Dilute nitrides such as GaInNAs are often grown by plasma-assisted molecular-beam epitaxy (MBE), and the plasma that provides reactive nitrogen also damages the semiconductor surface. Direct exposure to the plasma has been studied extensively, but here we report damage due to indirect exposure, while the shutter remains closed. The use of a protective arsenic cap on the wafer is found to prevent such indirect damage, resulting in a 2-3x increase in photoluminescence intensity, sharper features in transmission electron microscopy, and a 30% decrease in laser thresholds. This technique requires no changes to the MBE chamber, unlike a gate valve. (c) 2005 American Vacuum Society.