Optical anisotropy in GaAs/AlAs (110) superlattices

Short-period GaAs/AlAs superlattices grown along the [110] direction have orthorhombic symmetry and thus exhibit two different dielectric functions ε(ω) along the main axes perpendicular to the growth direction ([001] and [11¯0] direction). Spectroscopic ellipsometry as well as ab initio calculations have been used to determine the optical properties of these superlattices. The ${\mathit{E}}_1$ transitions split into two components with different strengths for the two principal polarizations. This effect can be qualitatively interpreted on the basis of a k⋅p model for the ${\mathit{E}}_1$ transitions and confinement arguments. The ${\mathit{E}}_2$ transition for [001] polarization is lower in energy than the one for [11¯0] polarization. In addition, we observe a transition slightly above ${\mathit{E}}_2$, which occurs predominantly in [001] polarization. Two other structures specific to the superlattices are identified, in analogy to the case GaAs/AlAs superlattices grown along [001]. Confinement effects on optical transitions for various superlattice periods are also discussed.