Abstract
Low-temperature epitaxial growth of high quality Si/ Si_<1-x>Ge_x /Si heterostructures at high Ge fractions on Si (100) is achieved by ultraclean low-pressure CVD using SiH_4 and GeH_4 gases. Lowering the Si_<1-x>Ge_x deposition temperatures and optimization of the Si_<1-x>Ge_x layer thickness are essential to prevent island growth and generation of misfit dislocations in the heterostructure at a higher Ge fraction. The SiH_4 and GeH_4 reaction rates are expressed by the Langrnuir-type equation, assuming that one SiH_4 or GeH_4 molecule is adsorbed at a single adsorption site, according to the Langmuir's adsorption isotherm, and decomposed there. It is found that the SiH_4 and GeH_4 adsorption rate constants become larger at the bond site of the Si-Ge pair than those at the others, while the SiH_4 surface reaction rate constant becomes the largest at the bond site of the Ge-Ge pair. With the B_2H_6 and PH_3 addition, changes of the deposition rate and the Ge fraction, and incorporation of B and P are also explained based on the modified Langmuir-type adsorption and reaction scheme associated with Henry's law, assuming that the high dopant partial pressure induces high coverage dopant adsorption which suppresses both the SiH_4 and GeH_4 adsorption/reaction on the surface.
