| The tip-sample junction, formed by scanning tunneling microscopy, has been investigated theoretically and experimentally. The samples used In this experimental are (a) uniformly doped p-type or n-type Si (100), with resistivity over a range from 0.18 to 25Ω-cm, and (b) a p/p+, epitaxially grown, sample with an uniformly doped substrate (ρ = 0.01 Ω-cm) and an 8 μm epitaxial layer (ρ = 15 Ω-cm). The silicon surfaces are either hydrogen terminated or covered with a layer of ultra-thin oxide.; We have found a Pt-Ir/air/p-Si diode follows a tunnel-generation-impact ionization model in the reverse bias region. Since mechanically cut tips, used in this work, can have complicated geometries, geometric effects, such as extended gates and concentration of the electrical field must be considered. In the presence of illumination, p/p+ junctions can be delineated successfully by taking advantage of the generation process.; In the forward bias region, a parabolic current-voltage relationship has been derived when the tip Fermi level is higher than the silicon conduction band edge. The data herein are in agreement with this model. It has been demonstrated that spectral shifts arising from the tip-induced band bending on the lightly doped silicon can be eliminated by forming an accumulation layer in p-type silicon or an inversion layer in n-type silicon by using a Pt-Ir tip. Illumination is also required for n-type silicon in order to eliminate shifts associated with deep depletion caused by tunneling leakage currents. Using the approaches described herein, energy gaps of approximately 1.1 eV are determined for both p-type and n-type silicon.; Scanning tunneling spectroscopy, without shifts related to band bending, has been utilized to study tip-induced gap states in a hydrogen-terminated surface and the samples with a layer of ultra-thin oxide. The separation dependence of scanning tunneling spectroscopy reveals a reversible interaction between the tip and sample, and suggests the evolution of a continuum of tip-induced gap states.; For the tip-sample contact case, a multi-step-tunneling model, along with the tip geometrical effect, is proposed to describe the electron transport through the surface bamer region, in which dense traps are induced in the band gap by the high-applied stress. Using this model, the unique behavior of measured resistance vs. material resistivity for the nanoscale radius tip can be explained successfully. |
