Theoretical study of Li and Na as n-type dopants for diamond

Phosphorus is the n-type dopant of choice for diamond, but results in a deep donor level and alternatives are being sought. One possibility is the incorporation of interstitial alkali metal impurities such as Li and Na. We present the results of density-functional calculations used to predict the stability and electrical activity of Li and Na in diamond. We find that although interstitial Li may be a good candidate donor, it is insoluble, mobile, and is likely to form complexes with other impurities rendering it electrically inactive. Specifically, codoping to form Li-B-Li and Li-O-Li complexes does not result in n-type activity. Additionally, interstitial Na is a deep donor, with Na-Na and Na-X-Na complexes generally following the properties of the Li counterparts. We conclude that alkali metal doping via interstitial Li or Na is unlikely to produce the elusive n-type material.