Deep traps in undoped n-type layers which had been grown, by means of organometallic vapor-phase epitaxy, onto sapphire substrates were studied by using temperature-dependent conductivity, photo-induced current transient spectroscopy, thermally stimulated current, electron beam-induced current, and band edge cathodoluminescence methods. Electron traps, with energy levels that were 0.1 to 0.2eV below the conduction band, were detected as well as hole traps with energy levels that were about 0.25, 0.5 and 0.85eV above the valence band edge. The use of cathodoluminescence and electron beam-induced current measurements showed that the deep recombination centers were distributed inhomogeneously, with a well-defined cellular pattern. Both the carrier lifetime and the luminescence intensity increased at cell walls, thus reflecting a lower density of recombination centers. On the other hand, the density of the main hole trap (0.85eV) increased. The photoconductivities of many samples revealed very long decay times at temperatures of between 100 and 300K. It was concluded that the effect was probably was not related to shallow donors, such as Si, but was instead associated with unidentified deep centers with a 0.2eV barrier to electron capture.

A.Y.Polyakov, N.B.Smirnov, A.V.Govorkov, M.Shin, M.Skowronski, D.W.Greve: Journal of Applied Physics, 1998, 84[2], 870-6