Undoped AlGaAs/GaAs quantum dots with thermally robust quantum properties

Abstract

In a modulation-doped AlGaAs/GaAs heterostructure, electrons in the two-dimensional electron gas (2DEG) are provided by ionization of Si dopants in the AlGaAs layer. To reduce the effect of Coulomb scattering between ionized dopants and electrons in the 2DEG, an undoped AlGaAs spacer is grown between the doped AlGaAs and the undoped GaAs layers, which reduces large-angle scattering, thus increasing the mobility and electron mean free path. In a traditional semiclassical picture of an open quantum dot, if the electron mean free path exceeds the dot width, transport becomes ballistic and the corresponding magneto-conductance fluctuations (MCF) are considered as a Fourier sum of periods arising from all possible Aharonov-Bohm loops that intercept the quantum point contacts and are formed by scattering from the dot walls alone. As a result, these devices, known as semiconductor billiards; were seen as ideal for studies of dynamical chaos in the quantum mechanical limit. However, modulation-doped devices are not without their problems. For example, it has recently been demonstrated that small-angle disorder scattering causes unpredictable changes in the device’s electronic properties each time it is cooled for use. This finding forces a careful reconsideration of our notions of ballistic transport in these devices. Another problem associated with modulation-doped devices is the temporal instability due to rapid switching of the dopants between ionized and de-ionized states, hindering the development of ultrasensitive quantum devices.

This thesis reports the development of undoped quantum dots, where the ionized dopants are removed and the 2DEG is populated electrostatically by applying a positive bias to a degenerately doped cap. Our “induced” devices produce MCF that are reproducible with high fidelity after thermal cycling to 300 K. By performing a comparative analysis between nominally identical undoped and modulation-doped billiards, we conclude that small-angle scattering dominates transport in dots. Our work has important implications for studies of quantum chaos and ballistic transport. Additionally, measurements of our small undoped quantum dot operating in the Coulomb blockade regime showed features of excited state transport and spin-dependent transport blockade.