| Semiconductor solid-state-lighting, which represents the future of human light source, has the tendency to replace the traditional bulb. Since 1960s, scientists have developed red, green light-emitting diodes (LEDs) based on solid semiconductors. After successfully fabrication of high-quality p-type GaN film, high-efficiency blue LED is commercialized. Up to now, researchers around the world have been still working hard to further improve the luminescence-efficiency of GaN based LED. Compared with GaN, ZnO has unique advantages such as:1) exciton energy is as large as 60 meV, which is two times of that of GaN and allows the realization of stable room-temperature stimulated laser; 2) ZnO single-crystal substrate as large as two inches is available and ZnO is amenable to low-temperature growth with a vast zinc resources in nature; 3) ZnO is bio-compatible and high-resist to the radiation. All these advantages promote ZnO as one of the hottest semiconductor in the world. As the problem of asymmetry doping widely exists in the area of wide-bandgap semiconductor, ZnO is suffered from p-type doping. Low resistivity, high hole mobility, high crystal quality and stable ZnO has been rarely reproducibly fabricated. This dissertation focuses on the donor and acceptor behavior in the ZnO film and nanowires. The detailed investigations can be divided into several parts:1) Because of the excellent crystal-quality of ZnO, we have investigated the donor behavior of Al, In in ZnO nanorods. The donor levels of the Al and In are 75 meV and 102 meV, respectively, which add difficulty to the p-type doping in ZnO. On the other hand, we found that the surface crystal quality of ZnO nanowires grown by MOCVD method is superior to that of nanowires grown by VPT method.2) Through theory and systematical experiments, we deduce that Na is a good acceptor dopant for ZnO. Utilizing PLD technique and Na doping, relative stable, low-resistivity p-type ZnO has been achieved and p-n homojunction LED has been realized. We pointed out that H is critical for promoting formation of Nazn acceptor, which has an acceptor level of-164 meV.3) Furthermore, through Na doping, we have realized p-type Zn1-xMgxO (0≤x≤0.25) films and found that an appropriate content of Mg (0.11≤x≤0.15) may enhance the p-type behavior. Alternatively, we have observed UV-blue emission from a p-ZnO:Na/n-ZnO junction containing a Zn0.9Mg0.1O/ZnO multi-quantum wells at room temperature. 4) Considering ZnO films are composed of numerous nanorods, we propose that negative charged adsorptions existed in the crystal grain boundaries degrade the hole mobility. After reducing the barrier height in the grain boundaries by UV illumination, we observed an enhanced p-type behavior or even a transition from n-type to p-type conductivity for ZnO:Na films. As for the p-type ZnO:Na film with a resisitivity of 13.8-19Ωcm, a hole concentration of 4.78×1018-4.66×1018cm-3 and a hole mobility of 0.12-1.42 cm2/V s, UV illumination can improve its p-type behavior, and a best p-type film with a resisitivity of 3.8 Qcm, a hole concentration of 2.09×1017 cm-3 and a hole mobility of 7.91 cm2/V s can be reproducibly obtained.5) We put emphasis on the growth of single-crystalline acceptor doped ZnO related materials. We have developed a PLD technique to grow highly-oriented phosphorus doped ZnO nanowires. However, we found that the phosphorous doping concentration in a single ZnO nanowire may be not uniform, as indicated by the varying defect density along the nanowire.6) Finally, regarding the disadvantages and advantages of a single nanowire or film growth technique, we demonstrated a new strategy to grow single-crystalline Na doped p-type ZnO or ZnMgO materials in combination of nano technique and film technique. The p-type behavior of the obtained single-crystalline p-type ZnO materials is stable over 11 weeks. |
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