Quantum transport in low-dimensional Si/SiGe and AlGaN/GaN systems

Abstract

In recent years the study of electronic properties of low dimensional mesoscopic systems has attracted considerable interest. One of the reasons for this is the opportunity it gives of investigating a wide range of new effects related to ballistic transport and phase coherence. Another reason can be found in the possibility it gives of fabricating nanostructures both for microelectronics and for possible applications in quantum computing and spintronics in general. The object of this thesis is the investigation of quantum transport in Si/SiGe- and AlGaN/GaN -based Quantum Point Contacts (QPCs). In particular, we focus on ballistic transport effects going beyond the oneelectron Landauer picture expected for a system of non-interacting electrons. Si-based nanostructures are one of the most important material systems for applications in spintronics and quantum information due to the weak spinorbit coupling and to the presence of nuclear zero spin isotopes, which make electron spin coherence time extremely long. However, silicon has a near degeneracy of orbital states in the conduction band, arising from multiple valley minima, which can enhance decoherence rates and make qubit operation in quantum computing more complicated. It has been shown that quantum confinement in nanostructures provides some amount of control over the valley splitting [1]. In this work, we have investigated quantum transport properties of strongly-confined Shottky-gated constrictions, made starting from Si-based 2DEG and focusing on the conductance behaviour of nanostructures with various geometries. Measurements have been made as a function of the gate voltage, the source-drain bias and the magnetic field. Our results reveal a complex framework due to the occurrence of deviations from the ideal quantized conductance behaviour. For instance, these can be due to backscat1

tering from impurities or transmission resonances, produced by multiple reflections, for the presence of an abrupt geometry of the confining potential. However our findings have revealed a zero-field energy valley splitting in our etched-nanostructures, due to the strong confinement generated by physical etching of the 2DEG heterostructures. In practice, in different devices we found a valley splitting energy of the order of ∼ 1meV that is comparable to values reported in literature. In the past ten years, due to developments in the field of AlGaN/GaN heterostructures, research has focused also on GaN -based 2DEG. The latter is in fact among the most promising materials for the study of properties related to ballistic transport and it is interesting from a technological point of view. GaN -based 2DEGs are a novel system in which the large band offset and the strong piezoelectric effect in this material system have been shown to generate an intrinsic high sheet density two-dimensional electron gas, ns ∼ 1013 cm−2 in our sample, with enhanced electron mobility [2, 3]. In addition, the relatively heavy mass of electrons makes GaN 2DEGs a convenient system for studying spin-polarized and electron-electron correlation effects. The strong spontaneous and piezoelectric polarization charge gives these systems a strong asymmetric electric field at the interface, which can also enhance the spin-orbit interaction, thus providing a spin-splitting energy of the conduction band states at zero-external field [4]. In this thesis we focused on the study of the electrical properties of an AlGaN/GaN 2DEG-system, exploiting both classical and quantum Hall effect. In our investigation, new interesting problems came out from the analysis of both Shubnikov-de Haas and low-field measurements: the occupancy of a second energy level of the 2DEG, the occurrence of a zero-field spinsplitting due to spin-orbit interaction and the occurrence of the key-feature of weak antilocalization [5], namely positive magnetoresistance. Electron quantum transport of mesoscopic devices on GaN -based heterostructures was also investigated. For these systems we measured the conductance as a function of the gate voltage and the magnetic field. In addition, we investigated the effect of deliberately introducing an asymmetry in the confining potential. We have obtained an interesting and rich framework in which we speculate the possibility of a zero-field spin-polarization as being due to the effect of asymmetry of the confining potential and the presence of a spin-

2

orbit coupling [6].

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