High‐Temperature Lasing Characteristics of ZnO Epilayers

Abstract

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...

Optical and morphological properties of MBE grown wurtzite CdxZn1−xO thin films

“…Similar emission has been observed in bulk ZnO [12], however the emission is generally found at shorter wavelengths around 565 nm. GaN has a yellow emission line which is attributed to deep level donor/acceptor states [18][19][20], and similar luminescence has been observed from Cd x Zn 1Àx O grown on Silicon substrates [17].…”

“…This should enable inexpensive device processing and engender new optical structures and devices based on ZnO ternary heterostructures. Lastly, the exceptional exciton binding energy of ZnO (60 meV) enables efficient photoluminescence at room temperature and has allowed researchers to realize high temperature UV lasing with epitaxially grown ZnO films [10][11][12]. While bulk ZnO itself offers many potential benefits, advanced heterostructures require the ability to tune the bang gap and confinement energies of the individual layers.…”

Complex Refractive Indices of Cd x Zn1−x O Thin Films Grown by Molecular Beam Epitaxy

“…Doped and undoped ZnO films of varying degrees of crystallinity exhibit good conductivity and are well explored for use as transparent conducting oxide (TCO) films. [35][36][37] Perhaps the most commonly emphasized semiconductor property of ZnO is its large freeexciton binding energy 38,39 ($60 meV), which is credited as enabling lasing in epitaxial layers at temperatures up to 570 K. 40 Thus, the persistence in ZnO-related research has occurred not only as a result of improvements in epitaxial growth and engineering methods, [5][6][7][9][10][11]16,21,[41][42][43][44][45] but also the continued exploration of its unique material properties.…”

Growth and characterization of ZnO nanorod arrays on boron-doped diamond films by low temperature hydrothermal reaction

“…Zinc oxide (ZnO), as a direct wide band gap (3.37 eV) semiconductor with a large exciton binding energy (60 meV), has been widely used in many fields [1], such as room-temperature [2] and high-temperature ultraviolet (UV) lasers [3], field emission displays [4], UV detectors [5], and gas sensors [6]. Generally, ZnO-based devices are grown on silicon [7], gallium nitride [8], indium-tin-oxide glass [9] and diamond substrates [10].…”