Investigations of the semi-insulating LiIn_(1-x)Ga_(x)Se_(2) solid solution for neutron detection

Abstract

Synthesis and single crystal growth methods for Li-containing chalogenides, specifically LiIn1-xGaxSe2, single crystals are discussed. This study elucidates the possibility of improving neutron detection by reducing the indium capture contribution; with the incorporation of the lithium-6 isotope, gallium substitution may overcome the neutron detection efficiency limitation of 6LiInSe2 due to predominant capture by the indium-115 isotope. As a figure of merit, the ternary parent compounds LiInSe2 and LiGaSe2 were included in this study. Quality crystals can be obtained utilizing the vertical Bridgman method producing quaternary compounds with tunable optical properties. Quaternary crystals of varying quality depending on gallium concentration, approximately 5 x 5 x 2 mm3 or larger in volume, were harvested, analyzed and revealed tunable absorption characteristics between 2.8-3.4 eV. The electronic structure and optical properties of the LiIn1–xGaxSe2 (x=0, 0.25, 0.5, 0.75, 1) solid solution were studied by density functional theory (DFT) with pure functionals. The exchange-correlation is treated within the local density approximation (LDA) and generalized-gradient approximation (GGA). The electronic structures for each respective compound are discussed in detail. Calculations reveal that gallium incorporation can be used to tune the optical-electrical properties of the solid solution and correlates with the lattice parameter. The band gap trend of the LiIn1–xGaxSe2 system follows a nonlinear behavior between the LiInSe2 and LiGaSe2 ternary boundaries. The bowing parameter is estimated to be on the order of 0.1–0.3eV at the -point. Low-temperature optical absorption revealed a 30% change in the temperature dependence of the band gap for the intermediate compound LiIn0.6Ga0.4Se2 compared to ternary boundaries and suggests the heat capacity to be another control element through strain.