Spin Injection, Transport, and Modulation in III-V Semiconductors.

Abstract

Spintronic devices, which aim to utilize the quantum mechanical spin properties of electrons, offer the potential of high-speed and low-power operation with performance possibly exceeding those of conventional charge-based devices. Spin-based devices may also provide a route for the integration and simultaneous operation of both memory and logic elements. Unfortunately, most spin devices studied today operate at cryogenic temperatures. For this technology to be successful, it is critical that the devices can operate at or near room temperature in the absence of an externally applied magnetic field. The main objective of this thesis was to experimentally demonstrate practical high temperature semiconductor-based spintronic devices. To achieve higher operating temperature of these devices, the channel length can be reduced to lower the average number of spin dephasing at the detecting end. Furthermore, the device must also exhibit a large magnetoresistance response at high temperatures. By changing the spin valve geometry to a vertically stacked MnAs/GaAs/MnAs structure instead of a conventional lateral geometry, an increase in magnetoresistance was achieved by a factor of 40 at T = 300 K. By adding a third gate terminal, the magnetoresistance of the vertical spin valve could be amplified by a factor of 500. Most spintronic devices require an externally applied magnetic field to manipulate its magnetoresistance, which may prohibit integration with conventional CMOS technology. We demonstrate and discuss the structure and physics of an InP-based three-terminal device that allows a systematic precession of spin by applying an electric-field across the channel. This prototype device shows that the magnetoresistance can be modulated by ~6% at T = 10K via spin-orbit coupling effects, and is one of the first experimental demonstrations of electrical spin precession. Finally, spin transport properties of a GaN nanowire grown by plasma-assisted MBE are studied. Because of its low spin-orbit coupling parameters, it is observed that the magnetoresistance of these nanowires are much larger compared to GaAs or InP based devices. Analysis of the nonlocal and Hanle precession data indicates a spin lifetime τsp ≈ 100 ps at T = 300 K, which corresponds to a spin diffusion length of λsf ≈ 270 nm.