Bridging experiment and computation to discover exotic thin-film semiconductors
Andrea Crovetto, Technical University of Denmark, Denmark
Experimental exploration of new semiconductor materials has followed a somewhat predictable path, dominated by charge-neutral elemental substitutions from single-element semiconductors (Si and Ge) to binaries (III-V and II-VI) to ternaries (I-III-VI2 and II-IV-V2 chalcopyrites, I-II-VII3 perovskites, etc.) to quaternaries (I2-II-IV-VI4 kesterites, I2-I-III-VII6 double perovskites, etc.). In addition, exploration of ternary and quaternary compounds is heavily biased towards single-anion, multi-cation materials.
An exciting opportunity to discover radically different types of semiconductors is to look at phosphorus-containing compounds. Phosphorus is one of the most versatile elements in the periodic table, as it can take a range of positive and negative oxidation states (+5 to -3) giving countless possibilities for materials discovery. In this talk, I will focus on two classes of P-containing compounds.
The first class is “P-rich phosphides”. In stark contrast to conventional semiconductors, P-rich phosphides contain bonds between nonmetallic atoms (here, phosphorus-phosphorus bonds). I will present the first successful thin-film synthesis [1] of any polycrystalline P-rich phosphide. The synthesized material is CuP2, a 1.5 eV band gap semiconductor with strong optical absorption and native p-type doping in an attractive range for thin-film heterojunction solar cells.
A second intriguing family of materials can be obtained by combining phosphorus with a more electropositive and a more electronegative species. Of particular interest are “phosphosulfides”, where sulfur is the more electronegative species. Many phosphosulfides are predicted to be stable semiconductors with direct band gaps in the visible and disperse band edges. Yet, there are less than five reports of phosphosulfides in thin-film form and hardly any optoelectronic characterization [2].
We have been studying these exotic semiconductors with an integrated experimental/computational work strategy inspired by the FAIR data principles. Density functional theory calculations indicate that many more phosphosulfide compounds should be thermodynamically stable than previously thought, including materials with previously unreported stoichiometric coefficients and structures.
Backed by a unique suite of combinatorial thin-film deposition setups with access to S and P sources, we have explored selected phosphosulfide phase diagrams by high-throughput experiments and we can now report the first thin-film synthesis of various compounds. Some of these semiconductors appear to be of high optoelectronic quality as inferred by measured photoluminescence decay times above 100 ns, which are in the same order of magnitude as state-of-the-art conventional direct band gap semiconductors.
References
[1] A. Crovetto, D. Kojda, F. Yi, K. N. Heinselman, D. A. LaVan, K. Habicht, T. Unold, A. Zakutayev, Journal of the American Chemical Society 2022, 144, 13334.
[2] L. A. Mittmann, A. Crovetto, J. Phys. Mater. 2024, 7, 021002.