Stimulated emission and optical gain in ZnO epilayers grown by plasma-assisted molecular-beam epitaxy with buffers

TLDR

High‑quality ZnO epilayers on sapphire were grown by plasma‑assisted MBE with an MgO buffer and their near‑threshold optical gain spectrum was measured using the variable stripe length method. The study shows that at room temperature, free exciton emissions dominate low‑excitation photoluminescence, while exciton–exciton scattering drives stimulated emission at intermediate excitation, producing a symmetric gain peak at 3.17 eV, and electron‑hole plasma contributes to gain above 220 kW cm⁻².

Abstract

We report the results of an experimental investigation on lasing mechanisms in optically pumped ZnO epilayers at room temperature. High-quality ZnO epilayers grown on sapphire by plasma-assisted molecular-beam epitaxy employing an MgO buffer were used. Free exciton emissions and their phonon replicas dominate the photoluminescence from low excited samples. Inelastic exciton–exciton scattering contributes to the mechanism of stimulated emission mainly at intermediate excitation. By using the variable stripe length method, we measured the near threshold optical gain spectrum of the ZnO epilayers. Different from the interband transition governed mechanisms, exciton–exciton scattering gives rise to a nearly symmetric gain spectrum with the peak at 3.17 eV. The electron-hole plasma emerges to contribute to the optical gain when excitation exceeds 220 kW/cm2.