Effect of Thermal Annealing Treatment and Defect Analysis on AgGaGeS₄ Single Crystals

AgGaGeS₄ is a new promising nonlinear-optical crystal for frequency-shifting a 1.064 μm laser into mid-IR. This quaternary compound single crystal has been successfully grown by a modified vertical Bridgman method. Although it has high transparency in the 0.5–11.5 μm spectral range, the nonideal transparency at 2.9, 4, and 10 μm restricts further optical experiments and applications. Therefore, in this work, AgGaGeS₄ wafers were annealed in vacuum and with a AgGaGeS₄ polycrystalline powder at different temperatures. After annealing, under certain conditions, the optical quality of AgGaGeS₄ wafers shows evident improvement, and it is found that volatile GeS₂ easily results in stoichiometric deviation, even decomposition, so that the choice of temperature plays a pivotal role in the annealing treatment. Conclusively, the results confirm that thermal annealing could effectively improve the optical quality of the as-grown AgGaGeS₄ crystal and annealings with a AgGaGeS₄ polycrystalline powder at 550 °C and in vacuum at 500 °C are optimum processes. After such treatment, the transmittance of the wafer is about 70% and the absorptions at 2.9, 4, and 10 μm have almost been eliminated. Besides, the binding energy tends to get smaller with increasing temperature and the Raman phonon frequency has scarcely changed, indicating that the thermal annealing processes only renovate the crystal structure by atomic diffusion or dislocation climbing but without changes in the main structure. At last, through Hall measurement and positron annihilation lifetime spectroscopy, we find that the carrier concentration has little change after annealing, while the cation vacancy sharply declines, and the trapping state of the positron is mainly attributed by the substitution of Ge⁴⁺ by Ga³⁺.