Abstract: A method for epitaxially growing a layer of III-V material on a wafer of a material such as silicon comprises the steps of placing the wafer (16') in a first ultra-high vacuum chamber (11), and epitaxially growing a transition layer such as germanium on the wafer. An intermediate high vacuum chamber (13) is used to transport the wafer 16' to a second ultra-high vacuum chamber (12), and the second chamber (12) is used to epitaxially grow a layer of III-V material over the transition layer. Gate valves (33 and 15) are sequentially opened and closed to that the second vacuum chamber (12) cannot be contaminated by gases or particles from the first vacuum chamber (11). Wafer transport from chamber (11) to (13) is achieved without exposure to the atmosphere or to significant pressure changes thus avoiding the waste of transfer time or the formation of native oxide on the wafer surface.