Unusual properties of the dominant acceptor in freestanding GaN

Photoluminescence (PL) study of high-purity undoped GaN revealed the yellow (YL) and green luminescence (GL) bands, attributed to two charge states of the same defect, presumably a gallium vacancy-oxygen complex. We have used time-resolved PL in a wide temperature range to study this defect. The GL band was visible only for short time (below 10-4 s) and then transformed into the YL band, consistent with the attribution of these bands to an acceptor binding two and one holes, respectively. The GL exhibits fast exponential decay (lifetime of 1.5 μs) above 30 K. In contrast to the YL band, where the lifetime decreases with increasing temperature due to increasing concentration of free electrons, the lifetime of the GL stays unchanged up to 70 K and then increases by a factor of 20 at 300 K. This anomalous behavior of the GL band is attributed to the existence of an excited state near the conduction band