Optical Properties of Vanadium Pentoxide Deposited by ALD

Article pubs.acs.org/JPCC

Optical Properties of Vanadium Pentoxide Deposited by ALD Erik Østreng,* Ola Nilsen, and Helmer Fjellvåg Centre for Materials Science and Nanotechnology, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway S Supporting Information *

ABSTRACT: Nanostructures of V2O5 find important technological applications in optics, catalysis, and lithium ion batteries. Their optical properties and surface roughness are important parameters in these respects. Here we report on atomic layer deposition (ALD) of V2O5 using the β-diketonate VO(thd)2 and ozone as precursors. In this work, X-ray diffraction, AFM, ellipsometry, and UV−vis-spectroscopy are used to show that the crystallographic orientation, optical properties, band gap, and surface roughness of the derived films are correlated and can be varied by controlling deposition temperature and film thickness. The band gap of the samples varies between 2.70 and 2.35 eV. The observed growth rate varies between 0.1 to 1 Å/cycle depending on deposition temperature and the number of cycles. This large variation in growth rate provides an interesting case of ALD growth, which can be rationalized in terms of a geometric crystal growth model.

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INTRODUCTION Vanadium oxides find use in a large range of applications such as catalysts for sulphuric acid production,1 catalysts for oxidative dehydrogenation of hydrocarbons,2 for reduction of NO with NH3,3 and as cathode material in rechargeable lithium ion batteries.4−6 This uses of V2O5 is extensively described in a recent review.7 V2O5 has been reported deposited by atomic layer deposition (ALD) from VTOP (vandyl-triisopropoxide, VO(OiPr)3 and water with thermal2,6,8 and plasma-enhanced9 ALD processes. Additionally, intercalation of lithium into V2O5 deposited by ALD10,11 has been studied, as well as the catalytic performance of oxidative dehydrogenation of vanadia coatings deposited by ALD.12 Deposition of V2O4 on carbon nanotubes by ALD is reported from vanadyl n-propoxide and acetic acid,13 and recently deposition of VO2 from tetrakis(ethylmethylamido)vanadium and ozone is described.14 Several groups have recently reported on the optical properties of thin films of V2O5 as deposited by different methods; PLD,15 spray pyrolysis,16 and sputtering.17 The optical properties have been related to particle size and film thickness.17,18 Spectroscopic ellipsometry was used to determine the dielectric function of nanocrystalline V2O5 and of strongly textured samples.19,20 In this study we investigate how the growth and optical properties of V2O5 deposited by ALD varies with deposition temperature and texture.

(AGA) in an OT-20 ozone generator supplied by Ozone Technology. The ozone/oxygen mixture, with a claimed ozone concentration of 15 vol %, was supplied at a rate of 500 cm3/ min during its pulse. The films were deposited using a sequence of 2 s VO(thd)2 pulse, 1 s purge, 3 s O3 pulse, and 2 s purge as basis unless otherwise specified. Nitrogen was used as a carrier gas supplied at an overall rate of 500 cm3/min. The carrier gas was generated with a Schmidlin UHPN3001 N2 purifier providing a mixture of N2 + Ar with a purity of 99.999% before being further purified by passing through P2O5 and a Mykrolis gas purifier. Films were deposited on soda-lime glass and on 5 × 5 cm2 single crystals of Si(111). In addition, fibers of fused silica were used as substrates for selected depositions. The soda-lime glass substrates were cleaned with ethanol, and the single crystals were blown free of dust with air and otherwise used as supplied. The silica fibers were pulled from a molten rod of fused silica to a diameter of ca. 0.5 mm and used directly. The growth mechanism and growth parameters were studied using a QCM (quartz crystal microbalance) based on two ATcut crystals with gold electrodes in a homemade holder and recorded with a MaxTek TM-400 data logger. The deposited films were characterized by X-ray diffraction (XRD) and X-ray reflectometry (XRR) using a Bruker AXS D5000 diffractometer with a Göbel-mirror to obtain a parallel Cu Kα beam. The films deposited on silica fibers were studied using synchrotron radiation XRD at the Swiss-Norwegian Beamline (SNBL) at ESRF (European Synchrotron Radiation Facility).

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EXPERIMENTAL SECTION Thin films were deposited in an F-120 Sat ALD-reactor (ASM Microchemistry) using VO(thd)2 and O3 as precursors. VO(thd)2 (thd = 2,2,6,6-tetramethylhepta-3,5-dione) was synthesized in house, as reported in ref 21 and sublimed inside the reactor at 125 °C. Ozone was prepared from 99.99% O2 © 2012 American Chemical Society

Received: May 9, 2012 Revised: July 30, 2012 Published: July 31, 2012 19444

dx.doi.org/10.1021/jp304521k | J. Phys. Chem. C 2012, 116, 19444−19450

The Journal of Physical Chemistry C

Article

In the synchrotron experiments, the wavelength was λ = 0.71998 Å, as determined by NIST LaB6. The measurements were performed in transmission mode with a MAR-345 image plate detector. Film thickness and optical properties were determined by a J.A. Woollam Alpha-SE spectroscopic ellipsometer and modeled with the CompleteEase program package. UV−vis spectroscopy was performed with a Shimadzu UV− vis 3600 in transmission mode, and band gaps were extracted from Tauc plots. The surface topology of selected samples was studied with atomic force microscopy (AFM) using a Park System XE-70. The measured data were analyzed using the XEI software package. Raman measurements were preformed with a SpectraPhysics Millennia Pro 12sJS Nd:YVO4 solid-state laser using 532 nm wavelength operating at 200 mW. Ellipsometry measures the difference in reflectance and the phase shift between light of different polarizations according to the fundamental relationship

Rp Rs

tan ψ·eiΔ =

time, and the growth rate was based on a linear regression over 16 cycles of deposition on the QCM crystals. The data are given as frequency decrease per cycle, which is proportional to the growth rate via the Sauerbrey equation.22 Results are presented in Figure 1, which shows that the process is self-

(1)

where ψ and Δ are measured from the ellipsometer, ψ is the angle with a tan value corresponding to the absolute value of the ratio of intensity for the parallel and normally polarized components of the reflected light, Δ is the phase shift between the components, and Rs and Rp are the reflectance coefficients of the components normal and parallel to the substrate normal, respectively. The relation is linked to the electronic structure and optical thickness of a thin film on a substrate through the dielectric function that is contained in the reflectance. In the model, we used between 1 and 3 Lorentzian oscillators to account for the anomalous dispersion around the band gap. The dielectric function was modeled as outlined in eq 2. ε = ε∞ +

∑ n

ε1 = 1 +

∑ n

ε2 =

∑ n

A n ·Γn·ω0 ω02

− ωn2 − i·ωn ·Γn

Figure 1. QCM-based results for film growth of V2O5 at 186 °C on variations in pulse and purge times based on 2s VO(thd)2 pulse, 1s purge, 3s O3 pulse, and 2s purge (when this parameter is not varied).

limiting at 186 °C. Figure 1 shows that a 2 s pulse of VO(thd)2 is sufficient to saturate the surface. A nitrogen purge of 0.1 s is necessary to prevent gas-phase reactions with the ozone. An ozone pulse of 3 s is needed to oxidize the organic part of the precursor, and a subsequent 1 s purge is required to keep the surface-limited growth. Figure 2 shows a more detailed QCM investigation of the growth process at 186 °C. Data were averaged for 100 cycles of depositions using the described optimized scheme: 2 s VO(thd)2 pulse, 1 s purge, 3 s O3 pulse, and 2 s purge. A

= ε∞ + ε1 + iε2

A Γω0(ω02 − ω 2) (ω02 − ω 2)2 + Γ 2ω 2 A Γ 2ωω0

(ω02 − ω 2)2 + Γ 2ω 2

(2)

where ε∞ is the dielectric constant at high energy, A is the amplitude, Γ is the broadening of the oscillator, ω0 is the energy at the resonance frequency, and ω is the energy of the incident light in electronvolts. Samples deposited for 2000 or more cycles were measured at 65, 70, and 75° incident angles, and these three data sets were simultaneously fitted. The data were furthermore fitted to a Lorentz oscillator model. The roughness, when necessary, was modeled with the Bruggemann effective media approximation (BEMA). For samples deposited using 500 cycles, a Cauchy-function was used to model the dispersion and extract a thickness using only data collected at wavelengths above 600 nm, where the material is relatively transparent.

Figure 2. Variations in mass as measured by QCM during one ALDcycle at 186 °C using 2s VO(thd)2 pulse, 1s purge, 3s O3 pulse, and 2s purge. The data are averaged over 100 consecutive cycles, and the average is displayed as a solid line, and the standard deviation is shown as the gray area. The absolute value (mass) is perturbed by thermal effects caused by the use of ozone.

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RESULTS The growth parameters for deposition at 186 °C were studied using in situ QCM analysis. Only one parameter was varied at a 19445



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plateau in the mass increase during the VO(thd)2 pulse is evident, proving self-limiting growth. It should be noted that the apparent mass increase in the purge period after the O3 pulse is most probably due to thermal effects caused by large amounts of ozone, as recently shown by Diskus et al.23 The variation in growth rate with temperature was studied with basis in samples deposited for 500 and 2000 cycles of VO(thd)2 and O3 according to the optimized pulsing procedure; see Figure 3. Gradient free films were obtained at

Figure 4. XRD data (λ = 1.5406 Å) for samples deposited on silicon using 2000 cycles of 2s VO(thd)2 pulse, 1s purge, 3s O3 pulse, and 2s purge at different temperatures. Miller indices given for V2O5 (space group Pmmn; a = 3.56, b = 11.52, c = 4.37 Å). The Si(111) peak has been excluded for clarity.

At 162 °C, the films are amorphous as deposited, whereas a broad 001 reflection is visible for deposition at 196 °C. For depositions between 206 and 225 °C, a broad set of 00l and h00 reflections along with narrow 0k0 reflections are visible. This indicates that the crystallites develop an orthorhombic shape that are elongated along the crystallographic b axis with the grains oriented with either top or side faces normal to the film surface. After a transition stage around 235 °C, the growth orientation changes slightly, and at 259 °C the 011 reflection becomes visible. Rocking curve measurements were performed on the 001 and 020 reflections of a sample deposited at 215 °C using 2000 cycles, showing a full width at half-maximum of 5.5 and 7°, respectively. An observed difference in thickness and roughness for samples deposited using 500 and 2000 cycles at 210 °C raises the question whether the crystallinity changes during growth. The thinner sample was found to be amorphous. This suggests that crystallinity develops during the growth. Surface Morphology. Figure 5 shows AFM pictures of the surface morphology. Films deposited at 167 °C are almost atomically flat, whereas for 186 °C, plate-like crystallites are beginning to form. This feature is more pronounced at 206 °C before it evolves into clearly orthorhombic shaped crystallites at 215 °C. For samples grown at 236 °C, the surface consists of plates with quite random shapes, whereas at 259 °C the plates are rather agglomerated. The measured surface roughness tends to increase in the temperature range where the orthorhombic crystallites are observed. The AFM pictures correlate well with the XRD data and the increased growth rate and porosity around 200 to 235 °C. The morphology depends on the number of deposition cycles, as shown in Figure 6. The platelike crystallites (deposition at 186 °C) evolve during the deposition and are only visible at 2000 and 5000 cycles in Figure 6. Previously, the ALD growth of crystalline films from seed objects of different aspect ratios with tetragonal symmetry have been modeled by Nilsen et al.26 The structure of V2O5 is orthorhombic and cannot be directly compared with results for tetragonal model systems; however, qualitative information can still be extracted. Experimentally we observe a change in growth rate as a function of the number of deposition cycles that is typical for aspect ratios around one, along with the expected

Figure 3. Evolution in growth rate with deposition temperature for films based on 500 and 2000 cycles. The full symbols symbolize a dense layer, and the open symbols represent the sum of the dense layer and a rough layer (modeled with effective medium approach). The deposition parameters used are 2s VO(thd)2 pulse, 1s purge, 3 s O3 pulse, and 2s purge.

growth temperatures from 162 to 235 °C. At higher temperatures, the films developed gradients, and some films deposited with 2000 cycles above 215 °C had a milky haze due to light scattering from the crystallites in the film. Because of high surface roughness of the films, it proved to be impossible to systematically derive film thicknesses by XRR. However, for the thinnest samples deposited on glass, XRR confirmed the assumptions done in the modeling of ellipsometry data. For the latter modeling, a two-layer model was adapted where the bottom layer represents the bulk of the film and the top layer consists of a mixture of the bulk and voids and was modeled with the BEMA. In Figure 3, the filled squares represent the thickness modeled as the bulk of the film and the open squares represent the sum of the bulk and the BEMA layer. The evolution of growth rate with temperature does not exhibit a characteristic plateau or an “ALD-window”, as normally seen for transition-metal oxides deposited from β-diketonatoprecursors, like, for example, iron24 and cobalt.25 The growth concurs still with the ALD principle of self-limiting reactions; see Figures 1 and 2. The large increase in growth rate for the samples deposited with 2000 cycles in the temperature range 200−250 °C is attributed to changes in surface morphology and crystallization during growth, as will be discussed below. Crystallinity. The crystallinity of the deposited films on silicon was studied with XRD in θ-2θ geometry. In general, these samples were amorphous when deposited below 196 °C, whereas polycrystalline V2O5 was found for films deposited at and above 196 °C. The diffraction patterns in Figure 4 show crystallinity and texture as a function of deposition temperature. 19446



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The band gaps of selected samples were estimated from Tauc plots of transmission UV−vis spectroscopy data. An indirect band gap was found at ∼2.5 eV for samples deposited below 235 °C and at ∼2.15 eV for samples deposited above 235 °C. Furthermore, a direct band gap was found at around 2.7 and 2.3 eV, respectively. The observed difference between direct and indirect band gaps was ∼0.2 eV, consistent with modeling,27 where indirect and direct band gaps are separated by ∼0.3 eV. The change in band gaps impacts the modeling of the ellipsometry data, where we assume that the band gap is modeled by a Lorentzian oscillator with energy comparable to the band gap estimated from absorption measurements. The anomalous dispersion around the band gap was modeled with two or three Lorentzian oscillators, as described in the Experimental Section. A suitable model must comprise at least two oscillators due to the two band gaps observed by UV−vis spectroscopy and the reported electronic band structure in the literature.20 The roughness of the samples was modeled by using a two-layer model: one dense layer and one layer with voids using the BEMA. The oscillator parameters of the BEMA layer were coupled to the dense layer to prevent unphysical results. The optical properties of the thin films depend notably on the deposition temperature. This is interesting because the crystallization of the samples occurs around 190 °C, which is relatively low with respect to the explored temperature span. The film roughness clearly correlates with the optical properties; however, this contribution is difficult to model properly. Additionally, the change in roughness is accompanied by a change in crystallite orientation, thereby exposing different surfaces with their variations in electronic structure.27 This will also affect the optical properties.

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DISCUSSION This article has reported the deposition of V2O5 by ALD from VO(thd)2. This new solid precursor has proven that it is possible to deposit crystalline V2O5 films at low temperature without the need for an annealing step, as is needed for VO(OiPr)3. The deposited films can also be tuned with respect to optical properties and crystallinity with different deposition temperatures. The precursor is also thermally stable to higher temperatures than the previously mentioned alkoxide and therefore increases the ease of deposition of complex vanadates due to better compatibility with other processes. The disadvantages when compared with the alkoxide are the lower growth rate, the need for ozone, and the fact that the precursor is solid instead of liquid. VO(thd)2 is not yet commercially available but is relatively easy to prepare in sufficient quantities. The temperature dependence of the growth rate is untypical for ALD growth. The growth rate (see Figure 3) shows an anomaly around 200−240 °C, where the growth rate depends on the number of cycles, being larger for films deposited during 2000 cycles than for 500 cycles. In the same temperature regime, a narrow 020 reflection in the diffraction pattern appears (Figure 4), and the surface roughness increases (Figure 5). The sample deposited using 500 cycles at 210 °C provides a representative example. It has similar growth rate and refractive index (and hence also density) as corresponding 500 cycles samples deposited at other temperatures. Importantly, this sample does not display any 020 reflection. Most probably, the (001) surface is favored during the initial 500 cycles, whereas (010) surface develops for thicker films and with significantly higher growth rate. The growth rate can be estimated from

Figure 5. AFM topography of films deposited on silicon using 2000 cycles of 2s VO(thd)2 pulse, 1s purge, 3s O3 pulse, and 2s purge at (a) 162, (b) 186, (c) 206, (d) 215, (e) 236, and (f) 283 °C. The height scale is 80 nm except for panels e and f, where the height scale is 150 nm. The graph shows the root-mean-square (rms) roughness, as derived from the respective AFM pictures.

trends in roughness and number of surface objects. To obtain the same effects by modeling for a growth rate of 30−60 pm/ cycle, the crystallites are required to be rather far apart, at least 5 nm. Optical Properties. To complement the AFM and XRD results and to obtain a broader perspective on the growth and properties of the samples, a detailed ellipsometry and UV−vis study was performed. To verify the evolution of growth rate with temperature as shown in Figure 3, especially above 196 °C, a more detailed discussion of the model is required. 19447



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Figure 6. AFM topography, growth per cycle (GPC), thickness, terminating objects, and roughness versus number of deposition cycles. The AFM images are 1.25 × 1.25 μm and z-scales equal. All samples are deposited on silicon using 2s VO(thd)2 pulse, 1s purge, 3s O3 pulse, and 2s purge at 186 °C.

Table 1. Parameters Derived from Modeling of Ellipsometry Data According to Equation 1 for Samples Shown in Figure 8a Tdep

thickness bulk

thickness BEMA

voids BEMA %

ω1

A1

Γ1

ω2

A2

Γ2

ε∞

162 186 215 235b

65.1 35.8 89.5 80.2

0 0 102.2 32.5

0 0 77.9 75.9

3.51 2.96 2.71 2.57

4.03 6.61 1.96 0.42

1.04 0.44 0.79 0.19

0 3.36 3.41 2.76

0 4.54 1.13 1.84

0 0.59 1.16 0.33

3.07 3.87 3.37 2.30

a ω, energy of the oscillator; A, amplitude; Γ, broadening; ε∞, dielectric constant at high energy. bThird oscillator at higher energy (3.61 eV) with high amplitude was introduced to ensure a reasonable fit. The third oscillator is assumed to model the split conduction band.27

packing of precursors on the surface in line with Ylilämmi.28 In our current estimate, we assume that the adsorbed specie is −VO(thd) with a diameter of the adsorbed fragment of ca. 6 Å (i.e., the width of the thd-ligand) that is free to rotate and can only react with one oxygen site on the surface. Such considerations lead to an upper limit for the growth rate of ∼50 pm/cycle for the 010 surface of V2O5. However, a growth rate of ∼100 pm/cycle is observed. This could indicate a

growth mechanism with more than one monolayer per cycle in the temperature regime 200−240 °C. Whether this could be true self-limiting growth or just surface-limited growth is open; however, one may speculate whether the catalytic properties of V2O5 could enhance the growth by oxidizing the precursor and thereby make room for more than one precursor molecule per absorption site per cycle, still at the monolayer coverage level. 19448



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systematically higher energies than for the band gaps. We note that the number of oscillators needed for a good fit changes and that the energy and broadening of the first oscillator decrease with increasing deposition temperature. For samples deposited at 235 °C, the optical spectra can be modeled by two oscillators representing transitions into the intermediate band and a strong, higher energy oscillator representing transitions into the conduction band. This picture fits quite well also for the roughest samples; however, the calculated broadening of the oscillators are much larger, which probably is a shortcoming of the applied model.

Note that the V2O5 (010) surface is catalytically active for oxidation of ammonia.3 It is interesting to note that the similar features are reported by Oukassi et al. for sputtered V2O5 films,20 who report that variations in the refractive index and film texture correlate with increase in deposition rate and porosity when a critical film thickness is reached. The band gap measurements in Figure 7 and the parametrization of the ellipsometry data in Figure 8 correlate well;

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CONCLUSIONS Thin films of V2O5 were deposited by ALD using VO(thd)2 and ozone. Large variation in growth rate, crystallinity, morphology, and optical properties have been observed as a function of deposition temperature. The optical properties depend on the deposition temperature that again governs the preferred orientation and crystallinity. The optical properties of V2O5 are revisited, and two band gaps are found and their energies are correlated with the energy of oscillators modeled from ellipsometry data.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. Tauc plots for samples deposited at 186 and 235 °C using 2000 cycles. The indirect band gaps is found to be 2.5 and 2.15 eV and the direct band gaps were found to be 2.7 and 2.35 eV for samples deposited at 186 and 235 °C, respectively.

Crystallization of amorphous samples investigated with synchrotron XRD. Raman spectroscopy of V2O5. This material is available free of charge via the Internet at http://pubs.acs.org.

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however, it should be noted that the energy of the oscillator is the peak of the absorption and not the onset, leading to

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +47 2285 5558. Fax: +47 22855441. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Niels Højmark Andersen for providing the experimental Raman data. The research leading to these results has received funding from the European Union Seventh Framework Programme ([FP7/2007-2013]) under grant agreement no. 227541.

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Figure 8. Refractive index and extinction coefficient as a function of wavelength for samples deposited using 2000 cycles at three different temperatures. Derived from the modeling of ellipsometry data; see Table 1. 19449



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Optical Properties of Vanadium Pentoxide Deposited by ALD

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