Studies of Ferromagnetic MnAs/GaAs Heterostructures and MnAs/InAs Spin LEDs.
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The hybrid semiconductor-ferromagnet structure has attracted much interest for spintronics applications which rely on spin injection/ tunneling from a ferromagnet into a semiconductor. The room temperature ferromagnetic metal MnAs is a key material in this dissertation work. MnAs was epitaxally grown on GaAs(001) and InAs(100) substrates by molecular beam epitaxy. The self-organized stripe form of MnAs/GaAs is discussed in the first part of the dissertation. The magnetic stripe pattern on MnAs is characterized using magnetic force microscopy (MFM) and magneto-optical Kerr effect (MOKE). With the saturation field applied to MnAs along its easy axis, a magneto-elastic stress on the periodic stripe pattern is created. The magneto-elastic strain causes a change of hysteresis measured with diffraction MOKE, not seen in either magnetization or typical MOKE measurement. The second part is a spintronic device study. Spin light emitting diodes are fabricated with MnAs/InAs heterostructures. This work focuses on measuring the injection of spins into the narrow gap semiconductor indium arsenide (InAs) from a MnAs spin-aligner and to study spin carrier transport and recombination mechanisms in an InAs quantum well. The experiment directly measures the optical polarization and compares the results to a rate equation simulation. The analysis shows that the spin-LEDs gave a maximum optical polarization of 17% at 7K. The final part of the dissertation deals with a new ferromagnetic metal MnGa/GaAs. Most electro-optical spintronic devices favor a spin polarizer aligned to the out-of-plane direction of the sample plane (perpendicular to the film plane). This is related to the quantization geometry for the states in the quantum well. The magnetization easy axis of MnGa is out-of-plane. The properties of MnGa are characterized with topography, composition, magnetization and metallicity measurements. The formation of Mn 2 As at the MnGa/GaAs interface is observed by X-ray diffraction (XRD). Topographic domains are measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The ferromagnetic and electronic properties of the MnGa films are characterized using vibrating sample magnetometer (VSM) and scanning tunneling microscopy/ spectroscopy (STM/ STS), respectively.