Image charges and their influence on the growth and the nature of thin oxide films

The polarization energies associated with localized charges near to boundaries between different dielectrics (the image terms in simple cases) can be important in several different ways. First, they influence the motion of mobile species near to boundaries between different materials, this being a well-understood feature. Secondly, they affect transfer across boundaries, and so modify injection processes. Thirdly, they affect the relative stability of different charge states and distributions of carriers over traps. We examine the behaviour in systems when a layer of one material is found on a substrate of another, with emphasis on the case where we have a metal and an oxide, in addition to a gas phase. We note that the terms from the polarization energy are substantial, and influence behaviour in oxide growth. This includes the distribution of charged defects within oxide layers. The same terms could stabilize small polarons in suitable cases, so that both electronic and ionic transport may be affected. We show the effect on oxidation kinetics in a new way, comparing the normalized oxidation rate for various mechanisms in a way which minimizes dependence on fitting parameters. Reference cases are evaluated numerically. One example (the modified parabolic case) shows Deal—Grove behaviour, but in which the linear term is not determined by interfacial reaction.