Ultra-wide spectral range (0.4–8 μm) transparent conductive ZnO bulk single crystals: a leading runner for mid-infrared optoelectronics

Abstract

Transparent conductive oxides have been widely applied to solar cells, liquid crystal displays, and other optoelectronic devices because of their excellent transparent conductive properties in the visible-near-infrared (Vis-NIR) region. At present, the transparent conductive oxide films (TCOFs) such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and aluminum-doped zinc oxide (AZO) are widely noticed and used. However, a high carrier concentration (≥1020 cm−3) of these TCOFs leads to a result that their transparency will be affected by the plasmons effect, and thus their transparent range will generally end around 2–3 μm. As such, they are more applicable in the Vis-NIR region but unable to meet the requirements of mid-infrared (MIR) optoelectronic devices. Faced with this situation, herein, we reported the native ZnO bulk single crystals with low background carrier concentration (1015–1016 cm−3) and high carrier mobility (μ > 300 cm2 V−1 s−1). These single crystals boast excellent transparent conductive properties (IR cutoff edge: 6–8 μm, Rsheet: 3.7–28.5 Ω/sq) with an advantage that they can be massively produced by a hydrothermal method at a low cost. Based on these merits, the native ZnO bulk single crystals bring a new possibility of breaking the bottleneck of MIR transparent conductivity.