We are developing techniques for the fabrication of quantum structures by epitaxial regrowth on laterally prepatterned heterostructures. In this study, we are reporting about the electrical properties of in situ chlorine etched and regrown interfaces. Molecular beam epitaxy grown GaAs layers are etched by chemical gas etching. The etching is performed at various sample temperatures, keeping the chlorine pressure constant. The etched surfaces are directly overgrown with the layer sequence of a modulation doped AlGaAs/GaAs high electron mobility transistor. The electrical properties are probed with DC-magneto-transport (MT) and cyclotron-resonance absorption (CR) measurements. We find the electron densities to be unaffected by the etching parameters. The MT mobilities of these structures exhibit a clear maximum with 190.000 cm2/Vs for the optimised etching temperature. The far infrared resonance of the optimised sample shows a single CR mode. Samples with interfaces etched at lower or higher temperatures exhibit a splitting of the CR. The splitting becomes the more pronounced the further the etching temperature differs from the optimum.