Low Energy Electron Diffraction During Pulsed Laser Annealing: A Time Resolved Surface Structural Study
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Low Energy Electron Diffraction During Pulsed Laser Annealing: A Time Resolved Surface Structural Study
R. S. Becker, G. S. Higashi, and J. A. Golovchenko AT&T Bell Laboratories, Murray Hill, New Jersey 07974
ABSTRACT
Nanosecond structural changes in a crystal lattice during pulsed laser annealing have been measured using time-resolved Low Energy Electron Diffraction (LEED). LEED is both structure and surface (- 10 A) sensitive. Lattice temperatures can be extracted from Debye-Waller like extinction coefficients. Combining these with nanosecond time resolution provides a surface probe for short-time dynamical processes. The technique is used to observe the time evolution of a Ge(l 11) surface during pulsed laser annealing. The results demonstrate rapid formation of a liquid layer and subsequent surface recrystallization and cooling.
In recent years the short lived ( 10i-3 to 10-6 seconds ) states of condensed matter that arise during fast pulsed laser annealing have been of increasing interest to researchers. Both the processes involved in the transformation and the short-lived states themselves have been of fundamental concern. Methods which have been heretofore brought to bear on this problem such as optical transmission and reflectivity [1-4], X-ray diffraction [5,61, Raman scattering [7-9], time-of-flight mass spectrometry [10], high-energy electron diffraction [ 11], stroboscopic electron microscopy [121, and second harmonic generation [13] have not been able to unambiguously answer the question of what, if any, surface atomic structural transformations occur. Optical transmission and reflectivity probe the state of the conduction electrons, second harmonic generation is symmetry sensitive but not surface sensitive, X-ray diffraction is structure sensitive but not surface sensitive, time-of-flight mass spectrometry does not resolve temporal processes, and Raman scattering has calibration problems. Some interesting experiments in picosecond high-energy ( - 20 keV ) electron transmission diffraction using a modified streak camera have been reported but are not surface sensitive and have not yet demonstrated repeatable before, during, and after states. There have been stroboscopic scanning electron microscope experiments performed at picosecond time scales, but these, while yielding temporal information, again, fail to provide structural information on the surface regime. It would appear that a structural probe well suited to the requirements of surface sensitivity and atomic structural sensitivity would be low-energy electron diffraction (LEED). With electron energies < 200eV, the probe beam penetrates only some three to five atomic layers. For an initial experiment using this new technique we describe an apparatus designed to study LEED on a nanosecond time scale on crystal surfaces during pulsed laser irradiation. We observe the (00) diffraction spot on (111) germanium and show how its intensity is extinguished under conditions associated with laser annealing. We further show how this intensity may be asso
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