Article
pubs.acs.org/JPCC
Alternative Low-Pressure Surface Chemistry of Titanium
Tetraisopropoxide on Oxidized Molybdenum
Alexis M. Johnson† and Peter C. Stair*,†,‡
†
Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States
Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States
‡
S Supporting Information
*
ABSTRACT: Titanium tetraisopropoxide (TTIP) is a precursor utilized in atomic layer
depositions (ALDs) for the growth of TiO2. The chemistry of TTIP deposition onto a
slightly oxidized molybdenum substrate was explored under ultrahigh vacuum (UHV)
conditions with X-ray photoelectron spectroscopy. Comparison of the Ti(2p) and C(1s)
peak areas has been used to determine the surface chemistry for increasing substrate
temperatures. TTIP at a gas-phase temperature of 373 K reacts with a MoOx substrate at
373 K but not when the substrate is at 295 K, consistent with a reaction that proceeds via a
Langmuir−Hinshelwood mechanism. Chemical vapor deposition was observed for
depositions at 473 K, below the thermal decomposition temperature of TTIP and within
the ALD temperature window, suggesting an alternative reaction pathway competitive to
ALD. We propose that under conditions of low pressure and moderate substrate
temperatures dehydration of the reacted precursor by nascent TiO2 becomes the dominant
reaction pathway and leads to the CVD growth of TiO2 rather than a self-limiting ALD
reaction. These results highlight the complexity of the chemistry of ALD precursors and demonstrate that changing the pressure
can drastically alter the surface chemistry.
■
kinetics.4,7,8 Many of these experiments are summarized in a
dedicated review by Puurunen.4 Studies of other ALD metal
precursors and corresponding growth processes of metal oxide
films, such as Nb2O5, HfO2, ZrO2, Al2O5, and TiO2, have also
been performed.9−12 Additional experiments have explored the
influence of different oxidizing precursors including H2O,
ozone, and O2 plasma.13−15
While TiCl4 is the common precursor for the growth of
TiO2, titanium tetraisopropoxide (TTIP) is an important
alternative. TTIP is preferable for materials intended for
catalytic applications since residual surface ligands can be
removed by calcination.16 In contrast with TiCl4, residual
surface chlorine induces Brønsted acidity that imparts
potentially undesirable catalytic functionality. While the thermal
decomposition of TTIP for chemical vapor deposition (CVD)
applications is the subject of a number of publications, a smaller
number have looked at the ALD chemistry of TTIP.5,17−19 An
enlightening study by Rahtu and Ritala combines mass
spectrometry, quartz crystal microbalance (QCM), and TPD
experiments to explore the reaction mechanism of TTIP
deposition onto both a predosed Ti(OCH(CH3)2)4−x covered
surface and SiO2 for TPD and QCM experiments, respec-
INTRODUCTION
Atomic layer deposition (ALD) is a technique that relies on
two sequential gas−surface reactions to grow materials in a
layer-by-layer fashion. This method is employed in the
semiconductor industry and is most commonly used to deposit
metal oxides.1 However, ALD has become an emerging
technique for the fabrication of novel and complex catalytic
systems and in the process has been extended to the growth of
metal films and nanoparticles on catalytic supports.2,3
ALD employs multiple cycles that are combined to build
materials with precise thickness control. One ALD cycle is
composed of two half cycles that result in the regeneration of
the initial surface groups, providing new reactive sites for the
following cycle. Given a finite number of surface reaction sites
and steric constraints imposed by the ligands, only a finite
number of precursor molecules can react, resulting in the selflimiting nature of the technique.4 Reactive sites on oxides
include hydroxyl groups, bridging oxygen atoms, and defect
sites.
There has been a significant amount of work directed toward
elucidating ALD chemistry, utilizing techniques such as X-ray
photoelectron spectroscopy (XPS), mass spectrometry, quartz
crystal microbalance (QCM), temperature-programmed desorption (TPD), DFT, and ab initio calculations.5,6 The most
studied system is the deposition of Al2O3 from trimethylaluminum (TMA) and H2O because it is widely used and
considered an “ideal” deposition process. Significant work has
been done to establish a mechanism for the reaction, identify
the source of self-limiting behavior, and understand the reaction
© XXXX American Chemical Society
Special Issue: John C. Hemminger Festschrift
Received: June 6, 2014
Revised: October 13, 2014
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tively.17 The current understanding is that the TTIP reaction
with a metal oxide surface follows eqs 1a and 1b
dosing, transferring into the HPC, and performing analytical
measurements.
The sample was a 1 cm diameter Mo(100) single-crystal disk
that was cut and mechanically polished to within 1° of the 100
plane. It was initially cleaned in a separate UHV chamber by
heating to 1500 K in 5 × 10−7 Torr of oxygen for 48 h to
remove bulk carbon and sulfur. Two 0.25 mm Mo wires were
used to suspend the crystal between two thermally isolated
copper leads at the bottom of the manipulator shaft. The
sample was resistively heated, and the temperature was
measured with a K-type thermocouple spot-welded to the
back of the crystal. Between TiO2 deposition experiments the
crystal was cleaned with argon ion sputtering using 2 keV ions
at normal incidence followed by brief annealing to 1073 K. This
procedure was repeated until the crystal was mostly free of
titanium. The crystal was subsequently annealed at 773 K under
600 L of O2 to remove carbide species that had formed during
the cleaning process. XPS was used to verify the chemical state
and composition of the crystal surface. Peak areas were
calculated with the appropriate sensitivity factor for comparison
between elements, and key elemental ratios are tabulated for
each experiment in the Supporting Information.25
TTIP Deposition. Titanium tetraisopropoxide (TTIP,
Sigma-Aldrich 97 and 99.999%) was purified using several
freeze−pump−thaw cycles before it was deposited onto the
Mo(100) single crystal in the UHV chamber by one of two
experimental methods: (1) dosing lines and (2) the Gemstar-6
ALD system. 97% pure TTIP was used in the dosing lines,
while 99.999% purity was used in the Gemstar-6 system;
however, both systems produced similar deposition chemistry
and are detailed below.26 Once a dose was complete, the
chamber was evacuated for 2−3 h until the pressure reached
10−9 Torr for analysis with XPS. A Mo(3d) satellite peak
overlaps with the C(1s) region and was removed through
subtraction of spectra that were taken of the base substrate
prior to precursor dosing.
Gas Delivery via Dosing Lines. The dosing lines were
used for experiments with the substrate heated to 573 and 473
K. The dosing line is evacuated with a mechanical pump to ∼10
mTorr and is comprised of a precursor vessel, a leak valve, and
a sapphire tube. The precursor vessel is isolated from the
dosing line by a needle valve. A leak valve (Granville-Philips)
separates the low vacuum side of the line from the UHV system
and is used to introduce gaseous precursors into the chamber.
The end of the dosing line is a sapphire tube with a 0.25 mm
tantalum wire coiled around it for resistive heating to control
the temperature of the gas entering the system. Prior to
deposition, the head space of the precursor vessels and the
dosing lines were evacuated and purged several times with N2
gas before directed dosing with either TTIP or H2O to a
pressure of 1 × 10−6 Torr. All precursor vessels and dosing lines
were kept at room temperature, ∼295 K, during the duration of
these experiments, while the sapphire tube was heated to 383
K. The vapor pressures of TTIP and water at room temperature
are 0.4 and 21 Torr, respectively.27,28 After each dose, the lines
and the chamber were purged with 1 × 10−5 Torr of N2 for a
minimum of 15 min. The QMS was used to verify the presence
of the intact precursor during the dose and the disappearance of
the precursor during the N2 purge.
Gas Delivery via the Gemstar-6. A commercial ALD
system (Arradiance Gemstar-6) was interfaced to the UHV
chamber through an auxiliary port on the system. One end of a
1/4″ OD stainless steel tube is inserted into the oven of the
2 − OH(s) + Ti(Oi Pr)4(g)
→ ( −O − )2 Ti(Oi Pr)2(s) + 2(CH3)2 CHOH(g)
(1a)
( −O − )2 Ti(Oi Pr)2(s) + 2H 2O(g)
→ ( −O − )2 Ti(− OH)2(s) + 2(CH3)2 CHOH(g)
(1b)
In this mechanism, half of the precursor ligands are removed
upon the initial reaction with surface hydroxyl groups (eq 1a),
and the other half are released with a subsequent water dose
(eq 1b).
Equations 1a and 1b can be generalized for many ALD
processes on metal oxide surfaces for which the majority of the
reactive sites are surface hydroxyl groups. However, as ALD
applications are broadening more nontraditional substrates are
being used, and the chemistry involved is less well understood.
In particular, only a few studies have explored reaction
mechanisms on noble metal surfaces, such as Cu and Pd, and
these studies predominantly involve the TMA/H2O process.20,21
In this study we use a slightly oxidized Mo(100) sample as
our substrate to develop a better understanding of how
deposition proceeds on alternative surfaces compared to
deposition on typical metal oxides such as Al2O3, TiO2, or
SiO2. In addition, it is advantageous to use MoOx as the base
substrate in order to minimize reactions with the alkoxide
ligands on TTIP. While molybdenum oxides can catalyze
alcohol dehydration to alkenes through an alkoxide intermediate, recent studies have shown that an alkoxide on the
surface of MoO3 would be more likely to recombine with a
hydrogen atom to desorb as the alcohol than dehydrate to the
alkene.22,23 As the Mo oxidation state decreases below 6+
dehydration of the alkoxide is even less probable. X-ray
photoelectron spectroscopy (XPS) was used to explore the
nature of the chemistry during TiO2 deposition by TTIP onto
MoOx surfaces at different temperatures. The ratio of Ti(2p)
and C(1s) XPS peak areas was used to determine whether the
TiO2 deposition followed CVD or ALD mechanisms at each
temperature. Surprisingly, we found that CVD can be the
dominant reaction mechanism even at temperatures used in
standard ALD growths that are below where TTIP decomposes.
■
EXPERIMENTAL METHODS
Instrumentation and Sample Mounting. All experiments were performed in a previously described three-level
ultrahigh vacuum (UHV) apparatus with a base pressure that
ranged from 5 × 10−10 to 2 × 10−9 Torr.24 Briefly, the upper
level includes an Extrel quadrupole mass spectrometer, low
energy electron diffraction (LEED), ion sputtering gun, heated
dosing lines, and a dual-anode Thermo/VG Microtech X-ray
photoelectron spectrometer with a hemispherical electron
energy analyzer. The middle level houses a high-resolution
electron energy loss spectrometer, and the bottom level
contains a high-pressure cell (HPC). Recently, an Arradiance
Gemstar-6 ALD system was interfaced to the top level of the
UHV apparatus. A 360° rotating XYZ manipulator with a 1 m
Z-translator was used to position the sample for sputtering, gas
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ALD system, and a fraction of the gas stream is directed into
the UHV system toward the surface of the Mo substrate. A
VCR gasket at the end of the stainless steel tube with a 75 μm
diameter laser-drilled hole reduces the pressure from 1 to 10
Torr to ∼10−7−10−6 Torr. The connecting line could be
evacuated with a turbo pump (Pfeiffer-Balzer 070) and heated
to 383 K. The temperatures of the TTIP and water containers
were 353 and 295 K, respectively. The corresponding vapor
pressure for TTIP is ∼2.2 Torr.28 The head space of the
precursor bottle was evacuated before dosing. Doses were
performed by first closing the isolation valve between the ALD
system and the mechanical pump evacuating it. The precursor
was then dosed into the ALD reaction chamber followed by
minimizing the carrier N2 flow to prevent large changes to the
partial pressures within the reaction chamber over the course of
the dose. After 15 s of equilibration time the gas was introduced
into the UHV chamber. After each dose was complete the
connecting line was purged with N2 for a minimum of 20 min,
and the concentration of precursor remaining in the chamber
was monitored with the QMS. The partial pressure of TTIP
and H2O entering the chamber was less than 3 × 10−7 and 3 ×
10−6 Torr, respectively, based on the changes in the base
pressure read by an ion gauge (Granville-Philips). This
experimental setup was used for the set of deposition
experiments in which the surface of the Mo crystal was heated
to 373 K.
■
RESULTS
Base MoOx Surface. A clean Mo (100) surface is extremely
reactive and will dissociatively adsorb gases at room temperature until a passivation layer is formed.29 The annealing
temperatures used in the study temporarily removed the
carbon, but were not high enough to completely desorb all of
the oxygen. The base Mo surface in these experiments was
passivated by both carbon and oxygen, as shown in the
representative XPS data in Figure 1 for the base substrate used
in the 573 K experiments. The integrated peaks from panels a,
b and c corresponding to Mo(3d), O(1s) and C(1s) spectra,
respectively, give a Mo:O:C ratio of 11:7:1. The binding
energies for the Mo(3d5/2) peak from the base substrates in this
work ranged between 227.9 and 228.0 eV with a full width halfmaximum (fwhm) of ∼1.1 eV, Figure 1a. The binding energies
for the O(1s) peak range from 530.4 to 530.7 eV with a
corresponding range of fwhm of 1.9−3.3 eV, Figure 1b. All
O(1s) spectra are well fit with two Gaussians, with peaks at
530.5 and 531.4 eV, consistent with O(1s) spectra from
molybdenum oxides found in the literature.30 The C(1s) peak
has a binding energy of 285.2 eV and a fwhm of 2.4 eV,
consistent with adventitious carbon.
After a brief anneal to 573 K the C(1s) signal disappeared
from the base MoOx surface but grew back to the concentration
shown in Figure 1c during the experiment. It is difficult with the
current XPS orientation, θ = 0°, to determine the exact
elemental ratio of the top few layers of the substrate. Previous
work from our laboratory has explored the effects of
chemisorbed oxygen on the structure, oxidation state, and
acidity of Mo(100).29,31−34 These early experiments used CO
dissociation to calibrate the O(1s) signal to establish a metric
for the expected intensity ratio of the Mo(3d5/2) to the O(1s)
peaks for increasing oxygen coverage ranging from 0.3 to 2
monolayers on a Mo(100) surface.35 For this work, the O/Mo
ratios are 0.06, 0.15, and 0.14 for the base substrate in the 573,
473, and 373 K experiments, respectively. These ratios
Figure 1. Representative (a) Mo(3d), (b) O(1s), and (c) C(1s)
spectra from the base Mo substrate.
correspond to oxygen surface coverages of 0.7, 1.7, and 1.5
monolayers. For oxygen coverages above 1.5 monolayers, the
two higher coverages in our case, it has been reported that the
surface has chemical and physical properties similar to MoO2.
Oxygen on surfaces with less than 1.5 monolayers is
chemisorbed.36,37 Because the surface structure and hence the
stoichiometry is not known we refer to the surface as MoOx.
The presence of chemisorbed oxygen implies that the Mo(100)
surface is passivated by a layer of oxygen atoms onto which the
adventitious carbon becomes adsorbed.
TTIP Deposition onto a 573 K MoOx Surface. The
results of TTIP deposition onto a 573 K base MoOx surface in
300 L (1 Langmuir = 1 × 10−6 Torr s) intervals are shown in
Figure 2. 573 K is above the reported TTIP thermal
decomposition temperature and would be expected to produce
chemical vapor deposition, whereby the growth of TiO2 is
dependent only on the integrated precursor exposure. Panels a
and b show the Ti(2p) and C(1s) spectra from the base surface
(black) and after three 300 L doses of TTIP (red). The
Ti(2p3/2) peak from the base Mo surface is insignificant, but
after TTIP exposure there is a large peak with a binding energy
of 459.4 eV and a peak splitting of 5.7 eV which is indicative of
TiO2 formation; however, there is charging in the sample which
leads to slightly higher binding energies.25 There is a persistent
C(1s) peak from adventitious carbon present on the base
surface, but this set of experiments also resulted in a
contribution at lower binding energies from carbidic carbon.
In Figure 2b, the C(1s) spectrum from the base Mo surface and
the total C(1s) spectrum taken after 900 L of TTIP are shown
in black and red. The C(1s) spectrum from the base surface
was subtracted from the total C(1s) spectrum after 900 L of
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Figure 2. Results for increasing 300 L doses of TTIP onto a 573 K Mo substrate. (a) Ti(2p) spectra taken before (black) and after (red) a total of
900 L of dosing time, (b) C(1s) spectra taken of the base surface (black) and the total C(1s) signal (red) after a total of 900 L of TTIP, (c) the
C(1s) signal added from 900 L of TTIP dosing (red) and the C(1s) spectrum from the isopropoxide in condensed vanadyl triisopropoxide (VOTP),
and (d) Ti (black squares) and C (open blue circles) surface densities as a function of TTIP exposure.
and is commonly used when growing TiO2 inside an ALD
reactor using TTIP as the metal precursor.13 Panels a and b
show the respective Ti(2p) and C(1s) spectra for the base
MoOx (black) and after four sequential 300 L of TTIP (red).
While the peak area is significantly lower at this temperature
than at 573 K, there is still a significant Ti(2p) signal from
deposited TiO2 which has a slightly lower binding energy of
459.0 eV, likely from reduced sample charging on a thinner
deposited metal oxide film. The C(1s) spectrum from the base
surface (black line in Figure 3b) is similar to the base signal in
Figure 2b but is lacking a contribution from carbidic carbon.
The red line represents the increase in C(1s) signal after four
300 L doses. The spectrum is bimodal and resembles the C(1s)
spectrum of condensed isopropoxide ligands, similar to Figure
2c. While the water doses reduced the intensity of the C(1s)
signal, the shape of the peak remained the same, indicating the
loss of exchangeable isopropoxide ligands.
The elemental ratios and surface densities calculated after
each dose are shown in Table S2 of the Supporting
Information. The base substrate had a Ti and C surface
density of 0.4 and 2.0 atoms/nm2. Figure 3c shows the changes
in Ti (black squares) and C (open blue circles) surface
densities above the base substrate value after each dose. The C
surface density grows in a manner similar to the case of the 573
K MoOx surface, in which it remains fairly constant with
increasing TTIP exposure. On the other hand, the Ti surface
density is almost linear with exposure time. These films are
slightly deficient in oxygen, which is consistent with previous
studies of TiO2 ALD with TTIP onto another clean surface,
Si(001), at 373 K.18 The Ti atom surface densities suggest that
40% of a full monolayer of titanium oxide was grown by the
TTIP to isolate the C(1s) signal due to the depositions alone.
The reported C(1s) spectra have been corrected in this
manner. The resultant C(1s) peak shape (Figure 2c) is bimodal
with similar peak ratios to that of the spectrum from an
isopropoxide ligand (blue) obtained in a previous study from
condensed vanadyl triisopropoxide.38
All elemental ratios and surface densities are summarized in
Table S1 (Supporting Information), with an error associated
with the integration and background subtraction process of
∼8%. The Ti and C surface densities for the base substrate
were 0.2 and 1.8 atoms/nm2, respectively. Figure 2d displays
the change in Ti (black squares) and C (open blue circles)
surface density above the base substrate value as a function of
precursor exposure time. As demonstrated by the graph, the Ti
surface density increases linearly with exposure time, while the
C surface density is approximately constant. The ratio of peak
areas, C(1s):Ti(2p), from the spectra after 900 L of TTIP is
∼1:4.9. The Ti(2p):O(1s) ratio indicates that at 573 K the
films are near stoichiometric. The reported density of ALD
grown TiO2 films at 573 K is 4.1 g/cm3, resulting in a Ti surface
density of 9.9 atoms/nm2.39 This is a conservative guide to
determine when a monolayer of TiO2 has been deposited. For a
substrate temperature of 573 K, after the second 300 L TTIP
dose a monolayer of TiO2 has been grown with 11.3 Ti atoms/
nm2.
TTIP and Water Deposition onto a 473 K MoOx
Surface. TTIP was deposited onto the base MoOx surface
held at 473 K. The results shown in Figure 3 are from four
sequential 300 L doses (1200 total L) of TTIP followed by two
sequential water doses of 300 and 600 L. The temperature of
the substrate is within the ALD window for TTIP deposition
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Reactions on MoOx at 295 and 373 K. TTIP was dosed
into the UHV chamber and onto a 373 K base Mo surface using
the Gemstar-6 system in three 20 L intervals followed by two
70 L intervals. After a total of 200 L of TTIP exposure the
surface was dosed with 360 L of water followed by another 20 L
dose of TTIP. The results are shown in Figure 4. The substrate
Figure 3. Results for increasing 300 L doses of TTIP onto a 473 K Mo
substrate. (a) Ti(2p) spectra taken before (black) and after (red) a
total of 1200 L of TTIP, (b) C(1s) spectra taken before (black) and
after (red) a total of 1200 L of TTIP, and (c) Ti and C surface
densities as a function of TTIP (black squares and blue circles) and
water (red squares and circles) exposure.
Figure 4. Results for increasing three 20 L and two 70 L doses of
TTIP onto a 473 K Mo substrate. (a) Ti(2p) spectra taken before
(black) and after (red) a total of 200 L of TTIP, (b) C(1s) spectra
taken before (black) and after (red) a total of 200 L of TTIP, and (c)
Ti (black squares) and C (open blue circles) surface densities as a
function of precursor exposure. Surface density changes from H2O
doses are shown in red.
end of the final dose. The C surface density is greater than the
Ti surface density for the first two doses with a C(1s):Ti(2p)
ratio of 1.3:1. This ratio implies that an average of 1.3 C atoms
are deposited for every Ti atom. After 1200 L of TTIP exposure
the Ti surface density is larger than that of C with a ratio of
1:2.3.
The red squares and red circles in Figure 3c represent the Ti
and C surface densities, respectively, after 300 L and then 600 L
of H2O. There is a 47% decrease in the C density upon the first
300 L of H2O, but there was no impact on the C density from
the subsequent 600 L H2O dose. As expected, the Ti surface
density increases only slightly due to partial removal of the
carbon overlayer.
temperature is well below reported decomposition temperatures for TTIP. Figure 4a shows the Ti(2p) spectra from a
base Mo surface and after 200 L of TTIP in black and red,
respectively. The binding energy, 458.9 eV, and peak splitting,
5.9 eV, are similar to the results displayed in Figure 3 and
consistent with a Ti−O containing species on the surface.
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control the temperatures of the gaseous precursors and the
substrate and therefore study substrate temperature effects in
more detail. In the current study we have explored the changes
in the chemistry of TTIP with increasing substrate temperatures while keeping the precursor gas temperature at 373 K.
On the time scale of our experiments, there is no evidence of
reaction with either water or TTIP when the substrate is held at
room temperature (295 K). However, deposition was observed
when the substrate temperature was increased to 373 K. Our
results indicate that thermal activation by the substrate is
critical as increasing the gas-phase temperature alone to 373 K
is insufficient for achieving growth. This means that the
precursor first accommodates to the surface temperature prior
to reaction and implies a Langmuir−Hinshelwood rather than
an Eley−Rideal mechanism for the surface chemistry.
Temperature Dependence of TTIP Deposition onto
MoOx. The growth mode and chemistry of TTIP deposition
onto a base MoOx surface was studied with XPS as a function
of substrate temperature. The shapes of the Ti(2p) and C(1s)
spectra, ratio of their peak areas, and Ti and C surface densities
after TTIP or water dosing are used to determine the type of
growth.
Atomic layer deposition and chemical vapor deposition are
related processes for the growth of materials. As mentioned
previously, ALD is a process that is composed of two sequential
self-limiting gas−surface reactions. Alternatively, CVD is the
growth of materials from chemical reactions of gas-phase
precursors. CVD can arise from a variety of mechanisms,
including gas-phase bimolecular reactions, thermal decomposition of the precursor or surface reactions that regenerate
reactive sites. Since CVD and ALD are similar, it can be hard to
distinguish between the two. One critical difference is selflimiting growth in ALD.
Chemical vapor deposition through thermal decomposition
of TTIP has been well studied.17,28,46−52 CVD products can
include TiO2, H2O, propene, H2, isopropanol, acetone, and
diisopropyl ether, depending on the reaction conditions.17,47,48,53,54 A common thermal decomposition mechanism includes propene as a product, as represented in eq 2.
Figure 4b displays the C(1s) peak from the base surface which
has a similar shape to previous adventitious carbon spectra. The
red line in Figure 4b represents the increase in C(1s) signal on
the surface after 200 L of TTIP dosing. While the peak shape
and binding energies are very similar to the spectra in Figures 2
and 3, the signal-to-noise ratio is too low for further peak shape
analysis.
Elemental ratios and Ti atom surface densities are reported
in Table S3 in the Supporting Information. Panel c depicts the
changes in the Ti and C surface densities (black squares and
open blue circles) above the base substrate values as a function
of precursor exposure time. Unlike the previous experiment at
473 K, the C surface density was consistently higher than the
corresponding Ti surface density. The C(1s):Ti(2p) ratio after
the first 20 L TTIP dose was approximately 5.8:1 and dropped
to 2.8:1 after a total of 200 L of TTIP exposure. Following 200
L of TTIP exposure, the surface was dosed with 360 L of water,
and the results are represented by the red markers in Figure 4c.
The Ti surface density remained the same, while the C surface
density decreased by ∼33%. The remaining signal had the same
shape as shown in Figure 4b, consistent with the loss of
isopropoxide from the surface. After the water another 20 L
dose of TTIP was applied which increased both the Ti(2p) and
the C(1s) signals. Films grown at 373 K are significantly oxygen
deficient. The Ti atom surface densities indicate that 17% of a
full monolayer of titanium oxide was grown by the end of the
final dose.
Two experiments with the surface at 295 K were also
performed. In the first experiment, 40 L of 388 K TTIP was
dosed onto a base room temperature MoOx substrate. No
titanium or carbon was deposited. The second experiment
involved dosing a room temperature TTIP-terminated surface
with 720 L of 388 K water. No detectable changes in the C(1s)
spectra were observed.
■
DISCUSSION
Several studies have demonstrated that ALD can be carried out
under UHV conditions (10−6 Torr) with pressures that are 6−7
orders of magnitude lower than in traditional ALD reactors
(0.1−10 Torr).12,18,40,41 However, it has been suggested that
the pressure difference between UHV conditions and traditional ALD can result in different chemistry.42,43 XPS, mass
spectrometry, secondary ion mass spectrometry (SIMS),
temperature-programmed desorption (TPD), IR spectroscopy,
and electron energy loss spectroscopy (EELS) are some of the
methods that have been used to study and verify self-limiting
growth on flat surfaces at lower pressures.5,8,12,40,44,45 The most
studied process is the growth of Al2O3 by deposition of
trimethylaluminum and water.4 IR and EELS experiments
conducted by Soto et al. and mass spectrometry experiments
performed by Juppo et al. have successfully observed ALD-like
behavior from lower-pressure dosing.8,43 A few other ALD
processes have been explored under UHV conditions, including
XPS studies by Roy et al. on the deposition of ZrO2 and
coupled XPS and SIMS experiments by Lemonds et al. to
investigate the chemistry of TaF5 and Si2H6 half-cycle
reactions.40,41 In both cases self-limiting deposition was
observed with substrate temperatures up to 523 K. Finally,
our own experiments investigating VOx deposition onto metal
oxide surfaces have observed ALD-like growth of vanadia in an
UHV system.38
Room Temperature TTIP Chemistry. Unlike traditional
ALD reactors, our UHV setup allows us to independently
Ti(OCH(CH3)2 )4(g) → TiO2(s) + 4C3H6(g) + 2H 2O(g)
(2)
For ALD of TTIP on a metal oxide surface, the reaction in the
first half-cycle is eq 1a. It is important to note that the reaction
terminates with some number of carbon-containing ligands
remaining on the surface, covering potential reactive sites. This
is responsible for the self-limiting behavior of ALD. Comparing
eq 1a and 2, one critical difference between CVD and ALD is
the lack of carbon expected to remain on the surface after CVD.
In our studies, we use this difference to determine the growth
mode. Not only do we observe surface carbon species but also
the shape of the C(1s) spectra is similar to the C(1s) spectrum
of condensed vanadyl triisopropoxide observed in previous
studies performed in the same apparatus. This indicates that the
carbon signal is predominantly composed of isopropoxide
ligands remaining on the surface after the reaction. Although
the Ti(2p) or Mo(3d) spectra cannot be used to distinguish
whether the isopropoxide ligands are bound to Ti or Mo atoms,
the decreases in C(1s) intensity after water dosing indicate that
exchangeable ligands are present.
The more informative XPS results are the C(1s) to Ti(2p)
peak area ratio and corresponding surface densities which
should indicate the ratio of ligands to titanium atoms remaining
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dose, the total C(1s):Ti(2p) ratio immediately decreases to
2.8:1, indicating that less than one ligand remains on the
surface per reacted TTIP molecule and there was an overall loss
of ligands from the first dose to the second. The ratio remains
relatively constant through 200 L of TTIP exposure. These
ratios are more suggestive of CVD-like growth of TiO2
compared to the first dose. These results suggest that ALD
chemistry takes place during the first dose because TiO2 is not
yet present on the surface, but following doses approach CVD
behavior as the TiO2 concentration builds up and the
dehydration reaction pathway becomes possible.
We have considered the possible role of the MoOx surface in
this alternative reaction pathway. MoO3 and to a lesser degree
MoO2 are also alcohol dehydration catalysts; however, recent
studies have shown that the surface alkoxides preferentially
desorb as the alcohol rather than dehydrate to the alkene.22,23
The presence of MoO3 was undetectable by XPS, and its
presence is not expected on Mo surfaces treated as described.
Therefore, substrate-catalyzed dehydration is not expected to
contribute to the overall dehydration reaction. To further
address the possibility of molybdenum oxide catalyzed
dehydration TTIP was deposited onto a 473 K MoOx substrate
with an oxygen coverage of 0.5 monolayer, a significantly lower
oxygen concentration than in the previous experiment.
Vibrational spectroscopy and surface polarizability studies
have shown that oxygen overlayers with this concentration do
not have the structure or electronic properties of molybdenum
oxide but rather behave as chemisorbed oxygen.34−36 This
surface will have even less activity than MoO2 for dehydration
of the alkoxide ligand. The elemental ratios, Ti atom surface
densities, and a description of the experiment are detailed in
Table S4 of the Supporting Information. Due to the low
reactivity of TTIP only 45% of a complete TiO2 monolayer is
deposited after a total of 2040 L of TTIP exposure. However,
we observe low C(1s):Ti(2p) ratios, indicating CVD-like
growth which is consistent with the results from the more
oxidized substrate. This suggests that the base MoOx substrate
does not play a significant role in the alternative reaction
pathway.
Implications for Atomic Layer Deposition Processes.
This alternative reaction pathway to ALD chemistry for TTIP
deposition is not restricted to MoOx substrates because the
active dehydration site is predominantly TiO2. However, it is
improbable that the dehydration reaction pathway plays a major
role during ALD processes in traditional reactors because the
precursor flux on the surface is 106−107 higher. Because of the
higher flux, all active dehydration sites are covered by deposited
TTIP molecules, and dehydration is quenched. This flux
dependence has possible implications for substrate architectures
with mass transport limitations, such as long nanoscale pores.
Our results suggest that CVD, instead of ALD, occurs at the
bottom of these pores because the flux of TTIP molecules may
be insufficient to inhibit the dehydration pathway. The CVD
reactions would then lead to a filling of the pores rather than a
thin conformal coating.
after each dose. For ALD-like reactions, we expect that there
should be, on average, 1−2 isopropoxide ligands per Ti atom
remaining on the surface if there is less than one monolayer
coverage. This would be reflected in the XPS spectra as a
C(1s):Ti(2p) ratio that is greater than or equal to 3:1. Instead,
for all doses of TTIP onto a 573 K MoOx surface the Ti(2p)
signal is greater than the C(1s) signal (Figure 2d), and the Ti
surface density increases linearly with TTIP exposure, therefore
indicating that TiO2 is being formed by CVD. This observation
is consistent with previous thermal decomposition studies.17,19,46,47,49,51
Chemical Vapor Deposition of TTIP at 473 K. At 473 K
all doses of TTIP also resulted in low C(1s):Ti(2p) atomic
ratios which decreased from 1.3:1 to 1:2.3 as the TTIP
exposure increased. This indicates that CVD also occurs at 473
K even though this is within the observed ALD temperature
window.13,55 Moreover, while thermal decomposition is a
common mechanism of CVD, 473 K is below the reported
TTIP thermal decomposition temperatures. We have considered the possibility of gas-phase reactions from residual water in
the chamber regenerating reactive sites from ligand exchange
causing uncontrolled growth, especially during the long
evacuation and data collection periods. However, this can be
ruled out by our experiment showing the absence of reaction
between dosed water and the 295 K TTIP-covered surface.
One possible CVD mechanism which explains our
observations is that, under UHV conditions at 473 K, we are
observing TiO2-catalyzed dehydration of TTIP molecules. As
far as we know, this reaction mechanism has not been proposed
before; however, it is well known that TiO2 is a dehydration
catalyst that is active for alcohol dehydration above 500 K.56−60
Recent TPD experiments of isopropanol on TiO2(110)
reported a low-temperature dehydration pathway around 300
K.58,59 It has also been established that the intermediate for the
reaction is the alkoxide which then undergoes β-hydride
elimination to form the alkene.56−58,60 While the adsorbed
species from TTIP deposition onto TiO2 is not exactly
equivalent to the reaction intermediate for isopropanol
dehydration on TiO2, the same mechanism and products,
H2O and C3H6, have been reported for TTIP decomposition.17,46,47,53,54 It is reasonable that the alkoxide ligand
chemistry of a deposited TTIP is similar to the chemistry of
the intermediate alkoxide formed during alcohol dehydration.
Thus, during low-pressure deposition, initially formed TiO2
species catalyze dehydration of subsequent TTIP molecules, to
form more TiO2 and regenerate reactive sites rather than
terminating the deposition reaction by saturating the surface
with isopropoxide ligands. This alternative dehydration pathway is always possible during TTIP deposition onto/or near
TiO2 but is likely suppressed under the higher-pressure ALD
conditions because the increased flux of TTIP molecules hitting
the surface leads to a more rapid surface saturation covering the
TiO2 sites active for dehydration. We propose that under the
experimental conditions of low flux and moderate temperatures
the ALD reaction kinetics are slow enough that dehydration
becomes the dominant pathway.
Lowering the MoOx substrate temperature to 373 K causes a
significant change in the growth mode. The integrated C(1s)
signal is larger than for the Ti(2p) signal for all TTIP and water
doses. After the first TTIP dose the calculated C(1s):Ti(2p)
ratio is 5.3:1 which means that on average 1−2 ligands are
remaining on the surface per reacted TTIP molecule, consistent
with ALD chemistry for the first dose. After the second TTIP
■
CONCLUSION
The chemistry of TTIP deposition on to a MoOx surface was
explored at a series of substrate temperatures in UHV
conditions. Compared to successful depositions onto a 373 K
MoOx surface, the lack of reactivity of 373 K TTIP with a 295
K surface indicates that the surface temperature is a critical
parameter. TTIP molecules must accommodate to the surface
G
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(26) The dosing lines and the Gemstar-6 system were both used to
deposit TTIP onto a MoOx substrate at 473 K. It was challenging to
quantitatively compare the total flux from each system as the dosing
methods are significantly different. In the Gemstar-6 the TTIP had to
prior to reaction and follow a Langmuir−Hinshelwood
mechanism.
CVD of TiO2 from TTIP was observed for substrate
temperatures of 573 and 473 K, which are, respectively, within
and below the reported thermal decomposition range for TTIP.
It is believed that, under the conditions of low pressure (10−6
Torr) and moderate temperatures (473 K), dehydration of
TTIP molecules by nascent TiO 2 not only becomes
competitive with the ALD reaction but actually becomes the
dominant reaction. At lower temperature, 373 K, ALD-like
chemistry is observed during the first TTIP dose, but XPS
results suggest that for subsequent doses the growth mode
transitions into CVD. In all cases, the influence of the MoOx
substrate appears to be negligible. While these results may not
directly impact traditional ALD processes, this type of
alternative chemistry may be relevant to high surface area
substrates with long nanopores, where mass transport to the
bottom of the pore is limited and CVD occurs in preference to
typical ALD reactions.
■
ASSOCIATED CONTENT
* Supporting Information
S
Details on elemental ratios and surface densities and effects of
the oxygen coverage on the base substrate on TTIP deposition.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: pstair@northwestern.edu.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This material is based upon work supported by the National
Science Foundation under Grant No. CHE-1058835. The
authors would also like to thank Kunlun Ding, Miles Tan,
Charlie Campbell, and Bruce Rosen for helpful discussions.
■
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