Atomic Layer Deposition of p-Type Bi2S3

Atomic layer deposition (ALD) of bismuth sulfide (Bi2S3) is demonstrated by the sequential exposure of bismuth(III) bis(2,2,6,6-tetramethylheptane-3,5-dionate) [Bi(thd)(3)] and hydrogen sulfide (H2S) at 200 degrees C. A saturated growth rate of 0.34-0.37 angstrom/cycle was observed by in situ quartz crystal microbalance (QCM) and verified by ex situ X-ray reflectivity (XRR) measurements throughout the ALD temperature window. As-deposited Bi2S3 films were found to be polycrystalline in nature without any preferential orientation. In addition to the direct band gap at ca. 1.56 eV normally seen for Bi2S3, we also found evidence for an indirect band gap at ca. 1.03 eV. Ultraviolet photoelectron spectroscopy (UPS) and Seebeck measurements strongly support degenerate p-type conductivity of the as-grown thin films, in contrast to the n-type nature normally found in the literature. Temperature-dependent (70-300 K) electrical resistivity measurements showed that, in the temperature range of 70-100 K, variable-range hopping (VRH) is the dominant carrier-transport process whereas, above 100 K, clear deviations from the VRH transport equation were observed, implying a crossover from localized states to a band-like transport process