Effect of subband splitting on Si inversion layers

Abstract

The subband structure of (100)Si inversion layers can be altered by either uniaxial stress or substrate bias due to deformation potential and quantizing surface field, respectively. The reduced splitting between E 0 and E 0 caused by compressive stress is shown to be exactly compensated by the additional surface field via substrate bias at high compressive stress. At low compressive stress (≲ 10 9 dyn cm −2) and low carrier concentration (< 10 12 cm 2) we observe a marked decrease of electron mobility at 4.2 K. The decrease in mobility can be recovered by suitable negative substrate bias. We attribute these results to the influence of the density of state tails of E 0 subband on the free electrons in the E 0 subband. At high stress, E 0 and E 0 cross. The measured E f crossing point from E 0 to mixed subband at various stresses and depletion charges agrees well with Takada and Ando's calculation. The results for [001] and [011] stresses indicate further splitting of the E 0 subband for the former. For (111) surfaces, the mobility increases with either eompressive or tensile stress in [011] direction. From the point of view of the influence of tail states of the higher subbands, this even function dependence of μ( P) seems to support 6-valley degeneracy in the absence of sttess.