SEMITIP, VERSION 5, Electrostatics and Tunnel Current Computations for a Probe Tip near a Semiconductor

R. M. Feenstra
Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213

This program computes the electrostatic potential and resulting tunnel current produced by a metallic probe tip near a semiconductor. This program is an updated version of the prior VERSION 4, VERSION 3, VERSION 2, and VERSION 1. Basic usage is described below, and additional details are available in the SEMITIP V5 Technical Documentation describing changes relative to VERSION 4. The major change in VERSION 5 relative to VERSION 4 is the added feature of self-consistency, important for situations of accumulation or inversion.

Some background information on the methods used in the program is contained in:

Self-consistency is achieved by computing both the charge densities and the tunnel current fully quantum mechanically, in the z-direction (normal to the surface), as described in Phys. Rev. B 80 075320 (2009). (In the lateral x- and y-directions, both the charge densities and the wavefunctions are semi-classical). For situations of semiconductor depletion, as commonly occurring in experiment, self-consistency occurs automatically in the computations (since the only charge densities are due to the background dopant atoms). However, for accumulation or inversion, self-consistency will in general produce a noticable effect on the tunnel current. See example 1 below for additional details.

A compiled version of the code, which should run on any Windows PC, is available in the file semitip_v5.exe. Input for the executable code comes from the file FORT.9. Download these two files, into filenames "semitip_v5.exe" and "fort.9". Then, run the code just by double clicking on it. Using a text editor, the input parameters in FORT.9 can be changed to whatever values are desired. In addition to the parameter values, this file also contains comments as to the meaning of each parameter.

Output from the program is contained in the following files (output depends on the value of the output parameter IWRIT as specified in the input file FORT.9):

All of the parameters in the program can be varied using the input file FORT.9, with the exception of the array sizes (NRDIM,NVDIM,NSDIM, and NEDIM), the specification of an auxiliary function other than a hemisphere which defines a protrusion on the end of the probe tip, and the specification of a surface state density other than a uniform or Gaussian shaped one. Modification of those parameters can be accomplished by changing the source code of the program. The source code is available, in the files:

All routines are written in Fortran. The source code can be downloaded directly from the above locations, and it can be compiled and linked on any platform. Sample input and output from the program is shown in the examples below.

Illustrative Examples of Running the Code

  1. n-type GaAs(110), with no surface states.
  2. n-type GaAs(110), viewing charge density images.
  3. n-type Si surface, plotting energies of inversion states.