Li and Na in diamond: A comparison of DFT models

Interstitial Li and Na have been suggested as alternatives for achieving shallow n-type doping of diamond. Experimental results have however been contradictory. We report ab initio   density functional theory modeling of Li and Na in diamond and compare results using periodic pseudopotential plane wave, and all-electron APW+loAPW+lo methods together with finite clusters using Gaussian basis sets. We show that although interstitial Li and Na are likely to behave as shallow donors, interstitial Li readily diffuses and it is energetically favorable for interstitial Li and Na to be trapped at existing vacancies in diamond. The resulting substitutional centers are not only passivated but also compensate other interstitial Li and Na donors. Na is shown to be incorporated in diamond as a substitutional acceptor, consistent with experimental observations.