High mobility electron gases in Si/Si_0.77Ge_0.23 quantum wells at 1.7 K

摘要

Calculations have been carried out to investigate the factors which limit the low temperature, low field mobilities of two dimensional electron gases formed in the X2-valley quantum wells of tensile strained Si/Si_0.77Ge_0.23 modulation doped structures. The electronic charge density in the system has been solved in conjunction with Poisson's equation to derive a self consistent solution for the bound sheet charge density. Details of the self consistent ground state wavefunction are fed into a simple calculation to derive the low field drift mobility. Remote ionized donor impurities in the supply layer and roughness at the SiGe spacer/Si well interface are found to be the main sources of electron scattering in the case of high mobility gases grown on SiGe virtual substrates at 800 ℃. The comparatively poor electron mobilities observed for experimental samples produced at the lower growth temperature of 600 ℃ show an inverse square law dependence of mobility on sheet carrier density, the characteristic for roughness scattering.