Sulfide compounds and mixed anion oxide-sulfide materials have potential as solar absorbers or transparent contacts. Improved techniques for depositing these materials in thin-film form are necessary to obtain greater compositional and phase control. Controlling the metal-sulfur ratio in sulfides and the oxygen-sulfur ratio in oxide-sulfides is a dominant challenge to thin film growth of these material systems.
Here, we report a deposition method with improved control over the sulfur content in thin-films through the addition of a radio frequency (RF) solids atom source (cracker) to a multiple-source sputtering system. This technique has enabled combinatorial growth of both sulfide and oxide-sulfide materials in thin film form. The growth conditions for sulfur containing compounds can quickly be refined using this system for combinatorial synthesis. Co-sputtering from one or two targets provides a compositional gradient across a stationary substrate.
In addition, a temperature gradient that is orthogonal to the composition gradient is induced across the substrate. An RF solids cracker is used to provide controllable amounts of activated sulfur across the entire substrate during the deposition. Typically, RF solids crackers are used in molecular beam epitaxy systems where the usual operating pressure is 10 to 10 Torr.
Here, we employ a RF solids cracker as an addition to our sputtering system where the typical operating pressure is 3 mTorr. Together, the composition gradient, orthogonal temperature gradient and activated sulfur source can be used concurrently to control the composition and phase of the deposited thin films. For this work, all films were deposited on 2”x2” glass substrates at a chamber pressure of 3 mTorr with only argon flowing as a process gas. The temperature gradient was 485°C to 375°C across 2”. The film composition was measured using both Rutherford backscattering spectrometry and x-ray fluorescence. X-ray diffraction was used for structural and phase determination.
The growth of sulfides is demonstrated using CuS and the growth of oxide-sulfides is shown with the BiOS system. In particular, CuS has been grown from both metallic Cu and ceramic CuO targets. BiOS films with tunable oxygen to sulfur ratios were grown from a BiO target. Further, the independent tuning of anion and cation ratios with this system is demonstrated by the growth of BiCuOS. The successful growth of both sulfide and oxide-sulfide compounds demonstrates the viability of this hybrid approach. These results also suggest that similar approaches with phosphides, oxide-phosphides and phosphide-sulfides would be achievable with hybrid deposition systems.
This work is supported by the U.S. Department of Energy, Office of Science, BES, under Contract No. DE-AC36-08GO28308 to NREL as part of the DOE Energy Frontier Research Center "Center for Inverse Design"
National Renewable Energy Lab, University of Colorado, Boulder
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