A Platform for Complementary Metal-Oxide-Semiconductor Compatible Plasmonics: High Plasmonic Quality Titanium Nitride Thin Films on Si (001) with a MgO Interlayer

By coupling photons into collective oscillations of free electrons, plasmonics enables the emergence of novel technologies with the combined capabilities of photonics and miniaturized electronics. In the past few decades, a large variety of plasmonicsbased applications have been demonstrated. These include nanolasers, interconnects, modulators, chemicaland bio-sensors, as well as light-emitting diodes and photovoltaic devices where plasmonics is used for efficiency enhancement. One of the most attractive materials alternative to noble metals that drive the plasmonics revolution, is titanium nitride (TiN), which has been investigated extensively due to its low-cost, gold-like, and tunable optical properties in the visible and near-infrared range, high thermal and chemical stability, high mechanical hardness, and bioand complementary metaloxide-semiconductor (CMOS) compatibilities. TiN has been widely used as a gate electrode in various CMOS devices. In the area of plasmonics, TiN-based waveguides, gyroidal metamaterials, nanohole metasurfaces, nanoantennas, and use of TiN nanoparticles for solar energy conversion and biomedicine have been reported. However, the majority of the demonstrations of TiN’s device potential in plasmonics have been on sapphire and bulk MgO substrates featured by their small lattice mismatch with TiN, enabling the best-performing plasmonic films. Even then, high deposition temperatures (not congruent with CMOS processes) were usually used to ensure the high structural quality of the TiN films. For example, using reactive sputtering and at a substrate temperature of 650 C, a peak plasmonic figure of merit (FOM1⁄4 ε 0/ε 00) of 4.5 has been demonstrated for TiN films on a bulk MgO substrate. Single-crystalline, highly metallic TiN films with an electron concentration of 9.2 10 cm 3 and a peak plasmonic FOM as high as 5.8 have been achieved on c-sapphire substrates by plasma-assisted molecularbeam epitaxy (PA-MBE) at a substrate temperature of 1000 C. However, realizing the true potential of TiN-based plasmonics through integration with the CMOS electronics necessitates Dr. K. Ding, D. Fomra, Dr. H. Morkoç, Dr. N. Kinsey, Dr. Ü. Özgür, Dr. V. Avrutin Department of Electrical and Computer Engineering Virginia Commonwealth University Richmond, VA 23284, USA E-mail: kding2@vcu.edu Dr. A. V. Kvit Materials Science Center University of Wisconsin-Madison Madison, WI 53706, USA

DOI: 10.1002/adpr.202000210 Titanium nitride (TiN) is highly attractive for plasmonics and nanophotonics applications owing to its gold-like but tunable optical properties. Its prodigious potential for plasmonics has been demonstrated on sapphire or bulk MgO. For a transformational impact, high optical quality TiN on Si is required instead, which would support the integration of nanophotonics with the complementary metal-oxide-semiconductor (CMOS) electronics. However, TiN grown on Si, even at elevated temperatures, lacks the optical quality needed, imposed by the large lattice mismatch between them. Here, a novel approach is reported wherein a thin MgO interlayer is inserted between TiN and Si. The improved crystalline quality enabled by MgO for TiN on Si(001) leads to a significant enhancement of the plasmonic figure of merit (FOM ¼ Àε 0 /ε 00 ) from 2.0 to 2.5 at telecommunication wavelength (peak FOM of 2.8), which is comparable to the widely accepted ultimate FOM obtained on bulk MgO grown under similar conditions. The TiN/MgO/Si structure enables the hybrid-plasmonic-photonic waveguide platform with sufficiently low losses, and thus long propagation lengths, for nanophotonic devices while providing additional practical advantages such as serving as a self-aligned robust etching mask. Thus, the muchanticipated potential of TiN on Si platform for CMOS compatible plasmonics is brought closer to reality.
conveying the high-performance results to the silicon platform. This requires the growth of high plasmonic quality TiN on Si substrates at CMOS-compatible temperatures. However, so far, only moderate plasmonic properties could be attained for TiN films grown on Si substrates using various deposition methods such as sputtering, [24,33,34] pulsed laser deposition (PLD), [35] and atomic layer deposition (ALD) [23] primarily due to the associated large lattice mismatch (21.9%). A relatively low resistivity of 1.5 Â 10 À5 Ω cm could be achieved by PLD only at temperatures as high as 700 C, [35] and TiN films grown on Si at lower temperatures of 250 C by ALD, although used to demonstrate fully CMOS-compatible plasmonic nanoantennas, exhibited weak metallic character (i.e., insufficiently large negative ε 0 of À24) and much higher loss (larger ε 00 of 41) compared with TiN films on MgO (ε 0 þiε 00 ¼ À30 þ 26i) at telecommunication wavelength 1550 nm. [23] Thus, despite significant efforts, the plasmonic quality of the heretofore reported TiN on Si does not yet satisfy the requirements for applications in integrated plasmonic devices due to a combination of high deposition temperatures or poor plasmonic performance.
In this study, we address the insufficient optical quality of TiN films on the silicon CMOS platform by the insertion of a thin (10 nm) MgO interlayer. Due to the close in-plane lattice constants of TiN (4.241 Å) and MgO (4.213 Å), this MgO interlayer significantly improves the crystalline and optical properties of TiN thin films on Si (001) substrates grown by plasma-enhanced atomic layer deposition (PE-ALD) at a moderate substrate temperature of 450 C, as compared with direct growth of TiN on Si under the same conditions. The plasmonic FOM of TiN on Si (001) at 1550 nm enhances from 2.0 to 2.5 (peak FOM from 2.4 to 2.8) with the MgO interlayer which is comparable with the films grown on bulk MgO (001) substrates. Investigations of the material microstructure revealed that even moderate structural quality MgO interlayer gives rise to a dramatic improvement in TiN structural perfection and thus optical quality via promoting spontaneous prevailing of the cube-on-cube epitaxial growth of TiN. This is enabled by the expansion of thermodynamically favorable (001)-oriented TiN grains winning the completion against other grain orientations due to the lowest surface energy of the TiN (001) plane. Beyond serving as a buffer layer for growth, the medium refractive index dielectric MgO layer between the metallic TiN and the high refractive index Si supports a low-loss plasmon mode in hybrid waveguide structures, [36] resulting in dramatically improved propagation lengths. By altering the thickness of the MgO interlayer (10-100 nm), the tradeoff between propagation length and electric field enhancement can be tuned in a wide range accordingly, making the TiN/MgO/Si structure qualified for integrated CMOS-compatible nanophotonics applications. Finally, we demonstrate the added benefit of TiN as a robust etching mask to realize Si waveguides with smooth vertical sidewalls to reduce optical scattering losses which otherwise arise from the patterning process.

Results and Discussion
To illustrate the effect of the MgO interlayer, TiN films with the same thickness of %100 nm were grown directly on Si (001), on Si (001) with a 10 nm MgO interlayer (deposited by PA-MBE, see Experimental Section for details), and on bulk MgO (001) substrates via PE-ALD. Growth parameters including plasma exposure time, substrate temperature, and chemisorption time of the PE-ALD process were optimized to obtain TiN films with the best optical properties (see Experimental Section for additional details). We note that our TiN films grown on sapphire at low deposition temperatures of %450 C have already been shown to exhibit high optical performance (peak FOM of 2.5) due to our optimized PE-ALD process involving prolonged plasma exposure and reduced chemisorption times to mitigate precursor decomposition. [37] In regard to the structural quality, Figure 1 shows the comparison of the X-ray diffraction (XRD) 2θ-ω and φ scans of the TiN films on Si (001) with and without the MgO interlayer. The TiN (002) peak in the XRD 2θ-ω scan along the growth direction is barely apparent for the film grown directly on the Si (001) substrate (Figure 1a), indicating the low crystalline quality. However, the (002) peak intensity is significantly enhanced when the MgO interlayer is introduced ( Figure 1b Interestingly, as shown from the scanning transmission electron microscopy (STEM) images in Figure 2, a high crystalline quality is not required for the MgO interlayer to achieve TiN thin films with substantially improved crystallinity. Figure 2c shows that the PA-MBE grown on 10 nm-thick MgO interlayer is composed of a 3 nm amorphous MgO sublayer, a 6 nm polycrystalline MgO sub-layer, and a 2 nm MgTiO or MgTiON sublayer from the Si substrate to the top. These findings are in a good agreement with the reflection high-energy electron diffraction (RHEED) evolution recorded in situ during the MBE growth ( Figure S1, Supporting Information). MgO interlayer initializes in the amorphous state due to the large lattice mismatch between Si and MgO and low substrate temperature (300 C) used during nucleation. As the growth proceeds at 680 C, textured polycrystalline MgO with (001) preferred orientation emerges. The intermediate MgTiO or MgTiON sublayer, as evidenced by energy dispersive X-ray (EDX) spectroscopy, is a result of intermixing between the MgO and TiN layers during the ALD growth procedure ( Figure S2, Supporting Information). However, this intermediate layer does not exhibit the spinel structure characteristic of Mg-containing complex oxides likely due to the relatively low ALD growth temperature. Inheriting from the MgO interlayer, the TiN film initializes in the realm of some competitions between different growth orientations for the first 20 nm. Beyond this region with competing forces, in agreement with the epitaxial relationships revealed by XRD, cube-on-cube epitaxial growth of (001)-oriented grains begins to prevail over the other orientations and eventually dominates, as confirmed by highresolution high-angle annular dark-field (HAADF) imaging (see Figure S3, Supporting Information). The thermodynamic driving force for the predominant expansion of the (001) orientation is the lowest surface energy of the (001) plane in the TiN film. [38] The improvement of the TiN crystalline quality with the MgO interlayer was further confirmed by the corresponding Fast Fourier Transform (FFT) pattern of the HAADF image showing mostly low-angle boundaries and the selective area diffraction (SAD) pattern taken from a 500 Â 500 nm 2 area showing clean diffraction reflexes inherent to relatively good crystalline material containing low-angle boundaries (see Figure S3, Supporting Information).
Based on these observations, it might seem feasible to develop a low-temperature TiN nucleation route to address the lattice mismatch issue for direct TiN deposition on Si. However, the low-quality TiN nucleation layer would very likely act as an  interlayer. It appears that the TiN initiation layer experiences a strong competition between different growth orientations, but then the 50-60 nm thick cube-on-cube epitaxial growth overwhelms other orientations and dominates. c) The MgO interlayer appears to be composed of three sublayers: amorphous MgO (juxtaposed to Si), followed by polycrystalline MgO, and MgTiO or MgTiON, finally culminating in TiN.
www.advancedsciencenews.com www.adpr-journal.com extremely lossy "dead" layer, although the quality of the top layer is high, which would only produce usable films when grown to thicknesses >100 nm. Here, we transfer the destructive interfacial layer from the TiN/Si interface to the MgO/Si interface. An interlayer between TiN and Si, consisting of dielectric MgO, regardless of its crystalline quality, does not strongly harm plasmon functionality beyond the inherent losses in the TiN layer. Instead, it provides additional opportunities and feasibilities to achieve efficient plasmonic devices, which will be discussed hereinafter. It should be noted that the PA-MBE method used here is not crucial (used here for convenience) and the thin MgO interlayer may be deposited using lowtemperature ALD, providing comparable crystalline quality and maintaining the CMOS process compatibility. [39] Moreover, other oxides common to CMOS technologies, such as TiO 2 , SiO 2 , and Al 2 O 3 , could also be investigated as the interlayer between TiN and Si if the lattice mismatch issue can be alleviated. For instance, the growth of TiN/Al 2 O 3 stack on Si for MOS capacitors had been demonstrated by several different methods such as in a single PE-ALD. [40] Even though ALD-grown Al 2 O 3 films on Si are typically amorphous, crystalline Al 2 O 3 using controlled process like annealing [41] could potentially be beneficial to improving the crystalline quality of TiN on Si.
Examining the optical properties determined via spectroscopic ellipsometry (see Figure 3), we note that the real part (ε 0 ) of the permittivity shows very weak substrate dependence for the TiN films investigated, whereas the imaginary part (ε 00 ) varies considerably with the substrate material. In general, higher crystalline quality should provide higher metallicity (more negative ε 0 ) in TiN films. However, other critical factors including stoichiometry and oxygen impurity concentration could have more significant effects on the metallicity. [33,42] Therefore, TiN films with better crystalline quality had not always exhibited higher metallicity. Here, in our TiN films, the similar metallicity is speculated to be due to similar stoichiometry and oxygen impurity concentration among the samples grown on different substrates while in the same reactor under the same ALD growth condition. In contrast, the large difference in ε 00 reflects that the optical losses in films on Si were significantly reduced due to the improvement in the crystallinity via the insertion of the MgO interlayer, and become comparable with those in the films grown on bulk MgO. Figure 4a shows the comparison of the FOM, which considers the tradeoff between losses and metallicity, for TiN films on different substrates. [43] The highest FOM was obtained for TiN grown on bulk MgO, which is consistent with the fact that bulk MgO possesses much better lattice matching to TiN than Si and higher crystalline quality compared with the MgO interlayer. Conversely, the lowest FOM curve was observed for the film grown directly on Si substrate due to the aforementioned high losses even though its metallicity is similar to that of the films grown on bulk MgO and Si with MgO interlayer. The unequivocal impact of the MgO interlayer on Si is the significant improvement of peak FOM from 2.4 to 2.8 (at %1300 nm), and the FOM at 1550 nm from 2.0 to 2.5. This improved performance, made possible by the thin MgO interlayer, nearly replicates that of TiN films grown on bulk MgO under the same conditions. As shown in Figure 4b, the use of MgO interlayer enables our TiN films to significantly outperform the plasmonic quality of other reported films grown on different substrates and by various deposition methods at CMOS compatible temperatures. More importantly, TiN films with an MgO interlayer exhibit the best performance among films grown on Si by PE-ALD [23] and reactive sputtering [34] at CMOS compatible temperatures, and even at higher than 600 C, [24,44] while also achieving their peak and fairly consistent performance in the telecommunications range (Figure 4a) and the solid and open connected dots in Figure 4b. It should also be mentioned that the usage of the 10 nm MgO interlayer shifts our data points to the high-performance edge of the growth temperature dependence trend (Figure 4b) for films grown on bulk MgO, even though our films were grown on a Si substrate.
In addition to the significant improvement of the TiN film quality, the TiN/MgO/Si stack inherently enables the realization of hybrid plasmonic-photonic waveguides (HPPWs), which have found versatile applications in devices such as modulators, [6,7,45,46] polarization control devices, [47][48][49] sensors, [50][51][52] and others. [53] As an example, Figure 5a shows the simulated electric field (E y ) profile of the fundamental mode in the HPPW structures with a   [5,54] As shown in Figure 5b, when the MgO interlayer thickness is varied between 10 and 100 nm, the propagation length of the HPPWs can be tuned between 1 and 25 μm, the higher end being   [6,7,45,46] polarization rotators in the studies by Caspers et al., Xu et al., and Kim and Qi, [47][48][49] and ring resonators for applications in sensing in the studies by Butt et al., Anderson et al., and Chamanzar et al. [50][51][52] The vertical blue lines represent propagation lengths of TiN on Si dielectric-loaded waveguides using the TiN films with the permittivity values reported in the literature (deposited at 250 C, [23] room temperature, [34] and 700 C, [24] respectively) without the MgO interlayer and the vertical red line represents the propagation length of TiN/MgO/Si HPPW with 100 nm thick MgO interlayer.
www.advancedsciencenews.com www.adpr-journal.com comparable with those reported in gold-based hybrid waveguides. [55] Moreover, increasing the MgO thickness from 10 to 100 nm decreases the electric field enhancement factor from 6.9 to 1.3 as expected. This is because a smaller gap size leads to a stronger optical coupling between the plasmonic and photonic modes, providing the ability to optimize electric field enhancement and propagation loss by simply altering the thickness of the MgO interlayer. The practical significance of the propagation length afforded by the TiN/MgO/Si stack is clearly shown in Figure 5c, when it is compared with the device lengths of some recently reported compact plasmonic devices including electro-optic modulators, polarization rotators, and ring resonators (circumferences are used as the device lengths) for sensing applications using common plasmonic metals such as Au, Al, Ag, and Cu. Device lengths for all of these demonstrations lie in the range from a few micrometers to a few tens of micrometers. Without an MgO interlayer, the losses in dielectric-loaded waveguides on the TiN/Si platform would make the realization of such applications extremely challenging when compared with those based on Au and Ag. As shown by the blue vertical lines, propagation lengths of TiN on Si dielectric-loaded waveguides without an MgO interlayer are all below 1 μm (using the permittivity values reported in the literature for films deposited at room temperature, [34] 250, [23] and 700 C [24] ), translating to more than 8 dB μm À1 of propagation loss. In contrast, due to the significantly improved plasmonic performance, the as-grown TiN/MgO/Si HPPWs can provide sufficiently long propagation lengths for all of the applications listed.
In addition to the propagation loss, optical scattering losses arising from the edge roughness produced during fabrication could significantly undermine the device performance or necessitate multiple complex fabrication steps to eliminate. [56] Compared with metals like Au, TiN is significantly easier to process and can provide high-quality vertical edges due to its robustness and high etching selectivity. As an example, the crosssectional and plan-view scanning electron microscopy (SEM) images of step-etch profiles in TiN/MgO/Si and Au/Si structures obtained using chlorine chemistry-based reactive ion etching (RIE) of TiN and Au, followed by fluoride-based RIE of Si are compared in Figure 6. Due to its crystalline nature and ease of etching in chlorine chemistry, a relatively vertical sidewall with a straight edge was achieved for the TiN/MgO/Si structure. In contrast, a highly sloped sidewall with a much higher edge roughness was obtained for the Au/Si structure. Another significant advantage of TiN is its ability to serve as a self-aligned hard mask for subsequent etching steps without any considerable physical or chemical etching. Therefore, it helps to both reduce the scattering loss and simplify fabrication processes for integrated nanophotonic structures. Although Au is also not reactive, its softness limits its ability to be used as a hard-mask, thereby requiring additional lithography steps, a separate hard mask, or an extremely thick resist to effectively pattern. As a result, our platform not only enables improved plasmonic performance of TiN with readily access to a CMOS compatible HPPW structure, but also provides inherent advantages in simplifying high-quality device fabrication and processing. www.advancedsciencenews.com www.adpr-journal.com

Conclusion
We have demonstrated the significant impact of an MgO nucleation interlayer to the plasmonic quality and device functionality of the TiN on Si platform for CMOS compatible plasmonics and nanophotonics. Using a 10 nm moderately crystalline MgO interlayer, the overgrown TiN films are shown to evolve from multiple oriented to thermodynamically favorable, (001)-oriented single crystal, thereby producing a high plasmonic FOM of 2.5 at 1550 nm and peak FOM of 2.8 due to the reduction of optical losses. The result is the translation of TiN to the bulk-MgO growth curve on a Si platform. Moreover, this tendency illustrates that under the PE-ALD growth conditions, a highly crystalline interlayer is not required to improve performance, which thereby opens a potential for scalable low-temperature deposition processes for TiN on Si. We have also shown that, in addition to acting as a nucleation layer, such a medium refractive index MgO interlayer between the metallic TiN and the high-index Si inherently supports a compact low-loss hybrid plasmonic mode and provides key practical processing benefits as a self-aligned etch mask for optical waveguides, thereby providing a framework to realize efficient CMOS-compatible plasmonic devices on Si.

Experimental Section
MgO Interlayer Growth: Growth of thin MgO interlayers on Si (001) substrates was carried out using PA-MBE equipped with a RHEED system so that the evolution of growth can be in situ monitored. A Knudsen cell was used to evaporate Mg, and a radio-frequency plasma source operating at 400 W was used as the source of reactive oxygen. Before loading into the MBE system, Si substrates were treated by a chemical cleaning process, which includes degreasing, boiling in dilute HCl and H 2 O 2 solution to remove metal ions, followed by removing the oxide layer in dilute HF, rinsing in deionized water, and blow-drying with nitrogen. After in situ degassing at 650 C for 10 min, the substrate temperature was reduced to 300 C for the deposition of a 2-3 nm MgO nucleation layer preceded by a 30 s Mg pre-exposure step to prevent oxidation of the Si substrate. Then the substrate temperature was raised, and a high-temperature MgO layer was deposited at 680 C for the subsequent TiN growth. The growth rate of the MgO layer was about 0.7 nm min À1 . As reported in the literature, highquality MgO thin films could be realized by low-temperature processes such as ALD and electron beam evaporation, [39,57] suitable for future development of fully CMOS compatible films for integrated devices.
TiN Growth: As a scalable and conformal technique for industrial-scale operation, a low-temperature plasma-enhanced atomic layer deposition (LT PE-ALD) system was used for the growth of TiN thin films on the MgO/Si composite described earlier. Tetrakis (dimethlamino) titanium (IV) (TDMAT) and nitrogen plasma were used as the Ti and N precursors, respectively. Each cycle of ALD consisted of a pulse of TDMAT on to the heated substrate followed by exposure to N 2 plasma. Deposition parameters of a plasma exposure time of 25 s, a chemisorption time of 0.5 s, and a substrate temperature of 450 C were used based on previous optimization routines to obtain the material with the best optical properties. [37] For a comparative study, the simultaneous growth of TiN directly on Si (001) without an MgO interlayer, and on bulk MgO (001) substrates was also carried out. The growth rate of the TiN films was about 1.3 Å per cycle, independent of the substrate material.
Reactive Ion Etching: The RIE was carried out in a Samco inductively coupled plasma (ICP) etching system (model: RIE-101 iPH) with photoresist as the mask. Chlorine chemistry-based RIE had been demonstrated to provide reasonable etching rates for both TiN and Au. [58,59] A gas mixture condition of Cl 2 /Ar ¼ 5/20 sccm with the ICP source power/bias power set at 300/100 W and the chamber pressure at 0.4 Pa was optimized for Au etching and carried out for TiN etching as well. A gas mixture condition of SF 6 /C 4 F 8 ¼ 20/14 sccm with the ICP source power/bias power set at 60/16 W and the chamber pressure at 0.5 Pa was carried out for Si etching.
Measurement: Optical characterization of the films was carried out using a J.A. Woollam M-2000 variable angle spectroscopic ellipsometer (VASE) by fitting spectroscopic ellipsometer data. [60,61] The thicknesses of the samples were measured via cross-sectional SEM and fixed in the ellipsometer model. The data from the ellipsometer were fit using a Bspline model, enforcing causality via the Kramers-Kronig (KK) relation. To maintain KK consistency, Complete Ease (software) fitted the psi and delta values to the imaginary component of the permittivity and subsequently calculated the real part of it by the KK relation. We added the experimental psi and delta curves of TiN films deposited on Si (001) with an MgO interlayer with fitted data as shown in Figure S4, Supporting Information. As shown, the model matched well with the measured psi and delta curves, with a mean square error of between 4 and 6. The structural properties were investigated using XRD double-axis 2θ-ω and φ scans. STEM measurements were carried out on a Titan microscope with a corrected electron optical systems (CEOS) probe aberration-corrector operated at 200 kV. STEM images were collected with a 24.5 mrad probe semiangle, 26 pA probe current with STEM resolution of %0.8 Å. EDX spectrum profiles were carried out at %200 pA probe current. The surface morphology of the layers was evaluated using atomic force microscopy (AFM). The etching profiles of TiN/MgO/Si and Au/Si structures were evaluated using SEM.
Simulation: The modal profiles, propagation lengths, and electric field enhancement factor of the HPPW structures using Si as the core material were simulated via the commercial software COMSOL Multiphysics at a wavelength of λ 0 ¼ 1300 nm. For all the structures, the designs closely replicated the scenarios of growths on a silicon on insulator (SOI) substrate. The Si core has a width of 400 nm and a height of 340 nm. The thickness of the MgO interlayer was set between 10 and 100 nm with a step of 10 nm. As confirmed by our ALD of TiN and the VASE measurements, the variation of the MgO interlayer does not show obvious change on the optical properties of the TiN films. The refractive indices of Si, SiO 2 , and MgO were set as 3.45, 1.47, and 1.90 at 1300 nm, respectively. The permittivity values of TiN films were taken from VASE measurements for our samples and from the corresponding reports for the reference samples. The propagation length was defined as the distance that the amplitude of the field attenuates to 1/e. The field enhancement factor was defined as the ratio between the peak amplitude of the normalized electric field |E| (|E| 2 ¼|E x | 2 þ |E y | 2 þ |E z | 2 ) in the hybrid area for the HPPW with TiN cap layer and that in the Si core for the SOI waveguide without a TiN cap.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.