LAUSR

Heterojunction bipolar transistors are used to measure vertical hole transport in narrow-band-gap InAs=InAs1−xSbx type-II superlattices (T2SLs). Vertical hole mobilities (μh) are reported and found to decrease rapidly from 360 cm=Vs at 120 K to approximately 2 cm=V s at 30 K, providing evidence that holes are confined to localized states near the T2SL valence-miniband edge at low temperatures. Four distinct transport regimes are identified: (1) pure miniband transport, (2) miniband transport degraded by temporary capture of holes in localized states, (3) hopping transport between localized states in a mobility edge, and (4) hopping transport through defect states near the T2SL valence-miniband edge. Region (2) is found to have a thermal activation energy of ε2 1⁄4 36 meV corresponding to the energy range of a mobility edge. Region (3) is found to have a thermal activation energy of ε3 1⁄4 16 meV corresponding to the hopping transport activation energy. This description of vertical hole transport is analogous to electronic transport observed in disordered amorphous semiconductors displaying Anderson localization. For the T2SL, we postulate that localized states are created by disorder in the group-Valloy of the InAs1−xSbx hole well causing fluctuations in the T2SL valence-band energy.