  Investigations that probe defects one at a time offer a unique opportunity to
observe properties and dynamics that are washed out of ensemble measurements.
Here we present confocal fluorescence measurements of individual defects in
Al-doped ZnO nanoparticles and undoped ZnO sputtered films that are excited
with sub-bandgap energy light. Photon correlation measurements yield both
antibunching and bunching, indicative of single-photon emission from isolated
defects that possess a metastable shelving state. The single-photon emission is
in the range 560 - 720 nm and typically exhibits two broad spectral peaks
separated by approximately 150 meV. The excited state lifetimes range from 1 -
13 ns, consistent with the finite-size and surface effects of nanoparticles and
small grains. We also observe discrete jumps in the fluorescence intensity
between a bright state and a dark state. The dwell times in each state are
exponentially distributed and the average dwell time in the bright (dark) state
does (may) depend on the power of the exciting laser. Taken together, our
measurements demonstrate the utility of a single-molecule approach to
semiconductor defect studies and highlight ZnO as a potential host material for
single-defect based applications.
