Electronic near-surface defect states of bare and metal covered n-GaN films observed by cathodoluminescence spectroscopy

Depth-dependent low energy cathodoluminescence spectroscopy (CLS) has been used to investigate the near-surface optical properties of n-type GaN epilayers grown under various growth conditions. Both bare and reacted-Mg/n-GaN and Al/n-GaN (annealed to 1000 degree C) surfaces were investigated. We find enhanced emission at approximately 1.4, 1.6, and 2.2 eV from states within the n-type GaN bandgap near the interface of the reacted Mg with the semiconductor, which correlates with previous measurements of Schottky barrier formation on the same specimens. No clear evidence for p-type doping at the reacted interfacial layer is apparent. For Al on n-type GaN, CLS emission is dominated before and after metallization by `yellow' emission, which correlates only weakly with the Fermi level stabilization energy. Instead, we observe emission above the GaN band edge emission at 3.85 eV, due either to deep level emission from AlN or to the formation of the alloy Al sub(x)Ga sub(1-x)N (x approximately 0.2) in the reacted near-surface region.