Lithium manganese oxide films fabricated by electron beam directed
vapor deposition
S. W. Jina兲 and H. N. G. Wadley
Department of Materials Science and Engineering, School of Engineering and Applied Science,
University of Virginia, Charlottesville, Virginia
共Received 17 August 2007; accepted 19 November 2007; published 17 December 2007兲
Lithium manganese oxide thin films have been grown using a gas jet based, electron beam directed
vapor deposition technique. The deposition rate could be controlled by the electron beam power as
well as the gas jet density and speed. This enabled films to be grown at deposition rates up to
16 nm/ s which is a significantly higher deposition rate than that reported for the growth of this
material by sputtering and other vapor deposition techniques. The lithium manganese oxide films
grown by this approach were slightly manganese deficient with a composition Li1+xMn2−yO4, where
0.08⬍ x ⬍ 0.125 and y ⬃ 0.2. After annealing in air at 700 ° C, thin films grown with a low jet speed
had a cubic spinel structure and were composed of very small size grains. The small grain size
resulted from the vapor phase formation of clusters and resulted in a nanocrystalline random film
texture. These films were highly porous with a spongelike interconnected pore network. The use of
a higher jet speed and lower growth pressure resulted in less vapor phase clustering. The films
contained small, uniformly distributed pores and exhibited a significant 具111典 texture. These
annealed films appeared well suited for use as the cathode layers in thin film Li/ Li-ion batteries.
© 2008 American Vacuum Society. 关DOI: 10.1116/1.2823488兴
I. INTRODUCTION
Thin film solid-state lithium batteries are widely used in
medical devices to provide power for microelectromechanical systems and as on-chip power sources because of their
light weight, small size, and high power storage density.1,2
Lithium–transition metal oxides such as LiCoO2 and
LiMn2O4 are widely used for the cathode material.3–5
LiCoO2 has a layered 共R-3m兲 structure, which facilitates
lithium insertion and extraction during battery operation.3 It
is widely used in commercial batteries in part because of its
high specific charge storage capacity 共⬃130 A h / kg兲 and excellent rechargeability 共⬎1000 cycles兲.3–5 However, LiCoO2
is costly, and it has significant toxicity issues.6 LiMn2O4 is a
candidate alternative cathode material for high energy density battery applications.7,8
Lithium–transition metal oxide films can be fabricated by
either electron beam evaporation,9–11 sputtering,11–16 or by
pulsed laser deposition.17 For example, reactive electron
beam evaporation has been used to synthesize LiMn2O4
films with a small grain size and good electrochemical
performance.9 Wang et al. have used magnetron sputtering to
grow Li4Ti5O12 thin films, which are an isostructure to
LiMn2O4.18 They have produced films that can be either relatively dense or composed of islandlike grains with interconnected grain boundary pores. They have examined the electrochemical properties and found that highly textured, porous
films with interconnected grain boundary pores exhibited superior electrode performance. The typical deposition rate of
the sputtering techniques used to fabricate these films is usually less than ⬃0.2 nm/ s.15
Author to whom correspondence should be addressed. Phone: 共434兲 8063277. Electronic mail: [email protected]
a兲
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J. Vac. Sci. Technol. A 26„1…, Jan/Feb 2008
An electron beam directed vapor deposition 共EB-DVD兲
technique has recently been developed for synthesizing binary metal oxides with controlled compositions and pore
fraction.19–21 In this approach, an electron beam is used to
evaporate a source material located in a water-cooled crucible positioned in the throat of a supersonic gas jet forming
nozzle. This jet entrains and transports the vapor to a substrate. The gas jet speed is determined by the pressure the
ratio of the pressure upstream and downstream of the nozzle
opening, and by the ratio of the specific heats of the gas used
to form the jet.22 Inert gases, such as helium or argon, sometimes doped with small amount of oxygen or other reactive
gases, are typically used to form the jet. Changing the upstream to downstream pressure ratio modifies the degree of
collimation of the vapor flux and the fraction of vapor deposited on a substrate.20 The technique has been used to
grow thick yttrium stabilized zirconia coatings at very high
deposition rates in excess of ⬃10 ␮m / min for use as thermal
barriers.19–21 These studies have indicated that film texture
and pore volume fraction are both sensitive functions of the
jet speed and the deposition chamber pressure.19,20
Here, we explore the synthesis of lithium manganese oxide films using this EB-DVD approach. The effects of gas jet
characteristics upon the crystal structure, texture, grain size,
and pore morphology of the films are all investigated. It is
found that nanoporous lithium manganese oxide films can be
grown at very high deposition rates with microstructures that
appear well suited for thin film battery applications. A companion article explores the use of the same approach to deposit lithium phosphorous oxynitride electrolytes used in
then film battery systems.
0734-2101/2008/26„1…/114/9/$23.00
©2008 American Vacuum Society
114
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S. W. Jin and H. N. G. Wadley: Lithium manganese oxide films
FIG. 1. Schematic illustration of EB-DVD technique. Pd and Pu indicate the
chamber 共downstream兲 and upstream nozzle opening pressure, respectively.
The pressure ratio, Pu / Pd, and ratio of specific heats of the gas determine
the speed of the gas jet used to inhibit lateral spreading of the evaporant
flux.
II. EXPERIMENT AND SIMULATION
A. Synthesis approach
A schematic illustration of the EB-DVD system used here
is shown in Fig. 1. A transonic rarefied gas jet is first created
by expanding helium gas through a 3 cm diameter nozzle.
The jet speed at the nozzle opening, U, depends on the ratio
of the specific heats of the gas, ␥ 共5 / 3 for He兲, the absolute
gas temperature T, and the Mach number M of the jet,22
U = M 冑␥RsT,
共1兲
where Rs is the specific gas constant 共2077 J / kg K for helium兲. The Mach number for the jet also depends on the ratio
of specific heats of the gas, ␥, and the upstream and downstream pressures. It can be found from the expression
冉
␥−1 2
Pu
M
= 1+
2
Pd
冊
␥/共␥−1兲
,
共2兲
where Pu / Pd is the ratio of pressures up and downstream of
the nozzle opening.23
The source material was placed in a 2.3 cm diameter
water-cooled crucible positioned in the nozzle throat to create an annular jet forming aperture. A 70 kV electron beam
was then used to evaporate 1.3 cm diameter LiMn2O4 source
ingots supplied by TCI Ceramics, Inc. 共Maryland兲. The
source to substrate distance was held constant for these experiments at 15 cm. Trials conducted before the film growth
campaign that was attempted indicated unstable evaporation
for electron beam power densities above 10 mA/ cm2. All the
films were therefore grown using a slightly reduced current
JVST A - Vacuum, Surfaces, and Films
115
density of 6.3 mA/ cm2. The resulting evaporant was entrained in the He supersonic jet and deposited on 共100兲 Si
substrates covered by a 30– 60 nm native oxide layer. The
substrate temperature during the deposition rose to around
70 ° C due to radiative heating from the evaporation source
and condensation of the vapor on the substrate.
Lithium manganese oxide films of various thicknesses
were grown under systematically varied gas flow conditions
to observe the effects of pressure ratio and gas density upon
film morphology. To investigate pressure ratio effects, the
pressure within the chamber 共Pd兲 was fixed as 13 Pa, and the
upstream pressures 共Pu兲 was adjusted by varying the capacity of the pumping systems.23 Results are shown for two
pressure ratios of 3.0 and 6.0 that were achieved in this way.
Films with different thicknesses were then fabricated by adjusting the deposition time. Results for a second set of films
synthesized using a fixed pressure ratio of 6 and a variable
chamber pressure are also presented to show the effect of the
deposition pressure on the microstructure of the films. Some
of the as-deposited films were also annealed at 700 ° C in air
for an hour to films to observe the stability of the structures.
The surface morphology and void fraction of as-deposited
and heat-treated films were determined from quantitative
analysis of scanning electron microscopy 共SEM兲 images using IMAGEJ image analysis software 共National Institutes of
Health, USA兲.24 The crystal structure and texture were measured using a Scintag x-ray diffractometer 共XRD兲. Lattice
parameters were calculated using a LAPOD software program
共University of Birmingham, UK兲.25 Complete pole figures
were used to determine the film texture and were generated
using a preferred orientation software package 共POPLA兲 共Los
Alamos National Laboratory, USA兲.26 The film’s composition was determined by a combination of Rutherford backscattering spectroscopy 共RBS兲 共Cornell Center for Materials
Research, USA兲, inductively coupled plasma-atomic emission spectroscopy 共ICP-AES兲, and x-ray photoelectron spectroscopy 共XPS兲.
B. Jet flow and vapor transport simulation
Significant changes in the morphology and texture of EBDVD films can occur as the jet flow conditions are varied.
This is consequence of gas phase collision during vapor
transport. As the gas jets momentum is varied by changing
the flow conditions, its effectiveness at entraining vapor depositing onto a substrate will change. Collisions within the
jet plume can also lead to cluster formation, which would
have a significant effect upon the morphology of a thin film.
Insight into this fundamental process can be gained from
directed simulation Monte Carlo 共DSMC兲 and is presented
below of a single metal species 共Cu兲 transport in a helium
flow. Such an approach is difficult to implement for the many
different atomic and molecular species present during deposition of LiMn2O4. However, several of the most important
phenomena are still encountered, and for a simple metal species such as copper, the key gas phase collision parameters
are well known.
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S. W. Jin and H. N. G. Wadley: Lithium manganese oxide films
DSMC is widely used to simulate the motions of particles
in rarefied gas flows.27 The major assumption of the DSMC
approach is that the motion and collisions of particles are
decoupled from each during finite simulation time steps. The
interactions of pairs of particles are then randomly selected
within a cell for collision and collided particles move in free
molecular motion. During each simulation step, such uncoupled collisions and movement of computational particles
through the grids of a simulation space are recorded.
A two-dimensional DSMC code 共ICARUS兲 developed at
Sandia National Laboratory was utilized to model the EBDVD process.27 The vapor atom velocity, mean free path,
energy, and incident flux/angle to the substrate were computed for the conditions used in experiments. The grid geometry incorporated the nozzle opening diameter and geometry,
the source material position, and substrate location used in
the DVD experimental setup.23 The cell size and number of
computational particles were carefully chosen to ensure adequate collision statistics.22
Two different DSMC problems were examined: one with
and one without clustering reactions. The former assumes
elastic collisions between the copper and helium atoms and
does not permit cluster formation. It was used to predict
vapor atom velocity, impact energy, incident angle, and investigate the monoatomic flux incident upon the substrate.
DSMC with clustering reactions were performed to investigate vapor atom clustering during the deposition under low
and high pressure ratio/chamber pressure. The ICARUS software allows several mechanisms of interaction to be
addressed.27 We used an elastic gas reaction, where the reaction is completely kinetic and charge transfer and electron
impact are ignored. Serial reactions were permitted, such as
Cu+ Cu= Cu2, Cu2 + Cu= Cu3 ¯ Cun + Cu= Cun+1. The elastic
collision reaction rate in ICARUS code has the form
k = A exp共−Ea / kBT兲, where k is the reaction rate, A is the
pre-exponential constant, Ea is the activation energy, kB is
the Boltzmann constant, and T is the absolute temperature. A
reaction rate of unity was used here since we are interested
only in comparisons of each set of experimental conditions.
Helium was used as the jet forming gas. The helium gas
flow rates were varied from 0.25⫻ 1024 / m2 s to 0.108
⫻ 1025 / m2 s depending on the upstream pressures. The vapor
inlet flux was fixed at 0.1⫻ 1024 / m2 s. The chamber pressure
was preset to either a pressure of 13 or 3.5 Pa. A variable
hard sphere model was used to calculate the collisional cross
section.27 A detailed report of simulation approach can be
found elsewhere.23
III. EXPERIMENTAL RESULTS
A. Deposition rate measurement
The jet flow conditions and deposition rates of lithium
manganese oxide films are summarized in Table I. All the
films were deposited using the same electron beam power
density 共6.3 mA/ cm2兲. The deposition rate increased rapidly
with both the pressure ratio and chamber pressure. Since the
electron beam current and therefore the source evaporation
J. Vac. Sci. Technol. A, Vol. 26, No. 1, Jan/Feb 2008
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TABLE I. Effects of deposition conditions upon film growth rates.
Experiment
1
2-1
2-2
2-3
Pressure
ratio
Chamber
pressure 共Pa兲
Deposition
rate 共nm/s兲
3
6
6
6
13
13
3.5
8
5.2
16.0
1.4
9.0
rates were similar for each experiment, the deposition rate is
a measure of the efficiency of a vapor entrainment in the gas
jets and its subsequent deposition on a substrate. As shown in
Sec. IV, increasing the gas jet speed and chamber pressure
共the gas jet density兲 increased the momentum of the jet and
improved the efficiency with which the atoms/molecules
evaporated by the source were entrained in the gas jet and
deposited on the substrate. The dependence of the deposition
rate upon the deposition conditions was well modeled by the
simulation approach.
B. Film morphology
1. As-deposited structures
The surface morphologies of as-deposited lithium manganese oxide films are shown in Fig. 2. In the case of the film
grown at pressure ratio of 3, the growth surface is typical of
a columnar film grown under conditions of kinetically constrained surface atom mobility.28 It consisted of primary
growth columns that were ⬃0.1 ␮m in diameter. Primary
intercolumnar pores about 0.1 ␮m in width surrounded the
growth columns. The structure was hierarchical in the sense
that the primary columns were made up of secondary growth
columns with very fine pores separating them. This fractal
surface morphology was generally observed on all of the
as-deposited lithium manganese films regardless of the deposition conditions.
Changing the pressure ratio from 3 to 6 had a significant
effect upon the fractal film morphology. The film surface
grown with a lower pressure ratio consisted of mounded
clusters and it was not possible to resolve a clear column tip
shape. As the pressure ratio was increased, a film surface
composed of more faceted fine column tips was observed.
The effect of chamber pressure 共gas jet density兲 on the
surface morphology of films grown with a pressure ratio of 6
can be seen in Figs. 2共b兲 and 2共c兲. The films were grown
using chamber pressures of 3.5 and 13 Pa. All the films had
a similar thickness of around 250 nm. It can be seen that
increasing the chamber pressure increased the surface breaking porosity but did not otherwise alter the basic surface
morphology except the clear column tip faceting in the film
grown at lower chamber pressure.
Cross-sectional images of fractured, as-deposited films
are shown in Fig. 3. All of the films had a columnar structure
with the growth columns oriented perpendicular to the substrate surface. The lower pressure ratio 共3兲 film, Fig. 3共a兲 had
larger width primary columns 共⬃100 nm in width兲. It was
the spherical tops of these that could be observed as mounds
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S. W. Jin and H. N. G. Wadley: Lithium manganese oxide films
117
FIG. 3. Cross-sectional images of as-deposited EB-DVD lithium manganese
oxide films: 共a兲 pressure ratio= 3, chamber pressure= 13 Pa; 共b兲 pressure
ratio= 6, chamber pressure= 13 Pa; and 共c兲 pressure ratio= 6, chamber
pressure= 3.5 Pa.
FIG. 2. Surface morphology of as-deposited lithium manganese oxide films:
共a兲 pressure ratio of 3, chamber pressure of 13 Pa; 共b兲 pressure ratio of 6,
chamber pressure of 13 Pa; and 共c兲 pressure ratio of 6, chamber pressure of
3.5 Pa.
in the surface SEM images, Fig. 3. The higher pressure ratio
共6兲, Figs. 3共b兲 and 3共c兲, had very fine columns.
2. Heat-treated structures
SEM images of films following heat treatment at 700 ° C
are shown in Fig. 4. The films underwent significant sintering. The secondary growth columns had agglomerated and
almost completely disappeared while the intercolumnar porosity had significantly coarsened.
JVST A - Vacuum, Surfaces, and Films
The film grown with the lower pressure ratio contained a
spongelike interconnected pore network. The growth columns and the interconnected network of intercolumnar pores
were coarser in the film grown with the higher pressure ratio.
As with the as-deposited films, the use of a higher chamber
pressures resulted in larger primary growth columns and
therefore a larger distance between the intercolumnar pores.
Cross-sectional images of the heat-treated films are shown in
Fig. 5. They exhibit analogous trends to those observed on
the outer surface. The film deposited with the lower pressure
ratio had a more porous structure with both intra and intercolumnar porosity. Films grown at higher pressure ratios and
with a higher chamber pressure, Fig. 5共b兲, had well defined
intercolumnar porosity with wide intercolumnar pores extending through the thickness of the film.
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S. W. Jin and H. N. G. Wadley: Lithium manganese oxide films
118
FIG. 5. Cross-sectional images of EB-DVD lithium manganese oxide films
annealed at 700 ° C for an hour: 共a兲 pressure ratio of 3, chamber vacuum
13 Pa; 共b兲 pressure ratio of 6, chamber vacuum of 13 Pa; and 共c兲 pressure
ratio of 6, chamber vacuum of 3.5 Pa.
XRD patterns of the as-deposited films had only broad/
diffuse peaks, which is indicative of a nanocrystalline structure. The film grown at a pressure ratio of 3 showed three of
FIG. 4. Surface morphology of lithium manganese oxide films fabricated
annealed at 700 ° C for an hour in air: 共a兲 pressure ratio of 3, chamber
pressure of 13 Pa; 共b兲 pressure ratio of 6, chamber pressure of 13 Pa; and 共c兲
pressure ratio of 6, chamber pressure of 3.5 Pa.
The measured void area as a function of the film thickness
is plotted in Fig. 6. The void volume fraction generally increased with film thickness. As anticipated from the SEM
images above, the annealed films grown with the lower pressure ratio of 3 yielded the largest void areas while films
deposited using a higher pressure ratio of 6 had a smaller
void area. It is clear that reduction of the chamber pressure or
increase of the pressure ratio results in denser films.
C. Crystal structure measurements
X-ray diffraction data for as-deposited and heat-treated
EB-DVD lithium manganese oxide films are shown in Fig. 7.
J. Vac. Sci. Technol. A, Vol. 26, No. 1, Jan/Feb 2008
FIG. 6. Effect of film thickness upon void volume fraction variations of
heat-treated lithium manganese oxide films.
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S. W. Jin and H. N. G. Wadley: Lithium manganese oxide films
FIG. 7. X-ray diffraction patterns of lithium manganese oxide films grown
with pressure ratios of 3 and 6 and chamber pressure of 13 Pa: 共a兲 as-grown,
pressure ratio of 3; 共b兲 as-grown, pressure ratio of 6; 共c兲 postannealed,
pressure ratio of 3; and 共d兲 postannealed, pressure ratio of 6.
these broad peaks at ⬃2␪ = 18°, 36°, and, 44°. These are
consistent with diffraction from the 共111兲, 共311兲, and 共400兲
planes of a spinel structure.29 In contrast, XRD pattern of the
film grown at a pressure ratio of 6 did not exhibit any of the
spinel structure peaks, but instead had a relatively sharp peak
at a 2␪ of ⬃36°. More thorough analysis of this diffracted
pattern has been performed, and it has been found that the
thin film had a disordered rock salt like structure probably
due to low substrate temperature and the smaller fraction of
clusters in the deposited flux.30
After heat-treatment in air at 700 ° C for an hour, examination of the XRD data indicated that all the films had fully
converted to a cubic spinel structure. The films grown with
the lower pressure ratio of 3, exhibited many peaks, which
matched those of the 共111兲, 共311兲, 共222兲, 共400兲, 共331兲, and
共511兲 planes of a cubic spinel. Films grown using the higher
pressure ratio had only two peaks corresponding to 共111兲 and
共222兲 planes of the cubic spinel structure regardless of the
chamber pressure. Pole figures were constructed to investigate the degree of the preferred orientation of films grown
with the lower and higher pressure ratios, Fig. 8. The former
had an almost random texture.31 Whereas the film fabricated
at the higher pressure ratio exhibited strong 具111典 texture.
The chamber pressure had a little effect upon this strong
具111典 texture when the films were deposited with a higher
pressure ratio.
119
FIG. 8. Pole figures of lithium manganese oxide films annealed 700 ° C in air
for 1 h. 共a兲 Grown at a pressure ratio of 3 and chamber pressure of 13 Pa.
共b兲 Pressure ratio of 6 and chamber pressure of 13 Pa. The texture strengths
of 共a兲 and 共b兲 are 1.07 and 2.96, respectively. A texture strength of unity
indicates random texture 共Ref. 31兲. The low pressure ratio sample had almost random texture while that deposited at higher pressure ratio was much
more textured with a predominance of 共111兲 planes coplanar with the coating surface.
D. Composition estimates
The lattice parameters of EB-DVD lithium manganese
oxide films were measured from XRD data. All of the annealed EB-DVD lithium manganese oxide films had a similar lattice parameter of 8.213 Å 共±0.02兲. This lattice parameter is slightly less than that of stoichiometric LiMn2O4,
8.247 62 Å. It is well known that there is a strong relationship between the lattice parameter and manganese oxidation
state, which is dependent upon composition.7,32 A lattice parameter that is smaller than that of stoichiometric LiMn2O4 is
consistent with an oxygen or lithium rich cubic spinel
structure.33 Compositions of several of the EB-DVD lithium
manganese oxide films were measured using XPS, ICP-AES,
and RBS and all were found to be Li1+xMn2−yO4 共0.8⬍ x
⬍ 0.125 and y ⬃ 0.2兲, consistent with a manganese deficient
spinel structure.
IV. DSMC RESULTS
Table II summarizes the nonreaction results and gives the
estimated vapor flux, incident angle, and adatom energies for
copper. It can be seen that the average vapor atom speed,
density and incident flux 共on the substrate兲 all increased with
TABLE II. Summarized DSMC results for the synthesis conditions used in the experiment. 共The incident flux and
ad-atom energy were obtained at the front of the substrate. The average incident angle is the measurement from
the normal to the substrate.兲
Average
Average
Average
Average
Average
Cu atom
Cu atom
Cu atom
Pressure
Chamber
Cu atom
Cu atom
ratio
pressure 共Pa兲 velocity 共m/s兲 density 共/m3兲 incident flux 共/m2 s兲 incident angle 共°兲 energy 共eV兲
6
6
3
13
3.5
13
JVST A - Vacuum, Surfaces, and Films
991
386
368
1.3⫻ 1019
0.43⫻ 1019
1.0⫻ 1019
1.3⫻ 1021
1.6⫻ 1020
5.1⫻ 1020
36.4
30.9
32.3
0.055
0.048
0.048
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S. W. Jin and H. N. G. Wadley: Lithium manganese oxide films
120
FIG. 9. Top: simulated vapor atom velocities for experimental processing
conditions studied here: 共a兲 pressure
ratio of 6, chamber pressure of 13 Pa;
共b兲 pressure ratio of 3, chamber pressure of 13 Pa; and 共c兲 pressure ratio of
6, chamber pressure of 3.5 Pa. Bottom: simulated vapor atom densities
under experimental processing conditions studied here: 共a兲 pressure ratio of
6, chamber pressure of 13 Pa; 共b兲 pressure ratio of 3, chamber pressure of
13 Pa; and 共c兲 pressure ratio of 6,
chamber pressure of 3.5 Pa.
the pressure ratio and chamber pressure. The incident flux
upon the substrate is correlated to the higher deposition rate.
Figure 9 shows the vapor atom velocity and density during the deposition. It is clear that the higher pressure ratio
and chamber pressure 共pressure ratio ⬃6, and chamber pressure ⬃13 Pa兲 generate faster and more focused vapor stream
to the substrate yielding a higher deposition rate. With the
same pressure ratio of 6, the lower chamber pressure 共3.5 Pa兲
inefficiently focused the vapor atoms due to the lower collision rate between the helium gas jet and vapor atoms. This
also resulted in a lower vapor atom speed. Other deposition
parameters such as the average incident angle 共from the normal to the substrate兲 and adatom energy 共near the substrate兲
were not so sensitive to the deposition conditions.
Figure 10 shows DSMC results when clustering was permitted. The critical factor, which governs the film microstructure, is not the absolute number of clustered atoms, but
the ratio of monomer to clusters to the substrate. Figure 10
clearly demonstrates that a lower pressure ratio and higher
chamber presser produce a much higher fraction of clustered
atoms near the substrate.
very high rate of film growth which was more than two orders of magnitude greater than that reported for the sputter
synthesis of lithium manganese oxide films.15,16 The fraction
of the emitted vapor reaching the substrate could be adjusted
by changing the pressure ratio and chamber pressure with
potentially significant consequences for those interested in
developing efficient, high rate deposition processes for the
growth of lithium manganese oxide thin films.
The effects of changes to the process conditions upon the
film morphology are less easily explained. Increasing the
pressure ratio during film deposition reduced the pore vol-
V. DISCUSSION
During sputtering 共and conventional electron beam deposition兲, the vapor flux angular distribution emitted from the
source is broadly distributed and only a small fraction
reaches the substrate. The use of gas jets to focus the vapor
from an electron beam evaporation source has resulted in a
J. Vac. Sci. Technol. A, Vol. 26, No. 1, Jan/Feb 2008
FIG. 10. Ratio of number of clustered atoms to monomers near the substrate.
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S. W. Jin and H. N. G. Wadley: Lithium manganese oxide films
ume fraction and increased the texture of the as-deposited
films. During physical deposition processes film structures
are usually manipulated by the adatom energy,34 the deposition rate,35 and the adatom incidence angle to the substrate34
which can all be modified to some extent by the chamber
pressure.36 However, we find that the increases in velocity of
the vapor species associated with the use of high chamber
pressures and high pressure ratios 共obtained from a DSMC
analysis; see Table II兲 were too small to account for the
significant changes in film structure. The simulations also
indicate minor changes to the average vapor incident angles
共Table II兲 and so this also cannot be responsible for the film
morphology differences observed as the chamber pressure
and the pressure ratio were varied. Finally, it is well known
that increasing the deposition rate usually produces more porous film morphologies. However, in the current study, the
films produced using the higher deposition rate 共pressure ratio of 6 and chamber pressure of 13 Pa兲 resulted in the
denser film morphology than those grown using the lower
deposition rate 共pressure ratio of 3 and chamber pressure of
13 Pa兲.
The most probable mechanism responsible for the morphological dependence upon pressure ratio is thought to be
vapor clustering during vapor phase transport within the gas
jets. The temperature of the vapor in the EB-DVD jet is low
due to the strong supersonic nozzle expansion, vapor atoms
are highly supersaturated because of the high melt surface
temperature, and there is a very high density of helium atoms
also present to remove the heat of formation of the clusters
that form during gas phase collisions of vapor atoms and
molecules.37 Once formed and stabilized in this way, the
clusters can grow very rapidly. In a study of a related gas jet
based process, Hill showed that the probability of cluster
formation and the average cluster size were both a sensitive
function of the time of flight of atoms to the substrate and the
vapor atom density.38 The DSMC analyses support this assertion and indicate that the use of a high pressure, low pressure ratio jet, and a high vapor atom density results in significant vapor phase clustering.
In the case of clusters formed under EB-DVD conditions,
their kinetic energy is low 共⬍0.5 eV/ atom兲 and the deposition process is analogous to low energy cluster beam deposition 共LECBD兲.39,40 Molecular dynamic simulations of this
process reveal that low energy clusters do not break apart
upon impact on the substrate.41 This results in polycrystalline
growth with clusters arrayed with random orientations.42 The
formation of clusters with a spinel structure during their free
flight to the substrate might also help to explain the appearance of this structure in the films grown under cluster deposition conditions. These observations are also clearly consistent with the grain structure and porosity of films grown
using a low pressure ratio where highly porous films with the
random texture were created.
The strong texture of films grown using a high pressure
ratio and at a low substrate temperature is commonly seen in
physical vapor deposition.43 Under high pressure conditions,
DSMC shows that the flux incident upon the substrate had a
JVST A - Vacuum, Surfaces, and Films
121
higher velocity and a lower fraction of clusters. Recent molecular dynamics simulations indicate that the reassembly of
vapor phase formed clusters on the surface is unlikely to be
significant at the deposition temperatures used here.44 Competitive grain growth by individual atom atom addition to
favored facets then occurs. The triangular facets that terminate the growth column of films with a 具111典 texture appear
to be of a 兵100其 type.45 The very limited kinetic opportunities
for atomic reassembly at the low growth temperature and
high deposition rate may then account for the observation for
the formation of a metastable rock salt structure in the asdeposited films.
As already mentioned, porous films with a larger surface
area are beneficial to electrochemical performance in thin
film batteries because they provide a larger surface area for
insertion and extraction of lithium ions.18,46 The surface morphology of Li4Ti5O12 film by sputtering and postannealing is
almost identical to that of EB-DVD postannealed film grown
at lower pressure ratio, and it had a significantly superior
electrochemical performance than denser films.18 From this
point of view, films, grown at lower pressure ratios, have a
preferred microstructure for electrochemical applications because of their larger void volume fraction. However, the effect of isolated uniformly distributed pores and strong 共111兲
texture in films grown at higher pressure ratio on the electrochemical property is unknown.
VI. SUMMARY
Lithium manganese oxide films were fabricated on Si substrates with high deposition rate 共from 1.3 to⬃ 20 nm/ s兲 using an electron beam directed vapor deposition technique.
The as-deposited films were composed of nanoscopic sized
grains with either a rocksalt during deposition at high pressure ratios or a cubic spinel structure when film growth was
conducted under low pressure ratio conditions. The films deposited at low pressure ratios are believed to have formed by
the deposition of small nanoclusters that are nucleated in the
vapor phase. Lattice parameter and composition measurements indicate that the films were slightly Mn deficient with
a composition; Li1+xMn2−yO4, where x and y ⬎ 0. The deposition rate could be controlled by varying the gas jet pressure
ratio and jet density. Significant film morphology changes
were observed to accompany variations of these processing
conditions. Increase of the pressure ratio increased the density and texture of the films. The rocksalt structures transformed to cubic spinel and the grain size and average pore
size were both increased by heat treatment of the asdeposited films in air at 700 ° C. This resulted in the formation of an interconnected spongelike 共open兲 pore network in
samples grown using lower pressure ratio deposition conditions. The pores became more isolated as the pressure ratio
was increased. The results suggest that thin porous lithium
manganese oxide films with a cubic spinel phase structure
can be rapidly synthesized by electron beam directed vapor
deposition followed by brief annealing at 700 ° C. The
growth rates were more than two orders of magnitude higher
122
S. W. Jin and H. N. G. Wadley: Lithium manganese oxide films
than those typically reported for films grown by sputter
deposition methods. Some appear well suited for use as the
cathodes of thin film lithium ion batteries.
ACKNOWLEDGMENTS
The authors thank Dr. Agnew for helping quantitative texture measurement and Dr. Menasian in Cornell Center
for Materials Research for Rutherford backscattering
spectroscopy.
1
J. B. Bates, N. J. Dudney, D. C. Lubben, G. R. Gruzalski, B. S. Kwak, X.
Yu, and R. A. Zuhr, J. Power Sources 54, 58 共1995兲.
2
D. Singh, R. Houriet, R. Giovannini, H. Hofmann, V. Craciun, and R. K.
Singh, J. Power Sources 97–98, 826 共2001兲.
3
K. Mizushima, P. C. Jones, P. J. Wiseman, and J. B. Goodenough, Mater.
Res. Bull. 15, 783 共1980兲.
4
M. Antaya, J. R. Dahn, J. S. Preston, E. Rossen, and J. N. Reimers, J.
Electrochem. Soc. 140, 575 共1993兲.
5
B. Wang, J. B. Bates, F. X. Hart, B. C. Sales, R. A. Zuhr, and J. D.
Robertson, J. Electrochem. Soc. 143, 3203 共1996兲.
6
C. P. Fonseca and S. Neves, J. Power Sources 135, 249 共2004兲.
7
Y. Xia, Y. Zhou, and M. Yoshio, J. Electrochem. Soc. 144, 2593 共1997兲.
8
M. S. Whittingham, Solid State Ionics 134, 169 共2000兲.
9
F. K. Shokoohi, J. M. Tarascon, and B. J. Wilkens, Appl. Phys. Lett. 59,
1260 共1991兲.
10
F. K. Shokoohi, J. M. Tarascon, B. J. Wilkens, D. Guyomard, and C. C.
Chang, J. Electrochem. Soc. 139, 1845 共1992兲.
11
J. B. Bates, N. J. Dudney, D. C. Lubben, G. R. Gruzalski, B. S. Kwak, X.
Yu, and R. A. Zuhr, J. Power Sources 54, 58 共1995兲.
12
J. B. Bates, N. J. Dudney, B. Neudecker, A. Ueda, and C. D. Evans, Solid
State Ionics 135, 33 共2000兲.
13
K. F. Chiu, F. C. Hsu, G. S. Chen, and M. K. Wu, J. Electrochem. Soc.
150, A503 共2003兲.
14
K. F. Chiu, H. C. Lin, K. M. Lin, and C. H. Tsai, J. Electrochem. Soc.
152, A2058 共2005兲.
15
N. J. Dudney, J. B. Bates, R. A. Zuhr, S. Young, J. D. Robertson, H. P.
Jun, and S. A. Hackney, J. Electrochem. Soc. 146, 2455 共1999兲.
16
K. H. Hwang, S. H. Lee, and S. K. Joo, J. Power Sources 54, 224 共1995兲.
17
C. Julien, E. Haro-Poniatowski, M. A. Camacho-Lopez, L. EscobarAlarcon, and J. Jimenez-Jarquin, Mater. Sci. Eng., B 72, 36 共2000兲.
18
C. L. Wang, Y. C. Liao, F. C. Hsu, N. H. Tai, and M. K. Wu, J. Electrochem. Soc. 152, A653 共2005兲.
19
D. D. Hass, J. F. Groves, and H. N. G. Wadley, Surf. Coat. Technol. 146,
J. Vac. Sci. Technol. A, Vol. 26, No. 1, Jan/Feb 2008
122
85 共2001兲.
D. D. Hass, P. A. Parrish, and H. N. G. Wadley, J. Vac. Sci. Technol. A
16, 3396 共1998兲.
21
H. Zhao, F. Yu, T. D. Bennett, and H. N. G. Wadley, Acta Mater. 54,
5195 共2006兲.
22
F. M. White, Fluid Mechanics 共McGraw-Hill, New York, 2008兲, p. 611.
23
D. D. Hass, Y. Marciano, and H. N. G. Wadley, Surf. Coat. Technol. 185,
283 共2004兲.
24
M. D. Abramoff, P. J. Magelhaes, and S. J. Ram, Biophotonics Int. 11, 36
共2004兲.
25
C. Dong and J. I. Langford, J. Appl. Crystallogr. 33, 1177 共2000兲.
26
J. S. Kallend, U. F. Kocks, A. D. Rollett, and H. R. Wenk, Mater. Sci.
Eng., A 132, 1 共1991兲.
27
T. J. Bartel and S. J. Plimpton, AIAA J. 92, 2860 共1992兲.
28
M. Ohring, The Materials Science of Thin Films: Deposition and Structure 共Academic, San Diego, CA, 2002兲, p. 495.
29
K. E. Sickafus, J. M. Wills, and N. W. Grimes, J. Am. Ceram. Soc. 82,
3279 共1999兲.
30
S. W. Jin, S. R. Agnew, and H. N. G. Wadley 共unpublished兲.
31
U. F. Kocks, C. N. Tome, and H.-R. Wenk, Texture and Anisotropy 共Cambridge University Press, Cambridge, UK, 1998兲, p. 121.
32
M. Morcrette, P. Barboux, J. Perriere, T. Brousse, A. Traverse, and J. P.
Boilot, Solid State Ionics 138, 213 共2001兲.
33
Y. Xia and M. Yoshio, J. Electrochem. Soc. 144, 4186 共1997兲.
34
X. W. Zhou, R. A. Johnson, and H. N. G. Wadley, Acta Mater. 45, 1513
共1997兲.
35
J. A. Thornton, J. Vac. Sci. Technol. 12, 830 共1975兲.
36
J. A. Thornton, J. Vac. Sci. Technol. 11, 666 共1974兲.
37
J. Steinwandel and J. Hoeschele, Particulate and Multiphase Processes,
General Particulate Phenomena Vol. 1 共Hemisphere Publication Inc., New
York, 1987兲, p. 571.
38
D. L. Hill, M.S. thesis, University of Virginia, 1994.
39
A. Perez, P. Melinon, V. Dupuis, P. Jensen, B. Prevel, J. Tuaillon, L.
Bardotti, C. Martet, M. Treilleux, M. Broyer, M. Pellarin, J. L. Vaille, B.
Palpant, and J. Lerme, J. Phys. D 30, 709 共1997兲.
40
E. Barborini, P. Piseri, A. Li Bassi, A. C. Ferrari, C. E. Bottani, and P.
Milani, Chem. Phys. Lett. 300, 633 共1999兲.
41
H. Haberland, Z. Insepov, and M. Moseler, Phys. Rev. B 51, 11061
共1995兲.
42
K.-H. Muller, J. Vac. Sci. Technol. A 3, 2089 共1985兲.
43
C. R. M. Grovenor, H. T. G. Hentzell, and D. A. Smith, Acta Metall. 32,
773 共1984兲.
44
M. Karl-Heinz, J. Appl. Phys. 61, 2516 共1987兲.
45
G. B. Cho, K. K. Cho, and K. W. Kim, Mater. Lett. 60, 90 共2006兲.
46
J. L. Shui, G. S. Jiang, S. Xie, and C. H. Chen, Electrochim. Acta 49,
2209 共2004兲.
20