Rectification Magnetoresistance In Ge-based Nonmagnetic Al/Ge Heterojunctions

Magnetoresistance (MR) is defined as the change of resistance of a material when it is placed in an external magnetic field. The first magnetoresistance was discovered by William Thomson in 1851, and in 1856, he discovered the anisotropic magnetoresistacne (AMR) in iron and nickel. Since then, the magnetoresistance was widely studied due to its significant values in theoretical research and technological application.Up to now, there are two kinds of MR materials have been extensively studied, magnetic materials and nonmagnetic materials. The MR research in magnetic materials has gained a great achievement in two kinds of magnetic systems:one is the artificial granular composites and multilayered films, in which the MR effects originate from the spin-polarized scattering or spin-polarized tunneling mechanisms and called GMR or TMR. The other one is the doped perovskite manganites, in which the MR is based on the transition from a high temperature paramagnetic insulator state to a low temperature ferromagnetic metal state and called CMR. The resistance of all the magnetic systems mentioned above decreases with the applied magnetic field, which is called negative magnetoresistance. In contrary, large positive MR effects were observed in nonmagnetic semiconductors Ag2Se and Ag2Te by Xu in 1997. It was found that the resistance of the material exhibits an unusual linear dependence on applied magnetic field. This extraordinary large positive MR is called extraordinary magnetoresistance effect (EMR). Since then, the MR research on nonmagnetic semiconductors is being taken more and more attention. There are several mechanisms of MR in nonmagnetic materials, including intrinsic inhomogeneity, change of materials’ band structure and geometric effect etc.In contrast with GMR and TMR, with no barkhausen noise, EMR has several advantages:good stability, high-degree precision, low-power dissipation. Therefore, the EMR effect is expected to lead to breakthrough in size of the electron device.In this thesis, we report a very different magnetoresistance in nonmagnetic Al/Ge Schottky heterojunctions that can be regarded as rectification magnetoresistance (RMR), which is due to the simultaneous implementation of the rectification and magnetoresistance in the same devices. As for magnetoresistance and rectification, they are two fundamental physical properties and respectively have wide applications in spintronics devices. Before the discovery of RMR, the study of magnetoresistance and rectification effects are relatively independent to each other. This leaves our understanding of the interplay between electrostatic response and spin dynamics incomplete and limits the further development of spintronics. RMR is a brand new MR, and provides us an alternative way towards advanced magnetoelectronics technologies.The main experiment and results are as follows:(1) The discovery of RMR:we prepare a nonmagnetic Al/Ge/Al circle Schottky Heterojunctions, then apply a pure small sinusoidal alternating-current to the nonmagnetic Al/Ge Schottky heterojunctions with Keithley 6221 source meter. We observed that the measured rectification voltage of the sample increases with the increase of the applied magnetic field. At room temperature and in a magnetic field of 6T, the sample show rectification voltage increases of up to 250%, which is greatly enhanced as compared with the conventional direct-current magnetoresistance of 70%. The findings of rectification magnetoresistance open the way to the new nonmagnetic Ge-based spintronics devices of large rectification magnetoresistance at ambient temperature under the alternating-current due to the simultaneous implementation of the rectification and magnetoresistance in the same devices.(2) Electrically Programmable Magnetoresistance in Nonmagnetic Ge-based Schottky Devices:the experiments were performed on Al/Ge/AI heterojunctions in a bar configuration with 5 mm in length and 2 mm in width. The used Ge substrate is single side polished,<100> orientation with resistivity of 55.6-59.4 Ω. cm. One Al/Ge electrode is purposely designed to be Schottky contact while the other electrode is Ohmic contact. We found that the RMR of the heterojunctions can be greatly tunable by simultaneously applying DC and AC current. Not only extremely large MR could be obtained, but also the sign of the MR could be reversed by changing the DC offset. The MR observed is 32500% when AC=0.1 mA and DC offset current=5.228 uA, while it is -530% when AC=0.1 mA and DC offset current=7.6 μA. This unambiguously indicates that simultaneous application of DC and AC is an effective method to manipulate the magneto-transport properties of Schottky devices.