W:Al2O3 Nanocomposite Thin Films with Tunable Optical Properties

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W:Al2O3 Nanocomposite Thin Films with Tunable Optical Properties Prepared by Atomic Layer Deposition Shaista Babar,† Anil U. Mane,† Angel Yanguas-Gil,† Elham Mohimi,‡ Richard T. Haasch,§ and Jeffrey W. Elam*,† †

Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60439, United States Department of Materials Science and Engineering, University of Illinois at Urbana−Champaign, Champaign, Illinois 61820, United States § Materials Research Laboratory, University of Illinois at Urbana−Champaign, Champaign, Illinois 61820, United States ‡

S Supporting Information *

ABSTRACT: A systematic alteration in the optical properties of W:Al2O3 nanocomposite films is demonstrated by precisely varying the W cycle percentage (W%) from 0 to 100% in Al2O3 during atomic layer deposition. The direct and indirect band energies of the nanocomposite materials decrease from 5.2 to 4.2 eV and from 3.3 to 1.8 eV, respectively, by increasing the W% from 10 to 40. X-ray absorption spectroscopy reveals that, for W% < 50, W is present in both metallic and suboxide states, whereas, for W% ≥ 50, only metallic W is seen. This transition from dielectric to metallic character at W% ∼ 50 is accompanied by an increase in the electrical and thermal conductivity and the disappearance of a clear band gap in the absorption spectrum. The density of the films increases monotonically from 3.1 g/ cm3 for pure Al2O3 to 17.1 g/cm3 for pure W, whereas the surface roughness is greatest for the W% = 50 films. The W:Al2O3 nanocomposite films are thermally stable and show little change in optical properties upon annealing in air at 500 °C. These W:Al2O3 nanocomposite films show promise as selective solar absorption coatings for concentrated solar power applications.

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INTRODUCTION The ability to design and engineer new materials with optical and electronic properties surpassing those of naturally occurring materials will enable breakthroughs in microelectronics and energy technology. One of the most useful attributes for any new material is tunability: easily adjustable material properties that can be optimized for a particular application or design. Transparent conducting oxides (TCOs) are a good example of tunable materials since the electrical conductivity can be adjusted over a broad range by controlling the concentration of charge carriers (e.g., electrons for ndoping) of an oxide semiconductor.1 However, tunability of the optical properties in TCOs is modest, since the band gap and dielectric constant are dominated by the oxide semiconductor material through most of the relevant parameter space. Nanostructured composites offer a means to greatly expand the range of optical properties available for materials and device design.2 For instance, cermets composed of metallic nanoparticles in a dielectric matrix can be designed to possess hybrid metal/ceramic properties, which in principle makes them wellsuited to bridge the gap in optical and electric properties between dielectrics and metals.3 One of the challenges of nanocomposite materials, though, is optical scattering: high contrast between the dielectric constant of the metal and the ceramic component can greatly increase the optical scattering © XXXX American Chemical Society

cross section, rendering the material translucent, which is unacceptable for many applications.4 It is, therefore, advantageous to identify tunable materials that can bridge the gap between metals and dielectrics without increasing optical scattering. One promising material class that meets these criteria is thin film nanocomposites comprising W and Al2O3 synthesized by atomic layer deposition (ALD). ALD is a powerful method for precisely doping thin film materials because the film growth is mediated by self-limiting surface chemical reactions and not by the fluxes of the incoming species.5,6 This provides precise control over the film thickness and also very conformal coating over high aspect ratio 3D structures.7,8 More importantly for this study, the self-limiting growth yields exquisite control over composition by alternating between the chemistries of two or more ALD materials in a controlled fashion.9−13 The ALD of W:Al2O3 mixed layers has been previously demonstrated in the form of both nanocomposites and nanolaminates.14−19 The ALD W:Al2O3 nanolaminate films have been implemented as hard X-ray mirrors17,18 and thermal barrier coatings.15 The ALD W:Al2O3 nanocomposite films have been utilized as Received: April 14, 2016 Revised: June 9, 2016

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The Journal of Physical Chemistry C tunable resistive films14,19 where the film resistivity can be varied by orders of magnitude by adjusting the W cycle percentage. Tunable resistive films composed of both ALD W:Al2O3 and Mo:Al2O3 nanocomposite layers have been used to manufacture large area microchannel plates and electron optical MEMS devices.19−21 To date, the optical properties of the ALD W:Al2O3 nanocomposites have not been explored. However, these previous works showed that the metallic nanoparticle size in the composite material was extremely small (1−2 nm),14,19,22 and this is a necessary condition to minimize optical scattering. Therefore, the ALD of W:Al2O3 nanocomposite films is a promising approach to synthesize new materials with optical properties in the dielectric-metal continuum and with a low diffuse scattering component. In this study, ALD W:Al2O3 nanocomposite films were prepared with W cycle percentages ranging from 0 to 100%. These films were characterized using spectroscopic ellipsometry (SE) and ultraviolet−visible spectrophotometry (UV−vis) to determine the optical properties. In order to correlate the optical properties with other physical and chemical properties of the films, we performed X-ray photoelectron spectroscopy (XPS), X-ray reflectivity (XRR), X-ray fluorescence (XRF), and scanning electron microscopy (SEM) measurements. W:Al2O3 nanocomposite films prepared by sputtering have been used previously as selective solar absorbing layers in concentrated solar power receivers where the coatings are operated at high temperatures.23,24 Consequently, we investigated the thermal stability of the nanocomposite layers at high temperatures in both inert and oxidizing environments and also measured the thermal conductivity of the ALD films to evaluate their potential as selective solar absorbing layers.

The composition and chemical states were determined by performing high-resolution XPS scans over the energy ranges corresponding to the elements of interest. The XPS measurements were performed both for the as-deposited films and after Ar ion sputtering for 6 min. This sputtering treatment is known to remove ∼24 nm from pure SiO2 films. The XPS spectra were not charge-compensated for the chemical state analysis. For XPS valence band studies of Al2O3, a Kratos Axis was used (Al Kα source). The film thickness and microstructural features were examined using high-resolution scanning electron microscopy (SEM, Hitachi S-4700), X-ray reflectivity (XRR), and spectroscopic ellipsometry (SE, J. A. Woollam Co. alphaSE). The optical properties of the films, including the real and imaginary components of the refractive index and the direct and indirect band gaps, were extracted from SE and ultraviolet−visible spectrophotometry (UV−vis, Cary 5000) measurements. The UV−vis measurements included both fractional transmission (T) and reflection (R) and were performed using an integrating sphere. These measurements were combined to obtain the fractional absorption, A = 1 − T − R, and the absorption coefficient was calculated from α = −log(1 − A)/t, where t is film thickness. A subset of the films were annealed at 450 °C in nitrogen for 4 h, at 500 °C in air for 1 h, and at 800 °C in air for 4 h, and the optical properties were remeasured before after the annealing treatments. Four point probe sheet resistance measurements were performed on films deposited on fused silica substrates to determine the electrical conductivity of the films, and time domain thermoreflectance spectroscopy (TDTR)16 was used on films deposited on Si(100) substrates to measure the thermal conductivity of the films.

EXPERIMENTAL SECTION Thin ALD W:Al2O3 composite films were prepared in a custom hot-walled viscous flow ALD reactor described elsewhere25 and using previously published methods.20,26 The precursors TMA (Aldrich, 99%), deionized H2O, Si2H6 (Voltaix, 99.999%), and WF6 (Aldrich, 99.9%) were used and maintained at room temperature. Films were deposited on quartz and p-type Si(100) substrates, and the deposition temperature was maintained at 200 °C. As-received substrates were loaded in the reactor without any further surface cleaning. The ultrahigh purity Ar carrier gas flow was set to 300 sccm, which provided a base pressure of 1.5 Torr in the ALD reaction chamber. The ALD timing sequence can be expressed as (t1-t2-t3-t4) where t1 and t3 are the dose times for the first and second precursor, respectively, t2 and t4 are the corresponding purge times, and all times are expressed in seconds. For the Al2O3 ALD, TMA and H2O were alternately pulsed into the flowing Ar carrier gas with the timing sequence (1-10-1-7), and the W ALD used alternating WF6 and Si2H6 exposures with the timing sequence (1-7-1-7). The partial pressures of the TMA, H2O, WF6, and Si2H6 during their respective dosing intervals were 0.2, 0.3, 0.1, and 0.25 Torr, respectively. We define the W cycle ratio as W% = W/(W + Al2O3) × 100, where W and Al2O3 are the relative number of WF6/Si2H6 and TMA/H2O ALD cycles performed, respectively. After the growth, the substrates were removed from the reactor at 200 °C, and as a result, the metallic portions of the film surface may have become oxidized, especially for the higher W% samples. The composition and chemical state of the films were measured by X-ray photoelectron spectroscopy (XPS) analysis using a Physical Electronics PHI 5400 (Mg Kα X-ray source).

RESULTS AND DISCUSSION Physical Properties. The microstructure, thickness, roughness and density of ALD W:Al2O3 composite films deposited on Si(100) substrates were determined using XRR, SEM, and ellipsometry. The thicknesses of films with W cycle ratio < 50% were extracted from ellipsometry, whereas, for the more optically absorbing films (>50% W cycle ratio), the thickness was determined using cross-sectional SEM. The growth per cycle (GPC) values derived from these thickness measurements are plotted versus W% in Figure 1. The dashed line in Figure 1 shows the rule-of-mixtures prediction for the GPC values of the composite films, which is a weighted average of the GPC values for the pure compounds. It is interesting to note that, for W% < 50, the GPC values are significantly below the rule-of-mixture predictions, whereas, for W% > 40, the agreement is much better. We attribute this behavior to the inhibition of the W ALD by the Al2O3.14 The XRR and SEM measurements for the ALD W:Al2O3 nanocomposite films are shown in Figure 2. The XRR data in Figure 2a were fit using parametrized models to extract the density, surface roughness, and interfacial roughness, and these values are given in Table 1. The density of the nanocomposite films was determined from the position of the critical angle in the XRR measurements. Above the critical angle, reflections from the air−film and film−substrate interfaces give rise to interference fringes. The period of these interference fringes and the drop in intensity are related to the thickness and roughness of the layers, respectively. The XRR measurements show clear fringes for W% < 50, indicating smooth films. For W % ≥ 50, the XRR interference fringes decrease in amplitude, indicating a rougher film surface. The density of the films

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Figure 3. Density of ALD W:Al2O3 nanocomposite films as measured by XRR (left axis), and relative W content as measured by XRF (right axis) versus W cycle ratio.

Figure 1. Growth per cycle (GPC) vs W cycle ratio for ALD W:Al2O3 nanocomposite films determined from spectroscopic ellipsometry, Xray reflectivity, and cross-sectional scanning electron microscopy (SEM) measurements. Dashed line shows the predicted GPC values using the rule-of-mixtures.

XRF (Figure 3, right-hand axis). It is curious that the XRR data for the 70% nanocomposite film show a peak near 2.75° in Figure 2a. This peak is not likely to be a measurement artifact since it appeared in multiple samples prepared under identical conditions. This peak may result from the emergence of a superstructure with a spacing of ∼2.5 nm at this particular composition. High-resolution cross-sectional transmission electron microscopy (TEM) could evaluate this hypothesis. Figure 2 shows SEM images for the 50 W% film (b) and the 70 W% film (c). The 50 W% film has a visibly rougher surface compared to the 70 W% film, and this is consistent with the XRR measurements (Table 1). Analysis of Composition and Chemical State. X-ray photoelectron spectroscopy (XPS) was used to analyze the composition of ALD W:Al2O3 composite films deposited on Si(100) substrates using 0, 30, 50, 70, and 100 W%. Survey XPS scans for the 0, 50, and 70 W% films recorded after 6 min of sputtering to clean the surface are shown in Figure 4a. These films are composed of W, Al, F, O, C, and Si. The F and C likely result from AlF3 and metal carbides, respectively.14,27 These compounds result from the unusual ALD chemistry that occurs when transitioning between the Al2O3 and metal ALD processes. The elemental composition of the films was extracted from peak-fitting of high-resolution XPS spectra using CasaXPS software, and the results are presented in Figure 4b. As expected, the Al and O concentrations decreased and the W concentration increased with increasing W%. In addition, the fluorine content decreased while the C and Si remained relatively constant for increasing W%. The W concentrations measured by XPS are significantly lower than the values predicted using a rule-of-mixtures formula in the range W% < 50. This finding is consistent with previous QCM studies that the Al2O3 ALD inhibits the W ALD.14 The high-resolution scans performed with only minimal sputtering revealed that, for W% < 50, the W is present mostly as an oxide, whereas, for W% ≥ 50, the W is mostly metallic. With prolonged sputtering, the W appeared to become more metallic, but we attribute this observation to sputtering-induced W-oxide reduction to form W metal.28 The Al is mainly bound to F and O, suggesting that the matrix is composed of aluminum oxyfluoride. The Si is mostly present in the oxide form, and the carbon is mostly present as graphite. However, below 30 W%, an additional C

Figure 2. (a) XRR measurements for ALD W:Al2O3 nanocomposite films. Cross-sectional SEM images for films prepared using W cycle ratios of (b) 50% and (c) 70%.

Table 1. Density, Growth per Cycle, Surface Roughness, and Interface Roughness for ALD W:Al2O3 Nanocomposite Films with W Cycle Ratios in the Range of 0−100% Calculated from Fitting the XRR Data in Figure 2a W cycles (%)

density (gm/cm3)

growth per cycle (Å/cycle)

surface roughness (nm)

interfacial roughness (nm)

0 30 50 60 70 100

3.2 3.7 6.6 12 14 17

1.2 1.1 2.8 3.3 3.4 5.0

0.7 1.0 3.4 1.3 1.2 1.7

0.7 0.8 0.7 0.5 2.0 0.8

increases with increasing W% as shown in Table 1 and Figure 3. This density increase agrees very well with the increase in W content of the films with increasing W% as determined from C

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Figure 4. (a) XPS surface survey scans after 6 min Ar ion sputter cleaning for ALD W:Al2O3 nanocomposite films prepared using W cycle ratios of 30, 50, and 70%. (b) Atomic concentration versus W cycle ratio for ALD W:Al2O3 composite films after 6 min Ar ion sputter cleaning.

Figure 5. (a) Absorption coefficient versus photon energy from UV− vis measurements of ALD W:Al2O3 nanocomposite films on quartz substrates having W cycle ratios between 0 and 60. (b) Direct and indirect band-gap energies calculated from Tauc plots of UV−vis absorption measurements versus W cycle ratio.

peak at high binding energy is observed, which indicates C−F bonding. Optical Analysis and Transport Measurements of ALD W:Al2O3 Nanocomposite Films. UV−vis Spectrophotometry. UV−vis spectrophotometry was performed on a series of ALD W:Al2O3 nanocomposite films deposited on fused silica substrates in the range of 0−40 W% (Figure 5a). The direct and indirect band gaps were calculated from these measurements using Tauc plots,29,30 and the results are shown in Figure 5b. These band-gap values were graphically calculated using the linear part of the absorption edge as shown in the Supporting Information (Figures S1 and S2). The direct band gap decreases from 5.1 to 4.2 eV between 10 and 40 W%, and the indirect band gap decreases from 3.5 to 1.9 eV in the same range (Figure 5b). The decrease in the direct band gap with increasing W% likely results from the formation of localized W d states within the Al2O3 band gap, as has been observed previously for other transition metals.31,32 To explore this possibility, additional XPS studies were performed to determine the valence band structure of the nanocomposite films (Figure 6). These measurements confirmed the interaction of W with the alumina matrix, resulting in the formation of W 5d states hybridized with the oxygen 2p band near the valence band edge at about 2.6 eV (boxed region in Figure 6). Earlier studies have correlated changes in the indirect band gap of nanocrystalline tungsten oxide films with quantum confinement effects related

Figure 6. XPS spectra of ALD W:Al2O3 nanocomposite films showing valence band structure for W cycle ratios in the range of 0−100%.

to the size of the WOx clusters.33−35 Previously, we established that the ALD W:Al2O3 nanocomposite films were composed of metallic, 1−2 nm nanoparticles in an amorphous matrix. Consequently, we attribute the decrease in indirect band gap D

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The Journal of Physical Chemistry C with W% to an increase in the size of the conducting nanoparticles in our films. Ellipsometry. We used spectroscopic ellipsometry to investigate the effect of composition on the optical constants (refractive index and extinction coefficient) of the ALD W:Al2O3 nanocomposite films. For materials comprised of two or more constituents or phases, it is often profitable to use effective medium theories such as those proposed by Maxwell− Garnett (M-G) and Bruggeman to obtain good approximations for the properties of the mixture, such as the volume fractions.30,36,37 We used the Bruggeman effective medium approximation (EMA) for the nanocomposite films with Al2O3 as the matrix modeled using a Cauchy dispersion model and W as the inclusion modeled using a B-spline formula. These models require accurate measurements of the optical constants for the individual constituent phases. Therefore, we first analyzed ellipsometric data for pure Al2O3 films using a Cauchy dispersion function, and for pure W using a Kramers−Kronig consistent B-spline function to obtain the optical constants for these films. The quality of the fitting was verified by noting that the mean squared error (MSE) for several sets of films having the same W% and increasing thickness was 400 nm, indicating that the metallic component of the film has oxidized. These findings imply that the maximum service temperature for these coatings in CSP would be in the range of 500−800 °C. Thermal and Electrical Conductivity. The thermal conductivity of ALD W:Al2O3 nanocomposite films deposited on Si(100) substrates was measured by time domain thermoreflectance spectroscopy (TDTR) as described elsewhere.16 The thermal conductivity increased with increasing W cycle ratio, and this trend matches quite well with the increase in electrical conductivity as measured by a four point probe as shown in Figure 10. The thermal conductivity data for the W% = 0 and 100 films in Figure 10 are taken from Costescu et al.16 Previously, ALD W deposited on packed beds of porous alumina and polymer particles was shown to increase the thermal conductivity of these particles.42 It is interesting to note that the thermal conductivity of the nanocomposite films is less than those of the pure Al2O3 and W films. In general, interface disorder scatters phonons at grain boundaries and at the interfaces between similar or different materials and this reduces the thermal conductivity. Differences in individual material properties determine the effective density of phonon states which also control the phonon scattering. Thus, it is expected that nanocomposite films with high interface densities will have reduced thermal conductivity compared to their pure counterparts.16,43 Tunable Band-Gap Coatings. The results shown in the previous sections are consistent with a transition at W% ∼ 50 from dielectric to metallic behavior for the ALD W:Al2O3

Figure 8. Thermal emissivity (left axis) and selective solar efficiency (right axis) versus W cycle ratio calculated for ALD W:Al2O3 nanocomposite films using absorption data in Figure 7a assuming T = 973 K and Q = 5 × 105 W/m2 (500 Sun concentration factor).

that, at W% = 10−20, the emissivity is relatively high and the efficiency is relatively low. Because of their higher band gap, these films are relatively transparent in the visible range and consequently must be very thick (30−45 μm) to achieve αs = 0.98, and this increases the IR absorbance also. The W% = 60 also has a high emissivity and low efficiency, but for a different reason. The metallic nature of this film provides a relatively flat absorption spectrum (Figure 5a), and the IR absorption leads to a lower efficiency. However, for the films of intermediate metal content (W% = 30−50), the combination of high visible absorption and low IR absorption yields low emissivities of ε = 0.23−0.28, and high efficiencies of ηsel = 0.95−0.96. For comparison, Pyromark 2500 (a common coating material for CSP receivers) shows ηsel = 0.94 and ε = 0.88 under these conditions.41 These results suggest that the W% = 30−50 films have favorable optical properties to serve as selective solar absorbing coatings for CSP receivers. F

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metallic nature of W in these films is supported by the changes in optical properties upon high temperature air annealing, which oxidizes the tungsten and reduces the free carrier concentration, generating a clear band gap around 350 nm (Figure 9). Finally, it is interesting to compare our ALD W:Al2O3 nanocomposite films with previous cermet materials. In our work, the optical properties for the ALD W:Al2O3 nanocomposite films with W% < 50 were well fit using an EMA model, but for W:Al2O3 cermets prepared by physical vapor deposition (PVD), this was not the case.46 A key difference is that the 1−2 nm particles in our ALD films are substantially smaller than those seen in some PVD cermets. Consequently, the main requirement for Bruggeman’s EMA mode, (homogeneity at a scale below optical wavelengths) is better satisfied in our case. The practical implication is that the broad optical tunability does not come at the expense of increased scattering, which is often a major limitation in the application of composite materials to optics.

Figure 10. Thermal conductivity as measured by TDTR (left axis) and electrical conductivity as measured by four point probe (right axis) versus W cycle ratio calculated for ALD W:Al2O3 nanocomposite films.

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nanocomposite films. Below W% = 50, the refractive index shows a weak, decreasing dependence with wavelength (Figure 7), a feature common to dielectric materials in which the dielectric constant in the visible range is dominated by the high energy interband transition between the conduction and the valence bands. The fact that the optical properties of these films can be fit well using an effective medium approximation (EMA) model is consistent with a nanocomposite structure composed of a conducting phase embedded in a dielectric matrix. This interpretation is supported by previous TEM measurements14,19,22 and by the thermal conductivity data (Figure 10), wherein the decrease in thermal conductivity at W% ∼ 50 can be attributed to an increase in phonon scattering, as previously reported for W/Al2O3 nanolaminates.16 Above 50%, our results indicate a material having optical, thermal, and electrical properties that are substantially metallic. This transition is marked by a sharp increase in the extinction coefficient and an absorption spectrum consistent with a high free carrier concentration. The valence band data (Figure 6) also support metallic behavior. The W% = 60, 70, and 100 samples are dominated by the W 5d component, as expected from W metal. Furthermore, the strong correlation between electrical and thermal conductivity at W% > 50 is consistent with a thermal conductivity dominated by the electron component, a characteristic of metals. Finally, no band gap could be identified in Tauc plots of the absorption data for W% ≥ 50. The ability to bridge the gap between dielectric and metallic behavior enables broadly tunable electrical conductivity spanning over 16 orders of magnitude, from 106 ohm−1 m−1 for pure W (Figure 10) to 10−10 ohm−1 m−1 for 10% W cycle ratio.14 While other materials systems, such as ZnO/Al2O3, have shown similar broad tunability,11 a key difference is that the ZnO conductivity results from doping and is, therefore, sensitive to substitutional impurities, oxygen vacancies, and oxygen partial pressure.44 In contrast, conductivity in the ALD W:Al2O3 nanocomposite films is dictated by structure, so that the resistance is extremely stable with time and environment.22 A second key difference is that changes in band gap for doped oxides are small and are dominated by the Burstein−Moss effect, wherein the band gap increases with increasing dopant concentration.45 However, Figures 6 and 7 show a reduction in band gap with increasing W content in the range of W% = 10− 40 before transitioning to a metallic regime above 50% W. The

CONCLUSIONS In conclusion, ALD W:Al2O3 nanocomposite films exhibit tunable optical properties where the direct and indirect band gaps decrease linearly with increasing W% in the range 10−40, and the refractive index shows a transition from dielectric to metallic character for W% ≥ 50. We attribute the decrease in direct band gap to the formation of W d states within the band gap of Al2O3. The decrease in the indirect band gap likely results from an increase in the size of the conducting WOx particles with increasing W%. This wide tunability, coupled with the low optical scattering intrinsic to 1−2 nm particles, makes the ALD W:Al2O3 nanocomposite films a promising material for future applications in optoelectronics and energy technology. In particular, the good thermal stability and favorable absorption spectrum make these materials promising candidates for selective solar absorbing films in concentrated solar power.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b03823. Tauc plots for ALD W:Al2O3 nanocomposite films prepared using W cycle ratios of 10, 20, 30, and 40% on quartz substrates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (630) 252-3520. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Argonne Laboratory Directed Research and Development (LDRD) project 2015-151-N0 and the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, through the SuNLaMP program. We wish to acknowledge Prof. David Cahill and Dr. Judith Kimling from the Materials Science and Engineering Department at the University of Illinois Urbana−Champaign for performing the thermal conductivity measurements. Electron microscopy was performed at the Electron Microscopy Center for Materials G

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Research (EMCMR) at Argonne National Laboratory. Use of the EMCMR was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357 operated by UChicago Argonne, LLC.

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