Theoretical basis for study of subsurface nanostructures by scanning tunneling microscopy

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So far, most studies using scanning tunneling microscopy (STM) have paid attention to the outermost layers of surfaces, and there were small number of works studying subsurface structures using STM.In this project I performed basic and theoretical investigations for the study of subsurface structures using STM.Main results obtained are as follows.1. I studied theoretically ballistic electron emission microscopy (BEEM) and clarified the factors determining the width of electron beams in BEEM.I found a new effect of transverse resonant tunneling. I developed a new method for constructing Bloch states by scattering calculations, and found important factors for the atomic-scale variation in BEEM.2. I took account of the effect of inelastic scattering by using a unitary and non-perturbative method, and investigated how deep in surfaces STM works. It was found that with the depth of subsurface interfaces, the conductance in an STM system approaches the value calculated on the boundary condition of Bloch states due to an averaging effect. The inelastic mean free path defined in the present study coincides with that derived from the relaxation time in the weak-scattering regime. In the strong-scattering regime, perturbation fails and the mean free path does not necessarily shorten with increase of the scattering strength.3. I calculated the surface-state conduction in flat and island surfaces with single and double STM tips. It was found that in the presence of surface states the conductance of flat surfaces does not much decrease even if the potential in the outermost layer of surfaces changes. The conductance of the double-tip STM is inversely proportional to the distance between double tips.As future development of this project, the theoretical study on the nano-scale surface conduction using multiple STM tips is promising.