Formation Of Second Phases And Defects In Growth Of Doped ZnO Nanofilms

ZnO semiconductors with excellent magnetic,optical and electrical performances are the ideal materials for multifunctional devices.The main limits for the applications are the stable control of p-type conductivity and the origin of room temperature ferromagnetism in ZnO.The effective doping and the control of defects formation are the main solutions.However,it is easy to form the second phases and the defects during the growth of the doped ZnO films,which can affect the magnetic,optical and electrical performances significantly.Thus,it is necessary to study the growth process of ZnO films systematically,revealing the formation mechanism of the second phases and the defects,which can attribute to controlling the magnetic and electrical properties in the doped ZnO films.Combined with the preparation method of electrodeposition,vacuum evaporation and the molecular dynamics simulation,the growth of the doped ZnO films and the formation mechanism of the second phases and the defects are studied in this paper.The effects of the dopants content,the growth modes of the metal films,the oxidation temperature and the high magnetic fields(HMF)conditions on the formation of the second phases and the defects are examined,and the relations between the formation mechanism of the second phases and the defects with the magnetic and electrical properties in the doped ZnO films are determined.The studies are divided into three parts:the study of the oxidation growth of doped ZnO films by methods of oxidizing the metal films prepared by electrodeposition and vacuum evaporation,the study of the in-situ growth of doped ZnO films by methods of RF atomic source assisting vacuum evaporation,the study of these growth process by the molecular dynamics simulation.The main results are as follows:(1)The effects of the growth methods on the formation mechanism of the second phases and the defects in the doped ZnO filmsThe formation mechanisms of the second phases and the defects in the ZnO:Co and ZnO:Cu films,which prepare by the oxidation growth and in-situ growth,are studied systematically.The doped ZnO films with oxidation growth have lower surface quality and crystallinity.While the doped ZnO films with in-situ growth have the smooth and compact surface,which shows higher surface quality and crystallinity of the doped ZnO films with oxidation growth.Both in the ZnO:Co films with oxidation growth and in-situ growth,Co dope into the ZnO with the Co2+state,and the exceed Co exist in the ZnO films forming the second phase Co metal.In the ZnO:Cu films with in-situ growth,Cu dope into the ZnO with the Cu2+and Cu+state,as the increase of Cu dopants content,Cu2+state changes to Cu+ state and a small amount of Cu2O phase forms in the films.In addition,the second phase of Cu-Zn is easy to form in the films,leading to the main defects transforming from VO to VZn.(2)The effects of high magnetic field on the oxidation growthThe interaction of magnetic energy and thermal energy can improve the crystallinity of the doped ZnO films during oxidation growth.However,the effects of HMF are decided by the surface morphology of Zn films.For the looser surface structure,HMF promotes the oxidation of Zn films and decreases the VO content.For the compact surface structure,HMF inhibits the oxidation of Zn films and increases the VO content.In addition,HMF suppresses the diffusion of Co and Cu elements during oxidation growth.The Lorentz force in high magnetic field can change the absorption positions of O2-,leading to the change of surface morphologies in doped ZnO films.(3)The effects of the second phases and defects on the magnetic and electrical propertiesIn the ZnO:Co films,the dopants Co2+ are not the origin of the ferromagnetism.The origin of ferromagnetism in ZnO:Co films has close relations with the second phase Co metal.The second phase Co metal attribute to the origin of ferromagnetism when the content of Co metal is lager.Otherwise,the VO is the origin of ferromagnetism.The ZnO:Cu films with Cu2+ show n-type conductivity.As the increase of Cu content,Cu+ ions become the major dopants and the ZnO:Cu films show p-type conductivity.In addition,lots of Vzn in the ZnO:Cu films can decrease the self-compensation effect.Thus,the Cu+ ions dopants and the VZn are the main reason for ZnO:Cu films showing p-type.