Spin Dynamics of Electrons Confined in Silicon Heterostructures

Abstract

The spin states of electrons confined in silicon heterostructures have shown promise as qubits for quantum information processing. Recently, a host of single and few electron silicon quantum dot device architectures have arisen as implementations for quantum computation. These devices often combine regions of low density two-dimensional (2D) electrons, localized electrons, and interfaces depleted of electrons. Electron spin resonance (ESR) is a unique tool for probing the spin dynamics of both mobile and localized electrons at silicon heterointerfaces and investigating the effects limiting the ability to control electrons and their spin states in these structures. We use a continuous wave ESR method to examine localized 2D electron band-tail states at Si/SiO2 interfaces in large area metal-oxide-semiconductor transistors. We compare two devices, fabricated in different laboratories, which display similar low temperature (4.2 K) peak mobilities. We find that one of the devices displays a smaller band-tail density of confined states and a shallower characteristic confinement. Thus, ESR reveals a difference in device quality, which is not apparent from mobility measurements, and is a valuable tool for evaluating the interface quality in Si/SiO2 heterostructures. Additionally, we use pulsed ESR techniques to study the spin dynamics of electrons confined in Si/SiGe heterostructures. For mobile 2D electrons, the density-dependent Dyakonov-Perel mechanism dominates spin relaxation. At low 2D densities, stronger electron-electron interactions cause an increase in the electron effective mass, leading to an increase in spin susceptibility. For very low densities, natural disorder localizes electrons at the silicon heterointerface. Naturally localized electrons in these structures display short spin relaxation times (< 0.1 ms). By electrostatically confining electrons to quantum dots, the spin relaxation time may be extended. We fabricate large-area dual-gated devices which confine electrons into ensembles of around 108 quantum dots in Si/SiGe heterostructures. By tailoring the device structure, a long electron spin relaxation time (T1 = 1.4 ms) is observed at 350 mK. Furthermore, a coherence time of 0.35 ms is measured. To our knowledge this is the longest T2 observed in a Si/SiGe quantum dot device.