Study of electric field close to a sample surface utilizing the electron standing wave excited in a vacuum gap of scanning tunneling microscopy

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By applying a voltage higher than a sample work function to a sample in scannging tunneling microscopy (STM), electrons field-emitted from an STM tip have positive kinetic energy near a sample surface. In this field emission regime, an electron standing wave (ESW) is excited in the vacuum gap under proper boundary conditions of a tunneling barrier and a potential near the sample. The eigenstates of ESW are determined by a potential near the sample surface. Thus the electric field near sample surface can be evaluated from the ESW excitation. The ESW can be detected from the differential conductance (dI/dV) versus the applied voltage curve : the peaks in the dI/dV curve correspond to the ESW excitation. We have obtained the dI/dV curves with ESW peaks for several samples : Au(111), Si(001)2x1, Si(111)7x7, Ge(001)2x1 and Si(001)2x1:H. To evaluate the field, there is a difficult problem that the tip shape regulates the electric field in the vacuum gap as a boundary condition. The thermal-field (T-F) treatment was applied for W tips to form a similar shape : the tip is heated under a high electric field resulting in expansion of {110} facets. The obtained dI/dV spectra were analyzed according to a model with a triangle potential, which has eigenvalues of the energy levels corresponding to the ESW spectra. It is concluded that the peak interval can be an index of the field evaluation.Furthermore, by raising the energy of the field emitted electron that irradiates sample surfaces, we have obtained electron energy loss spectra (EELS) and Auger electron spectra of backscattered electrons from semiconductors and metal surfaces with the same setup of field emission STM combined with an electron energy analyzer. This result exhibits the potential of elemental analysis on a nano scale with a combined instrument with the field emission STM.