Growth Of Single-Crystal ZnO By MOCVD, P-type Doping And Room-Temperature Electroluminescence Of ZnO-Based LED

Zinc oxide (ZnO) is a semiconductor with a direct wide band gap of 3.37 eV at room temperature. There has been a great deal of research interest in ZnO semiconductor for its prospects in optoelectronics applications lately. The large exciton binding energy of 60 meV paves the way for an intense near-band-edge excitonic emission at room and higher temperatures, because this value is 2.3 times that of the room-temperature thermal energy (k_BT=26 meV), much larger than GaN (25 meV), another wide-gap semiconductor (Eg~3.4 eV at room temperature) which is widely used for production of blue-ultraviolet and white light-emitting devices. The larger exciton binding energy makes ZnO obtain more efficient lasing by excitonic emission compared to other wide-band-gap semiconductors. Because exciton-exciton scattering-induced stimulated emission occurs at a threshold lower than that for the electron-hole plasma recombination. ZnO has some other advantages over GaN which are the availability of fairly high-quality ZnO bulk single crystals and perfect lattice match between ZnO and ZnO-based alloys (MgZnO and ZnCdO). However, one important problem should be overcome before ZnO could potentially make inroads into the world of optoelectronics devices: the growth of p-type-conductivity ZnO crystals with good properties. ZnO nanostructures of nanowires, nanotubes, nanobelts and nanorings etc. have attracted increasing attention due to their physical properties arising from quantum confinement (such as electronic quantum transport and enhanced radiative recombination of carriers). ZnO nanostructures have promising potentials in extensive applications and are the fundamental building blocks for fabricating nano-optoelectronics and nano-electronics devices, nanosized gas sensors, transducers, and field emitters etc.In this thesis, metal organic chemical vapor deposition (MOCVD) method was used to grow N-doped p-type and undoped ZnO thin films, ZnO nanowires, ZnO nanotubes and ZnO nanostructure networks. The realization of ZnO thin films with p-type conductivity make it successfully to fabricate of ZnO homojunction light-emitting diode. New results are as follows:1. One MOCVD equipment was developed especially for the growth of ZnO materials, and a patent about the equipment was authorized. ZnO thin films and nanomaterials have been grown by using this MOCVD system.2. Well-aligned ZnO nanowires have been grown by MOCVD method on Si substrates. The diameter and length of ZnO nanowires has been controlled by simply varying the reactant sources' flow rate and the growth time, respectively. Field emission measurements showed that the nanowires with sharp tips have excellent field emission properties. The mechanism of controlled-growth of the ZnO nanowires is proposed.3. Quasi-aligned one-dimensional (1-D) ZnO nanotubes have been grown on silicon substrates by MOCVD without using any catalyst. The nanotubes are of single-crystal structure and growing along [0001] direction. Room-temperature photoluminescence measurements of the ZnO nanotubes indicate strong ultraviolet emission and weak green band emission. A new growth mode for these ZnO nanotubes is proposed, which can be used to prepare other nanotubular structures.4. Quasi-three-dimensional (3-D) ZnO nanostructure networks have been grown on silicon substrate by MOCVD. The nanostructures are composed of 1-D nanowires and two-dimensional (2-D) nanowalls. The nanowalls in the nanostructures were growing along [0001] direction and vertically standing on the silicon substrate. The mechanism of synthesis of the ZnO nanostructure networks is also proposed.5. ZnO thin films have been grown on Si and sapphire (a-plane and c-plane) substrates by low-pressure MOCVD. It is showed that the crystal and electrical quality of the thin films grown on the a-plane sapphire substrates at a low substrate temperature of 350 °C was improved by using a ZnO buffer layer. The photoluminescence measurements indicate that the ZnO thin films grown at such a low substrate temperature have a strong UV emission.6. Nitrogen-doped /?-type ZnO thin films have been deposited by MOCVD method via using both NO and N2O as oxygen source, and NO was also used as N dopant source. Moderate N2O gas can effectively improve the electrical properties of the ZnO thin films by restraining the native donor defects.7. Nitrogen-doped p-type ZnO thin films have been grown by using a plasma-assisted MOCVD method. Important growth parameters such as substratetemperature, power of the plasma and the ratio of Zn/O were optimized to improve the properties of the ZnO thin films. The measurements of Hall effects, x-ray diffraction and photoluminescence show that the ZnO thin films are with good electric, crystal and optical properties.8. ZnO homojunction light-emitting diode has been fabricated by growing of p-type ZnO thin films on ?-type bulk ZnO substrate via plasma-assisted MOVCD method. A typical ZnO homojunction shows rectifying behavior with a turn-on voltage of about 2.3 V. Electroluminescence at room temperature has been demonstrated with band-to-band emission at I = 40 mA and defect-related emissions in the blue-green-yellow spectrum range.