Optical Properties of GeTe/Sb ₂ Te ₃ ‐Based Phase‐Change Memory Heterostructures Grown by Pulsed Laser Deposition

Optical properties of GeTe/Sb 2 Te 3 heterostructures are investigated with emphasis on the combined effects of thickness‐dependent sublayer behavior and interfacial diffusion. Spectroscopic ellipsometry shows that both GeTe and Sb 2 Te 3 thin layers exhibit pronounced, thickness‐dependent dielectric functions that differ substantially from those of thicker films. Consequently, an accurate optical description of nonperiodic heterostructures requires the use of multiple thickness‐specific dielectric‐function models for each constituent material. Interdiffusion at GeTe/Sb 2 Te 3 interfaces leads to the formation of ultrathin Ge–Sb–Te transition layers with compositions close to Ge 2 Sb 2 Te 5 , which are more structurally disordered than thick amorphous Ge 2 Sb 2 Te 5 . The effective refractive index and optical bandgap of the heterostructures span combinations not accessible in the individual materials, demonstrating that heterostructuring enables additional control over optical properties relevant for tunable photonic and optoelectronic applications.