Comparative Study of Carrier Lifetime Dependence on Dopant Concentration in Silicon and Germanium

The increasing use of thin germanium layers on silicon substrates as active layers in advanced electronic devices is the motivation for a comparative study of the dependence of the carrier recombination characteristics on excitation and dopant concentration in both semiconductors. Bulk and surface components of the carrier lifetime are separated by studying samples with different geometries and surface finishing. The bulk carrier lifetime is determined as a function of dopant concentration and in the case of germanium also as a function of the excitation level. The bulk lifetime observations are simulated by combining the lifetime according to Shockley-Read-Hall (SRH) theory with the lifetime resulting from Auger recombination above a doping concentration threshold. It is shown that the bulk carrier lifetime behavior in Si and Ge is very similar. Carrier lifetime changes in Si can be well described by a combined SRH and Auger recombination model, while lifetime variations in Ge fit well the SRH model, taking into account the lifetime dependence on the injection level. It is shown that the lifetime dependence on dopant concentration in germanium can be phenomenologically described by assuming a linear increase of recombination center concentration with dopant concentration. The results also illustrate the importance of surface passivation and of material quality which depends on the fabrication technology, when comparing carrier lifetimes obtained with different equipment and on different samples.