Thin Film Amorphous Electrolytes: Structure and ... - ACS Publications

This work presents a structural investigation of lithium-borate thin film electrolytes prepared by rf sputtering of Li-diborate targets. Chemically st...
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J. Phys. Chem. C 2007, 111, 8111-8119

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Thin Film Amorphous Electrolytes: Structure and Composition by Experimental and Simulated Infrared Spectra Efstratios I. Kamitsos,* Marc Dussauze, and Christos-Platon E. Varsamis Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou AVenue, 116 35 Athens, Greece

Philippe Vinatier and Yohann Hamon ICMCB-CNRS and ENSCPB, UniVersite´ Bordeaux I, 87 aV. Dr. A. Schweitzer, 33608 Pessac Cedex, France ReceiVed: December 15, 2006; In Final Form: March 22, 2007

Ionic conducting glasses in thin film forms are promising candidates for applications in microelectronics devices such as microbatteries and microsupercapacitors. In recent years, it was shown that physicochemical properties of thin films may differ substantially from those of the target bulk materials. Thus, it remains a challenge in science and thin film technology to control the properties of thin films in terms of chemical composition and conditions of manufacturing. This work presents a structural investigation of lithium-borate thin film electrolytes prepared by rf sputtering of Li-diborate targets. Chemically stable thin films ca. 1 µm thick were deposited on Si and gold-covered Si substrates under argon and their infrared spectra were measured in a broad spectral range (30-5000 cm-1). The spectra of thin films were modeled on the basis of rigorous expressions for reflectance and transmittance of the film/substrate bilayer system. The experimental results for thin films were compared with those simulated by using the optical response functions of bulk glasses xLi2O-(1-x)B2O3 (x ) 0.33, 0.30, 0.275, 0.25), which were prepared by the conventional melt quenching technique. The comparison of experimental and simulated spectra showed that rf sputtering of Li-diborate targets (x ) 0.33) leads to Li-borate thin films with lithium oxide content lower than that of the target material. Best agreement between experimental and simulated thin film spectra was obtained for x ) 0.275, after proper annealing of thin films to remove differences in thermal history between thin film and bulk glass states.

1. Introduction Solid electrolytes are promising materials for solid-state electrochemical devices including batteries, sensors, and electrochromic displays.1 Thin films of ionic oxide glasses, and in particular films of lithium ion-containing glasses, constitute a special family of solid electrolytes with great potential for technological applications.2-5 Recently, thin amorphous films were prepared by radio frequency (rf) magnetron sputtering in a wide range of compositions within the Li-borosulfate system.6 It was shown that rf-sputtering can extend greatly the glassforming region established previously by conventional meltquenching methods,7-9 and in addition, it can produce stable amorphous films with promising performance in microbattery devices.6 Thin film formation by rf-sputtering and other relevant techniques may lead to amorphous films with composition and properties depending on preparation conditions. For example, early studies on silicon dioxide, SiO2, have shown that this is indeed the case for thin films prepared by evaporation,10 dcsputtering,11 as well as rf-magnetron sputtering.12 The influence of preparation conditions has been observed also for films formed by pulsed laser deposition (PLD), including differences in composition between thin films and the parent target glass in lead-germanates,13 and variations in ionic conductivity in * Address correspondence to this author. E-mail: [email protected]. Phone: +30-210 7273 828. Fax: +30-210 7273 794.

lithium-silicate-vanadate thin films prepared by either PLD or rf-sputtering.14 A very recent study of thin films prepared by rf-sputtering in the lithium-phosphorus oxynitride (LiPON) system has demonstrated the need for careful adjustments of the preparation conditions (i.e., rf power, gas pressure, target density, and target-substrate distance) to maximize the ionic conductivity of the resulting thin films.15 It is noted that even less sophisticated thin film-forming techniquessin comparison to those mentioned abovesmay influence the structural characteristics of thin films. A comparative structural study of bulk glasses and melt-blown thin films in the AgI-silver borate system has shown systematic variations in the short-range order (SRO) structures between thin films and bulk glasses.16 Such differences were observed despite the fact that thin films and corresponding bulk glasses were prepared from the same melt and had the same chemical composition. Therefore, challenges in thin film science and technology involve understanding the role of preparation conditions on thin film composition/ chemistry and properties, and the establishment of structureproperty correlations in thin films relative to those in corresponding target/bulk materials. Infrared (IR) spectroscopy constitutes a powerful tool for the nondestructive structural characterization of glasses in thin film forms. Besides SiO210-12 and other amorphous thin films, IR has been applied for the structural investigation of film in the Ag-borate16 and Na-borate17 glass systems. IR spectroscopy is particularly informative for ionic glassy electrolytes because

10.1021/jp068617b CCC: $37.00 © 2007 American Chemical Society Published on Web 05/16/2007

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Figure 1. Schematic representation of the configurations employed for infrared measurements: (a) reflectance, R, for bulk glasses, (b) transmittance, T, and reflectance, R, for thin films deposited on Si wafer, and (c) reflectance-absorption, R-A, for thin films deposited on goldcovered Si substrate.

it provides information for the SRO structural units constituting the glass network, derived mainly from the mid-IR response, as well as for the interactions between cations and their network hosting sites on the basis of their localized vibrational modes active in the far-IR region.18,19 In addition, the far-IR activity of ionic glasses may be correlated with ion conduction properties because the optical conductivity in this region represents the maximum attainable ionic conductivity at high temperatures, where all cationic motions become delocalized and contribute to ionic conduction.17 Besides structural characterization, a quantitative analysis of infrared spectra can lead to the precise evaluation of the optical response functions of the material in the infrared spectral range. The required analysis of experimental spectra is now quite straightforward for bulk glasses, for which reflectance spectra can be measured easily in the entire infrared range from wellpolished slabs of materials (Figure 1a). The optical response functions of glass can be obtained by Kramers-Kronig (KK) transformation of the reflectance spectrum, as reported previously for alkali borate,19 germanate,20 and chalcogenide21 glasses. Besides KK inversion, the reflectance spectra can be analyzed by the reflectance fitting procedure. This method is based on modeling the dielectric function of the material, by employing for example, classical dispersion theory, and it was shown to give results in close agreement with those obtained by KK analysis.22 Compared to bulk glasses, a proper quantitative analysis of the IR spectra of thin films requires special attention due to complications arising from additional optical effects. For freestanding amorphous films with a thickness of ca. 1 µm, both transmittance, T(ν), and reflectance, R(ν), spectra can be measured. However, the KK transformation for semi-infinite samples is no longer valid due to multiple internal reflections of the incident electromagnetic wave inside the film. This effect results in the appearance of interference fringes in the measured T(ν) spectra which may affect strongly the vibrational spectrum profile. The proper formulation of the T(ν) spectrum now requires the use of rigorous expression, which takes fully into account the contribution of interference fringes16,23 and thus allows for a direct comparison of the true vibrational spectrum of thin film with that of the bulk material.16 For thin films deposited on substrates the acquisition of infrared spectra can be made in two configurations. First, when an infrared-transparent substrate is used for deposition (e.g., Si wafer) the IR spectra of the bilayer system can be measured in

Kamitsos et al. the transmittance and reflectance modes as shown in Figure 1b. Both T(ν) and R(ν) spectra are now perturbed by the combination of two interference effects due to multiple reflections in the thin film and in the substrate. Second, when the thin film is deposited on a metallic substrate the infrared measurement combines a double transmission through the film with an intervened reflection from the metallic surface (Figure 1c). This technique is known as reflection-absorption spectroscopy, R-A, and, to a first approximation, the measured spectrum at quasinormal incidence can be regarded as the transmittance spectrum of a film whose thickness is approximately double that of the measured film. It is noted that the R-A(ν) spectra are also affected by the appearance of interference fringes due to the thin film. Besides the interference effects, one should consider also the Berreman effect24 when the arrangements shown in parts b and c of Figure 1 are employed for infrared measurements under oblique incidence of the infrared radiation. This effect refers to the excitation of longitudinal optical (LO) modes, besides the expected transverse optical (TO) modes, when the infrared transmittance or reflectance spectra of thin films are measured with unpolarized light or with light polarized in the plane of incidence (p-polarization).25 Thus, when the excitation conditions, i.e., angle of incidence and polarization of light, favor the appearance of the Berreman effect more complicated spectral profiles are measured in the infrared. In the present work we report results of an infrared study of thin films deposited by rf-sputtering on Si wafers or on goldcovered Si (gold/Si) substrates using targets of the Li-diborate composition (Li2O-2B2O3). Within the Li-borosulfate system,6 the Li2O-2B2O3 composition was chosen for this study because it gives stable amorphous or glassy materials in thin film and bulk solid states. The primary goal of this work is to investigate the structure/chemistry of thin films with respect to characteristics of the bulk glass, which has the same nominal composition but was prepared by the conventional melt-quenching technique. The infrared spectra of both thin films and bulk glass were measured in a continuous and broad (30-5000 cm-1) spectral range, which facilitates the study of the nature of the SRO structural units and of the Li ion-site dynamics. Analysis of infrared spectra of thin films was made by employing the exact analytical expressions for reflectance and transmittance of a bilayer system (Figure 1b,c) by considering all multiple internal reflections in the film/substrate system. The complex refractive indices of substrates were calculated from their optical measurements and were employed as input for the fitting procedure. The optical properties of bulk glasses were obtained by Kramers-Kronig transformation of their specular reflection spectra and were also employed to simulate the infrared response of thin films. The results obtained from thin films and bulk glasses were compared and discussed with respect to the effect of preparation conditions on the composition thin films, and with reference to thermal histories of thin film and bulk states of these ionic glassy electrolytes. 2. Experimental Section 2.1. Sample Preparation. Li-borate thin films were developed by rf-sputtering, using a target of composition Li2O2B2O3. The target was prepared from reagent-grade powders of crystalline boron oxide (B2O3) and lithium metaborate (LiBO2). Stoichiometric amounts of the starting materials were mixed thoroughly in an agate mortar and pressed at 900 kg/ cm2. The pellet of 5 cm diameter was subsequently annealed twice at 800 °C for 12 h, with additional grinding and pressing between the two annealing processes.

Thin Film Amorphous Electrolytes Films were deposited under 1 Pa of argon with a target-tosubstrate distance of 6 cm and with rf power ranging from 1.5 to 2.5 W/cm2. Films were sputtered on silicon wafers (100 orientation, 0.6 mm in thickness) having both surfaces optically polished. Prior to Li-borate thin film deposition, half of the silicon wafer was covered by a layer of gold with thickness between 80 and 100 nm (Figure 1b,c). The duration of depositions was controlled to obtain films with thicknesses in the range 1 to 1.8 µm. Bulk Li-borate glasses xLi2O-(1-x)B2O3, with x ) 0.33 (diborate), 0.30, 0.275, and 0.25, were prepared from stoichiometric amounts of reagent-grade powders of Li2CO3 and B2O3. The mixed batches were melted in Pt crucibles at 10001200 °C for 30 min in an electric furnace. The bubble-free melts were subsequently quenched in a stainless steel mold to give clear glass disks 15 mm in diameter and ca. 2 mm in thickness. After annealing at Tg - 50 °C for 1 h, the glass disks were polished by using a polishing machine (Struers, LaboPol-1) equipped with the appropriate sets of abrasive papers and diamond suspensions to result in surface roughness better than ca. 0.25 µm. Glass layers of ca. 200 µm were removed by polishing to eliminate any gradient in the short-range order structure of the bulk glass.26 2.2. Infrared Measurements. Infrared measurements were performed on a Fourier-transform vacuum spectrometer (Bruker 113v) equipped with two sources (globar and Hg arc), two detectors (DTGS with KBr and polyethylene windows), and five different beam splitters, KBr for the mid-IR and four mylar films for the far-IR (with thickness 3.5-25 µm). This arrangement gives a continuous spectral coverage in the frequency range 30 to 5000 cm-1. All spectra were measured at room temperature and represent the average of 400 scans. Transmittance and reflectance spectra of thin films deposited directly on Si (Figure 1b) were measured either with low resolution (10 cm-1) or high resolution (0.2 cm-1). Films deposited on gold/Si substrates (Figure 1c) were measured in the reflection-absorption mode with a resolution of 10 cm-1. The specular reflection spectra of bulk glasses were measured with 2 cm-1 resolution, and all reflectance spectra were measured against a highly reflective aluminum mirror. To avoid the complications of the Berreman effect discussed in the Introduction and, thus, to keep the measured spectral profiles as simple as possible, all transmittance spectra were measured at normal incidence and the reflectance and reflection-absorption spectra were acquired at quasinormal incidence (θ ) 11°). Since the coupling of the LO modes with the electricfield vector is proportional to sin2θ,24 it is expected that the measured spectra will be practically free of LO contribution under normal or quasinormal excitation. The measured reflectance spectrum of gold film on Si wafer (Figure 2, RAu) shows that the gold layer can be considered as a quasiperfect mirror with reflectivity higher than 90%. Departures from the optimum reflectivity in the infrared can be attributed to the roughness of the deposited gold layer. Spectra for silicon wafer are also shown in Figure 2 in reflectance, RSi, and transmittance, TSi, and confirm its transparency in the infrared, with the sum RSi + TSi approaching the value of one. The bands observed in the transmittance spectrum of silicon at 565, 608, 740, 815, 890, 965, 1302, and 1450 cm-1 are attributed to lattice modes of Si,27 while the feature at 1107 cm-1 originates from oxygen impurities (asymmetric stretching vibration of Si-O-Si bridges). All bands of silicon wafer are weak and have absorption coefficient R ≈ 2 cm-1, with the exception of

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Figure 2. Infrared spectra measured with 10 cm-1 resolution for silicon wafer (transmittance, TSi, and reflectance, RSi) and for the gold layer on silicon (reflectance, RAu).

Figure 3. Infrared spectra of Li-borate thin films deposited from Lidiborate target (Li2O-2B2O3) on Si wafer (Tfilm/Si, Rfilm/Si) and on goldcovered Si substrate (R-Afilm/Au) in comparison with the specular reflectance spectrum of bulk Li2O-2B2O3 glass (Rbulk). Spectral resolution for thin films and bulk glass was 10 and 2 cm-1, respectively.

the feature at 608 cm-1, which appears to be stronger (R ≈ 10 cm-1). As shown in Figure 2, gold and Si substrates are characterized by quite flat optical responses and this makes them suitable for infrared measurements of thin films. Therefore, with the configuration of Figure 1b it is possible to measure both reflectance and transmittance spectra of thin films deposited on silicon, whereas the configuration of Figure 1c allows for measurements of reflection-absorption spectra of films deposited on gold. Figure 3 shows typical infrared spectra of a Liborate thin film deposited from the Li-diborate target on silicon (Tfilm/Si, Rfilm/Si) and gold (R-Afilm/Au) substrates in comparison to the reflectance spectrum of bulk 0.33Li2O-0.67B2O3 glass (Rbulk), which has the same nominal composition with the Lidiborate target. The measured infrared spectra clearly show differences in shape and position of peaks. This is not only observed between thin film and bulk glass responses as in Figure 3a, but also between infrared curves of thin films deposited on Si and gold substrates (Figure 3b). To understand the origin of such spectral differences, that is whether they are

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Figure 4. Schematic description of a bilayer optical configuration showing the propagation of light in the film/substrate system.

compositional/structural in nature or result from a combination of optical effects, we proceed with a detailed analysis of the measured spectral data. 3. Data Analysis 3.1. Theory. This section presents the formalism for the propagation of an electromagnetic field in a double layer system leading to the rigorous expressions for the measured optical spectra.28-34 In the following we consider normal incidence of light and the configuration vacuum-film-substrate-vacuum shown schematically in Figure 4. Each layer is characterized by thickness dj and complex refractive index n˜ j ) nj - ikj (j ) 0-3), while the phase change of the light beam traversing one layer is given by δj ) 2π/λnjdj, where λ is the wavelength in vacuum. The complex reflection and transmission Fresnel coefficients at each interface are defined respectively by:

rj )

n˜ j-1 - n˜ j n˜ j-1 + n˜ j

(1a)

tj )

2n˜ j-1 n˜ j-1 + n˜ j

(1b)

Then, the exact formulas for reflectance, R, and transmittance, T, for normal incidence of light to the bilayer system are obtained through the total reflection, rbi, and transmission, tbi, coefficients given by the following eqs:28 -2iδ1

rbi )

tbi )

r1 + r2e

-2i(δ1+δ2)

+ r3e

1 + r1r2e-2iδ1 + r1r3e-2i(δ1+δ2) + r2r3e-2iδ2 t1t2t3e-i(δ1+δ2) 1 + r1r2e-2iδ1 + r1r3e-2i(δ1+δ2) + r2r3e-2iδ2

(2a)

(2b)

with R ) rbirbi* and T ) tbitbi* (the asterisk denoting the complex conjugate) and the absorptance of the system being A ) 1 - (R + T). The above equations take into full account all multiple reflections in film and substrate. Following the procedure described in ref 32, the equations for reflectance, R, and transmittance, T, of the bilayer system can be written as:

R)

T)

D + F cos(2θ2) + G sin(2θ2) A + B cos(2θ2) + C sin(2θ2) t12t22t32e-2(γ1+γ2) A + B cos(2θ2) + C sin(2θ2)

refractive index and the thickness, respectively, of layers 1 or 2), γj ) 2π/λkjdj (kj is the imaginary part of the refractive index of layers 1 or 2), and the terms A, B, C, D, F, and G are functions of the parameters introduced above and are given analytically in the Appendix. As it will be shown later, the interference fringes due to silicon substrate used in this study have a period of ca. 2.5 cm-1, and thus the silicon interference pattern can be optically averaged by measuring low-resolution spectra (e.g., 10 cm-1). In this case, eqs 3 and 4 must be averaged with respect to the interference fringes of the substrate for their results to be directly comparable with low-resolution experimental measurements. The advantage of eqs 3 and 4 is that they can be easily integrated over the variable θ2, leading to substrate interference-free equations for the reflectance, Rav, and transmittance, Tav, of the bilayer system:

Rav )

[

(3)

(4)

where θj ) 2πnjdj/λ (nj and dj denote the real part of the

]

1 AB - BS AC - CS D-F 2 -G 2 2 S B +C B + C2

(5)

t12t22t32e-2(γ1 + γ2) S

(6)

Tav )

-2iδ2

+ r1r2r3e

Figure 5. Calculated real, n, and imaginary, k, parts of the complex refractive index for bulk Li2O-2B2O3 glass (a), silicon (b), and gold (c). For details see the text.

with S ) xA2+B2+C2. 3.2. Input for Calculations. To compare the spectral characteristics of thin films with those of bulk glasses of the same nominal composition, the complex refractive indices of bulk glasses were obtained through Kramers-Kronig analysis of the measured specular reflectance spectra. The calculated real, n, and imaginary, k, parts of the refractive index of the bulk glass having the diborate composition are depicted in Figure 5a. Optical properties of silicon can be determined by procedures reported previously.23,35,36 In this work we used the method based on the inversion of transmittance and reflectance spectra measured with low resolution,23,36 and the calculated n and k spectra for silicon are shown in Figure 5b. In the case of gold, the refractive index can be calculated by modeling the complex dielectric function with the Drude model and employing reported values for plasma frequency and the damping constant.37 The calculated n and k responses for gold are shown in Figure 5c. The calculated n and k spectra for bulk glass and for the silicon and gold substrates, as well as the substrate thickness,

Thin Film Amorphous Electrolytes

Figure 6. Simulated infrared transmittance spectra of a Li-diborate (Li2O-2B2O3) thin film (d1 ) 1 µm) on silicon wafer (d2 ) 600 µm). The blue envelope represents the complete spectrum calculated by employing eq 4, and the red line the averaged spectrum over substrate fringes obtained by eq 6. The inset shows both spectra in a narrow frequency range (1120-1255 cm-1) to highlight the interference pattern due to Si substrate.

were employed as input for simulations of the infrared spectra of thin amorphous films. With this procedure, the only adjustable parameter of simulations is the thickness of the glassy film. 4. Results 4.1. Transmittance Spectra of Thin Films on Silicon Wafer. The introduced model for the optical spectra of the bilayer system was first applied to simulate the transmittance spectrum of glassy thin film deposited on silicon wafer from the Li-diborate target. The calculated n and k spectra of bulk glass and Si and the thickness of Si substrate (d2 ) 600 µm) were used as input for the simulation. The transmittance spectrum of this bilayer system is given by eq 4 when interference fringes due to internal reflections in both film and substrate are considered, while eq 6 is applied to demonstrate averaging over the fringes due to Si substrate. Simulated transmittance spectra are shown in Figure 6 in the range 10-4500 cm-1 for a glassy film 1 µm in thickness. Simulation by eq 4 results in the blue-envelope spectrum, which is the complete infrared spectrum of the bilayer system. When eq 6 is used, the spectrum is averaged over substrate fringes and results in the simpler profile shown by the red-line spectrum in Figure 6. The inset of the same figure shows both spectra in a narrow frequency range (1120-1255 cm-1) and demonstrates clearly the interference pattern due to Si substrate. The obtained period of the interference resonance (∆ν ) 2.4 cm-1) is in agreement with that expected from the formula ∆ν ) 1/2nSidSi with nSi ) 3.4 (Figure 5) and dSi ) 0.6 mm. It is obvious from Figure 6 that the averaged spectrum, eq 6, is free of fringes related to the substrate and, thus, it represents the convolution of the true vibrational spectrum of the glassy film and its oscillatory interference pattern. In addition to these combined responses, the complete spectrum, eq 4, depicts also the modulated interference pattern of the Si substrate. It is observed that the amplitude of this modulation follows the

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Figure 7. Experimental infrared spectra of a thin film deposited on Si wafer (d2 ) 600 µm) from Li-diborate target (Li2O-2B2O3). The blueenvelope spectrum was measured with high resolution (0.2 cm-1) and the red-line spectrum with low resolution (10 cm-1). The inset shows clearly part of the interference pattern due to the Si substrate (blue), which is completely eliminated in the spectrum measured with low resolution (red).

pattern of the averaged spectrum, since for transparent substrates the key factor that determines this amplitude is the k value of the film. Experimental transmittance spectra of a Li-borate film (1.5 µm thick) deposited from the Li-diborate (Li2O-2B2O3) target on silicon wafer were acquired with low, 10 cm-1, and high, 0.2 cm-1, resolution and are shown in Figure 7. The highresolution spectrum shows clearly interference fringes due to Si with a period ∆ν ) 2.5 cm-1, in good agreement with the expected value. The low-resolution experimental spectrum presents interference fringes only from the film (see the broad envelope centered at ca. 2140 cm-1) superimposed on its vibrational spectrum. Comparison of measured spectra in Figure 7 with simulated spectra presented in Figure 6 and discussed above shows that measurements of thin film spectra with low resolution are equivalent to averaging over the interference pattern of the substrate, eq 6. Therefore, infrared measurements of thin films with relatively low resolution result in spectra free of substrate-interference fringes. 4.2. Reflection-Absorption Spectra of Thin Films on Gold-Covered Si Substrates. We focus now on the presentation of reflection-absorption spectra of Li-borate films deposited from the Li-diborate target on gold/Si substrates. For this configuration, multiple internal reflections occur only in the thin glassy film since the metal layer behaves like a perfect reflector. In Figure 8a we compare the experimental reflectanceabsorption, R-A, spectrum of a film on gold with the calculated transmittance spectrum, T, for a free-standing film with 1.80 µm thickness. The n and k values of the free-standing film are those of the bulk Li2O-2B2O3 glass, and this makes films having the same exact structure with the bulk glass. The thickness value of 1.80 µm was chosen in order to obtain comparable transmittance values for experimental and simulated spectra in the 800-1600 cm-1 frequency range. If the effect of the gold-thin film interface is neglected, the experimental R-A spectrum could be considered as the transmittance spectrum of a film having double thickness.

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Figure 8. (a) Experimental reflectance-absorption, R-A, spectrum of a Li-borate film deposited from Li-diborate target (Li2O-2B2O3) on gold-covered Si (red), and simulated transmittance, T, spectrum (blue) of a free-standing glassy film having the n and k values of the bulk Li2O-2B2O3 glass and thickness of 1.80 µm. (b) The same experimental R-A spectrum (red) compared with the simulated spectrum (green) using eq 3 and the n and k values of gold (thickness: 100 nm) and bulk Li2O-2B2O3 glass (thickness: 1.24 µm).

Figure 9. Low-resolution experimental transmittance spectra (10 cm-1, red) of Li-borate thin films deposited from a Li-diborate (Li2O-2B2O3) target on silicon wafer under different radio frequency powers (P). The calculated transmittance spectra (eq 6, blue) correspond to thin films having the Li2O-2B2O3 composition, and their indicated film thickness is the best value obtained by simulation of each experimental spectrum.

Before discussing differences between experimental reflectance-absorption and simulated transmittance spectra, it is instructive to refer to the origin of the main absorption envelopes in the transmittance spectrum at ca. 1350, 995, 710, and 380 cm-1 (Figure 8a). The high-frequency envelope at 1350 cm-1 is attributed to the asymmetric stretching vibration of boron-oxygen bonds in borate triangular units which may include both neutral triangles, BØ3, and charged metaborate triangles, BØ2O-, with Ø representing a bridging oxygen atom.16,19 The complex envelope peaking at 995 cm-1 originates from the asymmetric stretching vibrational modes of borate tetrahedral units, BØ4-, whereas the 710 cm-1 feature is associated with the deformation modes of various borate entities. While the mid-IR part of the spectrum is dominated by the vibrational activity of the network-building borate units, the rattling motion of Li ions in their local network sites is active in the far-IR and gives rise to the broad and asymmetric lowfrequency profile around 380 cm-1.16,19 The two strong high-frequency envelopes are also present in the experimental R-A spectrum and exhibit pronounced shifts toward lower frequency values, ca. 1295 and 890 cm-1, when compared to the corresponding bands in the transmittance spectrum. In addition, these borate stretching bands show a substantial change in relative intensity, while the feature at ca. 710 cm-1 appears also with lower relative intensity in the R-A spectrum. However, the most striking characteristic of the R-A spectrum is the apparent absence of the Li ion-site vibrational activity that is normally expected in the far-IR region. The spectral differences observed in Figure 8a show that no conclusions can be drawn from the direct comparison of reflection-absorption and transmittance spectra. This is because it is not certain at this point to what degree such differences are due mostly to optical effects or to structural changes from bulk to the thin film state. To clarify this situation, the R-A spectrum was simulated by eq 3, using as input for gold the n and k values of Figure 5 and a thickness of 100 nm. For the glassy thin film the n and

k values of bulk Li2O-2B2O3 glass were used (Figure 5), and the thickness was the only adjustable parameter. The comparison of experimental and simulated R-A spectra in Figure 8b demonstrates a close agreement in both band shape and position in the entire infrared range, with simulated film thickness equal to 1.24 µm. These results show that the far-infrared profile of the R-A spectrum, i.e., below ca. 700 cm-1, is governed mainly by the optical response of the gold substrate (i.e., its very high n and k value in this range, see Figure 5), which masks effectively the spectral characteristics of the glassy film. In conclusion, the main spectral differences observed between the experimental R-A spectrum and the calculated transmittance spectrum in Figure 8a should be attributed to optical effects, due to the high refractive index of the metal substrate in this range. Therefore, a search for possible structural variations between thin films and the corresponding bulk glass should be based on infrared measurements free from such strong optical effects from the metallic substrate, and this approach will be taken in the following section. 4.3. Effect of Preparation Conditions on Thin Film Characteristics. Films having the nominal lithium-diborate composition were deposited on silicon substrates with different rf deposition powers and typical transmittance spectra measured with 10 cm-1 resolution are shown in Figure 9. The characteristic features associated with the asymmetric stretching modes of the borate structural units are observed at ca. 1365 and 1010 cm-1 for triangular and tetrahedral borate units, respectively, with their deformation modes peaking at ca. 720 cm-1. The sharp and narrow feature measured at 610 cm-1 is due to the strongest lattice mode of the Si substrate (see Figure 2), which is visible because the Si substrate used here is more than 300 times thicker than the deposited thin films. In contrast to the R-A experimental spectra of thin films on gold (Figure 8b), all transmittance spectra of films show the presence of a broad band at ca. 350 cm-1 associated with the vibrational modes of lithium ions against their sites in the network.

Thin Film Amorphous Electrolytes

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Transmittance spectra were calculated by eq 6, using the n, k values of the bulk Li2O-2B2O3 glass and of the silicon substrate (d2 ) 600 µm), and are shown also in Figure 9. Comparison with the measured spectra demonstrates a close agreement in the range bellow 1700 cm-1, where the borate vibrational modes are mainly active, as well as above 1700 cm-1, where interference fringes due to multiple reflections in the films appear to have a dominant role. It is found also that the rf powers employed in this work (1.5 to 2.5 W/cm2) do not seem to affect, at least to a detectable degree, the infrared characteristics of thin films (Figure 9). 5. Discussion Having analyzed in detail the strong influence of optical effects on the infrared spectra of thin films, and especially for films deposited on gold substrates, we focus now on the comparison of calculated and experimental spectra of thin films deposited on silicon. The main message from Figure 9 is that the key characteristics of all experimental transmittance spectra agree well with those of the calculated spectra. This suggests that the nature and relative population of the short-range order (SRO) borate units in Li-borate films prepared by rf sputtering are close to those in the bulk glass of the same nominal stoichiometry. As noted above, the mid-IR profiles of the calculated transmittance spectra indicate the presence of charged borate tetrahedral units, BØ4-, with their B-O stretching vibration giving rise to the complex absorption envelope from ca. 800 to 1150 cm-1, and of borate triangles (BØ3, BØ2O-) with their corresponding vibration active in the high-frequency region (ca. 1150-1600 cm-1). Despite their dominant similarities, the experimental and calculated spectra of Figure 9 show subtle but systematic differences. To search for the origin of such differences we will rely on the very good agreement between experimental and calculated transmittance spectra above 1700 cm-1. This fact suggests that the nature and extent of optical effects should be the same in experimental and simulated thin film spectra, and thus spectral differences below ca. 1700 cm-1 should manifest real structural variations between films and bulk glass. The first spectral difference concerns the relative absorption of borate triangular units (1150-1600 cm-1) versus that of tetrahedral units (800-1150 cm-1). As noted in Figure 9, the absorption envelope due to borate triangles shows larger relative intensity in the experimental thin film spectra than in the bulk glass. In addition, this envelope exhibits in thin films enhanced absorption at ca. 1260 in comparison to the main absorption band centered at ca. 1370 cm-1. The second systematic difference is observed in the far-IR region. The broad band due to Li ion-site vibrations exhibits lower intensity in the measured film spectra and appears shifted to lower frequency in comparison to the bulk glass, i.e., ca. 350 cm-1 in films and ca. 375 cm-1 for bulk glass (Figure 9). Such combined spectral differences in both mid- and far-IR regions would be consistent with the Li ion content in thin films being smaller than that in the bulk Li-diborate glass.19 To examine the above hypothesis, we compare in Figure 10 the experimental transmission spectrum of the Li-borate film deposited under rf power of 1.50 W/cm2 with spectra calculated with the n and k spectral responses of bulk glasses xLi2O-(1 - x)B2O3 with x ) 0.30, 0.275, and 0.25, i.e., glasses with Li2O content lower than that of the nominal diborate composition (x ) 0.33). As observed in Figure 10, the bulk glass with composition 0.275Li2O-0.725B2O3 gives the best match in the

Figure 10. Comparison of the low-resolution experimental transmittance spectrum (10 cm-1, red) of a Li-borate thin film, deposited from a Li-diborate (Li2O-2B2O3) target on silicon wafer (P ) 1.50 W/cm2), with corresponding spectra calculated by using as input the optical responses (n, k) of bulk glasses xLi2O-(1-x)B2O3 with compositions x ) 0.30, 0.275, and 0.25 (blue). For details see the text.

mid-IR range between experimental and calculated spectra and, in particular, very good agreement in the region of borate tetrahedral (800-1150 cm-1) and triangular (1150-1600 cm-1) units. Nevertheless, a notable difference remains in the region of the Li ion-site vibration band in the far-IR even when comparison is made with the spectrum calculated for the 0.275Li2O-0.725B2O3 composition. The origin of this remaining difference may be related to structures of higher hierarchy than those at the local SRO structural units. The presence of chemical/structural inhomogeneities in glass has been proposed by various models even for glasses which, on the overall, are chemically homogeneous. According to the cluster-tissue model,38 fast cooling of a melt results in the formation of regions with high relative amorphicity (“connective tissue”), which surround regions of relatively better organization (“clustered pseudophases”). It was argued elsewhere39 that the metal ion-site vibration activity in the far-IR is sensitive to the relative participation of connective tissue and more ordered clustered pseudophases in the glass structure. Higher cooling rates, as in thin films vs the bulk glass, would enhance the relative participation of connective tissue and lead to a relatively more open glass structure. Thus, in terms of a potential energy landscape thin films are expected to occupy local energy minima of higher enthalpy and volume than that of the bulk glass.40 Therefore, it is of interest to track the response of thin films to annealing processes that may change their frozen-in energy. Results of an annealing experiment are shown in Figure 11 depicting the transmission spectra of a thin film before and after annealing for 15 min at 400 °C. The chosen annealing temperature is considerably lower than the glass transition temperature of the bulk state, Tg ) 500 °C for x ) 0.275,41 in order to avoid crystallization effects. The measured spectra of the film are compared with that calculated for a film having the composition 0.275Li2O-0.725B2O3. As shown in the inset of Figure 11 the spectra exhibit very similar interference patterns, indicating that changes induced by annealing in the

8118 J. Phys. Chem. C, Vol. 111, No. 22, 2007

Figure 11. Effect of annealing on the transmission spectrum of a Liborate thin film deposited on silicon wafer from a Li-diborate (Li2O2B2O3) target. Comparison is made with the calculated spectrum of a thin film having the structure of the bulk glass 0.275Li2O-0.725B2O3. The inset shows the spectra in a broader frequency range including their interference patterns.

spectral range below 1600 cm-1 should signal real structural rearrangements. The effect of annealing on the 800-1600 cm-1 envelop appears relatively small and it is manifested mainly by reduction in the relative intensity of the shoulder at 1075 cm-1 due to BØ4- tetrahedral units. Also, the deformation mode at 720 cm-1 reduces in intensity and shifts to 712 cm-1 after annealing. These observations indicate a rather limited modification of the SRO structure under the employed annealing conditions, and these are related mainly to rearrangements of BØ4- tetrahedral units. However, the Li ion-site vibrational dynamics appear more sensitive to annealing as manifested by the relative intensity increase of the far-IR absorption profile and its slight upshift upon annealing. In addition, an almost perfect agreement is observed now between the spectrum of the annealed thin film and that calculated for a film having the composition and structure of the bulk glass 0.275Li2O-0.725B2O3. These findings show clearly that annealing of the “hyperquenched” thin film amorphous state leads to change toward the bulk glassy state prepared under slower cooling. Notably, the largest effect has been observed for the lithium ion motion band, indicating that upon annealing lithium ions trigger limited SRO structural rearrangements to modify/create sites of a lower potential energy. 6. Conclusions Li-borate thin films having the nominal diborate composition, Li2O-2B2O3, have been prepared by rf-sputtering on Si and gold substrates. The infrared spectra of films were measured in the frequency range 30 to 5000 cm-1 and were analyzed to investigate the structure of the basic borate units which compose the glassy network and form also the sites occupied by the Li ions. The study of thin films was greatly facilitated by considering in parallel the infrared response of the bulk Liborate glasses. These glasses were prepared by the conventional melt quenching technique that, normally, leads to glass formation under lower cooling rates than those achieved by sputtering.

Kamitsos et al. It was shown that proper analysis of the infrared spectra of thin films requires implementation of the rigorous expressions that describe the optical response of thin films deposited on transparent (silicon) or metallic (gold) substrates. In particular, it was demonstrated that high-resolution experimental spectra of amorphous films on transparent substrates present a combined contribution of the vibrational response of glass and substrate and of the interference fringes due to multiple light reflections in the film and substrate. On the other hand, low-resolution experimental spectra could be simulated with the mathematical average of the complete expression over the fringes of the substrate. It was shown further that the transmission-absorption spectrum of a thin film deposited on a metallic substrate cannot be approximated by the transmittance spectrum of a film with double thickness. Instead, the rigorous expression should be used to describe accurately infrared spectra measured in the transmission-absorption mode. A key characteristic of such transmission-absorption spectra is the masking of typical vibrational bands in the far-infrared due to the dominating high values of the optical response functions of the metallic substrate. Transmittance spectra of “Li-diborate” thin films on silicon wafers were simulated by using as input the optical response functions of bulk glasses with composition xLi2O-(1-x)B2O3, x ) 0.33 (diborate), 0.30, 0.275, and 0.25, and of the silicon substrate. While the best agreement between experimental and simulated thin film spectra was obtained by using as input the optical response of the x ) 0.275 bulk glass, spectral differences remained in the far-infrared region resulting from the Li ionsite vibrational activity. It was found that such differences can be removed by annealing the thin films, a process that triggers mainly Li ion-site rearrangements toward sites of lower potential energy. As a conclusion, this work presented a detailed modeling of the infrared spectra of amorphous thin films deposited on substrates. The comparison of experimental and simulated spectra of Li-borate thin films allowed two main effects associated with the development of films by rf sputtering to be highlighted: (i) variations in composition between thin films and target material and (ii) differences in the energetics of sites occupied by Li ions, which can be removed by sub-Tg annealing of thin films. Acknowledgment. Support of this work by the EU through project NMP3-CT-2005-516975 is gratefully acknowledged. Appendix The parameters A, B, C, D, F, and G are defined as follows:

A ) 1 + |r1r2|2 exp(-4γ1) + |r2r3|2 exp(-4γ2) + |r1r3|2 exp[-4(γ1 + γ2)] + [2Re(r1r2) exp(-2γ1) + 2|r3|2 Re(r1r2*) exp(-2γ1 - 4γ2)] cos(2θ1) + [2Im (r1r2) exp(-2γ1) + 2|r3|2 Im(r1r2*) exp(-2γ1 - 4γ2)] sin(2θ1) B ) 2Re(r2r3) exp(-2γ2) + 2|r1|2 Re(r2r3*) exp(-4γ1 2γ2) + [2Re(r1r3) exp(-2γ1 - 2γ2) + 2|r2|2 Re(r1r3*) exp(-2γ1 - 2γ2)] cos(2θ1) + [2Im(r1r3) exp(-2γ1 2γ2) + 2|r2|2 Im(r1r3*) exp(-2γ1 - 2γ2)] sin(2θ1)

Thin Film Amorphous Electrolytes

C ) 2Im(r2r3) exp(-2γ2) - 2|r1|2 Im(r2r3*) exp(-4γ1 2γ2) + [2Im(r1r3) exp(-2γ1 - 2γ2) - 2|r2|2 Im(r1r3*) exp (-2γ1 - 2γ2)] cos(2θ1) + [-2Re(r1r3) exp(-2γ1 - 2γ2) + 2|r2|2 Re(r1r3*) exp(-2γ1 - 2γ2)] sin(2θ1) D ) |r1|2 + |r2|2 exp(-4γ1) + |r1r2r3|2 exp(-4γ2) + |r3|2 exp(-4γ1 - 4γ2) + [2Re(r1r2*) exp(-2γ1) + 2|r3|2 Re(r1r2) exp(-2γ1 - 4γ2)] cos(2θ1) + [-2Im (r1r2*) exp(-2γ1) - 2|r3|2 Im(r1r2) exp(-2γ1 - 4γ2)] sin (2θ1) F ) 2Re(r2r3*) exp(-4γ1 - 2γ2) + 2|r1|2 Re(r2*r3*) exp (-2γ2) + [2Re(r1r3*) exp(-2γ1 - 2γ2) + 2|r2|2 Re (r1*r3*) exp(-2γ1 - 2γ2)] cos(2θ1) + [-2Im(r1r3*) exp(2γ1 - 2γ2) + 2|r2|2 Im(r1*r3*) exp(-2γ1 - 2γ2)] sin(2θ1) G ) -2Re(r2r3*) exp(-4γ1 - 2γ2) - 2|r1|2 Im(r2*r3*) exp (-2γ2) + [-2Im(r1r3*) exp(-2γ1 - 2γ2) - 2|r2|2 Im (r1*r3*) exp(-2γ1 - 2γ2)] cos(2θ1) + [-2Re(r1r3*) exp(2γ1 - 2γ2) + 2|r2|2 Re(r1*r3*) exp(-2γ1 - 2γ2)] sin(2θ1) References and Notes (1) Vinatier, P.; Hamon, Y. Applications of ion transport in disordered solids. In Charge transport in disordered solids; Baranovski, S., Ed.; Wiley and Sons: New York, , 2006. (2) Levasseur, A.; Menetrier, M.; Dormoy, R.; Meunier, G. Mater. Sci. Eng. B 1989, 3, 5. (3) Bates, J. B.; Dudney, N. J.; Gruzalski, G. R.; Zuhr, R. A.; Choudhury, A.; Luck, C. F. Solid State Ionics 1992, 647, 53. (4) Julien, C.; Yebka, B.; Guesdon, J. P. Ionics 1995, 1, 316. (5) Souquet, J. L.; Duclot, M. Solid State Ionics 2002, 148, 375. (6) Joo, K. H.; Vinatier, P.; Pecquenard, B.; Levasseur, A.; Sohn, H. J. Solid State Ionics 2003, 160, 51. (7) Levasseur, A.; Kbala, M.; Brethous, J. C.; Reau, J. M.; Hagenmuller, P.; Couzi, M. Solid State Commun. 1979, 32, 839. (8) (a) Kamitsos, E. I.; Karakassides, M. A.; Chryssikos, G. D. J. Phys. Chem. 1986, 90, 4528. (b) Chryssikos, G. D.; Kamitsos, E. I.; Patis, A. P. J. Non-Cryst. Solids 1996, 202, 222. (9) Yamashita, M.; Terai, R. Glastech. Ber. 1990, 63, 13. (10) (a) Pliskin, W. A.; Lehman, H. S. J. Electrochem. Soc. 1965, 112, 1013. (b) Nakamura, N.; Mochizuki, Y.; Usami, K.; Itoh, Y.; Nozaki, T. Solid State Commun. 1984, 50, 1079. (11) (a) Koropecki, R. R.; Arce, R.; Bernardez, L. S.; Buitrago, R. J. Non-Cryst. Solids 1985, 74, 11. (b) Geotti-Bianchini, F.; Riu, L.; Gagliardi, G.; Guglielmi, M.; Pantano, C. G. Glastech. Ber. 1991, 64, 205. (12) Morimoto, A.; Noriyama, H.; Shimizu, T. Jpn. J. Appl. Phys. 1987, 26, 22.

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