Magnetic And Electrical Properties Of Ge-based Ferromagnetic Semiconductors And Their Heterojunction Diodes

Microelectronics industry has achieved great development in 20th. Based on the large scale integration, the processing and transmission of information are achieved by manipulating carries by applying electric field. However, in conventional information technology we only take advantage of the charge property of the electrons, while the spin degree of freedom of the electrons is neglected. The information storage technology, which is based on ferromagnetic materials, is achieved by utilizing the spin degree of freedom of electrons. It is a dream for most scientists to manipulate the charge and spin of electrons simultaneously by electric and magnetic field. In this way, spin dependent electronics (spintronics) emerges due to the desire of new generation information technology. Spintronics will be the core of the next generation information technology, by which the manipulating and storage of information can be achieved simultaneously. Spintronics has received considerable attention and scientific study, and it has achieved great development. There are two approachs to explore spintronics devices:firstly, the spin dependent magnetoresistance devices based on ferromagnetic materials have been successfully used in magnetic sensors, magnetic heads and Magnetoresistive random access memory;secondly, the transition metal doped magnetic semiconductors based on semiconductor materials have been well explored and achieved a great deal of development.Magnetic semiconductor are the key materials to explore spintronics devices, which is prepared by doping transition metal to III-V, II-VI and IV groups' semiconductor materials by conventional preparing methods. The carries are spin polarized in ferromagnetic semiconductors due to the exchange interaction between the polarized 3d electrons of magnetic ions and the carries. In the last century, the Curie temperature of the well explored ferromagnetic conductor europium chalcogenides is lower than 10K, and this material is awfully difficult to prepare. These characters are obstacles for europium chalcogenides used as spintronics devices. Until Ohno found that the Mn doped GaAs single films showed intrinsic ferromagnetism with Curie temperature about 110K, the passion of exploring new type ferromagnetic semiconductors is inspired again. The intrinsic ferromagnetism of GaMnAs is induced by hole carries. The theoretical calculations and experimental results evidently indicated that the GaMnAs are the recognized ferromagnetic semiconductor with intrinsic ferromagnetism. However, the Curie temperature of GaMnAs is much lower than room temperature, which hampers the application of GaMnAs ferromagnetic semiconductor in industry. At the same time, the Mn, Cr, Co doped ZnO, TiO2 and In2O3 were also extensively studied, but the magnetism can be influenced by oxygen vacancy easily. Therefore, the magnetic results of oxide ferromagnetic semiconductors reported by different research groups are different or even contradictorily.Ge-based FMS have attracted considerable attention for their good compatibility with present Si-based processing technology and their potential application in high performance devices due to the higher carrier mobility than Si. Theoretical calculation results indicate that the Curie temperature of Ge-based ferromagnetic semiconductor shoud be above 400K, which is another advantage to study Ge-based ferromagnetic semiconductors. Park et al. firstly reported that epitaxial single crystal MnxGe1-x films showed ferromagnetism and the Curie temperature increased linearly with Mn concentration from 25 to 116K. And the concentration of the hole carries as well as ferromagnetism can be tuned by applying gate voltage. The Curie temperature of samples sensitively depend on the concentration of transition metal, but the solution density of single films prepared in thermal equilibrium condition is very low, and it is easy to form clusters and second phase. In order to achieve the application of Ge-based ferromagnetic semiconductor in industry, we must improve the saturation magnetization and Curie temperature. To this end, we prepared a series of homogeneous amorphous Ge-based ferromagnetic semiconductor films and related heterojunction diodes with high doped concentrations (30-50 at %) under thermal nonequilibrium condition by magnetron co-sputtering technology. The electrical transport and magnetic properties have been systematically investigated. Our work will promote the application of Ge-based ferromagnetic semiconductors in spintronic devices. The details of our work as below:●Homogeneous amorphous MnxGe1-x:H films were synthesized under thermal nonequilibrium condition by magnetron co-sputtering technology with hydrogen in Ar atmosphere. Compared to the MnxGe1-x films without hydrogen, the MnxGe1-x:H films with hydrogen show higher concentration of hole carriers, larger conductivity, and higher saturation magnetization. Moreover, it was found that the anomalous Hall resistivity is proportional to the perpendicular magnetization. These electrical and magnetic properties indicate that the ferromagnetism of the MnxGe1-x:H films is intrinsic ferromagnetism mediated by the spin-polarized hole carriers.●Amorphous MnxGe1-x:H ferromagnetic semiconductor films prepared in mixed Ar with 20% H2 by magnetron co-sputtering technology show global ferromagnetism at low temperature. With increasing of measuring temperature the coercivity of MnxGe1-x:H changes from positive to negative, and then to positive, which has not been discovered in MnxGe1-x and other ferromagnetic semiconductors before. The changing of the coercivity is explained based on the magnetic coupling of the ferromagnetic regions with rich H, where the ferromagnetism with high Curie temperature is induced by carriers. Hydrogenation can tune the magnetic properties of MnxGe1-x film, which is essential for the application of MnxGe1-x films in spintronics devices.●Homogeneous amorphous FexGe1-x ferromagnetic semiconductor films with high Fe concentration were synthesized under thermal nonequilibrium condition by magnetron co-sputtering. The microstructure, magnetism, and Hall effects were systematically studied. The results indicate that FexGe1-x films have intrinsic ferromagnetism with high Curie temperature and magnetization. The saturation magnetization as a function of the temperature can be fitted by Bloch's spin-wave formula, and the Curie temperature is extrapolated up to 590K for Fe0.4Ge0.6 films. Quantitative analysis of the electrical transport reveals that FexGe1-x ferromagnetic semiconductor films show conductivity of weakly localized carriers (holes) on the metallic side. Moreover, the anomalous Hall resistivity is proportional to the magnetization for all the samples, indicating the carries are spin-polarized and the ferromagnetism is intrinsic. Therefore, FexGe1-x ferromagnetic semiconductors with high Curie temperature and magnetization have potential application in spintronic devices as a high efficient spin injection source.●FexGe1-x/Ge amorphous heterojunction diodes with p-type FexGe1-x ferromagnetic semiconductor have been grown on single-crystal Ge substrates of p-type, n-type and intrinsic semiconductor, respectively. All the heterojunction diodes show conventional p-n junction behavior like a rectification at the whole temperature range measured. No obvious difference was observed in the I-V curves of p-Fe0.4Ge0.6/p-Ge diode from 10K to room temperature. For the p-Fe0.4Ge0.6/n-Ge diode, good rectification maintains to room temperature. More interestingly, the I-V curve of p-Fe0.4Ge0.6/instrisic-Ge diode can be tuned by magnetic and electric fields, indicated by a large positive magnetoresistance. The resistances of the junctions decease with increasing temperature, suggesting a typical semiconductor transport behavior. The origin of the positive magnetoresistance is discussed based on the effect of the magnetic and electric field on the energy band structures.