Preparation, Ferromagnetism, Optical And Electrical Properties Of K-doped ZnO Films

Diluted magnetic Semiconductors (DMSs) with the ability of controlling thespin and charge simultaneously have attracted great attentions. Since roomtemperature ferromagnetism in Mn doped p-type ZnO was theoretically predicted byDielt et al. in2000, much attention was payed and quantity work were carried out onoxide semiconductors. However, the difficulty in ruling out the interference oftransition metal cluster and second impurity phases results in the great dispute in theorigin of ferromagnetism. The theory of d0ferromagnetism brought out by Coey etal. in2004was not only a challenge to the present theory of ferromagnetism, butalso an effective way of avoiding the influence on second magnetic phase.As the earliest predicted room-temperature ferromagnetism in the family ofwide band semiconductor, ZnO has excellent properties in electric, optics andmagnetic， and became the research focus in spintronic. We fabricated thepolycrystalline and epitaxial K doped ZnO (KZO) films and bilayers of Al dopedZnO (AZO) and KZO by Radio frequency magnetron sputtering method. We carriedout a systematic study on their structure, optic, electric and magnetic transportproperties. In addition, more attention was paid on the origin of ferromagnetism andp-type conductive behavior.The discovery of enhanced magnetism in pure ZnO film by annealing in airindicates that the origin of magnetism in ZnO films relates to Zn vacancy rather thanO vacancy. With a comparison of the photoluminescence spectrum, we found thatthe magnetism enhanced sample always accompanied by a strong green luminescepeak. The absence of p-type conductive behavior in polycrystalline KZO was due tothe abundant existence of interstitial defects like Kiand Zni. With the lower dopingof K (less than8%) in polycrystalline sample, the carrier concentration decreasedwith the increasing K contents, while its saturation magnetization enhanced. Thisresult is due to the occupancy of K+in Zn2+sites, which provides a hole and acts asan acceptor defect.An investigation in the epitaxial KZO films discovered a transition ofconductive behavior from n-type to p-type with the K doping content around6%.This transition could be ascribed to the transition of K+from the interstitial site to the occupancy site. The saturation magnetization of these films first increase, thendecrease with the increasing K contents and reach a maximum value at the content of8%, while the corresponding hole carrier concentration has the same tendency. Thisresult strongly indicates that the hole plays an important role in the modulation ofmagnetism of these samples. Finally, based on first-principle calculations, the Katoms on the substitutional sites which can induce the local magnetic moment withtheir neighboring oxygen atoms are responsible for the observed ferromagnetism.