Ultra-thin films of barium fluoride with low work function for thermionic-thermophotovoltaic applications

The deposition of barium fluoride thin and ultra-thin films on gallium arsenide substrates was performed by electron beam evaporation for analyzing the influence of film thickness and chemical composition on the work function of the resulting heterostructure. X-ray photoemission spectroscopy combined with ultraviolet photoemission spectroscopy measurements reveals that films of 2 nm nominal thickness and Ba/F = 1.0 stoichiometry ratio induce the achievement of a significantly low work function of 2.1 eV to the BaF/GaAs heterostructure. The significant reduction of the work function at least down to 3.0 eV is confirmed by a test thermionic converter operating at a cathode temperature of 1385 °C, where the heterostructure was applied as anode. The low work function, together with a negligible optical absorption, makes feasible the practical application of barium fluoride coatings on GaAs within hybrid thermionic-thermophotovoltaic devices.