Compositional dependence of direct transition energies in Si$_x$Ge$_{1-x-y}$Sn$_y$ alloys lattice-matched to Ge/GaAs

TL;DR

This study investigates the direct transition energies and structural properties of Ge-rich SiGeSn alloys with varying compositions, demonstrating potential for 1 eV absorption edges suitable for optoelectronic applications.

Contribution

It provides detailed spectroscopic and structural analysis of SiGeSn alloys with low Si and Sn content, expanding understanding of their optical properties and lattice matching capabilities.

Findings

Achieved 1 eV direct absorption edge with low Si and Sn fractions

Confirmed good agreement between predicted and measured lattice constants

Observed smaller shifts in higher-energy critical points compared to Ge

Abstract

Si$_x$Ge$_{1-x-y}$Sn$_y$ ternary alloys are a candidate material system for use in solar cells and other optoelectronic devices. We report on the direct transition energies and structural properties of Ge-rich Si$_x$Ge$_{1-x-y}$Sn$_y$ alloys with six different compositions up to 10 % Si and 3 % Sn, lattice-matched to Ge or GaAs substrates. The direct transitions occurring between 0.9 and 5.0 eV were investigated using spectroscopic ellipsometry (SE), and the resulting data was used to obtain the dielectric functions of the Si$_x$Ge$_{1-x-y}$Sn$_y$n layer by fitting a multi-layer model. Values for the $E_0$, $E_1$, $\Delta_1$, $E_0'$ and $E_2$ transition energies were then found by differentiating these dielectric functions to extract the locations of critical points. Structurally, the composition of the samples was measured using energy-dispersive X-ray measurements (EDX). The lattice constants predicted from these compositions are in good agreement with reciprocal space maps obtained through X-ray diffraction (XRD). The results confirm that a 1 eV direct absorption edge can be achieved using relatively low Si and Sn fractions ($<$ 10 % and $<$ 3 % respectively), while the higher-energy critical points show smaller shifts relative to Ge and match results previously observed or predicted in the literature.