Strong enhancement of direct transition photoluminescence with highly tensile-strained Ge grown by molecular beam epitaxy

Abstract

Highly tensile-strained layers of Ge were grown via molecular beam epitaxy using step-graded ${\text{In}}_x{\text{Ga}}_{1-x}\text{As}$ buffer layers on (100) GaAs. These layers have biaxial tensile-strain of up to 2.33%, have surface roughness of $<1.1\text{nm}$ , and are of high quality as seen with transmission electron microscopy. Low-temperature photoluminescence (PL) suggests the existence of direct-bandgap Ge when the strain is greater than 1.7%, and we see a greater than 100× increase in the PL intensity of the direct transition with 2.33% tensile-strain over the unstrained case. These results show promise for the use of tensile-strained Ge in optoelectronics monolithically integrated on Si.