There is not good selectivity between Al0.8GaAs and GaAs as dry etching using in
combination of BCl3/CL2/AR. Probably that would be true to InGaAs. However, you
can use diluted HF for etching Al0.8Ga0.2As which is selective to the GaAs as
well Al03.Ga0.7As and also it is isotropic.

Check out paper for this
Appl. Phys. A 88, 711–714 (2007)
Materials Science & Processing
Applied Physics A
p. kumar1
s. kanakaraju2
d.l. devoe1,

Sacrificial etching of AlxGa1−xAs for III–VMEMS surface micromachining
1 Department of Mechanical Engineering, University of Maryland, College Park, MD
20742, USA
2 Laboratory for Physical Sciences, 8050 Greenmead Drive, College Park, MD
20740, USA

Good luck
Parshant