SEMITIP, VERSION 2, Electrostatic 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 resulting from a metallic probe tip near a semiconductor, as described in the preprint here, submitted to Phys. Rev. B. This program is an updated version of the prior VERSION 1, described here. Documentation for VERSION 2 is available here, describing changes relative to VERSION 1.

A compiled version of the code, which should run on any Windows PC, is available in the file semitip_v2.exe. Input for the executable code comes from the file FORT.9. Download these two files, into filenames "semitip_v2.exe" and "fort.9". Depending on your browser this download can probably most easily be done by RIGHT CLICKING on the files when you see them listed (i.e. after clicking on the above links), and then clicking on SAVE TARGET AS and saving them into the directory of your choice using the ALL FILES filetype. 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.

TIP TO ENABLE SHORTER RUN TIMES: The finite-difference method used for the solution is an iterative one, with convergence parameters set in FORT.9, lines 23-25, as described in the documentation for VERSION 1. To minimize your run times it may convenient to initially use relaxed parameters, e.g., use IPMAX=2 on line 23, or limit the number of iterations on line 24, or increase the convergence parameters on line 25. Once you've zeroed in on the appropriate values for surface state density, tip radius, contact potential, etc. relevant to your problem, you can then tighten up the convergence to obtain more accurate results.

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

• FORT.10 - gives the numerical results for the following quantities:
  • tip radius of curvature
  • tip-sample separation
  • tip potential
  • opening angle of tip shank
  • PHI0 - the surface potential at a point directly opposite the tip apex
  • ratio of PHI0 to PHI00 where PHI00 is a 1-D value for PHI0
• FORT.11 - provides the potential along the central axis, as a function of z-distance (output for IWRIT>0)
• FORT.12 - provides the potential along the surface, as a function of the radial distance from the central axis (output for IWRIT>0)
• FORT.13 - gives the entire array of potential values (output for IWRIT>2)
• FORT.16 - gives an exact copy of the output to the console
• FORT.20, FORT.21, ... - contour lines of the potential (output for IWRIT>1)

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

• SEMITIP_V2.F - main routine.
• SEMITIP2.F - performs the detailed finite element solution of Poisson's equation.
• SEMIRHO.F - auxiliary routines for computing semiconductor charge densities.
• SURFRHO.F - auxiliary routines for handling surface charge densities.
• CONTR.F - plots contour lines of the computed potential.