Cathodoluminescence and Photoemission of Undoped and Mn Doped Lithium Tetraborate

Lithium tetraborate (Li2 B4 O7) crystals are being developed for possible use in solid state neutron detectors. The 6Li(n,a) 3H and 10 B(n,a)7 Li reactions are the basis for neutron detection, and a Li2 B4 O7 crystal enriched with Mn should show improved efficiency for neutron detection. There is a lack of fundamental characterization information regarding this useful material, particularly with regard to its electronic configuration. Photoemission spectroscopy was used to determine the energy level structure of the Mn doped Li2 B4 O7 crystals. Measurements were made using ultraviolet photoemission spectroscopy (UPS) using synchrotron radiation on two different beamlines. The first was the 3 m toroidal grating monochromator (TGM) beam line confirming the presence of Mn. Secondly, the normal incident monochromator (NIM) beamline studied the band gap states. Measurements were made at elevated sample temperatures to reduce charging effects. Photoemission studies provided evidence of Mn in the bulk crystal at 47.2 eV. Valence band analysis provided the presence of surface states but no acceptor sites. Cathodoluminescence studies were performed on undoped and Mn doped at beam energies from 5 to 10 KeV and at room temperature. Self trapped exciton emission states are evident in the undoped and Mn doped Li2B4O7 sample ranging in energies from 3.1 to 4.1 eV.