Growth of NiO and SiO₂ thin films

Abstract

It is now well established that thick films of transition-metal oxides such as NiO grow according to a modified Wagner mechanism by the diffusion of ions along grain boundaries in the oxide film. It is shown that the growth of thin films of this oxide is consistent with that of thick films if it is assumed that the same transport processes dominate, but that there is a strong electric field built in which assists injection of point defects into the oxide at the oxide/gas interface (Cabrera-Mott mechanism). Silica films are amorphous, and oxygen can be transported through them as a neutral interstitial molecule. When combined with reaction at the silicon/oxide interface this process accounts for many aspects of SiO₂ growth kinetics (Deal-Grove model). Very thin films, however, grow in dry oxygen at a faster rate than expected from this model; particularly at low temperatures and/or low pressures. Ionic oxygen motion through the SiO₂ network, acting in parallel with molecular diffusion, is explored as a possible explanation and is supported by some independent evidence.