The nitrides of most of the group IVb-Vb-VIb transition metals (TiN, ZrN, HfN, VN, NbN, TaN, MoN, WN) constitute the unique category of conductive ceramics. Having substantial electronic conductivity, exceptionally high melting points and covering a wide range of work function values, they were considered for a variety of electronic applications, which include diffusion barriers in metallizations of integrated circuits, Ohmic contacts on compound semiconductors, and thin film resistors, since early eighties. Among them, TiN and ZrN are recently emerging as significant candidates for plasmonic applications. So the possible plasmonic activity of the rest of transition metal nitrides (TMN) emerges as an important open question. In this work, we exhaustively review the experimental and computational (mostly ab initio) works in the literature dealing with the optical properties and electronic structure of TMN spanning over three decades of time and employing all the available growth techniques. We critically evaluate the optical properties of all TMN and we model their predicted plasmonic response. Hence, we provide a solid understanding of the intrinsic (e.g. the valence electron configuration of the constituent metal) and extrinsic (e.g. point defects and microstructure) factors that dictate the plasmonic performance. Based on the reported optical spectra, we evaluate the quality factors for surface plasmon polariton and localized surface plasmon for various TMN and critically compare them to each other. We demonstrate that, indeed TiN and ZrN along with HfN are the most well-performing plasmonic materials in the visible range, while VN and NbN may be viable alternatives for plasmonic devices in the blue, violet and near UV ranges, albeit in expense of increased electronic loss. Furthermore, we consider the alloyed ternary TMN and by critical evaluation and comparison of the reported experimental and computational works, we identify the emerging optimal tunable plasmonic conductors among the immense number of alloying combinations.

Conductive nitrides: Growth principles, optical and electronic properties, and their perspectives in photonics and plasmonics

Titanium nitride (TiN) has been widely used in the semiconductor industry for its diffusion barrier and seed layer properties. However, it has seen limited adoption in other industries in which low temperature ( 4 g/cm3), low stress ( 85% step coverage for aspect ratio of 10:1 were realized. Wet chemical etch data show robust chemical stability of the film. The properties of the film have been optimized to satisfy industrial viability as a Ruthenium (Ru) preseed liner in potential data storage and TSV applications.

Low-temperature (≤200 °C) plasma enhanced atomic layer deposition of dense titanium nitride thin films

The abbreviation “nano-Calphad” stands for “Calculation of Phase Diagrams for nano-systems.” Nano-systems contain at least one phase or at least one interface layer (film, complexion) with at least one of its dimensions being below 100 nm. The essential task of nano-Calphad is to introduce correctly the surface term into the equation for the Gibbs energy. In view of the controversy between the Kelvin and Gibbs equations, even this task does not have an obvious solution (in the present paper, the Gibbs method is preferred). However, there are many further questions to be addressed when the Calphad method is converted into the nano-Calphad method. This paper attempts to give an as full as possible list of all those problems, such as: (i) the definition of a new, independent thermodynamic variable, (ii) the extended phase rule, (iii) the curvature dependence of the interfacial energies, (iv) the dependence of interfacial energies on the separation between interfaces (including the problem of surface melting), (v) the role of the shapes and relative arrangement of phases, (vi) the role of the substrate (if such exists), and (vii) the role of segregation, taking into account its effect on the mass balance within multi-component nano-phases and its surface phase transition and complexion. It is also shown that the well-known meaning of the tie line in binary two-phase fields is lost in nano-systems. The issues related to the size limits of materials thermodynamics and the need for a more complete databanks on molar volumes and interfacial energies are discussed.

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Nano-Calphad: extension of the Calphad method to systems with nano-phases and complexions

Mechanical and corrosion-resistant properties of Ti–Nb–Si–N nanocomposite films prepared by a double glow discharge plasma technique

High power impulse magnetron sputtering (HiPIMS) is a novel physical vapor deposition (PVD) technique. HiPIMS is a low-temperature process, giving well-adherent coatings, better quality films, and droplet-free deposition. It provides enhanced gas dissociation and highly ionized plasma by inputting a high power during short pulses to the target. The target material is therefore not only sputtered but also ionized during the deposition process. In this study, TiN thin films were deposited using a uni-polar mode HiPIMS process. Nitrogen was used as the reactive gas to deposit TiN along with Ar gas. The deposition of TiN films was investigated by varying the duty cycles from 2 to 10% to have peak power density ranging from 208 to 1064 W/cm2. A high peak power density was obtained at a low duty cycle. DC magnetron sputtered TiN thin film (duty cycle = 100%) was also deposited for comparison. We demonstrate that the HiPIMS deposited TiN thin film exhibits a denser structure and smoother surface at low duty cycles. It was also found that the intense ion bombardment at low duty cycles deteriorates the film structure and their mechanical properties. Moderate ion bombardment at a duty cycle of 4.5% gives the highest (111)/(200) intensity ratio and the highest amount of Ti N bonding. This results in a film having the highest hardness, elastic modulus, and corrosion resistance of 29.3 GPa, 388.2 GPa, and 1.56E+ 06 (Ω/cm2), respectively. However, oxygen was found in the resulting films.

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Effect of duty cycles on the deposition and characteristics of high power impulse magnetron sputtering deposited TiN thin films

Substrate-temperature dependent structure and composition variations in RF magnetron sputtered titanium nitride thin films

The structure, mechanical, optical, and electrical properties of Ti1−x
 Nb
 x
 N thin films, 0 ≤ x ≤ 1, are reported. The films were deposited onto polycrystalline nuclear grade 316LN stainless steel substrate by radio frequency reactive magnetron sputtering in 100% N2 plasma. X-ray diffraction results revealed that Nb is soluble in TiN up to x = 0.77. From x = 0.26 to x = 0.77, these films stabilized in rock salt structure with (111) orientation. The hardness and Young’s modulus increased, whereas electrical resistivity decreased with an increase in the Nb concentration in the films. The highest value of hardness and Young’s modulus were 31 and 320 GPa, respectively, for x = 0.77, at which the value of the lowest resistivity of 56 μΩ-cm was measured. The films showed a broad reflectance band, with a minimum in reflectance that shifted to shorter wavelengths as a function of increasing x. The reflectance band extended from the ultraviolet (~250 nm) to the visible region (~750 nm) and the position of the reflectance shifted from 2.33 to 3 eV with an increase in x from 0 to 0.77.

Effect of Nb concentration on the structure, mechanical, optical, and electrical properties of nano-crystalline Ti 1−x Nb x N thin films

Conductive-atomic force microscopy study of local electron transport in nanostructured titanium nitride thin films

The optical reflectance, dielectric functions and phonon vibrational modes of Ti1−xNbxN (0≤x≤0.77) thin films are reported. Films of 500-nm thickness were deposited on 316LN nuclear grade stainless steel substrates by radio-frequency magnetron sputtering. The reflectance spectra of the films, in the energy range 1.5 to 5.5 eV, are fitted using the Drude–Lorentz model as the response of one Drude parameter and four Lorentz oscillators. It is demonstrated that the properties studied are dependent on Nb concentration, x, and exhibit a behavior transition threshold at x=0.5. The optical properties studied are closer to TiN for x 0.5. For example, the films showed a minimum in reflectance at ∼2.33 eV for values of x up to 0.5, corresponding to the Ti–N charge transfer band. Increase in Nb concentration beyond 0.5 caused a large shift in this energy to 3.2 eV, corresponding to the Nb–N charge transfer excitation. Similarly, the real part of the dielectric function is characterized by a screened plasma energy of 2.25 eV for values of x 0.5 (∼NbN). The energy at which the loss function reaches a peak value increases linearly for values of x from 0 to 0.41 and decreases very drastically for x>0.5. Phonon-vibrational modes of Ti1−xNbxN thin films studied by Raman spectroscopy show that Nb substitution in TiN results in first-order Raman scattering. The single-phonon acoustical peak at 270 cm−1 of TiN shifted to 265 cm−1 for x=0.77, while the two-phonon acoustical peak of TiN at 620 cm−1 shifted to 630 cm−1 for the same value of x. The reasons for the existence of a behavior transition threshold in Nb concentration are discussed.

Optical reflectance, dielectric functions and phonon-vibrational modes of reactively sputtered Nb-substituted TiN thin films

The nature of morphological instabilities in sputtered titanium and niobium nitride thin films grown on amorphous borosilicate glass and single-crystal Si (311) substrates is investigated. All the films were grown by RF magnetron sputtering at constant power and pressure but with thickness varying from 40 to 400 nm and substrate temperatures of 250–300 °C. The surfaces of the thin films can be divided into two areas: one in which the morphology is smooth with densely packed grains and the other in which there are morphological instabilities. A closer observation of the morphological instabilities reveals the coexistence of elastic strain-induced Asaro–Tiller–Grinfeld (ATG) type of instability and dendritic and snowflake structures due to diffusion-limited aggregation (DLA). The ATG instabilities extend over lengths of several tens of micrometers, whereas the DLA structures are confined to lengths of less than 10 µm in the same film. At low thickness (40–100 nm) only the elastic strain-induced instabilities emerge. High growth rates and a thickness of 150 nm are required to cause DLA and coexistence of the two kinds of instabilities. It has also been found that crystallization is not a prerequisite for the formation of dendritic structures.

K. Vasu

Papers

Substrate-temperature dependent structure and composition variations in RF magnetron sputtered titanium nitride thin films

Effect of Nb concentration on the structure, mechanical, optical, and electrical properties of nano-crystalline Ti 1−x Nb x N thin films

Conductive-atomic force microscopy study of local electron transport in nanostructured titanium nitride thin films

Optical reflectance, dielectric functions and phonon-vibrational modes of reactively sputtered Nb-substituted TiN thin films

Effects of elastic strain and diffusion-limited aggregation on morphological instabilities in sputtered nitride thin films