Study on manganese doped III-V semiconductors and spintronic devices
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Manganese doped III-V semiconductor materials, such as GaAs/Mn digital alloys and GaMnSb random alloys, are prepared with low temperature molecular beam epitaxy (LTMBE) technique. Magnetic properties of those materials are characterized with superconductive quantum interference device (SQUID) and vibrating sample magnetometer (VSM). The GaAs/Mn digital alloys show two ferromagnetic phases with the high T C phase persisting higher than 400K. Strong in-plane uniaxial anisotropy is observed for the low T C phase. Rotation measurements are carried out as a supplemental tool in the study of the two-phase behavior and anisotropy. Post-growth thermal annealing is performed on GaAs/Mn digital alloys and magnetization results are in agreement with X-ray results. In the study of GaMnSb random alloys, square like magnetic hysteresis loops are observed at low temperatures for the first time. The dependence of growth conditions such as J Sb /J Ga and Mn concentration are studied for the T C and H C in the GaMnSb random alloys. The coercive field is strongly temperature dependent and it is attributed to the temperature dependence of the cubic magnetocrystalline anisotropy. The time dependence of the coercive field reveals a thermal activation process. Some GaMnSb samples show the aging effect which is believed to be equivalent to a long time low temperature thermal annealing process. Another part of the thesis work is the implementation of spin-polarized transport. With major modifications to a traditional spin FET (Datta-Das), a device structure named Spin Bragg Filter (SBF) is proposed. In the design of a SBF, the ferromagnetic proximity effect and the Rashba effect are employed, for spin polarization and spin manipulation in a 2DEG system. An InAs layer provides the 2DEG channel and a MnAs layer on the InAs layer is patterned as the ferromagnetic reflectors. Device fabrication is carried out with electron beam lithography (EBL) technique and other techniques. Major difficulties are encountered in this multi-step fabrication process and we have identified the conditions, limitations for all processing steps. The solutions are suggested for the future work.