Impurity induced band-gap narrowing in p-type CuIn1-xGax(S,Se)2

Green's functions modelling of the impurity induced effects in p-type CuIn1-xGaxS2 and CuIn1-xGaxSe2 (x = 0.0, 0.5, and 1.0) reveals that: (i) the critical active acceptor concentration for the metal non-metal transition occurs at Nc approximate to 10(17)-10(18) cm(-3) for impurities with ionization energy of E-A approximate to 30-60 meV. (ii) For acceptor concentrations N-A>N-C, the hole gas of the metallic phase affects the band-edge energies and narrows the energy gap E-g=E-g(0)=Delta E-g. The energy shift of the valence-band maximum Delta E-v1 is roughly twice as large as the shift of the conduction-band minimum Delta E-c1. (iii) Delta E-v1 depends strongly on the non-parabolicity of the valence bands. (iv) Sulfur based compounds and Ga-rich alloys have the largest shifts of their band edges. (v) A high active acceptor concentrations of N-A = 10(20) cm(-3) implies a band-gap narrowing in the order of Delta E-g approximate to 0.2 eV, thus E-g=E-g(0)-0.2 eV, and an optical band gap of E-g(opt)approximate to E-g(0)-0.1 eV. (C) 2008 Elsevier B.V. All rights reserved.