Titanium Oxynitride Thin Films with Tunable Double Epsilon-Near

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Titanium oxynitride thin films with tuneable double epsilonnear-zero behaviour for nanophotonic applications Laurentiu Braic, Nikolaos Vasilantonakis, Andrei Mihai, Ignacio Jose Villar-Garcia, Sarah Fearn, Bin Zou, Neil McN. Alford, Brock Doiron, Rupert F Oulton, Stefan A. Maier, Anatoly V Zayats, and Peter K. Petrov ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07660 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 19, 2017

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ACS Applied Materials & Interfaces

Titanium oxynitride thin films with tuneable double epsilon-near-zero behaviour for nanophotonic applications Laurentiu Braic,† Nikolaos Vasilantonakis,‡ Andrei Mihai,∗,† Ignacio Jose Villar Garcia,¶ Sarah Fearn,¶ Bin Zou,¶ Neil McN. Alford,¶ Brock Doiron,§ Rupert F. Oulton,§ Stefan A. Maier,§ Anatoly V. Zayats,k and Peter K. Petrov¶ †Imperial College London, Department of Materials, Prince Consort Road, London SW7 2BP, UK ‡King’s College London, Department of Physics, Strand, London WC2R 2LS, UK ¶Imperial College London, Department of Materials, Prince Consort Road, London SW7 2BP, UK §Imperial College London, Department of Physics, Prince Consort Road, London SW7 2AZ, UK kKing’s College London, Department of Physics, Strand, London WC2R 2LS, UK E-mail: [email protected] Abstract Titanium Oxynitride (T iOx Ny ) thin films are fabricated using reactive magnetron sputtering. The mechanism of their growth formation is explained and their optical properties are presented. The films grown when the level of residual Oxygen in the background vacuum was between 5 nTorr to 20 nTorr exhibit double Epsilon-NearZero (2-ENZ) behaviour with ENZ1 and ENZ2 wavelengths tunable in the 700-850 nm

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and in the 1100-1350 nm spectral ranges, respectively. Samples fabricated when the level of residual Oxygen in the background vacuum was above 2E −8 Torr exhibit nonmetallic behaviour, while the layers deposited when the level of residual Oxygen in the background vacuum was below 5E −9 Torr, show metallic behaviour with a single ENZ value. The double ENZ phenomenon is related to the level of residual Oxygen in the background vacuum and is attributed to the mixture of T iN and T iOx Ny and T iOx phases in the films. Varying the partial pressure of nitrogen during the deposition can further control the amount of T iN , T iOx and T iOx Ny compounds in the films and, therefore, tune the screened plasma wavelengths. A good approximation of the ellipsometric behaviour is achieved with Maxwell-Garnett theory for a composite film formed by a mixture of T iO2 and T iN phases suggesting that double ENZ T iOx Ny films are formed by inclusions of T iN within a T iO2 matrix. These oxynitride compounds could be considered as new materials exhibiting double ENZ in the visible and near-IR spectral ranges. Materials with ENZ properties are advantageous for designing the enhanced nonlinear optical response, metasurfaces and non-reciprocal behaviour.

Keywords Plasmonics, Titanium Nitride, Titanium Oxynitride, thin films, Epsilon Near Zero, nonlinear photonics

Introduction As optoelectronic components become nano-dimensional, controlling the coupling between light and matter at the nanoscale has become a major technological challenge, as well as the subject of theoretical studies. Plasmonics has been established as the backbone of this new nanophotonic technology, exploiting the strong interactions between electromagnetic radiation and plasma oscillations in metallic nanostructures, which lead to the establishment of hybrid modes overcoming the diffraction limit. 1,2 Traditionally based on materials such 2 ACS Paragon Plus Environment

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as Ag and Au, it is now recognized that the materials base should be widened with a view to integration with semiconductor (e.g., CMOS-based) technology. In the search for alternative plasmonic materials, titanium nitride (T iN ) was proposed as a possible candidate to be used in the near infrared domain. 3,4 T iN films are one of the most studied systems, finding widespread use in a number of applications. 5–9 The suitability of T iN films for plasmonic application has recently been the subject of extended. 10,11 The colour of stoichiometric T iN films 12,13 hints at their similarity to gold. As confirmed by ellipsometric measurements, 14,15 the plasma frequency in recent plasmonics literature is 500 nm and the epsilon-near-zero (ENZ) behaviour (where the real part of permittivity is close to zero), extends up to around 1000 nm. In turn, its counterpart Titanium dioxide (T iO2 ) is a well-known insulator and oxide semiconductor with many applications in important research areas such as solar energy harvesting 16 and photocatalysis. 17 A number of intermediate phases of general composition T iOx Ny called Titanium oxynitrides are found midway between T iN and T iO2 . Preparation of Titanium oxynitrides through either oxidation of T iN or nitridation of T iO2 is not straightforward. On one hand, the oxidation of the T iN quickly leads to formation of T iO2 18 and on the other, nitrogen implantation in T iO2 leads to substantial reconstruction of the surface due to strong reduction and only a small amount (2-3%) of implanted nitrogen can be reached. 19 A major breakthrough, in terms of plasmonic applications, has been the development of ENZ metamaterials in the desired frequency range, which have many anticipated applications 20 as a result of their perfect absorber behaviour, 21,22 supercoupling effect, 23,24 radiation directivity, 25,26 and nonlinearity enhancement. 27,28 Recently, a broadband ENZ in the near-infrared has been reported for a multilayer of Indium Tin Oxide (ITO) films having different doping densities 21 as well as for nanorod-based metamaterials. 29 Another option to achieve broadband ENZ has been the addition of metallic inclusions to a dielectric film, with the filling factor changing along the direction of growth. 30 Metal-dielectric layers of varying thicknesses and geometries, and transparent conductive oxides, have also been suggested 26 . 31 Here, the mechanism of



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formation of titanium oxynitride thin films and their optical properties with tunable ENZ behaviour is investigated. We present the technological conditions for deposition of titanium oxynitride thin films with unusual double ENZ frequencies. It is shown that they can be modified by changing the film deposition conditions. We propose these oxynitrides as alternative plasmonic materials exhibiting double ENZ in the visible and near-IR spectral ranges.

Results and discussion A series of T iOx Ny thin films have been deposited on MgO and Si substrates by reactive sputtering of Ti targets using an Ar-N2 gas mixture(experimental details in supplementary). The sputtering system was equipped with Residual Gas Analyser (SRS), which was used to measure amount of residual gases in the vacuum chamber before deposition, and therefore to control the level of background Oxygen by pre-deposition chamber conditioning. In the first set of experiments, a series of 50 nm T iOx Ny films were sputtered on Si substrates using a gas mixture of 70%Ar+30%N2 (the total deposition gas pressure was 1.5 mTorr). A Residual Gas Analysis of the vacuum was carried out before starting the growth. Three different scenarios were studied: first deposition at room temperature, without any chamber preparation (baking or pre-sputtering of Titanium); second, prior deposition at room temperature, the chamber and substrates were baked at 250C for 2 hours; and third the chamber and the substrate were heated to 600C and kept at this temperature for 30 minutes while we performed a Ti pre-sputtering. The effect of the chamber preparation on the residual oxygen in the chamber can be seen in the (Table S2 in supplementary). The optical properties presented in Figure 1 (a and b), show the different behaviour of the real and imaginary parts of the dielectric constant of the deposited thin films obtained from ellipsometric measurements. When comparing these behaviours with the Residual Gas Analysis data (presented in supporting information Table 1), one can draw the conclusion that the reason for the


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different optical properties is linked to the level of residual Oxygen in the background vacuum. Samples fabricated when the level of residual Oxygen in the background vacuum was above 20 nTorr (High Residual Oxygen) at both room and high temperatures exhibited nonmetallic (T iO2 like) behaviour, while the layers deposited when the level of residual Oxygen in the background vacuum was below 5nTorr (Low Residual Oxygen), showed metallic (T iN like) behaviour with ENZ value around a wavelength of 600 (Figure 1a). This behaviour corresponds to what has already been reported for T iN films grown at high temperatures10. The films deposited with the level of residual Oxygen in the background vacuum between 5 nTorr to 20 nTorr (Medium Residual Oxygen) yield a 2ENZ behaviour with 2 ENZ frequencies around 600 nm and 1500 nm. This unusual phenomenon is not influenced by the films crystallinity. We attribute the appearance of double ENZ frequency to the mixture of T iN and T iOx Ny /TiOx phases in the films. 15

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Figure 1: (a) Real and (b) imaginary dielectric constants of the films deposited at the different partial oxygen content and temperatures as indicated in the panels. (c) and (d) Dielectric constants simulated using a Maxwell-Garnett model considering a composite formed by T iN inclusions in T iO2 (c) and T iO2 inclusions in T iN (d).



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In order to better understand the influence of the mixing of the TiN and TiOx phases on the optical properties of the Medium Residual Oxygen deposited samples, a MaxwellGarnett formalism has been used to simulate the experimentally observed optical properties. 32 Given a volume fraction, fi , of inclusions of permittivity, i , within a host medium of permittivity,h , the mediums effective permittivity, (ef f , is approximated by the formula:

(ef f − h )/(ef f + 2h ) = fi (i − h )/(i + 2h )

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In applying this effective medium model, the oxinitrite films can be treated as a composite of different phases: two scenarios were considered with either T iN inclusions in T iO2 films (i = T iN and h = T iO2 in Fig. 1c) or, the other way around, T iO2 inclusions inT iN films (h = T iN and i = T iO2 ) in Fig. 1d). We used literature data for T iN and T iO2 33 and the Maxwell Garnett model. The simulation results suggest that the double ENZ behaviour ellipsometry appears only for the case of T iN inclusions into the T iO2 matrix, while T iO2 inclusions in the T iN matrix do not result in double-ENZ behaviour. The latter can be easily understood as T iN inclusions would have a plasmonic polarizability while T iO2 inclusions would not. It should be noted also that in both cases, the presence of T iO2 for high T iN content appears to red shift the ENZ point. Therefore, even the Low Residual Oxygen deposited films with =0 near 700 nm could well include T iO2 (which is confirmed by the XPS and SIMS results and will be discussed below). For pure T iN films, the ENZ is expected to be close to 500 nm. These results of the Maxwell - Garnett modelling correspond remarkably well to our experimental results confirming the formation mechanism of T iOx Ny films as T iN inclusions into a T iO2 matrix.Maxwell-Garnett theory is applicable in situations where the volume fraction of inclusions is low as to maintain spatial separation of the spherical inclusions. As a result, figs. 1c and 1d must be considered together to explain our observations consistently. The simulation results produce a double ENZ behaviour only for the case of T iN inclusions in a T iO2 host, while T iO2 inclusions in a T iN matrix do not result in double-ENZ behaviour. It is therefore likely that the double ENZ behaviour ob6 ACS Paragon Plus Environment

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served arises from the plasmonic polarizability of T iN inclusions. While these observations suggest that T iN inclusions play an important role, the large volume fraction required to explain our observations in the model implies a far more complicated film morphology than suggested by the effective medium theory. The partial pressure of N2 (within the N2-Ar sputter gas mixture) (as well as the deposition temperature) govern the nitride saturation of the growing T iN film. Reducing the amount of nitrogen in the sputter gas mixture, results in the formation of non-saturated T iN layer, which forms TiOx and T iOx Ny phases due to residual oxygen inside the growth chamber. Varying the content of the deposition gas mixture, one can control the amount of T iN , T iOx and T iOx Ny in these films and therefore change the optical properties of the samples (see Figure 2, discussed in detail below). To explore the above hypothesis a second set of experiments was carried out. A series of thin films were sputtered at room temperature in a chamber with Medium Residual Oxygen level using sputter gas mixture with N2 partial pressure varying in the range between 5% and 30% of the total gas pressure. The samples optical measurement results are presented in Figure 2. 35

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Figure 2: The real (a) and imaginary (b) dielectric constants of the RT fabricated films under different N2 partial pressures. These films exhibit two distinct epsilon-near-zero frequencies (Figure 2a). The real part of the dielectric constant of the samples deposited at room temperature and nitrogen partial pressures between 5% and 30% decreases with the increase of wavelength from the ultra-violet range, becoming negative around 700750 nm, and starting to increase around 1000-1100 nm 7 ACS Paragon Plus Environment


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and finally becoming positive again after 1350 nm. For the samples fabricated at nitrogen partial pressures of 5%, the overall behaviour is the same, but the two epsilon near zero frequencies are 850 nm and 1200 nm. The imaginary part of these samples dielectric constant (Figure 1b) is nearly constant until 600 nm, and then significantly increases throughout the measured range showing losses, significantly higher than values reported for Au films (Figure 1c ). We attribute the tunable double ENZ behaviour of the room temperature deposited samples to the mixture of T iN and T iOx Ny /T iOx phases in the films. Moreover, the XPS studies show that for the Medium Residual Oxygen fabricated films, the ratio between T iN and T iOx does not depend on the depth, but we observed higher oxidation near the surface (which is normal after exposure to air). Disappearance of the double-ENZ behaviour in the samples deposited at high temperature is due to the (nearly) complete T iN structure saturation caused by the temperature enhanced nitrogen diffusion and low reactivity with available oxygen. In order to verify this hypothesis, we compared the Low Residual Oxygen deposited samples (grown at 600C) with the Medium Residual Oxygen deposited samples (grown at room temperature). Samples were placed in the XPS system and in-situ Ar ion milled for 20 seconds before an XPS spectra were acquired. The process of milling-sampling was repeated in cycles for up to 300 seconds total sputtering time. The ion sputtering rate has been estimated to be approx.0.26nm/min. The XPS results become constant (i.e., surveys stopped changing) after the third milling-sampling cycle. Figure 3 shows the Ti 2p (Figure 3a) and N 1s (Figure 3b) XPS spectra of samples deposited at 20% N2 partial pressure in vacuum chamber with Low Residual Oxygen, and Medium Residual Oxygen. The spectra show signals at the characteristic binding energies of T iNx , T iOx Ny , and T iOx . 34,35 Table 1 shows the calculated relative atomic percentages of the three different chemical states of Ti in the samples (the fitting procedures used to calculate these percentages are shown in the supporting information). Low Residual Oxygen deposited samples consist mainly of titanium nitride, with a small amount of titanium dioxide (3.5%) and oxynitride (9.6%).


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Figure 3: (a) Ti 2p XP spectra of samples deposited at 20% N2 partial pressure for 600C and room temperature after a few consecutive sputtering cycles. (b) N 1s spectra for the same conditions. The lines mark characteristic binding energies of different Ti chemical states. (c) and (d) SIMS profiles of 20% Nitrogen for TiON (c) and TiO (d). Medium Residual Oxygen deposited samples exhibit a considerably higher content of both titanium dioxide (10.1%) and titanium oxynitride (18.6%). These results are confirmed by Secondary Ion Mass Spectroscopy (SIMS) measurements on 30% N2 partial pressure samples (Figure 3c and d). SIMS depth profiling shows that the film composition is uniform, except for the enhanced secondary ion signals near the surface (no capping layer has been used) and at the interface with the MgO substrates. Table 1: Chemical binding of Ti in the samples fabricated at 20% N2 partial pressure Sample deposition temperature

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The higher content of oxidation states of Ti in the Medium Residual Oxygen fabricated samples, is due to the greater proportion of broken-bond defects or unreacted Ti atoms, to which residual oxygen then becomes attached. The N 1s peak at 397 eV for all the films (see supplementary figure 14S) in the present case can be attributed to nitrogen in the Ti-N bonds. 36 The very intense nature of this N 1s component peak is indicative of nitride formation in the film. The N 1s peak at lower binding energy (396 eV) has been associated to terminally bound nitrogen that is released during nitridation of Ti-O sites or in surface oxynitrides. 35,36 This peak has been noted by several authors for nitrides and oxynitrides of Ti grown by various methods. 36,37 In fact, in a theoretical paper, Graciani et al. 38 argue that the most stable way of growing good quality Ti oxynitrides is through nitrogen incorporation of the most stable alpha-TiO phase, which is isostructural with T iN . According to their calculations, it is much easier to implant N2 into the TiO than implant N2 into T iO2 , which is the usual way of growing T iOx Ny . 36 Our electrical measurements show the resistivity of the samples decreases as the intensity of the XPS T iN characteristic signal increases in comparison to the intensity of the characteristic T iOx Ny signal ; the characteristic T iN signal at approximately 455 eV increases in comparison with the T iO2 and T iOx Ny signals in the Ti 2p spectra (see Figure 3-S and Table S1 in the supplementary information). A full survey of the relative percentages of each of the Ti, O and N components found in the films can be found in the supplementary data.

The films electronic properties were studied using Kelvin Force Microscopy (KFM) and Hall effect measurements. To check the electrical film uniformity we performed KFM on both Low and Medium Residual Oxygen grown films (supplementary Figure15S) and found a complete uniformity in the electrical conductivity, with no microscopic segregated phases. We also checked the surface morphology (cf. Fig S-19 in supporting information) of samples both grown at room temperature and high temperature (600C) by scanning electron microscopy (SEM), for both medium and low residual oxygen content, and we haven’t dis-


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covered any apparent phase separation of different cystallites that might correspond to T iN and/or T iO2 . All samples grown at Medium Residual Oxygen exhibited electron concentrations in the order of 1022 carriers/cm3 (Supplementary Figure 16S), similar to the values encountered in the literature for titanium nitride and oxynitride. 34 The samples electron mobility (Figure 16S) increased from 0.03cm2 V −1 s−1 , for samples fabricated at 5% nitrogen partial pressure, to 0.1cm2 V −1 s−1 for samples fabricated at 30% nitrogen partial pressure, respectively. This is in agreement with earlier results for samples with similar Ti:O ratios. 34 Furthermore, such small mobility values are to be expected for sputtered polycrystalline films. Very recently, two studies regarding the effect of post growth vacuum annealing on RF sputtered T iN films, 39 and the effect of changing the deposition conditions (gas mixture, substrate temperature, biasing of substrate during growth) 40 have shown a wide tunability of the screened plasma wavelength in the 500-720 nm spectral range. This was attributed to a change in the crystallinity or composition with annealing temperature, but no double ENZ behaviour has been reported.

Conclusions To conclude, we have discovered unusual double ENZ properties that have not been reported for a transition metal nitride. The reason for its appearance is the level of residual Oxygen in the background vacuum that reacts with Ti during deposition. We have also confirmed that the double-ENZ behaviour appears only on the samples where TiN is incorporated in pre-existing T iO2 matrix. The two ENZ frequencies of the T iOx Ny films could be further tuned by changing the partial pressure of the nitrogen in the sputter gas mixture. We should emphasize that in our studies the double ENZ behaviour has been observed, in both thin amorphous T iOx Ny films on MgO and Si substrates, thus allowing one to fabricate, control and engineer tunable plasmonic and metamaterial devices, using CMOS compatible technology. Materials with ENZ properties are advantageous for designing the enhanced 11 ACS Paragon Plus Environment


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nonlinear optical response, 41 metasurfaces and non-reciprocal behaviour. 42

Supporting information Experimental procedures including : Thin film growth conditions, SEM, XPS with fitting procedures, Kelvin Force Microscopy, Hall Effect measurements, Transfer matrix calculations of ENZ modes.

Acknowledgement All authors acknowledge the EPSRC Reactive Plasmonics Programme Grant EP/M013812/1. S.A.M. further acknowledges the Royal Society and the Lee-Lucas Chair for funding. A.V.Z. acknowledges support from the Royal Society and Wolfson Foundation.We acknowledge support from the Henry Royce Institute made through EPSRC grant EP/R00661X/1.

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(42) Neira, A. D.; Olivier, N.; Nasir, M. E.; Dickson, W.; Wurtz, G. A.; Zayats, A. V. Eliminating Material Constraints for Nonlinearity with Plasmonic Metamaterials. Nat. comm. 2015, 6 .



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