Defect Characterization and Control in Metastable GeSn Optoelectronic Alloy Nanostructures

Nontechnical DescriptionAn important area of application for semiconductor materials is in optoelectronic devices; for example, in devices such as lasers or light-emitting-diodes (LED’s) that use electrons to stimulate the emission of light. There is great interest, both scientifically and technologically, in semiconductors that can emit light with wavelengths somewhat longer than visible light, i.e. in the mid-infrared part of the spectrum. Such light sources, when fabricated on silicon chips, could become key components in future ubiquitous chemical sensor networks, in speeding up data transfer between and on silicon chips, and in motion sensors required by autonomous vehicles. This research project focuses on a semiconductor material system, germanium-tin, that holds great promise for mid-infrared light emission on silicon chips. The efficiency of light emission by germanium-tin is limited by the presence of atomic scale defects that grow into the material when it is synthesized. This project characterizes the nature and number of such defects, and investigates methods for annihilating or altering them to minimize their effects on germanium-tin. Undergraduates are involved in these research activities, with special efforts made to recruit highly competitive undergraduate researchers from groups that are under-represented in the US science and engineering workforce. The project includes a partnership with Stanford’s RISE outreach program, to inspire high school students to consider further education and careers in STEM fields.Technical DescriptionExhibiting a direct bandgap at sufficiently large (x ~ 10 atomic %) tin composition, Ge(1-x)Sn(x) alloys hold great promise for mid-infrared (IR) light emitters and absorbers, while also being monolithically compatible with silicon electronic and photonic technologies. Previous research on germanium-tin epitaxial films grown on silicon has demonstrated mid-IR optically-pumped lasing, and there has been a gradual trend of increasing Sn content to access longer wavelength operation. The light emission characteristics of Ge(1-x)Sn(x) are still far from optimal. Low growth temperatures (< 300°C) used to promote high Sn content alloys cause large concentrations of acceptor-type vacancy defects to form. Strong pairing of Sn atoms with these vacancies is predicted theoretically and will result in enhanced non-radiative carrier recombination, reducing the efficiency of light emission and absorption. This project uses strain-engineered core-shell nanowire structures as a platform to study post-growth annealing to dissociate Sn-vacancy pairs and to annihilate vacancies incorporated in the Ge(1-x)Sn(x) shells during their growth. Shells of thickness up to 500 nm are of particular interest, to achieve wire structures capable of efficiently guiding mid-IR light. Synchrotron diffuse x-ray scattering is used to characterize trends in the relative concentration of vacancies bound to Sn atoms, divacancies, clusters and monovacancies in the alloys versus annealing time and temperature. A key goal is to understand the rates and mechanisms governing the approach to vacancy equilibrium in these alloys. Extended x-ray absorption fine structure analysis provides an additional probe of local bonding around Sn atoms and the stability of Sn-vacancy pairs. The project also examines atomic fluorine as a chemical vacancy passivant, building on prior experience with F passivation of Si surface states and vacancies in Ge. Coupling between x-ray and optoelectronic characterization of the core-shell wires can reveal fundamental insights into the connection between point defects and device-relevant properties. Temperature-dependent photoluminescence, photoconductivity and ultra-fast pump-probe measurements are used to probe Ge(1-x)Sn(x) band structure and the effects of different vacancy defect populations on carrier recombination dynamics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.