Magnetic And Transport Properties Of Ge-based And ZnO-based Magnetic Semiconductors

Spintronics has attracted considerable attention in current research fields of condensed physics,information science and new materials.The research in Spintronics mainly focuses on the processes associated with the charge and spin of electrons,such as the generation of spin-polarized current,spin injection,spin transportation,spin detection and spin controlment.The ultimate aim of Spintronics is to actualize a new type device referred as Spintronics devices.Spintronics has a huge application prospects since it makes a completely new model for information processing and storage and greatly enhances our ability to deal with information. The foremost thing to Spintronic devices actualization is to generate spin-polarized current.Therefore,magnetic semiconductor,which is as a key material to generate spin-polarized currents,has attracted intense research.Magnetic semiconductor is usually obtained by doping transition metal elements intoâ…¢-â…¤/â…¡-â…¥/â…¤semiconductors.The system of Mn doped GaAs has been widely and systematically studied among those materials,of which Curie temperature has reached 185K and magnetism originates from carriers-mediated magnetism.However,it does not fit for application due to its relative low Curie temperature.Therefore,the work in the magnetic semiconductor research fields has payed much attention on pursuing a magnetic semiconductor material with high Curie temperature in view of application.Considering the theoretical predictions that the Curie temperatures of Ge-based and ZnO-based magnetic semiconductor are above 400K and 300K,respectively,and the advantage of themselves as semiconductor materials,such as Ge is compatible with current Si-based processing technology,ZnO has wide application in light-emitting materials,piezoelectric material and transparent conductive films,the research towards Ge-based and ZnO-based magnetic semiconductor have attracted much more attentions.According to most of reported experimental results in Ge-based and ZnO-based magnetic semiconductor,the properties of samples are sensitive to the preparation methods and growth condition,the magnetic results of samplse are different from one another and even contradictory,the intrinsic magnetic origin of samples is still under debate.In order to solve these problem,we will do some researches on Ge-based and ZnO-based magnetic semiconductor through low temperature epitaxy and amorphous growth.Single crystal fabricated by low temperature epitaxy can efficiently get rid of influence of the secondary phase on ferromagnetic origin and facilitate the analysis of such issues as the origin of magnetism due to oneness in structure and purity in components.Amorphous growth is considered from the view of improving the solubility of tranzation metal element,which can increase Curie temperature and magnetization.The work of this paper is focused on the research of magnetic and transport properties in Ge-based and ZnO-based magnetic semiconductor.The main contents and results are as follows:●Magnetic anisotropy with different magnetization in in-plane and out-of-plane was found in Single crystal Ge_1-xMn_x(x=0.019,0.041,0.051,0.072,0.105) diluted magnetic semiconductor films fabricated on undoped Ge(001) substrates at low temperature by MBE,furthermore,a sign reversion of ordinary Hall effect coefficient with decreasing temperature was found in Ge_0.95Mn_0.05 film.The patterns of in-time reflection high energy electron diffraction(RHEED) and X-ray diffraction measurement imply that the samples keep single crystal structure until the Mn concentration is arrived at 0.072.The hysteresis loops measured at 5K show all of samples are ferromagnetic,moreover,a strong magnetic anisotropy,which was indicated by quite different magnetization in in-plane and out-of-plane under the same magnetic field,was found in our single crystal samples indicating a strong spin-orbit coupling interaction existed.The electrical transport of films is semiconducting and obeys Efros's variable range hopping law in the low temperature range.Anomalous Hall effects in Ge_0.95Mn_0.05 film were observed below the Curie temperature,indicating the carrier mediated intrinsic ferromagnetism.The sign of ordinary Hall effect coefficient is reversed in the low temperature range, which can be explained by the Aharonov-Bohm loops through an odd number of sites in hopping regime.The odd number of sites may come from three randomly distributed localized Mn acceptors sites,which is also the origin of AHE in the hopping regime.●Through doing researches towards rectifying character of Ge_1-xMn_x/Ge single crystal heterojunctions with different germanium substrates fabricated at low temperature by the molecular beam epitaxy,we found magnetic field can largely modulate the rectifying character of p-i junction in which a large positive magnetoresistance was also observed.The rectifying behavior of transport is found in all of junction samples.Theâ… -â…¤curves show that the best rectifying property was found in Ge_1-xMn_x/Ge with n-type doping Ge substrate,the largest current density was found in Ge_1-xMn_x/Ge with p-type doping Ge substrate,and the strongest response to magnetic field was found in Ge_1-xMn_x/Ge with non-doping Ge substrate. The large magnetoresistance and its dependence on temperatures are well understood by considering the band filling effect and the effect of magnetic field on the junction resistance.●The research towards magnetic and transport properties were done in amorphous Ge_1-xMn_x magnetic semiconductor films with high Mn concentrations prepared on liquid-Nitrogen-cooled glass substrates by ultra high vacuum MBE. Hysteresis loops measured at 5K show coexistence of ferromagnetism and paramagnetism.The maximum Curie temperature of 220K was found in Ge_0.48Mn_0.52 film.Moreover,exchange bias occurs in magnetization hysteresis loops for samples with higher Mn concentrations(x≥0.66),which can be explained by the antiferromagntic exchange coupling between ferromagnetic phase and antiferromagnetic phase.All the Ge_1-xMn_x magnetic semiconductor films show semiconducting transport behavior and anomalous Hall effects below the Curie temperature,indicating cartier-mediated ferromagnetism.●The intrinsic room temperature ferromagnetism was found in Co-doped wurtzite ZnO(Zn_1-xCo_xO) thin films grown on Al₂O₃(0001) substrates by oxygen plasma-assisted molecular beam epitaxy at the low growth temperature of 450℃. The epitaxial films of Co concentration at 0≤x≤0.12 are single crystal,which were examined by reflection high energy electron diffraction and x-ray diffraction.Both of optical transmission spectrum and in situ.x-ray photoelectron spectroscopy studies confirm the incorporation of Co²⁺ cations into wurtzite ZnO lattice.Magnetic measurements revealed that the Zn_1-xCo_xO thin films are ferromagnetic with Curie temperature T_C above room temperature,and the ferromagnetism shows intrinsic characteristic.The origin of magnetism can be explained by F-center model. Furthermore,all of films show semiconducting transport behavior.●The research towards magnetic and transport properties of as-grown and annealed inhomogeneous Zn_1-xFe_xO magnetic semiconductor films with high Fe concentration prepared by sputtering has been done.We found that Magnetic properties can be greatly modulated by controlling the composition inhomogeneity and subsequently fast annealing.Ferromagnetism and paramagnetism is co-existed in as-grown Zn_0.23Fe_0.77O film.Moreover,the positive switching field was observed in the as-grown Zn_0.23Fe_0.77O film,indicating an antiferromagnetic coupling existed between the neighboring local ferromagnetic regions which is mediated by thermally activated hopping carriers.The value of switching field increases with increasing temperature from 110 K to 300K.