Strategies For Enhancing Electrically Pumped Random Lasing From ZnO Film Based Light Emitting Devices On Si

Random lasing (RL) is a light-emitting phenomenon occuring in disordered gain mediums. Different from the conventional lasers constructed with gain media and well-defined resonant cavities, random lasers rely on the optical feedback provided by the multiple scattering in disordered gain materials. ZnO has a large exciton binding energy (-60meV) and a high refractive index, resulting in the high optical gain and strong multiple optical scattering. Consequently, the ZnO materials have been considered to be desirable for RL. Since the report of optically pumped RL from polycrystalline ZnO powders and films, a great many efforts have been expended in the field of RL from ZnO. In recent years, the electrically pumped RL from a diversity of ZnO-based devices have been demonstrated, shedding light on the practical application of RL. Obviously, the reinforcement of electrically pumped RL from ZnO films is a worthwhile subject. In this regard, low threshold current and high output optical power are the important pursuits for the electrically pumped RL from ZnO films.In this dissertation, the strategies to reinforce electrically pumped RL from ZnO film based light emitting devices on silicon have been intensively investigated. Electrically pumped RL from ZnO films has been reinforced by modifying the properties of ZnO films and designing new device structures. Moreover, the related physical mechanisms have been elucidated. The primary achievements in this dissertation are described as follows.(1) Electrically pumped RL actions of two metal-insulator-semiconductor (MIS) structured devices, using the ZnO film and Zn2TiO4-nanoparticle-incorporated ZnO film as the light-emitting layers, respectively, have been comparatively investigated. It is demonstrated that the device using the Zn2TiO4-nanoparticle-incorporated ZnO film exhibits a much smaller threshold current and a larger output power under the same injection current for the electrically pumped RL, which can be ascribed to the enhanced multiple light scattering within the ZnO film by the incorporation of Zn2TiO4nanoparticles into ZnO film.(2) Electrically pumped RL actions of two MIS structured devices, using the ZnO film and surface-textured ZnO film as the light-emitting layers, respectively, have been comparatively investigated. It is demonstrated that the device using the surface-textured ZnO film exhibits a much smaller threshold current and a larger output power under the same injection current for the electrically pumped RL, which can be ascribed to the enhanced multiple light scattering within the ZnO film by the surface texturing.(3) The electrically pumped RL actions of the ZnO film based MIS structured devices using SiOx(x≤2) films prepared by electron beam evaporation, radio-frequency sputtering and sol-gel methods, respectively, as the insulative layers have been investigated. It is found that the device using the SiO2film prepared by the sol-gel method exhibits the lowest RL threshold current. It is believed that the number and energy levels of defect states in the SiO2film prepared by the sol-gel method result in enough electrons accumulating in the conduction band of ZnO and enough holes generating in the valence band of ZnO, thus favorable for the electrically pumped RL.(4) The effects of multiple scattering in ZnO films and the band offset between ZnO and the insulator on the electrically pumped RL of ZnO film based MIS structured devices have been investigated. It is found that the stronger multiple scattering in ZnO films and the larger band offset between ZnO and the insulator lead to a lower RL threshold current.(5) The effect of ZnO film thickness on the electrically pumped RL of ZnO film-based MIS structured devices has been investigated. For the ZnO films with thicknesses below50nm, it is found that the RL threshold current increases with the increase of ZnO film thickness. Moreover, the output power of RL decreases with the increase of ZnO film thickness at small injection current regime, while it increases with the ZnO film thickness at large injection current regime. The mechanism underlying the above-mentioned results has been tentatively explored in terms of the two ingredients of RL, i.e. multiple light scattering and optical gain. For the ZnO films with thicknesses exceeding100nm, it is found that the RL threshold current decreases and the output power of RL increases with the increase of ZnO film thickness, which can be ascribed to the weaker optical absorption by the Si substrate and less optical leakage from the ZnO film resulting from the increase of ZnO film thickness.(6) The RL from the light-emitting device based on two-fold-tandem (double-) SiO2/ZnO-structure has been investigated. It is demonstrated that remarkable decrease in threshold current for electrically pumped RL from the double-SiO2/ZnO-structured device with respect to that in the case of single-SiO2/ZnO-structured device. Moreover, the former is of higher power conversion efficiency. In the double-SiO2/ZnO-structure, a waveguide is formed by the stacking SiO2/ZnO/SiO2, which enables photon confinement. Moreover, the electrons leaking out of the bottom SiO2/ZnO-structure are collected and partly involved in the radiative recombination in the top one. Furthermore, the RL photons generated in the bottom SiO2/ZnO-structure act as the stimuli to increase the stimulated emission rate in the top one. For the above-mentioned reasons, the RL performance of the double-SiO2/ZnO-structured device is substantially improved.