Intrinsic defects (vacancies and antisites) in neutron irradiated CdSiP2 crystals

Cadmium silicon phosphide (CdSiP2) is a nonlinear optical material widely used in optical parametric oscillators. Intrinsic defects (vacancies and antisites) are responsible for unwanted broad optical absorption bands in these crystals that degrade the performance of the devices. In the present work, optical absorption and electron paramagnetic resonance (EPR) spectra are acquired (at room temperature and 12 K, respectively) from a neutron-irradiated CdSiP2 crystal. After the irradiation, the crystal is highly absorbing from the band edge near 600 nm to beyond 1.3 μm because of overlapping defect-related absorption bands. Heating to 550 °C removes nearly all the absorption induced by the high-energy neutrons. EPR spectra show that a primary effect of the neutrons is the production of phosphorous vacancies. A broad EPR signal attributed to perturbed intrinsic defects is seen before heating the irradiated crystal. After warming to 550 °C, an EPR spectrum representing isolated neutral phosphorous vacancies (VP0) is observed without light and an EPR spectrum from neutral phosphorous-on-silicon antisites (PSi0) can be photoinduced with 633 nm light. The formation of the PSi0 antisites (occurring when phosphorous interstitials created by the neutrons are trapped at silicon vacancies) provides direct evidence that the neutrons displaced phosphorous ions. The VP0 and PSi0 donors were not detected in EPR spectra taken from the as-grown CdSiP2 crystal (i.e., before the neutron irradiation).