Atomic layer deposition of copper sulfide thin films

Atomic Layer Deposition (ALD) of copper sulfide (Cu_xS) thin films from Cu(acac)₂ (acac = acetylacetonate = 2,4-pentanedionate) and H₂S as Cu and S precursors is reported. Typical self-saturated reactions ("ALD window") are obtained in the temperature range T_dep = 130-200 °C for an average growth per cycle (GR) = 0.25 Å/cycle. The morphology, crystallographic structure, chemical composition, electrical properties and optical band gap of thin films were investigated using scanning electronic microscopy (SEM), X-ray diffraction under Grazing Incidence conditions (GI-XRD), X-ray reflectivity (XRR), energy dispersive spectrometry (EDS), Hall effect measurements, and UV-vis spectroscopy. The obtained copper sulfide films are heavily p-doped (charge carrier concentration ~ 10²¹ -10²² cm^{- 3}) with optical band gaps in the range of 2.2-2.5 eV for direct and 1.6-1.8 eV for indirect band gaps. Depending on the number of ALD cycles, multiphase compounds (made of digenite Cu_1.8S, chalcocite Cu₂S, djurleite Cu₃₁S₁₆ and covellite CuS) or single-phase digenite Cu_1.8S films are obtained via a growth mechanism that involves in-situ copper reduction and loss of sulfur by evaporation.