We studied the recombination at about 3.410eV in nominally undoped GaN and highly oxygen-doped GaN by photoluminescence (PL) and cathodoluminescence (CL). For this line we find an upper limit for the thermal activation energy of EA⩽21±3meV. In time-resolved PL, lifetimes of 300–480ps are observed indicating excitonic recombination. The defect giving rise to this emission can be created by Ar+ ion implantation. In conclusion, the 3.410eV luminescence is attributed to excitons bound to structural defects in hexagonal GaN.

We investigated the influence of the growth temperature on high temperature vapor phase epitaxy of GaN. An almost direct proportionality between the growth rate and the Ga vapor pressure is observed. At optimum conditions growth rates as high as 210 μm/h (T=1150°c) are achieved. The maximum growth rate is believed to be limited by the supply of ammonia and the starting composition of GaN. Under optimum GaN growth conditions AlGaN layers were grown starting from previously alloyed Al-Ga as well as from co-evaporation of Ga and Al. Adding Al leads to a significant reduction of growth rate and increases the homogeneity of the layers. However, in almost all cases phase separation is found. Besides the binary GaN and AlN phases an intermediate AlGaN phase appears