ZnO thin films deposited on sapphire, silicon, and fused silica substrates have demonstrated UV excitonic lasing at room temperature (RT) under optical excitation. [1][2][3][4][5] Because of the large binding energy (∼ 60 meV) of ZnO, it is also expected that the ZnO films can sustain excitonic lasing at high temperature. Recent studies of ZnO epilayers have observed spontaneous emission from free-excition (FE) radiative recombination as well as stimulated emission from exciton-exciton scattering (EES) and electron-hole-plasma (EHP) radiative recombination at temperatures up to ∼ 550 K.[2] ZnO nanosheets can also sustain photoluminescence (PL) at ∼ 870 K.[6]These studies have suggested the potential of using ZnO to fabricate high-temperature UV excitonic lasers. However, investigations on the temperature dependence of stimulated excitonic emission from ZnO films and ZnO/ZnMgO superlattices have shown that the corresponding characteristic temperature, which reflects the quality of the high-temperature performance of the lasers, is less than 90 K in a narrow range of temperature between 294 and 377 K. [7] In order to obtain high-quality excitonic lasing at high temperature, it is necessary to improve the characteristic temperature of the ZnO lasers. In this communication, strained ZnO epilayers with nanostructures are proposed to sustain coherent random lasing at high temperature. It is found that the ZnO epilayers can achieve lasing up to 570 K, and the corresponding characteristic temperature can be as high as 127 K. This is because i) the formation of ZnO nanostructures prevents the spreading of excited carriers over the pumped region and ii) the closed-loop path of light by random laser action allows the size variation of random cavities. Therefore, lasing can be obtained from the ZnO epilayers at high temperature. Two ZnO epilayers, sample A (ZnO(100 nm)/ SiO 2 (400 nm)/Si (substrate)) and sample B (ZnO(100 nm)/ MgO(200 nm)/ZnO(100 nm)/SiO 2 (400 nm)/Si (substrate)), were fabricated by the filtered cathodic vacuum arc technique and thermal annealing. [4,5] It is shown that the top (002)-oriented ZnO active layer of sample A (sample B) experiences tensile (compressive) strain along the c-axis.[5] Figure 1 shows the lasing spectra versus temperature, T, for the two samples under optical excitation at ∼ 2 × I th , where I th is the pump threshold intensity at the corresponding T. It can be shown that sample A (sample B) demonstrates RT EHP (FE) radiative recombination with discrete peaks at ∼ 393 nm (∼ 380 nm), and the corresponding lasing mechanism is attributed to random laser action. [5] With the increase of temperature, the shrinkage of the bandgap red-shifts the lasing peaks. Furthermore, it is observed that the full width at half maximum of the lasing peaks for both samples remains less than 0.4 nm for T up to 570 K. Hence, it is shown that coherent random lasing, which arises from the formation of closed-loop paths of light, [8] can be sustained inside the annealed ZnO epilayers with nanostructures at high...
