Surface Gated Quantum Dots in Shallow GaAs-AlGaAs Heterostructures

Borsosfoldi, Zoltan (1998) Surface Gated Quantum Dots in Shallow GaAs-AlGaAs Heterostructures. PhD thesis, University of Glasgow.

Abstract

In this thesis we demonstrate that surface gated quantum dots working in the high temperature regime (between 1.2 K and 7.7 K), can be reproducibly fabricated using the combination of state of the art electron beam lithography and shallow GaAs-AlGaAs heterostructures. Surface gated quantum dots with 4 electrodes and with nominal dot sizes between 100-350 nm were fabricated and characterised. We present the theory of electron transport in heterostructures with emphasis being put on electron transport in shallow GaAs-AlGaAs heterostructures where the two dimensional electron gas (2DEG) is 28 nm below the surface of the semiconductor. The theory of electron transport in point contacts (PCs) and in quantum dots is discussed in detail. We present the quantum dot design considerations for achieving device operation in the high temperature regime, design details, sample fabrication and sample handling procedures. The experimental setup for measuring our surface gated quantum dots and data acquisition technique is presented. Temperature dependence measurements between 1.2 K-9.6 K are presented for quantum dots with different nominal sizes. We discuss an improved measurement setup designed for electron transport measurements in quantum dots in the low conductance regime. We show in this work that theoretical calculations and detailed modelling can assist in the design of quantum dots for operation in the high temperature regime. We describe the optimisation of our quantum dots, which led to a fabrication and measurement of very small dots containing only a few electrons. At the time when our research work was carried out, the surface gated quantum dots detailed in this work showed the highest operating temperature reported in literature for devices of this type. The benefits of quantum dots operating at high temperatures are discussed. We show the existence of electron-electron interaction effects in these quantum dots at 1.2 K normally observed in surface gated quantum dots in the millikelvin temperature range. The effect of a magnetic field applied perpendicularly to the plane of our quantum dot in the few electron regime is reported and discussed. We show that surface gated quantum dots containing only a few electrons optimised for high temperature performance can be used for electron spectroscopy measurements and can yield interesting effects. We discuss the intrinsic drawbacks and difficulties associated with the operation of small surface gated quantum dots fabricated on shallow GaAs- AlGaAs heterostructures. The effects of the semiconductor surface and material imperfections on our quantum dot performance is also shown. Finally we discuss the prospect for using shallow GaAs-AlGaAs heterostructures in quantum dot devices and some possible future research directions in the field of surface gated quantum dots.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Andrew Long
• Keywords: Condensed matter physics, Quantum physics
• Date of Award: 1998
• Depositing User: Enlighten Team
• Unique ID: glathesis:1998-75403
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 20:14
• Last Modified: 19 Nov 2019 20:14
• URI: https://theses.gla.ac.uk/id/eprint/75403