Andrew Irvine  wrote :

> I want to do a wet etch which removes AlGaAs (x=0.5) but stops on GaAs
> (I guess 10x selectivity would be about the minimum).  The layer is 50nm
> thick, beneath a 5nm GaAs cap which I'd remove immediately beforehand
> with a non-selective etch.
>
> One etch which did it well in the past is concentrated HF, but we can't
> use any kind of HF any more for a couple of reasons, one obvious and one
> too dull to go into.
>
> Conc HCl?  Seems, according to some very sparse literature, to do the
> job for x>0.7, but it's failing miserably on my x=0.5 so far.

You could try KMnO4 : NaOH : H2O, 2:3:12 I've been meaning to try this don't
know if it will work, or try Ammonium Flouride and HCl mixture should work by
making HF in situ.

Let me know if the first one works.

Good luck.