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.

Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons

“…As a result, no improvement in the Q over the metallic case is expected. Values of Q in the range of 2-6 have been calculated for n-InGaAs [29], with similar results observed for heavily doped wide-gap oxides and nitrides such as AZO [30,31], GZO [32], ITO [30,31], and TiN [26], with Q values on the order 10 [120]. However, recent work within n-type CdO demonstrates Q on the order of 40, similar to plasmonic metallic systems, while offering resonances in the mid-IR, thus providing a promising alternative plasmonic material [33].…”

“…In heavily-doped, wide-gap semiconductors, γ∼0.05-0.1 eV at best, which leads to values that are generally lower than in noble metals (or comparable in the case of TiN) [27,120]. However, forthcoming results from n-type CdO are very promising with propagating FOMs on par with noble metals in the mid-IR [32]. The reduction in the FOM for propagating modes in SPhP materials can be understood by considering that while these materials have longer lifetimes and therefore lower damping, they also exhibit significantly reduced group velocities, v g due to their strong dispersion.…”

“…The search for alternative materials for nanophotonic and metamaterial applications can be broken into two major research thrusts: 1) identifying novel, lowerloss, free-carrier-based plasmonic materials and 2) exploring the use of dielectric materials. In the case of the former, efforts have focused on metal alloys [26][27][28], highly doped semiconductors [29][30][31][32][33] and more recently, graphene [34][35][36][37][38][39][40]. While the spectral range for localized plasmonic modes has been successfully pushed into the infrared using such materials, the plasmonic origin of the modes implies that they are still limited by the relatively fast electron/plasma scattering (typically on the order of 10-100 fs) [41][42][43].…”

Ultrabroadband terahertz conductivity of highly doped ZnO and ITO

“…Many studies focus on impurity doped zinc oxides [4][5][6][7], like aluminum-or gallium doped zinc oxide (AZO and GZO), and tin-doped indium oxide (ITO) [8] due to their low loss and metallic behavior in the near infrared. With the emerging of terahertz time domain spectroscopy (THz-TDS), the frequency resolved conductive properties of a range of conductors, including thin gold films [9], highly doped silicon [10,11] and ITO [8] have been investigated between 0.2 and 2.7 THz.…”

Fabrication of deep-profile Al-doped ZnO one- and two-dimensional lattices as plasmonic elements

“…Many studies focus on aluminum-or gallium-doped zinc oxides [4][5][6][7] or tin-doped indium oxide 8,9 due to their low loss and metallic behavior in the near infrared. One more advantage of TCOs is the possibility of tuning their permittivity by design through deciding the dopants or the ratio of different components 10 , thus constituting an advantage over metals having fixed permittivity values.…”