Photo-electron paramagnetic resonance investigation of the Cr³⁺ impurity and the (SCl)^2- defect in synthetic FeS₂ crystals

The method of photo-electron paramagnetic resonance has been used to investigate the photosensitivities of the Cr³⁺, Ni²⁺, and (SCl)^2- impurities and defects detected by electron paramagnetic resonance (EPR) in synthetic FeS₂ crystals grown by the method of chemical vapor transport by using ICl₃ as the transport agent. Cr³⁺ and (SCl)^2- are photosensitive, but Ni²⁺ is not, to IR illumination with photon energy less than the band-gap energy. A decrease (photoquenching) in the EPR signal of Cr³⁺ and an increase (photoinduction) in the EPR signal of (SCl)^2- were detected at low temperatures. At T≅110 K, the spectral response of the photoquenching of Cr³⁺ is the largest near λ=1425 nm^-1 (0.87 eV), which can be correlated with the 0.90 eV photoconductivity peak and the 0.87 eV IR absorption peak observed at low temperatures in the same batches of crystals. After several cycles of on-off IR illumination, the equilibrium population of Cr³⁺ is less than that of the virgin state, indicating the presence of hole traps. The transient and equilibrium behaviors of the (SCl)^2- defects are just the opposite, which indicates that a S^2- defect behaves as a hole trap. A band model has been established to explain the observed photoquenching of Cr³⁺, the photoconductivity peak, and the IR absorption peak.