Optical Properties of Plasmonic Ultrathin TiN Films

Abstract

Theoretical studies have been conducted to comprehend these characteristics in ultrathin gold films. [6,7] However, the fabrication of such thin films is generally quite difficult. For conventional plasmonic metals, such as silver and gold, continuous, smooth films with thicknesses lower than 10 nm are challenging to produce because of island formation and large defect concentrations which result from their high surface energy. [8][9][10] On the other hand, transition metal nitrides (TiN, ZrN, etc.) can be grown epitaxially on substrates such as c-sapphire and MgO, [11] enabling the formation of continuous ultrathin films down to 2 nm while maintaining high quality. [12] These materials have gained much interest for plasmonic applications for the visible and near-infrared region due to their tailorable optical properties and refractory quality. [13] TiN has already been shown to have potential applications in photovoltaics, [14,15] waveguiding, [16] modulators, [17,18] and nonlinear optical devices. [3,19] Although some of these devices use films of ≈10 nm, there is additional interest in the formation of ultrathin metallic films (<10 nm) which may facilitate additional applications. To support this, there have been efforts to grow ultrathin TiN films using atomic layer deposition (ALD). However, these films become dielectric below 5 nm and may not be useful for nanophotonic applications. [20] This is because TiN thin films grown using ALD with a TiCl 4 precursor are more prone to Cl impurities, which could result in a degradation of the metallic properties in thinner films. [21][22][23] In the present study, we have grown continuous, epitaxial, ultrathin TiN films that remain plasmonic in the optical range via DC reactive magnetron sputtering. Using sputtering to grow the film decreases the chance of contamination since only Ti, N, and Ar are introduced into the system, which helps maintain the metallic properties in ultrathin films. The dielectric permittivity of these films was extracted using spectroscopic ellipsometry, while Hall measurements give further insight into the optical behavior of the films. Results and DiscussionUltrathin TiN films with thicknesses of 2, 4, 6, 8, and 10 nm were grown on MgO using DC reactive magnetron sputtering. Due to the lattice matching between TiN (4.24 Å) and substrates such as MgO (4.21 Å) and sapphire (4.76 Å), high quality,Overcoming the challenge of growing ultrathin metallic films is of great importance for practical applications of nanoplasmonic structures. In the present work, epitaxial, ultrathin (<10 nm) films of plasmonic TiN are grown on MgO using DC reactive magnetron sputtering. The optical properties of the films are studied through variable angle spectroscopic ellipsometry and Hall measurements. As the film thickness decreases, they become less metallic and exhibit higher loss while still remaining plasmonic in the optical range. These trends are related to the decreasing carrier concentration in the thinner films and increased scattering, respectively. Howev...

Transdimensional epsilon-near-zero modes in planar plasmonic nanostructures

“…with C = εk c /(ε 1 +ε 2 ), to give the √ d thickness behavior in the ultrathin regime. This agrees well with the recent room-T measurements [4] and simulations [11] of ω p for stoichiometrically perfect TiN films of controlled variable thickness. Within its applicability domain, Eq.…”

“…New peculiar effects are revealed such as the plasmon mode degeneracy lifting and the dipole emitter coupling to the split epsilon-near-zero modes, leading to biexponential spontaneous decay with up to three-orders-of-magnitude increased rates.Transdimensional (TD) materials are ultrathin planar nanostructures composed of a precisely controlled finite number of monolayers [1]. Modern material fabrication techniques allow one to produce stoichiometrically perfect films of metals and semiconductors down to a few, or even a single monolayer in thickness [2][3][4][5][6]. TD materials make it possible to probe fundamental properties of light-matter interactions as they evolve from a single atomic layer to a larger number of layers approaching the bulk material properties.…”

“…Within its applicability domain, Eq. (3) can be used to obtain ε and/or k c with C being treated as a parameter to fit experimental data in terms of the standard local Drude model, which is typically the case in relevant experiments [4,26].…”

Finite-thickness effects in plasmonic films with periodic cylindrical anisotropy [Invited]

“…The thin film plasma frequency thus obtained, while being independent of k for relatively thick films, acquires the √ k spatial dispersion typical of 2D materials [32] and gradually shifts to the red as the film thickness decreases. This agrees with and thereby explains the recent plasma frequency measurements done on stoichiometrically perfect ultrathin TiN films of controlled variable thickness [24,26]. This theory was then used to demonstrate the remarkable confinement-induced features of the ultrathin films of finite lateral size [29].…”

“…With an extra feature of reduced dimensionality, optical metasurfaces offer new exceptional abilities of controlling incident electromagnetic flow which open up the door to the applications such as single-photon sources with directionally increased photon extraction useful for quantum information technologies and microscopy, for imaging and sensing as well as for probing the fundamentals of the light-matter interactions at the nanoscale [16][17][18][19][20][21]. A key to realizing these applications is the ability to fabricate ultrathin metallic films of precisely controlled in-plane anisotropy, periodicity and thickness, which is quite possible with the modern progress in nanofabrication techniques [22][23][24][25][26]. However, the strong vertical electron confinement entails new quantum effects [27][28][29] which are still to be explored for their importance in controlling the optical properties of ultrathin plasmonic films [30], especially in the presence of the in-plane anisotropy.…”