Electron and nuclear spin dynamics in n-GaAs

Abstract

The spin degree of freedom of charge carrier spins and the host’s nuclear spins in semiconductors are potential sources for the next generation of spintronics applications which motivate the deliberate investigation of the spin dynamics of well-controlled model systems like n-GaAs. Conduction electron spins are mobile in semiconductors and can be initialized, manipulated, and read out optically. Optical pumping with circularly polarized light can for example create a non-equilibrium electron spin polarization close to 100%. Nuclear spins are practically appealing as well due to their very long spin relaxation times. The mutual interaction between the electron and nuclear spin system is mediated via the hyperfine interaction. Indeed, through this interaction, a non-equilibrium spin polarization of electrons is transferred to the nuclear spins and results in dynamic nuclear polarization, which inter alia has an intricate dependence on the doping density. The main objectives of this thesis are measuring most accurately (i) the temperature dependence of the electron spin relaxation rate and (ii) the magnetic field, doping, and temperature dependence of the nuclear spin relaxation rate in a set of high quality n-GaAs samples from quasi-insulating over the metal-to-insulator transition up to the quasimetallic regime. The temperature dependence of the electron spin relaxation time is measured very accurately for three of the above-mentioned samples with the optical Hanle depolarization method. The measurements yield, in combination with a theoretical model, a quantitative insight into the efficiency of the different spin relaxation mechanisms. The longest electron spin relaxation time in n-GaAs results from an interplay of variable range hopping and hyperfine interaction for a doping concentration just below the Mott metal-to insulator transition at a finite temperature of ∼ 7 K. At higher doping densities the effect of these two mechanisms decreases such that they are negligible in the highest doped sample. For moderate and high temperatures, the description of the electron spin relaxation becomes unpretentious since the Dyakonov-Perel mechanism dominates over all other electron spin relaxation mechanisms. The Overhauser field from nuclear polarization intensifies or weakens the external magnetic field and affects the electron spin orientation. In order to measure the nuclear spin relaxation, a three-stage time-resolved detection of the Hanle effect is used. The method includes optical pumping and measuring the difference of the nuclear spin polarization before and after a dark (no laser light) interval of variable length. In this way, the nuclear spin system in the absence of excitation is investigated. The magnetic field dependence of the nuclear spin relaxation rate has a typical Lorentzian shape, showing the spin-spin interaction’s impact at lower magnetic fields. The strong field doping dependence of the nuclear spin relaxation rate can be explained quantitatively, considering the effective number of localized electrons over the entire density regime. Nuclear spin diffusion to the donor bound electrons increases the relaxation rate of the nuclear spin measured at 6:5 K and results in a distinct maximum at the metal-to-insulator transition. The rate in the very high doped sample increases due to the Korringa mechanism. The involved mechanisms explain the trend of the relaxation except for the very low doped sample. The temperature dependence of the lowest doped sample shows an electron spin relaxation channel affecting the nuclear spin relaxation, which is negligible at high doping densities.