Experimental study of 2D hole systems : coherent transport in quantum dots and magnetothermopower

Two-dimensional (2D) carrier systems built from semiconductor heterostructures have been at the center of a wide variety of experimental and theoretical research over the past decades. The quality improvement of GaAs/AlGaAs systems has allowed the observation of several peculiar ground states stabilized by the subtle interplay between carrier-carrier interaction, disorder and magnetic field. More recently, 2D systems in semiconductor heterostructures have also been used as a prime substrate for further confinement of the carriers to mesoscopic systems of major interest for the emerging fields of quantum computing and spintronics. This thesis addresses both magnetotransport measurements in hole open quantum dots (QDs) and thermopower studies of 2D holes in (311)A GaAs heterostructures. In the first part of this thesis, we describe the fabrication process for hole GaAs open QDs and investigate their magnetotransport properties at very low temperature T. Below 500 mK, the magnetoconductance of the open QDs exhibits clear signatures of coherent transport, namely magnetoconductance fluctuations and weak anti-localization. From these effects, we extract a T dependence for the dephasing time, together with an upper limit for the spin-orbit scattering time using the random matrix theory. Both the dephasing time and the spin-orbit scattering time are found to be much smaller than for electrons in similar systems. In the second part of this work, we report low-T thermopower measurements in the parallel magnetic field-induced metal-insulator transition (MIT) of 2D GaAs hole heterojunctions with different interface-dependent mobilities. When the magnetic field is increased, the diffusion thermopower decreases across the MIT. The reduction of the diffusion thermopower is more pronounced for the lower mobility sample where it reverses its sign. This behaviour indicates that the system does not undergo any ground state modification through the MIT but rather that the parallel magnetic field induces a dramatic change of the dominant hole scattering mechanisms. Finally, the last part of this thesis is devoted to the thermopower study of the insulating phase (IP) observed in 2D GaAs bilayer hole systems around the total Landau level filling factor n = 1. Our measurements show that the diffusion thermopower diverges with decreasing T in the IP. This divergence of the diffusion thermopower at low T indicates the opening of an energy gap in the system's ground state and suggests the formation of a pinned bilayer hole Wigner crystal around n = 1.(FSA 3) -- UCL, 200

Material, optical and electrical characterization of DC sputtered CuO by tuning oxygen concentration

To fabricate large area photodetection material with high sensitivity, low cost and easy processing for biomedical imaging and environmental applications, P type CuO layers with proper bandgap, high optical absorption and good electrical performance have been fabricated by DC reactive magnetron sputtering Considering that Copper oxide has three different phases CuO Cu 4 O 3 and Cu 2 O), it features the advantage of multiple bandgaps by tuning the oxygen concentration By XRD, Raman and SEM measurements, the phases of sputtered layers can be determined For the calculation of Bandgap and Urbach energy, UV VIS IR spectrophotometer has been used to measure the reflectance and transmittance of thin films Hall measurements have also been carried out to measure the carrier type, mobility, concentration and sheet resistance

Investigation and optimization of traps properties in Al2O3/SiO2 dielectric stacks

In this work, the properties of the interface traps between various Al2O3/SiO2 dielectric stacks and Si substrate are investigated by the conductance method. The trap state density and energy level distribution in the silicon bandgap are extracted for four stacks fabricated by different processes. Our results indicate that the trap state density can be optimized so that Al2O3/SiO2 can be advantageous for nano-scale transistors and resistive switching memory

Room-temperature DC-sputtered p-type CuO accumulation-mode thin-film transistors gated by HfO2

CuO grown by room-temperature direct current (DC) reactive magnetron sputtering is introduced to realize p-type thin-film transistors (TFTs), with a high-k HfO2 gate dielectric fabricated by atomic layer deposition (ALD). The devices work in accumulation mode (AM) with two apparent threshold voltages corresponding to the formation of buried channel and accumulation layer, respectively. CuO AM TFT with a channel length of 25 μm exhibits competitive on-off ratio (Ion/Ioff) of 1.3x10², subthreshold swing (SS) of 1.04 V dec-1, and field-effect mobility (µFE) of 1.1x10-3 cm2 V-1 s-1 at room temperature. By measuring a CuO metal oxide semiconductor (MOS) capacitor at room temperature, a high acceptor doping density (NA) of ~5x1017 cm-3, a high positive effective fixed surface charge density (Qf) of ~91012 cm-2 and a low interfacial trap charge density (Dit) of ~6x1010 eV-1 cm-2 at the HfO2/CuO interface are estimated. The µFE extracted from the accumulation regime appears lower than the Hall mobility measured for a similarly processed CuO layer on glass due to the increased hole concentration in CuO TFT, compared to a Hall concentration of ~1014 cm-3, following MOS process. SS appears limited by the decreased channel to gate capacitance (Ccg) related to the buried channel in AM TFTs, parasitic capacitance to ground and potentially very high interfacial traps at the non-passivated CuO/air interface

Compressing air costs - leakage

SIGLEGBUnited Kingdo