Localization and Band Gap Pinning in Semiconductor Superlattices with Layer Thickness Fluctuations

Abstract

We consider (AlAs)_n/(GaAs)_n superlattices with random thickness\nfluctuations Delta-n around the nominal period n. Using three-dimensional\npseudopotential plane-wave band theory, we show that (i) any amount Delta-n/n\nof thickness fluctuations leads to band-edge wavefunction localization, (ii)\nfor small Delta-n/n the SL band gap is pinned at the gap level produced by a\nsingle layer with ``wrong'' thickness n + Delta-n, (iii) the bound states due\nto monolayer thickness fluctuations lead to significant band-gap reductions,\n(iv) <001> AlAs/GaAs SL's with monolayer thickness fluctuations have a direct\nband gap, while the ideal <001> SL's are indirect for n<4.\n