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

Abstract

Spin angular momentum of electronic charge carriers is being explored currently for integration of non-volatile memory and processor in a single device. The main challenges in this research field are to integrate a spin polarized electron source to a semiconductor, and demonstrate an efficient spin injection, detection, transport, and manipulation mechanisms in such devices. At present, a large research effort is being dedicated to realize silicon based spintronic devices, because of its industrial dominance and expected long spin coherence length. Recently, it has been possible to inject and detect spin polarized electrons into silicon from a ferromagnet at room temperature. This has created possibilities to further investigate and understand the behavior of spin accumulation and depolarization in semiconductors with more details. This thesis aims to investigate the effect of the Schottky barrier, present at the interface, on spin injection and spin accumulation in silicon. Both electrical and spin transport measurements were performed on microfabricated Cobalt/SiO2 tunnel barrier/p-type silicon devices. 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 by changing the boron doping concentration in the silicon. These studies involve determining the carrier densities, mobility, diffusion coefficients, etc. in differently doped silicon substrates using Hall measurement technique, the Schottky barrier properties at the tunnel junction interfaces, as well as the change in the spin-signal with a magnetic field, temperature, and applied electrical bias voltage. With increasing the Schottky barrier width, a transition from direct spin polarized tunneling to an anomalous tunneling could be observed, giving rise to a change 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 and detection 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 extraction process with two step tunneling has also been proposed.