Investigation On The Formation Mechanism And Stability Of N-X Codoped P-type ZnO Thin Films

ZnO, with a band gap of 3.37 eV and the binding energy of exciton as high as 60 meV at room temperature(RT), has been attracted more and more attention as a strong candidate for ultraviolet(UV) light emitting diode(LED) and laser diode(LD). As an optoelectronic functional material, however, there are still some unsolved problems hindering its further applications in optoelectronic field. First, as-grown undoped ZnO always shows n-type conductivity, but its origin is still controversial. Second, the formation mechanisms of p-type conductivity of ZnO are not clear. Third, it has been proven to be very difficult to produce a stable and high quality p-type ZnO film. And the formation mechanism and stability of p-type ZnO have become the international difficult problems which need to be urgently solved.Hence, according to the present hotspots and difficulties on ZnO, in this thesis, our works are focused on undoped as well as Mg and Cd doped n-type ZnO thin films by radio frequency(RF) magnetron sputtering, three p-type doping, such as, N doped Zn-rich ZnO films, Ag-N codoped and C-N codoped ZnO films by combining RF magnetron sputtering with ion-implantation technologies. Combining with modern measure technologies and first-principles density functional calculations, the origin of n-type conductivity of undoped ZnO, the formation mechanisms and stability of p-type N doped Zn-rich ZnO, p-type Ag-N codoped Zn O and p-type C-N codoped ZnO were investigated. The conclusions are drawn as following:① It is proved that the anomalous Raman mode at 275 cm-1 in ZnO is be closely related to zinc interstitial(Zni) related defects. The concentration of Zni related defects in ZnO can be tuned by controlling Mg and Cd alloying content, where Zni related defects can be inhibited by alloying Mg and enhanced by alloying Cd impurity. Furthermore, the background electron concentration of ZnO is determined by Zni related defects which is the origin of the natural n-type conductivity in ZnO.② p-type ZnO:N film [ZnO:(Zn, N)] was successfully prepared under Zn-rich condition. The stability of p-type ZnO:(Zn, N) film is superior to p-type ZnO:N film prepared under normal condition. Zn-rich ZnO:N film contains a certain concentration of Zni defects which are easily trapped by NO acceptor to form Zni-2NO passive complexes, resulting in a decrease in the ionization energy of the acceptor and an improvement in the performance and stability of p-type ZnO:(Zn, N). The p-type film exhibited a stable conductivity-type over two month period after post-annealing, but apparent degradation of p-type characteristic was observed. It is demonstrated that the formation of(N2)O double-donor defects is one of the reasons for unstable p-type conductivity of ZnO:(Zn, N) film.③ p-type ZnO film, with resistivity of 92.57 Ω cm, carrier concentration of 3.072×1016 cm-3, and mobility of 2.2 cm2V-1s-1, was successfully prepared by Ag-N codoping method. The formation of AgZn-NO acceptor pair is responsible for the mechanism of p-type ZnO:(Ag, N) film. Although the p-type film exhibited a stable conductivity-type over four month period after post-annealing, apparent degradation of p-type characteristic was observed. It is demonstrated that the spontaneous formation of(N2)O double-donor defects at RT is one of the reasons for unstable p-type conductivity of ZnO:(Ag, N) film. Moreover, combining with theoretical and experimental studies, it is demonstrated that the observed RT ferromagnetism in ZnO:(Ag, N) films is neither directly induced by Ag nor N ions, but by oxygen vacancies defects.④ Our first principles calculations indicate that N ions of unoccupied oxygen vacancies are easily bonded to lattice oxygen atoms to form(NO)O donor complexes when nitrogen sources were incorporated into ZnO films by means of ion-implantation technology. In the process of post-annealing treatment, N in the complex is easier than O to diffuse to move “freely” in Zn O interstitial sites, and the mobile N ions would find oxygen vacancies to form NO acceptors. However, after the post-annealing treatment ended, the N ions would be deprived of energies and easily trapped by lattice oxygen or nitrogen around them to form(NO)O and(N2)O donor complexes, which goes against the p-type conductivity and its stability. Further theoretical and experimental studies indicate that the appropriate incorporation of a certain amount of carbon impurity can effectively trap interstitial N ions to form(CN)Zn acceptor complex, which avoids to form(NO)O donor complexes. Based on such theoretical scheme, the significant improvement of conductivity and stability of p-type ZnO films were successfully prepared by C-N codoping method, where the nitrogen was incorporated into the ZnO:C films by means of ion-implantation technology.