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PAPER
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Atomic layer deposition of lithium nitride and carbonate using lithium
silylamide
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Published on 14 June 2012 on http://pubs.rsc.org | doi:10.1039/C2RA20731A
Erik Østreng,* Ponniah Vajeeston, Ola Nilsen and Helmer Fjellvåg
Received 20th April 2012, Accepted 3rd May 2012
DOI: 10.1039/c2ra20731a
Lithium silylamide, LiN(SiMe3)2, has been explored as precursor for the successful deposition of thin
films of lithium nitride, Li3N, and of lithium carbonate, Li2CO3, by atomic layer deposition.
Deposition of Li2CO3 has been used as a tool in the method development as the compound is stable in
air, contrary to Li3N. Self limiting growth was demonstrated for both Li3N and Li2CO3. The
crystalline state of Li3N depends on the deposition conditions, and varies from amorphous to a phase
mixture of a-Li3N and b-Li3N. The growth rate of Li3N is 0.95 Å cycle21. The Li2CO3 is well
crystalline and highly oriented with (002) parallel to the substrate as deposited, and has, according to
XPS, a low content of silicon at deposition temperatures between 89 and 332 uC. The growth rate of
Li2CO3 is 0.35 Å cycle21. A geometrical model has been applied to rationalise the observed growth
rates. This is the first example of deposition of nitrides using silylamides, and the first route towards
lithium nitride by ALD.
Introduction
Lithium-containing functional materials have a wide range of
applications. Lithium enters into ferroelectric materials such as
in LiNbO3 and as a dopant in ZnO. However, the most intense
field of current developments relates to lithium-ion batteries.
Electrode and electrolyte materials containing lithium span
several material classes; phosphates like LiFePO4 and ‘‘LiPON’’,
carbonates Li2CO3, oxides LiCoO2 and (Li1 2 xLax)TiO3, and
nitrides like Li3N and Li1 2 xFexN.1–6 The present work on thin
films focuses on two such classes; deposition of carbonates,
which typically occur in SEI (solid electrolyte interface) layers,
and depositions of nitrides as electrolyte or electrode materials.
Lithium nitride (Li3N) has further been investigated in relation
to imide-/amide-based hydrogen storage materials7 and is a very
good lithium ion conductor,2 however, its breakdown voltage is
too low for practical use in batteries in its pure state.8
The growth of lithium containing thin films by ALD was
demonstrated by Putkonen et al.,9 followed by growth of ion
conducting lithium lanthanum titanate and lithium aluminium
oxide by Aaltonen.6,10 Recently, deposition of lithium silicate
has been reported by Hämäläinen et al.11 Lithium forms
monovalent cations, and as pointed out in Ref. 9 this gives rise
to challenges with respect to achieving self-limiting growth as
required for ALD-processes.
The currently explored precursor, LiN(SiMe3)2, adopts a
trimeric structure12 in the solid state, however, dimers exist in the
Centre for Materials Science and Nanotechnology, Department of
Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo,
Norway. E-mail: erik.ostreng@smn.uio.no; Fax: +47 2285 5565;
Tel: +47 2285 5558
This journal is ß The Royal Society of Chemistry 2012
gas phase according to gas electron diffraction.13 The hygroscopic nature of Li2O and its tendency to react with CO2,
represents a major challenge and makes deposition of pure Li2O
virtually impossible from a lithium precursor and water or
ozone. The range of precursors investigated for lithium ALD is
at this moment still limited. The need to develop novel material
chemistries for use in improved lithium-ion batteries, calls for
efforts to investigate new precursor systems.
Several nitrides of different metallic elements have previously
been deposited by ALD using different classes of precursors.
Examples are TiN, TaN, MoN and NbN which are deposited
from halides,14 whereas organometallic and metal–organic
precursors like alkyls, amines and amides are reported for
deposition of AlN,15 TiN,16 TaN,17 Hf3N4,18 Zr3N418 and
Mo2N.19
The current precursor, lithium silylamide, is an amide where
the functional groups are silyl groups and the carbon is replaced
by a metal, in this case lithium. Silylamide complexes are well
known as reagents in organic and metal–organic chemistry. Such
molecules have already been used in ALD, for example
homoleptic silylamides of bismuth,20 lanthanum21 and praseodymium22 have been used for growth of BiOx, Sr–Bi–Ta–O and
Bi4Ti3O12, La2O3 and LaAlO3, and PrOx respectively.
Bi[N(SiMe)3]3 was used at 190 uC, but the deposition of binary
BiOx was not sufficiently reproducible. La[N(SiMe3)2]3 and H2O
has been used to deposit La2O3 in the range 150 to 250 uC, with
the incorporation of 3.5 to 8 at% of silicon. Also the
Pr[N(SiMe3)2]3 precursor results in 4–12 at% silicon impurities
in the temperature range 200–300 uC. These studies also report
high contents of hydrogen, which decrease with increasing
deposition temperature.
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Heteroleptic silylamides like dichlorobis[(bis(trimethylsilyl)]
amido hafnium and zirconium have been studied for the
deposition of hafnium silicate23,24 and zirconium oxide25 in the
temperature ranges 150–400 uC and 150–350 uC, respectively.
Also these studies report silicon impurities that increase in
amount with increasing deposition temperature. Nam et al.
reports silicon contents as high as 30 at% when reacting
HfCl2[N(SiMe3)2]2 with water.23 Common for all these prior
studies is a high growth rate for the silylamides that decreases
with temperature. To our knowledge, no prior studies have
focused on the deposition of nitrides using silylamides.
In the present contribution we demonstrate the deposition of
Li3N and Li2CO3 from the silylamide, LiN(SiMe3)2. This
represents furthermore the first deposition of lithium nitride by
ALD and is the first example of the application of silylamide
precursors for nitride growth with ALD. In this respect, the
work may open up new possibilities within ALD precursor
chemistry, both generally for the deposition of nitrides and for
the nitrogen doping of lithium compounds such as LiPON.
As Li3N is air and moisture sensitive we use Li2CO3 as a model
system to study the chemistry of this precursor and determine the
temperature window and decomposition temperature of the
precursor. We will therefore first describe the growth of Li2CO3
before elaborating on the growth of Li3N.
software package. The total uncertainty of XRR is estimated to
be less than 1%.
Surface topography was studied for selected samples with
atomic force microscopy (AFM) using a Park Instruments XE70 and analyzed using the XEI software.
XPS spectra were collected with a Kratos Axis UltraDLD
instrument using monochromatic Al Ka X-ray radiation. The
resolution was 0.54 eV as determined by the full width at half
maximum of the Ag 3d5/2 peak. Low energy electrons were used
to compensate for surface charging on the Li2CO3 reference
sample. Energy referencing is based on the C 1s peak of
adventitious carbon set to 285.0 eV binding energy (BE). Peak
fitting was performed using Voight functions after subtraction of
a Shirley type background in CASA XPS. Instrument manufacturer’s sensitivity factors were employed for quantification.
TGA experiments were performed using a Perkin Elmer TGA
7 at a heating rate of 2 uC min21 in N2. Thermal decomposition
studies of the precursor were done using the equipment described
by Nilsen et al.26 The precursor was heated inside 8 mm sealed
quartz tubes at 115 uC for five days.
Raman spectroscopy was preformed with a Spectra-Physics
Millennia Pro 12sJS Nd:YVO4 solid state laser using 532 nm
wavelength operating at 200 mW.
Computational details
Experimental
Thin films were deposited in a F-120 Sat ALD-reactor (ASM
Microchemistry Ltd.) using LiN(SiMe3)2 (Aldrich 98%) Mo(CO)6
(Aldrich 98%), deionised H2O, CO2 (95%, AGA) and NH3 (Linde,
anhydrous 99.999%) as precursors. LiN(SiMe3)2 was sublimated
inside the reactor at 75 uC.
Nitrogen was used as a carrier gas in all experiments and
supplied at 500 cm3 min21. Nitrogen was generated with a
Schmidlin UHPN3001 N2 purifier that provides better than
99.999% N2 + Ar in the carrier gas. The carrier gas was further
dried by P2O5 and purified for remains of O2 by a Mykrolis gas
purifier.
Thin films were deposited on soda-lime glass, polished
titanium plates and 2.5 6 2.5 cm2 single crystal Si(100) wafers
polished for epitaxial growth, with native oxide. The substrate is
considered to have insignificant roughness compared to the
samples deposited. The soda-lime glass substrates were cleaned
with ethanol. The single crystals were blown dry with pressurized
air, and otherwise used as supplied.
Spectroscopic ellipsometry data were collected with a
Woollam Alpha SE ellipsometer between 380 and 900 nm and
analyzed by fitting a Cauchy model to the whole dataset.
Ellipsometry was used to determine the refractive index and
thickness of all Li2CO3 samples with an estimated uncertainty of
below 1%.
Characterization by X-ray diffraction (XRD) in h-2h-mode
was performed with a Bruker AXS D8 powder diffractometer
equipped with a Ge(111) monochromator providing Cu-Ka1
radiation and using a LynxEye detector. X-ray reflectometry
(XRR) and v-scans was performed using a Bruker AXS D8
diffractometer with a thin film stage and an asymmetric double
bounce Ge(220) monochromator and 0.2 mm slits to provide CuKa1 radiation. The XRR-data were fitted using the GENX
6316 | RSC Adv., 2012, 2, 6315–6322
The first-principles calculation was performed based on the
density functional theory and the pseudopotential methods, which
were implemented in the CASTEP code.27 Ultrasoft pseudopotentials were employed to describe the electron-ion interactions,
and the plane-wave cut-off energy was 600 eV. The exchange and
correlation terms were described with generalized gradient
approximations in the scheme of Perdew–Burke–Ernzerhof.28
The geometric optimization of the unit cell was carried out with
the BFGS minimization algorithm provided in this code. For each
phase, the lattice parameters and atomic positions were fully
optimized. The k-points were generated using the Monkhorst–
Pack method with a grid size of 10 6 10 6 4 and 10 6 10 6 8 for
the a- and b-phase respectively, structural optimization. Iterative
relaxation of atomic positions was stopped when the change in
total energy between successive steps was less than 1 meV cell21.
With this criterion, the forces generally acting on the atoms were
found to be less than 1023 eV Å21.
Density functional perturbation theory (DFPT)29 was used for
the Raman calculations. For the Raman calculation we have
used norm-conserving pseudopotentials with 850 eV energy cutoff for all atoms together with a 15 6 15 6 12, mesh of k points,
with the energy conversion threshold of 0.01 meV atom21,
maximum displacement of 0.001 Å and maximum force of
0.03 eV Å21, yielding a high accuracy for the energy and atomic
displacements. For Li and N atoms the valence states were
modelled using the 2s1 and 2s2, 2p3 electrons, respectively. In
general, in most of the cases the calculated and observed Raman
data vary within 5%.30,31
Results
The LiN(SiMe3)2 compound, being orange to yellow-brown and
sticky, reacts slowly with air and moisture. The compound must
nevertheless be stored and handled under inert conditions to
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assure sufficient reproducibility of experiments. Fresh precursor
was therefore transferred directly from a glovebox prior to each
experiment.
The precursor was studied by TGA in order to determine a
suitable sublimation temperature of 75 uC, at which the residue
was about 1 wt%. The precursor decomposition test resulted in a
white ring of decomposed precursor in the quartz tube at 375 uC
suggesting a potential maximum in the ALD-window around
375 uC, which is consistent with the prior art.21,22
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Reactions of LiN(SiMe3)2 with H2O
The initial experiments using alternating pulses of LiN(SiMe3)2
and H2O resulted in films with rather uncontrolled growth and
large gradients. These films were in addition highly reactive
towards air which made characterization extremely difficult. The
uncontrolled growth is likely caused by a reservoir effect where
the bulk of the film absorbs water providing a large supply of
hydroxide groups in addition to the self limiting surface reaction.
Any LiOH thereby formed in the bulk of the film may react with
the subsequent pulses of LiN(SiMe3)2 leading to a growth rather
limited by the thickness of the deposited film. This finding is
also supported by earlier results of Aaltonen.10 In order to
circumvent the problem, a pulse of CO2 was introduced after
each water pulse in order to form the stable and non-hygroscopic
Li2CO3. This allowed studies of the temperature window and
pulse parameters for the selected precursor in a simpler way than
directly studying the Li3N.
The pulse and purge parameters were optimized at a
deposition temperature of 186 uC, as shown in Fig. 1, resulting
in an LiN(SiMe3)2 pulse of 4 s and an H2O pulse of 0.25 s in
order to obtain surface limited growth.
It was found necessary to use 7.5 s CO2 pulses to provide
samples that were stable in air. Samples made using 2.5 and 5 s
CO2 pulses turned milky white over the course of a few days,
probably due to the reaction of unreacted Li2O or LiOH in the
film with ambient CO2.
The suggested wide ALD window from the thermal decomposition experiments, see above, was indeed observed experimentally in Fig. 2. Samples were deposited using 2000 cycles of a
Fig. 1 Pulse parameters for LiN(SiMe3)2, H2O and CO2 versus growth
per cycle (GPC) as measured by ellipsometry for films deposited using
2000 cycles at 186 uC. The other pulse and purge parameters were kept
constant during the screening are: 4 s Li—1 s purge—1 s H2O—1 s
purge—7.5 s CO2—1 s purge. Non-uniformities were measured to be
below 8%.
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Fig. 2 Ellipsometric measurements of the growth rate (GPC) and index
of refraction as a function of deposition temperature using pulse
parameters of a 4 s LiN(SiMe3)2 pulse, a 0.5 s H2O pulse and a 7.5 s
CO2 pulse, and 1 s purge. The error bars represent the variation over the
length of the chamber, ca. 10 cm.
4 s Li pulse, a 0.5 s water pulse and a 7.5 s CO2 pulse at
temperatures between 89 and 429 uC. Visually uniform films with
low silicon contents were achieved up to 380 uC, however nonuniformity was measured as high as 31% at 89 uC, but was below
8% in most cases. Fig. 1 and 2 show the average thickness over a
10 cm length and the error bar is the non-uniformity. At 429 uC
the resulting film was black, showed gradients and the extraction
of growth rate data was cumbersome. XPS analyses were
undertaken in order to clarify the impurity level of silicon as a
function of deposition temperature. 70 nm thick films of Li2CO3
were obtained on polished titanium substrates after 2000
deposition cycles, Fig. 3. Previous works indicate that thermal
decomposition of the precursor at higher temperatures will result
in increased silicon content.21,22 The present analysis shows that
the films deposited at temperatures up to 380 uC contain between
0.04 and 0.65 at% silicon impurities with no clear pattern in the
variation with deposition temperature. However, the sample
made at 428 uC contains as much as 5.8 at% silicon. The carbon
and oxygen levels in samples deposited between 89 and 380 uC
Fig. 3 Analyzed composition by XPS versus deposition temperature for
films deposited on titanium substrates after 2000 cycles using pulse
parameters of a 4 s LiN(SiMe3)2 pulse, a 0.5 s H2O pulse and a 7.5 s CO2
pulse, and 1 s purge.
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are consistent with the lithium carbonate powder used as
reference. The good correspondence of the lithium and oxygen
contents between the reference and the deposited films indicates
a low hydrogen content in the film. As the main possibility of
hydrogen incorporation will be as hydroxide or bicarbonate the
incorporation of hydrogen in the film will then be accompanied
by an increase in oxygen relative to lithium. The formation of
Li2CO3 films is further supported by the density as measured by
XRR (not shown) of 2.01 g cm23 compared to a theoretical
density32 of 2.10 g cm23. It was not possible to extract the
thickness of the deposited films from the XRR due to the high
roughness of the samples.
The crystallinity of the films deposited during 2000 cycles on
Si(100) at different temperatures was characterized with XRD,
Fig. 4. As expected, the crystallinity increases with increasing
deposition temperature. However, surprisingly the samples
deposited at 380 uC turned out be X-ray amorphous, but no
apparent explanation could be found. All crystalline samples
show only the (002) of the Li2CO3 phase (zabuyelite) proving
that this process yields an oriented film on silicon. Rocking curve
measurements (v-scan) of (002) for the sample deposited at
332 uC using 2000 cycles show a FWHM (full width at half
maximum) of 2.05u, supporting the claim of a highly oriented
growth.
Analysis of the topography by AFM shows that the
morphology varies notably with deposition temperature,
Fig. 5. The sample deposited at 138 uC is X-ray amorphous
even though AFM analysis shows structures that resemble
crystallites with dimensions of some 100 nm. At 331 uC larger
grains are formed with sizes up to 1 mm.
Depositions of Li–N
Deposition of Li3N was achieved on soda lime glass, silicon and
titanium substrates by alternating pulses of LiN(SiMe3)2 and
NH3. The deposition was facilitated by first depositing a 5 nm
Fig. 4 XRD data versus deposition temperatures for films deposited
using 2000 cycles and pulse parameters of a 4 s LiN(SiMe3)2 pulse, a 0.5 s
H2O pulse and a 7.5 s CO2 pulse, and 1 s purge. The sharp peak at 33u is
from the silicon substrate whereas the peak at 31.65u is identified as (002)
from Li2CO3, zabuyelite; the bars in the lower panel indicate the expected
diffraction from a (randomized) powder sample.
6318 | RSC Adv., 2012, 2, 6315–6322
Fig. 5 AFM images of Li2CO3 samples deposited on silicon(100) at
(left) 138 uC and (right) 331 uC using 2000 cycles. The RMS roughness is
measured to 12 and 48 nm respectively. The pulse and purge parameters
are: 4 s Li—1 s purge—1 s H2O—1s purge—7.5 s CO2—1 s purge.
adhesion layer of MoNx prior to the Li–N growth. The final
Li3N product was capped with a 20 nm layer of MoNx in order
to prevent reactions with the ambient air and to allow ex-situ
characterization. Initial studies proved that the Li3N films were
by far more reactive to ambient air than lithium oxide films. This
sandwiching of the Li3N film between two films of a denser
material also amplifies its signal in the XRR analysis, as the
reflectance comes from the difference in electronic densities
between the layers, thus making the analysis easier. Depositions
without the adhesion layer often resulted in powder-like
depositions rather than a homogeneous film, regardless of the
pulse and purge parameters; however successful depositions were
carried out on several different substrates without an adhesion
layer. With an adhesion layer, a continuous layer was always
formed.
The pulse parameters were optimized at 167 uC using
200 cycles of a 5 s LiN(SiMe3)2 pulse and a 5 s NH3 pulse,
modifying one parameter at a time (see Fig. 6 and 7). The
ammonia pulse could be as low as 1 s, however, then yielding
products with reduced uniformity. For a 2.5 s ammonia pulse,
the uniformity was optimal throughout the whole reactor
chamber. For longer ammonia pulses, a decrease in density
was observed. A satisfactory explanation is still lacking. The
shorter LiN(SiMe3)2 pulse required for growth of Li3N suggests
that the reaction between LiN(SiMe3)2 and the carbonate surface
is quite slow compared to the reaction between LiN(SiMe3)2 and
Fig. 6 Growth rate and density of Li3N measured by XRR as function
of LiN(SiMe3)2 pulse length. Samples are deposited at 167 uC using
200 cycles and a 5 s NH3 pulse and 1 s purge.
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Fig. 7 Growth rate and density of Li3N measured by XRR as function
of NH3 pulse length. Samples are deposited at 167 uC using 200 cycles
and a 5 s LiN(SiMe3)2 pulse and 1 s purge.
an –NHx-terminated surface, as the pulse required for self
limited growth of Li3N can be as short as 1 s.
When depositing LiN(SiMe3)2 and NH3 using 100, 200 and
500 cycles, a linear relationship between growth rate and
thickness of the Li–N layers were found, see Fig. 8. Linear
regression gives a growth rate of 0.95 Å cycle21.
Samples deposited at 167 uC were amorphous as deposited. In
order to obtain crystalline films, 95 nm thick films were annealed
at 300 or 600 uC for 1 min. However, these samples turned milky
already after a few minutes exposure to air after the annealing,
probably due to deterioration of the capping layer during
annealing. On the other hand, crystalline samples were obtained
by deposition of 1000 cycles of LiN(SiMe3)2 and NH3 at 332 uC.
X-ray diffraction shows the (110), (111) and (102) reflections of
a-Li3N, in addition to a few weak reflections attributed to
b-Li3N, see Fig. 9. The XRD-analysis also shows Li2CO3 and
LiOH, however, these are believed to result from post deposition
reactions with ambient air. The refined unit cell dimensions of
hexagonal a-Li3N are a = 3.696 Å and c = 3.895 Å, in good
agreement with literature.33
Fig. 8 Thickness as function of number of cycles for Li3N, deposited
using 5 s pulses of LiN(SiMe3)2 and of NH3 at 167 uC. Solid line
corresponds to a growth rate of 0.95 Å cycle21 according to linear
regression.
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Fig. 9 XRD pattern of a sample deposited with 1000 cycles at 332 uC,
using a 5 s LiN(SiMe3)2 pulse, a 5 s NH3 pulse and 1 s purges. The
pattern contains reflections identified as a-Li3N ( ), b-Li3N ( ), LiOH
( ), and Li2CO3 (+). The inset shows the (222) and (204) reflections from
a-Li3N.
N
Raman spectroscopy was used as an additional tool for
proving the chemical state of the amorphous samples of lithium
nitride, Fig. 10. For amorphous samples, the Raman spectra
confirmed the presence of both a- and b-Li3N, and the peak at
ca. 760 cm21 is attributed to a LiSi2N3 phase, however this
cannot be confirmed by XRD. The theoretically calculated
modes are all within a Raman shift error of ¡5%, in line with
expectations as described in the experimental section. The
relative intensities of the calculated modes concur well with
observations.
The silicon content of a sample of Li3N deposited at 168 uC on
MoNx-buffered titanium was analyzed with XPS. The sample
was deposited without a capping layer and allowed to oxidize
before the analysis to avoid effects from sputtering through the
Fig. 10 Raman spectra recorded for 100 nm thick, X-ray amorphous
sample deposited on SiO2 at 167 uC using 1000 cycles of a 5 s
LiN(SiMe3)2 pulse, a 5 s NH3 pulse and 1 s purges. Black line is measured
data, red, blue and pink lines are theoretically modelled spectra of a-Li3N
and b-Li3N, and LiN3Si2 respectively.
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capping layer. The silicon content was found to be ca. 6 atomic
percent when assuming a film composition of Li3N.
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Discussion
The differences in growth rate of Li2CO3 as a function of
temperature, as well the difference in growth rate between
Li2CO3 and Li3N, are considered in view of models by
Ylilammi.34 Obviously, the surface chemistry is rather different
between these cases. The carbonate surface is quite unreactive
while the |–NHx surface is more reactive towards LiN(SiMe3)2
due to its ability to react with protons at the surface. To explore
the surface chemistry we calculate the theoretical growth rate as:
np ~
adsorbed precursors
unit cells|pulse
nUC ~np
#Li per precursor
#Li per unit cell
GPC~nUC d
where np is the surface density of precursor molecules per unit
cell and per pulse, nUC number of lithium atoms deposited per
unit cell per pulse, GPC the growth rate per cycle and d the unit
cell dimension in the direction of the growth.
We here assume that the precursor ligand (–N(SiMe3)2) can be
approximated as a sphere with 6 Å diameter in correspondence
with the report by Fjeldberg,13 and that the reactive site is at the
centre of the anion. In the case of Li2CO3 growth, we assume
that the precursor is physisorbed on the surface with both
ligands intact and that the Li2CO3 film grows along the 001
direction leaving a rough surface. This indicates that the growth
is limited by surfaces at some angle to this growth direction,
having growth rates notably lower than the 001 direction. We are
unable to identify the specific terminating surfaces, and limit our
theoretical considerations to the 001 surface which should
represent the upper limit of what is observed along this direction.
We identify two possibilities regarding the arrangement of
precursor molecules on the surface, being either free to rotate
and spanning a circular area (case I), or being constrained to an
area that is better represented by an ellipse (II). In the case of
Li3N growth we assume that the densely packed (111)-surface is
exposed and furthermore that the precursor loses one ligand and
can be represented by a 6 Å diameter sphere, and bonds to either
every second (III) or every third nitrogen atom on the surface
(IV), see Fig. 11. The calculated surface densities and growth
rates resulting from these considerations are given in Table 1.
By comparing calculated and experimental growth rates in
Table 1, Case I corresponds well with the Li2CO3 growth rate at
331 uC, Case II corresponds quite well with the Li2CO3 growth
rate at 89 uC. The difference in growth rates between these
temperatures may indicate that elevated temperature provides
enough thermal energy to allow the precursor molecules to rotate
freely, while they are locked in at lower temperatures. Case III
corresponds well with the experimental growth rate of Li3N at
167 uC. This may indicate that the reason for the high growth
rate of Li3N is caused by the loss of one ligand of the precursor
when reacting with the surface. The reduction in growth rate of
Li2CO3 at high temperatures is probably due to rotation of
ligands, see Fig. 2.
The importance of a buffer layer underneath Li3N was
unexpected. In our case, layers of Mo-nitride turned out to be
successful. Previously, it is known that noble metals benefit from
similar adhesion layers, both metals35 and Al2O336 can act as
adhesion or nucleation layers. Aaltonen et al.35 reports that
Fig. 11 Schematic illustrations of cases I–IV, only the film anions are drawn, see text. Red dots symbolize oxygen atoms, blue dots nitrogen and the
black circle outlines the adsorbed precursor on the surface.
6320 | RSC Adv., 2012, 2, 6315–6322
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Table 1 Calculated and experimental growth rates based on the cases
I–IV illustrated in Fig. 11. The experimental data is taken from samples
deposited using 2000 cycles of Li2CO3 and 200 cycles of Li3N
Case
Material
Temp. (uC)
np
nUC
GPC (Å cycle21)
I
II
III
IV
Exp
Exp
Exp
Li2CO3
Li2CO3
Li3N
Li3N
Li2CO3
Li2CO3
Li3N
—
—
—
—
331 uC
89 uC
167 uC
0.16
0.29
0.5
0.33
0.04
0.073
0.33
0.22
0.23
0.42
1.1
0.73
0.23
0.41
1.1
platinum films do not grow at low temperature unless there is a
nucleation layer. It may be that the very reactivity of the Li3N
requires a barrier layer to prevent reactions with the substrate.
On the other hand, it may be equally likely that the chemistry for
depositing Li3N requires a nitride surface in order to form a
metal-nitrogen-metal bond. A full explanation requires experiments beyond the current data and will be highly relevant for the
deposition of nitrides from silylamide complexes in general.
There are few examples of deposition of orientated carbonates
using ALD in the literature. One such examples is given by
Nilsen et al.37 who deposited CaCO3 by using Ca(thd)2, ozone
and CO2. The calcite deposited was oriented in either with the
(104) plane normal to the growth direction at 350 uC or with
the (006) plane normal to the growth direction at 250 uC. For the
latter orientation, the CO322-groups are arranged parallel to the
growth direction. In our experiments, which show a strong (001)
orientation, the CO322 groups are also arranged in a plane
parallel to growth direction.
Silylamides are well known precursors in atomic layer
deposition of oxides and silicates. They have, however, not
previously been used as precursors in the growth of nitrides by
ALD. We have here proven deposition of Li3N and anticipate
that the process can be extended to nitrides more generally. It
must be a necessary, but not sufficient criterion, for thermal
ALD of any compound that the ligand of the cation precursor is
more basic than the anion precursor and thus able to strip the
anion precursor of hydrogen. For deposition of nitrides by ALD
using ammonia as nitrogen source, the ligand should therefore be
more basic than ammonia and thus enable the ligand to strip
ammonia of hydrogen. Considering that LiN(SiMe3)2 is a very
strong non-nucleophilic base,38,39 silylamides should be well
suited as precursors for deposition of other nitrides. Since
complexes with sufficient thermal stability are known for both
transition metals40 and rare earth metals,41 this may open up a
new field within low temperature growth of nitrides by ALD.
Conclusion
The precursor Li-silylamine has been explored and used to
deposit Li2CO3 and Li3N by ALD. The potential for deposition
of Li2O and LiOH is also considered. Self limiting growth and
deposition of amorphous and crystalline samples are proved
for Li2CO3 and Li3N. The obtained Li2CO3 is almost free of
silicon, crystalline above 235 uC and oriented and grows with the
(001) plane normal to the growth direction at temperatures
between 235 and 332 uC. The growth rate of Li2CO3 is about
0.35 Å cycle21. In the case of deposition of Li3N, it proved
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beneficial to deposit an adhesion layer and a capping layer of
MoNx in order to obtain uniform film growth and adhesion to
the surface, and to prevent sample oxidation in ambient air.
Crystalline samples of Li3N were deposited at 332 uC and
contained both a-Li3N and b-Li3N, and some impurities
considered as mainly Li2CO3 and LiOH. The presence of the
two Li3N-phases is supported by Raman-spectroscopy. The
growth rate of Li3N is 0.95 Å cycle21. The difference in growth
rates between Li2CO3 and Li3N, as well temperature dependence
of the growth rate, has been discussed in connection to packing
and chemistry of the precursor molecules at the surface.
Acknowledgements
The authors thank Martin F. Sunding for performing XPS
measurement and discussions. Dr Niels Højmark Andersen is
thanked for experimental Raman data.
The research leading to these results has received funding from
the European Union Seventh Framework Programme ([FP7/
2007-2013]) under grant agreement nu 227541.
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