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/cm(2) at 175 degrees C on a gold-covered QCM sensor. Using a film density of 5.07 g/cm(3) determined by X-ray reflectivity measurements, these mass gains are equivalent to SnS ALD growth rates of 0.22-0.24 angstrom/cycle. The ratio of the mass loss and mass gain ratio (vertical bar Delta m(2)/Delta m(1)vertical bar) from the H2S and Sn(acac)(2) reactions was vertical bar Delta m(2)/Delta m(1)| similar to 0.32 at 175 degrees 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 125-225 degrees C. X-ray fluorescence studies confirmed a Sn/S atomic ratio of similar to 1.0 for the SnS ALD films. X-ray photoelectron spectroscopy measurements revealed that the SnS ALD films contained oxygen impurities at 15-20 at% after air exposure. These oxygen-containing SnS ALD films displayed a bandgap of similar to 1.87 eV that is higher than the SnS bulk value of similar to 1.3 eV.