Preparation And Characterization Of P-type Na:ZnO Thin Films Deposited By DC Magnetron Sputtering

Zinc oxide (ZnO) is a II-VI compound semiconductor. It can be used in many areas due to its piezoelectric, thermoelectric, gas sensing and photoconducting multiple properties. Recently, ZnO has attracted great attention to application of optoelectronic devices 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.Different 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 impurities are further studied in this dissertation in order to find the best dopant for p-type ZnO among them. 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. But for 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 formationenergies of group-I elements, Na is the best candidate for p-type ZnO.Guided by theoretical analysis, Na-doped ZnO thin films were studied in this dissertation. The effects of substrate temperature, Na concentration and O₂ ratio of ZnO thins films were studied by XRD, SEM and Hall. It was found that the p-type ZnO thin film prepared at 500℃ and 0.2% Na contents in targets and 10 sccm O₂ ratio had better crystallinity and electrical properties. Hall results showed that the highest carrier concentrication was 1.07×10¹⁸ cm^-3 and the lowest resistive was 13.8 Ωcm. The stability of p-type conductive for Na-doped ZnO on the electrical properties didn't show obvious degradation after one month.