Minimization of Intrinsic Impurity Concentration in ZnGeP₂ Single Crystals via Directional Recrystallization

Area of Science:

• Materials Science
• Crystallography
• Nonlinear Optics

Background:

• Zinc germanium phosphide (ZnGeP2) is a key nonlinear crystal for mid-infrared applications.
• Performance limitations include residual absorption and intrinsic impurity phases.

Purpose of the Study:

• To improve the purity and optical properties of ZnGeP2 crystals.
• To investigate the effects of purification on optical characteristics and laser-induced damage threshold (LIDT).

Main Methods:

• Synthesis of polycrystalline ZnGeP2 via a modified two-temperature method.
• Purification using inclined directional recrystallization (up to three cycles).
• Single crystal growth using the vertical Bridgman method.
• Characterization through spectroscopy, absorption measurements, LIDT testing, and powder X-ray diffraction (PXRD).

Main Results:

• Repeated purification cycles enhanced optical homogeneity and near-infrared transparency.
• Absorption coefficient at 2.097 µm decreased from 0.45 to 0.30 cm⁻¹ after three cycles.
• Phase purity increased from 86.31% to 100% after three cycles, suppressing impurity phosphides.
• Laser-induced damage threshold (LIDT) decreased with increased purification.

Conclusions:

• Inclined directional recrystallization is an effective pre-growth purification method for ZnGeP2.
• A trade-off exists between reduced optical losses and damage resistance.
• The optimal number of purification cycles depends on the specific application requirements.