SummaryThis paper describes the atomic layer deposition of In2(S,O)3 films by using In(acac)3 (acac = acetylacetonate), H2S and either H2O or O2 plasma as oxygen sources. First, the growth of pure In2S3 films was studied in order to better understand the influence of the oxygen pulses. X-Ray diffraction measurements, optical analysis and energy dispersive X-ray spectroscopy were performed to characterize the samples. When H2O was used as the oxygen source, the films have structural and optical properties, and the atomic composition of pure In2S3. No pure In2O3 films could be grown by using H2O or O2 plasma. However, In2(S,O)3 films could be successfully grown by using O2 plasma as oxygen source at a deposition temperature of T = 160 °C, because of an exchange reaction between S and O atoms. By adjusting the number of In2O3 growth cycles in relation to the number of In2S3 growth cycles, the optical band gap of the resulting thin films could be tuned.
Thin films of Zn(O,S) were deposited by atomic layer deposition from diethylzinc, water (H2O), and hydrogen sulfide (H2S). First, a study on the influence of the H2S/(H2O+H2S) pulse ratio from pure ZnO to pure ZnS was performed at deposition temperature Tdep=120 and 200 °C. Zn(O,S) films had higher S content than expected, and this effect was stronger at Tdep=200 °C. Then, Zn(O,S) films have been synthesized over the range of temperature 120–220 °C at the constant H2S/(H2O+H2S) pulse ratio of 9%. For Tdep<180 °C, high and almost constant S content has been measured in the films. The significant increase of the S/(O+S) atomic ratio for Tdep>180 °C confirmed that exchange reactions occurred between the Zn(O,S) growing films and H2S. The grazing incidence x-ray diffraction patterns showed Zn(O,S) films with hexagonal wurtzite structures and with an optimum crystallization for temperatures Tdep=160–180 °C. Indeed, in this temperature range, well crystallized and large grains were obtained which was in good correlation with the film morphology determined by scanning electron microscope; and Hall effect measurements revealed low resistivities, high carrier concentrations (>1019 cm−3), and low mobilities. From these results, the authors propose the existence of a temperature range where the properties undergo significant changes while the atomic composition remains constant.
A comparative chemical analysis of InxSy and In2(S,O)3 thin films grown by atomic layer deposition (ALD) and plasma-enhanced ALD, respectively, was performed to understand the challenges and issues related to the assistance of plasma, especially for the implementation of these films as ultrathin (<50 nm) interfacial buffer layers in copper indium gallium diselenide (CIGS) solar cells. The films were synthesized using indium acetylacetonate [In(acac)3], hydrogen sulfide, and an Ar/O2 plasma as indium, sulfur, and oxygen precursors. Film growth mechanisms and chemistries were studied using gas phase measurements by quadrupole mass spectrometry and x-ray photoelectron spectroscopy for surface and in-depth characterizations. Distinctive signatures of thermal and plasma processes on the overall compositions of the films were evidenced, which were further discussed and explained. Added to this, the impact of the plasma on the underlying substrate, using silicon as a reference, was further investigated to identify its modification. This extensive study has led to a readjustment of the deposition conditions of In2(O,S)3 thin films and allowed promising implementation as buffer layers in CIGS solar cells.
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