Carrier Density Tunable CdO Thin Film Heterostructures by Ion Beam Irradiative Patterning for Mid-IR Plasmonic Metamaterials

Epitaxial cadmium oxide (CdO) thin films grown by high-power impulse magnetron sputtering (HiPIMS) are premier platforms for low-loss mid-IR optoelectronic devices. Herein, defect engineering and dopant activation for optically sharp, laterally patterned CdO heterostructures are executed by modulated carrier density profiles via ion beam irradiation. Low- (30 keV) to high-energy (1-3 MeV) ion irradiation are facilities to define CdO carrier density grating arrays unlike conventional TCO methods. Nanometer periodicity at 30 keV and micrometer resolution at 1-3 MeV are achievable. While optimal thermal activation, defect site imaging by TEM, and spectral analysis by micro-FTIR are ongoing, preliminary data indicate FIB-produced gratings by Ga³⁺ are locally activated by peak shifts and narrowing according to free carrier generation and mobility enhancement by Raman spectroscopy. Although defects affect local microstructure, epitaxy is maintained post-irradiation. Ion irradiation at 1-3 MeV develops a carrier density window of 2.5 x 10¹⁹ - 2.5 x 10²⁰ cm^-3 and mobilities upwards of 200 cm²V^-1s^-1 relative to displacement damage dose (DDD). Near- and far-field optical measurements identify local carrier density and mobility effects, as well as plasmon dispersion maps by ATR pair simulation to experimental data. Overall, these ion-beam modification methods show promising capabilities to design new lateral patterning techniques for spatially coherent CdO metamaterials in the mid-IR.