Growth of Large Single Crystals of Copper Iodide by a Temperature Difference Method Using Feed Crystal Under Ambient Pressure

A large single crystal of CuI was fabricated by the temperature difference method using NH4I aqueous solution. To explore the efficient crystal growth condition, we calculate the phase diagram of copper iodide components as a function of pH and redox potential by using thermodynamic simulation software, and we also measured the experimental solubility curve of CuI. Consequently, we could fabricate the largest CuI single crystal (8.6 × 9.1 × 14 mm3) to date. Crystal shape was also controllable by the stirring condition in the reactor; i.e., tetrahedron or cubic shape of crystals was formed under stirring or without stirring condition, respectively. Further, the highly pure and transparent CuI crystal was fabricated by the addition of sodium sulfite (Na2SO3) into NH4I aqueous solution. The electronic structure of our high quality CuI single crystal was determined using X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy, and the ionization potential of the CuI (001) facet was elucidated to be 5.2 ± 0.2 eV. Even though our CuI crystal was optically transparent, it exhibited low resistivity p-type behavior with high hall mobility (22.1–46.4 cm2 V–1 s–1). The acceptor level of our CuI crystal was also determined by the temperature dependence of the Hall effect property (0.15 ± 0.004 eV), which was consistent with the theoretical calculation on the CuI crystal model with p-type carriers. The present crystal growth method is quite facile and does not require any expensive fabrication equipment. Therefore, our CuI single crystal is expected to be useful as a p-type transparent substrate for the epitaxial growth of various electronic or optical devices, especially for materials with large lattice parameters