Heteroepitaxy

Heteroepitaxy is the process of growing a crystalline film on a crystalline substrate where the film material differs from the substrate. The deposited layer is crystallographically aligned with the underlying crystal, producing an epitaxial relationship. Heteroepitaxy is contrasted with homoepitaxy, in which the film and substrate are the same material.

A central issue in heteroepitaxy is lattice mismatch: if the lattice constants differ, strain is imposed on the film during growth. For small thicknesses, the film may remain strained and adopt the substrate lattice (pseudomorphic growth); beyond a critical thickness, misfit dislocations form to relieve strain, relaxing the layer. Thermal expansion mismatch and interfacial chemistry also influence defect density and interface abruptness.

Different growth modes describe how layers form: Frank–van der Merwe (layer-by-layer), Volmer–Weber (island growth), and Stranski–Krastanov (layer plus island after a few monolayers). Techniques such as molecular beam epitaxy, metal-organic chemical vapor deposition, and liquid phase epitaxy are used to grow heteroepitaxial systems with precise thickness control and abrupt interfaces.

Applications include semiconductor heterostructures such as GaAs/AlGaAs, GaN-based LEDs and lasers, InP-based devices, and GaN on silicon for high-electron-mobility transistors. Heteroepitaxy enables quantum wells, superlattices, and photonic structures; strain engineering can tune electronic and optical properties.

Challenges include the formation of defects like threading dislocations, interdiffusion at interfaces, and unwanted phases. Strategies to mitigate these issues involve buffer layers, composition grading, lattice-matched or closely matched material systems, and optimization of growth temperatures.