Epitaxial growth of spatially and spectrally controlled quantum dots and quantum dot molecules

Abstract

Epitaxially grown InAs quantum dots (QDs) have long been researched as a promising basis for qubits, and there have been numerous quantum operations proof-of-concept studies performed. However, the random nucleation of these QDs as well as inhomogeneity in their composition, size, and shape has made it difficult to fabricate arrays of these nanostructures for scalable device integration. Other quantum device technologies also suffer from these same types of materials issues. In this research, a molecular beam epitaxy (MBE) grown materials platform for site-controlled InAs QDs in combination with quantum dot molecules (QDMs) for built in spectral tunability is introduced. ☐ This research contains four main parts that are combined for the completed scalable, materials platform. First, nanofabricated substrates for epitaxial growth are designed and developed. Nanofabrication processes that are gentler on surfaces are utilized to help maintain an epitaxial surface for growth. There are two steps for the nanofabrication pattern: lithographic alignment markers for device patterning after growth, and a nanohole array for InAs QD nucleation during growth. Next, the substrates are prepared for epitaxial growth by undergoing various oxide cleaning and desorption steps. Wet chemical methods prior to loading into the MBE chamber are investigated. After the substrates are loaded into the MBE, thermal, atomic hydrogen, and gallium-assisted oxide desorption methods are investigated. Third, various parameters such as substrate temperature, arsenic overpressure, indium flux, and growth interrupts are investigated to obtain single, site-controlled InAs QDs in the patterned locations. Up to 89% single occupancy of QDs is found for a 10 µm nanohole spacing with the chosen growth conditions. This incredibly low density spacing is vital for implementation into photonic crystal cavity devices on the back end, which are about 5 µm wide structures. These spatially located QDs are then extended upward using a strain-based growth to grow a column of QDs, which creates a buffer layer away from defects at the growth interface while maintaining the spatial register. An optically active, QDM for integration into devices is grown at the top of the column. Finally, the growth and optical quality of the QDs are investigated.