First-principles Study On ZnO Doped With The Transition-metal Elements And The Physical Properties Of Metastable Phases

ZnO is one of semicondutors with band-gap energy and a hot topic in the research field of semiconducting materials.This dissertation focuss on the problems related to ZnO alloys doped with the elements of transition metals and the metastable phases of ZnO.According to Dietl e1 al"s predication,ZnO doped with the elements of transition metals may possess room-temperature ferromagnetism,thus is a candidate of diluted magnetic semicondutors,which has potential in the applications of spintronic devices.However,arguments are remined in theoretical and experimental studies of ferromagnetisnm in the ZnO doped with elements of transition metals.On the other hand,metastable phases of ZnO could be synthesized under some special conditions.Therefore,it is necessary to study the physical properties of these metastable phases for developing new ZnO-based f-unctional materials.The results and achievements in this dissertation are summarized as below:1.In previous studies of ZnO doped with Co,it is known that U correction could change the ferromagnetic(FM)and antiferromagnetic(AFM)orders of ZnCoO alloys,but the reasons were rnot further explored.In this study,the electronic structures of ZnCoO alloys were calculated using GGA+U and compared with absoption spectra of ZnCoO films and the results calculated using Tanabe-Sugano theory.Interband transition was found in good agreement with the experimental data.but the energy levels related to the highly localized Co 3d states are still incorrect.U correction made some 3d states separated from the valence band.thus,having impact on the formation energy of FM and AFM phases.Before U correction,FM is preferred.After U correction,AFM is preferred when Co content is low,and FM is preferred if Co content is high(?25%).The conclusions obtained from the calculations with U correction are consistent with most of experimental results;thus U correction seems to be necessary for the study of magnetism in the ZnO doped with the elements of transition metals.In addition,the calcualtions with U correction suggested that the AFM and FM phases in ZnCoO alloys are close in the formation energy,thus the AFM to FM transition is possible when the interaction between Co2+ ions is enhanced due to the increase in Co content and/or the Ruderman-Kittle-Kasuya-Yosida interaction is enhanced due to the increase in carrier density.Besides ZnCoO alloys,ZnO alloys doped with other transition metal elements(TM=Fe,Ni,Mn,V,Ti,Cr)were studied using the first-principles calculations with U correction.The U correction produced a change in the electronic structures of ZnTMO alloys similar to that of ZnCoO alloys,and then AFM changed to be preferred after U correction,but FM was not preferred in the cases of ZnTMO alloys with high TM contents.Therefore,we concluded that these ZnTMO alloys are not good candidates of diluted magnetic semiconductors,in good agreement with most of experimental conclusions.2.The ?-BN-type layered ZnO is one of metastable ZnO phases that have been synthesized in experiment,but the relevant experimental data are rare;thus it is necessary to study its physical properties for the potemtial applications in the future.In this dissertation,the ?-BN-type layered ZnO was studied in terms of electronic structure,phonon spectrum,dielectric function,infrared reflection,stiffness coefficient,bulk modulus,hardness,heat capacity,and Debye temperature and compared with those of wurtzite ZnO.The ?-BN-type layered ZnO was assigmed to be a semiconductor with indirect band gap,and the band-gap energy is slightly larger than that of' the wurtzite ZnO.The ?-BN-type layered ZnO exhibited a phonon spectrum similar to that of the wurtzite ZnO,but the discrepancy in the c-axis direction is observable,and thus having a wide infrared reflection band almost covering the band below 600 cm-1.In addition,the Raman peak observed in experiment was assigned to the optical mode ofE2ghigh.In comparison with the wurtzite ZnO,the layered ZnO was predicted to be a material behaving in a brittle manner,with a microhardness?3.6 times higher than that of the wurtzite ZnO.The temperature-dependent thermodynamic functions suggest that the layered ZnO has the thermal properties similar to those of the wurtzite ZnO,but having a little higher Debye temperature above room temperature.3.NiAs-and ê-BeO-type znic oxides are two new metastable phases reported in recent.Their electronic structures have been studied,but leaving the physical physical properties unknown.In this dissertation,GGA+U and LDA were mployed to study the physical properties of these two phases in terms of electronic structure,phonon spectrum,dielectric function,infrared reflection,stiffness coefficient,bulk modulus,and heat capacity and compared with those of wurtzite ZnO.The NiAs-type ZnO was found to be a semiconductor with heavy holes.The mechanical properties showed that its stiffness coefficients Cij increases with the increase in pressure.ZnO with ?-BeO structure is a semiconductor with direct band gap.The dielectric constant is about 2.1,which is slightly less than that of wurtzite ZnO.The dielectric function exhibited a blue-shift with the increase in pressure.In the optical modes,only is gniode Raman active at the center of the Brillouin zone.The reststrahlen band of the NiAs-type ZnO is about 250 cm-1,which is between the values of the wurtzite and the ?-BN-type phases of ZnO.Similar to the ?-BN-type layered ZnO,the?-BeO-type ZnO may have a negative thermal expansion coefficient at a low temperature.