PHOTOEMISSION FROM ALGAAS/GAAS SUPERLATTICES

We have performed photoemission experiments on AlGaAs/GaAs superlattices implanted close to a GaAs surface and on bulk GaAs. The samples are activated to negative electron affinity (NEA) by coadsorption of cesium and oxygen in ultrahigh vacuum. Each sample consists of a 50-period superlattice (52 angstrom/52 angstrom) covered by a GaAs cap of variable thickness d (d = 100 and 500 angstrom). In these samples, light absorption mainly occurs in the superlattice region where most of the electrons relax to the bottom of the first quantized miniband. The results demonstrate both thermalization to the first conduction miniband and photoemission from higher lying levels of the superlattice.