Impact of defects on photoexcited carrier relaxation dynamics in GeSn thin films.

We report the results of a study that was conducted to investigate the recombination paths of photoexcited charge carriers in GeSn thin films. The charge carrier lifetime was predicted as a function of temperature from a description of photoconductivity transients, assuming co-influence of Shockley-Read-Hall and radiative carrier recombination paths. We identify that dislocations are the source of a band of electronic states with the highest occupied state at EV+(85÷90)meV that acts as Shockley-Read-Hall centers determining the charge carrier lifetime. The photoluminescence (PL) and photoconductivity spectroscopy have been applied to distinguish between the contribution of both band-to-band and dislocation-related electron transitions. The PL band was found to demonstrate a low-energy shift of about 80±20meV relative to the edge of the photoconductivity spectra in the indirect bandgap GeSn films with dislocations. The role of a different nature deeper acceptor level at EV+(140÷160)meV in the recombination processes of the GeSn layers with better structural quality and the Sn content higher than 4 % was discussed. This detailed understanding of the recombination processes is of critical importance for developing GeSn/Ge-based optoelectronic devices.