In-situ surface, chemical, electrical characterization of the interfaces between ferromagnetic metals and compound semiconductors grown by molecular beam epitaxy

The growth of single crystal ferromagnetic metal/semiconductor heterostructures with atomically abrupt interfaces is desirable for use in spin-based transport devices where information is carried by the quantum spin-state of the electron rather than by its charge. Combining ferromagnetic metals with compound semiconductors is challenging due to the occurrence of solid state reactions at the interface during growth, which are believed to be detrimental to spin transport. Elemental ferromagnetic metals, such as Fe and Co, can be grown by molecular-beam epitaxy (MBE) as single crystal films on GaAs; however, they are not thermodynamically stable and reacted phases form at the interface. This research examines the structural, chemical, and electrical properties of Fe1-xCo x/GaAs interfaces as they form and determines under which growth conditions these interfacial reactions can be minimized.; The initial nucleation of Fe on GaAs surfaces is strongly influenced by the GaAs surface reconstruction, but results in little disruption of the reconstruction itself. Fe/GaAs reactions are reduced at lower MBE growth temperatures with a reacted layer thickness of approximately three monolayers at -15°C. A submonolayer coverage of arsenic acts as a surfactant and remains on the Fe surface during growth regardless of the growth conditions. Co is more reactive than Fe on GaAs and forms a reacted region composed of Co2GaAs, CoGa, and CoAs. ErAs can be used as an epitaxial diffusion barrier to minimize Fe-Ga-As and Co-Ga-As interfacial reactions for growth temperatures as high as 225°C.; Optical spin transport devices utilizing Fe1-xCo x contacts were fabricated and optimized to measure spin transport efficiencies. The signals measured by the devices depend critically on the formation of reaction products between the ferromagnetic metal contacts and the semiconductor. However, spin injection measurements show that in some instances, the interfacial reactions lead to the formation of interfacial layers with superior spin transport properties. These results are contrary to the commonly held belief that interfacial reactions are detrimental to spin injection. The sensitivity of spin-based devices to small changes in the interfacial structures of these ferromagnetic contacts may provide a new method for characterizing metal/semiconductor reactions.