Truly Transparent p‑Type γ‑CuI Thin Films with High Hole Mobility

The γ-phase of copper­(I) iodide (γ-CuI) is a p-type semiconductor with a wide bandgap (E g ≈ 3.1 eV). Conventionally, γ-CuI thin films have been synthesized by the iodination of Cu thin layers with iodine vapor. However, γ-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 γ-CuI films. The chemical reaction between Cu3N thin films and solid-phase iodine at 25 °C was found to yield highly transparent polycrystalline γ-CuI films with shiny appearance. The γ-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 γ-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 (ρ) of 3.1 × 10–2 Ω cm, hole density (n h) of 8.9 × 1019 cm–3, and mobility (μ) of 2.4 cm2 V–1 s–1. Mild heat treatment at 100–150 °C under an inert atmosphere was found to suppress n h and enhance μ. The heat-treated films had μ values of 9–10 cm2 V–1 s–1, which are comparable to those of other wide-bandgap p-type semiconductors grown epitaxially at high temperatures above 400 °C. These findings would assist studies on applications of γ-CuI thin films in transparent electronics.