> Also, does anyone know the etchant which will etch
> GaAs but not attack AlGaAs?
>
        there are several etchants like that:
        e.g. phosphoric acid based-
        Journal of Applied Physics -- October 1, 1998 -- Volume 84, Issue 7
pp. 3933-3938

        citric acid based -
        Journal of The Electrochemical Society -- January 2001 -- Volume
148, Issue 1 pp. G4-G9 ,
        Journal of Vacuum Science & Technology B: Microelectronics and
Nanometer Structures -- September 1990 -- Volume 8, Issue5 pp. 1122-1124

         Ammonium hydroxide based  (could not find a really good reference
right on the spot)

        From my experience, citric acid based solutions work quit nicely -
but that depends on you AlGaAs film composition and selectivity required

          Mike