Time-resolved absorption and luminescence following electron-hole pair creation in ZnO

We report transient absorption induced by electron-hole excitation in undoped ZnO. A laser pump/continuum probe method covers 2-300 ps, and an electron pulse with lamp transmission covers 8-300 ns. The broad absorption spectrum increases monotonically with wavelength from 900 to 1600 nm. Following a reasonable hypothesis that the free-carrier-like induced infrared absorption is proportional to the total number of free carriers, excitons, and shallow-trapped carriers in the sample, these data allow setting an upper limit oil the quantum efficiency of a specified lifetime component of luminescence. For the undoped commercial ZnO studied in this report, the quantum efficiency of room temperature excitonic luminescence is less than 5%. This means that there is significant room for improvement in applications. aiming to use room-temperature excitonic blue luminescence of ZnO for fast scintillators and light sources. Direct observation that a large majority of excitations are tied up for more than 100 nanoseconds in shallow traps confirms the premise for studies undertaken at Oak Ridge National Laboratory to improve the excitonic luminescence yield and decay rate of ZnO by donor doping. The preliminary results presented here on undoped ZnO suggest that induced absorption measurements should be a useful diagnostic of quantum efficiency while studying such dopant effects. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim