The authors have investigated the effect ofinsituhydrogen plasmas generated by electron cyclotron resonance on growth rate and carbon incorporation in GaAs and AlGaAs grown by metalorganic molecular‐beam epitaxy using triethylgallium and trimethylamine alane. Carbon backgrounds in GaAs grown at 500 °C were found to increase with increasing microwave power over the range 100–200 W and also with increasing H2flow at 175 W. Hydrogen plasmas did not significantly increase the growth rate for either GaAs or AlGaAs even at growth temperatures as low as 375 °C. As at higher temperatures, carbon levels were not reduced in GaAs grown at low temperatures though some improvement was observed for AlGaAs grown at 375 °C. Plasmas were found to be much more effective for cleaning substrates prior to growth. At 500 °C, low microwave powers and moderate exposure times were found to produce the lowest levels of interfacial C, O, and Si contamination.