We used a complement of nanoscale depth-resolved cathodoluminescence spectroscopy (DRCLS), atomic force microscopy (AFM), Kelvin-probe force microscopy (KPFM), and surface photovoltage spectroscopy (SPS) to identify how surface morphology, nanostructure, and polarity of ZnO surfaces correlate with electronically-active native point defects. ZnO is a promising optoelectronic semiconductor with a striking ability to grow nanostructures, but its electronic surface and interface states are not well understood despite their impact on transport and recombination. Previous near-surface DRCLS showed that nanoscale asperities on bare ZnO surfaces exposed to ambient atmosphere for several months can have high trapped charge densities that dramatically increase free carrier recombination and band bending . Not only do surface morphology and recombination velocity correlate within the outer tens of nanometers, but they associate with specific native point defects. SPS measures the filling and emptying of these states and thereby their energy level position in the band gap. In turn, CLS associates these optical transitions with oxygen and zinc vacancies, VO and VZn, respectively. Positron annihilation spectroscopy (PAS) showed that 2.1eV DRCLS emission correlates with VZn vs. depth, anticorrelating with 2.5 eV VO emission . Furthermore, our KPFM-based SPS allowed us to correlate DRCLS optical emissions with optical filling and emptying transitions of the same near-surface regions across ZnO surfaces on a nanometer scale. These localized measurements also reveal Zn vacancy concentrations that increase with proximity to the most common nanostructures - nanorods, nanoclumps, step edges, and starburst patterns. These defects in fact play a role in spontaneous growth of nanoscale surface features.
SPS spectra (fig. 1) display evidence for VZn charge emptying within fields of nano-“bumps” but not in flat ZnO surface areas. Near individual nanobumps, 2.1eV trap densities increase with proximity (fig.2) as do 2.1 eV vs. band gap nano-CLS spectra emissions. SEM-imaged hexagonal nano-pits display 2.1 eV intensities on the ZnO (0001) surface showing that growth of nanoscale features is mediated by the creation of sub-surface Zn vacancies as free surface Zn oxidizes. Furthermore, KPFM maps (fig. 3) before (a) vs. after (b) 2.25 eV illumination show increased potential that reveals large concentrations of VZn distributed non-uniformly around and extending away from AFM pit features. Low (<1 keV) electron beam energy DRCLS spectra and probe depths < 25nm show different deep level emissions dominating (fig.4) for the two polar surfaces. VZn emission dominates the Zn (0001) face while VO dominates O (000-1) spectra. These polarity affects manifest themselves not only in DRCLS but in potential maps and surface morphology as well. Our findings demonstrate the importance of polar effects in forming the surface and near-surface defects that in turn control the spontaneous formation of nanoscale asperities on ZnO surfaces.