Electrical spin injection into p-type silicon using SiO2-Cobalt Tunnel Devices: The Role of Schottky barrier

Sammanfattning: Spin angular momentum of electronic charge carriers is being explored currently forintegration of non-volatile memory and processor in a single device. The main challenges inthis research field are to integrate a spin polarized electron source to a semiconductor, anddemonstrate an efficient spin injection, detection, transport, and manipulation mechanisms insuch devices. At present, a large research effort is being dedicated to realize silicon basedspintronic devices, because of its industrial dominance and expected long spin coherencelength. Recently, it has been possible to inject and detect spin polarized electrons into siliconfrom a ferromagnet at room temperature. This has created possibilities to further investigateand understand the behavior of spin accumulation and depolarization in semiconductors withmore details.This thesis aims to investigate the effect of the Schottky barrier, present at the interface, onspin injection and spin accumulation in silicon. Both electrical and spin transportmeasurements were performed on microfabricated Cobalt/SiO2 tunnel barrier/p-type silicondevices. SiO2 tunnel barrier was grown by ozone oxidation that gives a much better interface because of the creation of very thin transition region, which makes it a robust method for high quality, extremely thin oxide layers. The Schottky barrier parameter has been tailored bychanging the boron doping concentration in the silicon. These studies involve determining thecarrier densities, mobility, diffusion coefficients, etc. in differently doped silicon substratesusing Hall measurement technique, the Schottky barrier properties at the tunnel junctioninterfaces, as well as the change in the spin-signal with a magnetic field, temperature, andapplied electrical bias voltage. With increasing the Schottky barrier width, a transition fromdirect spin polarized tunneling to an anomalous tunneling could be observed, giving rise to achange in the sign of spin accumulation. For the devices with larger Schottky barrier width,the effect of defects at the semiconductor/tunnel barrier interfaces on the spin injection anddetection mechanism were discussed. The Schottky barrier resistance is found to determine the spin transport behavior, which can be dominated by either direct tunneling or two-step tunneling. The role of local spin dipole formation at the interface during spin extractionprocess with two step tunneling has also been proposed.