Physicists from RIKEN have taken a step to enhance semiconductor performance, developing a single-crystal thin film of cuprous iodide. The film is atomically flat and free of any defects. Conventional approaches for fabricating thin films of cuprous iodide without any impurities typically necessitate depositing the film from a solution. Using cuprous iodide, however, which is a halide compound and a highly effective conductor and is stable above room temperature, a solution-based process is unable to generate a high-quality thin film. A team led Masao Nakamura from the RIKEN Center for Emergent Matter Science instead used molecular beam epitaxy. In the technique, a film is grown on top of a substrate, in a vacuum, and at an elevated temperature. Though the technique is commonly used in semiconductor manufacturing, it is difficult to use for cuprous iodide. The material is highly volatile, meaning that it evaporates easily in the epitaxial process and does not settle easily into the structure of the film. The team overcame that method by first growing its film at a lower temperature and then increasing it gradually in a two-step process. A thin film of cuprous iodide crystals (blue/purple) on an indium arsenide substrate (yellow). The sample’s purity was tested by shining photons onto the surface to create electron-hole pairs (red and blue spheres) and monitoring the light that was emitted (white rays). Courtesy of RIKEN Center for Emergent Matter Science. Another essential characteristic of the work involved the use of indium arsenide at its substrate; the physical spacing of indium arsenide lattices are similar to those of cuprous arsenide. A lattice spacing mismatch, Nakamura said, causes defects to form in the material. To test the purity of their sample, the physicists fired photons at the material’s surface using photoluminescence spectroscopy. The material absorbed the photons, causing it to excite its electrons to a higher energy state. That excitation caused the electrons to emit new photons. By then monitoring the behavior of the emitted light, the team determined that its process successfully generated a single-crystal film without defects. The team said it now plans to combine semiconductors made of different halides to look at the unknown properties that could emerge. Doing so, Nakamura said, will enable the exploration of novel functionalities and physics at the halide interfaces. The research was published in Applied Physics Letters (www.doi.org/10.1063/5.0036862).