Spectroscopy of Excitons, Bound Excitons and Impurities in h-ZnO Epilayers

Abstract

Photoluminescence (PL) and selective photoluminescence (SPL) experiments have been carried out to analyse the neutral donor bound exciton spectra of nominally undoped wurtzite ZnO epilayers and to study the electronic levels introduced by arsenic doping. The investigated samples were grown by molecular beam epitaxy on various substrates. The PL spectra of the undoped heterostructure layers exhibit little or no detectable deep emissions and are dominated by donor bound exciton recombination lines with distinct emission energies. The existence of residual strain distributions in the samples make possible the observation of resonant sharp lines due to the recombination of the bound excitons selectively created in their excited states. It is being shown that the first excited state spectra of the bound excitons consist of rotational states of the hole belonging to the (A) valence band. The PL spectra of ZnO:As samples present a new excitonic line as well as donor–acceptor pair emissions. The arsenic impurity is revealed to generate a shallow acceptor level in ZnO, an estimate of its binding energy is given.