Annealing of Ni/Al2O3/InGaAs structures results in a significant increase of the leakage current. The same treatment of Au/Ti/Al 2O3/InGaAs structures results in a slight increase in leakage only. Time of flight secondary ions mass spectrometry measurements detected an annealing induced indium outdiffusion into the dielectric layer. In structures having nickel metallization, the released indium accumulated at the Ni/Al2O3 interface. In structures having Au/Ti metallization, the indium atoms penetrated into the gold layer, and an indium free Al2O3 layer was observed. We attribute the observed different electrical behavior to the different indium diffusion behavior, and eliminate other explanations.