Optimization and emission wavelength control of In(Ga)As/GaAs quantum dots grown by molecular beam epitaxy

Abstract

The semiconductor quantum dot (QD) and its atom-like discrete energy level behavior provides improved and extended properties compared to the more traditional quantum well (QW) in its use in light-emitting devices in optoelectronics. QD research has evolved from classical light sources and absorbers to new types of non-classical, single-photon emitting light sources with applications in the next-generation of photonic integration. In this thesis the optimization process of the InAs/GaAs QD material system for applications as 1.3 um laser active media and single-photon emitters for quantum optics are presented in terms of epitaxial growth using molecular beam epitaxy (MBE). Upon presenting the fundamental theory of bulk semiconductor lattice and band structures, and their changes by quantum confinement in multiple dimensions, the actual epitaxial growth process of conventional layer-by-layer thin-film structures and growth mode shift to Stranski-Krastanov (SK) growth of QDs are explained. To provide deeper understanding of ultra-high vacuum growth conditions and requirements for high-quality layers with nanoscale precision, the components of a conventional MBE reactor are presented. Finally, the growth parameters that can be used to tailor the morphology and optoelectronic properties of semiconductor QDs, and the characterization methods implemented to study them, are discussed in terms of relevant applications. For the application of InAs QDs for single-photon emission, highly uniform QDs with a target density of 1-2E10 cm-2 and PL emission at 925-950 nm at 7 K are required, and their fabrication is well-established and repeatable in terms of MBE growth. In this thesis the main focus with this application was on the implementation of two-fold stacking of the QD layers, with an ultimate goal of achieving correlated growth of the QDs in a stack with as little alteration of QD properties as possible, so that when two single QDs are included in the same etched nanopillar, they can be coupled into the same cavity mode in a hybrid GaAs/Ag nanocavity. With the final growth parameters in the iteration, a 2-fold stacked QD sample was grown, with uniform properties of the two QD layers in terms of density and ground state energy, making it a successful initial test for future non-classical light source fabrication. For the application of laser active media beyond 1 um, the simultaneous requirement of large QD size, leading to a longer wavelength, and high enough QD density for laser operation becomes challenging due to the mutual interdependence of the growth parameters. The requirements for InAs/GaAs QDs lasing at 1.3 u m are high uniformity, a high QD density and PL emission at 1.24-1.25 um. With the best structure grown in the optimization iteration presented in this thesis, a peak photoluminescence wavelength of 1.22-1.23 um was achieved, but with a wide FWHM of 90-100 nm, which strongly appears to be caused by the presence of multiple QD size distributions which differ slightly in their ground state energies. The phenomenon is suspected to be a result of the yet unknown interaction between the InGaAs strain-reducing layer (SRL) and the QDs, and it is expected that in the next growth iteration the optimization of the SRL In composition and thickness could lead to minimization of the additional populations, and wavelength extension of the remaining narrow single population ground state peak to the target wavelength of 1.24-1.25 um.