Optical Properties of Gallium-Doped Zinc Oxide—A Low-Loss Plasmonic Material: First-Principles Theory and Experiment

Abstract

Searching for better materials for plasmonic and metamaterial applications is an inverse design problem where theoretical studies are necessary. Using basic models of impurity doping in semiconductors, transparent conducting oxides (TCOs) are identified as low-loss plasmonic materials in the near-infrared wavelength range. A more sophisticated theoretical study would help not only to improve the properties of TCOs but also to design further lower-loss materials. In this study, optical functions of one such TCO, gallium-doped zinc oxide (GZO), are studied both experimentally and by first-principles density-functional calculations. Pulsed-laser-deposited GZO films are studied by the x-ray diffraction and generalized spectroscopic ellipsometry. Theoretical studies are performed by the total-energy-minimization method for the equilibrium atomic structure of GZO and random phase approximation with the quasiparticle gap correction. Plasma excitation effects are also included for optical functions. This study identifies mechanisms other than doping, such as alloying effects, that significantly influence the optical properties of GZO films. It also indicates that ultraheavy Ga doping of ZnO results in a new alloy material, rather than just degenerately doped ZnO. This work is the first step to achieve a fundamental understanding of the connection between material, structural, and optical properties of highly doped TCOs to tailor those materials for various plasmonic applications.

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Plain Language Summary

When light hits a metal, the mobile electrons in the metal, driven by the light’s electric field, begin to oscillate at a particular frequency. These oscillations, called plasmons, can lead to extreme localization of light with strong field enhancement, and have been exploited for many applications, including molecular sensing and high-resolution microscopy. So far, metals such as gold and silver are commonly used in plasmonic devices because they suffer very small electric losses under dc-field conditions. In the technologically important near-infrared (NIR) range, however, their performance is severely limited by their large losses in this frequency range. Alternative materials that can replace the noble metals and enable low-loss plasmonic applications in the NIR are therefore highly desirable. Recently, transparent conducting oxides (TCOs) are emerging as such an alternative. Fundamental understanding of these new materials is only at its beginning. In this paper, we report a combined experimental and theoretical study of the optical properties of the doped forms of a widely used transparent conducting oxide, zinc oxide (ZnO).

The engineering advantage of ZnO, a semiconductor, is that the concentration of mobile electrons in it can be tuned across a broad range by (heavy) doping. It is precisely this feature that enables these materials to behave like metal with low loss when they interact with light in the NIR range. We have investigated the optical properties of gallium-doped ZnO in thin films, including its dielectric constant, its optical loss, and its performance measure in prototypical plasmonic devices and revealed how doping systematically modifies these properties. Moreover, we have, for the first time, connected the crystal structure of the doped oxide to the degree of doping and, in turn, to the optical properties. Indeed, we have found that ultraheavy doping of ZnO, used as a means to obtain low-loss plasmonic material, results in a new alloy material.

This work is the first step towards a fundamental understanding of how the material, structural, and optical properties determine the performance of highly doped transparent conducting oxides and how they must be optimized for various plasmonic applications.