Band gap and strain engineering of pseudomorphic Ge1−x−ySixSny alloys on Ge and GaAs for photonic applications

Abstract

The authors report the compositional dependence of the direct and indirect band gaps of pseudomorphic Ge1−x−ySixSny alloys on Ge and GaAs with (001) surface orientation determined from deformation potential theory and spectroscopic ellipsometry measurements. The effects of alloying Ge with Si and Sn and the strain dependence of the band gaps at the Γ, Δ, and L conduction band minima are discussed. Deformation potential theory predicts an indirect to direct crossover in pseudomorphic Ge1−y−xSixSny alloys on Ge or GaAs only for very high Sn concentrations between 15% and 20%. No indirect to direct cross-over in pseudomorphic Ge1−ySny alloys (x = 0) on Ge or GaAs was found for practically approachable Sn compositions (y < 25%). The predictions for the compositional dependence of the E0, E1, and E1 + Δ1 band gaps were validated for pseudomorphic Ge1−ySny alloys on Ge using spectroscopic ellipsometry. The complex pseudodielectric functions of pseudomorphic Ge1−ySny alloys grown on Ge by molecular beam epitaxy were determined from Fourier transform infrared and ultraviolet-visible ellipsometry in the 0.1–6.6 eV energy range of Sn contents up to 11%, to investigate the compositional dependence of the band gaps. Critical point energies and related parameters were obtained by analyzing the second derivative spectra of the dielectric function of the Ge1−ySny epilayers. Sn composition, thickness, and strain of the Ge1−ySny epilayers on Ge were characterized by high resolution x-ray diffraction. The E0, E1, and E1 + Δ1 band gaps of pseudomorphic Ge1−ySny alloys on Ge obtained from ellipsometry are in good agreement with the theoretical predictions.