We have quantified the impact of the PH3 mass-flow and the H-2 annealing scheme on the structural and optical properties of thin and thick Ge:P layers grown at 400 degrees C on 200mmSi(001) substrates. A transition from smooth, "mirror-like" to rough, "milky" surfaces was evidenced above a F(PH3)/F(GeH4) mass-flow ratio of 10(-3). Above that threshold, Ge: P layers were of lesser crystalline and optical quality. High P+ ion concentrations (e.g., more than 2 x 10(19) cm(-3)) were obtained in slightly tensile-strained, rather smooth Ge: P layers when 750 degrees C, 60 s anneals were used. In contrast, short thermal cycling between 750 degrees C and 875 degrees C led to out-diffusion and thus to P atoms loss. A more than 9 increase of the roomtemperature photoluminescence, together with a redshift due to bandgap narrowing, was evidencedwhen switching fromintrinsic to heavily phosphorous-doped Ge layers. Such a behaviour is in line with the exaltation of fast, direct transitions between the G valley of the conduction band and the heavy and light holes sub-bands of the valence band when the indirect L valley is occupied by electrons (n-type doping) and is close in energy to the G valley because of tensile strain. (C) 2015 Elsevier B. V. All rights reserved.