The ability of an ammonia purifier to reduce oxygen incorporation in AlGaN layers and to improve device performance was investigated. The employment of an ammonia purifier for GaN MOCVD processes showed a reduction of oxygen incorporation in AlGaN layers by a factor of almost 10. "Light output" and "forward voltage" were improved by a factor of 4 times and 10% respectively. Also, no difference in purifier performance was observed between a single use chemisorption-based purifier and a regenerable physisorption-based purifier. It was also demonstrated that the Nanochem(R) Regenerable Ammonia Purifier showed no sign of degradation in capacity after 50 regeneration cycles. (C) 2003 Elsevier B.V. All rights reserved.