**2. Conclusion**

To adopt an adequate strategy to produce Ge epilayers for optoelectronic applications, first it is important to emphasize that the thermal-induced tensile train is limited to a value lying in the range of 0.25–0.30%, which is far from the expected value to get direct band gap Ge. Thus, it would be preferable to set up growth conditions, which favor n-doping. In other words, heavy n-doping appears to be the parameter that is more important than the value of the tensile strain. This implies that an adequate growth strategy would be to grow and to n-dope Ge films at low temperatures followed by post-growth thermal annealing. Since, Ge films, which are grown and n-doped at low temperatures, usually contain a high density of point defects (vacancies and interstitials). Therefore, the primary role of post thermal annealing is to restore the crystalline quality and to activate dopants into substitutional sites, without taking care of the value of tensile strain in the film. The efficiency of phosphorus doping may depend on two main parameters: the dopant solubility in a matrix and the sticking coefficient of dopants on the film surface. These two parameters are probably competing. The sticking coefficient of an atom or a molecule on a substrate surface increases with decreasing the substrate temperature. Since our results reveal that P doping is more favorable at low substrate temperatures, it appears that the sticking coefficient of the P2 molecules is the dominant parameter determining the phosphorus doping level in Ge.
