Evidence of a Shockley-Read-Hall Defect State Independent of Band-Edge Energy in InAs/In(As,Sb) Type-II Superlattices

A set of seven InAs/In(As, Sb) type-II superlattices (T2SLs) are designed to have specific band-gap energies between 290 meV (4.3 mu m) and 135 meV (9.2 mu m) in order to study the effects of the T2SL band-gap energy on the minority-carrier lifetime. A temperature-dependent optical pump-probe technique is used to measure the carrier lifetimes, and the effect of a midgap defect level on the carrier-recombination dynamics is reported. The Shockley-Read-Hall (SRH) defect state is found to be at energy of approximately -250 +/- 12 meV relative to the valence-band edge of bulk GaSb for the entire set of T2SL structures, even though the T2SL valence-band edge shifts by 155 meV on the same scale. These results indicate that the SRH defect state in InAs/In(As, Sb) T2SLs is singular and is nearly independent of the exact position of the T2SL band-gap or band-edge energies. They also suggest the possibility of engineering the T2SL structure such that the SRH state is removed completely from the band gap, a result that should significantly increase the minority-carrier lifetime.