Electrical Characterization of Ultrathin RF-Sputtered LiPON Layers for

Research Article www.acsami.org

Electrical Characterization of Ultrathin RF-Sputtered LiPON Layers for Nanoscale Batteries Brecht Put,*,†,‡ Philippe M. Vereecken,†,§ Johan Meersschaut,† Alfonso Sepúlveda,† and Andre Stesmans‡ †

Imec, Kapeldreef 75, 3001 Leuven, Belgium Department of Physics, University of Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium § Centre for Surface Chemistry and Catalysis, University of Leuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium ‡

S Supporting Information *

ABSTRACT: Ultrathin lithium phosphorus oxynitride glass (LiPON) films with thicknesses down to 15 nm, deposited by reactive sputtering in nitrogen plasma, were found to be electronically insulating. Such ultrathin electrolyte layers could lead to high power outputs and increased battery energy densities. The effects of stoichiometry, film thickness, and substrate material on the ionic conductivity were investigated. As the amount of nitrogen in the layers increased, the activation energy of the ionic conductivity decreased from 0.63 to 0.53 eV, leading to a maximum conductivity of 1 × 10−6 S/ cm. No dependence of the ionic conductivity on the film thickness or substrate material could be established. A detailed analysis of the equivalent circuit model used to fit the impedance data is provided. Polarization measurements were performed to determine the electronic leakage in these ultrathin films. A 15-nm LiPON layer on a TiN substrate showed electronically insulating properties with electronic resistivity values around 1015 Ω·cm. To our knowledge, this is the thinnest RF-sputtered LiPON layer shown to be electronically insulating while retaining good ionic conductivity. KEYWORDS: Li-ion rechargeable battery, LiPON, impedance spectroscopy, thin film, RF sputtering conductivity (10−6 S/cm),9−11 and low electronic conductivity (electronic resistivity of ∼1014 Ω·cm). LiPON is currently used in commercial thin-film batteries and is the most frequently used solid electrolyte in the thin-film battery community.12,13 However, the use of this material in a practical configuration (electrolyte layer of 500 nm) leads to a cell resistance in the range from 10 Ω to 2 kΩ (as opposed to the resistance of a liquid cell, which is in the range of milliohms14). To lower this resistance, the electrolyte can be downscaled further. The reduction of the electrolyte thickness also results in an increase in the system’s energy density. As the electrolyte itself does not contribute to the battery’s energy content, any reduction of its thickness directly translates into an increased energy density. Furthermore, the use of a solid electrolyte also alleviates the need for stringent packing requirements needed to ensure safety when employing a liquid electrolyte. Obviously, the system’s energy density is further increased by reducing the amount of nonactive material. An issue met when reducing the electrolyte’s thickness is the onset of electronic conduction, which needs to be avoided. This effect is well-known in oxides with reduced thickness and has

1. INTRODUCTION Rechargeable lithium-ion batteries dominate the current battery market. They are in widespread use because of their high energy and power densities. The classical lithium-ion battery systems employ a liquid electrolyte to facilitate Li-ion transport between the electrodes. However, the presence of this organic solvent poses a significant safety risk, restricts the temperature window, and prevents easy miniaturization. Switching to a solid-state electrolyte could relieve these issues. As such, solid-state thin-film batteries might offer an alternative for the current “wet” lithium-ion batteries. In addition to their increased safety and operational range, they allow easy miniaturization and on-chip integration,1−3 which enables their use for medical implants and various sensor systems. However, stable solid electrolytes with high ionic conductivities are currently not available. Materials with high ionic conductivities have been reported but generally have a limited stability window.4−6 For this reason, electrolytes with lower ionic conductivity are used in thin-film configuration, which reduces the distance over which the Li ions need to be transported and, hence, the ionic resistance. Currently, lithium phosphorus oxynitride glass (LiPON) is the most commonly employed solid-state electrolyte. Since the discovery of LiPON in the 1990s by Bates and co-workers, the material has received continued attention,7,8 mainly because of its wide stability window (0−5 V vs Li+/Li),9,10 reasonable ionic © XXXX American Chemical Society

Received: December 21, 2015 Accepted: March 1, 2016

A

DOI: 10.1021/acsami.5b12500 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. ERD depth profiles of (a) a 30-nm LiPON layer on a 40-nm TiO2/30-nm TiN/Si substrate and (b) a 50-nm LiPON layer on a 60-nm TiN/Si substrate. In panel a, the lithium signal is detected throughout the TiO2, indicating the formation of a lithium-depleted LiPON layer, whereas in panel b, a sharp interface of LiPON with TiN is seen without any lithium outdiffusion.

even claimed for LiPON layers down to 12 nm.26 However, in the last case, no compelling evidence was provided. In this work, the effects of reducing the LiPON film thickness from 240 to 15 nm were investigated systematically. Critical parameters such as the ionic and electronic conductivities were evaluated for different LiPON stoichiometries and substrates. The ionic and electronic conductivities were evaluated by solid-state impedance spectroscopy (SIS) and polarization measurements, respectively. LiPON layers of 15 nm were shown to be electronically insulating while retaining a good ionic conductivity.

been described by the onset of different tunneling mechanisms.15 LiPON is typically deposited by reactive sputtering from a Li3PO4 target, but other deposition methods such as ion-beamdirected assembly (IBDA),16 atomic layer deposition,17 and electron-beam evaporation18 have also been demonstrated. When Li3PO4 is sputtered in a nitrogen plasma, nitrogen is incorporated into the layer, leading to the formation of LiPON.7,8,19 The incorporation of nitrogen into the film improves both the stability and ionic conductivity of the material. This is caused by the shortening and additional cross-linking of the phosphate chains.9,11 Shorter chains lead to a more favorable environment for Li-ion transport. The incorporation of nitrogen into the LiPON layers also leads to a reduction of the lithium content and a smaller fraction of Li ions participating in conduction.17,19,20 Hence, the conductivity will exhibit a trade-off between the increased conductivity caused by the decreasing activation energy and a reduction in conductivity due to a decrease in the amount of (mobile) lithium. In the present work, LiPON layers were fabricated by sputtering, which still provides the highest ionic conductivity reported so far.9 Atomic layer deposition (ALD) of LiPON layers has been demonstrated recently but with lower ionic conductivity than for sputtered layers.17,21 Use of ALD could facilitate the employment of LiPON in 3D battery systems. Such large-area 3D substrates increase the thin-film battery capacity while retaining a small total footprint.22 However, the practical realization of such a battery requires a uniform, conformal, and defect-free coating with a thin solid electrolyte layer. It can be imagined that, in the case of LiPON, a uniform nitrogen composition will also be a key requirement. In a 3D battery, the use of thin electrolyte layers is a matter of necessity. A 3D battery uses nanostructured templates, such as silicon pillars,2 metal meshes,1 or etched trenches.23 Such structures provide only a limited space for stack deposition. In addition, if one wants to retain a favorable electrode-toelectrolyte fraction, a reduction of the electrolyte’s thickness is desired. It is therefore anticipated that the electrolyte layer will undergo extreme downscaling in solid-state 3D batteries.24 Contradictory reports can be found on the onset point of electronic conduction in LiPON thin films. In one work, the onset was argued to occur below 100 nm;24 in another, it was claimed to occur below 50 nm;25 and electronic insulation was

2. EXPERIMENTAL SECTION LiPON layers were deposited by RF sputtering (Pfeiffer, Spider) from a 4-in. Li3PO4 target (Praxair, 99.9% purity). Various depositions were performed under conditions of different power (100−250 W) and varying N2 and Ar flows (between 0 and 40 sccm). Before each deposition, at least 1 h of presputtering was performed. During the deposition of the LiPON layer, the pressure was kept at 3 × 10−3 Torr; otherwise, the chamber base pressure was kept at 3 × 10−6 Torr. Different substrates were used for LiPON deposition, namely, 60−90 nm of sputtered TiN (Endura) on Si, 40 nm of TiO2 on 30 nm of TiN (both ALD-deposited, ASM Pulsar 3000) on Si, or 80 nm of sputtered Pt (home-built tool) on TiO2 on 30 nm of SiO2 on Si. The morphology of the deposited layers was checked using scanning electron microscopy (SEM, Nova). The LiPON stoichiometry was, in turn, determined by elastic recoil detection (ERD), making use of a primary ion beam of Cl4+ accelerated to 8 MeV by a 2 MV tandem accelerator. In this setup, the forward recoiled and scattered ions were detected with a time-of-flight energy (ToFE) telescope.27 The thickness of the films was determined by a combination of SEM, profilometry, and spectroscopic ellipsometry (only for the thinnest films). The combination of these techniques was used to calculate the sputter rates [Figure S1 in the Supporting Information (SI)]. Electrochemical experiments were carried out in an Ar glovebox (O2, H2O < 1 ppm). For this purpose, a three-electrode Teflon cell was clamped on top of the sample using a Kalrez O-ring, and a Luggin capillary was used to connect the cell with the reference-electrode compartment. One molar LiClO4 (battery-grade, Sigma-Aldrich) in propylene carbonate was used as the electrolyte. An autolab (Metrohm) potentiostat with a frequency-response analyzer module (frequency range from 0.01 Hz to 1 MHz) was used for all electrochemical impedance spectroscopy (EIS) testing. EIS was applied to samples during the optimization of the sputtering process because it does not require the deposition of metal contacts. For the “dry” electrical characterization of the LiPON layers, metal−electrolyte−metal (MEM) capacitors were fabricated. Gold B


Research Article


Figure 2. Ionic conductivities of LiPON films sputtered for 40 min under different conditions of N2 flow and RF power. Ionic conductivity was determined by wet (red circles) and dry (blue asterisks) impedance spectroscopy measurements. (a) Effects of N2 flow on the ionic conductivity of LiPON layers sputtered at 150 W (or 1.85 W/cm2). A clear optimum is present at 30 sccm N2, corresponding to a conductivity of 2.2 × 10−7 S/cm. (b) Effects of varying RF power on the LiPON ionic conductivity at a fixed N2 flow of 30 sccm. The highest conductivity (obtained by dry measurements) of 1 × 10−6 S/cm was found at ∼2.75 W/cm2. The solid curves guide the eyes. metal dots were deposited through a shadow mask by thermal evaporation (Alcatel). The dots had diameters varying from 100 to 500 μm, allowing the determination of perimeter leakage. Electrical measurements were carried out in a micromanipulated cryogenic probe station that was kept under a vacuum (10−4−10−5 Torr) to avoid effects of moisture during the measurements. Care was taken to avoid exposure of the LiPON layers to the ambient for prolonged periods to prevent contamination of the surface.28 Heating and cooling (using liquid N2) was carried out over a range from 200 to 350 K. Current−voltage and polarization measurements were performed using an Agilent 4156C parameter analyzer. Dry solidstate impedance spectroscopy (SIS) was carried out using an HP4284A precision LCR meter over a frequency range from 20 Hz to 1 MHz. For each conductivity value reported in this work, a minimum of five different metal dots were measured. An ac signal was always applied to the bottom contact (TiN or Pt) to limit noise. Impedance spectroscopy results were analyzed by fitting an equivalent circuit to the data, using MEISP software (Kumho Chemical Laboratories). The obtained fits had a χ2 value in the range from 1 × 10−3 to 1 × 10−5. The relative standard deviation for all fitted values remained below 5%.

detrimental for the thinnest LiPON layers as a significant fraction of their total lithium content will be lost. Figure 1b represents the deposition of a similar LiPON (50nm) layer on a (60-nm) TiN current collector. No lithium outdiffusion is seen here, confirming the previously reported barrier properties of a TiN current collector.29,30 Similar experiments were also conducted on a thin platinum layer (30nm thickness); no lithium outdiffusion was detected in this case either (Figure S2 in the SI). Therefore, it is concluded that the platinum layer also acts as an adequate blocking layer. Both TiN and platinum can therefore function as current collectors in the further characterization of thin LiPON layers. The morphology of different LiPON layers was investigated by SEM, the results of which are depicted in Figures S3 and S4 (see SI). A dense, closed layer without pinholes or crystallites could be seen in all cases. The morphology was independent of the tested sputtering conditions. Results similar to those shown here were found for all tested samples. To optimize the ionic conductivity (σion), its dependence on the N2 flow and sputtering power was investigated. The ionic conductivity was determined using both dry solid-state impedance spectroscopy (SIS) [performed using metal− electrolyte−metal (MEM) capacitors] and wet electrochemical impedance spectroscopy (EIS) measurements. When performing EIS, the ionic conductivity can be determined without experiencing the effects of possible electronic leakage because the liquid electrolyte is electronically insulating. The results of these measurements are summarized in Figure 2, and some of the results of the dry impedance measurements are also summarized in Table 1. Ionic conductivity was extracted by fitting an equivalent circuit model to the impedance spectroscopy response. The same model was used for both wet and dry impedance analysis. This was possible because the difference in contact type (blocking vs nonblocking) could be accommodated by a change in the n parameter of the CPEBlock. More details on the fitting and model parameters are provided in the next section (see Figure 3). Figure 2a shows the dependence of the ionic conductivity on the N2 flow rate at a fixed radio-frequency (RF) sputtering power of 150 W (or ∼1.85 W/cm2). Note that a minimum flow of 25 sccm was applied; thus, when the N2 flow was zero, a pure Ar flow of 25 sccm was applied. A clear increase in ionic conductivity was seen with increasing N2 flow, reaching an

3. RESULTS AND DISCUSSION 3.1. Process Characterization. In this work, thin LiPON films were deposited by RF sputtering on different substrates. As the ionic conductivity of LiPON is closely linked to its stoichiometry, elastic recoil detection (ERD) measurements were conducted to determine the material composition. In this way, the interface with the substrate can be seen, along with the possible effects of lithium outdiffusion. The elemental depth profiles, constructed from ERD measurements on different substrates, are shown in Figures 1 and S2 (SI). Figure 1a shows the depth profile of a 30-nm LiPON layer (as-deposited) on a substrate composed of (from top to bottom) 40 nm of TiO2, 30 nm of TiN, and silicon. The LiPON layer was sputtered at 150 W (∼1.85 W/cm2) under a flow of 20 sccm N2 and 5 sccm Ar (considered the standard sputtering condition in this work). A clear outdiffusion of lithium from the LiPON layer into the TiO2 layer occurred. The presence of lithium was detected throughout the TiO2 layer up to the TiN current collector, which acted as a barrier for Li-ion diffusion. Naturally, this diffusion out of LiPON into the substrate will affect the ionic conductivity of the material because a lithium-depleted layer remains. The impact of such outdiffusion will be most C


Research Article


Table 1. Overview of the Stoichiometries and Ionic Conductivities of Various LixPOyNz Layers Sputtered for 40 min under Different Sputtering Conditions gas flow (sccm) 25 Ar 20 N2/5 Ar 30 N2

power (W) 150 150 225

stoichiometry Li2PO2.8 Li2.6PO2.13N0.8 Li2.14PO1.85N0.91

σion 300 Ka (S/cm) −8

Ea (eV)

(3 ± 2) × 10 (1 ± 0.2) × 10−7 (1 ± 0.1) × 10−6

− 0.63 ± 0.03 0.53 ± 0.02

Ionic conductivity (σion 300 K) determined at 300 K by dry impedance measurements as the average of the measurements of a minimum of five metal dots. a

Figure 3. (a) Complex-plane plot of the impedance response measured at 0 V of a 30-nm LiPON layer sputtered on TiN using the standard recipe. The solid curves represent the fitting results based on the circuit model depicted in the inset. A good fit was obtained for all of the different temperatures studied. (b) Changes in ionic conductivity, calculated from the sum of R1 and R2, for layers with varying thicknesses as a function of inverse temperature. Data points consist of averages of the ionic conductivity for a minimum of five metal dots per temperature. Little difference is seen between the various layers, in either the slope or the magnitude of the ionic conductivity.

optimum conductivity of 2.2 × 10−7 S/cm at 30 sccm of N2. A similar optimum was reported previously.11 Figure 2b shows the effects of the RF sputtering power on the ionic conductivity (with the N2 flow fixed at 30 sccm). From 1.8 to 2.8 W/cm2, the ionic conductivity increases, reaching a maximum of 6 × 10−7 S/cm. With a further increase in power, the conductivity is reduced again. The combination of the sputtering parameters of a N2 flow of 30 sccm and the application of a power of 225 W (2.7 W/cm2), leading to the optimum ionic conductivity, is henceforth referred to as the best known method (BKM). During sputtering of LiPON, nitrogen is incorporated into the Li3PO4 film in two different ways: First, it can form a double bond with a P atom (NP). In this case, it is considered a doubly coordinated nitrogen. The second possibility is incorporation in a triply coordinated way (N< ).7 The ratio between these two types of nitrogen incorporation influences the ionic conductivity.11 The largest increase in conductivity is obtained by incorporating triply coordinated nitrogen. Because the coordination of the nitrogen is influenced by the sputtering power, an optimum in conductivity with varying RF power has been reported.11,31 The smaller impact of doubly coordinated nitrogen on the ionic conductivity can be attributed to its less efficient cross-linking capabilities compared to those of triply coordinated nitrogen. The stoichiometry of LiPON layers deposited under different conditions was determined using ERD, the results of which are compiled in Table 1. For sputtering in pure Ar plasma at a power of 150 W, a layer stoichiometry of Li2PO2.8 was found, slightly lithium- and oxygen-poor compared to the ideal Li3PO4 target material. When the nitrogen content in the flow was increased, namely, to 20 sccm N2/5 sccm Ar, a change in the

stoichiometry to Li2.6PO2.13N0.8 was measured, a compositional value lying in the range of other stoichiometries (LixPOyNz, with 2.6 < x < 3.5, 1.9 < y < 3.8, and 0.1 < z < 1.3) reported before for different LiPON layers.9,20,32,33 Subsequently, the gas flow was changed to 30 sccm N2, and the power was increased to 225 W (i.e., BKM conditions); under these conditions, the stoichiometry changed to Li2.14PO1.85N0.91, and the amount of incorporated nitrogen further increased. Compared to the layer sputtered at 150 W, the amounts of both Li and O in the BKM layer decreased. Such a decrease in lithium and oxygen contents is commonly observed as more nitrogen becomes incorporated in the layer.9,19,20 As is evident from Table 1, there was a clear increase in ionic conductivity with increasing N2 content. The lowest conductivity value was found for the Li2PO2.8 layers, which have an ionic conductivity (σion 300 K) of 3 × 10−8 S/cm at 300 K, similar to values reported elsewhere. 9 When the amount of incorporated nitrogen was increased, the conductivity rose. The highest value found in this work was 1 × 10−6 S/cm for a 240-nm-thick Li2.14PO1.85N0.91 layer on TiN. This value is lower than the maximum value reported for LiPON (3 × 10−6 S/cm), but equal to what is typically found as best value.9,11,19,20 The rise in conductivity with increasing nitrogen content is associated with a reduction in the activation energy for Li-ion transport, decreasing from Ea = 0.63 eV to Ea = 0.53 eV, in line with other observations.9,11 As explained above, the incorporation of nitrogen into Li3PO4 layers leads to a more favorable environment for Li-ion motion, which lowers its activation energy.20 The formation of NP bonds instead of PO bonds also explains the reduced oxygen content as measured by ERD for the BKM material (see Table 1). From the above results, we conclude that LiPO(N) solid electrolyte layers were successfully fabricated. The sputtering D


Research Article


Table 2. Overview of the Impedance Analysis of LiPON Films Deposited under Different Experimental Conditionsa standard TiN

a

standard Pt

BKM TiN

LiPON thickness (nm)

Ea (eV)

σ300 K (S/cm)

Ea (eV)

σ300 K (S/cm)

Ea (eV)

σ300 K (S/cm)

15 30 70 130 240

0.6 ± 0.01 0.58 ± 0.02 0.64 ± 0.03 0.63 ± 0.03 0.7 ± 0.03

2 ± 0.3 × 10−7 2.5 ± 0.6 × 10−7 1 ± 0.3 × 10−7 1 ± 0.8 × 10−7 2.5 ± 0.4 × 10−7

0.60 ± 0.015 0.59 ± 0.01 0.64 ± 0.03 − −

2 ± 0.9 × 10−7 2 ± 0.5 × 10−7 2 ± 0.3 × 10−7 − −

0.57 ± 0.005 − 0.52 ± 0.02 − 0.53 ± 0.02

2 ± 0.1 × 10−7 − 5 ± 0.2 × 10−7 − 1 ± 0.1 × 10−6

Calculated conductivity values are averages of a minimum of five metal dots measured at 300 K.

conductivity, and an associated drop in activation energy, was observed under the optimized sputtering conditions. The values presented here agree well with those reported elsewhere.7−10,26,32 3.3. Equivalent Circuit Model Analysis. In this section, the equivalent circuit model is discussed in greater detail. In addition, physical counterparts for the components of the equivalent circuit model are proposed. The circuit model used is shown in the inset in Figure 3a. It consists of a resistance (Rs) in series with two R/CPE branches and finally followed by another constant phase element in series, added to account for the capacitive polarization due to the ionic blocking contact. The impedance of the constant phase elements, ZCPE, is modeled as34 1 e−inπ /2 ZCPE = Q 0ωn (1)

parameters were optimized to obtain the highest possible ionic conductivity, reaching a value of 1 × 10−6 S/cm for the Li2.14PO1.85N0.91 material in this work, well in line with literature reports. 3.2. Effects of LiPON Film Thickness and Substrate Material on Ionic Conductivity. In this section, the effects of the substrate and LiPON layer thickness on the ionic conductivity is evaluated. In addition the equivalent circuit used to analyze the impedance spectroscopy data is introduced as well. Figure 3a shows, in a complex-plane plot, an example of the impedance results observed at 0 V on a 30-nm LiPON layer deposited on a TiN substrate and contacted with a gold dot (500 μm in diameter). The characteristic solid electrolyte behavior is observed: The impedance shows an intercept on the real axis at high frequency, two semicircles in the mediumfrequency range, and a “vertical” response at low frequency. The intercept with the real axis is modeled by a resistor (R), each semicircle is modeled by a constant phase element (CPE) in parallel with a resistor, and the vertical response is modeled by a single CPE, resulting in the equivalent circuit model depicted in the inset in Figure 3a. The first (R1/CPE1) branch of the model is attributed to the semicircle at higher frequencies, and the second branch is attributed to that at lower frequency. Two semicircles were introduced in the model, because it was impossible to adequately fit the impedance response using only a single semicircle model. The solid curves in Figure 3a represent the best fit to the impedance spectroscopy data, showing that all features of the impedance response could be adequately fitted (see also Figure S5 in the SI). A similar model has been used in other reports to analyze thin-film LiPON impedance data.25 The final ionic conductivity value was extracted from the sum of R1 and R2. Note also that all films studied exhibited a vertical impedance response at low frequency characteristic of a capacitor structure (MEM), indicating that even the 15-nm LiPON film is electronically insulating. Figure 3b shows the extracted ionic conductivity (on a logarithmic scale) versus the inverse temperature for LiPON layers of varying thicknesses deposited on different substrates (TiN or Pt). As expected, the logarithm of the conductivity increased linearly with increasing inverse temperature, in accordance with an Arrhenius-type behavior. No significant difference in the magnitude of the ionic conductivity or in the slope of the temperature dependence was seen between layers with different thicknesses or contacted by different current collectors. Table 2 presents an overview of the quantitative properties obtained from impedance analysis for the different LiPON films deposited on Pt and TiN under the different sputtering conditions examined (see also section 3.1). Little influence of the type of substrate on the ionic conductivity or the activation energy was found. A clear increase in ionic

In this equation, ZCPE is the impedance of the CPE, i represents the imaginary number, Q0 is a fitting parameter, n takes a value between 0 and 1, and ω = 2πf, where f is the applied frequency. As n changes from 0 to 1, the CPE response evolves from a pure resistor to an ideal capacitor. Interface roughness typically results in a deviation from ideality,35 which can be taken into account by a reduction in the n value of the CPE. Table 3 summarizes the variation in the equivalent circuit model parameters as the contact size and substrate material are Table 3. Summary of the Most Relevant Circuit Model Parameters, Inferred through Fitting of the Impedance Response at Room Temperature of a 30-nm-Thick LiPON Layera Sputtered on Different Substratesb substrate

Au dot diameter (μm)

Rs (Ω)

R1 (Ω)

R2 (Ω)

C1 (nF)

C2 (nF)

TiN TiN Pt Pt

300 500 300 500

70 60 75 65

820 377 1000 395

23000 4800 33000 10000

0.48 1.1 0.8 1.4

27 45 24 65

χ2 1 1 2 6

× × × ×

10−3 10−3 10−4 10−4

a

Standard recipe; 20 sccm N2, 5 sccm Ar. bMaximum standard deviations found from the fitting were ±0.2 Ω for the resistances and ±0.005 nF for the capacitances.

varied for 30-nm thick LiPON layers fabricated using the standard recipe (150W, 20sccm N2, 5sccm Ar). Note that the χ2 values for the model vary between 1 × 10−3 and 2 × 10−4, indicating a reliable fit in all cases. The first parameter in the model that we discuss is the series resistance (Rs). A series resistance value of about 70 Ω was found in all cases, a value that was also extracted from I−V measurements after electrical breakdown of the LiPON layer,15 equivalent to a normalized resistance of 0.13 Ω·cm. E


Research Article


Figure 4. (a) Capacitance density versus layer thickness as obtained from impedance data taken on LiPON films of various thicknesses grown on TiN under standard sputtering conditions. Capacitance values were extracted from the constant phase elements in the equivalent circuit model shown as inset in Figure 3. C2 indicates an inverse dependence on the layer thickness, as expected, whereas C1 and C3 are largely independent of the LiPON thickness. The solid lines guide the eyes. (b) Inferred areal resistance value versus temperature, where R1 and R2 correspond to the elements shown in the inset circuit Figure 3. The two resistances decrease with increasing temperature and show equal exponential dependences on temperature.

The second set of equivalent components are those that constitute the first R/CPE branch, namely, R1 and CPE1 (consisting of C1 and the parameter n, as shown in eq 1). This branch of the model is associated with the high-frequency semicircle. The value of R1 was found to vary between 377 and 1000 Ω and to scale inversely with the contact size. The Q0 value of the CPE element was taken as C1 as n was found to lie between 0.9 and 1. The C1 capacitance value was around 1 nF and scaled with the dot area, as expected for a capacitor. However, the capacitance density (i.e., capacitance/dot area) did not scale with layer thickness as one would expect for a typical capacitor (Figure 4a). Instead, C1 appeared to be independent of the thickness; which will be addressed below. The third set of equivalent components consists of the elements R2 and CPE2. The resistance varied between 33 and 5 kΩ (see Table 3) and was the dominant impedance contribution to the ionic conduction. When the conductivity was calculated from R2, a value in the range of 1.9 × 10−7 S/cm was found. As expected for ionic conduction, the R2 value decreased exponentially with increasing temperature, as seen in Figure 4b. The values of C2 (extracted from CPE2) were found to lie in the range from 24 to 65 nF and again to scale with the dot diameter (see Table 3). C2 also scaled with layer thickness (Figure 4a), as expected for a classical parallel-plate capacitor. This indicates that R2 and CPE2 correspond to the bulk response of the electrolyte, where CPE2 corresponds to its dielectric capacitance and R2 to the ionic resistance. The capacitance extracted for C2 matches well with the values calculated for a parallel-plate capacitor with a LiPON dielectric of dielectric constant ϵr = 13.36 The final circuit element depicted in the inset of Figure 3 to be discussed is the CPEBlock element accounting for the blocking contact (C3). This can be attributed to the formation of an electrochemical double layer (EDL). Because LiPON layers have a large amount of mobile ions, double layer formation is likely to occur. The creation of such an electrochemical double layer finds its origin in the workfunction difference between the metal contacts as depicted in Figure 5. The attribution of C3 to the electrochemical double layer is justified by its independence of the layer thickness (shown in Figure 4). The value for C3 varied only from 21 to 17

Figure 5. Schematic representation of the creation of an electrochemical double layer under influence of the work-function difference between the metal contacts. μ̃ e corresponds to the electrochemical potential (or Fermi level) of the materials.

nF when the layer thickness was varied over an order of magnitude. C3 is thus largely independent of the layer thickness. Furthermore, the values found here for C3 lie in the range anticipated for the formation of an electrochemical double layer.37 Unfortunately, the assignment of the branch consisting of R1 and C1 could not be done unambiguously. To gain more insight into the nature of R1/CPE1, the temperature dependence of R1 was investigated, giving the results plotted in Figure 4b. R1 was found to show an exponential dependence on the temperature, indicative of ionic conductivity. Note that the slopes of both R1 and R2, and therefore also the associated activation energies, are roughly equal. The extracted Ea values lie around 0.6 eV, pointing to Li-ion conductivity through LiPON.7,8 Indeed, surface layers such as Li2O or Li2CO3 would give significantly larger activation energies. In addition, the calculated conductivity values (assuming transport over 30 nm) corresponding to R1 are too large (5 × 10−6 S/cm) for these LiPON layers (see Table 2). Conversely, if one assumes a uniform conductivity of 2 × 10−7 S/cm through the layer, a layer thickness of 2 nm is extracted from the measured R1 resistance. When this layer thickness is used to calculate the dielectric F


Research Article


Figure 6. Relaxation behavior of the current density upon application of a 3-V bias to a 500-μm gold dot on Pt/LiPON/Au structures. (a) Transient current densities for 30- and 70-nm-thick LiPON layers on a platinum substrate (b) Results for two 15-nm LiPON layers, one on a TiN current collector and one on a Pt current collector. The 15-nm layer on TiN exhibits a clear relaxation behavior evolving to a low current density. For the case of the Pt substrate, a typical time-dependent dielectric breakdown response is seen that eventually results in hard breakdown.

constant of C1, a value of ϵr = 5 is found, a physically reasonable value for a dielectric layer. Based on these considerations, two phenomena can be thought of as possible origins of the second semicircle, namely, the presence of an interface layer and the occurrence of grain boundaries. The latter is commonly observed in crystalline electrolytes such as LiLaTiO3.38 However, the deposited LiPON is a glass, and no crystallites could be distinguished in the SEM images (cf. Figures S3 and S4 in the SI) or by XRD (Figure S6 in the SI). TEM studies performed elsewhere26 did not reveal any crystallites either. Therefore, the presence of grain-boundary conductivity seems to be unlikely in this system. The other plausible cause of the high-frequency semicircle (associated with R1/CPE1) is a change in the electrolyte/metal interface. The layer thickness of 2 nm calculated from the observed conductivity provides credibility to an interface-linked phenomena. The buildup of a highly lithium enriched layer at one interface is accompanied by the creation of a lithium-poor layer at the other side, as shown in Figure 5. Different lithiumpoor stoichiometries of LiPO(N) have commonly been observed and are known to have a different ionic conductivity, for example, Li4P2O7 (σion = 2.3 × 10−7 S/cm)39 and LiPO3 (σion = 3 × 10−9 S/cm).40,41 The creation of such layers would naturally result in a distinct semicircle related to the changed conductivity and its associated capacitance. Therefore, the first R1/CPE1 branch can be attributed to the interface layer that is created as a result of the formation of an electrochemical double layer. Note that, in the work reported here, the ionic conductivity was calculated from the sum of R1 and R2, because the total impedance originating from a thin electrolyte layer can experience a significant contribution from the creation of an interface layer. However, the effect of the interfacial resistance R1 on the ionic conductivity remains limited, as illustrated in Table S1 (SI). From the above results, we conclude that the equivalent circuit model used to analyze the impedance data needs two R/ CPE branches to enable adequate fitting of the impedance data. The first branch is attributed to an interface phenomenon, the second to the bulk response of the LiPON layer.

3.4. dc Polarization Measurements. In the final section of this work, we assess the electronic conductivity of LiPON layers of varying thicknesses. Electronic conduction is typically evaluated by polarization measurements [also known as galvanostatic intermittent titration technique (GITT) or isothermal transient ionic current (ITIC), modified versions of the Hebb−Wagner method42−44]. A constant potential is applied over the layer under investigation, causing the flow of a transient current. In first instance, this current consists of a combination of dielectric polarization and ionic and electronic currents. However, because the stack under investigation consists of two blocking contacts on a solid electrolyte, the ionic current gradually fades out, the reason being the development of both lithium-depleted and lithium-enriched regions (see Figure 5). This prevents a continuous ionic current, so eventually, only the electronic current remains. During the experiments performed in the current work, a constant 3-V bias was applied across the LiPON layersa value particularly chosen because it is the potential a solid electrolyte experiences in a battery stack. Typical examples of such stacks are LiMn 2 O 4 /LiPON/Li 4 Ti 5 O 12 , 45 Li 2 Mn 2 O 4 /LiPON/ Nb2O5,46 and LiCoO2/LiPON/Li.47 Any LiPON layer, independent of its thickness, thus has to be stable under a constant 3-V bias. The 3-V bias was applied to the layers for a minimum time span (relaxation time) of 12 h, a typical time used in such measurements.25,26 In the case of ultrathin layers, shorter relaxation times were applied initially, but this did not result in a stable current. When we tried to fit the acquired data to a single exponential term, as has been done elsewhere,18,43,44 no satisfactory fit could be obtained. Only upon using a sum of two exponential functions (double-exponential) could a decent fit be obtained. Such a double-exponential fit can be linked to the equivalent circuit shown in Figure 3. When comparing the time constants extracted from the double-exponential fitting with the RC time constant values obtained from the equivalent circuit, a good agreement was found for these terms (data shown in SI). This provides further credibility to the model outlined above and the proposed assignment of its components. Figure 6 shows the evolution of the observed current upon application of a 3-V (positive) bias to the 500-μm-diameter gold dot on a Pt/LiPON/Au structure. It shows the typical G


Research Article

ACS Applied Materials & Interfaces relaxation behavior that has been reported before.18,42−44 The final current that is reached is assumed to be solely composed of an electron current. Figure 6a shows that both a 30- and a 70-nm layer relax to a current value of approximately 5 × 10 −9 A/cm2, in clear contrast with the findings of Li et al.,25 who measured a dominant electronic conductivity in LiPON layers thinner than 50 nm. The results shown here (cf. Figure 6) clearly demonstrate that the samples are electronically insulating. The stability of the LiPON layers to these potentials was confirmed by SIS measurements before and after the polarization. No significant changes in impedance characteristics were detected after the stressing (see Figure S7 in the SI). Figure 6b shows a 3-V stressing measurement on 15-nm LiPON layers deposited on Pt and TiN substrates upon application of a 3-V bias (corresponding to a field as high as 2 MV/cm). The LiPON layer on TiN shows a decrease in current, exposing its electronically insulating properties. To our knowledge, this is the first detailed current relaxation analysis under a 3-V dc bias of a 15-nm thin LiPON layer, unambiguously demonstrating the layer to be electronically insulating. Upon calculation of the electronic resistivity, a value of 2.5 × 1015 Ω·cm was found. Yet, for 15-nm LiPON layers sputtered on a platinum current collector, a different behavior was detected. Whereas the layer initially shows the same relaxation behavior as on TiN, after ∼4 h, a progressive breakdown was seen. Sudden jumps in current appeared, eventually culminating in hard breakdown, after which the current reached the compliance level. This behavior is similar to that found in a typical time-dependent dielectric breakdown (TDDB) measurement, which is commonly used to assess dielectric quality.15,48 To our knowledge, this is the first time this phenomenon has been described for solid-state Li-ion electrolytes. Because the 3-V potential is too low to start the chemical dissociation of LiPON,9 the breakdown here could be caused by the accumulation of defects similar to a dielectric breakdown mechanism with oxygen vacancies, for example. When these are able to form a percolative path, hard breakdown is observed. As such, it appears that the latter type of degradation has to be taken into account when determining the ultimate scaling limit for solid electrolytes in thin-film batteries. The time to breakdown at 3 V is directly linked to the lifetime of the electrolyte.15,48 Different scenarios can be devised to explain the observed substrate-dependent (Pt vs TiN) breakdown characteristics of LiPON layers. A first possible cause might be the different defect densities created when fabricating devices on platinum and TiN substrates. These will influence the starting defect density and, thus, the lifetime of the solid electrolyte because fewer defects have to be generated before reaching the critical amount needed to cause breakdown. Second, Pt is known to have a higher work-function than TiN;49,50 therefore, the band structure will differ from the situation shown in Figure 5. In this case, the built-in field of the electric double layer will be smaller because Au and Pt have similar work-functions, thus allowing more facile breakdown. Finally, although no initial intermixing could be traced when LiPON was deposited on Pt (see SI), Li can alloy with Pt upon negative polarization.51−53 Li−Pt alloy formation thus might occur upon the application of the 3-V bias (positive applied to the Au dot). This would change the LiPON composition and thus induce defects in the structure. The electronic resistivity values of LiPON layers of various thicknesses deposited on TiN and Pt substrates, inferred from the polarization measurements, are summarized in Table 4. To

Table 4. Summary of the Electronic Resistivity Data Extracted from Polarization Measurements Performed under Application of a 3-V Bias to LiPON Layers of Various Thicknesses Deposited on TiN and Pt Substrates ρa (Ω·cm) layer thickness (nm)

deposition conditions

TiN

Pt

15 30 70 240

standard standard standard BKM

(2.5 ± 2) × 1015 (2 ± 2) × 1015 − (1.8 ± 0.1) × 1013

breakdown (1.5 ± 0.6) × 1014 (7 ± 0.8) × 1013 −

a Calculated resistivity values based on averages of the final 50 leakagecurrent data points.

calculate the resistivity of a layer, the average current of the last 50 data points of the current-versus-time trace were used. All of the layers showed resistivity values above 1013 Ω·cm, thus more than 6 orders of magnitude higher than their ionic conductivities. These high resistivity values clearly indicate the electronic insulating properties of LiPON layers, even for layers as thin as 15 nm. Layers thinner than 15 nm were not evaluated here. The extracted resistivity values are several orders of magnitude higher than what has been reported for other electrolyte materials such as LiTaO3 (σelec ≈ 10−11 S/ cm)54 and Li3xLa2/3−xTiO3 (σelec ≈ 10−8−10−9 S/cm).38 Note also that the effective current density values found here are better than or comparable to other values reported for thicker LiPON layers.26,55,56

4. CONCLUSIONS AND FINAL REMARKS We have investigated the effects of layer thickness, stoichiometry, and substrate material on the properties of LiPON layers. We found that the substrate has to be a Li-ionblocking layer to prevent outdiffusion during deposition. No effect of the thickness on the ion conductivity was observed. With increasing nitrogen content, an increase in the ionic conductivity and a reduction in the activation energy were found. The equivalent circuit used to analyze the impedance data of the measured LiPON layers has been discussed in detail, and physical counterparts for the components were proposed. dc polarization measurements were performed to unambiguously determine the electronic conductivity. Layers down to 15 nm were shown to be electronically insulating. Dielectric breakdown was detected after long-time stressing for the first time on LiPON layers. The results obtained in this work outperform previous reports by Li et al.25 and Ruzmetov et al.24 These works reported sharp increases in the electronic conductivity of LiPON at thicknesses of 50 and 100 nm, respectively. In the latter report, nanowire batteries consisting of LiCoO2/LiPON/ Si were fabricated. These nanowires were over 8 μm long, and a 100-nm LiPON layer was deposited as an electrolyte by physical vapor deposition. It is thus likely that a thickness gradient is present over the length of the nanowire. In addition, crystalline LiCoO2 contains sharp crystallite edges that can significantly enhance local electric fields. These factors will contribute to the electronic leakage through the 100-nm LiPON layer. Batteries with a LiPON thickness of 100 nm have been shown elsewhere to be capable of retaining stable battery charges.45,46 H


Research Article


(8) Wang, B.; Kwak, B.; Sales, B.; Bates, J. Ionic Conductivities and Structure of Lithium Phosphorus Oxynitride Glasses. J. Non-Cryst. Solids 1995, 183, 297−306. (9) Yu, X.; Bates, J. B.; Jellison, G. E.; Hart, F. X. A Stable Thin-Film Lithium Electrolyte: Lithium Phosphorus Oxynitride. J. Electrochem. Soc. 1997, 144, 524−532. (10) Bates, J. B.; Dudney, N. J.; Gruzalski, G. R.; Zuhr, R. A.; Choudhury, A.; Luck, C. F.; Robertson, J. D. Fabrication and Characterization of Amorphous Lithium Electrolyte Thin Films and Rechargeable Thin-Film Batteries. J. Power Sources 1993, 43−44, 103− 110. (11) Nimisha, C.; Rao, K. Y.; Venkatesh, G.; Rao, G. M.; Munichandraiah, N. Sputter Deposited LiPON Thin Films from Powder Target as Electrolyte for Thin Film Battery Applications. Thin Solid Films 2011, 519, 3401−3406. (12) Power in System on Chip Needs a New Energy Storage Solution. Cymbet Corporation, 2014. http://www.cymbet.com/pdfs/ Cymbet-PowerSoC-2014-Presentation.pdf (accessed Dec 2015). (13) Advantage of Thin Film Batteries. Excellatron, 2010. http:// www.excellatron.com/advantage.htm (accessed Dec 2015). (14) Xu, K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303−4417. (15) Degraeve, R.; Kaczer, B.; Groeseneken, G. Degradation and Breakdown in Thin Oxide Layers: Mechanisms, Models and Reliability Prediction. Microelectron. Reliab. 1999, 39, 1445−1460. (16) Vereda, F.; Clay, N.; Gerouki, A.; Goldner, R.; Haas, T.; Zerigian, P. A Study of Electronic Shorting in IBDA-Deposited LiPON Films. J. Power Sources 2000, 89, 201−205. (17) Kozen, A. C.; Pearse, A. J.; Lin, C.-F.; Noked, M.; Rubloff, G. W. Atomic Layer Deposition of the Solid Electrolyte LiPON. Chem. Mater. 2015, 27, 5324−5331. (18) Liu, W.; Fu, Z.; Li, C.; Qin, Q. Electrochem. Solid-State Lett. 2004, 7, J36−J40. (19) Hamon, Y.; Douard, A.; Sabary, F.; Marcel, C.; Vinatier, P.; Pecquenard, B.; Levasseur, A. Influence of Sputtering Conditions on Ionic Conductivity of LiPON Thin Films. Solid State Ionics 2006, 177, 257−261. (20) Fleutot, B.; Pecquenard, B.; Martinez, H.; Letellier, M.; Levasseur, A. Investigation of the Local Structure of LiPON Thin Films to Better Understand the Role of Nitrogen on their Performance. Solid State Ionics 2011, 186, 29−36. (21) Nisula, M.; Shindo, Y.; Koga, H.; Karppinen, M. Atomic Layer Deposition of Lithium Phosphorus Oxynitride. Chem. Mater. 2015, 27, 6987−6993. (22) Baggetto, L.; Niessen, R.; Roozeboom, F.; Notten, P. High Energy Density All-Solid-State Batteries: A Challenging Concept Towards 3D Integration. Adv. Funct. Mater. 2008, 18, 1057−1066. (23) Oudenhoven, J. F. M.; Baggetto, L.; Notten, P. H. L. All-SolidState Lithium-Ion Microbatteries: A Review of Various ThreeDimensional Concepts. Adv. Energy Mater. 2011, 1, 10−33. (24) Ruzmetov, D.; Oleshko, V.; Haney, P.; Lezec, H.; Karki, K.; Baloch, K.; Agrawal, A.; Davydov, A.; Krylyuk, S.; Liu, Y.; Huang, J.; Tanase, M.; Cumings, J.; Talin, A. Electrolyte Stability Determines Scaling Limits for Solid-State 3D Li Ion Batteries. Nano Lett. 2012, 12, 505−511. (25) Li, J.; Dudney, N.; Nanda, J.; Liang, C. Artificial Solid Electrolyte Interphase to Address the Electrochemical Degradation of Silicon Electrodes. ACS Appl. Mater. Interfaces 2014, 6, 10083−10088. (26) Nowak, S.; Berkemeier, F.; Schmitz, G. Ultra-thin LiPON FilmsFundamental Properties and Application in Solid State Thin Film Model Batteries. J. Power Sources 2015, 275, 144−150. (27) Laitinen, M.; Rossi, M.; Julin, J.; Sajavaara, T. Time-of-Flight Energy Spectrometer for Elemental Depth ProfilingJyväskylä Design. Nucl. Instrum. Methods Phys. Res., Sect. B 2014, 337, 55−61. (28) Nimisha, C. S.; Rao, G. M.; Munichandraiah, N.; Natarajan, G.; Cameron, D. C. Chemical and Microstructural Modifications in LiPON Thin films Exposed to Atmopsheric Humidity. Solid State Ionics 2011, 185, 47−51.

The good electronic insulating properties for the ultrathin layers reported here also disagree with the findings of Li et al., who reported sub-50-nm layers fabricated on Au substrates to be electronically conductive. One possible explanation for this discrepancy can be a difference in LiPON stoichiometry that might affect the layer’s electric characteristics. Another explanation might originate from the lithium loading of the gold substrate used during the application of a bias because gold is also a lithium-alloying electrode. This effect will most severely impact the thinnest films. In addition, the flat band structure will result in a reduced breakdown field, as has been shown herein.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b12500. Graph showing the sputtering rates for different recipes, ERD depth profile obtained using a Pt current collector, SEM images of the LiPON layers, Nyquists plot on a log−log scale and enlargement of the high-frequency region, XRD measurements of LiPO and LiPON samples, table comparing ionic conductivities for different resistances, SIS data before and after polarization measurements, fit of a double-exponential function to the polarization trace and associated fitting parameters (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Dr. Bivragh Majeed for the SEM images. This work was partially supported by IWT-Flanders (Belgium) under SBO project SOS-Lion (18142). We also thank Praxair for providing the LiPO sputter targets.

■

REFERENCES

(1) Zhang, H.; Yu, X.; Braun, P. V. Three-Dimensional Bicontinuous Ultrafast-Charge and -Discharge Bulk Battery Electrodes. Nat. Nanotechnol. 2011, 6, 277−281. (2) Vereecken, P. M.; Huyghebaert, C. Conformal Deposition for 3D Thin-Film Batteries. ECS Trans. 2013, 58, 111−118. (3) Rubloff, G. W.; Kozen, A. C.; Lee, S. B. From Nanoscience to Solutions in Electrochemical Energy Storage. J. Vac. Sci. Technol., A 2013, 31, 058503. (4) Kamaya, N.; Homma, K.; Yamakawa, Y.; Hirayama, M.; Kanno, R.; Yonemura, M.; Kamiyama, T.; Kato, Y.; Hama, S.; Kawamoto, K.; Mitsui, A. A Lithium Superionic Conductor. Nat. Mater. 2011, 10, 682−686. (5) Klingler, M.; Chu, W.; Weppner, W. Coulometric Titration of Substituted LixLa2−xTiO3. Ionics 1997, 3, 289−291. (6) Yokokawa, H. Thermodynamic Stability of Sulfide Electrolyte/ Oxide Electrode Interface in Solid-State Lithium Batteries. Solid State Ionics 2016, 285, 126−135. (7) Bates, J. B.; Dudney, N. J.; Gruzalski, G. R.; Zuhr, R. A.; Choudhury, A.; Luck, C. F.; Robertson, J. D. Electrical Properties of Amorphous Lithium Electrolyte Thin Films. Solid State Ionics 1992, 53−56, 647−654. I


Research Article


(49) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2008. (50) Lima, L. P. B.; Dekkers, H. F. W.; Lisoni, J. G.; Diniz, J. A.; Van Elshocht, S.; De Gendt, S. Metal Gate Work Function Tuning by Al Incorporation in TiN. J. Appl. Phys. 2014, 115, 074504. (51) Put, B.; Vereecken, P. M.; Mees, M. J.; Rosciano, F.; Radu, I. P.; Stesmans, A. Characterization of Thin Films of the Solid Electrolyte LixMg1−2xAl2+xO4 (x = 0, 0.05, 0.15, 0.25). Phys. Chem. Chem. Phys. 2015, 17, 29045−29056. (52) Honbo, H.; Momose, H. Analyses of Passivation Films on Lithium and Lithium Alloys by Electrochmical Quartz Crystal Microbalance. J. Electroanal. Chem. 2010, 638, 269−274. (53) Wibowo, R.; Ward Jones, S. E.; Compton, R. G. Kinetic and Thermodynamic Parameters of the Li/Li+ Couple in the Room Temperature Ionic Liquid N-Butyl-N-methylpyrrolidinium Bis(trifluoromethylsulfonyl)imide in the Temperature Range 298−318 K: A Theoretical and Experimental Study Using Pt and Ni Electrodes. J. Phys. Chem. B 2009, 113, 12293−12298. (54) Glass, A.; Nassau, K.; Negran, T. Ionic conductivity of quenched alkali niobate and tantalate glasses. J. Appl. Phys. 1978, 49, 4808. (55) Vereda, F.; Goldner, R.; Haas, T.; Zerigian, P. Rapidly Grown IBAD LiPON Films with High Li-Ion Conductivity and Electrochemical Stability. Electrochem. Solid-State Lett. 2002, 5, A239−A241. (56) Le Van-Jodin, L.; Ducroquet, F.; Sabary, F.; Chevalier, I. Dielectric Properties, Conductivity and Li+ Ion Motion in LiPON Thin Films. Solid State Ionics 2013, 253, 151−156.

(29) Knoops, H. C. M.; Baggetto, L.; Langereis, E.; van de Sanden, M. C. M.; Klootwijk, J. H.; Roozeboom, F.; Niessen, R. A. H.; Notten, P. H. L.; Kessels, W. M. M. Deposition of TiN and TaN by Remote Plasma ALD for Cu and Li Diffusion Barrier Applications. J. Electrochem. Soc. 2008, 155, G287−G294. (30) Freixas, J.; Eustache, E.; Roussel, P.; Brillard, C.; Deresmes, D.; Nuns, N.; Rolland, N.; Brousse, T.; Lethien, C. Sputtered Titanium Nitride: A Bifunctional Material for Li-Ion Microbatteries. J. Electrochem. Soc. 2015, 162, A493−A500. (31) Roh, N.; Lee, S.; Kwon, H. Scr. Mater. 1999, 42, 43. (32) Choi, C.; Cho, W.; Cho, B.; Kim, H.; Yoon, Y. S.; Tak, Y. RadioFrequency Magnetron Sputtering Power Effect on the Ionic Conductivities of Lipon Films. Electrochem. Solid-State Lett. 2002, 5, A14−A17. (33) Park, H. P.; Nam, S. C.; Lim, Y. C.; Choi, K. G.; Lee, K. C.; Park, G. B.; Lee, S.-R.; Kim, H. P.; Cho, S. B. Effects of Sputtering Pressure on the Characteristics of Lithium Ion Conductive Lithium Phosphorous Oxynitride Thin Film. J. Electroceram. 2006, 17, 1023− 1030. (34) Orazem, M. E.; Frateur, I.; Tribollet, B.; Vivier, V.; Marcelin, S.; Pebere, N.; Bunge, A. L.; White, E. A.; Riemer, D. P.; Musiani, M. Dielectric Properties of Materials Showing Constant-Phase-Element (CPE) Impedance Response. J. Electrochem. Soc. 2013, 160, C215− C225. (35) Huggins, R. A. Simple Method to Determine Electronic and Ionic Components of the Conductivity in Mixed Conductors: A Review. Ionics 2002, 8, 300−313. (36) Fu, Z.-W.; Liu, W.-Y.; Li, C.-L.; Qin, Q.-Z.; Yao, Y.; Lu, F. Highk Lithium Phosphorous Oxynitride Thin Films. Appl. Phys. Lett. 2003, 83, 5008. (37) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 2001. (38) Inaguma, Y.; Liquan, C.; Itoh, M.; Nakamura, T.; Uchida, T.; Ikuta, H.; Wakihara, M. High Ionic Conductivity in Lithium Lanthanum Titanate. Solid State Commun. 1993, 86, 689−693. (39) Pradel, A.; Pagnier, T.; Ribes, M. Effect of Rapid Quenching on Electrical Properties of Lithium Conductive Glasses. Solid State Ionics 1985, 17, 147−154. (40) Martin, S.; Angell, C. Dc and ac Conductivity in Wide Composition Range Li2O−P2O5 Glasses. J. Non-Cryst. Solids 1986, 83, 185−207. (41) Martin, S. Ionic Conduction in Phosphate Glasses. J. Am. Ceram. Soc. 1991, 74, 1767−1784. (42) Hartmann, P.; Leichtweiss, T.; Busche, M.; Schneider, M.; Reich, M.; Sann, J.; Adelhelm, P.; Janek, J. Degradation of NASICONType Materials in Contact with Lithium Metal: Formation of Mixed Conducting Interphases (MCI) on Solid Electrolytes. J. Phys. Chem. C 2013, 117, 21064−21074. (43) Frenning, G.; Nilsson, M.; Westlinder, J.; Niklasson, G.; Mattsson, M. S. Dielectric and Li Transport Properties of Electron Conducting and Non-conducting Sputtered Amporphous Ta2O5 Films. Electrochim. Acta 2001, 46, 2041−2046. (44) Mattsson, M. S.; Niklasson, G. A. Isothermal Transient Ionic Current as a Characterization Technique for Ion Transport in Ta2O5. J. Appl. Phys. 1999, 85, 8199. (45) Nakazawa, H.; Sano, K.; Baba, M.; Kumagai, N. Stability of Thin-Film Lithium-Ion Rechargeable Batteries Fabricated by Sputtering Method without Heating. J. Electrochem. Soc. 2015, 162, A392− A397. (46) Nakazawa, H.; Sano, K.; Abe, T.; Baba, M.; Kumagai, N. Charge−Discharge Characteristics of All-Solid-State Thin-Film Lithium-Ion Batteries Using Amorphous Nb2O5 Negative Electrodes. J. Power Sources 2007, 174, 838−842. (47) Song, S.-W.; Hong, S.-J.; Park, H. Y.; Lim, Y. C.; Lee, K. C. Cycling-Driven Structural Changes in a Thin-Film Lithium Battery on Flexible Substrate. Electrochem. Solid-State Lett. 2009, 12, A159−A162. (48) Martin, A.; O’Sullivan, P.; Mathewson, A. Dielectric Reliability Measurement Methods: A Review. Microelectron. Reliab. 1998, 38, 37− 72. J


Electrical Characterization of Ultrathin RF-Sputtered LiPON Layers for

Recommend Documents