Imperfection : holes and defects in silicon quantum dots

Abstract

Spurred on by the promise of a quantum “speed-up” for certain classes of computational problems, interest in quantum computation has seen a great surge over the past decades. One of the approaches is confining carriers in “quantum dots”, small regions in a semiconductor, such that quantum properties become observable. The first step in creating quantum dots in semiconductors is having exact control over the confinement potential. However, defects, coming about by disorder or other fabricational issues are of great influence on quantum dot formation. This work concerns the properties of hole quantum dots under the influence of, and the charge transport through, these defects. We thoroughly explore the silicon-based heterostructure of these devices and most prominently look at the Si/SiO2 interface and its associated defects. We explore the origin of these defects, and devise methods of eliminating them. We find that the ability to passivate defects at the Si/SiO2 interface hinges primarily on controlling the dewetting at higher temperatures. We introduce an ALD-grown Al2O3 layer, which strikes two birds with one stone. The layer both provides hydrogen for the annealing process and prevents dewetting. The introduction of Al2O3 introduces a layer of negative fixed charge, which can be eliminated or controlled by exposure to a UV-ozone oxidation process. We also find that by supplanting Al by Pd as the electrode material, the formation of an interfacial layer between the metal and the oxide is prevented We show that we can indeed make a hole quantum dot in intrinsic silicon, by demonstrating single-hole tunneling through a two-dimensional hole gas. Furthermore, by using Al2O3 grown by atomic-layer deposition in an annealing process, we passivate the majority of the electrically active defects. The most likely candidates for the observed defects are Pb centers, E′ centers, or unintentional quantum dots created by thermally induced strain. Using the Al2O3 oxide, and the passivating properties of the hydrogen contained therein, we are able to create intentional quantum dots of at least 180 nm. These quantum dots show many charge transitions, indicating the low level of disorder in the devices. We also study the g-factor anisotropy a hole quantum dot in silicon. We do this by studying the Zeeman splitting in a magnetic field capable of rotating 360∘ over all 3 degrees of freedom. Using two methods of fitting the data to our model, we extract similar anisotropies for the g-tensor (g∥*≈ 2.2, g⊥*≈ 4), indicating that the g-factor is roughly twice as strong out-of-plane than in-plane. This is consistent with the prediction that light-holes and heavy-holes are oriented preferentially in a 2D quantum well. A proof-of-principle of a single-layer depletion-mode hole quantum dot is demonstrated. The negative fixed charge in ALD-grown Al2O3 enables the operation in depletion-mode by inducing a 2D hole gas. Characterization of the charge-offset stability of this device indicates that the device is extremely stable, with the charge-offset stability having an upper bound at Q0 = 0.04e and a lower bound of Q0 = 0.005e. This compares favourably to previously known results for all-Si based devices (Q0 < 0.01e) and Al-based devices (Q0 < 0.15e).