Oxygen contamination has been observed inIn0.5Ga0.5Pand(Al0.23Ga0.77)0.5In0.5Pfilms grown by solid source molecular beam epitaxy with elemental phosphorus. Using a conventionalP4cracking zone temperature of 950 °C, spike contamination levels as high as1×1019 cm−3were observed at growth interrupted interfaces with the resultant deactivation of silicon doping pulses. By reducing the phosphorus cracking temperature to 700 °C, the oxygen level in InGaP was reduced to below the secondary ion mass spectrometry background level of3×1016 cm−3.No measurable accumulation of oxygen was observed at growth interrupted interfaces and efficient silicon pulse doping was obtained. InGaP films grown at the lower cracking temperature exhibited improved mobilities and enhanced photoluminescence intensities. An oxygen level in(Al0.23Ga0.77)0.5In0.5Pof less than1.5×1017 cm−3was obtained with good mobilities and luminescence. Efficient silicon pulse doping in AlGaInP was demonstrated. The oxygen contamination is in the phosphorus flux and is likely a volatile phosphorus oxide such asP4O6.