Why can CuInSe2 be readily equilibrium-doped n-type but the wider-gap CuGaSe2 cannot?

The wider-gap members of a semiconductor series such as diamond-->Si-->Ge or AlN-->GaN-->InN often cannot be doped n-type at equilibrium. We study theoretically if this is the case in the chalcopyrite family CuGaSe2-->CuInSe2, finding that: (i) Bulk CuInSe2 (CIS, E-g=1.04 eV) can be doped at equilibrium n-type either by Cd or Cl, but bulk CuGaSe2 (CGS, E-g=1.68 eV) cannot; (ii) result (i) is primarily because the Cu-vacancy pins the Fermi level in CGS farther below the conduction band minimum than it does in CIS, as explained by the doping limit rule; (iii) Cd doping is better than Cl doping, in that Cd-Cu yields in CIS a higher net donor concentration than Cl-Se; and (iv) in general, the system shows massive compensation of acceptors (Cd-III,V-Cu) and donors (Cl-Se,Cd-Cu,In-Cu). (C) 2004 American Institute of Physics.