Experimental and theoretical studies on gadolinium doping in ZnTe

We studied the effects of Gd doping on the structural and optical properties of ZnTe films grown by pulsed laser deposition. We found that a small amount of Gd doping yields a reduction in the ZnTe lattice constant with no change in the fundamental band gap of the material. When the doping level increases above 7% the lattice constant becomes more or less constant, while the band gap increases abruptly (by as much as 50 meV). Theoretical calculations based on ZnTe supercells containing either isolated defects or defect complexes show that the reduced lattice constant can be attributed to the presence of defect complexes involving substitutional Gd ions and neighboring vacancies. The insensitivity of the band gap to low Gd concentration can be explained by self-compensation of these defects, while the band-filling effect probably explains the increase in the band gap energy.