Spintronic semiconductor devices based on Mn doped GaAs

The effects of the strong sp-d exchange interaction, ferromagnetic ordering and large spin fluctuations on the electrical transport properties of various spintronic semiconductor devices have been studied both theoretically and experimentally. The studied devices, which either have a ferromagnetic Mn doped GaAs layer or a Mn doped quantum dot as a central part of the device structure, included pn- and Schottky diodes, Esaki-Zener tunnel diodes, resonant tunnelling diodes, and ferromagnetic single electron transistors consisting of ferromagnetic quantum dots. The modeling of the spintronic devices utilized the advanced Green's function techniques, such as Keldysh Green's functions, which allowed accurate modeling by combining the quantum mechanically calculated electronic structure of the devices with the quantum statistical transport theory. This way the effects of scattering and collisional broadening of the energy levels could also be conveniently included in the models. The models predicted strongly spin-dependent transport and large changes in the magnetotransport properties of the spintronic semiconductor devices at temperatures close to the ferromagnetic ordering temperature or in moderate magnetic fields. The model for the ferromagnetic quantum dots predicted Kondo-like resonances in the conductance at high temperatures. In the experimental part of the work the ferromagnetic thin films, pn-junctions, Esaki-Zener tunnel diodes, Schottky diodes, and resonant tunnelling diodes were fabricated using Molecular Beam Epitaxy technique for the growth of the Mn doped GaAs layers. The electrical and magnetic properties of these devices were studied by measuring the I-V characteristics, Hall effect, magnetoresistance, and magnetization as a function of temperature and magnetic field. The main result was the observation of the tunnelling anisotropic magnetoresistance effect (TAMR) in the Esaki-Zener tunnel diodes and the resonant tunnelling diodes. The effect was observed at very low bias voltages, which might allow the realization of ultra low-power spintronic devices. The developed models explained the measured magnetotransport properties. As an example, applying the spin-disorder scattering model a good quantitative agreement was obtained between the measured and calculated resistance and magnetoresistance in Mn doped GaAs layers in wide temperature and magnetic field ranges.