ELECTRONIC CONSEQUENCES OF RANDOM LAYER-THICKNESS FLUCTUATIONS IN ALAS GAAS SUPERLATTICES/

We study the effects of a few types of atomic disorder on the electron ic and optical properties of AlAs/GaAs (001) and (111) superlattices: (i) atomic intermixing across the interfaces; (ii) replacing a single monolayer in a superlattice by one containing the opposite atomic type (isoelectronic delta doping); and (iii) random layer-thickness fluctu ations in superlattices (SL). Type (i) is an example of lateral disord er, while types (ii) and (iii) are examples of vertical disorder. Usin g three-dimensional empirical pseudopotential theory and a plane-wave basis, we calculate the band gaps, electronic wave functions, and opti cal matrix elements for systems containing up to 2000 atoms in the com putational unit cell. Spin-orbit interactions are omitted. Computation ally much less costly effective-mass calculations are used to evaluate the density of states and eigenstates away from the band edges in ver tically disordered SLs. Our main findings are: (i) Chemical intermixin g across the interface can significantly shift the SL energy levels an d even change the identity (e.g., symmetry) of the conduction-band min imum in AlAs/GaAs SLs; (ii) any amount of thickness fluctuations in SL s leads to band-edge wave-function localization; (iii) these fluctuati on-induced bound states will emit photons at energies below the ''intr insic'' absorption edge (red shift of photoluminescence); (iv) monolay er fluctuations in thick superlattices create a gap level whose energy is pinned at the value produced by a single delta layer with ''wrong' ' thickness; (v) (001) AlAs/GaAs SLs with monolayer thickness fluctuat ions have a direct band gap, while the ideal (001) superlattices are i ndirect for n<4; (vi) there is no mobility edge for vertical transport in a disordered superlattice, because all the states are localized; h owever, the density of states retains some of the features of the orde red-superlattice counterpart. We find quantitative agreement with expe riments on intentionally disordered SLs [A. Sasaki, J. Cryst. Growth 1 15, 490 (1991)], explaining the strong intensity and large red shift o f the photoluminescence in the latter system. We provide predictions f or the case of unintentional disorder. (C) 1995 American Institute of Physics.