Intrinsic point defects (vacancies and antisites) in CdGeP2 crystals

Cadmium germanium diphosphide (CdGeP2) crystals, with versatile terahertz-generating properties, belong to the chalcopyrite family of nonlinear optical materials. Other widely investigated members of this family are ZnGeP2 and CdSiP2. The room-temperature absorption edge of CdGeP2 is near 1.72 eV (720 nm). Cadmium vacancies, phosphorous vacancies, and germanium-on-cadmium antisites are present in as-grown CdGeP2 crystals. These unintentional intrinsic point defects are best studied below room temperature with electron paramagnetic resonance (EPR) and optical absorption. Prior to exposure to light, the defects are in charge states that have no unpaired spins. Illuminating a CdGeP2 crystal with 700 or 850 nm light while being held below 120 K produces singly ionized acceptors ( V Cd -) and singly ionized donors ( Ge (+)(Cd) ), as electrons move from V-Cd( 2 -) vacancies to Ge (2 +) (Cd) antisites. These defects become thermally unstable and return to their doubly ionized charge states in the 150-190 K range. In contrast, neutral phosphorous vacancies ( V (0)(P) ) are only produced with near-band-edge light when the crystal is held near or below 18 K. The V (0)(P) donors are unstable at these lower temperatures and return to the singly ionized V (+)(P) charge state when the light is removed. Spin-Hamiltonian parameters for the V (-)(Cd) acceptors and V (0)(P) donors are extracted from the angular dependence of their EPR spectra. Exposure at low-temperature to near-band-edge light also introduces broad optical absorption bands peaking near 756 and 1050 nm. A consistent picture of intrinsic defects in II-IV-P-2 chalcopyrites emerges when the present CdGeP2 results are combined with earlier results from ZnGeP2, ZnSiP2, and CdSiP2.

Automated summary

Cadmium germanium diphosphide (CdGeP2) crystals, belonging to the chalcopyrite family, have versatile terahertz-generating properties. CdGeP2 crystals exhibit unintentional intrinsic point defects including cadmium vacancies, phosphorous vacancies, and germanium-on-cadmium antisites. These defects can be studied using electron paramagnetic resonance (EPR) and optical absorption techniques. Illuminating CdGeP2 crystals with specific wavelengths of light at low temperatures produces different charged states of the defects. Additionally, exposure to near-band-edge light introduces broad optical absorption bands.