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

R. Andrew Wall, Kyle C. Lipke, K. B. Ucer, R. T. Williams, D. Millers, K. Smits, and L. Grigorjeva

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-carrierlike
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 on
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.

Physica Status Solidi (C) Current Topics in Solid State Physics

DOI 10.1002/pssc.200879896

phys. stat. sol. (c) 6, No. 1, 323–326 (2009)

pdf-iconDownload PDF