Localization and band-gap pinning in semiconductor superlattices with layer-thickness fluctuations

We consider (AlAs)n/(GaAs)n superlattices with random thickness fluctuations ∆n around the nominal period n. Using three-dimensional pseudopotential plane-wave band theory, we show that (i) any amount ∆n/n of thickness fluctuations leads to band-edge wavefunction localization, (ii) for small ∆n/n the SL band gap is pinned at the gap level produced by a single layer with “wrong” thickness n+∆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 SL’s the reductions are 166, 67, 29, and 14 meV for 〈111〉 SL’s, and 133, 64, 36, and 27 meV for 〈001〉 SL’s, (iv) 〈001〉 AlAs/GaAs SL’s with monolayer thickness fluctuations have a direct band gap, while the ideal 〈001〉 SL’s are indirect for n < 4. PACS numbers: 73.20.Dx,71.50.+t Typeset using REVTEX