Calculation of tunneling current across Trapezoidal potential barrier in a Scanning Tunneling Microscope

TL;DR

This paper presents a detailed quantum mechanical model for calculating tunneling currents in STM using non-WKB solutions, considering temperature effects, tip geometry, and bias voltage, to improve understanding of tunneling phenomena.

Contribution

It introduces a comprehensive method combining Airy function solutions and hyperboloid modeling to accurately compute STM tunneling currents across various conditions.

Findings

Calculated tunneling currents vary with bias voltage, tip distance, and curvature.

Temperature influences tunneling probability through Fermi factors.

Lateral resolution depends on bias voltage and tip radius.

Abstract

The Planar Model of the Electrode-Vacuum-Electrode configuration for STM in which electrode surfaces are assumed to be infinite parallel planes, with atomic size separation and vacuum between them, is used to calculate tunneling current densities for both low and high bias voltages. Non WKB, Airy function solutions for the Schr\"odinger Equation for the trapezoidal barrier in the tunneling region are used to calculate the tunneling probability. Temperature dependent Fermi Factors for each electrode are introduced and the calculation involves integration over the electron energies. In order to convert the current densities obtained in the planar model to tunneling currents the tip and sample surfaces are modelled as confocal hyperboloids, and the tip sample distance is replaced by the length of the line of force (field line). The current is found by integrating the current density over a finite area of the tip. The calculated tunnel currents for a few electrode pairs at room temperature are plotted for several values of bias voltage, tip sample distances, and tip radii of curvature. Pauli Effects are studied as a function of bias voltage and tip-sample distance. Some estimate of lateral resolution and its dependence on bias voltage and tip radius is also presented.