FREE-ELECTRON LASER INTERNAL PHOTOEMISSION MEASUREMENTS OF HETEROJUNCTION BAND DISCONTINUITIES

Internal photoemission (IPE) measurements of the conduction band disco ntinuities at GaAs/amorphous-Ge interfaces have been carried out at th e Vanderbilt Free-Electron Laser (FEL). IPE is a simple and direct mea surement capable of unprecedented accuracy. The FEL is uniquely equipp ed for semiconductor spectroscopy due to its brightness, tunability, a nd spectral range, 1-8 mum (0.15-1.2 eV). IPE uses FEL photons of suff icient energy to optically pump electrons over the conduction band dis continuity, producing a photocurrent. No complex modeling is required to extract the discontinuity from the experimental data, since it coin cides with photocurrent threshold energy. Both linear and Fowler curve fits of the threshold region agree within an accuracy of +/-5 meV. Th e IPE measurements are in agreement with previous conventional vacuum photoemission measurements that were limited to +/-100 meV accuracy. U nlike conventional vacuum photoemission, IPE does not have the require ment that interfaces must be grown and measured in situ. Since FEL-IPE utilizes subband pp photons, it can be applied to buried interfaces, as well as interfaces under bias. Because of its accuracy and simplici ty, systematic use of this technique over a broad range of semiconduct or interfaces promises to resolve long-standing disagreements between competing semiconductor band-lineup theories.