A novel submount substrate with high thermal conductivity for optoelectronic devices is proposed. The substrate is fabricated by depositing an Al-based insulating film on a copper substrate. AlN films were deposited by RF reactive sputtering using an Al target (5N) in a gas mixture of Ar and N 2 . Al 2 O 3 films were deposited by oxygen-ion-assisted electron beam (EB) evaporation. Many conductive paths were detected in the AlN films.These defects were introduced in the AlN film during a photolithography process for fabricating electrode patterns because the alkaline developer dissolved the film. An Al-OH peak in Fourier transform infrared spectroscopy (FT-IR) spectra suggested that the Al(OH) 3 formation was one of the reasons for the presence of conductive paths in the AlN film. In the case of Al 2 O 3 films, no conducting path was detected in electrical measurements, and no marked change in surface morphology was observed in scanning electron microscopy (SEM) images, after treatment with the alkaline developer. The Al 2 O 3 /Cu structure is a candidate for the novel submount substrate with high thermal conductivity.