Electronic and optical properties of corrugated GaAs/AlAs superlattices

Abstract

The tight-binding method is applied to the analysis of the electronic and optical properties of corrugated GaAs/AlAs superlattices recently grown by molecular beam epitaxy on (311) A GaAs oriented substrates. These structures are of great interest because the facetting of the flat surface leads to the formation of quantum wires by a lateral corrugation with period of 3.2 nm. The model uses a sp3s * atomic basis including the spin-orbit coupling and second-neighbor interactions. The Hamiltonian parameters are fitted so as the energy bands of the two semiconductors near the Γ and X band edges are accurately described. The unit cell of quantum-wire superlattices is two-dimensional and lead to a very large tight-binding basis containing 320xN local orbitals for a superlattice of N layers along the growth direction [311]. The electronic band structure is calculated in order to investigate the nature of the lowest conduction bands and these features are explained in terms of zone folding and lateral corrugation. For corrugated superlattices with layers of equal widths larger than 3.2 nm, the main contribution to the lowest energy conduction state is due to the cation s orbitals and is identified as coming from the Γ-valley of GaAs. For thinner widths, the cation p orbitals of the AlAs layers give the major contribution to the lowest conduction state. This situation is quite similar to the crossing of the Γ- and X-like states of (100)-grown superlattices which present a type I-type II transition near 3 nm. Comparison with non-corrugated (311) superlattices with (311) flat faces shows that corrugation increases the confinement of the lowest conduction bands, more localizing the Γ-electrons in GaAs wells and the X-electrons in the AlAs barriers. The two upper valence states are localized in GaAs layers. They result from mixing of the bulk states due to the reduced symmetry of the quantum wires and show an appreciate in-plane anisotropy. The calculated value of the Γ- and X-like state crossing is in agreement with the experimental observations. Optical properties of (311) corrugated superlattice are studied and the calculated interband transitions account for the photoluminescence data.