Electrical characterization of amorphous LiAlO 2 thin ﬁ lms deposited by atomic layer deposition

LiAlO 2 thin ﬁ lms deposited by atomic layer deposition (ALD) have a potential application as an electrolyte in three-dimensional (3D) all-solid-state microbatteries. In this study, Li-ion conductivity of such ﬁ lms is investigated by both in-plane and cross-plane methods. LiAlO 2 thin ﬁ lms with a Li composition of [Li]/ ([Li] + [Al]) ¼ 0.46 and an amorphous structure were grown by ALD with thicknesses of 90, 160 and 235 nm on di ﬀ erent substrates. The electrical characterization was conducted by impedance spectroscopy using inert electrodes over a temperature range of 25 – 200 (cid:2) C in an inert atmosphere. In-plane conductivities were obtained from ﬁ lms on insulating sapphire substrates, whereas cross-plane conductivities were measured from ﬁ lms on conducting titanium substrates. For the ﬁ rst time, comparison of the in-plane and cross-plane conductivities in these ALD LiAlO 2 ﬁ lms has been achieved. More comparable results are obtained using a cross-plane method, whereas in-plane conductivity measurements demonstrate a considerable thickness-dependence with thinner ﬁ lm thickness. The room-temperature conductivity of the LiAlO 2 ﬁ lms has been determined to be in the order of 10 (cid:3) 10 S cm (cid:3) 1 with an activation energy of ca. 0.8 eV.


Introduction
Future Li-ion battery designs may rely on the utilization of suitable solid-state electrolytes.3][4] For a conventional two-dimensional (2D) design using thin lm electrolytes, a room temperature conductivity above 10 À6 S cm À1 (ref.5 and 6) is required such as the state-ofthe-art lithium phosphorus oxynitride (LiPON) thin lm.However, this 2D design suffers from a limited capacity per footprint area (A h cm À2 ) and a low power density.Therefore, the current focus for all-solid-state batteries is the realization of three-dimensional (3D) microbattery designs, which can appreciably enhance the active electrode area inside the batteries and thus increase the power density. 1,7,8A key factor is a thin lm process capable of depositing homogeneous and pinhole-free lms onto complex structures with large surface areas.Atomic layer deposition (ALD) has proven to be a promising technique for such applications. 9It is based on selflimiting gas-to-surface reactions that ensure highly conformal growth over complex geometrical shapes. 10][13] The deposition of Li-containing thin lms by ALD was rst reported in 2009 (ref.9) for the formation of Li 2 CO 3 .5][16] The exploitation of thin lm solid-state Liion electrolytes by ALD is still considered immature, and there are very few reports on the Li-ion conductivity of the deposited lms.Liu et al. obtained a room-temperature conductivity of 2 Â 10 À8 S cm À1 for a Li 5.1 TaO z ALD thin lm. 12Aaltonen et al. deposited Li 2 O-Al 2 O 3 thin lms, which were targeted as a barrier layer between the Li anode and the lithium lanthanum titanate [ (Li, La) x Ti y O z , LLT] electrolyte, 11 and they obtained an ionic conductivity of 1 Â 10 À7 S cm À1 at 300 C for the lm subjected to a post-annealing process at 700 C for 5 h, which may have induced crystallization of the asdeposited amorphous structure.Recently, Park et al. 17 investigated LiAlO 2 ALD lms with a thickness of 50 nm grown onto quartz substrates and obtained a promising room-temperature conductivity of 5.6 Â 10 À8 S cm À1 in ambient air.Very recently, Kozen et al. deposited LiPON by ALD and obtained a conductivity of 1.45 Â 10 À7 to 3 Â 10 À7 S cm À1 with increasing N content (1.8% to 16.3%). 18However, this conductivity was extracted from the impedance measured in a LiPON/organic liquid electrolyte/Li metal coin cell using LiPON as a working electrode, and it is not unambiguous to assign the result to the bulk conductivity of the electrolyte.It can be noted that in some of the aforementioned studies, the conductivity of Li-containing thin lms is measured at higher temperature (>100 C).This may be due to high measured resistances that are beyond the instrument's capability, arising from low ionic conductivity and the commonly used in-plane geometry.
Characterization of electrical conductivity of thin lms or membranes can be performed through two different geometrical congurations, as demonstrated in Fig. 1.One comprises cross-plane conductivity of lms deposited either directly onto a conducting substrate (e.g.stainless steel, Ti) or onto a conducting interlayer (e.g.platinum) coated on an insulating support substrate (e.g.sapphire, Al 2 O 3 ). 19,20The conducting substrate or the interlayer serves as the bottom electrode, and the top electrode is deposited on the surface of the lm.The cross-plane conductivity conguration has been widely applied for the characterization of solid-state electrolytes in Li-ion batteries; however, most reports are on bulk lms with thicknesses in the micron range. 19,21For thin lms, the application of this geometry is limited by short-circuits due to inherent pinholes, micro-cracks, structural changes, and thermal effects at elevated temperatures as well as damage caused by experimental handling.An alternative conguration is the in-plane geometry, 22,23 where lms are deposited onto an insulating substrate, with 2 or 4 co-planar electrodes on the lm surface.This conguration gives rise to a wider choice of substrates and circumvents the aforementioned short-circuit issues, but does not reect the conductivity in the direction intended for electrolyte application and can be affected by anisotropy caused by the texture and other structural effects such as grain boundaries and interfacial lattice mismatch.Moreover, in-plane measurements on resistive systems may be inuenced by a parasitic parallel surface, interface, and substrate conductance.From a practical point of view, characterization of conductivity on thin lms with a thickness down to the nanometer-scale introduces extra challenges.It is also necessary to carefully control the atmosphere and temperature range to maintain a pristine amorphous state.
The present study aims to investigate the electrical conductivity of LiAlO 2 thin lms prepared by ALD, which has been suggested as an alternative electrolyte 11 or protection layer 24 showing good stability with currently used electrode materials.Both in-plane and cross-plane measurements were performed on LiAlO 2 thin lms deposited onto insulating sapphire and conducting Ti substrates, respectively.Room-temperature conductivity is readily measured, and the temperaturedependent ionic conductivities were obtained up to 200 C. Results from in-plane and cross-plane methods were compared with three different lm thicknesses.

Experimental
Thin lms of nominal composition LiAlO 2 were prepared by Atomic Layer Deposition (ALD) using a modied process based on the one described previously.Electrodes comprising a 100 nm Pt lm on a 5 nm Ti adhesion layer were deposited by e-beam evaporation (Leybold DC V 6-12 kV).The growth rate was 0.2 A s À1 for Ti and 0.3-0.5A s À1 for Pt.Parallel band electrodes were made on the surface of LiAlO 2 lms on a sapphire substrate for in-plane conductivity measurements.Multiple round electrodes with a diameter of 3 mm were applied to the lms on the Ti substrate for cross-plane conductivity measurements.These Pt electrodes were rstly examined to exclude the area in the lm from pinholes.Au wires were attached on top of the Pt electrodes using Ag paste (Aldrich, 735825-25G) and dried at 120 C for 1 h in ambient air to obtain soer contacts and minimize risks of lm damage.The prepared samples were placed in a ProboStat sample holder (NorECs, Norway) for the conductivity measurements, and an outer steel tube was used to shield the system and contain the controlled atmosphere.The congurations of in-plane and cross-plane measurements utilized in this study are illustrated in Fig. 1.The Pt electrode/lm cross-section of a 90 nm lm on a Si (111) substrate was cut by Focused Ion Beam (FIB) (FEI Helios NanoLab) and observed by SEM-EDX (HITACHI SU8230-Bruker Quantax).Scanning electron microscopy (Quanta 200 FEI) was used for the top view of the electrodes aer electrical measurements.
The electrical conductivity was measured by two-electrode impedance spectroscopy (Novocontrol Alpha-A + POT/GAL 15V 10A, Novocontrol Technologies) over a frequency range from 1 MHz to 0.05 Hz with an AC amplitude of 50 ca.70 mV (50 mV rms).Room-temperature conductivities were measured in ambient air, and the temperature-dependent measurements were performed in dry Ar from room temperature to $200 C at heating and cooling rates of 2 C min À1 .At each temperature, a stabilization period of 30 min was applied prior to data acquisition.The impedance spectra obtained were analyzed in terms of equivalent circuits using ZView2 soware (Scribner Associates Inc.).The conductivity was calculated by the resistance obtained from the data tting R and the in-plane or crossplane geometrical considerations, as shown in eqn ( 1) and ( 2), respectively:

Results and discussions
The amorphous structure of the LiAlO 2 lms investigated in this study is conrmed by XRD, as shown in Fig. 2. Both the asdeposited lm and the lm annealed at 600 C exhibit amorphous structures.The transition to a crystalline structure is observed in the lms annealed to 950 C. The FIB-cut 90 nm LiAlO 2 lm on a Si (111) substrate aer Pt deposition exemplies the cross-sectional morphology of an electrode/lm interface (Fig. 3), displaying a good connection between Pt layer and LiAlO 2 lm.Top-view SEM images of the electrode contacts shows that the e-beam evaporated Pt electrodes exhibited good adhesion and stability under the experimental conditions of this study (Fig. 4).No distinct delamination of the Pt layer or agglomeration of Pt grains was observed aer the thermal cycles irrespective of the substrate.Only minor cracks/ssures were visible close to the Ag/Pt boundary, which is believed to have only a negligible effect on the measurements.
The electrical conductivity of the as-deposited LiAlO 2 lms with three thicknesses, 90, 160 and 235 nm, was investigated by impedance spectroscopy.Fig. 5 shows the Nyquist plots of the in-plane impedance of the 160 nm LiAlO 2 lm on a sapphire substrate.Typical impedance spectra comprise one semi-circle in the high-frequency region and a low-frequency inclined line.The high-frequency semi-circle can be ascribed to the response of the bulk lm, presumably reecting Li-ion conduction.6][27] The equivalent circuit used to model the impedance spectrum is shown as an inset in Fig. 5, in which R b represents the bulk resistance of the lm and is used to calculate the conductivity using eqn (1).The constant phase element CPE e takes into account the capacitive contribution of the electrodes associated with the accumulation of charge carrying ions at the electrolyte/ electrode interface.Due to the small cross-sectional area of the conductivity pathway, resulting from the small lm thickness, the geometrical capacitance of the bulk lm becomes very small and is completely masked by the stray capacitance CPE stray from the substrate and setup. 28,29he in-plane conductivities of the 90, 160 and 235 nm LiAlO 2 lms obtained from impedance spectra are shown as a function of temperature in Fig. 6.The conductivities increase exponentially with increasing temperature, indicating a thermally activated conduction mechanism.However, anomalously high conductance was observed when measured in ambient air or at the beginning of heating in dry Ar, as indicated by the curves in Fig. 6.This is attributed to residual water remaining from the ALD cycling or surface adsorbed water from ambient air, giving rise to an additional fast conduction pathway along the surface.When measured in dry atmospheres (here dry Ar), it is permanently eliminated aer initial heating.The obtained conductivities otherwise show good reproducibility between increasing and decreasing temperature and can be interpreted and tted according to an Arrhenius-type behavior for diffusing carriers: where E a denotes the activation energy, s 0 is the pre-exponential factor, k is Boltzmann constant, and T is the absolute temperature.The activation energies and pre-exponential factors obtained from cooling are listed in Table 1, along with the roomtemperature conductivities.It can be seen that the in-plane conductivity varies pronouncedly with different lm thicknesses.The thinnest lm of 90 nm exhibits the highest conductivity of $10 À9 S cm À1 , which is one order of magnitude higher than the others.This is in agreement with the abovementioned thickness-dependence in previous reports.However, it is not evident here that simple "thickness to conductivity" relation can be drawn since the 235 nm lms does not exhibit the lowest conductivity.Fig. 7 shows the cross-plane complex impedance of a 160 nm LiAlO 2 thin lm on a Ti substrate.In this geometry, both contributions of the bulk thin lm and the electrodes can be observed at low temperature (Fig. 7(a)).It can be tted using the equivalent circuit depicted in Fig. 7(c).A high-frequency semicircle associated with the contribution from the bulk thin lm is represented by the bulk resistance R b in parallel, with   a capacitive CPE g mainly representing the geometrical capacitance of the thin lm material.The low-frequency linear part of the curve is typical of a blocking electrode (CPE e ), indicating the ionic characteristics of the conduction.It can be noted that the stray capacitance, which was dominating in the in-plane impedance spectra, is relatively small here in the cross-plane geometry compared to the capacitive contribution of the bulk lm and can thus be neglected.Loss tangent plots help to distinguish between the high-and low-frequency regions, as shown in Fig. 7(b).The peak value in the tan d spectra (indicated by the arrows) separates the electrode process towards lower frequency.It evolves to higher frequency with increasing temperature, as exemplied here from 24 to 61 C, due to a decrease in the time constant of the corresponding bulk and electrode transport processes.
The reproducibility of the conductivities obtained with the cross-plane conguration was examined by measuring two electrodes on the same thin lm sample at each temperature, as exemplied in Fig. 8(a) for the 160 nm LiAlO 2 lm.The good agreement indicates a good uniformity in the lm and the deposited electrodes.During the rst heating process (Fig. 8(a), open symbols), there was no remarkable conductivity enhancement at low temperature, and reproducible results were observed over the entire temperature range (RT -200 C).In comparison to the in-plane measurement, the cross-plane method thus proves to be less sensitive to adsorbed water, which is reasonable since the cross-plane conduction pathway within the lm lies between the Ti substrate and Pt electrode, without the surface being exposed to ambient air.Moreover, the lms and electrodes exhibit a good thermal stability over the investigated temperature range.Fig. 8(b) shows the Arrhenius plots of conductivity obtained from 90, 160 and 235 nm LiAlO 2 thin lms.A good agreement is evidenced using the cross-plane method without considerable thickness-dependence. Compatible room-temperature conductivity is obtained for 90, 160 and 235 nm LiAlO 2 lms being 2.4 Â 10 À10 , 2.8 Â 10 À10 and 2.5 Â 10 À10 S cm À1 , respectively.
A summary of the conductivity in these LiAlO 2 lms is presented in Fig. 9 which shows the comparison of the thickness and geometry congurations.The room-temperature conductivities, activation energies and the pre-exponential factors obtained from the Arrhenius relation are listed in Table 1.The ionic conductivity of materials with a single charge carrier can be expressed in terms of the charge mobility and the concentration of charge carriers, with the diffusion coefficient being derived by the Nernst-Einstein relationship: where c i denotes the concentration of mobile ions of charge z i e and charge mobility m i , and D i is the random (or self) diffusion  View Article Online coefficient.A hopping mechanism with a correlated jump phenomenon has also been proposed for amorphous materials in relation to the short and long range order effects on the mobility of lithium ions in the disordered structure. 30In accordance with the diffusion model, the activation energies reported in Table 1 reect the tting of the Arrhenius equation in the form of eqn (3).In terms of Li ions, which have a concentration in the material independent of temperature, the activation energy of ca.0.8 eV reects the mobility and diffusivity of the Li ions.On a more mechanistic level, it is still open whether the energy reects only the barrier for disordered Li ions (or defects) to jump or also contains formation of disorder (defects) as such.The variation of in-plane conductivities in the 90, 160 and 235 nm lms might be due to the structural modulation or the hetero-interface effect such as lm/substrate mist, which commonly leads to a thickness-dependence in the in-plane conductivity. 31One may also consider the variation in charge carrier density to be related to the space charge layer (SCL), which will be more pronounced in thin lms with a thickness comparable to the length of the SCL region.For example, Li et al. studied the in-plane conductivity of LiPON thin lms 32 and reported what was interpreted as a transition from ionic conductivity to mixed ionic-electronic conductivity when the lm thickness was reduced from 50 nm to 40 nm due to the electronic conduction induced by an enlarged SCL region in heterojunctions.Furthermore, a surface enrichment of C or H from the ALD may vary with the number of pulsing cycles, resulting in differences in surface composition and conduction in as-deposited lms of different thickness.The in-plane conductivities in our study might be inuenced by all the aforementioned factors.One may also notice that for thicker lms of 160 and 235 nm, the in-plane and cross-plane conductivities are within the same order of magnitude, whereas a distinct discrepancy is demonstrated in the 90 nm lms.This leads to a suspect conductivity enhancement from reduced lm thickness.Further work with ultrathin and thicker lms would help to clarify this "thickness to conductivity" relation, which is, however, beyond the scope of this study.Reproducible conductivities are obtained by a cross-plane method and independent of the lm thickness.This may indicate that in this study, the cross-plane method better represents the bulk conductivity of the lms, and thus the room temperature conductivity can be drawn to be $10 À10 S cm À1 .Nevertheless, from a practical point of view, the experimental difficulties of measuring the 90 nm lm have been drastically increased.There was a larger probability of short-circuiting because of pinholes and the lm became more vulnerable to the damage caused by sample handling and thermal effects.
Overall, the ALD LiAlO 2 thin lms investigated in this study with amorphous structures and Li cation ratios [Li]/([Li] + [Al]) ¼ 0.46 exhibit room-temperature conductivities larger than 10 À10 S cm À1 .These are considerably higher than those reported for single-crystalline 33    2), probably beneting from the amorphous nature with an isotropic conduction.Compared to crystalline LiAlO 2 , the increased disorder in the glassy or amorphous structure gives rise to an increased ionic mobility and suppressed electronic mobility, resulting in predominantly ionic transport characteristics, 35 as well as enhanced ionic conductivities.A better agreement has been found in an early report on bulk Li 2 O-Al 2 O 3 glasses, 35 in which the extrapolated room-temperature conductivity is 3 Â 10 À11 S cm À1 for a Li cation ratio [Li] Recently, Park et al. 17 reported an extrapolated s RT ¼ 5.6 Â 10 À8 S cm À1 for an ultrathin (50 nm) ALD LiAlO 2 lm, which was obtained by in-plane measurements over 300-400 C in ambient air.Though the exact Li content is lacking, one can assume based on their previous work on ALD processes 24 that this enhanced conductivity value may partially result from a high Li cation percentage approaching 0.82, which can only be achieved in relatively thin lm.Therefore, it is reasonable to believe that the comparatively low Li cation ratio (0.46) in our study accounts for the lower conductivity but on the other hand, allows for thicker lms to be obtained (90, 160 and 235 nm).With respect to the application in solid-state batteries, the conductivities of the ALD LiAlO 2 lms reported in this study are still inadequate for use as a solid-state electrolyte.Further modications in the ALD processes, for example altering the pulse ratios between the Li and Al cycles in order increase the Li content, could be considered for improving the conductivity. 36

Conclusion
The Li-ion conductivity of amorphous LiAlO 2 lms deposited by atomic layer deposition (ALD) has been investigated by impedance spectroscopy on lms deposited on insulating sapphire and conducting Ti substrates using in-plane and crossplane geometries, respectively.The conductivity in these lms exhibit ionic conduction, presumably by Li ions, with an Arrhenius-type activated temperature dependency.The roomtemperature conductivity of LiAlO 2 lms is on the order of $10 À10 S cm À1 , and the activation energy is ca.0.8 eV.In-plane and cross-plane methods have been compared: in-plane conductivities exhibit stronger thickness-dependence particularly with a thin lm thickness (90 nm here), which can be tentatively attributed to effects of the large interface and surface of that geometry.Better reproducibility is achieved using the cross-plane geometry, showing close conductivity values in 90, 160 and 235 nm lms, once short-circuiting from lm damage, as a result of fabrication or handling, can be avoided.
11 All lms were deposited using an ASM F-120 Sat reactor at 225 C. Trimethyl aluminium (TMA) (Witco GmbH, 98%) was used as the aluminium source and lithium trimethyl silanolate (LiTMSO) (Aldrich, 97%) as the lithium source.Ozone (O 3 , concentration $200 g N À1 m À3 ), generated by an IN USA AC series ozone generator from oxygen (99.6% O 2 , AGA) and water (H 2 O, 25 C), was used as the oxygen source.N 2 (g) was used as a pulse and purge gas, generated by a Schmidlin-Sirocco 5 generator (99.999% purity considering N 2 + Ar).LiAlO 2 was deposited from the pulsing sequence [TMA (0.5 s pulse/3 s purge) + O 3 (3/5) + LiTMSO (5/2) + H 2 O (0.5/5)].The lms were deposited onto conducting Ti (ASTM, Astrup AS), insulating sapphire (a-Al 2 O 3 (0001), UniversityWafer) and Si (111) (Coating And Crystal Technology Inc.) substrates.The latter was used as a reference sample for thickness, structure and composition analyses.The lm thickness was determined by spectroscopic ellipsometry (alpha-SE, J. A. Woollam Co., using the Cauchy function) using lms on Si substrates.The structure of the as-deposited lms and lms annealed at 600 C and 950 C in air was investigated by X-ray diffraction (XRD, Bruker D8 Discover) analysis of the lm on a Si (111) substrate.The elemental composition was determined by the time-of-ight elastic recoil detection analysis (TOF-ERDA), yielding an atomic ratio of Li : Al ¼ 1 : 1.16, denoted as LiAlO 2 in this study, with a lithium cation content ratio [Li]/([Li] + [Al]) close to 0.46 for the as-deposited lms.

Fig. 1
Fig. 1 Sketches of the (a) in-plane and (b) cross-plane geometrical configurations for thin film conductivity measurement.L: distance between the parallel band electrodes, l: length of band electrodes, d film : film thickness, A: electrode area.Red arrows indicate the current pathway.

Fig. 4
Fig. 4 Top-view SEM images of electrode contacts on top of LiAlO 2 thin films after conductivity measurements: (a) band electrode for a film on a sapphire substrate; (b) circular electrode for a film on a Ti substrate; (c) boundary of Ag/Pt.

Fig. 5
Fig. 5 Impedance spectra of a 160 nm LiAlO 2 thin film on a sapphire substrate obtained by in-plane measurements.The equivalent circuit used for data fitting is shown as an inset, with the solid lines representing the fitting results.

Fig. 6
Fig. 6 Conductivities of LiAlO 2 thin films on sapphire substrates obtained by in-plane measurements in dry Ar during heating (open symbols) and cooling (solid symbols).Curves represent the anomaly seen during the first heating, attributed to adsorbed water.

Fig. 7
Fig. 7 Impedance spectra of a 160 nm LiAlO 2 film on a Ti substrate obtained by cross-plane measurements at low temperatures: (a) Nyquist plots; (b) loss tangent spectra: 3 0real and 3 00imaginary part of the complex permittivity; (c) the corresponding equivalent circuit.

Fig. 8
Fig. 8 Conductivities of LiAlO 2 thin films on the Ti substrate obtained by the cross-plane measurement in dry Ar, (a) from two different electrodes E1 and E2 on the same 160 nm LiAlO 2 thin film during heating (open symbols) and cooling (solid symbols).(b) Comparison of 90, 160 and 235 nm LiAlO 2 films.

Fig. 9
Fig. 9 Conductivities of 90, 160, and 235 nm LiAlO 2 thin films obtained by in-plane (open symbols) and cross-plane (solid symbols) measurements during cooling in dry Ar.The grey area emphasizes the cross-plan conductivities.

Table 2
Comparison of conductivities of different types of LiAlO 2 (*extrapolated from high temperature results)