| dc.description.abstract | This thesis presents a set of experiments conducted on GaAs/AlGaAs heterostructures fabricated into Hall bars and a lateral quantum dot device. The Hall bar experiments seek to further the understanding of the complexity of the GaAs valence band of a two-dimensional hole gas (2DHG). Meanwhile, the quantum dot experiments aim to improve the performance of an RF-QPC readout employed for charge detection. These experiments are significant for the development of future quantum technologies that utilize spins in semiconductors, in fields such as spintronics and quantum computing.
Firstly, we study Hall bar devices formed in a 2DHG of a dopant-free GaAs/AlGaAs heterostructure. The hole sheet density is adjustable via a metal top-gate that induces and confines holes to the GaAs/AlGaAs interface. We employ magnetotransport measurements to assess both the device quality and the underlying physics of the valence band. By measuring several devices, we identify fabrication parameters that lead to parasitic parallel conduction and gate hysteresis. In our devices, featuring an asymmetric quantum well hosting a 2DHG, Rashba spin-orbit interaction (SOI) leads to a beating pattern in the Shubnikov-de Haas (SdH) oscillations of the two subbands of the heavy-hole (HH) valence band. We employ Fourier analysis to isolate the frequencies present in the SdH oscillations. The effective masses of the two spin-orbit split HH subbands are extracted via the temperature dependence of the isolated oscillations, without invoking assumptions regarding the band structure. This analysis is performed for different values of hole density and ranges of magnetic field. The HH splitting induced by Rashba SOI is evaluated and compared to modulation-doped samples in literature. Moreover, we extract the out-of-plane g factor from thermal-activation measurements in the quantum Hall regime.
Secondly, we investigate non-linear magnetotransport in a Hall bar device, also formed in a 2DHG of a dopant-free GaAs/AlGaAs heterostructure. We observe the SdH phase inversion phenomenon and explore the empirical relationship between the hole temperature and applied dc current through the Hall bar. In the quantum Hall regime, comprehensive maps of the differential resistances as a function of current and magnetic field (B) are generated to show the evolution of quantum Hall breakdown reaching filling factor ν = 1. By adjusting the top-gate voltage, the hole sheet density is incremented to reveal how the quantum Hall breakdown characteristics evolve with hole density. A Zener-type tunneling model is employed to describe the size of the transport diamonds in current. We examine the magnetic field dependence of the critical current compiled for different values of hole density. The zero current anomaly (ZCA) phenomenon is observed and discussed. Hysteresis observed in the quantum Hall breakdown regime of a two-dimensional electron gas (2DEG) has been linked with dynamic nuclear polarization (DNP) through the interplay of electron and nuclear spins via the hyperfine interaction. Motivated by examining the strength of the hyperfine coupling between hole and nuclear spins, we investigate hysteresis near breakdown in our 2DHG.
Finally, we present measurements on a lateral double quantum dot (DQD) device formed in a 2DEG of a modulation-doped GaAs/AlGaAs heterostructure. These devices demonstrate potential as a platform for advancing quantum information technologies, particularly when employed as single spin qubits. The fast, single-shot readout of spin states is crucial, and using spin-to-charge conversion techniques, only the charge state readout becomes essential. Thus, we design and setup an optimized radio frequency quantum point contact (RF-QPC) charge sensor for the quantum dot device. Charge sensitivity measurements are conducted using a standard QPC sensor, followed by the RF-QPC. Additionally, we characterize a superconducting quantum interference device (SQUID) amplifier for integration into the RF-QPC readout. We successfully demonstrate a carrier nulling technique, specifically to accommodate the SQUID amplifier's limited dynamic range, and enable its integration into the RF-QPC readout. After Incorporating the SQUID amplifier into the RF-QPC readout, critical issues are identified and future work to address them is discussed. | en |
