RESUMO
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+.
RESUMO
AgGaGe nS2( n+1) crystal is a series of quaternary nonlinear optical materials for mid-IR laser applications of converting a 1.064 µm pump signal (Nd:YAG laser) to 4-11 µm laser output, but only AgGaGeS4 has attracted the most attention, remaining the other promising AgGaGe nS2( n+1) crystal whose physicochemical properties can be modulated by n value. In this work, AgGaGe nS2( n+1) ( n = 2, 3, 4, and 5) polycrystals are synthesized by vapor transport and mechanical oscillation method with different cooling processes. High-resolution X-ray diffraction analysis and refinement have revealed that all the four compounds are crystallized in the noncentrosymmetric orthorhombic space group Fdd2, resulting in the excellent nonlinear optical property, and the distortion of tetrahedron with the variation of n value causes the discrepancy of physicochemical property. Besides, using the modified Bridgman method, AgGaGe nS2( n+1) single crystals with 15 mm diameter and 20-40 mm length have been grown. We have discussed the structure and composition of AgGaGe nS2( n+1) by XPS spectra and analyzed the three kinds of vibration modes of tetrahedral clusters by the Raman spectra. The Hall measurement indicates that the AgGaGe nS2( n+1) single crystals are p-type semiconductor, and the carrier concentration decreases with the increasing n value. All the transmittances of as-grown AgGaGe nS2( n+1) samples exceeds 60% in the transparent range, especially the transmittance of AgGaGe2S6, is up to 70% at 1064 nm, and the band gap of as-grown crystal increases from 2.85 eV for AgGaGe2S6 to 2.92 eV for AgGaGe5S12. After a thermal annealing treatment, the absorptions at 2.9, 4, and 10 µm have been eliminated, and the band gap changed into the range of 2.89-2.96 eV.