Investigation On Impurity Defect Behavior And P-type Doping Of Low-dimensional ZnO

As a novel II-VI direct band gap compound semiconductor with many advantages such as large exciton binding energy (60 meV) and wide band gap (3.37 eV), ZnO has been considered one of the most promising materials for short wavelength optoelectronic devices. In recent years, many researchers have already realized p-type doping and electroluminescence of ZnO. However, the poor stability upon p-type conduction and lack of luminous efficiency will limit the pratical application of ZnO. The research on defect engineering of ZnO, which means the control of the impurities and defects and its physical mechanisms, can contribute to realize p-type doping of ZnO and have a certain guiding significance on device physics research and the applications of ZnO in various fields.In this paper, the common defects in ZnO such as surface states, H defects, Cu defects and Na defects were studied considering the background of defect engineering. The p-type ZnMgO:Na films were prepared by PLD and the Na acceptor level was also obtained. The main contents of this paper include:1. The ZnO nanorods were modified by Al2O3 coating and H plasma treatment. It can be found that Al2O3 coating only led to the suppression of the defect emissions, while the near-band-emission (NBE) was greatly enhanced by more than 200 times after H plasma treatment. Evidence was provided that the surface states have a large impact on the defect emissions. Al2O3 coating can passivate the surface states, leading to the the suppression of the defect emissions. H plasma treatment can passivate the nonradiative recombinations centers which quench the NBE, thus resulting in the great enhancement of NBE. The present results have an important significance on improving the luminous efficiency of ZnO and understanding the behaviors of the surface defects.2. The low-temperature PL spectrum dominated by an extremely strong violet emission, was obtained in high energy H plasma treated ZnO nanorods. It can be found that the violet emission was determined by at least two emitting channels according to time-resolved and excitation density-dependent PL results. The above view was confirmed by annealing experiments, which also give evidence that the violet emission is related to H. The ZnO nanorods were treated by H plasma with different power and the violet emission appeared when the plasma power exceeded 30 W. It can be thought that hydrogen may be bound to the intrinsic defects more easily under high energy H plasma treatment, leading to the formation of H-related complex defects, resulting in the violet emission. Moreover, the violet emission disappeared after 400 ℃ annealing due to the escaping of hydrogen. The present results have an important significance on understanding the behaviors of H-related defects in ZnO.3. P-type Cu-doped ZnO films were synthesized by a hydrothermal method combined with post-annealing process. The formation of the films can be explained by a Cu-adsorbed face-selective crystal growth inhibition mechanism. The p-type conductivity was obtained by Hall-effect measurements, which can be verified by a p-n homojunction with well-defined rectifying characteristic. The ionization energy of 326 meV of the CuZn acceptor can be calculated by temperature-dependent PL, which is too large to make most of the holes ionize, thus resulting in a weak p-type conductivity with relatively low hole concentration of 3.11×1015 cm-3. In addition, a large number of intrinsic defects and Cu-related complex defects introduced by the thermal diffusion of Cu may also affect the conductivity of the sample, making it difficult to realize p-type doping.4. P-type Na-doped ZnMgO films were prepared by pulsed laser deposition. The large lattice mismatch by Na-doping can be relieved by Mg alloying, leading to the improvement of the surface morphology of the samples. The ionization energy of NaZn acceptor was calculated to be about 460 meV according to the temperature-Hall and temperature-PL results. It is worth mentioning that the NaZn acceptor energy level was firstly reported in this paper.