The topic of electron-hole (e-h) recombination dynamics in zinc oxide (ZnO) is of great importance to clarify its the excitonic properties, while still not completely explored. In fact, time-resolved photoluminescence (TRPL) is very often employed as a crystalline quality characterization tool, while TRPL studies may indeed give also quantitative information, once the experimental results are analyzed using a physically reasonable recombination model. We show that sub-nanosecond time-resolved analysis of photoluminescence in as-received (0001) ZnO single crystals evidences clear signature of bimolecular recombination dynamics at high injection (Mott density) regime. We studied the decay kinetics using a general expression of time-resolved photoluminescence signal resulting from a second order e-h recombination rate. Analysis based on effective lifetime calculation allows to determine the fundamental parameters characterizing the recombination dynamics (unimolecular and bimolecular lifetime, free-carrier concentration). Oxygen annealing of the samples leads to a transition from a bimolecular kinetics characterizing an e-h photogenerated plasma to a double exponential kinetics. Analyses carried out though Atomic Force and Scanning Kelvin Probe Microscopy allow interpretatioin of this phenomenon as an effect of oxygen adsorption at surfaces, while ruling out the oxygen vacancies as origin of green luminescence.
Electron-hole recombination kinetics in ZnO: transition from bimolecular to unimolecular regime
L Santamaria;
2010
Abstract
The topic of electron-hole (e-h) recombination dynamics in zinc oxide (ZnO) is of great importance to clarify its the excitonic properties, while still not completely explored. In fact, time-resolved photoluminescence (TRPL) is very often employed as a crystalline quality characterization tool, while TRPL studies may indeed give also quantitative information, once the experimental results are analyzed using a physically reasonable recombination model. We show that sub-nanosecond time-resolved analysis of photoluminescence in as-received (0001) ZnO single crystals evidences clear signature of bimolecular recombination dynamics at high injection (Mott density) regime. We studied the decay kinetics using a general expression of time-resolved photoluminescence signal resulting from a second order e-h recombination rate. Analysis based on effective lifetime calculation allows to determine the fundamental parameters characterizing the recombination dynamics (unimolecular and bimolecular lifetime, free-carrier concentration). Oxygen annealing of the samples leads to a transition from a bimolecular kinetics characterizing an e-h photogenerated plasma to a double exponential kinetics. Analyses carried out though Atomic Force and Scanning Kelvin Probe Microscopy allow interpretatioin of this phenomenon as an effect of oxygen adsorption at surfaces, while ruling out the oxygen vacancies as origin of green luminescence.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
