Electrostatic exciton trap in a thin semiconductor membrane for optical coupling to a GaAs spin qubit

Descamps, Thomas Daniel Marian; Bluhm, Jörg (Thesis advisor); Dunin-Borkowska, Beata Ewa (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

Interfacing stationary matter qubits with flying photonic qubits is of major interest in quantum computing and quantum communication technology as it is a necessary requirement to create a distributed quantum computer or a quantum network. Assuming a spin-based stationary qubit, Gallium-Arsenide (GaAs) spin qubit in gate-defined quantum dots (GDQD) would be a promising candidate for a spin-photon interface as the direct bandgap of GaAs allows interaction with light. However, as the confinement in GDQD results from electrostatic potentials, either holes or electrons can be confined but not jointly. Since the spin of the photo-electron is entangled with the spin of the photo-hole after photon absorption, the loss of the latter would lead to decoherence of the former. To provide a coherent transfer of information between the spin qubit and the photonic qubit, one solution is to modify the conventional heterostructure and gate architecture to embed an exciton trap as an intermediary. In this thesis, we implement the exciton trap by including a quantum well (QW) in the heterostructure and by patterning two local metal gates vertically aligned on both sides of the heterostructure. The QW provides the confinement along the growth direction while an electric field applied between the opposite metal gates confines the exciton in-plane by the quantum-confined Stark effect. Patterning these gates on the two sides of the 220 nm heterostructure involves a demanding fabrication process as the substrate has to be removed. To characterize the devices, a dilution refrigerator was customized to enable spin qubit and photoluminescence measurements at millikelvin temperatures. As a first approach to accumulate the two-dimensional electron gas (2DEG) in the QW needed to form GDQD, double-side doping of the barriers was considered. Hall measurements showed that a high mobility electron gas (above 1e6 cm2/(Vs)) can still be obtained on a thinned heterostructure. Quantum point contacts and single GDQD can also be formed. The thin doped heterostructure showed the photoluminescence expected from a QW filled with electrons populating the lowest sub-band of the 2DEG. The formation of an exciton trap by the quantum-confined Stark effect with top and bottom local gates is also possible provided depletion of the 2DEG underneath the trapping gates. Although the doped heterostructure demonstrated the basic prerequisites to create the aforementioned spin-photon interface, the undesired optical excitations of the DX centers added to the poor material quality pushed for the development of a second approach based on dopant-free heterostructures. On undoped heterostructure, the photoluminescence properties of exciton traps with a global grown back-gate were first investigated to reproduce already-known behaviors. From this foundation, the uncommon exciton traps consisting of local metal gates vertically aligned on the top and bottom side of the wafer were characterized. The behavior was close to the expectation for some samples but others showed significant discrepancies probably originating from a photocurrent or an undesirable exciton dissociation in the trap. To accumulate a 2DEG in the QW, the first option relied on the field effect created by a global back-gate underneath the channel and the ohmic contacts. Hall measurements were performed on heterostructure with and without GaAs substrate to characterize the accumulated 2DEG which had a mobility above 1e5 cm2/(Vs). The strong variability in the quality of the ohmic contacts added to the thick barrier between the channel and the back-gate pushed for a second option. Based on the field effect created by a top-gate and the local modulation doping under the ohmic contacts, preliminary results on substrate also demonstrated a 2DEG mobility above 1e5 cm2/(Vs), but the lack of reproducibility added to an undesirable leakage current advocate for further developments.