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Preparation of lithium containing oxides by solid state reaction of atomic layer deposited thin films Elisa Atosuo, Miia Mäntymäki, Kenichiro Mizohata, Mikko J. Heikkilä, Jyrki Räisänen, Mikko Ritala, and Markku Leskelä Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03586 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 9, 2017

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Chemistry of Materials

Preparation of lithium containing oxides by solid state reaction of atomic layer deposited thin films Elisa Atosuoa, Miia Mäntymäki*a, Kenichiro Mizohatab, Mikko J. Heikkiläa, Jyrki Räisänenb, Mikko Ritalaa, Markku Leskeläa a

Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, FI-00014 Helsinki, Finland b

Department of Physics, University of Helsinki, P.O. Box 43, FI-00014 Helsinki, Finland

ABSTRACT: Lithium containing multicomponent oxides are important materials for both lithium-ion batteries and optical applications. In most cases thin films of these materials are desired. Atomic layer deposition (ALD) is a thin film deposition method that is known to deposit high quality films by sequential self-limiting surface reactions. However, the reactivity of lithium ions during the deposition process can pose challenges for the control of the film growth and even destroy the self-limiting nature of ALD completely. In this paper, we have studied the combination of atomic layer deposition and solid state reactions for the generation of lithium containing multicomponent oxide films. Atomic layer deposited transition metal oxide thin films were covered with ALDgrown lithium carbonate and the films were annealed to produce lithium tantalate, titanate and niobate. Lithium carbonate was chosen as the source of lithium because it is easy to deposit by ALD and can be handled in air. The films were analyzed as-deposited and after annealing using grazing incidence X-ray diffraction (GIXRD), field emission electron microscopy (FESEM) and time-offlight elastic recoil detection analysis (ToF-ERDA). By this method we were able to produce crystalline and very close to stoichiometric films of LiTaO3, Li2TiO3 and LiNbO3. The films showed only small amounts of carbon and hydrogen impurities after annealing. After prolonged annealing at high temperatures, lithium silicates began to form as a result of lithium ions reacting with the silicon substrates.

Introduction The increasing demands for both portable electronics and electric vehicles have made lithium-ion batteries a hot topic in materials chemistry. Higher energy densities and capacities are required, which means that new materials are needed for the next generation of batteries. In particular, all-solid-state thin film batteries are studied extensively, to be used in small batteries for cellphones, laptops and for on-chip integration. To make batteries smaller and all-solid-state, thin film deposition techniques are needed to provide excellent film quality, easy and accurate thickness control and good conformality, which enables not only 2D- but 3D-battery construction. 3Dbatteries could provide both higher energy and power densities without compromising the small footprint area required especially of batteries in portable devices.1, 2 A thin film deposition method able to provide high quality films also in 3D-structures is atomic layer deposition (ALD). ALD relies on self-limiting surface reactions, which produce the excellent conformality needed for 3D-batteries.3 In addition, the cyclic nature of the film growth in ALD makes thickness control straightforward. However, despite being an excellent method for depositing both binary and ternary thin films, ALD faces some challenges related to the deposition of lithium-containing materials. Most of the materials needed for all-solid-state batteries are ternary,

quaternary of even higher oxide materials. Due to the high reactivity and diffusivity of the lithium ion, depositing these materials in the correct stoichiometry can be challenging even with ALD.4-6 Of particular concern is the “reservoir effect” seen in the most commonly used LiOtBu + H2O process, where water absorbs into the forming Li2O/LiOH film, destroying the self-limiting nature of the ALD growth.5 Despite this challenge, many potential lithium-ion battery materials have already been deposited by ALD by thoroughly mixing subcycles of Li2O and other oxides, resulting in materials such as Li4Ti5O126, 7, LiTaO34, and LiNbO35. To overcome the difficulties in depositing ternary lithium oxides by the common subcycle method, we have combined ALD with solid state reactions to obtain the desired lithium material in a more simple fashion. An atomic layer deposited transition metal oxide film is covered with Li2CO3 also deposited by ALD with a robust process utilizing Lithd (lithium 2,2,6,6-tetramethyl-3,5-heptanedionate) and ozone. Another reason for choosing Li2CO3 is that the carbonate films can be easily handled in air. The resulting stack is annealed in air to decompose the carbonate and to form a ternary lithium containing oxide. Thus, the composition control of the resulting ternary material is achieved via thickness control of the binary oxide and lithium carbonate and by choosing a proper annealing temperature.

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Experimental Results and discussion Film deposition Li2CO3 films Li2CO3 thin films were deposited in an ASM Microchemistry F-120 hot-wall flow-type ALD reactor onto TiO2, Ta2O5 and Nb2O5 films. These films had been deposited by ALD onto single-crystalline silicon wafers using water and either titanium tetrachloride, tantalum ethoxide or niobium ethoxide, as described in the literature.8-10 The TiN buffer layer was deposited using TiCl4 and NH3 plasma at 300 °C in a Beneq TFS-200 reactor with a remote plasma configuration.11 Lithd (Volatec Oy) and ozone, generated by a Wedeco GmbH Modular 4 HC ozone generator, were used as precursors for the Li2CO3 depositions as previously described in the literature.12 The ozone concentration was 100 g/Nm3 with a flow rate of 30 L/h. The lithium precursor was evaporated inside the reactor at 192 °C. The pulse time for Lithd was 1.5 s with a 2 s purge, and ozone was pulsed for 2.5 s with a 3.5 s purge time. All deposition experiments were done at 225 °C at a pressure of approximately 5 mbar. The pulsing of the lithium precursor was done by inert gas valving, with gaseous N2 as the pulse and purge gas. The N2, obtained from liquid N2, had as an impurity less than 3 ppm of H2O and O2 each.

Film characterization Film thicknesses were analyzed with X-ray reflectivity (XRR) measurements using a PANalytical X’Pert Pro MPD X-ray diffractometer. Film thicknesses were also studied from film cross-sections, which were imaged using a Hitachi S4800 FESEM instrument. A muffle furnace was used for the annealing of the films. All annealings were done in air with a maximum heating rate of approximately 9 °C/min. For most samples, the holding time at the selected temperature was 2 hours. The cooling was done slowly inside the furnace, usually over-night (10-14 hours). The crystallinity of the films was studied by grazing incidence X-ray diffraction measurements conducted with the PANalytical X’Pert Pro MPD X-ray diffractometer using parallel beam optics and an 1° incident angle. In situ high temperature XRD (HTXRD) measurements were also conducted using an Anton-Paar HTK1200N oven. PANalytical Highscore Plus v. 4.5 was used for XRD phase identification using ICDD and ICSD databases. The morphology of the films before and after annealing was studied by scanning electron microscopy with the Hitachi S4800 FESEM instrument. For the FESEM imaging, the samples were coated with approximately 3 nm of Au/Pd by sputtering. The composition of the films was studied with time-of-flight elastic recoil detection analysis (ToF-ERDA). The ToF-ERDA measurements were performed with 50 MeV 127I and 40 MeV 79 Br beams from the 5 MV EGP-10-II tandem accelerator at the University of Helsinki.13 The detection angle was 40° and the sample was tilted 15° relative to the beam direction.

In the majority of the experiments, Li2CO3 films were deposited using 3000 cycles of Lithd and ozone onto the transition metal oxide films. This resulted in Li2CO3 films of approximately 26-28 nm in thickness on silicon, as determined by both XRR and FESEM cross-sections (Supporting Information, Table S1). The as-deposited Li2CO3 films are crystalline and as such very rough, which made fitting of the XRR results difficult. Roughnesses of the order of 1.5 to 2.3 nm were determined for 3000 cycles Li2CO3 films deposited onto silicon. Putkonen et al. also reported a large rms roughness of 19 nm for their 120 nm film.12 To verify the XRR thickness results, FESEM cross-sections were prepared of Li2CO3 films on top of Ta2O5 (Figure 1). In Fig. 1a), 3000 cycles of Li2CO3 have been applied on 50 nm of Ta2O5. In Fig. 1b), the Li2CO3 film is sandwiched between two 40 nm Ta2O5 films. Based on the images it appears that our Li2CO3 is slightly thicker than the 28 nm obtained from most of the XRR measurements on silicon. However, this might be an effect of the high roughness of the films (Fig 1c). In any case, based on these analyses it appears that the growth rate of our Li2CO3 films was only one third of that reported by Putkonen et al. in their Li2CO3 paper.12 We can only speculate as to why there is such a large difference. For example, our smaller ozone concentration might affect the growth. In addition, our F-120 reactor has a different configuration (cross-flow cassette) than the one used by Putkonen et al. (Sat, with a more open tube).

Figure 1. FESEM cross-sections of 3000 cycles of Li2CO3 a) on top of 50 nm of Ta2O5 and b) between 40 nm of Ta2O5. In c), 3000 cycles of Li2CO3 has been deposited onto 50 nm Ta2O5 and the sample has been tilted to better reveal the surface roughness in plain view. In the cross-sections a) and b), the arrow points to the Li2CO3 layer.

To study the behavior of Li2CO3 during annealing in air, a 26 nm film on silicon was measured with elastic recoil detection analysis before and after annealing in air. After the an-


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neal, carbon had been removed from the film (Supporting Information, Table S2). In the following experiments, the same 3000 cycles of Li2CO3 were applied onto transition metal oxide films and subsequently annealed in air. The expected metal ratios, as calculated from film thicknesses, can be found in the Supporting Information, Table S3.

Li2CO3/Ta2O5 Tantalum oxide was of interest in this work because lithium tantalate, LiTaO3, is not only a possible lithium-ion conducting material when in a proper form,4, 14 but also a well-known optical, ferroelectric and piezoelectric material15, 16 with potential uses in, for example, optical waveguides.17 Ta2O5 films were deposited using tantalum ethoxide and water and were amorphous as-deposited.9 3000 cycles of Li2CO3 was deposited onto a 50 nm tantalum oxide film, and the resulting film was studied both before and after annealing in air. High temperature XRD measurements (Figure 2) were used to study the behavior of the film stack during heating. Already at 445 °C a peak belonging to LiTaO3 starts to emerge.18 Thus, lithium is quite mobile in the film stack already at moderate temperatures. However, diffraction peaks originating from Li2CO3 can still be seen at 565 °C.19 At 655 °C only two phases of lithium tantalate are present. The cubic Li3TaO4 phase 20 is an interesting one, as it is not at all as well known as LiTaO3.21, 22 The Li3TaO4 phase is more clearly present in the diffractogram measured at room temperature after annealing (Fig. 3). Its presence indicates a lithium excess in the film after annealing. When a smaller amount of Li2CO3 was deposited onto slightly thinner Ta2O5 film of 43 nm, no Li3TaO4 was observed in the X-ray diffractogram (Supporting Information, Figure S1). Interestingly, the phase was also absent when the 50 nm Ta2O5 film was annealed at 750 °C (Supporting Information, Figure S2) No crystalline Ta2O5 was observed in any of our annealing experiments, although the material has been reported to crystallize at 700 °C and above.23, 24 Our first experiments revealed that annealing temperatures higher than 750 °C can lead to lithium silicate formation since the stacks are deposited onto single crystalline silicon (Supporting Information Fig. S1). In most of the later experiments, temperatures below 700 °C were used for annealing. It is worth noting that this temperature is lower than those needed for melting and thermal decomposition of Li2CO3.25 However, formation of ternary lithium oxides from Li2CO3 has been reported to occur already at these lower temperatures.26 Clearly, reactions with metal oxides lower the decomposition temperature of Li2CO3, as is also seen in our experiments.

Figure 2. High temperature X-ray diffractograms of a Li2CO3/Ta2O5 film stack. The annealing was done in air. The LiTaO3 phase begins to emerge at 445 °C, and Li2CO3 has completely disappeared at 655 °C.

The same film stack was studied also by annealing it in a muffle furnace in air at 650 °C for 2 hours (Figure 3). This annealing corroborates the HT-XRD measurement in that after annealing at 650 °C only LiTaO3 and Li3TaO4 phases are present. Importantly, despite the long annealing time, no lithium silicates can be seen in the diffractogram. This means that although lithium is diffusing in the film, it is not reacting with the silicon substrate even after a long annealing time at this temperature.

Figure 3. X-ray diffractograms of a 3000 cycle Li2CO3 film deposited onto 50 nm Ta2O5 and measured as-deposited and after annealing for two hours in air at 650 °C.

FESEM imaging was used to study the film stack before and after annealing (Figure 4). The as-deposited film showed a very rough surface as was already demonstrated in Fig. 1c), with flakes protruding upwards from the film surface. After annealing at 650 °C, the film showed a smoother surface with small crystallites. Interestingly, the film does not show cracking or crater formation, which could have formed as a result of



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carbon leaving the inner parts of the Li2CO3 film as carbon dioxide during the annealing.

Figure 4. FESEM images of Li2CO3 film deposited by ALD onto Ta2O5, before (a) and after (b) annealing in air at 650 °C.

The Li2CO3/Ta2O5 samples were also studied with ERDA to examine both the elemental ratios and the distribution of lithium in the films. As can be seen in Table 1, the films are close to stoichiometric Li2CO3 on Ta2O5 as deposited. Both before and after annealing, the films contain small amounts of fluorine and sodium impurities. Sodium is most likely an impurity originating from the metal precursors. Fluorine in very small amounts is often seen in films deposited by ALD utilizing ozone. It has been postulated that the fluorine could originate from Teflon parts in the ozone line, fluoropolymer gaskets or vacuum grease used in parts of the reactor.27-29 After annealing at 650 °C the films are lithium-rich compared to the stoichiometric LiTaO3, which was already deduced from the X-ray diffractograms. Based on the calculations of Table S3, the films should be close to stoichiometric, thus indicating that the Li2CO3 film is indeed slightly thicker on Ta2O5 than it is on silicon (see also Figure 1). Carbon has been removed reasonably well already at 650 °C. Another striking aspect of the ERDA results was that the amount of hydrogen was very high even after annealing at 650 °C. However, the hydrogen content was reduced from 11.4 to 2.6 at% when the film was annealed at 750 °C for only 30 minutes. The carbon content also decreased after annealing at 750 °C. Although a higher annealing temperature was used for this sample, no lithium silicates were seen in the X-ray diffractogram. One can postulate that the explanation for the high hydrogen content in the 650 °C sample is the lithium excess in the film5: lithium is possibly forming lithium hydroxide in the film, since this particular sample was stored in ambient air for some time before the ERDA measurement.

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The ERDA depth profiles (Figure 5) revealed that lithium carbonate is initially formed on top of the tantalum oxide film. After the anneal lithium and tantalum are mixed, with only a little carbon present in the film. As with many other ternary lithium materials deposited by ALD5, 6, lithium is somewhat enriched on the film surface.

Table 1. Elastic recoil detection analysis results (at%) of a 3000 cycle lithium carbonate film deposited onto a 50 nm tantalum oxide film. The films were analyzed as-deposited and after annealing for 2 hours at 650 °C in air and 30 minutes at 750 °C in air. Li2CO3/Ta2O5 as-deposited

Li2CO3/Ta2O5 after anneal at 650 °C

Li2CO3/Ta2O5 after anneal at 750 °C

Li

15.1 ± 0.6

15.2 ± 0.5

18.3 ± 0.7

Ta

11.3 ± 0.2

10.6 ± 0.2

12.4 ± 0.2

O

56.4 ± 0.8

57.9 ± 0.7

62.7 ± 0.9

C

8.6 ± 0.4

3.7 ± 0.3

2.8 ± 0.3

H

6.8 ± 0.5

11.4 ± 0.7

2.6 ± 0.3

F

1.0 ± 0.1

0.4 ± 0.1

0.12 ± 0.04

Na

0.8 ± 0.1

0.8 ± 0.1

1.1 ± 0.1

Li:Ta:O = 1.4 : 1 : 5.5

Li:Ta:O = 1.5 : 1 : 5.1

Elemental ratios

Li:C:O = 1.8 : 1 : 3.3 (Ta2O5 assumed stoichiometric)

Compared to the lithium tantalate deposited by ALD with the subcycle approach4, our process produced more closely stoichiometric LiTaO3 with much lower amounts of carbon impurities despite the same deposition temperature. In addition, our process produces crystalline films, which are more desirable for optical applications than amorphous films.16, 30 However, one might argue that amorphous films would be more desirable for lithium-ion battery applications because of their better ionic conductivities.14


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Figure 5. ERDA depth profiles of Li2CO3/Ta2O5 films before (a) and after annealing in air at 650 °C (b) and 750 °C (c).

Li2CO3/TiO2 Titanium dioxide films were studied as underlayers for Li2CO3 in an effort to make Li4Ti5O12, a possible anode material for lithium-ion batteries.31, 32 The TiO2 films were deposited using TiCl4 and water8, and the films were anatase as deposited. Li2CO3 was deposited onto a 54 nm TiO2 film using the same 3000 cycles of Lithd and O3 as in the tantalum oxide experiments. The as-deposited stack showed XRD reflections belonging to both Li2CO3 and the anatase phase of TiO2.19, 33 (Figure 6). After annealing in air at 650 °C for 2 hours, the film showed peaks belonging to both anatase and the titanate phase Li2TiO3.34 Despite the high temperature annealing, the high temperature phase of TiO2, rutile, is not seen in the X-ray diffractogram. It must be noted that the most intense reflections of Li2TiO3 and Li5Ti4O12 are very close to each other.34, 35 However, we were able to verify the 2:1 composition of the film from ERDA results (Table 2). Li2TiO3 is a well-known impurity phase in the synthesis of the more desirable Li4Ti5O12.36

Figure 7. FESEM images of Li2CO3 film deposited by ALD onto TiO2, before (a) and after (b) annealing in air at 650 °C.

ERDA measurements (Table 2) revealed that, similar to the tantalum oxide experiments, also here the as-deposited film is close to stoichiometric Li2CO3. Unlike in the Ta2O5 case, however, both the carbon and hydrogen impurities are below 1.5 at% already after annealing at 650 °C. Moreover, the resulting film is very close to stoichiometric Li2TiO3, with only a small excess of oxygen. As can be seen from the ERDA depth profiles (Figure 8), lithium mixed well with the TiO2 film, leaving very little carbon impurities in the annealed film. Despite the surface morphology resembling that of anatase, no TiO2-rich layer is formed on the surface during annealing.

Table 2. Elastic recoil detection analysis results (at%) of a 3000 cycle lithium carbonate film deposited onto a 54 nm titanium oxide film. The films were analyzed as-deposited and after annealing for 2 hours at 650 °C in air.

Li Figure 6. X-ray diffractograms of a 3000 cycle Li2CO3 film deposited onto 54 nm TiO2 and measured as-deposited and after annealing for two hours in air at 650 °C.

FESEM imaging (Figure 7) revealed that the as-deposited film stack had an even rougher and flakier surface than the Li2CO3/Ta2O5 films. After annealing the roughness decreased considerably. The resulting film showed more column-like crystallites, compared to the flakes of the as-deposited film. Also, in the annealed film the crystallite size was larger than in the tantalum oxide experiments. The surface morphology of the annealed film resembles somewhat that of anatase, although anatase is tetragonal, whereas Li2TiO3 is monoclinic.37

Ti O C H F Cl

Li2CO3/TiO2 as-deposited

Li2CO3/TiO2 after anneal at 650 °C

20.0 ± 0.5

28.4 ± 0.8

12.1 ± 0.2

15.5 ± 0.3

56.5 ± 0.6

53.9 ± 0.8

9.9 ± 0.3

0.5 ± 0.1

1.1 ± 0.1

1.5 ± 0.2

0.33 ± 0.04

0.20 ± 0.05

0.04 ± 0.01

-

Li:C:O = 2.0 : 1 : 3.3 Elemental ratios

(TiO2 assumed stoichiometric)


Li:Ti :O = 1.8 : 1 : 3.5


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Figure 8. ERDA depth profiles of Li2CO3/TiO2 films before (a) and after annealing in air at 650 °C (b).

Based on the ERDA results and the calculations presented in Table S3, it is evident that 3000 cycles of Li2CO3 produced more lithium in the film than was expected. It turned out that, indeed, the Li2CO3 growth rate on TiO2 was significantly higher than on the other materials studied (Supporting Information, Figures S3 and S4). Whereas 3000 cycles produced 28 nm of Li2CO3 onto silicon, a TiO2 film in the same run was covered with slightly over 100 nm of Li2CO3, as determined from a FESEM cross-section. The reason for this high growth rate is, for the moment, unclear. For example, the effect of the rough TiO2 starting surface should not affect growth much after the first few nanometers. In any case, it was found that the growth rate seems to stay constant as a function of Li2CO3 cycles. Thus, controllability is still obtained. To achieve the less lithium containing Li4Ti5O12 phase, we also applied 1500 cycles of Li2CO3 onto a slightly thinner TiO2 film of 44 nm. Based on the Li2O-TiO2 phase diagram of ref. [38], a higher molar ratio of TiO2 should lead to both Li2TiO3 and Li4Ti5O12 formation in the temperature range used in our experiments. However, using a thinner Li2CO3 layer led to only monoclinic Li2TiO3 and anatase based on the X-ray diffractogram of the annealed sample. In addition, ERDA revealed that the annealed film had a Li:Ti ratio of 4 : 6 (Supporting information Figure S5). It would appear that obtaining the spinel phase of lithium titanate in our experimental setup is not entirely straightforward. Shen et al. reported that Li4Ti5O12 is only formed after annealing Li2CO3 and TiO2 powders at high temperatures of 800 °C and above.36 Meng et al. reported on obtaining Li4Ti5O12 after annealing an ALD film with an initial Li:Ti ratio of 2:1 at 850-950 °C.7 The film was deposited onto Ndoped CNTs using the TiO2 and Li2O/LiOH ALD processes. Meng et al. claimed that the decrease in the metal ratio is a result of lithium vaporization, which has been previously reported to occur as lithium peroxide.39 Shen et al. also report on the importance of a lithium excess in the starting materials,

although they used a much smaller excess of 3 % in their powder samples compared to the thin film samples prepared by Meng et al..36 In our experiments, annealing at 800-900 °C lead to lithium diffusion into the silicon substrate. To prevent the diffusion, we deposited a stack with a TiN buffer layer between the substrate and the TiO2/Li2CO3 films. An approximately 90 nm TiO2 film was deposited onto 27 nm of TiN, and 3000 cycles of Li2CO3 were applied on top of the stack. TiN was chosen as the lithium-ion buffer layer based on an earlier report on its good lithium-ion blocking characteristics.40 A number of HT-XRD measurements were made on this sample during heating in air. Our first experiments showed that the rutile phase of TiO2 emerged at 775 °C. Shortly after this, the TiN peaks disappear, indicating a failure in the buffer layer. As a result, lithium silicates such as Li2SiO3 and Li2Si2O7 began to crystallize. Thus, the TiN-layer was not thick enough to block lithium diffusion into the substrate at these high temperatures. The stability of TiN at high temperatures depends on the film thickness.41 In our case, a possible failure mechanism is oxygen diffusion from air through the films to TiN, where titanium oxide begins to form. Due to the possibility of TiN failure, our later experiments with this sample focused on lower temperatures. To follow the possible reaction of Li2TiO3 to Li4Ti5O12, the Li2TiO3 reflection at 20.4° (2θ) was followed. As seen in Figure 9, this small peak begins to disappear above 660 °C and has completely vanished at 700 °C. Therefore, a sample was first heated to 670 °C and X-ray diffractograms were measured many times at this temperature. This isothermal measurement revealed Li4Ti5O12 formation during a heating time of 460 minutes. The spinel phase is retained after cooling to room temperature. However, it must be noted that some lithium silicate formation also resulted from this heat treatment because of TiN failure (shoulder peak at 18.8°, for example). Based on these results, it can be concluded that obtaining Li4Ti5O12 by solid state reaction is possible but not at all trivial when thin films are involved.


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Figure 9. HT-XRD of a Li2CO3 – TiO2 – TiN film stack, measured in air. Above 750 °C the rutile phase of TiO2 begins to form, accompanied by the destruction of TiN. Formation of Li4Ti5O12 (appearance of a reflection at 35.3°) and disappearance of Li2TiO3 (decreasing peak at 20.4°) can be seen during an isothermal measurement at 670 °C.

Li2CO3/Nb2O5 Lithium niobate was studied in this work because of its similarity to lithium tantalate, including its piezoelectricity, ferroelectricity, large electro-optic coefficient and high ionic conductivity in the amorphous state.14, 17, 42 The niobium oxide thin films were deposited using niobium ethoxide and water and were amorphous as deposited.10 As with the previous film materials, 3000 cycles of Li2CO3 was deposited onto a 54 nm Nb2O5 film at 225 °C, and the resulting film stack was annealed at various temperatures in air. Despite the similarities between Nb2O5 and Ta2O5 and the resulting multicomponent oxides, in these experiments the two metals behaved quite differently. The X-ray diffractogram of the as-deposited Li2CO3/Nb2O5-film revealed that no crystalline Li2CO3 was present (Figure 10). However, a 23 nm top layer was visible on the Nb2O5 film in an XRR measurement (Supporting information, Figure S6). After annealing in air at 650 °C for 2 hours, the film only showed reflections belonging to the crystalline lithium niobate, LiNbO3.43 Interestingly, even after annealing for 2 hours at 800 °C, no lithium silicates were seen in the diffractogram, indicating that lithium is quite stable in the niobate. However, in a sample consisting of 54 nm of Nb2O5 and 6000 cycles (approx. 51 nm) of Li2CO3, silicate formation was observed at 800 °C (Supporting information, Figure S7). This result indicates that the amount of lithium has a large effect on its reactivity and mobility in niobate films.

Figure 10. X-ray diffractograms of a 3000 cycle Li2CO3 film deposited onto 54 nm Nb2O5 and measured as-deposited and after annealing for two hours in air at 650 °C and at 800 °C.

The XRD results are corroborated by the FESEM images (Figure 11), in that the as-deposited film has very little surface features. After annealing the film shows scarcely spaced crystallites on top of a polycrystalline film. Unlike in the previous experiments, the film does show some cracks, most likely caused by either the FESEM sample preparation or the crystallization itself.



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Table 3. Elastic recoil detection analysis results (at%) of a 3000 cycle lithium carbonate film deposited onto a 54 nm niobium oxide film. The films were analyzed as-deposited and after annealing for 2 hours at 650 °C and at 800 °C in air.

Figure 11. FESEM images of Li2CO3 film deposited by ALD onto Nb2O5, before (a) and after (b) annealing in air at 650 °C.

Li Nb

ERDA results (Table 3) show that stoichiometric Li2CO3 did not form on top of Nb2O5, but instead the films were lithium-rich compared to their carbon content. After annealing at 650 °C the films were close to stoichiometric LiNbO3 with a small niobium excess (metal ratio Li : Nb = 0.85), and only small amounts of impurities. The metal ratio is very close to that approximated in Table S3, indicating that even if the deposited layer is not stoichiometric, crystalline Li2CO3, the amount of lithium deposited was consistent with the calculation for a 3000 cycle Li2CO3 film. With the common subcycle ALD process5, the best films were obtained with a pulsing ratio of 1 pulse of Li to 2 pulses of Nb, resulting in a film with a Li : Nb ratio of ~ 0.72. Therefore, our films are closer to the correct stoichiometry. Due to the low solid solubility of Nb2O5 into LiNbO3, the niobium excess found in both the subcycle process and our method is most likely either in the form of amorphous niobium-rich niobate LiNb3O85 or amorphous Nb2O5 at LiNbO3 grain boundaries. After annealing at 800 °C, our films appear quite oxygen rich. This could be due to difficulties in separating the interfase between the film and the native oxide of the substrate in the ERDA analysis.

O C H F Na

Elemental ratios

Li2CO3/Nb2O5 as-deposited

Li2CO3/Nb2O5 after anneal at 650 °C

Li2CO3/Nb2O5 after anneal at 800 °C

14.7 ± 0.6

16.5 ± 0.6

14.0 ± 0.5

17.7 ± 0.3

19.5 ± 0.4

15.9 ± 0.3

60.5 ± 0.9

62.3 ± 0.9

68.8 ± 0.9

5.2 ± 0.4

0.7 ± 0.1

0.6 ± 0.1

1.7 ± 0.3

0.8 ± 0.1

0.7 ± 0.1

0.17 ± 0.04

0.11 ± 0.04

0.03 ± 0.02

0.08 ± 0.03

0.10 ± 0.03

0.12 ± 0.03

Li:C:O = 2.8 : 1 : 3.1 (Nb2O5 as-

Li:Nb:O = 1 : 1.2 : 3.8

Li:Nb:O = 1 : 1.1 : 4.9

sumed stoichiometric)

The ERDA depth profiles reveal that some lithium had already diffused into the upper part of the Nb2O5 film during the Li2CO3 deposition, before any annealing. Still, most of the lithium and also the carbon resided on the surface layers of the Nb2O5 film, as was also evidenced by XRR. This could mean that the few surface features seen on the as-deposited film are in fact related to Li2CO3 formation, without forming an actual crystalline carbonate film. This growth mechanism, in which lithium diffuses into a metal oxide film already during the Li2CO3 deposition, is similar to that seen by Miikkulainen et al. for lithiation of manganese and vanadium oxides.44 These oxides could be converted to their lithiated states simply by exposing the oxide films to either Lithd and ozone or LiOtBu and water. Miikkulainen et al. hypothesized that the reaction has to do with a redox reaction involving the transition metal.44, 45 Although niobium is predominantly found in the oxidation state +V, it can form oxides also with oxidation numbers +IV and +II.46 Therefore, a similar redox process is most likely effective here as well. Although tantalum shares the same oxidation states, it is reported to be much less inclined to form oxides with these lower oxidation states.46 This could explain the differences in the reactivity of niobium and tantalum oxides in these experiments. After annealing, lithium and niobium are very well mixed with no clear enrichment of lithium on the film surface (Figure 12), as opposite to the lithium niobates deposited by the conventional ALD approach.5


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Figure 12. ERDA depth profiles of Li2CO3/Nb2O5 films before (a) and after annealing in air at 650 °C (b) and 800 °C (c). All authors have given approval to the final version of the manuscript.

Conclusions

Funding Sources We have demonstrated that ternary lithium oxide thin films can be deposited by combining ALD and solid state reactions. By depositing Li2CO3 on top of transition metal oxide films and annealing the resulting film stack, we were able to obtain crystalline and close to stoichiometric lithium tantalate, niobate and titanate with only very small amounts of impurities. Using the robust Li2CO3 ALD process avoids complications in the growth process related to the hygroscopicity of Li2O and the high diffusion of lithium ions. In addition, this method makes the handling of the films simpler because Li2CO3 is stable in air. In the tantalum and titanium oxide cases the mixing of the metals occurred only after the annealing step, while with niobium oxide lithium is already diffusing into the film during the Li2CO3 deposition process, similarly to what has been observed before for manganese and vanadium oxides.44 Next, it would be interesting to study how this deposition process works for other ternary lithium oxides. On the other hand, more work still needs to be put into depositing the Li4Ti5O12 spinel phase instead of the Li2TiO3 phase predominantly obtained in this work.

Financial support from ASM Microchemistry Oy is gratefully acknowledged. This work was also supported by the Finnish Centre of Excellence in Atomic Layer Deposition.

ACKNOWLEDGMENT Dr. Marianna Kemell and Mr. Leo D. Salmi are thanked for help with the FESEM cross-section imaging. Mrs. Maarit Mäkelä and Ms. Jennifer Ott are thanked for the TiN layer deposition.

ASSOCIATED CONTENT Supporting Information Available. Thicknesses of Li2CO3 films on silicon and TiO2 using XRR and FESEM cross-sections. ERDA results on as-deposited and annealed Li2CO3 on silicon. Calculations on metal ratios based on film thicknesses and densities. X-ray diffractograms of LiTaO3, Li2TiO3 and LiNbO3 films with different Li2CO3 amounts. High-temperature XRR measurement of Li2CO3/Nb2O5. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * e-mail [email protected], tel. +358-2941 50225

Author Contributions



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