Attracting shallow donors: Hydrogen passivation in (Al,Ga,In)-doped ZnO
  The hydrogen interstitial and the substitutional Al_Zn, Ga_Zn and In_Zn are all shallow donors in ZnO and lead to n-type conductivity. Although shallow donors are expected to repel each other, we show by first principles calculations that in ZnO these shallow donor impurities attract and form a complex, leading to a donor level deep in the band gap. This puts a limit on the n-type conductivity of (Al,Ga,In)-doped ZnO in the presence of hydrogen. 
Masahiko Matsubara, Mozhgan N. Amini, Rolando Saniz, Dirk Lamoen, Bart   Partoens
cond-mat.mtrl-sci