In accordance with a method of forming a halide perovskite thin film, a first halide perovskite material is chosen from which a halide perovskite thin film is to be formed. An epitaxial substrate formed from a second halide perovskite material is also chosen. The halide perovskite thin film is epitaxially formed on the substrate from the first halide perovskite material. The substrate is chosen such that the halide perovskite thin film formed on the substrate has a selected value of at least one property. The property is selected from the group including crystal structure stability, charge carrier mobility and band gap.

A method of forming a perovskite superlattice includes providing a single crystal substrate. The single crystal substrate is exposed to a precursor composition having ions and molecules therein of which a perovskite is composed to thereby form a perovskite superlattice on the single crystal substrate. The perovskite superlattice includes at least one series of layers having alternating inorganic slabs and organic spacers. The single crystal substrate and the inorganic slabs have lattice constants that differ from one another by less than a prescribed amount.

A composite substrate includes: a base substrate and an -Ga.sub.2O.sub.3 crystal film that is provided on the base substrate, has a thickness of 10 m or more, and has at least one alkali metal element content of 1.210.sup.15 atoms/cm.sup.3 or more and 1.010.sup.18 atoms/cm.sup.3 or less.

The present disclosure provides a crystal preparation method, including: obtaining a first wafer after a first treatment, using the first wafer after the first treatment as a first seed crystal to grow a precursor crystal based on a top-seeded solution growth method; performing a second treatment on the precursor crystal; and using the precursor crystal after the second treatment as a second seed crystal to grow a target crystal based on a physical vapor transport method.