Encapsulating Bis(-Ketoiminato) Polyethers. Volatile, Fluorine-Free Barium Precursors for
Metal-Organic Chemical Vapor Deposition
Daniel B. Studebaker, Deborah A. Neumayer, Bruce J. Hinds, Charlotte L. Stern, and
Tobin J. Marks*
Department of Chemistry, Materials Research Center, and Science and Technology Center for
Superconductivity, Northwestern University, Evanston, Illinois 60208-3113
ReceiVed October 1, 1999
The synthesis, characterization, and incorporation in volatile metal-organic chemical vapor deposition (MOCVD)
precursors of a new class of linked -ketoiminate-polyether--ketoiminate ligands is presented. These ligands
are designed to encapsulate alkaline-earth cations having low charges and large ionic radii. Barium complexes
having the general formula Ba[(RCOCHC(R′)N)
2
(R′′)] (R ) tert-butyl or CF
3
;R′ ) tert-butyl, methyl, or CF
3
;
R′′ )-(CH
2
CH
2
O)
4
CH
2
CH
2
- or -(CH
2
CH
2
O)
5
CH
2
CH
2
)-) were prepared and characterized by
1
H and
13
C
NMR spectroscopy, elemental analysis, and mass spectrometry. Single-crystal X-ray diffraction analysis of
2,2,5,25,28,28-hexamethyl-9,12,15,18,21-pentaoxa-4,25-diene-6,24-diimino-3,27-pentacosadionatobarium(II) reveals
a monomeric, nine-coordinate, tricapped trigonal prismatic coordination geometry. Single-crystal X-ray structural
analysis of 1,1,1,24,24,24-hexafluoro-4,21-ditrifluoromethyl-8,11,14,17-tetraoxa-3,21-diene-5,20-diimino-2,23-
tetracosadionatobarium(II)‚2DMSO reveals a monomeric, ten-coordinate, distorted tetracapped trigonal prismatic
coordination geometry. Volatility data are presented for these barium complexes, demonstrating viability as MOCVD
precursors. In addition, it is demonstrated that thin epitaxial films of BaTiO
3
can be grown on (001) MgO by
low-pressure MOCVD techniques using one of these barium complexes and Ti(dipivaloylmethanate)
2
(isopro-
poxide)
2
as precursors.
Introduction
Metal-organic chemical vapor deposition (MOCVD) is the
process of choice for the growth of ceramic thin films for
numerous solid-state electronic devices.
1
Large-scale film
growth by MOCVD takes advantage of simpler, less costly
equipment, ready scalability, and higher throughput as compared
to conventional physical vapor deposition (PVD) techniques.
Furthermore, MOCVD produces conformal coatings over steps,
vias, and other surface topographies, in contrast to PVD which
is a line-of-sight technique. Sol-gel processing, a less expensive
film growth technique, can suffer from voids, poor control of
film thickness and smoothness, and carbon contamination.
Therefore, MOCVD is widely used to reproducibly grow high-
quality thin films for a wide spectrum of applications. Critical
to the efficiency of any MOCVD process are the properties of
the metal-organic precursors. These species must be volatile,
exhibit stable vapor pressures, and cleanly decompose to the
desired product at useful substrate temperatures.
1c,d,f
Many
compounds which satisfy the above criteria are used to produce
high-quality thin films containing the respective metal centers.
Unfortunately barium, critical to high-temperature superconduc-
tors
2
such as YBa
2
Cu
3
O
7-x
and Tl
2
Ba
2
CaCu
2
O
7-x
, ferroelectrics
3
such as BaTiO
3
and (Ba
1-x
Sr
x
)TiO
3
, nonlinear optical materials
4
such as -BaB
2
O
4
, and collosal magnetoresistive materials
5
such
as La
0.66
Ba
0.33
MnO
3
, does not currently have a molecular
precursor which conforms to all of the above requirements.
Although MOCVD growth of the above materials has been
successfully accomplished with varying degrees of success using
known barium complexes,
6
current generation precursors exhibit
significant deficiencies.
7
Thus, while homoleptic -diketonates
of group II and transition elements are often useful precursors,
the simple -diketonates of Ba
2+
are typically oligomeric
because of the low Ba
2+
ionic charge:radius ratio.
8
This tendency
for oligomerization substantially reduces molecular volatility.
Monomeric barium -diketonates can be formed by saturating
the metal coordination sphere with polydentate neutral Lewis
base functionalities such as tetraglyme (tetraethylene glycol
dimethyl ether). However, when tetraglyme is coordinated to
barium(II) -diketonates having nonfluorinated ligands such as
Ba(dpm)
2
(dpm ) dipivaloylmethanate) (Figure 1A), the ensuing
complex (Figure 1B) readily loses tetraglyme upon attempted
* To whom correspondence should be addressed at the Department of
Chemistry.
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10.1021/ic991161a CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/28/2000