Effects of external stress on the electronic structure and optical properties of [001]- and [111]-growth-axis semiconductor superlattices

We analyze the effects of an external compressive stress on the electronic structure of [001]- and [111]-growth-axis semiconductor superlattices. Lattice-matched and strained-layer superlattices are considered. We treat cases where the external stress is applied in a direction perpendicular to the growth axis of the superlattice. In the case of [001]-growth-axis superlattices, the application of a stress perpendicular to the growth axis can cause a strong admixture of the light- and heavy-hole states, resulting in transition energy shifts that are nonlinear functions of the magnitude of the applied stress. This behavior has its physical origin in deformation-potential effects described by the strain Hamiltonian. Additional effects can arise in [111] strained-layer superlattices where the application of an external stress can also cause a modulation of the internal strain-induced electric fields in addition to deformation-potential effects. These internal electric fields arise because of the piezoelectric nature of III-V zinc-blende-structure semiconductor materials. Since the strain-induced internal electric fields are proportional to the off-diagonal components of the internal strain tensor, they vanish in strained-layer superlattices grown along the [001] orientation.Consequently, [111]- and [001]-growth-axis strained-layer superlattices can exhibit very different behavior when subjected to an external compressive stress applied in a direction perpendicular to the growth axis. In fact, while only deformation-potential effects are present in [001]-growth-axis superlattices, numerical calculations show that the response of [111]-growth-axis strained-layer superlattices to an external stress can be dominated by the internal field effects. We illustrate these ideas by considering the electronic structure and optical properties of lattice-matched Ga0.47{In}0.53$As-${Al}0.48In0.52As superlattices grown along the [001] axis and strained-layer Ga_1-x{In}ₓ$As-${Al}_{1{{-}}y}InyAs superlattices grown along the [001] and [111] axes. In the case of [111] strained-layer superlattices, we treat cases where the Ga_1-x{In}ₓ$As quantum wells are either in biaxial compression (x=0.53, y=0.30) or in biaxial tension (x=0.53, y=0.74) due to the lattice mismatch between the alloys. We indicate how the piezomodulation of the internal piezoelectric fields by the application of an external stress can be used to change the optical response of [111]-growth-axis strained-layer superlattices.