Aluminium oxide and titanium oxide films were deposited using the Atomic Layer Deposition method on n-type 4H Sic and p-type Si {001} substrates, with doping 6.10(15) cm(-3) and 2.10(16) cm(-3), respectively, and on 1.2 kV PiN 4H Sic diodes for passivation studies. The Al2O3 and Sic interface was characterised for the existence of an effective negative charge with a density of 1.10(12) -2.10(12) cm(-2). The dielectric constant of Al2O3 as determined from capacitance-voltage data was about 8.3. The maximum electric field supported by the Al2O3 film was up to 7.5 MV/cm and 8.4 MV/cm on Sic and Si, respectively.