ICl and IBr provide rapid etching of the ternary allows InGaP, AlInP, and AlGaP under electron cyclotron resonance conditions. The rates are almost independent of microwave power in the range 400 to 1000 W, with typical values of similar to 1.5 mu m/min with ICl/Ar and similar to 0.4 mu m/min with IBr/Ar. At low microwave powers (less than or equal to 750 W), the etched surface morphologies are quite smooth and there is little degradation of photoresist masks. High Al content AlxGa1-xP (x greater than or equal to 0.7) alloys appear to be good candidates as etchstop layers in InGaAUP device structures when using these plasma chemistries.