Journal
JOURNAL OF LUMINESCENCE
Volume 128, Issue 5-6, Pages 792-796Publisher
ELSEVIER
DOI: 10.1016/j.jlumin.2007.11.010
Keywords
ZnO; luminescence efficiency (or -yield); laser processes; stimulated emission; exciton; electron-hole plasma
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In linear optics, we report on measurements of the absolute external quantum efficiency of bulk ZnO and powders using an integrating sphere. At low temperature the near band edge emission efficiency can reach 0.15 in the best samples. For deep center luminescence this value may be even higher. When going to room temperature (RT) the quantum efficiency drops by about one order of magnitude. From time resolved luminescence measurements we deduce the lifetime of the free and bound excitons to be in the sub ns regime and find for the latter a systematic increase with increasing binding energy. Concerning lasing, we discuss the role of excitonic processes and the recombination in an inverted electron-hole plasma (EHP). While excitonic processes seem well justified at lower temperatures and densities, doubts arise concerning the concept of excitonic lasing at RT in ZnO. The densities at laser threshold at RT are frequently close to the Mott density or above but below the density at which population inversion in an EHP is reached. We suggest alternative processes which can explain stimulated emission in this density regime in an EHP at RT. (C) 2007 Elsevier B.V. All rights reserved.
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