Crystal Growth, Characterization, and Thermal Annealing of Nonlinear Optical Crystals AgGaGeSe ( = 1.5, 1.75, 2, 3, 4, 5, and 9) for Mid-infrared Applications.

The new quaternary single crystals AgGaGeSe ( = 1.5, 1.75, 2, 3, 4, 5, and 9) have high nonlinear optical property and can be used for mid-IR laser applications in high power. However, only AgGaGeSe and AgGaGeSe have been grown on a large scale and studied in detail. In this work, the AgGaGeSe ( = 1.5, 1.75, 2, 3, 4, 5, and 9) crystals (Φ 20 mm × 40 mm and Φ 40 mm × 100 mm) were grown by the modified Bridgman method. The crystal structure was studied by X-ray diffraction and the Rietveld refinement method. The composition and morphology were characterized by scanning electron microscopy and metallurgical microscopy. The chemical state and vibration modes of surface elements were characterized by X-ray photoelectron spectroscopy and Raman spectra, and the electrical property was investigated by the Hall effect measurement, which indicates that all the single AgGaGeSe crystals are n-type semiconductors. The transmittance of all as-grown AgGaGeSe crystal wafers exceed 65% in the transparent range, and the band gap increases from 2.05 eV for AgGaGeSe to 2.14 eV for AgGaGeSe. Besides, after being annealed in two different conditions, the wafers show different changes. We discovered a special decomposition phenomenon during the annealing process and found the more appropriate annealing method at last. In addition, the absorption peaks at 4.2, 10, and 14.9 μm of wafers have been nearly eliminated, and the quality of most crystals has been improved.