Atomic Layer Deposition of a Magnesium Phosphate Solid Electrolyte

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Atomic Layer Deposition of a Magnesium Phosphate Solid Electrolyte Jin Su, Tohru Tsuruoka, Takuji Tsujita, Yu Nishitani, Kensuke Nakura, and Kazuya Terabe Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b01299 • Publication Date (Web): 21 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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Chemistry of Materials

Atomic Layer Deposition of a Magnesium Phosphate Solid Electrolyte Jin Su†, Tohru Tsuruoka*†, Takuji Tsujita‡, Yu Nishitani‡, Kensuke Nakura‡, and Kazuya Terabe† †

International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Technology Innovation Division, Panasonic Corporation, Kadoma City, Osaka 571-8501, Japan ABSTRACT: Atomic layer deposition   (ALD) was used to fabricate magnesium phosphate thin films as a magnesium-ion conducting solid electrolyte. The deposition was carried out at lower deposition temperatures, ranging from 125 to 300 °C. The film exhibits an amorphous nature and excellent step coverage, even in narrow trenches with a higher aspect ratio. The growth rate and the ionic conductivity were found to increase with a decrease of the deposition temperature. This can be explained by increased disordering of the phosphate matrix, giving rise to enhanced hopping conduction of magnesium ions. The film deposited at 125 °C showed an ionic conductivity of 1.6 × 10-7 S cm-1 at an ambient temperature of 500 °C, with an activation energy of 1.37 eV. These values are better than those of the film deposited at 250 °C as well as of a sputtered film. Our results indicate that ALD has great potential for the fabrication of magnesium-based solid electrolyte films.

■ INTRODUCTION Ion batteries are essential for their wide usage in mobile electronic devices, electric vehicles, and electrical grid energy storage systems, as well as for the storage of electricity generated from intermittent renewable wind or solar energy.1-8 However, current commercial ion batteries cannot satisfy the increasing requirements of the ever-growing range of applications.9-13 Increased energy densities, cycle lives, and safety levels are critically required in an affordable cost range. Solid state electrolytes (SSEs) can provide improvements in the energy densities of ion batteries without any additional modification to anode and cathode materials, because there are no liquid electrolytes and solid electrolyte interphases cannot form at the interface between the electrodes and the electrolytes.14-15 SSEs have some additional advantages, such as decreased battery volume, greater flexibility for battery design and management, higher thermal stability and safety due to their noncombustible properties.16-20 Atomic layer deposition (ALD) is a promising technique for SSE-based ion batteries because it can accurately deposit controlled thickness, pinhole free, high uniformity electrolyte films onto patterned substrates with high aspect ratios.21-29 In particular, three-dimensionally (3D) structured electrolytes are very important for achieving higher performance in ion battery applications, due to their higher specific surface areas providing higher energy densities.21-24 Recently, the ALD technique was applied to the fabrication of amorphous SSE films for Lithium (Li)-ion battery applications such as Li3PO423, nitrogen-doped Li3PO4 (LiPON)24,25, Li4SiO426, LixNbyO27, and LixAlySizO28,

and demonstrated excellent conformality on 3D patterned substrates. To enhance conductivity, it is important to construct fast ion conduction pathways with lower activation energies in amorphous glass networks. Therefore, the ALD process for amorphous electrolytes is crucial for realizing SSEs with high ionic conductivity. Magnesium (Mg) ion conducting SSEs are promising candidates for future energy storage and conversion devices. Mg is an alkaline earth metal, with a higher theoretical specific volumetric capacity of 3830 mAh cm-3, compared with the theoretical value of 2060 mAh cm-3 for Li metal, and therefore shows a high degree of promise in practical applications for high-energy-density ion batteries.30 Furthermore, Mg elements, as conducting ion sources, have attracted considerable attention as a competitor to Li elements, owing to the natural abundance of Mg stored in the earth’s crust.12, 30 One of the most important advantages of Mg metal is its chemical stability. It can be kept stable at room temperature in ambient atmosphere due to the formation of a surface passivation layer.31 Ikeda et al. fabricated MgxZry(PO4)z compounds by sintering a mixture of magnesium carbonate, zirconium oxide, and ammonium dihydrogen phosphate at 1350 °C, and observed ionic conductivities up to 2.9 × 10-5 S cm-1 at 400 °C.32 Imanaka et al. found ionic conductivities up to 6.9 × 10-3 S cm-1 at 800 °C by an increase of the Zr2O(PO4)2 content in Mg1+xZr4P6O24+x + xZr2O(PO4)2 composites.33 Higashi et al. prepared crystalline Mg(BH4) and Mg(BH4)(NH2) hydrides, and observed ionic conductivities on the order of 10-6 S cm-1 at 150 °C.31 Aubrey et al. fabricated porous metal-organic frameworks (MOFs) of Mg2(2,5dioxidobenzene-1,4-dicarboxylate) and Mg2(4,4′-

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dioxidobiphenyl-3,3′-dicarboxylate), and these MOFs exhibited ionic conductivities up to 0.25 mS cm-1 at room temperature.34 Although higher ionic conductivities have been reported for the various electrolyte materials described above, to our knowledge, there have been no reports on the fabrication of thin film Mg-ion conducting SSEs. Here, we report plasma-assisted ALD growth and characterizations of magnesium phosphate thin films as an SSE. The ALD process was developed at relatively lower deposition temperatures, ranging from 125 °C to 300 °C. The ionic conductivities and the activation energies of the deposited films were evaluated. It was found that the films show amorphous nature and excellent step coverage, even in narrow trenches with a higher aspect ratio. The film deposited at 125 °C exhibited higher ionic conductivity of more than two orders of magnitude than that exhibited by the film deposited at 250 °C below the ambient temperature of 300 °C. We discuss herein how the process temperature affects the matrix structure and the ionic conductivity of ALD-grown magnesium phosphate films. ■ EXPERIMENTAL SECTION The magnesium phosphate films were deposited on Si, SiO2covered Si, Pt-coated Si, and quartz substrates, where each substrate was used for a specific characterization method. The substrates were cleaned by acetone and ethanol ultrasonication for 10 min, followed by plasma ashing in oxygen atmosphere to remove any remaining contaminant species. The substrates were then transferred to an ALD system (SPLEAD Co. Ltd.), in which the base pressure was less than 10-3 Pa, and a process pressure of 40 Pa was maintained via Ar gas flow with 40 sccm. The MgPO films were deposited at deposition temperatures, Td, ranging from 125 °C to 300 °C, using Bis(ethylcyclopentadienyl)magnesium (Mg(EtCp)2) and Tris(dimethylamino)phosphine (TDMAP) as the magnesium and phosphorus sources, respectively. H2O and O2-plasma were used as oxidants for Mg(EtCp)2 and TDMAP, respectively. These precursors were delivered to the deposition chamber via a 10 sccm Ar carrier gas flow. The O2-plasma was produced by the direct plasma configuration, in which RF voltage was applied between the showerhead and the substrate holder. To estimate the growth rate, the thickness of films deposited on SiO2-covered Si substrates was measured using a J. A. Woollam M-2000 and EC-400 spectroscopic ellipsometer in the wavelength region of from 400 to 1000 nm. Fourier transform infrared spectroscopy (FTIR) measurements were carried out to analyze the chemical bonding state of the films deposited on 100-nm thick Pt coated Si substrates. FTIR was utilized, in an attenuated total reflectance mode, using a Thermo Fisher Nicolet 4700 spectrometer in the wavenumber region of 5004000 cm-1 with a 2 cm-1 resolution. The chemical bonding state and composition of the films deposited on Si substrates were analyzed by X-ray photoelectron spectroscopy (XPS, PHI Quantera). Survey and high-resolution spectra were collected using a monochromatic Al Ka X-ray source. The energy of C 1s peak (284.6 eV) was used for binding energy calibration. XPS data were analyzed using CasaXPS software for composition estimation and peak fitting. The crystallinity of the films deposited on quartz substrates was measured by X-ray diffraction (XRD) using a Rigaku SmartLab. The morphology and structure of the films deposited on Si substrates were analyzed by scanning electron microscopy (SEM, Hitachi, SU8230), with an acceleration voltage of 10 kV and an emission

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current of 10 µA. Surface and cross-sectional images were obtained without conducting layer coating. Energy dispersive X-ray spectroscopy (EDX) scans were used to obtain the elemental mapping analyses of the films. The density of the MgPO films deposited on Si substrates was evaluated by X-ray reflectivity (XRR) measurements using a Rigaku SmartLab. The film density was estimated from the fitting of the XRR curves to theoretical curves, calculated based on a multilayer structure model in which magnesium phosphate layers with different thicknesses and densities are stacked on a Si substrate. The transport characteristics were evaluated by electrochemical impedance spectroscopy (EIS) using a Solartron 1260 analyzer, equipped with a 1296 dielectric interface, in a frequency range of from 1 MHz to 10 mHz, with an AC amplitude of 5 mV. For this measurement, cross-point structured cells of Pt/magnesium phosphate/Pt were fabricated on quartz substrates. First, 5-nm thick Ti and 50-nm thick Pt were deposited, by electron-beam evaporation, as the adhesion layer and the bottom electrode. Then, an 80-100 nm thick magnesium phosphate film was deposited on the bottom electrode by ALD. Finally, a 50-nm thick Pt was deposited by radio-frequency (RF) sputtering under an Ar pressure of 0.43 Pa with an RF power of 50 W. Cells with different sizes, ranging from 0.1×0.1 to 2×2 mm, were prepared. The EIS measurements were carried out in an Ar filled glovebox with < 0.2 ppm of O2 at a pressure of 5×10-4 Pa. During the measurements, the ambient temperature was controlled from 200 to 500 °C. ■ RESULTS AND DISCUSSION We developed a ternary process for magnesium phosphate film. Figure 1 illustrates the proposed schematic of our ALD process. The process starts from the termination, with hydroxyl groups (-OH) on the substrate surface, by a H2O pulse, as shown in Figure 1A. Then, Mg(EtCp)2 is introduced into the deposition chamber to form a metastable surface covered with ethylcyclopentadienyl ligands (Figure 1B). After that, another H2O pulse is introduced to remove the ethylcyclopentadienyl ligands and form Mg-OH units on the surface (Figure 1C). These three steps are regarded as the initial ALD half process for preparing the Mg-OH-terminated surface. The main ALD process is initiated with a TDMAP pulse, where the reaction of TDMAP with -OH units leads the termination with dimethylamino phosphate ligands (Figure 1D). Then, an O2plasma pulse is applied to provide O radicals onto the surface, in order to remove the dimethylamino phosphate ligands and, presumably, cross-linking phosphorous atoms, forming Mg-OP bonds (Fig. 1E). This step may also form -OH units, which act as active sites for the next Mg(EtCp)2 chemisorption. Subsequently, a Mg(EtCp)2 pulse is introduced to again form a metastable surface with the ethylcyclopentadienyl ligands (Figure 1F). Finally, the main process is completed by a H2O pulse to remove the ethylcyclopentadienyl ligands and again form a surface terminated with Mg-OH units (Figure 1G). The magnesium phosphate film can be deposited by repeating the main processes shown in Figures 1D to 1G. Note that Figure 1 illustrates only the reaction at the surface. The molecular structure of the film for the repeated cycles is omitted. In the strict sense, the molecular structures from the final step at the first cycle to the initial step at the second cycle should be as illustrated in Figure S1 of Supporting Information.


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Figure 1. Schematic of the proposed ALD magnesium phosphate process. (A) Initial surface terminated with hydroxyl groups; (B) Metastable surface, with ethylcyclopentadienyl ligands, after a Mg(EtCp)2 pulse; (C) Formation of Mg-OH units by a H2O pulse; (D) a TDMAP pulse for P incorporation; (E) O2-plasma to cross-link P atoms and form Mg-O-P bonds; (F) Mg(EtCp)2 pulse followed by H2O pulse to form again the Mg-OH-terminated surface. Steps D to G result in one MgPO deposition cycle. The oxidation of the dimethylamino phosphate ligands (Figure 1E) is very important to the fabrication of magnesium phosphate film. If H2O or O2 is used as the oxidant after TDMAP, the film cannot be deposited. This means that these oxidants do not create the necessary available sites for the subsequent reaction with Mg(EtCp)2. On the other hand, it was found that using H2O-plasma also makes it possible to successively deposit the film. However, FTIR analysis shows that the film deposited with H2O-plasma contains slightly higher concentrations of O-H than the film deposited with O2plasma. Thus, we decided to adopt O2-plasma for the oxidation after TDMAP, as illustrated in Figure 1. The magnesium phosphate films were deposited with 150 deposition cycles, based on the process sequence shown in Figure 1. Figures 2a and 2b represent the growth-per-cycle (GPC) plotted as a function of the TDMAP and Mg(EtCp)2 pulse times, respectively, evaluated at a deposition temperature of 250 °C. In Figure 2a (2b), the Mg(EtCp)2 (TDMAP) pulse times were fixed at 4 (3) s. The time for H2O and O2-plasma pulses was set at 2 s. The purge time after the Mg(EtCp)2 and TDMAP pulses was set at 20 s, while after the H2O and O2plasma pulses it was set at 15 s. The GPC was found to be almost constant up to a pulse time of 4 s for both TDMAP and Mg(EtCp)2, implying self-limiting behavior, although the GPC seems to increase slightly with a decrease of the Mg(EtCp)2 pulse time (the reason for which will be discussed later). Note that the GPC is 1.3 Å/cycle for a magnesium oxide (MgO) film deposited by a thermal ALD process of alternative Mg(EtCp)2 and H2O pulses at the same deposition temperature, as indicated by the closed circles in Figures. 2a and 2b. This means that the GPC of the ALD magnesium phosphate film (except for shorter (Mg(EtCp)2 pulse times) becomes lower than that of the ALD MgO film (we also confirmed self-limiting behavior for the MgO process). To check the short-range chemical bonding structures, FTIR measurements were carried out for the films deposited on Pt/Si substrates under the same conditions. The results are shown in Figures 2c and 2d. For a MgO film, a single sharp peak was

observed at 723 cm-1, which corresponds to the characteristic vibrational mode of Mg-O bonds.35 With increased TDMAP pulse time and a fixed Mg(EtCp)2 pulse time of (4 s), this peak was shifted to the lower wavenumber side, showing significant decrease and broadening, and reached a peak at 663 cm-1 with a small shoulder at 617 cm-1. These features may be associated with the bending vibration of O-P-O or O=P-O bonds36,37, but we believe that they originate from the asymmetric stretching vibration of Mg-O-P bridges, according to the transition of the absorbance peak. In addition, a large peak at 1165 cm-1, accompanied by a shoulder at 965 cm-1, appeared with the introduction of TDMAP. These peaks are attributed to the symmetric stretching vibration of PO32- ions in the pyrophosphate (Mg2P2O7) and the asymmetric vibration (nas) of P-O-P bonds, respectively.38 If the Mg(EtCp)2 pulse time was increased with a fixed TDMAP pulse time (3 s), a similar peak evolution was observed in the FTIR spectra, with the exception of the Mg(EtCp)2 pulse time of 1 s, as shown in Figure 2d. Almost no signals related to the stretching vibrations of C-O (carbonate CO32- anions) and O-H bonds were observed in the regions of 1400-1600 and 3000-3700 cm-1, respectively, which indicates that the films contain very low concentrations of carbon- and hydroxylrelated impurities. However, only the film with a 1 s Mg(EtCp)2 pulse time shows large C-O and O-H contributions, with a broad band in the region of 750-2000 cm-1. This trend becomes more significant for lower deposition temperatures, as shown in Figure S2 of Supporting Information. The GPC increased steeply with a decrease of the Mg(EtCp)2 pulse time at a deposition temperature of 125 °C, and the corresponding FTIR spectra show large C-O and O-H bands up to 1 s Mg(EtCp)2 and 2 s TDMAP pulse times. These residual impurities probably come from incomplete reactions of the surface during the ALD process or from contamination by exposure of the films to air after deposition. The results indicate the importance of using the appropriate amounts of Mg(EtCp)2 and TDMAP precursors in the ALD process. By considering both the GPC and the FTIR data at all the deposition temperatures, we determined the


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MgO

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Figure 2. Growth-per-cycle (GPC) of magnesium phosphate films (closed triangles) as a function of the pulse time of (a) TDMAP and (b) Mg(EtCp)2, evaluated at 250 °C. The GPC for an MgO film, plotted with closed circles. (c, d) The corresponding FTIR spectra of magnesium phosphate films deposited on Pt-coated Si substrates. optimum pulse times for Mg(EtCp)2 and TDMAP to be 4 s and 3 s, respectively. The pulse time dependence of Figures 2a and 2b were obtained with the said Mg(EtCp)2 (4 s) and TDMAP (3 s) pulse times, respectively. Figure 3a plots the GPC as a function of the deposition temperature. As the deposition temperature was decreased from 300 °C, the GPC was initially almost constant at ~1.15 Å/cycle down to 200 °C and then increased to ~1.41 Å/cycle at lower deposition temperatures. There is no apparent process window in the deposition temperature range observed. The variation of GPC around 200 °C was evidenced by FTIR spectra, as shown in Figure 3b. As the deposition temperature decreased, a new peak started to emerge at 1250 cm-1 at 200 °C and grew to the same strength as the peak for PO32- (at 1165 cm-1) at 125 °C. This peak is attributed to the asymmetric stretching vibration of PO2- ions in the metaphosphate (MgP2O6).38 The shoulder for the P-O-P bond at 965 cm-1 also increased with a decrease of the deposition temperature. Another peak appeared at 770 cm-1 below 150 °C, which is attributed to the symmetric stretching vibration (ns) of P-O-P linkages. On the other hand, the relative intensities of the two peaks at 617 and 663 cm-1 reversed as the deposition temperature decreased. The strengthening of the lower-wavenumber peak may suggest elongation of Mg-O-P bonds. The appearance and variation of several absorption peaks imply that the chemical bonding state is significantly changed depending on the deposition temperature: the pyrophosphate unit is dominant at higher deposition temperatures, whereas a mixture of the pyrophosphate and

metaphosphate units are formed at lower deposition temperatures. Note that a small shoulder was observed at ~1050 cm-1 for all the deposition temperatures, which is attributed to nas of PO43- ions in the orthophosphate (Mg3P2O8).38 This implies the presence of a small amount of orthophosphate in the deposited films. The FTIR spectra showed an absence of O-H and C-O stretching vibration bands in all the films deposited at the deposition temperatures used. This indicates that our ALD process can effectively eliminate carbon- and hydroxyl-related impurities and, in ambient atmosphere, the chemical stability of the deposited magnesium phosphate films is much higher than Li-based SSEs such as Li3PO438 and LiPON39. XPS measurements were performed to quantitatively determine the chemical composition of the magnesium phosphate films deposited on Si substrates. Figure S3 of Support Information represents survey spectra measured for films deposited at different deposition temperatures. It was not possible to avoid oxidation and carbonization of the film surface, even though the films were only exposed to air for a short period of time (< 30 s) when being transferred from the ALD system to the XPS apparatus. If the film surfaces were etched slightly by Ar+ sputtering for 60 s, carbon signals were significantly reduced in all the films, as shown in Figure S4 of Support Information. This may suggest that carbon-related impurities come from contamination by exposure of the films to air after deposition or from incomplete chemical reactions during the ALD process, which is consistent with the FTIR observations.


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Figure 3. (a) GPC of magnesium phosphate films deposited at different deposition temperatures, from 125 °C to 300 °C. The corresponding (b) FTIR and (c) high-resolution XPS spectra of O 1s peaks (NBO = non-bridging oxygen; BO = bridging oxygen). (d) Atomic composition versus deposition temperature, evaluated from the XPS spectra. High-resolution XPS spectra of Mg 2p and P 2p peaks of films deposited at 125 °C and 250 °C are depicted in Figure S5 of Supporting Information. Single sharp, symmetric peaks were detected at 51.1 eV for Mg 2p and at 134.2 eV for P 2p in both films, revealing a single chemical environment of Mg and P in each film. There was no obvious shift of these Mg 2p and P 2p peaks, also indicating the single chemical state of these elements regardless of the deposition temperature. These peaks were not affected by the Ar+ sputtering. Figure 3c shows high-resolution XPS spectra of O 1s peaks measured for the films deposited at 250 °C and 125 °C. The peaks can be deconvoluted into three components located at 533.2, 531.8, and 529.6 eV. The peak at 533.2 eV belongs to bridging oxygen (BO) in P-O-P linkages (one oxygen atom bound to two P atoms), and the peak at 531.8 eV can be attributed to non-bridging oxygen (NBO) in Mg-O-P bonds (one oxygen atom bound to one P atom and one Mg ion, denoted as Mg2+…O--P in Fig. 3c).40 The NBO/BO ratio increased with increased deposition temperatures. This trend is opposite to that observed for ALD-grown lithium borate-carbonate films41, but is consistent with the prediction based on molecular dynamics calculations42. The small peak at 529.6 eV is associated with oxygen in an Mg-O environment, in agreement with the result obtained for MgO.43 These three deconvoluted peaks are located at the same binding energy positions and with similar peak shapes in both films, although the relative intensities are

different. Their relationship to the FTIR spectra is discussed later. The atomic composition of the deposited films was evaluated from the survey spectra of Figure S3, as shown in Figure 3d. The elemental compositions calculated from Figure 3d are summarized in Table I. The relative composition of the film deposited at 300 °C was estimated to be Mg3P2O6.3. The O/P ratio is slightly lower than the stoichiometric Mg3(PO4)2, suggesting that the film is slightly oxygen-deficient. We see that the compositions of Mg and O relative to P decreased with a decrease of the deposition temperature: the composition of the film deposited at 125 °C changed to Mg2.4P2O5.4. The Table I. Elemental composition of ALD magnesium phosphate films deposited from 125 to 300 °C, determined by XPS. Atomic composition (%) Mg

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Figure 4. (a) Thickness of the magnesium phosphate films plotted as a function of the number of deposition cycles, measured for 125 °C and 250 °C. The solid lines were obtained via linear regression analysis and both coefficients of determination, R2, equal to 0.999. SEM images taken for the top surface of 100 nm-thick films deposited at 125 °C (b) and 250 °C (c).

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500nm (d)

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Figure 5. (a) Cross-section SEM images of a 100-nm thick magnesium phosphate film, deposited at 250 °C on a patterned trench structure fabricated on a Si substrate. Magnified views at the bottom (b) and the sidewall (c) of the trench. The corresponding EDX mapping images for the Mg K edge (d), P K edge (e), and O K edge (f), measured at the sidewall (c). temperature variation in the relative composition will be discussed later. One reason for selecting TDMAP as the precursor is because it has a high content of nitrogen in its skeletal structure, which may enable the creation of nitrogen-doped magnesium phosphate (magnesium phosphorous oxynitride), similar to LiPON. However, the XPS analyses revealed that no nitrogen remained in the deposited films, suggesting that nitrogen is removed completely in the oxidation of TDMAP by O2-plasma (from Figure 1D to 1E). It is necessary for us to develop another process to dope nitrogen into the magnesium phosphate matrix. We also examined the impact of RF power on the O2-plasma in the ALD process of magnesium phosphate films. FTIR

spectra were measured for films deposited with different RF powers (15, 30, and 50 W) at deposition temperatures of 250 °C and 125 °C, as shown in Figure S6 of Supporting Information. At 250 °C, the spectral shape was almost the same for the films deposited with different RF powers, and almost no C-O and OH absorption bands were observed, indicating significantly smaller impurity concentrations in the films. In contrast, at 125 °C, large bands related to C-O and O-H bonds appeared below 30 W. Hence, there is a trade-off between the RF power and the deposition temperature; i.e. a lower RF power is sufficient at higher deposition temperatures whereas a higher RF power is needed at lower deposition temperatures. From the results of Figure S6, we adopted an RF power of 50 W for all


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the deposition temperatures, in order to reduce the concentration of impurities. The thickness of deposited films was linearly dependent on the number of deposition cycles, up to more than 800 cycles, as shown in Figure 4a. The GPC is calculated to be 1.41 and 1.21 Å/cycle for deposition temperatures of 125 °C and 250 °C, respectively, whose values are consistent with the results of the pulse time dependence (Figures 1a and 1b). Figures 4b and 4c show SEM images measured on the top surface of 100 nm-thick films deposited at 125 °C and 250 °C, respectively. The film deposited at 125 °C exhibited a very smooth surface (Figure 4b). As the deposition temperature increased, the film surface became slightly rough (Figure 4c). However, the root mean square (RMS) roughness of both film surfaces was estimated to be less than 1 nm, as measured by atomic force microscopy (Figure S7 of Supporting Information), indicating that the films deposited at the two deposition temperatures possess very smooth surfaces and similar surface RMS roughness. The XRD patterns of the films deposited on quartz substrates did not present any crystalline peaks, suggesting an amorphous nature regardless of the deposition temperature, as shown in Figure S8 of Supporting Information. One of the important features of the ALD process is the ability to produce conformal deposition of uniform thin films on 3D micro- and nano-structures with high aspect ratios. To examine the conformality of our magnesium phosphate ALD process, patterned trench structures were fabricated on a Si substrate by electron-beam lithography and reactive-ion etching, in which the width and depth are ~1 and ~5 µm, respectively. Figure 5a shows a cross-sectional SEM image of the patterned trench structures, deposited by a magnesium phosphate film with 820 deposition cycles at 250 °C. The image reveals that the deposited film is very uniform, at the bottom as well as the sidewall regions of the trench, as can be seen in Figures 5b and 5c. The film thickness was measured to be ~100 nm, which is in good agreement with the thickness on planar substrates. In order to investigate elemental distribution in the deposited film, the EDX mapping images of Mg, P, and O elements were measured at the sidewall of the trench of Figure 5c. The results, presented in Figures 5d-5f, show that all the elements are distributed uniformly in the film. Similar results were obtained for films deposited at other deposition temperatures. These observations demonstrate the excellent conformality of our ALD process. The electrical conductivity of the deposited films was evaluated by EIS measurements on cross-point structured Pt/magnesium phosphate/Pt cells with different cell sizes. The EIS data, measured at 400 °C and 500 °C, are plotted in Figure 6a. The closed triangles show the results for the cell with a film deposited at 125 °C, while the open triangles indicate the results for the cell with a film deposited at 250 °C. The thickness was estimated to be 107 nm and 82 nm for the films deposited at 125 °C and 250 °C, respectively. The EIS data consist of one semicircle in the high-frequency region and on inclined tail at the low-frequency region. The former corresponds to the bulk resistance of the magnesium phosphate SSE film and the latter is attributed to polarization of the electrode-electrolyte interface.44,45 In this case, Mg ions work as the charge diffusion carrier.32 Such features are typically representative of ionic conductors if the measurement is applied for an ionic conductor in an open circuit with ion blocking electrodes.23,45 When the measurement temperature was raised from 400 °C to 500 °C,

(a)

o

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T ( C) 400 300 250

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1000/T (K )

Figure 6. (a) Typical EIS curves measured for Pt/magnesium phosphate/Pt cells with magnesium phosphate films deposited at 125 °C (closed triangles) and 250 °C (open triangles), with fitting curves based on the equivalent circuit model as illustrated in the inset. The blue and red curves represent the results measured at 400 °C and 500 °C, respectively. (b) Arrhenius plots of the ionic conductivity for the two films, measured in the ambient temperature range of between 200 °C and 500 °C. The solid lines are fitted to the experimental data using the Arrhenius equation. the semicircle became smaller by more than one order of magnitude, as shown in the upper inset. This means that the conductivity increases more than ten times when the temperature is raised by 100 °C. Figure 6a also displays the curves fitted to the EIS data, based on an equivalent circuit model, as illustrated in the lower inset. The equivalent circuit model consists of three series components; one contact ohmic resistance (Ro), which was attributed to the high-frequency limiting resistance of the electrode, and two parallel resistors and constant-phase elements (CPE), with one (RB and CPEB) corresponding to the



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Figure 7. Film density and growth rate of magnesium phosphate thin films deposited at 125 °C and 250 °C . The expected molecular structures for the two deposition temperatures are schematically illustrated on the right (250 °C) and left (125 °C) sides respectively. “bulk” of the magnesium phosphate film and the other (RI and CPEI) corresponding to the resistive reaction layer between the Pt layer and the magnesium phosphate film. Because the deposited films exhibit an amorphous nature, as shown in Figure S8, it is reasonable to assume no grain boundaries for the ion transport. Therefore, the resistance of the reaction layer is likely to originate from polarization at the interfaces between Pt and magnesium phosphate. The ionic conductivity s of the magnesium phosphate films can be calculated simply by substituting the resistance of the bulk resistance into the following equation, s = d/(ARB), where d is the thickness of the magnesium phosphate film, and A is the geometric electrode area of the Pt/magnesium phosphate/Pt cell. From the fitting to the EIS data, the bulk resistances RB were estimated to be 146 kΩ and 129 kΩ at 500 °C, and 2.49 kΩ and 4.25 MΩ at 400 °C for the films deposited at 125 °C and 250 °C, respectively. The Arrhenius plot of the ionic conductivity, calculated for the films deposited at the two deposition temperatures, is displayed in Figure 6b. It was found that the ionic conductivity is estimated to be 1.6 × 10-7 and 1.8 × 10-7 S cm-1 at the ambient temperature of 500 °C for the films deposited at 125 °C and 250 °C, respectively, indicating that both films exhibit similar conductivities at higher ambient temperatures. As the ambient temperature was decreased, the ionic conductivity of the film deposited at 250 °C decreased more steeply than in the film deposited at 125 °C. At ambient temperatures below 300 °C, the film deposited at 125 °C showed a conductivity value more than two orders of magnitude larger than in the film deposited at 250 °C. From the Arrhenius plots of Figure 6b, the activation energies (EA) for the ionic conductivity were also evaluated by using the Arrhenius equation, sT = B exp [-EA/(kBT)], where B is a preexponential factor, T is the absolute temperature, and kB is the Boltzmann's constant. From linearly fitting to the experimental data, the activation energies are determined to be 1.37 eV and 1.78 eV for the films deposited at 125 °C and 250 °C, respectively. These EA values are similar to those reported for Mg(BH4)(NH2) hydrides (1.31 eV)31, MgZr4P6O24 single phase (1.41 eV)33, and MgxZry(PO4)z sintered compounds (0.83 - 2 eV)32, suggesting that the conductivity observed in our magnesium phosphate films originates from Mg-ion conduction similar to the above mentioned Mg-based electrolytes. Since the ionic conductivities of the two ALD films are almost the same

at the ambient temperature of 500 °C, the activation energy seems to be determined by the deposition temperature during the ALD process. For comparison, we also examined the ionic conductivity of RF-sputtered magnesium phosphate film, which was deposited from a Mg3(PO4)2 target in Ar atmosphere at room temperature. The result is shown together with the data for ALD films in Figure S9 of Supporting Information. The sputtered film exhibited similar conductivity at 500 °C, but lower conductivity at lower ambient temperatures, resulting in an activation energy even higher than in the ALD film deposited at 250 °C. This result indicates that the ALD process can achieve higher ionic conductivity than can be achieved by the sputtering process. In particular, the ALD process at lower deposition temperatures significantly improves the transport characteristics of the magnesium phosphate film. The thermal stability of the deposited film was also investigated. The XPS analyses revealed that the elemental composition was almost maintained up to 400 °C. However, the composition of P decreased drastically after heating to 500 °C, suggesting that a portion of the P atoms desorb from the film at this temperature. The electrical conductivity was reproducible below 400 °C, but decreased after heating to 500 °C (EA increased from 1.37 to 1.75 eV), as shown in Figure S10 of Supporting Information. This reduction of conductivity can be attributed to decomposition of the phosphate matrix by the thermal desorption of P atoms, which results in decreased conduction of Mg ions. In order to explain the higher conductivity of ALD magnesium phosphate films deposited at lower temperatures, we need to consider how the film structure is affected by the deposition temperature. In general, phosphate glass (P2O5) is formed with PO4 tetrahedra, which are connected through P-OP linkages, resulting in a polymeric structure.46,47 The PO4 tetrahedra are connected to their adjacent counterparts by three vertices corresponding to P-O- bonds, while the remaining vertex is occupied by a double-bonded oxygen P=O.48,49 The main structure of the phosphate consists of BO of P-O-P bonds. The addition of Mg atoms de-polymerizes the phosphate network by converting the BO to NBO of Mg-O-P bonds.41,50, The transport of Mg ions in this glass structure is considered to occur due to thermally activated hopping between NBOs.41,51 Such a transport mechanism is mainly due to the diffusion of mobile ions through the potential minima in the glassy network.


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The FTIR spectra in Figure 3b show that the films deposited at higher temperatures exhibit a dominant absorption band of pyrophosphate (PO32-) ions with small peaks of Mg-O-P bonds and P-O-P linkages. A small peak corresponding to orthophosphate (PO43-) ions is also observed. This means that the film is formed by pyrophosphate (Mg2P2O7) with a small amount of orthophosphate (Mg3P2O8), as schematically illustrated on the right side of Figure 7. With decreasing deposition temperature, a peak corresponding to metaphosphate (PO2-) ions appeared. At 125 °C, the strength of the metaphosphates became comparable to that of the pyrophosphates, and additional peaks related to P-O-P linkages were also clearly observed. The increased number of P-O-P linkages is also evidenced by the increased intensity of the XPS peak for BO of P-O-P linkages at the deposition temperature of 125 °C, as seen in Figure 3c. Therefore, the film consists mainly of mixed chemical forms of pyrophosphate (Mg2P2O7) and metaphosphate (MgP2O6) with chain and ring structures. The increased asymmetric stretching of P-O-P linkages may indicate the presence of triphosphate (P3O9) and tetraphosphate (P4O12) rings38,50, as schematically illustrated on the left side of Figure 7. Thus, it is concluded that the film deposited at higher temperatures is relatively well-arranged compared to the film deposited at lower temperatures. The latter also seems to be more disordered. To examine the degree of disordering of the ALD-grown magnesium phosphate films, XRR measurements were performed to determine the film density of 100 nm-thick films deposited at 125 °C and 250 °C. The obtained XRR curves, with fitted curves, are presented in Figure S11 of Supporting Information. The film densities were estimated to be 2.84 and 2.91 g cm-3 for the films deposited at 125 °C and 250 °C, respectively. These values are lower than the theoretical value (3.06 g cm-3) of a crystalline (farringtonite) Mg3(PO4)2 structure52, which seems to be reasonable because of the amorphous nature. Figure 7 plots the film density and GPC as a function of the deposition temperature. We can clearly see the opposite tendency between these two physical parameters; the film density decreases by 2.4%, whereas the GPC increases by 16%, with a decrease of the deposition temperature. These results support the schematics of the expected molecular structures of the ALD-grown films, as illustrated on both sides of Figure 7. Note that the film density of the sputtered film was estimated to be 2.95 g cm-3, which is higher than the values for both of the ALD films. The XRD results revealed that both films are amorphous (Figure S8), but the XRR results indicate that the film density is lowered at lower deposition temperatures (Figure 7). In addition, the absorbance peaks related to the Mg-O-P bridges are likely to elongate the bond length with decreasing deposition temperature, as seen in Figure 3b. These observations suggest that lower-temperature deposition can create more space in the phosphate network, which acts as conduction pathways for Mg ions, as illustrated on the left- side of Figure 7. Thus, the higher conductivity observed for the film deposited at lower temperatures can be attributed to the enhanced hopping conduction of Mg ions in the disordered phosphate matrix. On the other hand, the NBO/BO ratio decreased at lowered deposition temperatures, as seen in Figure 3c. The reduction of NBO can lower the ionic conductivity, as has been reported for ALD-grown lithium borate-carbonate films.41 However, our results showed that conductivity becomes higher for lower deposition temperatures. This indicates that the

deposition temperature, rather than the formation of NBO, has a stronger influence on the conductivity. Our results suggest that ALD has great potential for controlling the local structure of Mg-ion conducting SSE films. It is difficult to discuss quantitatively how the proposed structural change results in the exact stoichiometry found in the deposited films. However, it can be seen that the formation of metaphosphates is qualitatively consist with the variation in compositions estimated from the XPS analyses (Table I), in which the compositions of Mg and O relative to P are decreased for lowered deposition temperatures. Further investigation is required to clarify the correlation between the matrix structure and the chemical composition in the glassy phosphate network. Finally, we comment on the impact of the direct plasma configuration on our ALD process. During the oxidation by O2plasma, ion etching should occur on the substrate surface simultaneously, because both radicals and ions are produced under the current plasma conditions. Indeed, if the pulse time of O2-plasma was increased, the GPC decreased and the absorption bands for phosphates also decreased significantly. This indicates that a longer pulse time of O2-plasma leads to the ion etching of phosphates rather than oxidation of TDMAP. We are now constructing a new RF plasma system to produce only radicals, so as to realize selective oxidation of TDMAP in the ALD process. ■ CONCLUSIONS In this study, a new plasma-assisted ALD process was successfully developed to deposit magnesium phosphate electrolyte films using Mg(EtCp)2 and TDMAP precursors with H2O and O2-plasma oxidants, in a deposition temperature range of between 125 °C and 300 °C. The films showed an amorphous nature, regardless of the deposition temperature, and excellent step coverage in narrow trenches 1 µm wide and 5 µm deep. It was found that the film deposited at higher temperatures has a relatively well-arranged pyrophosphate matrix, while the film deposited at lower temperatures has a disordered matrix consisting of pyrophosphates and metaphosphates with chain and ring structures. The film deposited at 125 °C exhibited an ionic conductivity of 1.6 × 10-7 S cm-1 at the ambient temperature of 500 °C, with an activation energy of 1.37 eV. These values are better than those found in sputtered magnesium phosphate film. This improved conductivity originates from the enhanced hopping conduction of Mg ions between non-bridging oxygens in the disordered amorphous phosphate matrix. Although the conductivity level is much lower than ALD-grown lithium phosphate and LiPON films2325 , we expect that it will be possible to improve the conductivity by optimizing growth processes such as nitridation and by the addition of alloying elements. Our results demonstrate the good controllability of the molecular structures of the phosphate matrix and the usefulness of ALD technique in realizing highly conductive Mg-based SSE films.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI:



Pulse time dependence at 125 °C, survey and high-resolution XPS spectra of ALD magnesium phosphate films, RF-power dependence, AFM images, XRD patterns, electrical conductivity of ALD and sputtered magnesium phosphate film, and XRR curves with fitting curves calculated based on a multilayer model.

AUTHOR INFORMATION Corresponding Author * Tohru Tsuruoka, E-mail: [email protected]

Author Contributions T.T.(Tsujita), Y.N., and K.N. initiated the ALD solid electrolyte research. T.T.(Tsuruoka), T.T.(Tsujita), and K.T. planned the research. T.T.(Tsuruoka) and J.S. developed the ALD process. J.S. fabricated and characterized all the films. T.T.(Tsujita) conducted XPS measurements. J.S. and T.T.(Tsuruoka) wrote the manuscript. All authors discussed and approved the final version of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors would like to thank A. Ohi and T. Ohki of the MANA Foundry at NIMS for their assistance in the fabrication of the patterned Si substrate. This work was supported by the PanasonicNIMS Center of Excellence for Advanced Functional Materials.

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Atomic Layer Deposition of a Magnesium Phosphate Solid Electrolyte

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