Ordering of Epitaxial Semiconductor Nanostructures Using In Situ Pulsed Laser Interference Patterning

Low-dimensional semiconductor nanostructures have received enormous research attention by virtue of their unique electronic structure and have shown major potential for applications in nanoelectronics, nanophotonics, and optoelectronics. In particular, III-V semiconductor quantum dots (QDs), quantum dot molecules (QDMs) and quantum rings (QRs) are deemed to be promising building blocks for quantum information processing and communications. Self-assembly during epitaxial growth has enabled the production of these structures with high crystalline and optical quality. However, self-assembly also comes with stochastic nucleation and size inhomogeneity, which can limit their potential for device integration where precise positioning and nanostructures with predictable and ideally identical electronic properties are demanded. Site-controlled growth of nanostructures using ex situ lithographic techniques presents an attractive approach; nevertheless, this involves complex fabrication processes and the resulting properties of the structures have not, in general, matched those of random self-assembled nanostructures. This dissertation seeks to develop an innovative approach to laterally align high-quality epitaxial semiconductor nanostructures using an in situ patterning process based on the direct application of optical methods. In this work, an in situ technique combining nanosecond pulsed direct laser interference patterning (DLIP) with molecular beam epitaxy (MBE) growth is introduced, which offers a fast, high-efficiency route to realise the lateral ordering of semiconductor nanostructures. In the first part, the epitaxial growth and characterisation of Stranski-Krastanov (S-K) InAs QD and QDM arrays on GaAs substrates are investigated. The nanoisland arrays induced by single-pulse four-beam DLIP are observed to act as preferential nucleation sites for InAs QDs and result in a site occupancy dependent on the growth and interference parameters. The influences of both the DLIP conditions and the epitaxial growth parameters on the ordering of InAs/GaAs QDs are discussed. Precisely ordered arrays of single InAs QDs are fabricated for the first time using this in situ and non-invasive approach. The patterned QD arrays exhibit strong photoluminescence (PL) and a narrow full width at half maxima (FWHM), indicating good size uniformity and high optical quality. The second part of the dissertation explores the fabrication of ordered GaAs/AlGaAs QD and QR arrays using the droplet epitaxy (DE) approach combined with in situ DLIP. The DE approach has emerged as an attractive method to create lattice-matched self-assembled QDs with certain advantages compared to strain-driven nucleation processes. Regular arrays of Ga droplets are initially formed on nanoisland-templated AlGaAs surfaces, which are subsequently crystallised into GaAs crystals under an arsenic flux. By optimising the growth parameters, including the deposited Ga amount, the growth temperature, and the arsenic beam equivalent pressure, highly ordered arrays of single GaAs QDs and QRs can be obtained. High optical quality and excellent size homogeneity are attained according to the low-temperature PL spectra, in which a record-narrow PL emission FWHM of ~17 meV from patterned GaAs QD arrays is observed. In the final part of the dissertation, initial studies of the selective area growth (SAG) of GaAs droplets and nanocrystals on Si (100) & (111) substrates, and the growth and characterisation of type-II GaSb QDs on GaAs substrates employing in situ DLIP are demonstrated. These initial investigations show that DLIP is able to structure a silicon substrate to create Si nanoisland arrays. These islands can serve as preferential nucleation sites for Ga droplets, which can then be crystallised under arsenic exposure. Further deposition of GaAs results in the formation of periodic GaAs nanocrystals on the surface, with the size and site occupancy depending on the interference and growth parameters. The lateral ordering of S-K GaSb QDs on GaAs substrates has also been obtained, with the QD nucleation again subject to DLIP-induced nanoisland arrays. Low-temperature PL spectra of the patterned ordered arrays of GaSb QDs exhibit a comparably narrow FWHM of ~50 meV and reveal the characteristics of type-II band alignment.