Microscopic Origins of Electron and Hole Stability in ZnO
  A novel and direct method is proposed to assess the doping limits of semiconducting materials. Applied to the case of ZnO, our first-principles calculations demonstrate that p-type ZnO is thermodynamically unstable. 
C. Richard A. Catlow, Alexey A. Sokol and Aron Walsh
cond-mat.mtrl-sci