Thin-film InGaAs metamorphic buffer for telecom C-band quantum dots and optical resonators

Abstract

The advent of quantum cryptography applications holds the prospect of opening a new chapter of telecommunication. One vital building block for these technologies is access to efficient non-classical light sources. Recent breakthroughs have been reported in the effort of attaining high-quality single-photon emission from quantum dots inside the crucial telecom C-band. One attractive route in this regard is to apply additional strain-engineering to the established InAs-on-GaAs material system by inserting a metamorphic buffer. This approach has already demonstrated promising optical properties. However, an integration of these quantum dot emitters into an advanced photonic structure to enhance extraction efficiency and to utilize beneficial cavity effects is still missing. This thesis aims at establishing an InGaAs metamorphic buffer that facilitates compatibility with conventional photonic cavity structures as well as common lithography fabrication methods. For this purpose, a next-generation buffer design is proposed and discussed. Its non-linear, strain-optimized content grading enables maximum lattice transition at minimal thickness. This thin-film design is then realized via metal-organic vapour-phase epitaxy. Here, a comprehensive optimization of growth parameters is conducted to attain maximum crystalline quality. This process includes fine-tuning the quantum dot emission to 1550nm wavelength and an optimization for maximum brightness plus minimal fine-structure splitting. Furthermore, the completed layer stack is characterized structurally and design resiliencies within the buffer layer are explored. Markedly, a minimum feasible stable thickness of 170nm is found. Moreover, benchmark emission properties like single-photon purity, linewidth and decay time consistently exhibit favorable comparability to their traditional quantum dot system counterparts. Finally, the integration of the thinfilm metamorphic structure into various exemplary advanced photonic cavities is investigated. Critically, the feasibility of necessary design adaptations is examined, determining flexibilities and limitations. The presented results constitute a significant step towards the fabrication of high-quality single-photon sources inside the telecom C-band based on semiconductor quantum dots. Accordingly, the obtained progress will boost the real world implementation of emerging communication technologies based on non-classical light.