CBED Pattern Simulations

TEM-CBED Pattern Simulations

EMsoft provides a pair of programs for the simulation of TEM Convergent Beam Electron Diffraction (CBED) patterns. The first program, EMLACBED, computes a so-called "master CBED pattern" for a particular zone axis and a series of thickness values. For each contributing reflection, a complete diffraction disk is stored with a given angular resolution and convergence angle. This latter value is typically chosen to be relatively large (i.e., 20-30 mrad or larger). Once the master CBED pattern has been computed, the second program, EMCBEDpattern [to be written as of 12/13/18], can be used to compute realistic CBED patterns in which the convergence angle can be set; one can also change the Laue center position within a range of half the initial convergence angle to mimic the effect of beam tilt. The sections below describe both programs in detail. The output file produced by the EMLACBED program can be visualized using the CBEDDisplay IDL program; look under the IDL Scripts tab for more information.

The EMLACBED program

The first CBED program uses the standard BetheParameters name list file as well as a regular input name list with the following parameters (use EMLACBED -t to obtain a copy of the name list template files):

 &LACBEDlist
! the name of the crystal input file MUST be specified
 xtalname = 'undefined',
! microscope accelerating voltage [kV]
 voltage = 200.,
! zone axis direction indices
 k = 0 0 1,
! foil normal direction indices
 fn = 0 0 1,
! minimum d-spacing to be taken into account [nm]
 dmin = 0.025,
! maximum HOLZ layer number for the output file 
! (this does not affect the number for the actual computations)
 maxHOLZ = 2,
! incident beam convergence angle [mrad]
 convergence = 25.0,
! output will be diffraction disks inside a square of area (2*npix+1)^2
 npix = 256,
! number of thicknes values to be used in the output
 numthick = 10,
! starting thickness value [nm]
 startthick = 10.0,
! thickness increment value [nm]
 thickinc = 10.0,
! smallest intensity for which a diffraction disk needs to be added to the output file
 minten = 1.0E-6,
! number of threads to be used for computation
 nthreads = 1,
! name of output HDF5 file (relative to EMdatapathname)
 outname = 'undefined'
 /

Most of the parameters in this input file are self-explanatory. The output file is in HDF5 format, and contains the intensities for each reflection for the entire illumination cone. In a future generation of this program, we will use full diffraction symmetry groups, so that only the unique information will be computed for a single member of each family of reflections.

Note that this program also requires that the Bethe potential parameters be set correctly. The default values for these parameters (in the BetheParameters.template file) are optimized for the EBSD modality in the SEM, so they are far too small for use with typical TEM accelerating voltages. The following are reasonable values for 80-300 kV:

 &Bethelist
! strong beam cutoff
 c1 = 20.0,
! weak beam cutoff
 c2 = 40.0,
! complete cutoff
 c3 = 500.0,
! double diffraction reflections: maximum excitation error to include [nm^{-1}]
 sgdbdiff = 1.00
 /