In this study, we characterize GaAs interfaces processed by ion beam etching (IBE), chemical assisted ion beam etching (CAIBE), and chemical gas etching (CGE). The etched interfaces are overgrown with GaAs or AlGaAs using molecular beam epitaxy. Interfaces in Si-doped GaAs are characterized with capacitance-voltage (C-V) profiling. Furthermore, we use a two-dimensional electron gas (2DEG), located directly at the etched interface in modulation doped AlGaAs/CaAs heterostructures, to probe the interface quality with far infrared (FIR) transmission spectroscopy and magnetotransport measurements. Applying a CGE process with suitable parameters we can remove more than 100 nm GaAs and generate clean surfaces with 2DEGs similar to those of nonetched reference samples. in addition we can reduce the interface state density of an IBE or CAIBE etched surface significantly applying a subsequent CGE process, which removes the ion damaged layer. However the two-dimensional electron systems at such interfaces show characteristic FIR absorption of dot-like potentials, which is a clear sign of a contaminated or disturbed surface.