Preparation Of P-type ZnO Thin Films And Study On The Doping Mechanisms

Zinc oxide (ZnO) is II-VI compound semiconductor. It can be used in many areas due to its piezoelectric, thermoelectric, gas sensing and photoconducting multiple properties. Recently, the application of ZnO in optoelectronic devices has attracted great attention because ZnO can be used to fabricate blue or ultraviolet light emitting diodes (LED) and laser diodes (LD) due to its direct wide-bandgap (3.37 eV) at room temperature. In particular, ZnO has higher exciton binding energy (60 meV), compared with 24 meV for GaN and 26 meV for the thermal activation energy at room temperature, which permits efficient excitonic emission processes at room temperature, therefore ZnO has great development potential in the field of optoelectronic devices. Its practical applications in this field depend on the fabrication of high quality n-type and p-type ZnO thin films. To date, high quality n-type ZnO thin films have already been achieved by doping with group-III elements. Howerver, undoped ZnO can easily generate some defects acting as donors, so it is difficult to obtain p-type ZnO due to self-compensating effect, which heavily limits the development of ZnO in the field of optoelectronic devices. Hence, it will add a considerable impetus to the development of ZnO if we find some appropriate acceptor impurities to realize high quality p-type ZnO by theories and experiments.Many research groups theoretically calculated various possible acceptor elements, but the results are not uniform and even some are conflicting with the experiments. Based on the density functional theory (DFT), group-I and group-V impurities are further studied in this dissertation in order to find the best dopant for p-type ZnO among them. In group-V elements, the calculated acceptor ionization energy of N is 0.31 eV, which is much smaller than that of P (0.77 eV) and As (0.89 eV), and the acceptor formation energy of N is lowest among them. So N element is ideal dopant for p-type ZnO in group-V elements. Compared with group-V elements, the group-I elements are complicated. The acceptor ionization energies of Li and Na calculated by DFT are 0.11 eV and 0.16 eV, respectively. The difference between them is small. Butfor K element, the acceptor ionization energy is relatively high and its value is about 0.29 eV. In the view of ionization energy, K element can be excluded. The calculated formation energies show that it is easy to form interstitial Li instead of substitutional Li, compared with Na element. Considering ionization energies and formation energies of group-I elements, Na is the best candidate for p-type ZnO. Compared with N, however, Na is worse elemental dopant source for p-type ZnO because of the formation of compensating interstitials.Thus N is the best dopant for p-type ZnO in theory.Guided by theoretical analysis, N-doped p-type ZnO thin films were first prepared in our experiments. In this dissertation, there are two methods to realize N-doped ZnO. One is thermal oxidization of Zn₃N₂ thin films. It can overcome the difficulty that N is not easy to dope into ZnO, and also has simple process and good reproducibility. Oxidation temperature and time were studied in experiments. The results of XRD and UV-Vis spectra showed that Zn₃N₂ completely changed to ZnO at 350℃. Hall results indicated that ZnO thin film, prepared at 400℃ for 3 hour, had the best p-type properties. Carrier concentration and resistivity of the film was 1.22×10¹⁷ cm^-3 and 83.2 Ωcm, respectively. The other is DC reactive magnetron sputtering with N₂ as dopant source. Hall results showed that when substrate temperature was fixed, hole concentration of as-grown thin films incrcased with the N₂ flow increasing and the thin film prepared at 20 sccm had the highest hole concentration. But if N₂ flow was higher than 20 sccm, the conduction type would change to n-type. When N₂ flow was fixed at 20 sccm, ZnO thin films obtained at 400℃ showed the best electrical properties. The hole concentration of ZnO thin films prepared by this way was 5.58×10¹⁷ cm^-3, with the lowest resistivity of 8.44 Ωcm.The electrical properties of N-doped p-type ZnO thin film were not stable in our experiments. So Na-doped ZnO thin films were also studied in this dissertation. The effects of substrate temperature and Na contents in target on properties of ZnO thins films were studied by XRD, SEM and Hall. It was found that the p-type ZnO thin film prepared at 550℃ and 0.2% Na contents in target had better crystallinity and electrical properties. Hall results showed that the highest carrier concentrication was1.07×10¹⁸ cm^-3 and the lowest resistive was 27.6 Ωcm. The stability of p-type conductive for Na-doped ZnO was better than that of N-doped ZnO and the electrical properties didn't show obvious degradation after one month.