1 University College London, Kathleen Lonsdale Materials Chemistry, Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, UK. c.r.a.catlow@ucl.ac.uk
We present a fundamental method to assess the doping limits of hetero-polar materials; applied to the case of ZnO, we show clearly that electrons are stable and holes are unstable under the limits of thermodynamic control.