Influence of LO and LA phonon processes on thermal-nonequilibrium excitation and deexcitation dynamics of excitons in GaN, AlN, and ZnO

Abstract

The 1S-exciton density NX(1) in GaN, AlN, and ZnO is calculated for thermal-nonequilibrium states where the temperatures of an electron–hole system and phonons are different in the range of 10–400 K. The ratio of NX(1) in AlN to that in ZnO reaches 21 despite the similar exciton-binding energies of AlN and ZnO, which is due to the higher rate of excitation by LO-phonon absorption in ZnO. This result reveals that thermal-nonequilibrium states significantly affect the validity of evaluation methods for physical parameters such as internal quantum efficiency of radiation. The ratio of NX(1) in AlN to that in ZnO is enhanced from 2.2 to 18 by the occupation of states of the principal quantum number n from 2 to 5. This result demonstrates the importance of the discussion on the n≥3 states which have not been taken into account in other analyses. The main reason for the decrease in NX(1) is found to be the increase in the temperature of LO phonons rather than LA phonons, which indicates the importance of LO-phonon control in light-emitting devices. The results for general thermal-nonequilibrium states are nontrivial because the mechanisms of the population balance are complicated owing to the several-n occupation and the transition rates determined by various factors. Our analyses and discussions quantitatively unveil the LO- and LA-phonon effects on the thermal-nonequilibrium excitation and deexcitation dynamics of excitons and provide the basis for design of highly efficient light-emitting devices particularly in the ultraviolet region.