P型與n型氧化鋅稀磁半導體之結構、電性、光學與磁性質之研究

Abstract

In this study, we grew n-type ZnCoO, ZnCoO:Al, and p-type ZnCoO: N, ZnCoO:(Al, N) thin films by using rf magnetron co-sputtering system, and investigated the structural, electrical, optical and magnetic properties to clarify the origin of room temperature ferromagnetism (RTFM) of ZnO-based diluted magnetic semiconductor. Five parts are included in this experiment. The first part is the fabrication of Zn1-xCoxO thin films and investigations of the physical properties. The Zn1-xCoxO thin films were fabricated in Ar atmosphere by rf magnetron co-sputtering system. According to the results of the structural and optical measurements, the Co2+ ions successfully substituted Zn2+ ions sites of ZnO, proving that rf magnetron co-sputtering system can effectively fabricate ZnO-based diluted magnetic semiconductors. The results of magnetic and electrical measurements respectively present paramagnetism and n-type conduction with electron concentration of 10^19 cm^-3. In the second part, in order to raise the n-type carrier concentration of ZnCoO, a series of ZnCoO:Al with different Al concentration were produced. The results of experiments show that Al-doping indeed promotes the carrier concentration but it has a solubility limit of 6 at.%. Besides, only the ZnCoO:Al with high carrier concentration has room temperature ferromagnetism. In the third part, we fabricated Co doped and (Co, N) co-doped ZnO thin films, and compared the physical properties of them. The sputtering gas of pure argon was used for the ZnCoO film; two other different gases (a mixture of Ar and N2 (gas flow-ratio is 15:5 sccm) and a pure N2) were used for ZnCoO:N films. Without N-doping, the films showed n-type electrical conduction. Upon N-doping, the films became poorly conductive and even exhibited p-type conduction for pure N2 sputtering-gas. Paramagnetism was observed in the film with no N-doping (ZnCoO), but superparamagnetism and then a mixture of ferromagnetism plus paramagnetism were observed with increasing N-doping. In the fourth part, in order to raise the hole-concentration of ZnCoO, we used the Al-N method to fabricate p-type ZnCoO:(Al, N) thin films, and compared the physical properties with the n-type ZnCoO:Al thin films. The results show that the carrier concentrations of ZnCoO:Al and ZnCoO:(Al, N) are 5.34×10^20 cm^-3 and 5.27×10^13 cm^-3 respectively. Both films possess RTFM, but only the ZnCoO:Al exhibits anomalous Hall-effect signals. In the fifth part, we tried to do p-type Zn1-xCoxNyO1-y diluted magnetic semiconductor by using N2O for sputtering gas. Due to the charge compensation, the resistances of samples were too large to determine the conduction type and obtain the carrier concentration. However, we still can confirm that Co2+ and N3- ions successfully substitute Zn2+ and O2- sites for the films by structural and optical measurements. In the case of magnetic property, only the film with lower atomic concentration of cobalt possesses RTFM, and the other films with higher Co concentration present paramagnetism. We consider the mechanism of RTFM is suitable to be explained by BMP model, and the ferromagnetism is sensitive to the cobalt content of ZnCoO films. Finally, we summarize all the results of experiments and suggest that there are two distinct ferromagnetic mechanisms that can give rise to ferromagnetism in ZnO:Co. The magnetic polarons and carrier-mediated exchange mechanisms are suitable in insulating and conductive ZnCoO films respectively.