Atomic Layer Engineering of Epsilon‐Near‐Zero Ultrathin Films with Controllable Field Enhancement

Abstract

Enhanced and controlled light absorption, as well as field confinement in optically thin materials, are pivotal for energy‐efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films can support the so‐called epsilon near zero (ENZ) modes in a frequency region of near‐zero permittivity, which can lead to the perfect light absorption and ultrastrong electric field intensity enhancement (FIE) within the films. To achieve full control over absorption and FIE, one must be able to tune the ENZ material properties as well as the film thickness. Here, engineered absorption and FIE are experimentally demonstrated in aluminum‐doped zinc oxide (AZO) thin films via control of their ENZ wavelengths, optical losses, and film thicknesses, tuned by adjusting the atomic layer deposition (ALD) parameters such as dopant ratio, deposition temperature, and the number of macrocycles. It is also demonstrated that under ENZ mode excitation, though the absorption and FIE are inherently related, the film thickness required for observing maximum absorption differs significantly from that for maximum FIE. This study on engineering ENZ material properties by optimizing the ALD process will be beneficial for the design and development of next‐generation tailorable photonic devices based on flat, zero‐index optics.

High‐Quality Plasmonic Materials TiN and ZnO:Al by Atomic Layer Deposition

“…[ 101 ] Up to date, several groups reported on the tuning of ENZ wavelength in the ranges 1320–2300, [ 94 ] 1450–1545, [ 101 ] 1500–1700, [ 95 ] 1550–1700, [ 96 ] 1600–2200, [ 99 ] and 1800–2400 nm. [ 97 ] As an example, Figure 8 shows ENZ wavelength dependences on substrate temperature, dopant concentration, and thickness for AZO layers grown by ALD on silica and silicon substrates by Gurung et al [ 96 ] The ENZ wavelength blueshifts as the substrate temperature increases from 225 to 250 °C (Figure 8a,d), which can be explained by the increase in doping efficiency (the ratio of electrically active Al species to total Al content in the matrix) as well as an increase in the grain size, and the improvement in crystal structure of AZO with increasing deposition temperature. The better crystallinity of the films should result in a reduction of scattering centers for electrons, and thus an increase in carrier mobility.…”

“…Figure 8b,e shows that as the Al content increases (dopant ratio ZnO:Al decreases), the ENZ wavelength (as well as loss) first decreases and then increases again. Gurung et al [ 96 ] explained this finding by doping saturation as well as the formation of Zn vacancies at high Al concentrations. As the Al content of AZO increases, the carrier concentration increases until it reaches a maximum value.…”

“…Unfortunately, the ALD‐grown AZO often exhibit poor optical quality and insufficient carrier concentrations due to low doping efficiencies at high doping levels (>10 20 cm −3 ). [ 96,101,103 ] For example, about 60% of Al atoms in AZO layers deposited by reactive magnetron sputtering [ 86 ] and over 90% of Al atoms in CVD‐grown AZO [ 92 ] were found to be electrically active, while only about 13% Al worked as dopants in the ALD‐grown AZO films. [ 103 ] The poor doping efficiency (the ratio of electron concentration to total Al content in the matrix) is a serious limitation of ALD, which results in lower quality of ALD‐grown AZO and GZO films compared with those grown by the other methods.…”

“…[ 105,106 ] For this reason, the conventional ALD route for deposition of AZO films consists of two alternating steps: 1) zinc and oxygen precursor alternating pulses for ZnO growth followed by 2) a doping cycle of aluminum and oxygen precursor alternating pulses, which obviously results in the formation of AlO x /ZnO layered structure. [ 95,96,103 ]…”

Tailoring Infrared Absorption and Thermal Emission with Ultrathin Film Interferences in Epsilon‐Near‐Zero Media

“…The high field enhancement at PA may be effectively exploited for a variety of applications, e.g., designing high-efficiency photodetectors. [25,62]…”