The effect of growth conditions and N₂/O₂ ambient on LO-phonon replicas during epitaxial growth of ZnO on c-sapphire

Abstract

High quality heteroepitaxial thin films of ZnO:N were grown by pulsed laser deposition using a two-step growth method and annealed in situ at different temperatures and ambient conditions. Films were analyzed by X-ray diffraction (XRD), electrical measurements, and photoluminescence experiments at low temperatures to investigate the effect of nitrogen doping. The XRD results demonstrate epitaxial growth on the c-sapphire substrates, with average grain size of 57 nm. Photoluminescence spectra reveals a peak at 3.061 eV (405.1 nm) which is part of the longitudinal-optical-phonon replicas of excitons bound to neutral acceptors ${\text{A}}_{1}^{0} \,{\text{X}}_{\text{A}}$ at 3.348 eV (370.4 nm), attributed in recent investigations to a newly reported donor–acceptor pair. Electrical resistivity and Hall effect measurements were performed using standard four point van der Pauw geometry at room temperature. Fresh films exhibited a resistivity of 3.1 × 10⁻³ Ω cm, a carrier density of 1.3 × 10¹⁹ cm⁻³, and a mobility of 53 cm²/V s. During approximately 2 weeks the as-deposited films presented a p-type behavior, as shown by the positive sign of the Hall constant measured. Thereafter, films reverted to n-type. From electrical measurements and photoluminescence spectra, the acceptor energy was determined to be 150 meV, in close agreement with reported values. These results are consistent with those presented in the literature for high purity crystals or homoepitaxial thin films, even though samples for the present study were processed at lower annealing temperature.