Electroluminescence in the Classical and Quantum Regime in Undoped GaAs/AlGaAs Heterostructures

Abstract

Quantum information processing holds the promise to radically change the way we perform computations and transmit information. In the realm of quantum computing, there has been enormous progress in the last few decades in a huge variety of quantum systems and it is unclear which platform will be the leading system to execute quantum computations. Conversely, photons have always remained the front-runner for the long distance transfer of quantum information since photons travel at the speed of light and have limited mechanisms of decoherence (as compared to other carriers of quantum information) when traveling over long distances. The method used to generate single photons remains the pertinent open question.

Current state-of-the-art single-photon sources (SPSs) are optically-active quantum dots driven by an external laser source. For laboratory-scale experiments, they have proven fruitful in order to demonstrate key components of a quantum network, as well as performing fundamental tests on the nature of quantum mechanics. However, one challenge associated with these optically active quantum dots is two-qubit interactions since the quantum dots are usually spatially isolated. Conversely, two-qubit interactions for spin qubits in gate-defined quantum dots is routinely achieved via the Heisenberg exchange interaction. Thus, it would be highly desirable to have a way to convert the quantum information of the spin state of gate-defined quantum dots to photon polarization. Furthermore, for the prospects scaling of the technology, it would be highly desirable for this quantum information transfer to be all-electrical in order to leverage conventional multiplexing techniques.

In the first part of this thesis, we outline our proposal for an all-electrical SPS where single-photon emission is driven by electroluminescence (EL) at the single-charge to single-photon level. In order to control carriers at the single-charge level, we propose using non-adiabatic single-electron pumps (SEPs) previously investigated as quantized current sources for metrology. We have also previously developed a lateral p—n junction whose geometry allows direct integration with a SEP. We compare our proposed SPS to existing electrically-driven SPS in the literature, highlighting anticipated strengths of our proposed device, including a fabrication process compatible with standard semiconductor fabrication techniques.

Given the key role SEPs play in our proposed SPS, we describe the established theory underpinning the high fidelity operation of SEPs. We also highlight practical considerations for the operation of SEPs, including device fabrication challenges faced during the course of this research, and demonstrate how to measure and characterize a SEP.

A secondary focus of this thesis has been investigating EL from lateral p—n junctions in regimes where there was no attempt to control carriers at the single-charge level. While measuring lateral p—n junctions, we noticed an unconventional form of EL that did not require a forward bias to be applied. By swapping the polarity of the top gate voltage of our ambipolar induced devices, existing carriers recombine radiatively with incoming carriers of the opposite charge. Due to the flow of carriers in and out of the device, we called this form of luminescence the tidal effect. We develop a model to explain the non-monotonic frequency-dependent EL intensity and perform temperature-dependent measurements to identify the species responsible for the observed EL.

We also further investigate a similar phenomenon when two adjacent top gates are periodically swapped with a phase difference between the two signals. We demonstrate that this form of EL is more efficient over larger areas than the tidal effect, and therefore may be more suitable for general illumination purposes.

Lastly, we also performed the first EL measurements from lateral p—n junctions in single heterojunction interfaces. Despite the lack of a bottom barrier in these devices, our measurements suggest that carrier recombination is occurring near the interface. We characterize the EL spectra and observed the so-called H-band, a type of space-indirect exciton created in proximity to a populated single heterojunction interface, which has only previously been observed in photoluminescence experiments. Time-resolved EL experiments suggest reduced dimensionality of neutral excitons. We show that the lifetime of the H-band can be tuned electrically. We also demonstrate that the tidal effect can also be observed in these single heterojunction interfaces.