Time-resolved surface photovoltage measurements at n -type photovoltaic surfaces: Si(111) and ZnO(10 1¯ 0)

The time-resolved change in the surface potential upon photoexcitation has been measured in two $n$-type photovoltaics, Si (111) 7\ifmmode\times\else\texttimes\fi{}7 and ZnO ($10\overline{1}0$), using two different laser pump-synchrotron x-ray probe methodologies. Taken together, these experiments allow the dynamics of the surface photovoltage (SPV) to be monitored over timescales of subnanoseconds to milliseconds. The timescales for the photoinduced change in the SPV are dramatically different in the two samples, with measured SPV decay time constants of 6.6 \ensuremath{\mu}s for Si and up to 1.2 ms (dependent on surface oxygen concentration) for ZnO. The carrier dynamics at the Si (111) 7\ifmmode\times\else\texttimes\fi{}7 surface are well modeled by a self-decelerating relaxation model involving the recombination of carriers by thermionic emission across the surface depletion layer on nanosecond timescales. In the case of ZnO ($10\overline{1}0$), a persistent photoconductivity (PPC) is observed, which is influenced by oxygen annealing conditions during sample preparation. Persistent photoconductivity is also observed when the surface is illuminated with subband-gap (405 nm) radiation, revealing that defect states approximately 340 meV above the valence band edge are directly associated with the PPC. We demonstrate that, under the conditions of our experiment, PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO.