Selenium vacancies in BaGa4Se7 crystals

Barium gallium selenide (BaGa4Se7) is a recently developed nonlinear optical material with a transmission window extending from 470 nm to 17 μm. A primary application of these crystals is production of tunable mid-infrared laser beams via optical parametric oscillation. Unintentional point defects, such as selenium vacancies, cation vacancies (barium and/or gallium), and trace amounts of transition-metal ions, are present in BaGa4Se7 crystals and may adversely affect device performance. Electron paramagnetic resonance (EPR) and optical absorption are used to identify and characterize active defects in BaGa4Se7 crystals grown at BAE Systems. Five distinct defects, each representing an electron trapped at a selenium vacancy, are observed with EPR (there are seven crystallographically inequivalent selenium sites in this monoclinic crystal). One defect is seen at room temperature before illumination. The other four are seen at lower temperature after exposure to 532 nm laser light. Each singly ionized selenium vacancy has a large, nearly isotropic, hyperfine interaction with 69Ga and 71Ga nuclei at one neighboring Ga site, which indicates a significant portion of the unpaired spin resides in a 4s orbital on this adjacent Ga ion. Optical absorption bands peaking between 430 and 750 nm are produced by the 532 nm light. These photoinduced bands are assigned to the selenium vacancies.