Scientists at the Japan Institute for Physical and Chemical Research (RIKEN) have created a thin film of copper iodide. The atomically flat sample will lay the foundation for the production of better quality semiconductors.
Semiconductors are at the heart of many optoelectronic devices, including lasers and LEDs. Engineers have long wanted to use copper iodide (an example of a halide compound) in semiconductors. This compound is excellent at conducting signals and is stable above room temperature. The problem is that it is difficult to make a really thin film of copper iodide without impurities. The usual method is to apply a film from a solution. “But the dissolution process is not suitable for creating a high quality thin copper iodide film,” explains Masao Nakamura of the RIKEN Center for New Materials Research.
Instead, Nakamura and his collaborators used an alternative method, molecular beam epitaxy, in which a film is gradually grown over a substrate at elevated temperatures and in a vacuum. Molecular beam epitaxy is already widely used in semiconductor manufacturing. However, this method is difficult to use for copper iodide. The fact is that this material is very volatile and easily evaporates during the process, and does not settle in the form of a film. To solve the problem, scientists tried growing the film at a lower temperature and then increasing it. It was this two-stage process that proved to be the most effective, the author of the study notes.
To improve the quality of the film, the scientists used indium arsenide as a substrate. Its structure is similar to the lattice of copper iodide. This is important, because if the lattice spacing is not matched, many defects are formed in the material.
The authors of the development checked the purity of their sample using photoluminescence spectroscopy. This method involves shooting photons (or light particles) at the surface of the material. They are absorbed by the material, exciting its electrons to a higher energy state and causing them to emit new photons.
Monitoring the light emitted allowed the team to determine that they had created a defect-free monocrystalline film.