ECASIA SPECIAL ISSUE PAPER Work function and negative electron affinity of ultrathin barium fluoride films Alessio Mezzi 1 | Eleonora Bolli 1,2 | Saulius Kaciulis 1 | Matteo Mastellone 3,4 | Marco Girolami 3 | Valerio Serpente 3 | Alessandro Bellucci 3 | Riccardo Carducci 3,5 | Riccardo Polini 3,5 | Daniele M. Trucchi 3 1 Institute for the Study of Nanostructured Materials, ISMN-CNR, Rome, Italy 2 Department of Industrial Engineering, University of Rome “Tor Vergata” , Rome, Italy 3 Institute of Structure of Matter, ISM-CNR, Rome, Italy 4 Department of Basic and Applied Sciences for Engineering, University of Rome “La Sapienza” , Rome, Italy 5 Department of Chemical Science and Technologies, University of Rome “Tor Vergata” , Rome, Italy Correspondence Alessio Mezzi, Institute for the Study of Nanostructured Materials, ISMN-CNR, Rome 00015, Italy Email: alessio.mezzi@cnr.it Funding information EU research and innovation program Horizon 2020, FET-OPEN, Grant/Award Number: 737054 Thin films of barium fluorides with different thicknesses were deposited on GaAs substrate by electron beam evaporation. The aim of the work was to identify the best growth conditions for the production of coatings with a low work function suitable for the anode of hybrid thermionic-photovoltaic (TIPV) devices. The chemical composition and work function ϕ of the films with different thicknesses were investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet pho- toelectron spectroscopy (UPS). The lowest value of ϕ = 2.1 eV was obtained for the film with a thickness of 2 nm. In the valence band spectra of the films at low kinetic energy, near the cutoff, a characteristic peak of negative electron affinity was present. This effect contributed to a further reduction of the film's work function. KEYWORDS barium fluorides, negative electron affinity, UPS, work function, XPS 1 | INTRODUCTION In the last years, following the increasing interest on electron-emission- based devices, 1 a hybrid thermionic-photovoltaic (TIPV) converter was proposed and developed in the frame of European H2020 project AMADEUS for the conversion of stored thermal energy. 2 A TIPV device is constituted of a hot thermionic cathode, able to emit electrons and photons at an operating temperature in the range of 1700–2000 K, and a cooled thermionic anode coupled with a thermo-photovoltaic (TPV) cell. The anode has to guarantee an efficient collection of electrons and the optical transparency to the infrared (IR) radiation emitted by the cathode and absorbed by the TPV cell (made of GaAs or InGaAs) that could convert the absorbed radiation into electricity. 3 Generally, in order to improve the thermionic device performance, the materials with a low work function ϕ must be selected for both cathode and anode. 4,5 For a TIPV device, the thermionic anode must be developed taking into consideration three specific characteristics: (1) ϕ lower than that of the thermionic cathode; (2) capability to collect the electrons thermally emitted by thermionic emitter; (3) optical transparency for the black- body radiation from the emitter. A good strategy for this development is represented by the deposition of a functional coating on theTPV sur- face. Usually, alkali metal and their oxides (e.g., Cs and Cs 2 O) were used for reducing the work function of typical substrates of thermionic appli- cations, such as refractory metals. 6,7 Alternatively, barium compounds are more stable at high temperatures than cesium ones. In particular, barium fluoride (BaF 2 ) was found to be an interesting alternative for the role of collector coating on GaAs-based TPV due to its high thermal resistance, 8,9 over 600  C. The deposition of BaF 2 on GaAs substrates by molecular beam epitaxy (MBE) was reported in literature 10 for realiz- ing a dielectric layer for microelectronics. Moreover, BaF 2 demonstrated a low work function (ϕ = 2.7 eV) as top layer in three-layer cathodes 11 and on silicon, 12 but according to our knowledge not on GaAs. In the present work, thin films of BaF 2 were deposited on GaAs substrates by electron beam evaporation. The nominal thickness of Received: 3 October 2019 Revised: 2 April 2020 Accepted: 26 May 2020 DOI: 10.1002/sia.6832 Surf Interface Anal. 2020;1–7. wileyonlinelibrary.com/journal/sia © 2020 John Wiley & Sons, Ltd. 1