James L. Truitt

Coulomb blockade in a silicon/silicon–germanium two-dimensional electron gas quantum dot

We report the fabrication and electrical characterization of a single electron transistor in a modulation doped silicon/silicon–germanium heterostructure. The quantum dot is fabricated by electron beam lithography and subsequent reactive ion etching. The dot potential and electron density are modified by laterally defined side gates in the plane of the dot. Low temperature measurements show Coulomb blockade with a single electron charging energy of 3.2 meV.

Si/SiGe Quantum Devices, Quantum Wells, and Electron-Spin Coherence

Silicon quantum devices have progressed rapidly over the past decade, driven by recent interest in spintronics and quantum computing. Spin coherence has emerged as a leading indicator of suitable devices for quantum applications. In particular, the technique of electron-spin resonance (ESR) has proven powerful and flexible for probing both the magnitude and the nature of spin scattering, when compared to theoretical predictions. Here, we provide a short review of silicon quantum devices, focusing on silicon/silicon-germanium quantum wells. Our review touches on the fabrication and lithography of devices including quantum dots, and the development of Schottky top gates, which have recently enabled the formation of few-electron quantum dots with integrated charge sensors. We discuss recent proposals for quantum-dot quantum computing, as well as spin- and valley-scattering effects, which may limit device performance. Recent ESR studies suggest that spin scattering in high-mobility Si/SiGe two-dimensional electron gases may be dominated by the D’yakonov and Perel’ mechanism arising from Bychkov–Rashba spin-orbit coupling. These results rely on theoretical predictions for the dependence of the coherence time T 2 * on the orientation of an external applied magnetic field. Here, we perform ESR experiments on a series of samples fabricated by different methods, including samples recently used to obtain few-electron quantum dots. While we observe some similarities with recent experiments, we find that for five out of six samples, the angular dependence of T 2 * was far larger than the theoretical predictions. We discuss possible causes for this discrepancy, but conclude that the theoretical understanding of these samples is not yet complete.

Probing a single quantum dot by pulsed and continuous microwave radiation

Using both picosecond millimeter-wave impulses and continuous microwave radiation, we probe the electronic structure and dynamic response of single-electron tunneling in structured quantum dots, essential for understanding their applications as computational elements, emitters and detectors. Under pulsed microwave radiation, the dots capacitance is greatly reduced compared to the equilibrium values. In contrast, the dots capacitance remains constant under continuous microwave radiation. This phenomenological reduction of capacitance stems from the faster pumping of electrons by pulsed microwave excitation as compared to the slow relaxation of electrons.