Hi guys.

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.

And thoughts?

Thanks!

Andy