Impact of seed layer on material quality of epitaxial germanium on silicon deposited by low pressure chemical vapor deposition

The impact of the growth conditions of the germanium seed layer on the material quality of epitaxial germanium grown on (100) silicon by Low Pressure Chemical Vapor Deposition is studied. In order to obtain a smooth surface morphology, a thin Ge seed layer is grown at low temperature, followed by a thick Ge cap layer at high temperature. An optimal seed deposition condition of 335 deg. C and 4 kPa is identified. Seed layer growth at lower temperatures (e.g. 320 deg. C) leads to the formation of crystallographic defects, while growth above 350 deg. C produces unacceptable surface roughening associated with rapid Ge surface diffusion. Seed growth pressures above 6 kPa are found to lead to gas phase nucleation. A qualitative growth model for the Ge seed layer at 335 deg. C and 4 kPa is also described. It is demonstrated that a Ge seed layer thickness greater than 30 nm is required to obtain smooth Ge films. For seed layers at or below 30 nm thicknesses, the lowered thermal stability of this thin film produces severe islanding during the transition to the cap growth temperature (650 deg. C). In situ doping with boron above ∼1019 cm-3 in the seed layer enhances the seed growth rate and lowers the Ge/Si interfacial oxygen level. For in situ annealed 2 μm-thick Ge films deposited on this seed layer, a threading dislocation density of ∼2 x 107 cm-2 is achieved, along with a surface roughness of ∼1.6 nm