Chemical environment and functional properties of highly crystalline ZnSnN2 thin films deposited by reactive sputtering at room temperature

Zinc tin nitride (ZnSnN2) thin films have been deposited on glass and silicon substrates using a reactive co-sputtering process. Although the deposition temperature was limited to the room temperature, the films show a highly crystallization level and a strong preferred orientation in the [001] direction. The film composition, measured using energy dispersive X-ray spectroscopy and electron probe microanalysis, indicates a possible tin understoichiometry (or a zinc and a nitrogen overstoichiometry). As confirmed by transmission electron microscopy, the main oxygen contamination of the films results from oxidation of the grains boundaries after air exposure of the samples. X-ray photoelectron spectroscopy and Mossbauer spectrometry have been used to determine the chemical environment of atoms in the ZnSnN2 crystals. Both methods confirm that Sn4+ ions are bonded to nitrogen atoms and that the oxygen contamination results in the formation of Sn2+ ions. Zinc tin nitride exhibit an electron mobility at room temperature close to 3.8 cm(2) V-1 s(-1) and an optical band gap of 1.8 eV as measured independently from UV-visible spectrometry and ellipsometry. The results obtained in the present study confirm the suitability of ZnSnN2 thin films as an Earth abundant material for absorber layer in photovoltaic devices.