Site-Controlled III-Nitride Quantum Dots

Abstract

Group III-nitride semiconductor quantum dots (QDs) exhibit large exciton binding energy (> 26 meV) and band offsets, making them an ideal candidate to exploit various quantum optical effects at the high temperature including single-photon emission, strong-coupling, indistinguishable photon generation and polariton lasing. These phenomena can lead to future quantum information technology. The practical use of the III-nitride QDs as quantum light sources requires the addressability of a single QD, both in its position and emission energy. To date, most semiconductors QDs are epitaxially grown by the self-assembled processes such as the Stranski-Krastanov growth which possess very limited control over the QDs’ positions and dimensions, making them difficult to be utilized at the device level.

In this thesis, we investigate novel processes for the fabrication of site- and dimension-controlled III-nitride QDs. Two lithography-based techniques have been considered including selective area epitaxy (SAE) and top-down etching. In SAE, the formation QDs is controlled by the pre-patterned mask openings. Different source supply and growth mechanisms determine QD’s growth morphology. Morphology evolution in SAE is studied experimentally which qualitatively agrees with the theoretical phase-field model. The non-uniformity of the InGaN thickness was found to be the origin of the broad photoluminescence (PL). In the top-down etching approach, InGaN QDs are formed by etching a patterned InGaN single quantum well. Each QD is disk-shaped and embedded in a nanopillar. Strong and distinct PL signal of a single quantum disk was observed even at room temperature. The emission was found to exhibit characteristics from a discrete energy state that is homogeneously broadened. The single InGaN QD was extensively studied using micro-PL. A model based on 2-dimensional Poisson’s equation was developed to quantitatively explain the large blue shift observed in the experiment. The saturation of the PL linewidth at high temperatures was also interpreted using a sidewall charge center model.

To demonstrate the scalability and device integration of the site-controlled III-nitride QDs, large-area nanolithographic processes and photonic-crystal optical cavities have been developed. Pattern shrinkage by spacer and by electrodeposition were introduced and demonstrated, with the former aiming at sub-10 nm patterning and the latter at large-scale nanofabrication.