VALENCE-BAND STRUCTURE OF GEXSI1-X FOR HOLE TRANSPORT CALCULATION

We have calculated the hole densities of states and the velocities as functions of energy in strained and relaxed p-type GexSi1-x layers gro wn in (001) Si substrate. It is shown that the nonparabolic and nonsph erical effects are very large in the energy range of (0, 0.2 eV) measu red from the heavy hole band edge. Deeper into the valence band, the b ands gradually become parabolic and spherical. For most applications, the impurity doping concentration is below 10(20) cm(-3). For 10(20) c m(-3) p-type doped Si, the Fermi level is 77.3 meV at 77 K. It is ther efore concluded that the nonparabolic and nonspherical effects must be taken into proper consideration when investigating the transport prop erties of p-type GexSi1-x samples. The calculated data of both relaxed and strained GexSi1-x valence band structures are curve fitted and a data library is built up for further study of the hole transport prope rties. The mobility and the diffusion coefficient are largely affected when the doping concentration is increased. It is found that at high doping concentration the contributions from the light hole and spin sp ilt-off bands become very important, they can become even larger than the contribution from the heavy hole band, even their densities of sta tes are smaller than that of the heavy hole band.