Spin Transport in Silicon Nanowires with an Intrinsic Axial Doping Gradient

This dissertation is focused on electrical spin injection and detection at the nanoscale dimensions that semiconductor nanowires offer. Semiconductor spintronics is the natural extension of metallic spintronics for applications in semiconductor industry. After the tremendous impact of the giant magnetoresistance effect (GMR) in hard disk read heads, semiconductor spintronics has been thought as the key ingredient for the realization of spin field-effect transistors (Spin-FETs). The advantages of spintronic devices would include non-volatility, enhanced data processing speeds, decreased electric power consumption and facilitation of quantum computation. The primary goal of this research is to study spin dynamics and spin-polarized transport in semiconductor nanowire (NW) channels, specifically in phosphorus (P) doped silicon (Si) nanowires (NWs). The interest in one-dimensional (1D) nanoscopic devices is driven by the rich spin-dependent physics quantum confinement engenders, and the eventual miniaturization of the spintronic devices down to nanoscales. One of the most important aspects to achieve efficient spin injection from a ferromagnet (FM) into a semiconductor (SC) is the interface between the two materials. This study is focused primarily on this effect and how it can be tuned. In this work, we peform systematic spin transport measurements on a unique type of P-doped Si NWs which exhibit an inherent doping gradient along the axial direction. On a single NW, we place a series of FM electrodes, which form contacts that evolve from Ohmic-like to Schottky barriers of increasing heights and widths due to the pronounced doping gradient. This facilitates rigorous investigation of the dependence of the spin signal on the nature of the FM/SC interface. The advantage of using a single NW to study the afformentioned effects is that possible complications during the fabrication process are minimized compared to experiments that use multiple different devices to perform such experiments. 2-terminal (2T), nonlocal 4-terminal (4T) and 3-terminal (3T) spin valve (SV) measurements using different configurations of FM electrodes were performed on the Si NWs. In addition, 3T and nonlocal 4T Hanle measurements were performed. The collected data reveal distinct correlations between the spin signals and the injector and detector interfacial properties. These results were possible due to the unique inhomogeneous doping profile of our Si NWs. This study reveals a distinct correlation between the spin signals and the FM/Si NW injector interfacial properties. Specifically, we observe a decreasing injected current spin polarization due to diminishing contribution of the d-electrons, thus the necessary tunneling contact for efficient spin injection and its properties are being investigated and analyzed. The results demonstrate that there is an optimal window of interface resistance parameters for maximum injected current spin polarization. In addition, they suggest a new approach for maximizing the spin signals by making devices with asymmetric interfaces. To the best of our knowledge, this is the first report of electrical spin injection in SC channels with asymmetric interfaces.