Abstract
ABSTRACTPhotoluminescence (PL) spectra of GaN epitaxial layers grown on vicinal (0001) 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 (0001) substrates exhibit a broad PL emission resulting from high free carrier concentrations. As derived from Secondary Ion Mass Spectroscopy (SIMS), exact (0001) epilayers reveal very high concentrations of oxygen (1.5×1019 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.
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