Arrays of quantum-light-emitting diodes with site-controlled pyramidal quantum dots

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Chung, Tung-Hsun
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University College Cork
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Quantum information technology is an interdisciplinary subject, merging quantum mechanics and information science. In this field, the building blocks are quantum bits (qubits), which are superposition quantum states of simple two-level systems. Among all the platforms for the preparation of qubits, the polarization directions of single-photons are attractive as information carriers for practical realizations, due to reduced photon decoherence as well as the fact that they can be manipulated by convenient optical components. Moreover, the request for entangled sources from quantum communication and quantum computation can be satisfied by polarization-entangled photon emitters. In combination of their atomic-like energy structure and mature development in foundries/labs, epitaxial semiconductor quantum dots (QDs) have been exploited to deliver quantum light sources, such as indistinguishable single-photons and polarization-entangled photon pairs, by both optical and electrical injection (with triggering on demand). However, conventional self-assembled QDs nucleate randomly and possess finite values of finestructure splitting (FSS) in the excitonic states, mainly due to low crystal symmetry, which adds hurdles to entanglement reconstruction and significantly limits their scalability and potential for further integration. In this thesis, pyramidal QDs (PQDs) were grown by metalorganic vapor phase epitaxy (MOVPE), starting from site- controlled pyramidal recesses photolithographydefined on GaAs (111)B substrates. Their inherent properties enable them to be controlled spatially and provide them with high crystal symmetry, i.e., close-to zero FSS, suggesting an alternative for the solution of the abovementioned difficulties encountered by selfassembled QDs. However, the non-planar feature of PQDs makes it challenging to embed them into light-emitting diodes (LEDs). Here, we developed and designed a fabrication process which successfully overcame the configuration-induced processing complexity for the preparation of on demand single-photon and entangled-photon sources by electrical injection. xiv Indeed, the main achievement in this thesis is the fabrication of quantum LEDs with site-controlled PQDs which are able to generate single-photons and polarizationentangled photon pairs triggered on demand. In single-photon emission, the value of g(2) (0) could be reduced to 0.078 0.066 when combined with a time-gating technique under pulse exciation. On the other hand, the fidelities to the expected maximally polarizationentangled state were 0.85±0.04 under continuous excitation and 0.823±0.019 under pulse excitation by assistance of time-gating, with 75 % of the intensity maintained in a 1.5 ns window. The prepared entangled source was also importantly proved to violate Bell’s inequalities. Moreover we worked around finding solutions to some challenging issues concerning PQDs. The system in this study was an In0.25Ga0.75As QD sandwiched by GaAs barriers. One issue in our MOVPE-grown PQDs is a dominant negatively-charged environment. We explored a number of methods as detailed in the text which proved effective in suppressing at specific conditions the probability of capturing excess electrons, strongly improving the polarization-entangled photon pair emission via the biexcitonexciton decay process, and improving our sources. Also, a previous growth model on the AlGaAs/GaAs system developed in our group was expanded to the current system to understand the mechanism of indium segregation on both InGaAs V-grooved quantum wires and PQDs. The simulation successfully suggested consistent growth temperature-dependent emission energy evolution with the reported experimental results, in which an unexpected QD redshifting paired by a lateral quantum wires blueshifting with increasing growth temperature was observed. In addition, a new faceting at the pyramidal recess base during MOVPE growth was observed and reported for the first time. Altogether our results justify the PQD system as a promising platform to generate quantum light sources meeting a number of important requirements: e.g. spatial control, high fidelity, trigger on demand, and electrical injection.
Pyramidal quantum dots , Polarization-entangled photons , Single-photon sources , Quantum-light-emitting diode
Chung, T-H. 2016. Arrays of quantum-light-emitting diodes with site-controlled pyramidal quantum dots. PhD Thesis, University College Cork.
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