Raman spectroscopy and atomic force microscopy study of interfacial polytypism in GaP/Ge(111) heterostructures

Highlights: • Interface and surface of GaP/Ge(111) are investigated by Raman spectroscopy and AFM on the same site. • Spatially resolved (SR) Raman study revealed the distribution of residual strain in GaP layer. • Asymmetry ∼358 cm⁻¹ is assigned to topographic variations across the surface of GaP microstructure. • Dominance of WZ phase at interface & ZB phase at surface are confirmed by SR polarized Raman study. • Results provide a basis for identification of allotropes at entangled heterostructure interfaces. Effects of lattice and polar/nonpolar mismatch between the GaP layer and Ge(111) substrate are investigated by spatially resolved Raman spectroscopy. The red shifted transverse optical (TO) and longitudinal optical (LO) phonons due to residual strain, along with asymmetry to TO phonon ∼358 cm⁻¹ are observed in GaP/Ge(111). The peak intensity variation of mode ∼358 cm⁻¹ with respect to TO phonon across the crystallographic morphed surface of GaP micro structures is associated with the topographical variations using atomic force microscopy mapping and Raman spectroscopy performed on both in plane and cross-sectional surface. Co-existence of GaP allotropes, i.e. wurtzite phase near heterojunction interface and dominant zinc-blende phase near surface is established using the spatially resolved polarized Raman spectroscopy from the cross sectional surface of heterostructures. This consistently explains effect of surface morphology on Raman spectroscopy from GaP(111). The study shows the way to identify crystalline phases in other advanced semiconductor heterostructures without any specific sample preparation.