Research On The P-type Doping Related Problems In ZnO Films

Zinc oxide (ZnO) has a dirct wide band gap (3.37 eV) and a lager exciton binding energy (60 meV), which promises efficient ultraviolet light emitting diodes and low-threshold lasing diodes may be realized in ZnO. However, reliable and highly conductive p-type ZnO is still one of the biggest obstacles to ZnO based optoelectronic devices. It is speculated that the diffuculty in p-type doping of ZnO is mainly caused by the strong compensation caused by the relatively high residual electron concentration and the donor-related defects induced by the doping process. In this thesis, a detailed investigation on the origin of the above two compensation has been carried out, and the following results are obtained:1. Temperature-dependent Hall measurement has been employed to study the origin of the high residual electron concentration in undoped ZnO films, and it is found that a degenerate layer exists at the ZnO/sapphire interface due to the large lattice mismatch between them. The degenerate have a huge impact on the electrical properties of the ZnO films, and is one of the most important origins of the residual electron concentration in undoped ZnO films. The above results have been verified by Hideo Hosono et al., who demonstrated that a conductive degenerate layer existing at the ZnO/sapphire interface will increase the electron concentration and reduce the electron mobility in the ZnO film.2. Although p-type ZnO films have been obtained using nitrogen as dopant, the p-ZnO films obtained usually have poor electrical properties, which hinder the future development of ZnO. In this thesis, in order to improve the electrical properties of p-type ZnO film, an simple approach has been proposed to increase the doping concentration of nitrogen in ZnO by exposing the ZnO:N films in the ambient of nitrogen plasma periodically. Hall measurements and photoluminescence spectroscopy indicate that this approach is helpful in improving the nitrogen concentration in ZnO films. Under the optimized conditions, p-type ZnO film with a hole concentration of 1.68×1018 cm-3 has been achieved. The above results may provide a simple route to p-ZnO films with improved electrical properties.3. The main source of compensation donor in our nitrogen doped ZnO films has been analyzed, and it is found that the substituted N molecule (N2)O donors is the one of the most important source of donor compensation, and they can be removed by thermal treatment in O2 atmosphere. The above results may lay a solid ground for improving the reproducibility of the N-doped p-type ZnO films. The above results have been confirmed by Sallet et al. and by McCluskey et al. in their review paper recently, respectively.