Study On The Controllable Growth Of ZnO Films By MOCVD And ZnO-Based Light-Emitting Devices

As an important II-VI group semiconductor, ZnO has received considerable attentionfor its potential applications in short-wavelength optoelectronic devices because of its largedirect band gap (3.37eV) and high exciton binding energy (60meV). However, up to nowhigh-performance ZnO-based light-emitting diodes (LEDs) and lasing diodes (LDs) havenot been achieved yet. This is mainly limited to the controllable growth of high-qualityZnO films, especially the stable and reproducible p-type ZnO materials. In order to solvethe fore-mentioned problems, our research has been focused on the controllable preparationof high-quality ZnO films, p-type ZnO and design and fabrication of ZnO-basedoptoelectronic devices. The detailed research contents are as follows:The effects of light irradiation intensity on the growth model of ZnO films have beenstudied by a photoassisted metal-organic chemical vapor deposition (PA-MOCVD) system.As the light irradiation intensity increases, the microstructure of ZnO changes from athree-dimensional (3D) nanorod form to2D continuous dense films form. Andsingle-crystal ZnO films were obtained at a light irradiation intensity of2000W, with a lowfull width at half-maximum (284arcsec) of (0002) ω-rocking curves. As for themorphology evolution of the resulting ZnO formation, we proposed a possiblephotofacilitated nucleation mechanism. In addition, we have investigated the effects of lightirradiation intensity on the optical and electrical properties of ZnO films. This part ofresearch lays foundations for the fabrication of high-performance ZnO-based LEDs andLDs.We have studied in detail the effects of growth temperature and reaction pressure onthe growth dimension of ZnO epilayers. A controllable growth of ZnO nanostructures withdifferent dimensions was achieved using a catalyst-free growth mehod. As the processingtemperature increases, the microstructure of ZnO epilayers changes from a3D nanorodsform to a2D nanowall networks (NNWs) form. On the basis, the2D ZnO NNWs formevolves into a2D dense films form along with the increase of reaction pressure. We systematically studied the effects of growth parameters on the growth of thermodynamicsand kinetics of ZnO epilayers.We have performed the research on controllable preparation, growth mechanisms andoptical properties of2D ZnO NNWs and1D ZnO nanowires (NWs). Additionalexperiments on time-dependent morphology tracking were carried out to investigate thegrowth mechanisms of two structures. In addition, we achieved the controllable growth ofZnO NNWs on micro-hole size and density by adjusting the oxygen flow rate. And possiblecontaminants adsorption behaviors were proposed to explain the deep-level emissionvariation for the produced ZnO NNWs. Moreover, well-aligned1D ZnO NWs were alsoprepared by optimizing the low-temperature nucleation layer, reaction pressure and lightirradiation intensity. And the growth mechanisms were also investigated by time-dependentmorphology tracking experiments.A heterojunction LED based on n-ZnO NNWs/p-GaN has been demonstrated to testthe application potential of ZnO NNWs as emitting layer in LEDs. At12mA, the lightoutput power of the diode reached1.38μW, and only~8.08%decay in the light outputpower was observed after a running time of8.5hours, indicating a good device stability.Temperature-dependent EL measurements were also carried out to verify the sensitivity ofEL performance to temperature. In addition, a heterojunction LED based on n-ZnONWs/single-quality films/p-GaN structure have been designed and fabricated to guaranteethe internal quantum efficiency and light extraction efficiency. The introduction of1D ZnONWs modified the carrier injection and recombination processes, and also substantiallypromoted the light extraction efficiency of the studied diode.The p-type ZnO have been prepared by out-diffusion of As atoms from a GaAsinterlayer, and the typical electrical properties of ZnO:As films show hole concentration of1.21×1017cm-3and mobility of0.78cm2/V s, respectively. We detailed analyzed the originof p-type conductivity for As doping, and two vertical conducting LEDs structures(p-ZnO/n-SiC and p-ZnO/n-GaN/n-SiC) were fabricated to verify the p-type conductivityof ZnO:As films. The electroluminescence performances of two diodes are remarkable interms of their low emission onset and good device stability.Two ZnO/Si heterojunction diodes have been fabricated to test their applicationfeasibility in Si-based optoelectronics. High-resistance MgZnO layer was introduced intothe n-ZnO/p-Si heterojunction as the electron-blocking layer to improve the electron-holerecombination in ZnO layer, and an enhanced light efficiency by~31%was realized finally.Further, we achieved an electrically pumped lasing in ZnO from ann-MgZnO/i-ZnO/SiO2/p-Si asymmetric double heterostructure. By introducing ahollow-shaped SiO2pattern, the lateral diffusion of injection current was prevented and ultimately lowered the threshold of the LDs. We focused on the analysis of resonance modeof the LDs and carrier injection and confinement configuration of the proposed asymmetricdouble heterostructure.Metal-insulator-semiconductor LDs have been prepared based on ZnO/MgO core/shellstructures by using1D ZnO NWs and2D ZnO NNWs as the gain medium. Both two LDsexhibit low threshold currents at forward bias. We firstly studied the effect of MgO shellthickness on the optical properties of ZnO NWs, and the enhancement of ultravioletemission showed a critical thickness of15nm. A hollow polygonal microcavity whisperinggallery type resonant mode was originally proposed to interpret the lasing characteristics ofthe ZnO NNWs-based LDs. In addition, we attempted to introduce low-resistivity p-NiO asthe hole-providing layer to improve the device performance of Au/MgO/ZnO LDs.However, no lasing has been achieved in the n-ZnO/MgO/p-NiO diode, but bidirectionaldriven spontaneous radiation characteristics were demonstrated.