Complex excitonic recombination kinetics in ZnO: Capture, relaxation, and recombination from steady state

Abstract

The kinetics of relaxation and recombination processes of excitons in an epitaxial-grown thick ZnO layer has been examined using time-resolved cathodoluminescence. The unique feature of this technique allows the full analysis of excitation from thermal equilibrium into true steady state and the relaxation back into thermal equilibrium. The luminescence at 5K is characterized by a rich structure of excitonic lines: XA, I1, I2, I6, I8, and I9 as well as the excited states I6*, I8*, and I9* are clearly resolved. The efficient capture of the free excitons by impurities is directly visualized during the onset as well as the decay. This capture feeds the neutral impurity bound excitons I8 and I9, the initial decay of which becomes delayed. The ionized impurity bound excitons I1 and I2 exhibit a very fast initial decay due to the carrier capture by the impurities followed by a persistent, significantly slower nonexponential component.