Strain-engineering the Bi incorporation and the optical properties of GaAsBi alloys

The development of new semiconductor materials with dilute Bismuth has aroused great interest among researchers in the recent years. The GaAsBi alloy exhibits a band gap reduction of up to 90 meV/%Bi, a strong enhancement of spin-orbit splitting and an almost temperature-insensitive band gap, which are attractive properties for infrared lasers, photodetectors and terahertz optoelectronic applications. Bi content ranging from 6% to 11% in bulk GaAsBi epilayers covers the most important telecommunication bands (1.3µm to 1.55µm). Moreover, recent theoretical calculations predicted a topological insulator (TI) phase in Bi-doped III-V semiconductor at concentrations higher than 19%, which would launch the III-V semiconductors as a new TI platform for integrated spintronics applications. However the e↵ective use of GaAsBi alloys is still limited, due to the difficulty in growing a high-quality material with well-controlled properties. The large miscibility gap between GaAs and GaBi, and the strong tendency of Bi towards surface segregation requires non-standard growth conditions, that induce the simultaneous incorporation of defects that a↵ect negatively the optical properties. Another important issue is how and to what extent the compressive strain induced by the GaAs substrate influences the incorporation of Bi and the physical properties of the GaAsBi film. The most used approach in literature to obtain Bi incorporation during epitaxial growth of GaAsBi is to act on the kinetic parameters of growth, such as flux ratios, growth rate and temperature. In this work, we applied an innovative strategy, never explored in literature, in order to improve Bi incorporation into the alloy. Our growth approach is the strain-engineering of the pseudomorphic growth of GaAsBi on GaAs(001) substrates by means of suitable intermediate InGaAs bu↵er layers acting as stressors. All samples studied were grown by Molecular Beam Epitaxy with a set of growth param eters (substrate temperature, As/Ga flux ratio, growth rate and di↵erent Bi fluxes) that we optimized for the growth of GaAsBi films directly on GaAs under compressive strain. Next, we demonstrated that such growth conditions can be successfully transferred for the growth of GaAsBi under tensile strain on relaxed InGaAs bu↵er layers. Moreover, we were able to obtain the first GaAsBi film ever grown in quasi-matched condition with a Bi content as high as 7.2%. In this thesis, we present a complete morphological, structural and optical characterization of GaAsBi films under both compressive and tensile strain conditions. AFM measurements showed that, at higher Bi concentration, segregation and formation of Bi clusters cannot be avoided on the surface of compressive samples, while they are always absent for tensile samples. Symmetric !-2✓ X-ray di↵raction and Reciprocal Space Maps allowed us to determine the Bi content and the strain conditions of GaAsBi films. Optical characterization revealed a drastic dependence on the strain in Photoluminescence: given the same amount of Bi concentration, tensile strain gives rise to a much larger redshift of the energy emission compared to that of compressive samples. This result demonstrates that PL emission can be controlled not only by varying the Bi concentration, but also by varying the strain conditions, thus adding a new parameter for tailoring the optical properties of this material. Furthermore, another important and surprising result is the increase of PL signal intensity for GaAsBi samples grown under tensile strain with respect to compressive ones, which preserve the optical properties for Bi concentration as high as 7.2%. Optical characterization is quite encouraging about the probability of obtaining GaAsBi alloys with Bi content in the range 10-20%, and with reliable optical properties. Such result would also open at the chance to search for the predicted non-trivial topological phase in this material.