Saturated Miniband Transport in Semiconductor Superlattices

The conduction bands of semiconductor superlattices allow us to study electron dynamics in regimes which are not easily accessible in naturally occurring materials. We have examined several aspects of electron transport in the limit where the electron energy spans the band-width of allowed states of a super-lattice miniband. In particular, we will focus on electron transport along the growth direction of GaAs/AlGaAs superlattices. In one set of experiments, tunneling cyclotron resonance, where the electron must tunnel coherently through the superlattice barriers, is used to measure the tunneling mass deep inside the forbidden gap of AlAs barriers. The mass is found to be strongly renormalized (0.09m_{e}) from the band edge value (0.15m_{e }) commonly in use. In a second set of experiments we demonstrate thermal saturation of miniband transport for the first time. The frequency dependent miniband conductivity quenches as the temperature is raised so that kT is much greater than the miniband width. Under these conditions the miniband tends to become uniformly occupied, although not full, an applied electric field cannot change the momentum distribution of the electrons (i.e. no current flows) and the conductivity vanishes.