Hi Christopher

First of all you can etch These layers by plasma, wet and even by Ion Milling depending on ( what you look for &) the Layer thickness to etch and the surface crystallographic plan to be etched...

How deep do you want to etch? ... on which Crystal plan?

I guess the best way to do is laser ablation method by a micromachining systems....(femto second laser gives better results than nano-second)

Yes Nurhan, why not laser ablation for small areas; for whole Wafer plasma is Interesting. Concerning Wet etching: H3PO4 can be a solution except if you want to etch the "c" plan.

This is a very thorough study, you may find useful:

http://link.aip.org/link/JAPIAU/v84/i9/p5248/s1&Agg=doi