Structural and electronic properties of in-situ phosphorous-doped Ge layers grown by reduced pressure-chemical vapour deposition

• In-situ phosphorous doping with PH3 of Ge thick layers grown at 400 °C, 100 Torr.
• Too high PH3 flows yield lesser structural and electronic quality Ge:P layers.
• 750 °C, 1 min H2 bakes instead of short thermal cyclings to be used after growth.
• Room temperature photoluminescence intensity: factor of 9 increase in Ge:P layers.
• Band gap narrowing evidenced for heavily in-situ phosphorous-doped Ge layers.

We have quantified the impact of the PH3 mass-flow and the H2 annealing scheme on the structural and optical properties of thin and thick Ge:P layers grown at 400 °C on 200 mm Si(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 × 1019 cm− 3) were obtained in slightly tensile-strained, rather smooth Ge:P layers when 750 °C, 60 s anneals were used. In contrast, short thermal cycling between 750 °C and 875 °C led to out-diffusion and thus to P atoms loss. A more than 9 increase of the room temperature photoluminescence, together with a redshift due to bandgap narrowing, was evidenced when switching from intrinsic to heavily phosphorous-doped Ge layers. Such a behaviour is in line with the exaltation of fast, direct transitions between the Γ 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 Γ valley because of tensile strain.