A “superlattice” model for a smooth GaAs/AlAs (001) heterointerface

Abstract

The effect of a smooth interface potential on the electronic states in GaAs/AlAs (001) structures is investigated using the pseudopotential method. In this approach, the transition region between GaAs and AlAs is assumed to be a layer corresponding to a half-period of the (AlAs)2(GaAs)2 superlattice, with the potential of this layer being close to the real potential near the heterointerface. In this case, the intervalley mixing occurs at two boundaries and in the transition layer rather than at one boundary, as in the model with a sharply cut-off potential. It is shown that a smooth potential has an appreciable effect on electron tunneling in structures with thin layers. This effect is especially important in the case where short-wavelength X states are involved. For one GaAs/AlAs (001) boundary, the transition layer acts as a quantum well localizing the charge density of a mixed Γ-X state near the boundary. In structures with a layer thickness of less than 2 nm, the differences in the resonance energies obtained in the models with a smooth heterointerface and with a sharp heterointerface can be as high as ∼0.1 eV. The envelopes of the wave functions associated with Γ1(1), Γ1(2), and Γ3(1) superlattice valleys and with Γ1, X1, and X3 valleys of GaAs and AlAs are analyzed. It is shown that the matching matrices for the envelope functions at the GaAs/(AlAs)2(GaAs)2 and (AlAs)2(GaAs)2/AlAs boundaries depend only weakly on the electron energy near the bottom of the conduction band and that the probability densities calculated using these functions agree with the results of many-band calculations. Therefore, these functions can be used to construct a model with a smooth interface potential in the framework of the effective-mass method.