Atomic layer deposition of crystalline Bi2O3 thin films and their conversion into Bi2S3 by thermal vapor sulfurization

Abstract

Crystalline Bi₂O₃ thin films have been grown on Si and quartz substrates at 300 °C in an atomic layer deposition (ALD) chamber using Bi(thd)₃ (thd: 2,2,6,6-tetramethyl-3,5-heptanedionato) and H₂O as precursors. Post-growth thermal sulfurization of such Bi₂O₃ films results in orthorhombic Bi₂S₃ at elevated temperatures. Atomic-force microscopy reveals that the surface roughness of Bi₂O₃ increases with growth cycles, and the roughness of Bi₂O₃/Si is generally larger than those of Bi₂O₃/quartz grown under the same conditions. However, after sulfurization, the surface morphologies became nearly similar in spite of their different substrates and/or thicknesses. The evolution of X-ray diffraction patterns as a function of growth cycles provide evidence that the growth starts with nonstoichiometric β-phase Bi₂O_2.3, and then transfers to α-Bi₂O₃ with the increase in growth cycles; the β-to-α growth transition occurs earlier on Si than that on a quartz substrate. Optical absorption spectroscopy reveals a bandgap of 3.0–3.5 eV for the as-grown Bi₂O₃ thin films, which is narrowed to 1.5 eV for the resultant Bi₂S₃ after sulfurization. X-ray photoelectron spectroscopy indicates that both the as-grown Bi₂O₃ and the sulfurized Bi₂S₃ thin films are n-type semiconductors with the valence band maxima located at about 2.1 and 1.0 eV below their Fermi levels, respectively. These observations, together with the large absorption coefficient of Bi₂S₃ in the wavelength range of visible light, suggest a significant potential for Bi₂S₃ thin films and/or Bi₂S₃/Bi₂O₃ heterojunctions in various optoelectronic devices, e.g., for solar energy conversions.