Mid-Wave to Near-IR Optoelectronic Properties and Epsilon-Near-Zero Behavior in In-Doped CdO

Abstract

Abstract Body: Cadmium oxide (CdO) is one of the most promising material systems for low-loss infrared (IR) plasmonics to date. Through reactive co-sputtering using high power impulse magnetron sputtering (HiPIMS) and radio frequency (RF) sputtering, doped CdO demonstrates high mobility films with tunable carrier concentrations across a range corresponding to epsilon-near-zero (ENZ) modal frequencies spanning the mid-IR and reaching into the near-IR. Specifically, indium-doped CdO (In:CdO) yields carrier densities ranging between 2x1019 cm-3 and 5x1020 cm-3 while maintaining mobilities between 300-400 cm2/Vs across this range, thus allowing for the widest accessibility of the IR spectrum (1650-5325 cm-1) of a single plasmonic material grown by sputtering. Fully accessing the spectrum of plasmonic applications, including surface plasmon oscillations, ENZ modes, and strong coupling phenomena requires that one maintains these appealing transport properties over a film thickness range spanning a few tens to a few thousands of nanometers. Extensive CdO fabrication experiments using high-power impulse magnetron sputtering reveal a strong dependence of carrier density and mobility on film thickness when thickness drops below 100 nm, a particularly significant range for ENZ modes. We attribute this behavior to well-reported surface charge accumulation and depletion layers caused by Fermi level pinning. In this presentation, we demonstrate the reversal between surface charge accumulation and depletion in In:CdO. Further, we show how these surface effects can influence optical properties, and how they can be controlled by understanding and manipulating CdO defect chemistry. Lastly, we will show preliminary results of plasmonic tunability via ferroelectric switching.