Detection of Magnetic Resonance on Shallow Donor - Shallow Acceptor and Deep (2.2 eV) Recombination from GaN Films Grown
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There has been much activity lately by several groups to study and, if possible, to identify intrinsic defects in undoped epitaxial layers of GaN grown by either organo-metallic chemical vapor deposition (OMCVD) or molecular beam epitaxy (MBE). In addition, there has been some work to obtain more detailed information on the participation of these defects in the recombination processes observed from these GaN films. Both sha!low, effective-mass like and deep states of donor or acceptor character have been revealed from a variety of techniques. The origin of these states in undoped GaN, either due to residual impurities or non-stoichiometric defects such as vacancies, anti-sites, or interstitials, is a subject of much discussion. Much of this work has focused on GaN films grown on sapphire (A12 0 3 ) substrates, typically with the use of AIN buffer layers. Since the in-plane lattice constant mismatch is - 1% between AIN and 6H-SiC as compared to the - 13% difference between AIN and sapphire, the expected lower degree of strain in the GaN layers grown on 6H-SiC should lead to improved film quality. Recently, it has been reported that GaN films grown with the use of high-temperature AIN buffer layers on 6HSiC substrates has resulted in a significant reduction of the dislocation density in these films after the first micron of growth [1]. We have previously reported PL and ODMR experiments performed on both undoped and Mg-doped GaN films grown on (0001)-oriented sapphire (A12 0 3 ) substrates. Both shallow and deep states were revealed by magnetic resonance obtained by detection on the 2.2 eV broad emission band from undoped GaN layers [2-4] and on emission from heavily Mg-doped GaN films [3,4]. Preliminary results obtained for undoped GaN layers grown on 6H-SiC substrates were presented recently [4]. In this work we discuss in more detail the results of PL and ODMR studies of undoped wurtzite GaN layers grown by OMCVD on 6H-SiC. One of the goals of this work is to compare the 667
Mat. Res. Soc. Symp. Proc. Vol. 395 01996 Materials Research Society
magnetic resonance obtained from GaN films grown on 6H-SiC with the results obtained from layers grown on sapphire substrates. In addition, it was found from this work that the PL observed from these films was found to be quite dependent on the layer thickness. In particular, a rather strong shallow donor - shallow acceptor band was observed from films less than 1 jimthick (in addition to a stronger donor-bound excitonic peak near the band-gap energy and a relatively weak 2.2 eV emission band also observed from these thin films and for layers greater than 2 jim). The first observation of magnetic resonance on this SD - SA recombination from undoped GaN is reported in this work. Thus, the thin GaN layers grown on 6H-SiC provided the opportunity to contrast the magnetic resonance detected on the 2.2 eV deep emission band with that found on shallow donor - shallow acceptor recombination from the same film. In addition, the ODMR studies of the SD-SA recombination allowed us to
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