LOCALIZATION AND BAND-GAP PINNING IN SEMICONDUCTOR SUPERLATTICES WITHLAYER THICKNESS FLUCTUATIONS

We consider (AlAs)(n)/(GaAs)(n) superlattices with random thickness fl uctuations Delta n around the nominal period n. Using three-dimensiona l pseudopotential plane-wave band theory, we show that i) any amount D elta n/n of thickness fluctuations leads to band edge wave function lo calization, ii) for small Delta n/n the SL band gap is pinned at the g ap 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 mono layer SLs the reductions are 166, 67, 29, and 14 meV for [111] SLs, an d 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 ide al [001] SLs are indirect for n < 4.