Spin Transport and Nanomagnetism in Semiconductor Heterostructures

Semiconductor spintronics is widely regarded as a viable pathway to overcome many of the physical limitations of the present micro-electronics technology. Electrical spin injection into a semiconductor, coherent transport and manipulation of the injected spins in the semiconductor channel, and detection of the transported spins via another probe are indispensable ingredients of the proposed spin-based field effect transistors (spin-FETs), which may offer transformative new functionalities not present in a conventional FET. The primary goal of this dissertation is to study spin transport and understand the spin dynamics in a semiconductor. A most unique aspect of this research is the use a persistent photoconductor, Si-doped Al₀.₃.3Ga₀.₇As, as the spin transport channel material. Utilizing the persistent photoconductivity of Al₀.₃.3Ga₀.₇As:Si, we can tune the carrier density of the spin channel in the device in situ by photo-doping, spanning a broad range from the insulating to metallic state. This facilitates investigation of the spin accumulation and transport properties on one and the same sample over orders of magnitude variation in the carrier density in the spin channel. Exploitation of the persistent photoconductivity circumvents the issues that arise when using multiple samples; we significantly reduce fabrication time required to make many replicas at differing doping levels, and more importantly, avoid sample-to-sample variations influencing the measurements. 3-terminal (3T) and nonlocal 4-terminal (4T) Hanle measurements have been performed on our lateral devices with patterned Fe electrodes for spin injection and detection, and Al₀.₃.3Ga₀.₇As:Si as the spin transport channel. The measurements were conducted over a broad range of carrier densities across the insulator-to-metal transition (IMT). The bias current and channel carrier density dependencies of 3T and nonlocal 4T Hanle signals have been measured and systematically compared. Although their magnitudes differ by about an order of magnitude, the 3T and nonlocal 4T Hanle signals exhibit broad similarities in many aspects, including their dependencies on the carrier density and bias current. The specific resistance of the injecting junction plays a very important role in spin injection/extraction between a ferromagnet and semiconductor. Using the measured channel resistivity and junction specific resistance, the experimentally obtained Hanle signal magnitudes could be compared with the expectations based on spin accumulation in the theoretical framework of Fert and coworkers. In our devices with an interface of epitaxial graded Schottky junctions, the 3T Hanle magnitudes are in general agreement with the theoretical expectations. Specifically, the experimental values are smaller than the theoretical expectation in the insulating state, attain good agreement in the vicinity of the IMT, and become greater than the theoretical values by several factors deep into the metallic state. The observed discrepancies away from the IMT in our devices should be contrast with the results in spin devices with oxide tunnel barriers, where the 3T Hanle signals are often several orders of magnitude greater than the theoretical predictions. Furthermore, in our devices, the 3T Hanle signals are consistent with an exponential spatial decay of the spin accumulation based on the nonlocal 4T Hanle measurements. The spin lifetimes in the AlGaAs channel are determined from the 3T and nonlocal 4T Hanle curves, via separate fits to the Lorentzian function and the one dimensional spin drift-diffusion model. The two approaches yield similar values of spin lifetime, from 2 to 4 ns, and similar evolution with the carrier density. This comprehensive set of measurements covering large ranges of carrier density and bias current revealed broad and striking similarities between the 3T and nonlocal 4T Hanle signals in our devices. The results provide strong evidence that the 3T Hanle measurements in devices structures like ours with an epitaxial Schottky junction are indeed manifestation of spin accumulation, rather than due to other spurious effects. The research has shed considerable light on a number of pertinent issues in spin relaxation and spin transport in semiconductors. The second line of research of this dissertation is to utilize the high-sensitivity semiconductor Hall magnetometry techniques to study the static and dynamic magnetic properties of InAs quantum dots (QDs) doped with Cr or Mn. The goal of this research was to measure the magnetization of a small array of, or even an individual, QDs via an integrated micro/nano Hall magnetometer based on the 2-dimensional electron gas (2DEG) GaAs/AlGaAs heterostructure. The experiment was expected to facilitate a direct correlation of the measured magnetic properties of the QDs with their structural/chemical characteristics, which may provide insight on the fundamentally important issue of the origin of ferromagnetism in diluted magnetic semiconductors. A fabrication procedure was developed, which produced devices enabling Hall gradiometry measurements. However, due to the persistent difficulty of producing ohmic contacts to the 2DEG while limiting the sample temperature, the signal-to-noise ratio of the devices was not adequate for measuring the magnetization of the QDs.