SPA4 RHEED software
The SPA4-program allows in connection with the SPA-RHEED instrument and the hardware belonging to the SPA-RHEED a comfortable
electronic record of high resolution low energy electron diffraction patterns. The diffraction pattern of the surface is scanned by means of electrostatic deflection plates with a channeltron
detector. The six independent deflection voltages (double octopole of the gun and detector in x- and y-direction) are controlled by the SPA4-program with six 16Bit D/A converters. The electrons
detected in the channeltron are brought to TTL-level with a pre-amplifier and are then recorded with a counter. D/A-converter and counter are placed in the computer on an ISA-bus-extension module of
the BurrBrown Company. This module is connected to the hardware (power supply, preamplifier) by only three BNC-cables.
The SPA4-program allows different measuring modi:
- Optimisation of the electron gun adjustments and the resolution of the whole instrument. The parameter for these scans may be
modified quickly by using the function keys. At the same time a ratemeter is simulated. This modus allows no recording and storage of data.
- Recording of 1-dimensional cuts (1D-scan) of the diffraction patterns. Here all the parameters of the scan may be adjusted
independently. It is possible to use deflection voltages (in volt) or reciprocal lattice units . The scan may be displayed either in a linear or a logarithmic scale. The storage takes place in an
especially compact data format with high precision and dynamics. But the data may be also saved as an ASCII file for further consideration and evaluation with different software.
- Recording of two-dimensional diffraction pattern (2D-scans). All the parameters of the 2D-scans may be also adjusted
optionally as discussed in the paragraph above. The intensity is displayed on a grey-scale with 64 levels and again it is possible to choose between an either linear or logarithmic scale. For saving,
the data of every measuring point is compressed to 2 Byte. Additionally the 2D-scans may be converted to other file formats in use (Tiff, Byte, PostScript).
- On top of that it is possible to create complete measuring programs (macros) with any parameter of 1D-, 2D-scans, spot
measurements etc. for time dependent or time critical measurements (for example evaporate-, adsorption or reaction experiments). These programs are either repeated automatically or may be started by
hand if necessary. The parameters for the measuring programs (macros) may be modified even during measuring. On this occasion the data is saved automatically during measuring.
Some of the measuring modi discussed above show additional sub-menus. They allow a first evaluation and analysis of the data as
well as storage in different data formats and a print-out of the measuring data.
- the so called cursor menu permits a first evaluation and analysis of the measuring data of 1D- and 2D-scans:
1. Smoothing of spot profiles, which are
disturbed by high noise level. By using a special logarithm, the width of the spots is not deteriorated. 2. Spikes induced by static's may be cancelled. 3. Determination of spot width and position.
- a further sub-menu allows the saving and continuous loading of 1D- and 2D-scans by offering direct access to all directories
and sub-directories and all existing floppy disc drives.
- with the SPA4-program the direct output of diffraction patterns, spot profiles or intensity behaviour to a plotter, printer or
a file is possible. Several types of printers are supported. The best quality, however, is achieved by using a 600dpi PostScript printer (especially for half tone images).
This software has been developed for SPA-LEED measurements at the Institut für Festkörperphysik in Hannover. The adaptions for SPA-RHEED were made there by H. Frischat and at Focus.
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