Photoconductivity of GeSn thin films with up to 15% Sn content

We have used temperature-dependent photoconductivity (PC) with different excitation wavelengths and intensities to study the photoexcited charge-carrier transport within GeSn/Ge/Si heterostructures. The evolution of the PC spectra with temperature was analyzed between 10 and 200 K. These strained GeSn films were grown with high enough Sn content such that the band gaps were direct. As such, the relationships between the band gaps and the temperature were determined using photoconductivity spectroscopy. As a result, an anomalous, linear blueshift of the PC spectral edge was found with increasing temperature. This was attributed to the variation of the GeSn band gap within the film, due to variations in strain and the increased contribution to PC from the region of highest Sn content. The change in photocurrent with excitation intensity and temperature demonstrates that conduction occurs predominantly through the GeSn and Ge layers under low optical pumping and through the Si substrate under high optical pumping. A phenomenological model of photoconductivity in GeSn as it depends on strain was proposed. This detailed understanding of the transport of photoexcited carriers along the GeSn layers is critical for developing GeSn/Ge-based optoelectronic devices.