Low energy electron diffraction using an electronic delay-line detector

A low energy electron diffraction (LEED) instrument incorporating a delay line detector has been constructed to rapidly collect high-quality digital LEED images with low total electron exposures. The system uses a position-sensitive pulse-counting detector with high bias current microchannel plates. This delay-line detector combined with a femtoampere electron gun offers a wide range of flexibility, with electron dosing currents ranging from 0.15 pA to 0.3 fA. Using the highest current setting and collecting 1x106 counts per image, individual LEED images can be completed in 4 s with an acquisition rate of 250 kHz and a total electron exposure of 5x106 electrons. Under the latter conditions, images can be collected in 20 min with an acquisition rate of 1 kHz with a total electron exposure of 2x106 electrons. An angular width of 0.13 deg. at 108 eV is demonstrated, which means that domain sizes as large as 600 A can be resolved, depending on the surface quality of the crystal. The system electronics collect 2048x2048 pixel images with a spatial resolution of about 75 μm. The dynamic range of this system is 32 bits/pixel (limited only by physical memory). The construction of the detector results in a 'plus'-shaped artifact, which requires that, for a given sample orientation, two images be taken at a relative angle of 45 deg. Identical current-voltage curves from an MgO(111)1x1 H terminated sample, taken during several hours of exposure to the low current electron beam, demonstrate minimal electron induced H desorption