The present invention provides a process for producing a layer of a crystalline material comprising a perovskite or a hexahalometallate, which process comprises: (i) exposing a substrate to a vapour comprising a first precursor compound in a first chamber to produce a layer of the first precursor compound on the substrate; and (ii) exposing the layer of the first precursor compound to a vapour comprising a second precursor compound in a second chamber to produce the layer of a crystalline material, wherein the pressure in the second chamber is above high vacuum. The invention also provides a process for producing a layer of a crystalline material comprising a perovskite or a hexahalometallate, which process comprises exposing a layer of a first precursor compound on a substrate to a vapour comprising a second precursor compound in a chamber to produce the layer of a crystalline material, wherein the pressure in the chamber is greater than high vacuum and less than atmospheric pressure. The invention also provides a process for producing a semiconductor device comprising a layer of a crystalline material, which process comprises producing said layer of a crystalline material by a process as according to the invention.