The impact of donors on recombination mechanisms in heavily doped Ge/Si layers

Heavy n-type doping has been proposed as a route to achieve positive optical gain in germanium layers since it is supposed to enhance the Γc carrier density. Nevertheless, the impact of donor impurities on the excess carrier lifetime in heavy doped epitaxial Ge/Si layers has not yet been addressed in the literature. To elucidate this point, we investigate the optical properties of heavily doped Ge layers selectively grown on silicon by means of photoluminescence (PL) experiments and theoretical modelling. A self-consistent multi-valley effective mass numerical model for simulation of PL spectra has been implemented, taking into account the influence of dopants on the non-radiative recombination dynamics. Upon comparing measurements and modelling, we find a linear increase in the defect-related Shockley-Read-Hall (SRH) recombination rate as a function of the donor density. The non-radiative lifetime decreases from ∼30 ns in intrinsic Ge/Si samples to ∼0.1 ns for a doping density in the 1019 cm-3 range. As a consequence, we find that SRH is the dominant non-radiative recombination process up to a donor density of ∼5 × 1019 cm-3. Despite this reduced lifetime, we observe an overall positive impact of doping on the radiative recombination rate for donor densities up to an "optimal" value of ∼3 × 1019 cm-3, with a ×7 intensity enhancement compared to the intrinsic case. A further increase in the donor concentration brings about a worsening of the optical emission.