THE COMBINED EFFECTS OF STRAIN AND HEAVY DOPING ON THE INDIRECT BAND-GAP OF SI AND GEXSI1-X ALLOYS

Both boron and phosphorus cause shrinkage in the lattice constant of Si. Therefore heavily doped Si:B and Si:P epitaxial layers grown on intrinsic Si substrates are strained provided no misfit dislocations are present at the interface. The strain is a biaxial extension and has a hydrostatic component as well as a shear component. Ge(x)Si1-x layers matched to Si or Ge substrates are also strained. The strain causes band splitting which in its turn causes narrowing of the fundamental indirect band gap of the layers. The hydrostatic component also causes band gap shrinkage. In addition the strain modifies and increases the band gap narrowing due to heavy doping effects as compared with the unstrained case. In this paper the narrowing of the fundamental indirect band gap due to the combined effects of strain and heavy doping is calculated for Si and Ge(x)Si1-x epilayers. The results are used to explain the photoluminescence of heavily doped Si:B strained layers grown by MBE on intrinsic Si substrate. It is shown that the band gap narrowing due to heavy doping in strained Ge(x)Si1-x layers must be taken into account in the design of double heterostructure bipolar transistors with Ge(x)Si1-x base layers.