X-ray photoelectron spectroscopy study of lithium thio-germanate thin films

In this study, the compositions of and chemical speciation in lithium thio-germanate thin films grown by radio frequency (RF) magnetron sputtering were determined by means of X-ray photoelectron spectroscopy (XPS). Three different targets, nLi₂S + GeS₂ (n = 1, 2, and 3) for RF sputtering were prepared. Thin films of ∼1 μm thickness were grown on single crystal Si [111] substrates using 50 W power and ∼3.3 Pa Ar gas atmospheres. As is commonly observed for RF sputtered non-oxide films, the surfaces of these lithium thio-germanate films were contaminated with both C and O at ∼10 at %. Etching the top surface of the films with Ar removed all of the surface carbon and reduced the oxygen contamination down to an average value of ∼5 at %. After such surface etching, the compositions of the films were found to agree very closely with the nominal composition of the target from which it was made. The Li₂GeS₃ (n = 1) thin film was found as expected to exhibit a 2:1 non-bridging sulfur (Ge-S^-Li ⁺, NBS) to bridging sulfur (Ge-S-Ge, BS) ratio. As the Li ₂S content increased, the Li₄GeS₄ and Li ₆GeS₅ thin films, n = 2 and 3, exhibited as expected, 100% NBS. For the S2p core peaks, the binding energy of S in GeS₂ is the highest due to the nearly covalent bonding in the BS structure, Ge-S-Ge. As the Li₂S content increased, the binding energy of S in the thin films progressively decreased due to the progressive increase in the fraction (and number) of NBS (more ionic), Ge-S^-Li⁺, in the films.