Differences in the Surface Charging at the (100) and (110) Surfaces of Li 2 B 4 O 7
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Differences in the Surface Charging at the (100) and (110) Surfaces of Li2B4O7 David Wooten,1 I. Ketsman,2 Jie Xiao,2 Ya. B. Losovyj,2,3 J. Petrosky,1 J. McClory,1 Ya. V. Burak,4 V.T. Adamiv,4 and P.A. Dowben2 1
Air Force Institute of Technology, 2950 Hobson Way, Wright Patterson Air Force Base, OH 45433-7765, U.S.A. 2
Department of Physics and Astronomy and the Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln, P.O. Box 880111, Lincoln, NE 68588-0111, U.S.A. 3
J. Bennett Johnston Sr. Center for Advanced Microstructures and Devices, Louisiana State University, 6980 Jefferson Highway, Baton Rouge, LA 70806, U.S.A. 4
Institute of Physical Optics, Dragomanov 23, Lviv 79005, Ukraine.
ABSTRACT From angle resolved photoemission, the (100) surface termination of Li2B4O7 is significantly more polar than the (110) surface termination although the accepted dipole orientation of this pyroelectric crystal is along (001). Consistent with the surface termination, the surface charging at the surface of (100) is significantly greater than observed at (110) and plays a role in the surface photovoltage effects. Because of the different interfaces formed, device properties likely depend upon crystal faces of lithium borate. INTRODUCTION With the fabrication of a semiconducting boron carbide, a material suitable for the fabrication of solid state neutron detectors [1-7], there has been a resurgence in the development of boron based semiconductors for neutron detection. In addition to the boron carbides [1-7], possible boron based semiconductors for solid state neutron detectors include boron nitrides [8,9], boron phosphides [10-11], and the lithium borates [12,13]. Although Li2B4O7 lithium borate has a much larger (6.3 to 9.3 eV) band gap [14,15] than the boron carbides, boron nitrides, or boron phosphides, this class of materials has distinct advantages. The lithium borates are typically both translucent (when undoped) and can be isotopically enriched to a high degree. Li2B4O7 can be isotopically enriched to 95 at% 6Li and 97.3 at% 10B [13] from the natural 7.4 at% 6Li and 19 at% 10B. Since lithium tetraborate (Li2B4O7) single crystals are pyroelectric and piezoelectric [16], surface termination and interface effects must also be seriously considered in solid state device design. This is a critical consideration as doping will serve to enhance differences along different crystal directions [17]. Doping of the Li2B4O7 is certainly possible [13], and essential for solid state neutron detection applications, given the very high resistivities of undoped crystals. These undoped resistivities are on the order of 1010 Ω.cm [12].
EXPERIMENT The Li2B4O7 single crystals were grown from the melt by the Czochralski technique as described elsewhere [13,15]. Clean surfaces were prepared by several methods including resistive heating and combinations of sputtering and subsequent annealing. The electronic structure and stoichiometry were similar in all cases. Combined ultraviolet photoemission (UPS) and
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