Optical absorption in the defect-related region of highly efficient non-linear ZnGeP2 crystals under e-beam irradiation and post-irradiation anneals has been investigated.
Partially irreversible changes of the absorption were found in the spectral range 0.9–2.5 μm (0.5–1.3 eV) after irradiation and subsequent low-temperature anneals. Data obtained do not support the vacancy model for ZnGeP2 absorption in the 0.5–1.3 eV range.
The least squares fit for the parameters of the theoretical dependence of optical absorption cross-section to the experimentally measured ZnGeP2 optical absorption coefficient spectra show that the defect-related absorption in 0.5–1.3 eV region is caused by deep donor levels with energy position E=Ev+(0.85–0.90) eV.
Significant changes in the energy spectrum of the dominant optically active centers have been observed under influence of e-beam irradiation and post-irradiation anneals.
Based on the optical absorption measurements obtained for as-grown, annealed and e-irradiated ZnGeP2 crystals, a model of point defect interactions has been proposed. This takes into account both the reversible interactions, such as the formation of donor-acceptor pairs, and the irreversible interactions of a quasi-chemical type.
The behavior of the energy spectrum of the optically active defects is discussed in terms of the modes of interaction between the initial point defects and those generated by irradiation. The analysis performed showed that the best agreement with experimental data is reached when it is assumed that optical defect-related absorption in the 0.5–1.3 eV range related mainly to the disordering defect in the cation sublattice of ZnGeP2, namely, to atoms of Ge substituting for Zn.
Defect concentration profiles created by e-irradiation in ZGP crystals of different thickness were calculated. The optimum conditions for providing a uniform defect distribution with depth in irradiated ZnGeP2 samples were determined.
The optimal e-beam irradiation fluences, giving maximum ZnGeP2 enlightenment, allowed us to reduce the defect-related absorption down to a value of 0.01 cm-1 at 2 μm.