  Epitaxial III-V semiconductor heterostructures are key components in modern
microelectronics, electro-optics and optoelectronics. With superior
semiconducting properties, halide perovskite materials are rising as promising
candidates for coherent heterostructure devices. In this report, spinodal
decomposition is proposed and experimentally implemented to produce epitaxial
double heterostructures in halide perovskite system. Pristine epitaxial mixed
halide perovskites rods and films were synthesized via Van der Waals epitaxy by
chemical vapor deposition method. At room temperature, photon was applied as a
knob to regulate the kinetics of spinodal decomposition and classic coarsening.
By this approach, halide perovskite double heterostructures were created
carrying epitaxial interfaces and outstanding optical properties. Reduced
Fr\"ohlich electron-phonon coupling was discovered in coherent halide double
heterostructure, which is hypothetically attributed to the classic phonon
confinement effect widely existing in III-V double heterostructures. The
ability to develop coherent double heterostructures in halide perovskites paves
an avenue to exploring halide perovskite-based quantum wells and superlattices
for high-performance and low-cost optoelectronics, electro-optics and
microelectronics.
