| ZnO, an II-VI compound semiconductor, was considered as a promising material for ultraviolet light-emitting diodes, laser diodes and photodetectors, due to its many excellent physical properties, such as a wide band gap of 3.37 eV and a large excitonic binding energy of 60 meV at room temperature.To realize application of ZnO-based devices, both n-type and p-type ZnO with good conductivity and high quality are of the essence. It has been proven that stable n-type ZnO films can be prepared easily, doped by doner like Al and Ga. Therefore, study on preparation and properties of p-type ZnO thin films has become a very important issue for applications of ZnO in photo-electronic devices.It is well known that a few groups have reported the p-type ZnO is obtained. Many dopants, such as N, P, As, Sb, Li, were employed to prepare the p-type ZnO films.But the growth of reproducible p-type ZnO remains a big challenge, and some physical questions (such as stability, optical and electrical properties, and acceptor dopant) of p-type ZnO can not still be resolved very well. Otherwise,the low carrier concentrations is the most significant obstacle for ZnO-based optoelectronic devices practically. The key to obtain the high quality device is producible low resistivity and reproducible p-type ZnO.Among many dopant, Li and N are considered as the best candidates for producing p-type ZnO due to small strain effects and shallow acceptor levels of substitutional LiZn and NO acceptors based on first-principle calculation. In order to improve the stability and electrical properties of p-type ZnO, several groups tried to fabricate Li-N dual-doped ZnO films(ZnO: (Li, N)) by various techniques such as RF-magnetron sputtering, two-step heat treatment, pulsed laser deposition, and plasma-assisted molecular-beam epitaxy (P-MBE). Hang-Ju Ko just obtained n-type Li-N dual doped ZnO by P-MBE method using Li3N as dopant, however, Ye et al fabricated the ZnO: (Li, N) films with good p-type conduction by pulsed laser deposition. X.H. Wang also obtained the p-type ZnO: (Li, N) films, but resistivity is high and hole density is low.Although Li-N dual doped p-type ZnO have been investigated widely, its electrical and optical properties still need enhance, and mechanism of the p-type conductivity is not clear yet. It's reported that the hole in the film is produced by the cluster acceptor dopant of LiZn-2N-2O by the measurements of XRD and XPS, combining with the dopant theory. However,recently,it's also reported that the dual-acceptor complex LiZn-NO is unlikely to form due to their repulsive interaction.To resolve the problems about Li-N dual doped p-type ZnO mentioned above, in this thesis, by X-ray diffraction (XRD), photoluminescence (PL) measurement, and Hall Effect measurement,we investigate the structure, electric and optic properties of ZnO: (Li, N) thin films prepared on quartz substrates by radio-frequency magnetron sputtering, using diffetent molar ratio of Ar/O2 as sputtering gases respectively. And we investigate the effect of different ambient on the growth of Li-N dual doped p-type ZnO. Then the ZnO films doped by different dopant had been made, to study the effect of N on the p type electric conduction of ZnO: (Li, N) thin film. The details are as follows:( 1 ) The ZnO: (Li, N) thin films prepared on quartz substrates by radio-frequency magnetron sputtering, using diffetent molar ratio of Ar/O2 as sputtering gases . And the as-grown films were annealed in vacuum for 30 min at 600℃. By investigating the structure, electric and optic properties of ZnO: (Li, N) thin films, we find small change of the molar ratio of Ar/O2 could change the structure and properties of the ZnO: (Li, N) thin films largely.(2)When the molar ratio of the Ar/O2 is 60:1, it's proved that which growth ambient avails the growth of p-type ZnO: (Li, N) thin films, via analysing the structure, constitution, electric and optic properties of the ZnO: (Li, N) thin films. And unless that condition, the film can't grow with high c-axis preferential orientation.(3)The optical properties of the p-type ZnO: (Li, N) thin film was also investigated by temperature-depent PL, the different emission bands in UV region were assigned, and the optical acceptor level of LiZn was calculated to be131.6 meV above the covalence band.(4)The pure ZnO and the Li doped ZnO thin films were prepared under the same gowth and annealing condition with the p-type ZnO: (Li, N) thin film. By X-ray diffraction (XRD), photoluminescence (PL) measurement, and Hall Effect measurement,we investigate the structure, electric and optic properties of the films with different dopants. It's found that because of the dopant N, the crystal quality of the p-type ZnO: (Li, N) thin film has been enhanced. And the additional introduction of N may help compensate the single Lii donor defects. So that there are more LiZn defects as accptors in the ZnO: (Li, N) thin film. Which improve the p-type conductivity and stability of ZnO thin films. |
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