Fabrication And Characterization Of Na-related Doped And N Doped P-type ZnO Films

ZnO, a kind ofⅡ-Ⅵsemiconductor, has a wide direct band gap of 3.37eV at room temperature, it has high exciton binding energy of 60meV, which is much higher than the kinetic energy at room temperature. With strong excitonic emission at room temperature, ZnO is suitable for fabricating short-wavelength optoelectronic devices, such as violet-blue light-emitting diodes (LEDs), laser diodes (LDs) and ultraviolet (UV) photodetectors. In order to realize ZnO-based optoelectronic devices, stable p-type ZnO is highly needed. Due to intrinsic defects or impurities, ZnO is naturally n-type semiconductor. On the other hand, p-type ZnO is very difficult to achieve. Various kinds of acceptor dopants have been used to fabricate p-type ZnO, however, the best dopant choice has not been determined yet. In this thesis, we focuse on the fabrication of high-quality p-type ZnO film by Na-related doping and N doping, and also attempt to find the best acceptor dopant for ZnO. The work includes:1. Li, Na-codoped p-type ZnO thin films were successfully prepared by PLD. The effect of substrate temperature, O₂ pressure on the structrucal, electrical and optical properties of the films are presented and discussed in this article. The optimal electrical preperties of the Li, Na-codoped p-type ZnO film were achieved at the substrate temperature of 600℃and the oxygen pressure of 21Pa. The optimized film presented a resistivity of 99.81～104.5Ωcm, a carrier concentration of 1.03×10¹⁶～2.96×10¹⁶cm^-3 and a Hall mobility of 2.11～6.03cm²V^-1s^-1. Deduced from the experimental data concerning dual-doping idea, we demonstrate that Na acceptor element is superior to Li acceptor element, considering that Na is preferred to substitute Zn sites in ZnO lattice. However, it is not a good method for preparing high-quality p-type ZnO thin films through Li-Na codoping, possibly due to the fomation of excess intersitial Na or Li during the competition process between Li and Na.2. Na-doped ZnO thin films were successfully prepared by PLD. The effect of laser energy on the structrucal, electrical and optical properties of the films are presented and discussed in this article. The optimal electrical properties of the Na-doped p-type ZnO film were achieved at the substrate temperature of 500℃, the oxygen pressure of 40Pa and the laser energy of 280mJ. The optimized film presented a resistivity of 20.25～20.88Ωcm, a carrier concentration of 1.29×10¹⁷～3.53×10¹⁷cm^-3 and a Hall mobility of 0.848～2.38cm²V^-1s^-1. We could get ZnO film of optimal crystal quality and p-type conduction in proper laser energy, however, laser energy has limited effect on the properties of ZnO films. Because of the good electrical property of Na-doped p-type ZnO films, we are sure Na doping is a good method of fabricating p-type ZnO films.3. N-doped ZnO thin films were successfully prepared by MOCVD. The effect of substrate temperature on the crystallinity, electrical and optical properties of the films are presented and discussed in this article. The optimized electrical properties of the N-doped p-type ZnO film were achieved at the substrate temperature of 390℃and the growth pressure of 20Pa. The optimized film presented a resistivity of 84.7～93.7Ωcm, a carrier concentration of 5.54×10¹⁶～1.04×10¹⁷cm^-3 and a Hall mobility of 0.64～1.2cm²V^-1s^-1. The XPS spectra of N1s for the ZnO:N film grown at 390℃reveals N acceptor in ZnO:N films is highly compensated and this explains that the conduction type of films could be reversed in a narrow temperature range. Due to the high-compensation phenomenon of N acceptor, it is proposed that N is not suitable to be the acceptor element in ZnO compared to Na element.