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 epila yers grown under various growth conditions. Both bare and reacted-Mg/n-GaN and Al/n-GaN (annealed to 1000 degrees C) surfaces were investigated. We fi nd enhanced emission at similar to 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 semico nductor, which correlates with previous measurements of Schottky barrier fo rmation on the same specimens. No clear evidence for p-type doping at the r eacted interfacial layer is apparent. For Al on n-type GaN, CLS emission is dominated before and after metallization by "yellow" emission, which corre lates only weakly with the Fermi level stabilization energy. Instead, we ob serve emission above the GaN band edge emission at 3.85 eV, due either to d eep level emission from AlN or to the formation of the alloy AlxGa1-xN (x a pproximate to 0.2) in the reacted near-surface region.