ZnO-based Double Heterostructure And Multiple Quantum Well Electroluminescent Devices

Zinc Oxide (ZnO) is one of the II-VI semiconductor materials with a wide direct band gap of3.37eV and a large exciton binding energy of60mV. In addition, ZnO material is environmentally friendly and shows high thermal stability, low cost and abundant availability. For these advantages, ZnO is considered as one of the most promising materials for generating high-efficiency short-wavelength excitonic luminescence and fabricating semiconductor lasers with a low thresholds. However, the choice of dopant and growth technique remains controversial and the reliability of p-type ZnO is still under debate. To avoid the difficulties in p-type doping of ZnO, kinds of/p-type conductive material have been used with n-ZnO in electroluminescent devices. So far, ZnO-based electroluminescent devices usually show low luminescence efficiency and poor stability. Mechanisms of electroluminescence (EL) and random lasing remain unclear. In this doctoral thesis, ZnO-based single heterojunction, double heterojunction and multiple quantum well (MQW) have been prepared to fabricate light-emitting diodes (LEDs) and laser diodes (LDs). The solution of improving luminescence performance and stability has been put forward. The mechanism of EL has been studied systematically.First, LEDs based on ZnO/NiO single heterojunctions were fabricated using a radio frequency magnetron sputtering system on commercially available n+-GaN/sapphire substrates. The GaN was used as an electron injection layer and a suitable substrate for their similar physical properties as ZnO. With the help of the electron blocking layer of i-ZnMgO inserted between the ZnO and NiO layers, the intensity of the～370nm emission has been greatly enhanced and the threshold current decreases from～70mA to as low as～23mA. Due to the effective barrier of the ZnO/ZnMgO interface, electrons will be confined in the n-ZnO layer and the advantages of the large exciton binding energy of ZnO can be fully used. Random lasing has also been observed.To improve the performance of single heterojunction electroluminescent devices, LEDs with MgZnO/ZnO/MgZnO double heterojunction structure have been fabricated and the room temperature EL spectra have been investigated. With the help of double heterostructure, LEDs show better visible EL performance than that of LED with ordinary p-i-n structure. By replacing ZnO film with ZnO nanorod arrays in this double heterostructure, strong ultraviolet EL emissions around370nm and380nm was achieved by taking the advantages of ZnO nanorods with large carrier injection rate, relaxed strain to the substrate and good light extraction. The ZnO-nanorod-based double heterostructured LED exhibits superior stability with an intensity degradation of less than3%over8hours.In the MgZnO/ZnO/MgZnO symmetric double heterostructure, the large AEc of MgZnO/n-ZnO is harmful for the electron-injection from n-ZnO to i-ZnO. To solve this problem, ZnO/GaN-based LEDs with improved asymmetric double heterostructure of Ta2O5/ZnO/HfO2have been fabricated. EL performance has been enhanced by the HfO2electron blocking layer and further improved by inserting a Ta2Os hole blocking layer. The emission originations have been identified, which indicated that the Ta2O5/ZnO/HfO2asymmetric structure could confine carriers in the activ i-ZnO layer more effectively so that the radiation from GaN can be suppressed. The LED only losts-30%of its EL intensity after a continuous operation over160hours which is longer than that demonstrated in previous reports.Inspired by the commercialized GaN-based MQW LED, in order to realize high-efficient ZnO luminescence, MQW structure has been introduced into the ZnO-based LEDs and pure ultraviolet random lasing has been obtained. A ZnO/ZnMgO MQW ultraviolet random laser diode was fabricated with an electron injection layer of n-GaN and a hole injection layer of p-NiO using a radio frequency magnetron sputtering system. The scanning electron microscopic images show good interface of MQW. EL measurements revealed that the diode exhibited pure UV random lasing centered at-370nm. The device has a very low threshold current density of4.7A/cm2and extremely weak visible emission. Light scattering from the grain boundaries of the ZnO nanocrystalline film may form close loops to establish gain. When the optical gain of the emitted light can exceed the loss in the close-loops under enough bias voltage, random lasing action occurs.The study in this doctoral thesis provides innovative ideas, experimental experience and scientific reference for development of ZnO-based electroluminescent devices. One can expect that ZnO-based electroluminescent devices will achieve application to market in the near future.