Tunneling escape time of electrons from the quasibound Stark localized states in ultrathin barrier GaAs/AlAs superlattices.

Abstract

Band-edge optical absorption spectra in two series of monoperiodic GaAs/AlAs superlattice diodes are investigated as a function of bias voltage by low-temperature photocurrent spectroscopy. In the superlattices the well width is fixed at 11 or 22 monolayers (ML) and only the ultrathin barrier thickness is varied between 2 and 6 ML. In the limit of high electric field conditions where the fundamental Stark ladder transition dominates, the linewidth of the heavy-hole excitonic peak is found to increase systematically with decreasing the barrier thickness. That is, the linewidth for the superlattice with thinner barriers is broader than that with the thick barrier superlattice, which is basically determined by the usual inhomogeneous broadening. This enhancement of the spectral linewidth by more than 10 meV is due to a lifetime broadening of the quasibound Stark localized states. This resonance energy broadening due to the rapid tunneling escape of electrons is used to determine the tunneling time by the uncertainty principle. The tunneling escape time thus determined as a function of barrier thickness shows good agreement with values deduced from the confined electron level broadening for the biased double-barrier quantum-well structures based on simple transfer matrix calculations.