ION MIGRATION IN CHALCOPYRITE SEMICONDUCTORS

Quantitative data are presented that show partial ionic conductivity of Cu or Ag in ternary and quaternary electronic semiconductors, with idealized stoichiometry CuxAg1-xInSe2. A trend of increasing facility of ionic motion with increasing Ag content was observed. Ionic transference numbers up to 0.13 and 0.55 were measured for CuInSe2 and AgInSe2, respectively. This trend can be correlated with the degree of compactness of the structure. It is supported by results from measurements of effective values of chemical diffusion coefficients, obtained by a potentiostatic current decay technique. Those results show a generally higher diffusivity in AgInSe2 than in CuInSe2. A clear trend of increasing diffusion coefficient (up to 10(-7) cm2/s) with decreasing concentration of IB metal was observed. No obvious general correlation is seen between net electronic carrier concentration (or resistivity) and diffusion coefficients, except that overall the highest diffusion coefficients are found for Cu-poor (or Ag-poor) samples which are also the most resistive. The effect of temperature (between 20 and 100-degrees-C) on the diffusivity is small. On the basis of our observations we conclude that diffusion occurs predominantly via a vacancy mechanism and suggest that this possibility of coexistence of significant ionic mobility with true semiconductivity can be useful for electronic doping by native defects.