Truly Transparent p-Type γ-Cul Thin Films with High Hole Mobility

The gamma-phase of copper(I) iodide (gamma-CuI) is a p-type semiconductor with a wide bandgap (E-g approximate to 3.1 eV). Conventionally, gamma-CuI thin films have been synthesized by the iodination of Cu thin layers with iodine vapor. However, gamma-CuI films fabricated by this method have a rough surface and thus a frosted-glass-like appearance, which make it difficult to apply this material to transparent electronics. In this paper, a simple new method is proposed for the synthesis of truly transparent p-type gamma-CuI films. The chemical reaction between Cu3N thin films and solid-phase iodine at 25 degrees C was found to yield highly transparent polycrystalline gamma-CuI films with shiny appearance. The gamma-CuI films fabricated by this method had root-mean-square roughness values of 8-12 nm, which are less than one-third of those for gamma-CuI films synthesized by the conventional method. As a result, specular transmittance of >75% in the visible region was attained. An as-prepared film had a resistivity (rho) of 3.1 x 10(-2) Omega cm, hole density (n(h)) of 8.9 x 10(19) cm(-3), and mobility (mu) of 2.4 cm(2) V-1 s(-1). Mild heat treatment at 100-150 degrees C under an inert atmosphere was found to suppress n(h) and enhance mu. The heat-treated films had mu values of 9-10 cm(2) s(-1), which are comparable to those of other wide-bandgap p-type semiconductors grown epitaxially at high temperatures above 400 degrees C. These findings would assist studies on applications of gamma-CuI thin films in transparent electronics.