Anisotropy of infrared spectral absorption in uniaxially-stressed n-GaP

A theoretical and experimental investigation of free-carrier absorption (FCA) in unstressed (optically isotropic) and uniaxially-stressed (optically anisotropic) n-GaP in the quantum frequency regions has been carried out. At a pressure P = 10 ⁴ kgf cm ^-2, parallel to the [100] direction, and at T = 300K, practically all the electrons previously uniformly distributed throughout all the equivalent valleys migrated into one lowered valley. This permitted the investigation of the components of the FCA coefficient at parallel (α _∥) and perpendicular (α _⊥) orientations of the field axis relative to the long axis of the mass tensor ellipsoid of the lowered valley. It is shown that the experimentally-observed singularities of FCA in n-GaP (varying frequency dependences α ₀, α _∥, α _⊥, low value of the ratio {α _⊥}/{α _∥}⁽¹⁾ and frequency dependence with the power less than {3}/{2}) can be explained within the framework of FCA quantum theory only if the intermediate states in the upper conduction band Δ ₂ and the intervalley scattering anisotropy are taken into account.