Larsena a Research Institute for Materials, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands. b Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland. c High Pressure Research Center, Polish Academy of Science, ul. Sokolowska 29/37, 01-142 Warsaw, Poland.
ABSTRACT Photoluminescence (PL) spectra of GaN epitaxial layers grown on vicinal (000 1 ) GaN substrates (i.e. N-polar) demonstrate high optical quality showing free exciton transitions in addition to narrow bound-exciton peaks (line width 1-2 meV). In contrast, N-polar films deposited on exactly oriented (000 1 ) substrates exhibit a broad PL emission resulting from high free carrier concentrations. As derived from Secondary Ion Mass Spectroscopy (SIMS), exact (000 1 ) epilayers reveal very high concentrations of oxygen (1.5x1019 atom/cm3), which are about one order of magnitude higher then in the layers grown on a 4°-off substrate. It is therefore concluded that oxygen is responsible for the high free carrier concentration and could be a dominant shallow donor in the homoepitaxial N-polar films. It is also shown that incorporation of oxygen strongly depends on the polarity of the films (Ga or N). Concentrations of other impurities, such as Si, C and H are found to be similar for homoepitaxial films of both polarities being consistent with the results of exciton-related PL.
INTRODUCTION It has been recently recognized that the polarity of GaN is important for the built-in electric field in group-III-nitride heterostructures and therefore can be used for optimizing the performance of high frequency/high power electronic devices [1]. As a consequence, the material and optical properties of GaN of both polarities are of great interest. Up to now, however, most attention has been paid to Ga-polar layers, mainly due to fairly good morphological properties of this films as compared to N-polar samples [2,3]. It has recently been shown that epilayers deposited on Npolar GaN substrates can also exhibit a smooth surface and narrow photoluminescence (PL) lines, provided that they are grown on misoriented substrates [4,5]. However, the understanding of the structural differences of epilayers of both polarities (e.g. different density of intrinsic/extrinsic defects, dislocations and/or inversion domains) and the relation to the optical spectrum still need to be clarified. This contribution reports on a comprehensive study of shallow-impurity-related photoluminescence (PL) in homoepitaxial (000 1 ) GaN films (N polarity). The observed PL features are correlated to the compositional properties of the samples, as derived from Secondary Ion Mass Spectroscopy (SIMS) analysis. The impurity incorporation in the N-polar films grown on exact and misoriented GaN single crystals will be discussed and compared to similar Ga-polar films and to our standard N-p
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