Recombination Dynamics in ZnO

Time-resolved PL is a nondestructive, powerful technique commonly used for the optical characterization of semiconductors. The free carrier or exciton lifetime, an important parameter related to material quality and device performance, can be measured by TRPL spectroscopy. The exciton lifetimes will vary with crystal quality, becoming longer as the quality improves. The efficiency of the radiative recombination is strongly related to decay time of the particular transition. 

Reynolds et al. [175] measured the recombination lifetime of the allowed (Fs) and forbidden (Fg, allowed by induced strain) free excitons at 2 K in a strained singlecrystal ZnO grown by the hydrothermal method as 259 and 245 ps, respectively. The lifetime for the F5 exciton was slightly higher, 322 ps, for an unstrained sample. They noted that free-exciton lifetimes are determined not only by the radiative decay but also by the nonradiative decay and capture processes leading to bound excitons [66]. Evidently, the measured single exponential decays reflect the effects from all three. 

The fast decay constant is smaller for the as-received sample (170.4 ps) and most probably represents the effective nonradiative recombination at room temperature. The slow-decaying component is attributed to the radiative lifetime of the free exciton. The 0.86 ns value measured for the as-received sample is reasonably consistent with the 0.97 ns value measured by Koida et al. [176] for single-crystal ZnO. The relative magnitude of the slow-decaying component to the fast-decaying component (A2/A1 = 0.094 for 540 pj cm ) for the as-received sample suggests that the nonradiative processes are dominant. It has been proposed that the nonradiative 

FX and DBF denote the free and donor-bound excitons, respectively. (After Ref. [50].) 

The spontaneous recombination times observed by Ozgiir et al. [152] for RF-sputtered ZnO thin films are comparable to other values reported in the literature. Guo et al. [119] reported 30 ps room-temperature excitonic recombination times for ZnO thin films grown on Si by OMVPE. Koida et al. [176] measured recombination 

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