  The UV enhancement of Al-coated ZnO single crystals with a wide range of
carrier densities is systematically studied using depth-resolved
cathodoluminescence (CL) and photoluminescence (PL) as well as valence band
X-ray photoemission spectroscopy (VB-XPS). AN up to 17-fold enhanced PL UV
emission for Al-coated ZnO with the highest carrier density (2 x 10^17 cm^-3)
was measured, which falls to a 12-fold increase for the lowest carrier density
(3 x 10^13 cm^-3). Depth-resolved CL measurements confirm that the enhancement
is strongest near the metal coating-ZnO interface consistent with an increased
UV emission due to an exciton-localized surface plasmon coupling mechanism.
Correlative CL, PL and VB-XPS studies reveal that a number of additional
effects to the presence of the Al surface coating also contribute to the UV
enhancement factor. These include increased UV enhancement due to the formation
of a surface depletion layer induced by the metal surface coating, which also
passivates competitive non-radiative surface recombination channels found in
uncoated ZnO. Significantly, it was established that the magnitude of the
emission enhancement factor can be raised in a controlled way by reducing the
thickness of the depletion layer by increasing the carrier density. The
contribution of these effects collectively provides an explanation for the
large span of enhancement factors reported in the literature.
