A-C Properties of Anodic Oxide Films on Silicon

Low resistivity (∼0.02 ohm cm), (111)‐oriented, p‐type silicon slices were anodized in a solution of 0.04 N KNO 3 in ethylene glycol containing 2% water, and the dielectric behavior of the system Si / anodic SiO 2 / electrolyte has been studied. It was found that the reciprocal capacitance decreased proportionally to the square root of time during which the sample was immersed in a measurement electrolyte. Denning a “diffusion constant,” 2 D = ( ε ε o · A ) 2 ∂ ( 1 / C ) 2 / ∂ t , it was found that D was not affected by the p H of the measurement electrolyte but increased ( i ) with the water content of the electrolyte, ( ii ) with temperature, and ( iii ) with anodic bias. An activation energy of 0.66 ev has been determined between 0° and 75 °C. It is suggested that water, after entering the anodic oxide, dissociates into hydroxyl groups which migrate into the oxide and cause the effective dielectric thickness to decrease. Partial dehydration was possible by annealing or by evacuation. The frequency dependence of the capacitance, when measured in an electrolyte of low water content, ( i ) increased with film thickness for oxide layers formed at the same current density, and ( ii ) decreased with increasing formation current density for constant film thickness.