Xizhu Zhao

The search continues for alternative nontoxic n-type electron transport layers in optoelectronic thin-film devices. Indium oxysulfide, In2(O,S)3, represents one promising material for this application, especially when paired with chalcogenide absorber layers. The ternary nature of the composition allows for electrical conductivity and optical bandgap tuning by tailoring the sulfur to oxygen ratio in the oxysulfide alloy. However, thin films of In2(O,S)3 are typically deposited only by chemical bath deposition or plasma-enhanced atomic layer deposition. We report deposition of thin films of In2(O,S)3 in a custom-built thermal reactor using only water vapor and hydrogen sulfide as the coreactants. This advance is enabled by the use of a recently reported, highly reactive indium formamidinate precursor. As shown by x-ray photoelectron spectroscopy, the composition can be tuned from pure In2O3 to pure In2S3 by varying the ratio of cycles employing water or hydrogen sulfide. The oxygen to the sulfur ratio in the film can be controlled by altering the dose sequence, although films typically contain more sulfur than would be expected naively from the percentage of hydrogen sulfide doses in the deposition recipe. Rutherford backscattering spectrometry confirms the composition is sulfur-rich relative to the dosing ratio. Structural characterization indicates films are relatively amorphous in nature. Electrically, these films offer reasonably constant electron mobility at different O:S ratios, with an electron concentration tunable over 4 orders of magnitude. These oxysulfide films possess a higher indirect bandgap than their oxygen-free indium sulfide counterparts, indicating higher transmittance to blue light. These indium oxysulfide films may be suitable candidates for electron transport layers in thin-film solar cells where their wider bandgap might result in higher optical transparency and thus short circuit current density, while the tunability of their conduction band offset with an absorber layer may result in higher open circuit voltage.

Tin monosulfide, SnS, absorbs visible light and holds promise for thin-film photovoltaics. However, the optoelectronic properties of this material vary among the different structural phases, and control over the phase of vapor deposited SnS thin films is not well understood. In order to study the phases and crystallographic orientations of SnS films, films with thicknesses of 90 nm–750 nm were prepared by atomic layer deposition (ALD) at temperatures between 80 °C and 200 °C on amorphous silicon dioxide (a-SiO₂) and single-crystal sodium chloride (NaCl). We show that the crystal structures and orientations of the ALD-SnS thin films vary with deposition temperature, film thickness, and substrate. We confirm the presence of metastable cubic π-SnS in co-existence with the thermodynamically stable orthorhombic α-SnS and find that the π phase is more prevalent at lower deposition temperatures. The films grown on a-SiO₂ are textured, the degree of texturing increases with lower temperature or higher thickness, and the deposited phase is also thickness dependent. Upon annealing, which is known to promote SnS grain growth, all films revert to orthorhombic α-SnS. The films grown on the NaCl(100) substrate exhibit a much higher degree of texturing and show different preferred orientations dependent on the phase: π-(400) and α-(111) or α-(040). In addition, we demonstrate a proof-of-concept device made from the highly oriented SnS grown on NaCl.

Indium oxide is a major component of many technologically important thin films, most notably the transparent conductor indium tin oxide (ITO). Despite being pyrophoric, homoleptic indium(III) alkyls do not allow atomic layer deposition (ALD) of In₂O₃ using water as a co‐precursor at substrate temperatures below 200 °C. Several alternative indium sources have been developed, but none allows ALD at lower temperatures except in the presence of oxidants such as O₂ or O₃, which are not compatible with some substrates or alloying processes. We have synthesized a new indium precursor, tris(N,N′‐diisopropylformamidinato)indium(III), compound 1, which allows ALD of pure, carbon‐free In₂O₃ films using H₂O as the only co‐reactant, on substrates in the temperature range 150–275 °C. In contrast, replacing just the H of the anionic iPrNC(H)NiPr ligand with a methyl group (affording the known tris(N,N′‐diisopropylacetamidinato)indium(III), compound 2) results in a considerably higher and narrower ALD window in the analogous reaction with H₂O (225–300 °C). Kinetic studies demonstrate that a higher rate of surface reactions in both parts of the ALD cycle gives rise to this difference in the ALD windows.