Surface-acoustic-wave-defined dynamic quantum dots

• Main Author: Astley, Michael Robert
• Published: University of Cambridge 2008
• Subjects: 530.412
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596199

The strain associated with a surface acoustic wave (SAW) propagating across a piezoelectric medium creates a travelling electric potential. Gallium Arsenide is such a piezoelectric material, and so SAWs can be used with existing semiconductor technologies for creating complex low-dimensional nanostructures. A SAW travelling along an empty quasi-one-dimensional channel creates a series of dynamic quantum dots which can transport electrons at the SAW velocity (∼ 2800 ms−1 ), allowing high-frequency operations to be carried out on the electron without the need for fast pulsed-gate techniques. Such dynamic quantum dot devices can provide valuable insights into fundamental physical phenomena and could have technological applications in quantum information processing. This thesis details investigations into SAW-defined dynamic quantum dot devices. Chapter 1 introduces the scientific background to the experiments described in this thesis; Chapter 2 provides details of the processing and measurement techniques used to perform these experiments. Chapter 3 consists of a study into the effect that reflections have on the acousto-electric current generated in a SAW channel. Reflections create a modulation to the channel entrance potential which is critical in determining the magnitude of the acousto-electric current. As the frequency of the SAW is varied, a particular reflection creates a periodic interference with the main SAW driving the current which can be observed in the Fourier transform of the acousto-electric current’s frequency dependence. The period of these oscillations is directly related to the distance which the reflection has travelled relative to the main SAW, which allows the principle reflection mechanisms to be characterised. Reflections persisted on a SAW device for large amounts of time, giving rise to much of the “noise” seen in the frequency dependence, and the pattern of reflections was found to be chaotic. Chapters 4-8 show the results obtained with a device where two SAW channels were linked by a tunnel barrier. This device allowed quantum mechanical tunnelling of electrons from the dynamic quantum dots to be observed over a subnanosecond timescale. Chapter 5 describes how the escape rates of the electrons from dynamic quantum dots can be measured using a rate equation analysis, and these rates are fit to a simple tunnelling model to derive the addition energies of the dynamic quantum dots. In Chapter 6 the tunnelling current was found to contain low-visibility oscillations, which cannot be explained by simple models. It is thought that these oscillations are caused by the non-adiabatic time-evolution of the electron wave function when the tunnel barrier is lowered suddenly. Chapter 7 shows how a crosstalk current through a short constriction is sensitive to local potential changes in an analogous manner to a quantum point contact, and how this effect can be used to detect the occupation of dynamic quantum dots in a nearby SAW channel. Chapter 8 collects some minor observations which have been made whilst studying the tunnel barrier device. In Chapter 9 I present the conclusions of the experiments presented in this thesis, and provide some ideas for future directions this work may take.