Polar face dependence of the ultrafast UV reflectivity of ZnO single crystal

The transient reflectivity of an similar to 40 fs, 340 nm pulse incident on each face of a ZnO single crystal is reported as a function of carrier density. Three exponentially damped carrier relaxation components can be identified in these responses. Only the Zn face reflectivity reveals a subpicosecond decay (200-300 fs) due to electron scattering. In contrast, the O face reflectivity exhibits an similar to 2 ps absorptive decay attributed to the trapping of carriers by O-surface defects at the earliest times. The other decay components are consistent with electron cooling via acoustic phonons and exciton recombination for both faces. (C) 2008 American Institute of Physics.