Recombination mechanisms in highly doped CdS:In

Time-resolved luminescence measurements of CdS:In cyrstals in the near-band-gap region are presented for a wide range of indium doping levels, i.e., for concentrations between 10¹⁷ and 10²⁰ cm^-3. Interactions of the indium dopants lead to a broad luminescence band near the band-gap energy and alter the luminescence dynamics. A decrease of the lifetimes of both donor and acceptor bound exciton complexes is observed at medium doping levels (10¹⁷--10¹⁸ cm^-3). This is explained by the reduction of the effective binding energies leading to shorter decay constants of these complexes. The decay of the donor-acceptor-pair recombination luminescence is also investigated for indium concentrations below the Mott density. A statistical model proposed by Thomas and Hopfield is used to determine the electronically active impurity concentration. At higher doping levels (up to 10²⁰ cm^-3) the degeneracy of the conduction band leads to a luminescence band attributed to radiative non-k-conserving band-to-band transitions. The measured lifetimes reflect the number of occupied states in the conduction band and the number of free states in the valence band. A nonexponential fit reveals the concentration of electronically active impurity centers. A correlation between photogenerated and doping-generated carriers is established for high excitation densities.