Thin film solar cells made from earth-abundant, non-toxic materials are needed to replace the current technology that uses Cu(In,Ga)(S,Se) 2 and CdTe, which contain scarce and toxic elements. One promising candidate absorber material is tin monosulfide (SnS). In this report, pure, stoichiometric, single-phase SnS films were obtained by atomic layer deposition

Atomic layer deposition (ALD) of tin oxide (SnO x ) films was achieved using a newly synthesized tin precursor and hydrogen peroxide. We obtained highly pure, conductive SnO x films at temperatures as low as 50 °C, which was possible because of high chemical reactivity between the new Sn precursor and hydrogen peroxide. The growth per cycle is around 0.18 nm/cycle in the ALD window up to 150 °C, and decreased at higher temperatures. Self-limited growth was demonstrated for both the Sn and O precursors. Thickness is linear in the number of cycles, with an induction period of not more than a few cycles. Rutherford backscattering spectroscopy (RBS) and X-ray photoelectron spectroscopy (XPS) measurements showed that the composition ratio of O/Sn is ∼2 and that the films do not contain any detectable carbon or nitrogen impurities. X-ray and electron diffraction analyses identified crystallites of SnO2 with the rutile structure and average grain size 5−10 nm. The density of the films is 83% of the bulk rutile phase. The surfaces are very smooth, with roughness about 3% of the film thickness. The lowest resistivity is about 10−2 ohm·cm. The mobility is over 7 cm2/V·s, and the free electron concentration reaches nearly 1020 cm−3. The dependence of mobility on temperature suggests that grain boundary scattering is the dominant electron scattering mechanism. The optical transmission of a 100 nm film is 87.8% and its absorption is 3.3% when averaged over the wavelengths from 400 to 800 nm. Over 80% uniformity of thickness was achieved inside holes with aspect ratios up to 50:1. This successful low temperature growth of conductive nanocrystalline SnO x films by ALD allows it to be exploited in transparent electrodes for displays, organic light emitting diodes, solar cells, conductive and protective coatings on plastic, microchannel electron multiplier plates, or as a semiconductor layer in transparent transistors.

Abstract(Sn,Al)O x composite films with various aluminum (Al) to tin (Sn) ratios were deposited using an atomic layer deposition technique. The chemisorption behavior of cyclic amide of tin(II) and trimethylaluminum were analyzed by Rutherford backscattering spectroscopy. Both precursors showed retarded and enhanced chemisorption on Al 2 O 3 and SnO 2 surfaces, respectively. The films show highly anisotropic electrical conductivity, i.e. much higher resistivity in the direction through the film than parallel to the surface of the film. The cause of the anisotropy was investigated by cross-sectional transmission electron microscopy, which showed a nanolaminate structure of crystalline SnO 2 grains separated by thin, amorphous Al 2 O 3 monolayers. When the Al concentration was higher than ~35 at.%, the composite films became amorphous, and the vertical and lateral direction resistivity values converged toward one value. By properly choosing the ratio of SnO 2 and Al 2 O 3 subcycles, controlled adjustment of film electrical resistivity over more than 15 orders of magnitude was successfully demonstrated.

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