<title>Semiconductor spectroscopy and ablation processes with the Vanderbilt FEL</title>

Abstract

This paper describes the results of novel experiments made possible by the recently commissioned Vanderbilt Free-Electron Laser (VU-FEL_, the brightest tunable midinfrared FEL in the world. We emphasize two classes of experiments, novel semiconductor spectroscopies and wavelength-dependent laser ablation studies. Both take advantage of the high brightness and the tunability of the FEL. We have recently demonstrated the feasibility of measuring semiconductor heterojunction band discontinuities using the internal photoemission (IPE) technique and report the results for the cases of GaAs/GaAlAs and a-Ge/GaAs. The basic physical mechanism of IPE is that a photocurrent is produced by optically pumping electrons over the conduction band discontinuity (Delta) E<SUB>c</SUB>. A photocurrent threshold is observed when the photon energy exceeds (Delta) E<SUB>c</SUB>. Because IPE is optical in nature, (Delta) E<SUB>c</SUB> can be determined with unprecedented accuracy (5 meV). By comparison, the best known direct method of measuring band discontinuities (UPS or XPS) achieves accuracies of only 100 meV.