= Solution
Use a patterned resist or hard mask and \b[anisotropic chlorine-based <reactive-ion etching>], for example an appropriate $\mathrm{Cl_2/BCl_3}$ <inductively coupled plasma> process for GaAs/AlGaAs. Ion-assisted removal gives a well-defined mesa with less lateral undercut than an isotropic wet etch. Control etch time or endpoint to obtain the specified approximately $1.2\,\mu\mathrm m$ depth.
The mask must survive the etch and protect the active aperture. Control <ion> damage and sidewall roughness, remove residues and passivate the sidewall where required. Compatibility with the metal stack determines whether the mesa is etched before or after front-contact deposition.
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