LOCALIZATION AND BAND-GAP PINNING IN SEMICONDUCTOR SUPERLATTICES WITH LAYER THICKNESS FLUCTUATIONS

We consider (AlAs)(n)/(GaAs)(n) superlattices with random thickness fluctuations Delta n around the nominal period n. Using three-dimensional pseudopotential plane-wave band theory, we show that i) any amount Delta n/n of thickness fluctuations leads to band edge wave function localization, ii) for small Delta n/n the SL band gap is pinned at the gap level produced by a single layer with <<wrong>> thickness n + Delta n, iii) the bound states due to monolayer thickness fluctuations lead to significant band gap reductions, e.g., in n = 2, 4, 6, and 10 monolayer SLs the reductions are 166, 67, 29, and 14 meV for [111] SLs, and 133, 64, 36, and 27 meV for [001] SLs, iv) [001] AlAs/GaAs SLs with monolayer thickness fluctuations have a direct band gap, while the ideal [001] SLs are indirect for n < 4.