Tin Monosulfide Thin Films Grown by Atomic Layer Deposition Using Tin 2,4-Pentanedionate and Hydrogen Sulfide

Tin monosulfide (SnS) was grown by atomic layer deposition (ALD) using sequential exposures of tin(II) 2,4-pentanedionate (Sn(acac)2) and hydrogen sulfide (H2S). In situ quartz crystal microbalance (QCM) studies showed that the SnS ALD mass gain per cycle was 11−12 ng/cm2 at 175 °C on a gold-covered QCM sensor. Using a film density of 5.07 g/cm3 determined by X-ray reflectivity measurements, these mass gains are equivalent to SnS ALD growth rates of 0.22−0.24 Å/cycle. The ratio of the mass loss and mass gain (|Δm2/Δm1|) from the H2S and Sn(acac)2 reactions was |Δm2/Δm1| ∼ 0.32 at 175 °C. This measured ratio is close to the predicted ratio from the proposed surface chemistry for SnS ALD. The SnS ALD was self-limiting versus the Sn(acac)2 and H2S exposures. The SnS ALD growth rate was also independent of substrate temperature from 125 to 225 °C. The SnS ALD growth on Al2O3 ALD substrates displayed nucleation problems and smaller growth rates. These differences may be caused by site blocking by the Al(acac)* surface species. X-ray fluorescence studies confirmed a Sn/S atomic ratio of ∼1.0 for the SnS ALD films. X-ray photoelectron spectroscopy measurements revealed that the SnS ALD films contained oxygen impurities at 15−20 atom % after air exposure. These oxygen-containing SnS ALD films displayed a band gap of ∼1.87 eV that is higher than the SnS bulk value of ∼1.3 eV. In addition, these SnS ALD films produced very weak photoluminescence at room temperature. SnS ALD may be useful to fabricate photovoltaic or solar conversion devices.