SUBPIXEL DETECTION IN VIDEO RHEED IMAGE-ANALYSIS

A CCD camera and computer data acquisition system is used for surface investigation by means of the reflection high energy electron diffraction (RHEED) method. Accuracy of these measurements, especially of lattice parameter determination, is limited by camera spatial distortion, high noise, and diffraction-spot broadening. In order to resolve this problem a detection method of a virtual camera subpixel detector is proposed. The conversion function between a real camera and a perfect virtual pinhole model camera is established using a method of reference calibrate grid. The RHEED intensity diffraction patterns are approximated by analytical functions which fit the simple diffraction intensity profiles well. Then the centres of the diffraction lines and points are calculated with high (subpixel) precision. The accuracy of the method is demonstrated by analysis of RHEED diffraction patterns obtained from a Pd(111) surface. It is shown that the error of determination of the diffraction spot position is lower than 0.4%. The use of the method is shown for the example of Pd epitaxial film growth on a NaCl substrate where increased vertical lattice parameters are found.