Room-temperature stimulated emission in two-dimensional MgxZn1−xO/ZnO heterostructures under optical pumping

Two series of MgxZn1−xO/ZnO multiple quantum wells with 18 and 27 at.% of magnesium content in the barrier layers and well widths Lw from 1 to 20 nm have been grown by the pulsed laser deposition method. Stimulated emission is observed in the photoluminescence (PL) spectra excited by the pulsed laser (λexc = 248 nm). The pump density threshold of stimulated emission depends nonmonotonically on the well width, which is associated with an increase of the internal quantum efficiency of the two-dimensional structures caused by a reduction of the radiative lifetime of excitons with decreasing well width, as has been shown by the time-resolved photoluminescence spectra analysis. A minimum value of the radiative lifetime τr = 355 ps was obtained for the Mg0.27Zn0.73O/ZnO MQW with a well width Lw = 2.6 nm. It has been established that the deep energy levels produced by active surface states with carrier recombination forming the green band in the PL spectrum (λGB = 510 nm) are metastable.