Vapour deposition process for the preparation of a chemical compound

A physical vapour deposition (PVD) process for the preparation of a chemical compound, wherein the process comprises providing each component element of the chemical compound as a vapour, and co-depositing the component element vapours on a common substrate, wherein:

• the vapour of at least one component element is provided using a thermal cracking effusion source;
• the vapour of at least one other component element is provided using a plasma source, said plasma source being a source for said at least one other component element in the plasma phase;
• and
• at least one further component element vapour is provided;
• wherein reaction of the component elements to form the chemical compound occurs on the surface of the substrate, and not in the vapour phase prior to deposition.