Phosphorus and carrier density of heavily n-type doped germanium

The threshold current density of n-type, tensile-strained Ge lasers strongly depends on the electron density. Although optical net gain analyses indicate that the optimum electron density should be on the order of 1 x 10(20) cm(-3) to get the lowest threshold, it is not a simple task to increase the electron density beyond the mid range of 10(19) cm(-3). The present paper analyzes the phenomenon where electron density is not proportional to phosphorus donor density, i.e., "saturation" phenomenon, by applying the so-called amphoteric defect model. The analyses indicate that the saturation phenomenon can be well explained by the charge compensation between the phosphorus donors (P+) and doubly negative charged Ge vacancies (V2-). Published by AIP Publishing.