Photoluminescence properties of ZnO epilayers grown on CaF2(111) by plasma assisted molecular beam epitaxy

Abstract

Photoluminescence (PL) properties of ZnO films grown on ${\mathrm{CaF}}_{\mathrm{2}}\mathrm{(111)}$ with a low-temperature buffer layer by plasma assisted molecular beam epitaxy are investigated, in which lattice misfit tensile strain is expected to be compensated by the thermal compressive strain. The low-temperature buffer layer is further introduced to accommodate lattice strain leading to the growth of almost strain-free and high quality ZnO films. PL spectra of ZnO layers measured at 10 K are dominated by neutral-donor bound exciton emission at 3.366 eV ${\mathrm{(I}}_4)$ with a linewidth of 12 meV. Commonly observed deep level emission at around 2.3 eV is negligibly small in intensity. Free exciton emission develops as temperature is raised and eventually dominates at temperatures higher than 70 K. Detailed study on temperature-dependent PL spectra indicates that the energy position of the free exciton emission is located at the same energy as bulk materials suggesting the growth of strain-free ZnO layers. Consequently, stimulated emission due to exciton–exciton scattering is observed at 77 K.