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Abstract
Core-shell slanted nanocolumnar thin films (SCTFs) were produced by e-beam glancing angle deposition of SiO2 and subsequent atomic layer deposition (ALD) of TiN. Conformity of the ALD film over the slanted columns was confirmed by transmission electron microscopy and energy dispersive X-ray spectroscopy mappings. Angle resolved spectrophotometry characterization revealed angle-dependent transmission of the films, akin to their fully metallic counterparts. We found that such inclined core-shell nanocolumn films enable tailoring of the angular selectivity independently from the SCTF thickness and density.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Thin films with angular-selective (AS) transmission can yield significant improvements in optical performance suitable for applications such as ophthalmics, fenestration, as well as optoelectronic technologies. For instance, such films can be used to correct certain visual impairments, to tune solar transmission as a function of the sun’s position in the sky [1], or to attenuate parasitic light sources coming from specific angles.
Slanted columnar thin films (SCTFs) fabricated through glancing angle deposition (GLAD) are an effective means of obtaining AS optical properties [2–8]. Most notably, metal SCTFs have been shown to exhibit high optical transmission when incident light is parallel to the metal columns, and low transmission when it is perpendicular to them [1,2]. As an example, Mbise et al. reported evaporated chromium SCTFs with 52% luminous transmittance at a positive angle of 60°, and 24% at the negative angle equivalent [2]. For selected wavelengths and for p- polarised light, the transmission of such films can reach values which are ten times higher at positive angles then at negative angles [2].
The observed angular selectivity results from the geometry of the optically absorbing columns which leads to a higher absorption in one direction and a lower absorption in the other. Precise properties of GLAD-based SCTFs can, however, be difficult to predict solely from deposition parameters. Indeed, in GLAD coatings, the columns’ inclination is adjusted by varying the angle of the incident material flux; the latter, however, also affects the film density and growth rate [4]. In addition, even with identical glancing angles, film porosities and column angles vary significantly for different materials [9], substrate temperatures, deposition rates [4] and other parameters. The density of SCTFs also depends on their thickness, due to extinction and broadening of the columns during their growth [4,10]. As a result, manufacturing GLAD-based AS coatings is a major challenge if independent tailoring of either the angle of the columns, the opacity and/or the film color is required.
Additionally, GLAD SCTFs are limited to materials that can be deposited by physical vapor deposition (PVD) [4]. In fact, the most AS SCTFs are obtained through evaporation [2] as it combines long throw distances and mean free paths [4]. As the evaporated materials are limited, this discourages the use of certain optically important materials for manufacturing AS films. Most notably, this includes titanium nitride (TiN), which is of interest for its plasmonic properties [11–13], high thermal stability [14], good mechanical properties [13], and compatibility with CMOS technology [15].
In this paper, AS optical coatings are made by depositing conformal ALD layers of TiN over optically transparent SiO2 SCTFs. Since selectivity is mainly absorption-based, the combination of GLAD and ALD provides an additional degree of freedom to independently adjust the microstructural and optical characteristics, leading to adjustable angular selectivity. ALD functionalization of SCTFs has been reported in the literature for passivation of functional columns [16–18], decoupling of device microstructure and surface chemistry [19,20], and for the fabrication of tubular magnetic nanostructures [21]. To our knowledge, we present the first application of this technique that adds optical functionality to existing SCTFs. Consequently, we compare the AS performance of these films to conventionally fabricated SCTFs and discuss the advantages of the proposed approach.
2. Experimental methodology
2.1 GLAD deposition of the SiO2 cores
SiO2 SCTFs were prepared on B270 glass substrates using electron beam evaporation in a pilot scale Leybold Optics box coater (Boxer Pro) equipped with a custom substrate holder allowing for a glancing angle of 87° [22]. SiO2 1-5 mm pellets of 99.99% purity were used as the source material. Prior to deposition, the process chamber was pumped down to a base pressure of 2 × 10−6 Torr. The angle of the columns was subsequently characterized by scanning electron microscopy (SEM) of the film cross-section (see Fig. 1). The measured column angle was then used in conjunction with variable angle spectroscopic ellipsometry measurements using a J.A. Woollam Co. RC2-XI and subsequent Bruggeman modeling [23] to determine the thickness, porosity and refractive index of the films (see Table 1).
Fig. 1. Scanning electron micrograph of a SiO2 slanted columnar thin film taken prior to ALD functionalization.
Table 1. SiO2 SCTF characteristics prior to ALD functionalization. Thickness, porosity and refractive index were determined by ellipsometry whereas the column angle was measured by SEM imaging.
2.2 Deposition of TiN shells by ALD
The SiO2 GLAD films were subsequently functionalized with different thicknesses of titanium nitride using titanium tetrachloride (TiCl4) and ammonia (NH3) precursors [24] in an ALD-150LX system from Kurt J. Lesker Company. The exposure time for both the TiCl4 and the NH3 were previously optimized to yield saturated ALD growth for a sample surface temperature of 335 °C.
Three different samples were made using 400, 500 and 600 cycles of ALD. The thicknesses were predicted to be of 5 ± 1 nm, 6 ± 1 nm and 7 ± 1 nm, respectively, using the known growth per cycle (GPC) of the process (0.12 ± 0.02 Å/cycle). The GPC was obtained from a 500-cycle TiN sample deposited on a Si wafer with a 1050 nm thermal oxide [25] known to improve thickness measurements for thin absorbing films by interference-enhancement during ellipsometric modeling [26].
2.3 Characterization of core-shell SCTFs
Transmission electron microscopy (TEM) measurements were taken over a core-shell column using a JEOL JEM-2100F microscope operated at 200 keV.
Spectral angular transmittance values, ${T_p}$ and ${T_s}$, were assessed for polarizations parallel (p) and perpendicular (s) to the incidence plane, respectively using an Agilent Cary 7000 Universal Measurement Spectrophotometer (UMS) in which film columns were oriented parallel to the incidence plane. Subsequently, the luminous transmittance ($T_p^{lum}, \,T_s^{lum}, \,T_u^{lum}$, where u indicates unpolarized light) was obtained for each incidence angle and polarization from an average of the spectral transmittance weighted to the CIE 1931 2° standard observer and CIE E standard illuminant [2,27].
3. Results and discussion
3.1 TEM analysis of shell conformity
While ALD is known for conformal deposition, results can vary significantly with subtle differences in substrate surface chemistry [26] and process conditions. In this section, we demonstrate the presence of distinct TiN shells and SiO2 cores in our core-shell SCTFs using TEM. As can be seen in Fig. 2(a), from the TEM micrographs of a representative TiN-coated nanocolumn, a clear and distinct dark shell of TiN is visible around the SiO2 nanocolumn. The thickness of the shell was estimated to be 7 $\pm $ 1 nm for the 600-cycle sample, and 4 ${\pm}$ 1 nm for the 500-cycle sample. In both cases, no significant variation of the shell thickness was observed along the column length. The measured thicknesses are within the range predicted by the known growth rate of the ALD process (7 ± 1 nm and 6 ± 1 nm, respectively), indicating that optimal ALD growth occurred conformally around the SiO2 columns. The slightly lower thickness for the 500-cycle shell may indicate a longer nucleation delay [25] on the SiO2 columns than on the thermal oxide used for GPC determination or the presence of deposition within the columns.
Fig. 2. Micrographs of a TiN-coated SiO2 nanocolumn obtained by transmission electron microscopy. (a) TEM bright field nanocolumn image used for measuring the TiN shell thickness. (b) Selected-area electron diffractogram taken over the TiN-coated SiO2 nanocolumn shown in (a). Mappings of (c) titanium and (d) silicon EDS counts over the nanocolumn shown in (a).
In Fig. 2(c) and 2(d), EDS mappings confirm that the shell observed in Fig. 2(a) is indeed composed of titanium, or of a titanium compound. As can be seen, the counts of the titanium element are concentrated on the edges of the nanocolumn, whereas the counts of the silicon element are concentrated in the center, suggesting the presence of a distinct core and shell.
The crystalline structure of the shell was assessed by a selected-area diffraction measurement (SAED) (see Fig. 2(b)). The presence of rings suggests that the darker region appearing in Fig. 2(a) is caused by the presence of a polycrystalline layer of titanium nitride enveloping the SiO2 column: on the edges of the column, multiple grains diffract the incident electrons, leading to darker edges. Dark spots can also be seen in the center of the column, corresponding to random diffracting grains present on the surface or on the bottom of the column. The interplanar distances associated with the first three rings of the diffractogram are 2.39 Å, 2.08 Å and 1.48 Å. These distances correspond to the (111), (200) and (220) planes of cubic TiN [28] albeit the observed offset may likely be caused by oxidation [29] and/or a non-stoichiometric Ti/N ratio [30,31]. Considering these results, it is concluded that the ALD deposited TiN envelops the SiO2 columns leading to the formation of core-shell SCTFs.
3.2 Angular selectivity of core-shell SCTFs
While SiO2 SCTFs normally do not exhibit AS in the visible spectrum, the presence of an absorbing TiN shell in the core-shell SCTFs results in angular selective behaviour. As an example, the spectral transmission in the visible region for the 500-cycle sample is shown in Fig. 3(a) for three angles of incidence: $0^\circ $, $+ 60^\circ $ and $- 60^\circ .\; $One can see that ${T_p}({ + 60^\circ } )> {T_p}({0^\circ } )> {T_p}({ - 60^\circ } )$ for all wavelengths. Such asymmetric behaviour is true for all angles of incidence but for p-polarization only, while for s-polarization: ${T_s}(\theta )= {T_s}({ - \theta } )$. The angular selectivity of these films can be defined in two ways: a) the ratio of transmitted light at a positive and negative angle which gives the relative AS: $S(\theta )= T({ + \theta } )/T({ - \theta } )$, and b) the difference in transmitted light at a positive and negative angle which gives the absolute AS: $\Delta T(\theta )= T(\theta )- T({ - \theta } )$. Relative AS is useful for predicting signal to noise ratios, in optoelectronic applications for instance, whereas absolute AS is useful for indicating differences in perceived light intensity, or differences in transmitted solar energy, for ophthalmic and energy applications.
Fig. 3. (a) p- (left y-axis) and s-polarized (right y-axis) transmission spectra of a SiO2 slanted columnar thin film coated with 500 cycles of TiN by ALD measured at three different angles. Transmission at positive and negative angles are identical for s-polarized light; (b) relative AS and (c) absolute AS at 60° for core-shell SCTFs with 400, 500 and 600 cycles of ALD.
${S_p}({60^\circ } )$ and $\Delta {T_p}({60^\circ } )$ are plotted in Fig. 3(b) and 3(c) for the 400-, 500- and 600-cycle samples as a function of wavelength. Taking the 500-cycle sample as an example, it can be seen that both these values vary significantly as documented by the peaks at 360 nm and 520 nm, respectively. A high transmission region above 500 nm corresponds to rather low values of ${S_p}$, but to relatively high $\Delta {T_p}\; > \; 0.4$. The gradually increasing TiN absorption [12] at shorter wavelengths leads to a lower transmission and hence to a decrease of $\Delta {T_p}$. Meanwhile, ${S_p}$ rises sharply, up to a maximum value of 17. Depending on the application, the balance between these two values for a fixed wavelength can be adjusted by the thickness of the absorbing shell.
Similar observations are true for the overall luminous angular selectivity of SCTFs, as it can be seen in Fig. 4, where polarized and unpolarized angular luminous transmittance, ${T_{lum}}$, are presented for all three shell thicknesses as well as for an uncoated SCTF. As expected, the uncoated film exhibits no observable difference in transmission between positive and negative angles. However, the angular transmission of the coated SiO2 SCTFs is asymmetrical for p-polarization and symmetrical for s-polarization. For the coatings with 500 and 600 cycles of TiN, the angular position of the unpolarized light transmission maximum is measured at 50°, which is in close agreement with the 44° angle of the columns’ inclination and consistent with observations of metal SCTFs [2].
The measured $S_u^{lum}({60^\circ } )$ (i.e. $\frac{{T_u^{lum}({ + 60^\circ } )}}{{T_u^{lum}({ - 60^\circ } )}}$) values are 1.08, 1.75 and 2.48 respectively for 400, 500 and 600 cycles of ALD. Therefore, it is deduced that increasing the thickness of the shell leads to higher measured $S_u^{lum}$ and lower measured $\Delta T_u^{lum}$, similarly to the effect of increasing the thickness of a pure metal SCTF [2]. Contrarily to the latter, however, increasing the shell thickness does not significantly affect the size and shape of the columns. Thus, core-shell SCTFs can have different selectivity with an identical morphology. In addition, metallic nanocolumnar angular selective coatings are typically limited to below 100 nm, as higher thicknesses lead to very low visible transmission [2]. This is not the case for the core-shell coatings studied here since longer columns can be compensated for by reducing the thickness of the absorbing shell. This offers an opportunity to control the interference in the film and other thickness-related effects, as the film thickness and absorption can be independently adjusted. In our case, this allows one to tailor the spectral response of the films. Indeed, the high values of $\Delta {T_p} > 0.3$ are reached at shorter wavelengths for the 400 cycles sample, in the middle of the visible for the 500 cycles one, and in longer wavelengths for the 600 cycles (see Fig. 3(c)). This tunability is achieved without a significant loss in overall angular selective performance.
Fig. 4. Luminous transmittance (Tlum) as a function of incidence angle ($\theta $) for SiO2 SCTFs coated with, from left to right, 0, 400, 500 and 600 cycles of TiN using ALD. The curves present s- and p-polarizations as well as unpolarized light (u).
The overall AS performance of our samples is, in fact, comparable to the best results (highest ${S^{lum}}({60^\circ } )\; $at given ${T^{lum}}({60^\circ } )$) known in the literature. Indeed, Fig. 5 shows a comparison between our core-shell films and selected results from other authors. Films offering high ${T^{lum}}({60^\circ } )$, in conjunction with high ${S^{lum}}({60^\circ } )$, are most desirable for AS applications, thus films trending towards the upper right part of the figure offer a better performance. As seen in the figure, the core-shell samples are similar in performance to evaporated Cr films and perform substantially better than sputtered Cr SCTFs, due to the structure they inherit from evaporated SiO2 SCTFs.
Fig. 5. Relative luminous angular selectivity ($T_u^{lum}({ + 60^\circ } )/T_u^{lum}({ - 60^\circ } )$) vs. maximum transmittance ($T_u^{lum}({ + 60^\circ } )$) of unpolarized light. TiN-SiO2 core-shell SCTFs values are compared with data published by Mbise et. al. for evaporated (e) and sputtered (s) Cr SCTFs as well as reactive DC sputtered Al2O3 SCTFs with metallic Al inclusions [2].
Indeed, this exemplifies another key advantage of the core-shell SCTF technique: while evaporation of good quality AS TiN SCTFs is technically challenging, the core-shell method enables one to incorporate the optical properties and temperature- and corrosion-resistant qualities of a material such as TiN into a AS film while leveraging the structure of a well established SCTF material such as SiO2.
4. Conclusions
In conclusion, conformal deposition of TiN shells was achieved over SiO2 SCTFs by ALD. The resulting core-shell films presented pronounced AS properties, similar to metal SCTFs. Contrarily to the latter, absorption and thus angular selectivity can be tailored independently from the column length. Furthermore, the use of ALD to deposit absorbing shells enables one to use new materials, such as durable metal nitrides, in the fabrication of GLAD-based AS coatings.
Funding
Natural Sciences and Engineering Research Council of Canada (IRCPJ 433808-11).
Acknowledgements
This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Multisectorial Industrial Research Chair in Coatings and Surface Engineering (MIC-CSE) program, and by Essilor International who provided evaporation deposition equipment. The authors also thank Mr. Sébastien Chénard and Mr. Francis Turcot for their technical assistance, Julien Gagnon for assisting with GLAD deposition and Dr. Richard Vernhes for stimulating discussions.
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