Hi, I am looking for some etchant of GaAs which can stop on
Al(0.15)Ga(0.85)As. Citric Acid and Hydragen Dioxide don't have enought
selectivity. I was told Amonium Hydraoxide and Hydragen Dioxide can do this
job, if I adjust it to right PH value. any clue about this problem? Or where
can I find the detail study about the Amonium etchant on GaAs/AlGaAs system.

Thanks a lot
Qingwei