Photoconductivity of the Single Crystals Pb₄Ga₄GeS₁₂ and Pb₄Ga₄GeSе₁₂

Quaternary semiconductor materials of the Pb₄Ga₄GeS(Se)₁₂ composition have attracted the attention of researchers due to possible use as active elements of optoelectronics and nonlinear optics. The Pb₄Ga₄GeS(Se)₁₂ phases belong to the solid solution ranges of the Pb₃Ga₂GeS(Se)₈ compounds which form in the quasi-ternary systems PbS(Se)−Ga₂S(Se)₃−GeS(Se)₂ at the cross of the PbGa₂S(Se)₄−Pb₂GeS(Se)₄ and PbS(Se)−PbGa₂GeS(Se)₆ sections. The quaternary sulfide melts congruently at 943 K. The crystallization of the Pb₄Ga₄GeSe₁₂ phase is associated with the ternary peritectic process L_p + PbSe ↔ PbGa₂S₄ + Pb₃Ga₂GeSe₈ at 868 K. For the single crystal studies, Pb₄Ga₄GeS(Se)₁₂ were pre-synthesized by co-melting high-purity elements. X-ray diffraction results confirm that these compounds possess non-centrosymmetric crystal structure (trigonal symmetry, space group ). The crystals were grown by the vertical Bridgman method in a two-zone furnace. The starting composition was stoichiometric for Pb₄Ga₄GeS₁₂, and the solution-melt method was used for the selenide Pb₄Ga₄GeSе₁₂. Obtained value of the bandgap energy for the Pb₄Ga₄GeS₁₂ and Pb₄Ga₄GeSе₁₂ crystals is 1.86 and 2.28 eV, respectively. Experimental measurements of the spectral distribution of photoconductivity for the Pb₄Ga₄GeS₁₂ and Pb₄Ga₄GeSе₁₂ crystals exhibit the presence of two spectral maxima. The first lies in the region of 570 nm (2.17 eV) and 680 nm (1.82 eV), respectively, and matches well the optical bandgap estimates. The locations of the admixture maxima at about 1030 nm (1.20 eV) and 1340 nm (0.92 eV), respectively, agree satisfactorily with the calculated energy positions of the defects V_S and V_Se.