Synthesis of group 13 sesquialkoxides and their application as precursors to crystalline oxide films

Article abstract of DOI:10.1021/om0608657

The reaction of Me₃Ga with an excess of 4-methylbenzyl alcohol (4-MeC₆H₄CH₂OH) in toluene, under reflux conditions, resulted in the formation of the sesquialkoxide [Ga{MeGa(OCH ₂C₆H₄Me-4)₃}₃] (1). In contrast, reaction of Me₃In with excess of 4-methylbenzyl alcohol (4-MeC₆H₄CH₂OH) under the same conditions afforded another type of sesquialkoxide, [In{Me₂In(OCH ₂C₆H₄Me-4)₂}₃] (2). Compounds 1 and 2 have been characterized by NMR, FTIR, and elemental analysis, and the structure of 1 was determined by X-ray crystallography. The structure of 1 consists of a central Ga³⁺ ion coordinated by three [MeGa(OCH ₂C₆H₄Me-4)(μ-OCH₂C ₆H₄Me-4)₂]^- units such that it has a coordination number of six. The surrounding metal ions are four coordinate. ¹H NMR data for 1 indicate that in solution two isomers (1:1) are present, namely, the C₁- and C₃-symmetrical isomers, but in the solid state 1 exists as the C₃-symmetrical isomer. Compound 1 possesses the correct ratio of Ga:O atoms (2:3) for that found in gallium oxide (Ga₂O₃), and LPCVD of 1 afforded crystalline Ga ₂O₃ films at 600°C. These results are surprising, as Ga₂O₃ films are typically only crystalline above 700°C.

Full text of DOI:10.1021/om0608657

Organometallics 2007, 26, 403-407
403
Synthesis of Group 13 Sesquialkoxides and Their Application as
Precursors to Crystalline Oxide Films
†
†
,†
†
Siama Basharat, William Betchley, Claire J. Carmalt,* Sarah Barnett,
†
‡
Derek A. Tocher, and Hywel O. Davies
Department of Chemistry, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, U.K., and
Epichem Limited, Power Road, Bromborough, Wirral CH62 3QF, U.K.
ReceiVed September 21, 2006
The reaction of Me
reflux conditions, resulted in the formation of the sesquialkoxide [Ga{MeGa(OCH
In contrast, reaction of Me In with excess of 4-methylbenzyl alcohol (4-MeC CH
conditions afforded another type of sesquialkoxide, [In{Me In(OCH Me-4)
and 2 have been characterized by NMR, FTIR, and elemental analysis, and the structure of 1 was
3
Ga with an excess of 4-methylbenzyl alcohol (4-MeC
6
H
4
CH
2
OH) in toluene, under
Me-4) ] (1).
OH) under the same
2 3
} ] (2). Compounds 1
2
C
6
H
4
3 3
}
3
6
H
4
2
2
2
C H
6 4
3
+
determined by X-ray crystallography. The structure of 1 consists of a central Ga ion coordinated by
-
three [MeGa(OCH
six. The surrounding metal ions are four coordinate. H NMR data for 1 indicate that in solution two
isomers (1:1) are present, namely, the C - and C -symmetrical isomers, but in the solid state 1 exists as
the C -symmetrical isomer. Compound 1 possesses the correct ratio of Ga:O atoms (2:3) for that found
in gallium oxide (Ga ), and LPCVD of 1 afforded crystalline Ga films at 600 °C. These results are
surprising, as Ga films are typically only crystalline above 700 °C.
2 6
C
H
4
Me-4)(µ-OCH
2 6 4 2
C H Me-4) ] units such that it has a coordination number of
1
1
3
3
2
O
3
2 3
O
2 3
O
Introduction
and are intrinsically noncorrosive so can be stored almost
indefinitely when kept in a dry atmosphere. Furthermore, metal
alkoxides are ideal for use in aerosol-assisted (AA)CVD due
to their solubility in a wide range of organic solvents and in
low pressure (LP)CVD as a result of their high volatility.10
Precursors previously employed for the CVD of Ga2O3 and
In2O3 have either contained an excess of oxygen (e.g., gallium
Gallium oxide (Ga2O3) is thermally stable, an electrical
insulator at room temperature, and semiconducting above 400
C. It is considered to be one of the most ideal materials for
°
1
,2
application as thin-film gas sensors at high temperatures. At
temperatures above 900 °C the electric conductivity changes
depend on the concentration of oxygen; hence the oxygen
1
1
or indium tris(alkoxides) ) or been used in the presence of an
oxygen source (e.g., O2 or H2O). Gallium tris(alkoxides),
3
concentration can be detected. Above 400 °C, gallium oxide
thin film operates as a surface-control-type sensor to reducing
[
Ga(OR)3]n, were first reported in 1964 in separate papers by
4
gases, e.g., CO and EtOH. Therefore, it may be possible to
1
2
13
Mehrotra and Mehrotra and Funk and Paul. Gallium tris-
ethoxide) and gallium tris(isopropoxide) were isolated from
5
switch the function of the sensor with temperature. Indium
(
oxide films are both transparent to visible light and conductive
and find application as transparent conductive oxide (TCO)
coatings. Dopants (e.g., tin or fluorine) can be used to increase
the conductivity of the films and to make them more suitable
the reaction of GaCl3 with 3 equiv of NaOR. An extensive series
of gallium and indium tris(alkoxides) were subsequently pub-
lished and prepared via alkoxide/alcohol exchange reactions.1
This series of compounds provided the first information
regarding possible structures of the gallium tris(alkoxides).
4-16
6
for applications such as in solid-state optoelectronic devices.
The versatility of metal alkoxides as convenient precursors
to metal oxides via chemical vapor deposition (CVD) processes
is widely recognized;7-9 they are easy to prepare and purify
i
However, Oliver and Worrall also demonstrated that [Ga(O Pr)3]
existed in solution as an equilibrium mixture of tetramer and
1
7,18
dimer.
*
Corresponding author. E-mail: c.j.carmalt@ucl.ac.uk. Tel: +44 (0)207
Indium oxide precursors incorporating alkoxide ligands
include [In(OCMe(CF3)2)3(H2N Bu)] and [Me2In(OC(CF3)2-
6
79 7528. Fax: +44 (0)207 679 7463.
t
†
University College London.
Epichem Limited.
‡
(
1) Fleischer, M.; H o¨ llbauer, L.; Meixner, H. Sens. Actuators, B 1994,
8-19, 119-124.
2) Ogita, M.; Yuasa, S.; Kobayashi, K.; Yamada, Y.; Nakanishi, Y.;
Hatanaka, Y. Appl. Surf. Sci. 2003, 212-213, 397-401.
3) Li, Y.; Trinchi, A.; Wlodarski, W.; Galatsis, K.; Kalantar-Zadeh, K.
Sens. Actuators, B 2003, 93, 431-434.
4) Binions, R.; Carmalt, C. J.; Parkin, I. P.; Pratt, K. F. E.; Shaw, G. A.
Chem. Mater. 2004, 16, 2489-2493.
5) Ogita, M.; Saika, N.; Nakanishi, Y.; Hatanaka, Y. Appl. Surf. Sci.
999, 142, 188-191.
6) Rees, W. S. CVD of Nonmetals; VCH: Weinheim, 1996; pp 172-
74.
(9) Crosbie, M. J.; Wright, P. J.; Davies, H. O.; Jones, A. C.; Leedham,
T. J.; O’Brien, P.; Critchlow, G. W. Chem. Vap. Deposition 1999, 5, 9-12.
(10) Basharat, S.; Carmalt, C. J.; King, S. J.; Peters, E. S.; Tocher, D.
A. Dalton Trans. 2004, 3475-3480.
1
(
(
(11) Carmalt, C. J.; King, S. J. Coord. Chem. ReV. 2006, 250, 682-
709.
(
(12) Mehrotra, R. C.; Mehrotra, R. K. Curr. Sci. 1964, 33, 241.
(13) Funk, H.; Paul, A. Z. Anorg. Allg. Chem. 1964, 330, 70-77.
(14) Booch, H.; Paul, A.; Funk, H. Z. Anorg. Allg. Chem. 1965, 337,
145.
(15) Chatterjee, S.; Bindal, S. R.; Mehrotra, R. C. J. Indian Chem. Soc.
1976, 53, 867.
(16) Bindal, S. R.; Mathur, V. K.; Mehrotra, R. C. J. Chem. Soc. 1969,
863.
(17) Oliver, J. G.; Worrall, I. J. Inorg. Nucl. Chem. Lett. 1969, 5, 455.
(18) Oliver, J. G., Worrall, I. J. J. Chem Soc. A 1970, 2347.
(
1
1
1
9
(
(
7) Mehrotra, R. C.; Singh, A.; Sogiani, S. Chem. ReV. 1994, 94, 1643-
660.
(
8) Chandler, C. D.; Roger, C.; Hampden-Smith, M. J. Chem. ReV. 1993,
3, 1205-1241.
1
0.1021/om0608657 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 12/13/2006

4
04 Organometallics, Vol. 26, No. 2, 2007
Basharat et al.
CH2NHMe)]2.19,20 The precursors [Ga(hfac)3] (hfac ) hexafluo-
using the KEVEX system. UV-vis spectra were recorded in the
range 200-1100 nm using a Helios double-beam instrument. Film
solubility was assessed by immersing a 1 cm × 0.6 cm piece of
glass into solvent (water, dichloromethane, ether, toluene, acetone,
and concentrated nitric and hydrochloric acids). Hardness scratch
tests were conducted using a brass stylus and a stainless steel
scalpel.
t
roacetoacetonate), [Ga(O Bu)3]2, and [Ga(OCH(CF3)2)3(HNMe2)]
have been used to deposit Ga2O3 films via LPCVD in the
presence of O2 at 450 °C.2
1-23
However, the formation of GaF3
was observed in the mass spectrum of [Ga(hfac)3], and the
presence of fluorine in Ga2O3 films could induce problems for
gas sensor applications, due to changes in baseline resistance
and sensor drift. Thus, there is a real need for new Ga2O3 and
In2O3 CVD precursors.
Group 13 sesquialkoxides24-27 are tetrameric compounds,
Synthesis of 1. A solution of 4-methylbenzyl alcohol (1.78 g,
14.6 mmol) in toluene (5 mL) was added dropwise to a stirred
solution of Me Ga (0.75 g, 6.5 mmol) in toluene (5 mL) at room
3
temperature. After refluxing the resulting solution for 24 h, the
solvent was removed in vacuo to yield a brown, viscous liquid.
THF (8 mL) was added, and after 48 h at -20 °C colorless crystals
which possess a M:O ratio of 2:3, the desired ratio for M2O3.
Thus, complexes of this type are ideal precursors to group 13
oxides. Indeed, the formation of Al2O3 using an aluminum
sesquialkoxide has been indicated, although no details were
1
of 1 were formed (1.2 g, 52%), mp 102 °C. H NMR (400.14 MHz,
CD
-
2
4
2
Cl
0.91, -0.89 (s, 3 × 3H, GaCH
.34 (m, 2 × 27H, OCH CH
.39-5.59 (m, 2 × 18H, OCH
2
): δ -1.24, (s, 9H, GaCH
3
, C
, C
, C
3
-symmetric isomer), -1.09,
-symmetric isomer), 2.28-
2
7
reported. Although a number of aluminum sesquialkoxides
3
1
have been reported, analogous examples for Ga and In are
C H
2 6 4
3
3
- and C
CH , C
1
-symmetric isomer),
- and C -symmetric
limited and no deposition studies have been reported.2
4,27,28
The
2
C
6
H
4
3
3
1
synthesis of sesquialkoxides usually involves a ligand redistribu-
tion reaction between metallanes and earth metal alkoxides. We
have investigated the reaction of metallanes (Me3Ga and Me3In)
with ROH (R ) OCH2C6H4Me-4) in a 4:9 ratio, which afforded
two different types of sesquialkoxides. The complexes possess
a 2:3 ratio of M:O, and preliminary CVD reactions are also
reported.
13
1
isomer), 7.02-7.49 (m, 2 × 36H, OCH
2
C
6
H
4
CH
): δ -11.6, -11.0 (GaCH
), 67.8, 68.7, 69.2, 69.6 (OCH
3
). C{ H} NMR
), 22.6. 27.4
CH ), 128.0,
(
(
100.61 MHz, CD
OCH CH
2
Cl
2
3
2
C
6
H
4
3
2
C
H
6 4
3
128.6, 128.7, 130.2, 130.7, 130.9, 131.0, 131.5, 132.0, 137.3, 138.9,
-
1
139.4 (OCH C H CH ). FTIR (cm ): 2358 m, 2341 m, 1652 w,
2
6
4
3
1558 w, 1456 s, 1377 s, 1261 m, 1010 m, 804 m. Anal. Calc for
Ga : C, 63.69; H, 6.41. Found: C, 63.93; H, 6.75.
Synthesis of 2. A solution of 4-methylbenzyl alcohol (0.85 g,
.03 mmol) in toluene (5 mL) was added dropwise to a stirred
solution of Me In (0.5 g, 3.12 mmol) in toluene (5 mL) at room
C
75
H
90
O
9
4
7
Experimental Section
3
General Procedures. All manipulations were performed under
a dry, oxygen-free dinitrogen atmosphere using standard Schlenk
techniques or in an MBraun Unilab glove box. All solvents were
distilled from appropriate drying agents prior to use (sodium for
toluene; sodium benzophenone for THF). Trimethylgallium and
trimethylindium were obtained from Epichem Ltd. All other
reagents were procured commercially from Aldrich and used
without further purification. Microanalytical data were obtained at
University College London (UCL). NMR spectra were recorded
temperature. After refluxing the resulting solution for 24 h, the
solvent was removed in vacuo to yield a white solid. THF (8 mL)
was added, and after 48 h at -20 °C colorless crystalline 2 was
isolated (0.7 g, 70%), mp 210 °C. 1H NMR (400.14 MHz,
CD Cl ): δ -0.52, (s, 18H, InCH ), 2.32 (s, 18H, OCH C H CH ),
2
2
3
2
6
4
3
5.07 (d, AB J ) 11.2 Hz, 6H, OCH C H CH ), 5.20 (d, AB J )
2
6
4
3
11.2 Hz, 6H, OCH C H CH ), 7.13 (m, 12H, OCH C H CH ), 7.27
2
6
4
3
2
6
4
3
13
1
(m, 12H, OCH C H CH ). C{ H} NMR (100.61 MHz, CD Cl ):
2
6
4
3
2
2
δ -1.1 (InCH ), 21.1 (OCH C H CH ), 64.6 (OCH C H CH ),
3
2
6
4
3
2/6
2
6
4
3
4
on a Bruker AMX400 spectrometer at UCL, referenced to CD
2
Cl
2
,
2 6 4 3 2 6 4 3
127.9 (C , OCH C H CH ), 128.6 (C , OCH C H CH ), 137.6
(C , OCH C H CH ), 141.0 (C , OCH C H CH ). FTIR (cm ):
3/5
1
-1
which was dried and degassed over molecular sieves prior to use;
2
6
4
3
2
6
4
3
1
13
H and C chemical shifts are reported relative to SiMe
4
(δ 0.00).
2682 w, 1463 s, 1377 s, 1261 w, 1085 m, 1073 m, 1012 m, 997 m,
798 m, 721 w. Anal. Calc for C H O In : C, 50.81; H, 5.68.
FT-IR spectra were obtained on a Shimadzu FTIR-8200 instrument.
Thermal gravimetric analysis (TGA) was performed under a
72
54
6
4
Found: C, 50.11; H, 5.32.
-
1
dinitrogen atmosphere with a heating rate of 10 °C min and on
a Netzsch STA 449C instrument. Raman spectra were acquired on
a Renishaw Raman System 1000 using a helium-neon laser of
wavelength 632.8 nm. The Raman system was calibrated against
the emission lines of neon. X-ray powder diffraction patterns were
measured on a Siemens D5000 diffractometer using monochromated
Low-Pressure CVD of 1. Compound 1 (0.1 g) was loaded into
the sealed end of a quartz tube (50 cm length × 25 mm diameter)
in the glovebox. Glass (70 mm × 6 mm × 2 mm) substrates were
placed carefully along the inside of the tube. The tube was then
placed in a furnace such that 35 cm was inside the furnace and the
end containing the sample protruded by 5 cm. The tube was heated
to a temperature of 600 °C under dynamic vacuum (3.0 kPa). The
tube was slowly drawn into the furnace, ca. 1 cm/30 min, until the
sample started to melt. Once the compound had decomposed, the
furnace was allowed to cool to room temperature. Translucent light
gray films were deposited on the glass substrates.
1 1
Cu KR radiation (λ ) 1.5406 Å). The diffractometer used glancing
incident radiation (1.5°). Samples were indexed using Unit Cell
and compared to database standards. EDXA was obtained on a
JEOL 25, and SEM was obtained on a Hitachi S570 instrument
(
19) M ˆı inea, L. A.; Hoffman, D. M. J. Mater. Chem. 2000, 10, 2392-
395.
20) Chou, T.; Chi, Y.; Huang, S.; Liu, C.; Carty, A. J.; Scoles, L.;
Udachin, K. A. Inorg. Chem. 2003, 42, 6041-6049.
21) Battiston, G. A.; Gerbasi, R.; Porchia, M.; Bertoncello, R.;
Caccavale, F. Thin Solid Films 1996, 279, 115-118.
22) M ˆı inea, L. A.; Suh, S.; Bott, S. G.; Liu, J. R.; Chu, W. K.; Hoffman,
Crystal Structure Determination and Refinement. Crystals
of 1 were obtained at -20 °C from a concentrated solution of THF.
A single crystal was mounted on a glass fiber, and all geometric
and intensity data were taken from this sample on a Bruker SMART
APEX CCD diffractometer using graphite-monochromated Mo KR
radiation (λ ) 0.71073 Å) at 150 ( 2 K. Data reduction and
integration was carried out with SAINT+,29 and absorption
2
(
(
(
D. M. J. Mater. Chem. 1999, 9, 929-935.
(
(
(
23) Valet, M.; Hoffman, D. M. Chem. Mater. 2001, 13, 2135-2143.
3
0
24) Neum u¨ ller, B. Chem. Soc. ReV. 2003, 32, 50-55.
corrections were applied using the program SADABS. The
25) Chitsaz, S.; Iravani, E., Neum u¨ ller, B. Z. Anorg. Allg. Chem. 2002,
31
structure was solved by direct methods using SHELXS-97 and
6
28, 2279-2285.
26) Atwood, D. A.; Jeiger, J. A.; Liu, S.; Rutherford, D.; Wei, P.; Tucker,
R. C. Organometallics 1999, 18, 976-981.
27) Munoz-Hernandez, M.; Wei, P.; Liu, S.; Atwood, D. A. Coord.
Chem. ReV. 2000, 210, 1-10.
28) Chitsaz, S.; Neum u¨ ller, B. Z. Anorg. Allg. Chem. 2001, 627, 2451-
459.
(
(29) SAINT+, Area detector control and data integration and reduction
software; Bruker AXS: Madison, WI, 2001.
(30) Sheldrick, G. M. SADABS; University of G o¨ ttingen: Germany, 1997.
(31) Sheldrick, G. M. SHELXS-97; University of G o¨ ttingen: Germany,
1997.
(
(
2

Gallium Sesquialkoxides
Organometallics, Vol. 26, No. 2, 2007 405
Scheme 1
Table 1. Crystallographic Data for Compound 1a
developed using alternating cycles of least-squares refinement and
difference Fourier synthesis.32 All non-hydrogen atoms were refined
anisotropically. One of the methylbenzoxo groups (C22-C28) was
found to be disordered and was modeled over three sites with ratios
chemical formula
CCDC deposit no.
fw
C81H102Ga4O10.
618654
1522.51
150(2)
T (K)
80.6:3.8:5.6. The benzyl rings were restrained to be flat, and the
space group
a (Å)
b (Å)
R3h
bond lengths and angles of each part were restrained to be the same.
Additionally, the thermal parameters for equivalent atoms were
constrained. Hydrogen atoms were placed in calculated positions
and their thermal parameters linked to those of the atoms to which
they were attached (riding model) except for the methyl groups of
C8 and C10, which were treated with a rigid rotor model, and C18,
which was treated with an idealized disordered methyl model. Nine
regions of highly disordered THF in the unit cell were accounted
for using SQUEEZE,33 which calculated 309 electrons, giving an
average of 34 electrons per THF (THF has 40 electrons). See Table
15.6095(19)
15.6095
55.392(7)
120
c (Å)
γ (deg)
3
V (Å )
11688(2)
6
1.298
0.71073
1.424
0.0352
0.0930
Z
Fcalcd (g cm-3)
λ (Å)
µ (mm-1)
R b
wR2
1
c
1
for selected crystal data.
a Bruker SMART APEX diffractometer ω rotation with narrow frames,
2 b
Crystal data in CIF format, CCDC 618654, for 1 have been
deposited with the Cambridge Crystallographic Data Centre.
graphite monochromated Mo KR radiation, refinement based on F . R1 )
c 2 2 2 2 2 1/2
-1
∑
|Fo| - |Fc|/∑|Fo|. wR2 ) {∑[w(Fo - Fc ) ]/∑[w(Fo ) ]} ; w
)
2
2
2
σ (Fo ) + (aP) + bP.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Results and Discussion
2 6 4 3
C H Me-4)}
] (1)a
[Ga{MeGa(OCH
In an attempt to isolate a molecular precursor to Ga2O3, with
the correct Ga:O ratio (2:3), the reaction between Me3Ga and
excess 4-MeC6H4CH2OH was carried out. After workup of the
reaction mixture and recrystallization from THF at -20 °C,
colorless crystals of 1 were obtained (Scheme 1). Analytical
and spectroscopic data for 1 were consistent with the formation
of the sesquialkoxide [Ga{MeGa(OCH2C6H4Me-4)3}3] (1). The
Bond Lengths
Ga(1)-O(2a)
Ga(2)-O(3)
Ga(2)-O(2)
1.962(1)
1.812(1)
1.916(1)
Ga(1)-O(1)
Ga(2)-O(1)
Ga(2)-C(10)
1.973(1)
1.912(1)
1.940(1)
Bond Angles
O(2b)-Ga(1)-O(2)
O(2)-Ga(1)-O(1)
O(2)-Ga(1)-O(1)
O(3)-Ga(2)-O(2)
O(3)-Ga(2)-C(10)
O(2)-Ga(2)-C(10)
95.93(5)
75.96(5)
93.92(5)
107.30(6)
121.83(8)
119.10(7)
O(2b)-Ga(1)-O(1)
O(2a)-Ga(1)-O(1)
O(3)-Ga(2)-O(1)
O(1)-Ga(2)-O(2)
O(1)-Ga(2)-C(10)
95.60(5)
166.54(5)
104.41(6)
78.48(5)
1
H NMR of 1 shows the presence of two species, the C3-
symmetrical and C1-symmetrical isomers, in a 1:1 ratio. An AB
spin system was observed in the NMR of 1 due to the chirality
of the complexes (diastereotopic H atoms). The structure of 1
117.01(8)
a
Symmetry operations for equivalent atoms: a ) -x+y, -x, z; b )
-
y, x-y, z.
was determined by single-crystal X-ray crystallography (Figure
+
1
) and can be described as a Ga3 ion coordinated with three
-
metallate [MeGa(OCH2C6H4Me-4)(µ-OCH2C6H4Me-4)2] units.
The central Ga atom in 1 is six-coordinate with three four-
coordinate gallium atoms occupying the periphery. Thus, the
central Ga atom is chelated in a bidentate fashion through two
oxygen atoms of the three [MeGa(OCH2C6H4Me-4)(µ-
-
OCH2C6H4Me-4)2] units. The Ga-O distances (Table 1) are
slightly longer around the central Ga atom (1.962(1) and
.973(1) Å) than the bridging Ga-µO bond lengths for the
terminal four-coordinate gallium atoms (1.916(1) and 1.912(1)
Å). This is due to the increase in atomic radii with increasing
coordination number. As expected, the terminal Ga-O bond
distances are the shortest (1.812(1) Å). The Ga2O2 rings in 1
are planar with small inner ring angles at Ga (75.96(5) and
1
(
32) Sheldrick, G. M. SHELXL-97; University of G o¨ ttingen: Germany,
997.
33) Spek A. L. Acta Crystallogr. 1990, A46 (Suppl.), C34.
1
(

406 Organometallics, Vol. 26, No. 2, 2007
Basharat et al.
Figure 2. TGA of 1.
successfully achieved. The TGA of 2 shows an overall weight
loss of 55% between 20 and 600 °C, which corresponds well
with the calculated weight loss of 56% for In2O3 formation. It
is worth noting that TGA at atmospheric pressure gives only
an indication of the potential of the precursors to form bulk
metal oxides.
LPCVD of 1 was investigated, the details of which are
described in the Experimental Section. Compound 1 deposited
gray-colored films on glass substrates at 600 °C. The film
produced was analyzed by WDX (wavelength dispersive X-ray
analysis), powder XRD, Raman, and UV-vis measurements.
The WDX data showed the presence of gallium and oxygen in
the film. Significant breakthrough of the excitation volume
through the coating to the underlying glass was observed, and
so accurate quantitative analysis by WDX was difficult.
However, WDX data showed that carbon contamination was
Figure 1. Molecular structure of 1 (hydrogen atoms and minor
disorder components are omitted for clarity) showing thermal
ellipsoids at the 50% probability level.
78.48(5)°) and angles greater than 90° at O. The structure of 1
is similar to the related Ga complex [Ga{MeGa(OCH2Ph)3}3],
and all bond lengths and angles are comparable.25
The reaction of Me3In and an excess of 4-MeC6H4CH2OH
afforded, after workup, colorless crystalline 2 (Scheme 1).
Spectroscopic and analytical data for 2 indicated that the indium
sesquialkoxide [In{Me2In(OCH2C6H4Me-4)2}3] had formed.
Unfortunately, X-ray-quality crystals of 2 could not be ob-
1
-2%. By scanning electron microscopy (SEM) the gallium
oxide films show the formation of agglomerates of ap-
proximately 0.1 µm. All the films were investigated using
Raman microscopy. In all cases no Raman scattering was
observed, and it is thought that gallium oxide is a poor Raman
scatterer. However, Raman spectroscopy indicated the absence
of graphitic carbon in the films. The gallium oxide films passed
the Scotch tape test and could not be abraded with a brass stylus
but could be marked with a stainless steel scalpel; this is typical
1
tained. However, the H NMR of 2 shows the presence of only
one set of resonances for the In-Me group and one for the
aromatic protons in OCH2C6H4Me-4 and the methyl group in
OCH2C6H4Me-4. These data are consistent with the formation
of a symmetrical structure rather than the same type of sesqui-
alkoxide as observed for 1 (Scheme 1). The OCH2 moieties are
diastereotopic, as shown by the presence of two closely spaced
1
0
of Ga2O3.
1
X-ray powder diffraction of the film obtained from 1 showed
a diffraction pattern consistent with the monocline â-Ga2O3
structure (Figure 3). Determination of the lattice constants gave
a ) 12.26(2) Å, b ) 3.038 Å, and c ) 5.81(1) Å, which are in
doublets in the H NMR of 2. Therefore, it is likely that
compound 2 possesses D3 symmetry similar to those of related
indium and aluminum complexes, of the type [M{Me2M-
OR)2}3] structurally characterized.25,26,28 Formation of the two
(
1
0
agreement with those reported previously for Ga2O3 thin films.
different sesquialkoxides for gallium and indium (compounds
and 2) is due to differences in the reactivity of Me3Ga versus
3
4
Conducting a Tauc plot of the UV/visible data indicated that
the films had a indirect band gap of 4.3 eV, comparable to other
1
Me3In. Thus, the more reactive Me3Ga gives with 4-MeC6H4-
CH2OH the unsymmetrical sesquialkoxide [Ga{MeGa(OCH2C6-
H4Me-4)3}3], whereas the less reactive Me3In yields the
symmetrical sesquialkoxide [In{Me2In(OCH2C6H4Me-4)2}3].
In order to study the decomposition pathway of compounds
4
values for Ga2O3 of 4.2-4.9 eV.
Given the successful formation of Ga O films from 1,
2
3
LPCVD of 2 was investigated. In contrast to 1, compound 2
afforded black films at 600 °C. Powder XRD of these films
indicated that decomposition had occurred, and rather than
forming In2O3, indium metal was deposited. These results are
in contrast to the TGA of 2, which indicated clean decomposi-
tion of 2 to In O . However, the melting point of 2 is
1
and 2, thermal analyses were carried out. The TGA (thermal
gravimetric analysis) of 1 (10 °C/min from 20 to 600 °C) shows
a weight loss of 72% (calculated weight loss 73% for Ga2O3
formation from 1). This behavior indicates that compound 1
may be a suitable single-source precursor for the deposition of
Ga2O3 thin films. The TGA of 1 (Figure 2) is clean, although
the decomposition of 1 does not occur in clean, discrete stages
but is continuous from the onset at ca. 110 °C throughout the
temperature range studied. The decomposition of 1 begins at a
higher temperature than the melting point, suggesting that
transport of the intact molecule to the substrate could be
2
3
significantly higher than the gallium complex 1, suggesting that
decomposition of 2 has occurred before the melting point. Thus,
transporting the precursor intact to the substrate without
decomposition would not be possible.
(34) Tauc, J., Ed. Proceedings of the International School of Physics;
Enrico Fermi, Course XXXIV, The Optical Properties of Solids, 1966.

Gallium Sesquialkoxides
Organometallics, Vol. 26, No. 2, 2007 407
Figure 3. XRD of Ga
O
2 3
thin films deposited from 1 by LPCVD at 600 °C.
Conclusions
Ga2O3 films from 1 at the temperature of deposition (600 °C)
is significant since all previous reports have found that only
amorphous Ga2O3 films are obtained below 700 °C.1
The synthesis and characterization of two different group 13
sesquialkoxides, [Ga{MeGa(OCH2C6H4Me-4)3}3] and [In{Me2-
In(OCH2C6H4Me-4)2}3], have been described. The structure of
the gallium derivative, [Ga{MeGa(OCH2C6H4Me-4)3}3], has
also been reported. The formation of crystalline Ga2O3 films
from this complex was achieved. The formation of crystalline
0-13
Acknowledgment. We thank the EPSRC for a DTA (S.B.).
OM0608657