Generation of Spin Polarization in Side-Gated Indium Arsenide Quantum Point Contact

This dissertation explores the use of side-gated semiconductor quantum point contacts (QPCs) to generate strongly spin polarized current by purely electrical means. An anomalous conductance plateau (at conductance value, G = 0.5 × (2e2/h)) was observed [P. Debray et al., Nature Nanotechnology 4, 759 (2009)] in an asymmetrically-biased, InAs quantum point contact with in-plane side gates, at 4.2 Kelvin in the ballistic transport regime. This was a clear experimental signature of spontaneous spin polarization in the narrow channel of the QPC. A non-equilibrium Green's function (NEGF) simulation revealed that three ingredients are necessary to create the spin polarization: an asymmetric lateral confinement, a lateral spin-orbit coupling (LSOC) induced by the lateral confining potential of the QPC, and a strong electron-electron interaction.;In order to use such QPC devices to make all-electric spin valves, it is necessary to probe the sensitivity of these three ingredients to the bias difference between the two side gates that confine the electrons in the narrow channel. Towards that goal, this thesis shows experimentally that the anomalous conductance plateaus are quite robust and remain over a wide range of asymmetric bias voltages. In addition, a very systematic study is conducted to understand the appearance of conductance anomalies which range from 0.4 to 0.7 × (2e2/h) depending on the biasing conditions. These results are interpreted as evidence for the sensitivity of the QPC spin polarization to the defects (surface roughness and impurity (dangling bonds) scattering) generated during the etching process that defines the QPC side walls or gates. This assertion is supported by NEGF simulations of the conductance of a QPC in presence of dangling bonds on its walls. We also show that a spin polarization over 90% can be achieved despite the presence of these defects. NEGF simulations show that the maximum spin polarization is not necessarily reached where the conductance of the channel is equal to 0.5 × (2e2/h). We propose developing QPC devices in series at elevated temperature as a mean to build an all-electric spin valve.