InGaAs(100) channel surfaces that were originally covered with a protective As2 layer are thermally decapped in-situ in a high vacuum atomic layer deposition system prior to Al2O3 deposition. The As2 decapping process is monitored by chamber pressure change and in-situ x-ray photoelectron spectroscopy. A pressure spike is observed during the decapping process and the completion of the As2 desorption is confirmed by a chemical shift in As 3d bonding to the lower binding energy by 0.8 eV. Post-metallization forming gas anneal improves the interface properties and lowers the density of defects at the oxide/InGaAs interface by passivating interface defects and border traps. Thermal evaporation of the gate metal further reduces the frequency dispersion in weak inversion and accumulation compared to the same metals deposited by e-beam evaporation.