Shallow and deep centers were studied by means of temperature-dependent Hall-effect and photoluminescence measurements in 2 sets of undoped n-type samples that had been grown by means of organometallic vapor-phase epitaxy. The 2 series were grown under differing conditions and had electron concentrations which differed by several orders of magnitude. The compositional dependences of the ionization energies of the dominant donors in these 2 sets of samples were very different; thus indicating that different types of center were involved. In both cases, they were thought to be related to some form of native defect. These defects behaved like H-like donors at low Al concentrations, and became increasingly deep with increasing Al content. The shallow-deep transition occurred at an x-value of about 0.2 in low-conductivity AlxGa1-xN, and at an x-value of about 0.5 in high-conductivity samples. Several photoluminescence bands were detected, and it was shown that a band at 3.05eV was due to a radiative transition between deep donors in the upper part of the band gap, and holes in the valence band or on shallow acceptors. In the case of a yellow luminescence band at 2.25eV, it was demonstrated that the band consisted of 2 overlapping bands and that the predominant band was due to a transition between native donors and a C-related deep center.

A.Y.Polyakov, M.Shin, J.A.Freitas, M.Skowronski, D.W.Greve, R.G.Wilson: Journal of Applied Physics, 1996, 80[11], 6349-54