Vapor Growth of Indium Iodide (Inl) on the International Space Station

Physical vapor transport (PVT) of indium monoiodide (InI) was conducted at the International Space Station. The flight experiments were preceded by ground-based crystal growth experiments in order to optimize the applied thermal profiles and other processing conditions. InI is part of a group of heavy metal iodides that are promising as room temperature gamma-ray and X-ray radiation detectors. Advantages of InI are it’s relatively high atomic number and density, to provide high stopping efficiency, and it’s large bandgap (2.0 eV), to give high electrical resistivity and low leakage current. It has low toxicity, a low melting point temperature (365 °C), no solid phase transition, and no tendency to form polytypes. InI has been previously grown by Bridgman and Czochralski crystal growth methods. Although PVT is slower than these melt growth techniques, it has potential advantages in that it should reduce inclusions and impurities since it is based on sublimation, reduce intrinsic defects due to lower growth temperatures, and reduce dislocation densities due to reduced thermal stress.

Ground-based growth experiments utilized an ampoule in a vertical setup with a fused quartz frit holding the feed material above a cone-shaped fused quartz growth chamber with a nucleation tip. Both closed and semi-closed setups were used. The ampoules were placed in a 7-zone furnace originally built for Bridgman growth. The furnace settings and ampoule position were chosen in such a way that the feed material and growth chambers were initially nearly isothermal (~315°C), and a slow translation into the colder section of the furnace initiated nucleation and growth. Several single crystals, up to 2.5 cm long, were obtained. There were two flight experiments, and these were conducted using the SUBSA furnace located in the Microgravity Science Glovebox (MSG). The SUBSA furnace had a single heated zone with no translation capability. Silver cladding was put around the flight ampoules to decrease the otherwise large temperature gradient. Material was successfully transported but the resulting samples were polycrystalline. Lessons learned from the development of the flight experiments will be described.