Investigation of excitons in AlGaN/GaN multiple quantum wells by lateral photocurrent and photoluminescence spectroscopies

Abstract

We report on the investigation of excitons in Al0.2Ga0.8N/GaN multiple quantum wells (MQWs) by lateral photocurrent, and photoluminescence (PL) spectroscopies over the temperature range from 9 to 300 K. The MQWs were deposited homoepitaxially by plasma-assisted molecular-beam epitaxy on a (0001) GaN template grown by hydride vapor phase epitaxy. Excitonic peaks in the photocurrent spectra due to the bulk GaN template and the MQW structure were observed up to room temperature. The PL excitonic peak in the MQWs was strongly Stokes shifted over the whole temperature range, a result attributed to recombination via disorder-induced excitonic band-tail states in the MQWs, due primarily to well/barrier interface roughness fluctuations. A theoretical calculation estimates these fluctuations to be 0.7±0.2 monolayers. The temperature dependence of the Stokes shift indicates that, at higher temperatures, the excitons in the PL experiments are in thermal equilibrium with the lattice before recombining. At lower temperatures, the data suggest a nonthermal exciton distribution, which we attribute to exciton trapping in local potential minima. The PL intensity was found to be strongly quenched by two thermally activated mechanisms with activation energies of 33±6 meV and 165±33 meV. The splitting of the exciton, which gives rise to the photocurrent, was correlated with these two quenching mechanisms, and a model was developed to account for the temperature dependence of the excitonic photocurrent in the MQWs.