Room-temperature stimulated emission in two-dimensional Mg_xZn_1−xO/ZnO heterostructures under optical pumping

Abstract

Two series of Mg x Zn 1−x O/ZnO multiple quantum wells with 18 and 27 at.% of magnesium content in the barrier layers and well widths L w 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 (λ exc = 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 τ r = 355 ps was obtained for the Mg 0.27 Zn 0.73 O/ZnO MQW with a well width L w = 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 (λ GB = 510 nm) are metastable.

Dimensional effects in exciton and defect-related luminescence of ZnO-based step quantum wells

“…As the electron-hole gas is localized in quantum wells, a reduction of the QW width leads to a rise in charge carriers concentration in them at optical excitation. This favors further interaction of charge carriers, so the recombination rate is built up, which leads to a growth of quantum efficiency [7]. As this takes place, the integral intensity of the green band I GB in the range of well widths from 12 to 6 nm remains almost unchanged and amounts to ≈20% of the integral I tot .…”

“…Previously, we have established that the deep energy levels, recombination of charge carriers from which forms the green band of I GB in PL spectra, are metastable with the life time τ = ± 51. 5 1.3 GB μs [7]. Thus, at small values of the quantum well width L W (1-5 nm) the charge carriers realize a nonradiative transition to deep metastable levels and then radiatively recombine, forming a green band that leads to quenching of the exciton band.…”

“…The recent results of investigations on production of lowdimensional structures based on zinc oxide suggest a possibility of fabrication of high-temperature low-threshold lasers with optical pumping [4][5][6][7], and the quantum confinement effects observed in ZnO nanostructures [8] films rises with an increase in the cadmium content. As a result, the Fermi level is displaced to the bottom of the conduction band.…”

Gain-Guiding Anisotropic Polarized Amplified Spontaneous Emissions from C-Plane ZnO/ZnMgO Multiple Quantum Wells

“…Thus, ZnO heterostructures [ 4 , 5 ] or ZnO-based quantum wells (QWs) [ 6 ] have been well-designed or actively developed to produce practical optoelectronic devices, such as highly efficient LEDs or LDs in the blue/UV spectrum. Scientists are also interested in carrier dynamics [ 7 ], acoustic phonon generation [ 8 ], and room temperature amplification spontaneous emission (RT-ASE) [ 2 , 9 , 10 , 11 ] from ZnO/ZnMgO MQWs. In previous reports, low-threshold RT-ASE from ZnO bulk, thin film [ 12 , 13 ], and MQWs [ 9 ] has been produced based on the exciton–exciton scattering (ex–ex scattering) instead of the electron-hole plasma recombination.…”

“…In previous reports, low-threshold RT-ASE from ZnO bulk, thin film [ 12 , 13 ], and MQWs [ 9 ] has been produced based on the exciton–exciton scattering (ex–ex scattering) instead of the electron-hole plasma recombination. Furthermore, the excitation threshold of ASE and the radiation lifetime of ZnO/ZnMgO MQWs have been investigated in relation to the good thickness [ 10 ] and Mg concentration [ 11 ].…”