We report scanning tunneling microscopy (STM) studies of Si substitutional donors (Sio,) in GaAs that reveal delocalized and localized electronic features corresponding to Sip, in the top few layers of the (110) cleavage surface. The delocalized features appear as protrusions a few nm in size, superimposed on the background lattice. These features are attributed to enhanced tunneling due to the local perturbation of the band bending by the Coulomb potential of subsurface SiG,. In contrast, STM images of surface SiG, show very localized electronic structures, in good agreement with a recent theoretical prediction [J. Wang et al. , Phys. Rev. 8 47, 10329 (1993)l. PACS numbers: 61.16.Ch, 68.35,0v, 71.55.Eq, 73.20.Hb Shallow impurities in semiconductors, such as Si substitutional donors (SiG, . ) in GaAs, are of scientific and

As the effective stress increases, low-permeability rock undergoes fairly small porosity changes, but significant decrease in the permeability. Empirical relationships based on laboratory-measured data, typically exponential or power laws, have been proposed to describe the stress-permeability, stress-porosity, and permeability-porosity relationships. However, these approximations yield poor fitting in low effective stress ranges, or unreasonable prediction for certain effective stresses. In this study, we develop a series of theoretical models for the essential relationships among the porosity, permeability and the effective stresses for lowpermeability sedimentary rock, based on the concept of Two-Part Hooke Model (TPHM). The TPHM conceptualizes an intact rock into a soft part and a hard part, which comply with the natural-strain-based and engineering-strain-based Hooke's law, respectively. The derived relationships are validated by the experimental data from the literature. The comparisons show that the theoretical predictions agree well with the experimental results. The soft-part, comprising of only a small portion of the rock body, is responsible for the significant permeability reduction in low stress levels. The high stress-sensitivity of permeability is mainly attributed to the micro-crack (soft-part) closure in the intact rock.

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