ELECTRON-LOCALIZATION MECHANISMS IN GAAS/GA0.7AL0.3AS SUPERLATTICES

The effects of miniband width (DELTA), applied electric field. and atomic layering thickness disorder present in real semiconductor superlattices (SL's) have been studied experimentally via transverse magnetotransport experiments on i set of n-type modulation-doped GaAs/Ga0.7Al0.3As SL's. X-ray diffraction and transmission-electron microscopy studies show that the thickness of any given layer may vary randomly by +/- 1 atomic monolayer from the well-defined average layer thickness. Current-voltage traces show a large drop in the high-field differential conductivity with a characteristic voltage that is proportional to DELTA in the range 10 < DELTA < 30 meV, but which is nearly independent of DELTA for DELTA greater than or similar or 30 meV. This behavior is consistent with a crossover from a Wannier-Stark to a negative effective-mass field-induced localization mechanism. Furthermore, magnetotransport data at low temperatures clearly show small disorder-induced localization corrections to the conductivity in the smaller bandwidth SL's. Scattering length scales derived from the data indicate that the dominant source of disorder is the layer thickness variations. This disorder introduces a mobility edge which is smaller than DELTA in the samples studied, but which is comparable to the Fermi energy so that the smallest bandwidth SL is very near a metal-insulator transition.