Activation enthalpy of recombination-enhanced vacancy migration in Si

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Abstract
As reported by Watkins and co-workers, recombination-enhanced migration is observed in Si for positively charged vacancies, but not for negatively charged vacancies. Although there is a component of the recombination-enhanced vacancy migration that is independent of temperature (athermal), we show that there is another component which obeys a simple Arrhenius behavior with an activation enthalpy of 0.07 eV. These facts are rather mysterious if one believes that the normal thermal activation enthalpies for vacancy migration measured at the temperatures used in these experiments are relevant parameters for the recombination-enhanced migration process. We show that if, instead, one accepts as relevant parameters the ballistic model values for the enthalpy of vacancy migration at high temperatures, then a simple explanation of the facts is readily apparent. At high temperatures, thermally generated carriers recombine at vacancies even in the absence of electrical or optical injection. The implications of this on the temperature dependence of the vacancy contribution to self-diffusion are discussed.