Effect of surface morphology on the optical properties of InAs/Ge (1 1 1)

Highlights: • Nanotextured InAs(1 1 1) shows reduced value of dielectric constants compared to bulk. • Anisotropic shape of nanotextured InAs shows relaxation in Raman selection rule. • 234 cm⁻¹ mode observed in InAs/Ge is identified as plasmon-LO phonon coupled mode(L_{_}). • Temperature dependent Raman shift curve shows a slope value of −0.017 cm⁻¹ K⁻¹ for L_{_}. • Carrier concentration of 5–8 × 10¹⁷ cm⁻³ for the surface accumulation layer is estimated. - Abstract: In the present work, the effect of surface morphology of nanotextured InAs/Ge (1 1 1) epilayers on their optical properties is presented. Spectroscopic ellipsometry measurement of dielectric constants using effective medium approximation reveals that nanotextured InAs layers have reduced value of dielectric constant compared to the bulk in the energy range of 1.6–6 eV, which is attributed to its morphology and quantum confinement effect. The anisotropy in the shape of the InAs nanostructures also gives rise to anisotropic dielectric constant leading to partial relaxation in polarization dependent Raman selection rule. A phonon mode observed at ∼234 cm⁻¹ in the InAs/Ge heterostructure is identified as plasmon-LO phonon coupled mode originating from the accumulated surface of InAs and this is found to be independent of the morphology of InAs structures. This identification is also corroborated with the temperature dependent phonon frequency shift of the coupled mode. Carrier concentration of 5–8 × 10¹⁷ cm⁻³ for the surface accumulation layer is estimated with the help of the frequency of the coupled mode.