Work function and negative electron affinity of ultra-thin barium fluoride films

In the last years, a hybrid thermionic-photovoltaic (TIPV) direct converter has been proposed and developed for the conversion of concentrated solar energy in the frame of European H2020 project AMADEUS. In order to improve the device performance, the active materials must be characterized by a low work function. Not only, for the thermionic anode must be selected a material with specific characteristics: 1) work function lower than that of the cathode; 2) optical transparency to the blackbody radiation emitted by the emitter. A good strategy is represented by the deposition of a functional coating on the TPV surface. For this reason, BaF2 thin films have been deposited on GaAs substrates by electron-beam thermal evaporation. The thickness of the BaF2 coating has been varied from hundreds of nm down to 1 Å, in order to select the sample with the best performance. The influence of the thickness on the chemical composition and on the work function has been investigated by XPS and UPS, respectively. In particular, the work function values have been calculated from the signal cut-off in acquired He I valence band spectra. In order to avoid the low kinetic energy cut-off caused by the spectrometer, a negative bias V has been applied to the samples. The obtained results revealed that a low work function of 2.1 eV can be reached, when the thickness of the film is of ≈ 1 nm, which is a promising value for a practical application in hybrid TIPV devices. A very interesting effect appeared in the ultra-thin layers of BaF2 with a thickness of only few Å:a new peak positioned at low kinetic energy, near the signal cut-off, appeared in the valence band spectra. This result could be interpreted by considering that conductive band maximum moves above the vacuum level, resulting in negative electron affinity.