A novel diamine adduct of zinc bis(2-thenoyl-trifluoroacetonate) as a promising precursor for MOCVD of zinc oxide films

Article abstract of DOI:10.1021/ic051175i

A novel diamine (N,N,N′,N′-tetramethyletilendiamine) adduct of zinc bis(2-thenoyl-trifluoroacetonate) has been synthesized in a single-step reaction. Single-crystal X-ray diffraction studies of Zn(tta) ₂·tmeda provide evidence of a mononuclear structure with a six-coordinated zinc ion. The thermal behavior of this adduct points to mass-transport properties suitable for its application to MOCVD processes. This novel compound has been successfully applied as a precursor for the deposition of ZnO films on (100) Si and quartz substrates. The good quality of the deposited films indicates that the adduct is a very attractive precursor for MOCVD applications.

Full text of DOI:10.1021/ic051175i

Inorg. Chem. 2005, 44, 9684−9689
A Novel Diamine Adduct of Zinc Bis(2-thenoyl-trifluoroacetonate) as a
Promising Precursor for MOCVD of Zinc Oxide Films
Graziella Malandrino,*,† Manuela Blandino,† Laura M. S. Perdicaro,† Ignazio L. Fragala`,*,†
Patrizia Rossi,‡ and Paolo Dapporto‡
Dipartimento di Scienze Chimiche, UniVersita` di Catania, and INSTM, UdR Catania, Viale A.
Doria 6, I-95125 Catania, Italy, and Dipartimento di Energetica, “S. Stecco” UniVersita` di
Firenze, Via Santa Marta 3, I-50139 Firenze, Italy
Received July 13, 2005
A novel diamine (N,N,N
synthesized in a single-step reaction. Single-crystal X-ray diffraction studies of Zn(tta)2
′
,N
′
-tetramethyletilendiamine) adduct of zinc bis(2-thenoyl-trifluoroacetonate) has been
tmeda provide evidence of
‚
a mononuclear structure with a six-coordinated zinc ion. The thermal behavior of this adduct points to mass-
transport properties suitable for its application to MOCVD processes. This novel compound has been successfully
applied as a precursor for the deposition of ZnO films on (100) Si and quartz substrates. The good quality of the
deposited films indicates that the adduct is a very attractive precursor for MOCVD applications.
Introduction
(MOCVD).11 Among them, MOCVD offers several advan-
tages, including the production of high-quality films through
a fine-tuning of various processing parameters associated
with simpler, less costly equipment, ready scalability, and
higher throughput as compared to conventional physical
vapor deposition (PVD) techniques.12
The success of a MOCVD process depends critically on
the availability of volatile, thermally stable precursors that
exhibit high and constant vapor pressures to achieve uniform
and reproducible film growth.
Zinc oxide (ZnO) is one of the most promising oxide
semiconductor materials because of its good optical, electri-
cal, and piezoelectrical properties. Recently, ZnO has been
extensively studied because of its various potential applica-
tions, such as gas sensors, photodetectors, light-emitting
diodes, and laser systems.1
ZnO films are required for all these applications. There-
fore, a high throughput technique that may represent a
potentially scalable process is required for the fabrication
of films. These films have been grown using a large variety
of techniques, including sublimation,2 pulsed-laser deposition
(PLD),3 spray pyrolysis (SP),4 atomic-layer deposition
(ALD),5 molecular beam epitaxy (MBE),6 chemical-bath
deposition (CBD),7 sol-gel,8 electrodeposition,9 oxidation
of zinc films,10 and metal organic chemical vapor deposition
Previous studies on MOCVD processes of ZnO films have
reported the use of liquid dimethyl-Zn13 and diethyl-Zn14
complexes or of the solid acetate,15 alkoxide,16 and acetyl-
(7) Drici, A.; Djeteli, G.; Tchangbedji, G.; Derouiche, H.; Jondo, K.; Napo,
K.; Bernede, J. C.; Ouro-Djobo, S.; Gbagba, M. Phys. Status Solidi A
2004, 201, 1528-1536.
(8) Shaoqiang, C.; Zhang, J.; Feng, X.; Xiaohua, W.; Laiqiang, L.;
Yanling, S.; Qingsong, X.; Wang, C.; Jianzhong, Z.; Ziqiang, Z. Appl.
Surf. Sci. 2005, 241, 384-391.
* To whom correspondence should be addressed. E-mail: gmalandrino@
dipchi.unict.it (G.M.), lfragala@dipchi.unict.it (I.L.F.).
† Universita` di Catania.
(9) Yoshida, T. Electrochemistry 2004, 72, 44-48.
(10) Zhenguo, J.; Shichao, Z.; Chao, W.; Kun, L. Mater. Sci. Eng., B 2005,
117, 63-66.
‡ Universita` di Firenze.
(1) Materials Today; June 2004: special issue on ZnO films and
nanostructures.
(2) Tribolate, R.; N’tep, J. M.; Barbe, M.; Lemasson, P.; Mora-Sero, I.;
Munoz, V. J. Cryst. Growth 1999, 198/199, 968-974.
(3) Fan, X. M.; Lian, J. S.; Guo, Z. X.; Lu, H. J. Appl. Surf. Sci. 2005,
239, 176-181.
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Troyon, M.; Amrani, M. Mater. Chem. Phys. 2004, 83, 43-47.
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A 2005, 23, 118-125.
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2004, 269, 356-361.
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2004, 16, 1661-1664.
(12) Hitchman, M. L.; Jensen, K. F. Chemical Vapor Deposition: Principles
and Applications; Academic Press: London, 1993.
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1998, 4, 51-59.
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A.; O’Brien, P.; Critchlow, G. W. J. Mater. Chem. 1994, 4, 1249-
1253.
9684 Inorganic Chemistry, Vol. 44, No. 26, 2005
10.1021/ic051175i CCC: $30.25
© 2005 American Chemical Society
Published on Web 11/25/2005

NoWel Adduct of Zinc Bis(2-thenoyl-trifluoroacetonate)
acetonate zinc17 complexes as precursors. Nevertheless, these
precursors have some drawbacks because of their pyrophoric
nature, in the case of the dialkyl zinc complexes,18 and
because of the effects of crystallite sizes on the precursor
evaporation rate and, hence, on the film growth rate, in the
case of solid source.19 Note that the crystallite sizes may be
responsible for changes in evaporation rates in different
experiments and, even more importantly, in fluctuations
during the same experiment.
Table 1. Crystal Data and Refinement Parameters of Complex
Zn(tta)2‚tmeda
formula
mol wt
C22H24F6N2O4S2Zn
623.92
T (K)
298
λ (Å)
0.71069
monoclinic, P21/c
11.947(2)
cryst syst, space group
a (Å)
b (Å)
5.368(3)
c (Å)
15.132(3)
â (deg)
92.94(2)
2774.6(9)
4, 1.494
1.106
V (Å3)
Recently, the syntheses of the Zn(hfa)2(H2O)2L adducts
with L ) diglyme, triglyme, and tetraglyme (H-hfa )
1,1,1,5,5,5-hexafluoro-2,4-pentanedione, diglyme ) bis(2-
methoxyethyl)ether, triglyme ) 2,5,8,11-tetraoxadodecane,
tetraglyme ) 2,5,8,11,14-pentaoxapentadecane) have been
reported. Although, these precursors have good thermal
stability and volatility, water molecules are present in the
coordination sphere independently from the polyether length,
i.e., regardless of the number of polyether oxygen atoms,
the central metal ion is coordinated by H2O.20 The most
recent study on Zn MOCVD precursors regards the synthesis
of diamine adducts of the Zn(hfa)2 moiety reported by Marks
et al.21 These precursors are water-free, thermally stable,
volatile, and in a liquid state at processing temperatures.
In the present work, we have synthesized and investigated
the mass-transport properties of a new water-free zinc adduct,
the Zn(tta)2‚tmeda (htta ) 2-thenoyltrifluoroacetone, tmeda
) N,N,N′,N′-tetramethylethylendiamine). The 2-thenoyltri-
fluoroacetone has been chosen as the anionic ligand because,
as for the Cd precursor that is useful for solution routes,22 it
could represent a multipurpose single source for ZnO or ZnS
phases. The thermogravimetric analyses have shown that this
complex is thermally stable and volatile. The functional
validation of the Zn(tta)2‚tmeda complex as a MOCVD
precursor has been assessed by applying it to the MOCVD
of high-quality ZnO films on quartz and Si (100) substrates.
Z, dcalc (g cm-3
)
µ (mm-1
)
2θ range for data collection (deg)
no. of reflns collected/unique
data/params
8.5-64.7
22925/9044 [R(int) ) 0.0326]
2663/339
final R indices [I > 2σ(I)]
R indices (all data)
R1 ) 0.0684, wR2 ) 0.1970
R1 ) 0.2028, wR2 ) 0.2695
of (0.1 °C. Isothermal investigations were carried out at 20 Torr.
The cylindrical sample boat (12.56 mm2 cross sectional area) was
filled with ∼15 mg of the zinc adduct. The melting points were
measured in air with a Koffler microscope. Infrared transmittance
spectra were recorded using a Jasco FT/IR-430 spectrometer as
Nujol mulls between NaCl plates. The instrumental resolution was
2 cm-1. 1H and 13C NMR spectra were recorded on a Varian Inova
500 spectrometer.
Synthesis. An aqueous solution of Zn(CH3COO)2‚2H2O (2.064
g, 9.40 mmol) in 50 mL of H2O was added to a CH2Cl2 solution
(40 mL) containing htta (4.177 g, 18.80 mmol) and tmeda (1.418
mL, 9.40 mmol). The resulting solution was vigorously stirred in
a separating funnel. The Zn(tta)2‚tmeda, insoluble in the CH2Cl2/
H2O mixture, precipitates, and is recovered by adding EtOH (20
mL). The solution was left to crystallize, and the Zn(tta)2‚tmeda
crystals were washed with small quantities of pentane, in which
the adduct is insoluble. The yield was 75%. Melting point: 128-
130 °C at 760 Torr.
Anal. Calcd for C22H24F6N2O4S2Zn: C, 42.35; H, 3.88; S, 10.28;
N, 4.49. Found: C, 42.12; H, 4.01; S, 10.64; N, 4.30.
X-ray Crystallographic Study of Zn(tta)2‚tmeda. A crystal
having the dimensions 0.15 × 0.22 × 0.44 mm3 was used for the
data collection. Intensity data collection was performed using an
Oxford Xcalibur diffractometer with graphite-monochromated Mo
KR radiation (λ ) 0.71069 Å), at 298 K.
The cell parameters, the intensity data, and the reduction have
been performed using the program package CRYSALIS, version
1.17.23 Intensity data were corrected for Lorentz and polarization
effects. The absorption correction was made using an analytical
method based on faces indexing.
Experimental Section
Reagents. The chemical reagents H-tta and tmeda were pur-
chased from STREM Chemicals. Zn(CH3COO)2‚2H2O was pur-
chased from Carlo Erba.
General Procedures. Elemental microanalyses were performed
in the Analytical Laboratories of the University of Catania using a
Carlo Erba elemental analyzer EA 1108. Thermogravimetric (TG)
analyses were performed by using a Mettler Toledo TGA/SDTA
851e. Dynamic thermal investigations were carried out under a
purified nitrogen flow (30 sccm) at atmospheric pressure using a 5
°C/min heating rate. The weight of the sample was varied in the
range 12-15 mg. The temperature was measured with an accuracy
Zn(tta)2‚tmeda crystallizes in the monoclinic system, space group
P21/c, Z ) 4, a ) 11.947(2) Å, b ) 15.368(3) Å, c ) 15.132(3)
Å, â ) 92.94(2)°.
The structure was solved by direct methods using the SIR97
program,24 and was refined by full-matrix least squares against F2
using all data (SHELX97)25 to R1 ) 0.0684 for 2663 reflections [I
> 2σ(I)]; the number of refined parameters was 339. Anisotropic
thermal parameters were used for the non H-atoms. All the hydrogen
(17) Sato, H.; Minami, T.; Miyata, T.; Takata, S.; Ishii, M. Thin Solid Films
1994, 246, 65-70.
(18) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; Wiley-Interscience: New York, 1999.
(19) Hitchman, M. L.; Shamlian, S. H.; Gilliland, D. C.; Cole-Hamilton,
D.; Nash, J. A. P.; Thompson, S. C.; Cook, S. L. J. Mater. Chem.
1995, 5, 47.
(23) (a) CrysAlis CCD, version 1.171.pre23_10 beta; Oxford Diffraction
Ltd.: Oxford, U.K.; release 21.06.2004 CrysAlis171.NET (compiled
June 21, 2004, 12:00:08). (b) CrysAlis RED, version 1.171.pre23_10
beta; Oxford Diffraction Ltd.: Oxford, U.K.; release 21.06.2004
CrysAlis171.NET (compiled June 21, 2004, 12:00:08).
(24) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(25) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
(20) Gulino, A.; Castelli, F.; Dapporto, P.; Rossi, P.; Fragala`, I. Chem.
Mater. 2000, 12, 548-554.
(21) Ni, J.; Yan, H.; Wang, A.; Yang, Y.; Stern, C. L.; Metz, A. W.; Shu,
J.; Wang, L., Marks, T. J.; Ireland, J. R.; Kannewurf, C. R. J. Am.
Chem. Soc. 2005, 127, 5613-5624.
(22) Malandrino, G.; Finocchiaro, S. T.; Rossi, P.; Dapporto, P.; Fragala`,
I. L. Chem. Commun. 2005, 5681-5683.
Inorganic Chemistry, Vol. 44, No. 26, 2005 9685

Malandrino et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
Zn(tta)2‚tmeda
Zn(1)-O(1)
Zn(1)-O(2)
Zn(1)-O(3)
Zn(1)-O(4)
Zn(1)-N(1)
Zn(1)-N(2)
2.062(3)
2.078(3)
2.055(3)
2.093(3)
2.177(4)
2.188(4)
O(1)-Zn(1)-O(2)
O(1)-Zn(1)-O(3)
O(1)-Zn(1)-O(4)
O(1)-Zn(1)-N(1)
O(1)-Zn(1)-N(2)
O(2)-Zn(1)-O(3)
O(2)-Zn(1)-O(4)
O(2)-Zn(1)-N(1)
O(2)-Zn(1)-N(2)
O(3)-Zn(1)-O(4)
O(3)-Zn(1)-N(1)
O(3)-Zn(1)-N(2)
O(4)-Zn(1)-N(1)
O(4)-Zn(1)-N(2)
N(1)-Zn(1)-N(2)
86.25(12)
169.55(12)
85.20(12)
89.62(14)
96.90(15)
89.06(12)
95.83(13)
170.34(14)
89.59(15)
85.99(12)
96.37(14)
92.39(15)
92.52(15)
174.32(15)
82.23(16)
present low-cost synthetic procedure gives a water-free and
nonhygroscopic adduct. The complex is stable in air, and
its melting point is 128-130 °C at 760 Torr. These are
important issues for source precursors used for CVD ap-
plications for which low-cost chemicals that may be ma-
nipulated on open benches are an important objective.
X-ray Crystallographic Study of Zn(tta)2‚tmeda. The
Zn(1) ion is hexacoordinated; the six donor atoms are four
oxygen atoms of two tta anions and two nitrogen atoms of
one tmeda molecule (Figure 1). The coordination geometry
may be described as distorted octahedral (see Table 2).
All the Zn-O and Zn-N bond distances are in agreement
with those retrieved from the Cambridge Structural Database
(CSD, version 5.25)28 for zinc complexes having acetylac-
etonate derivatives and tmeda as ligands. The retrieved ranges
are 2.002-2.161 Å (mean value ) 2.076 Å) and 2.086-
2.411 Å (mean value ) 2.172 Å) for Zn-O and Zn-N
distances, respectively.
The tta anions are characterized by extended conjugation,
as shown by the fact that all the atoms of each tta anion,
except the fluorine ones, lie on a plane. The maximum
deviation from the mean plane featuring O(3) and O(4) as
donors is 0.060(8) Å for C(9). The second tta ligand appears
to be a little bit more distorted with respect to the mean plane
(C(4) shows the highest deviation, 0.370(8) Å). The angle
between the two mean planes is 84.57(8)°.
Figure 1. ORTEP view of the compound Zn(tta)2‚tmeda.
atoms were introduced in calculated positions, and were refined
according to the linked atoms; the overall temperature factor
converged to 0.145(5) Å2.
Geometric calculations were performed by PARST97,26 and
molecular plots were produced by the program ORTEP3.27 Table
1 reports crystallographic data and structure refinement parameters.
In Figure 1 an ORTEP3 drawing of the complex is reported.
MOCVD Experiment and Film Characterization. ZnO films
were prepared in a reduced-pressure, horizontal, hot-wall MOCVD
reactor from the Zn(tta)2‚tmeda precursor contained in a resistively
heated alumina boat. Quartz and Si (100) substrates were used for
the depositions. In this study, the precursor evaporation temperature
was kept at 170 °C, whereas the reactor temperature was kept at
650 °C. Ar (150 sccm) and O2 (150 sccm) flows were used as carrier
and reaction gases, respectively. The mass flows were controlled
with an MKS 1160 flowmeter using an MKS 147 electronic control
unit. Depositions were carried out for 60 min. The total pressure
in the reactor was about 3 Torr.
θ-2θ X-ray diffraction (XRD) patterns were recorded on a
Bruker-AXS D5005 θ-θ X-ray diffractometer, using Cu KR
radiation operating at 40 kV and 30 mA. Film-surface morphology
was examined using a LEO iridium 1450 scanning electron
microscope (SEM). Energy-dispersive X-ray analysis (EDX) spectra
were obtained using an IXRF detector.
The tmeda molecule, acting as a chelating ligand, shows
a gauche conformation; the dihedral angle N1-C19-C20-
N2 has a value of 55.3(7)°. With regard to the relative
disposition of the three ligands, the mean plane defined by
the tmeda atoms N1, C19, C20, and N2 forms angles of 78.8-
(3) and 72.1(2)° with the mean planes defined by all the
non-hydrogen atoms of the two tta anions (with the exception
of the fluorine atoms).
Results and Discussion
Synthesis. The adduct Zn(tta)2‚tmeda has been prepared
through immediate reaction from zinc acetate bihydrate,
2-thenoyltrifluoroacetone, and N,N,N′,N′-tetramethylethyl-
endiamine. The synthetic route to the complex is reported
in the following scheme
In the CSD, we retrieved the complex Co(tta)2‚tmeda.29
This complex is quite isomorphous and isostructural with
Zn(tta)2‚tmeda. The RMS value found superimposing the
metal cation as well as the six donor atoms is 0.031 Å.
Finally, if we consider the plane defined by the metal
cation and the CH carbon atoms of the two tta anions, the
two thiophene rings lie on opposite sides with respect to it.
Zn(CH3COO)2‚2H2O + 2htta + tmeda f
Zn(tta)2‚tmeda + 2CH3COOH + 2H2O
The adduct is soluble in ethanol and acetone, whereas it is
insoluble in solvents such as CH2Cl2, H2O, n-pentane, and
n-hexane. The reaction can be easily carried out on open
benches, without the need for a controlled atmosphere. The
(28) Allen, F. H. Acta Crystallogr., Sect. B 2002, 58, 380-388.
(29) Tzavellas, L. C.; Tsiamis, C.; Kavounis, C. A.; Cardin, C. J. Inorg.
Chim. Acta 1997, 262, 53-59.
(26) Nardelli, M. Comput. Chem. 1983, 7, 95-98.
(27) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.
9686 Inorganic Chemistry, Vol. 44, No. 26, 2005

NoWel Adduct of Zinc Bis(2-thenoyl-trifluoroacetonate)
Scheme 1
Table 4. Peaks in the 13C NMR Spectrum of the Zn(tta)2‚tmeda
Adduct
Figure 2. Structure of the Zn(tta)2‚tmeda adduct compared to that of the
homologous Cd(tta)2‚tmeda.
1
Table 3. Peaks in the H NMR Spectrum of the Zn(tta)2‚tmeda Adduct
In the crystal lattice, no intermolecular contacts are present.
A cis configuration has been instead observed for the Cd-
(tta)2‚tmeda complex (Figure 2). This difference may be
responsible for the completely different behavior in terms
of thermal properties of the Zn and Cd homologues, as the
Zn(tta)2‚tmeda is highly thermally stable and volatile whereas
the Cd(tta)2‚tmeda analogue decomposes, leaving a CdF2
residue.22
The 13C NMR spectrum (Table 4) of the Zn(tta)2‚tmeda
adduct shows a singlet at δ ) 46.65 and a singlet at δ )
56.29 that are associated with carbons of the methyl and
methylene groups, respectively, of the tmeda. A singlet at δ
) 89.60 represents the resonance of the C in the R position
of the tta ring, whereas the singlets between δ ) 128 and
133 represent the resonances of the Ca, Cb, and Cc carbons
of the thiophene ring. The singlet at δ ) 146.34 is associated
with the resonance of Cd, whereas the quartet at δ ) 119.15
(1J ) 286.2 Hz) is associated with the resonance of carbons
of the -CF3 groups. Finally, the singlet at δ ) 183.46 and
the quartet at δ ) 172.06 (2J ) 31.4 Hz) represent resonances
of the carbonilic carbons linked, respectively, to the thiophene
ring and to the -CF3 group.
The thermal behavior of the present Zn precursor has been
investigated by dynamic and static thermogravimetric (TG)
analyses. The TG profile recorded at atmospheric pressure
and isothermal curves measured at 20 Torr in a purified
nitrogen flow are reported in Figures 3 and 4, respectively.
The thermogravimetric first derivative (DTG) (Figure 3)
consists of a single peak, thus indicating that the precursor
evaporates quantitatively in the 200-320 °C range, with
about 5% residue left at 450 °C. It can be concluded that
the Zn(tta)2‚tmeda precursor shows a clean vaporization with
a low residue, as required for MOCVD applications.
Isothermogravimetric curves have been evaluated in the
150-190 °C interval (Figure 4) under a reduced pressure
(20 Torr) of pure N2. In this temperature range, the adduct
is melted; thus the linearity of vaporization for all the
investigated temperatures indicates that this compound may
Physicochemical and Mass-Transport Properties of Zn-
(tta)2‚tmeda. The Fourier transform infrared (FT-IR) spec-
trum of the present Zn(tta)2‚tmeda shows no band at 3600
cm-1, thus providing evidence of no coordinated water
molecules and hence a complete saturation of the zinc center
by the tmeda nitrogen atoms. Peaks at 1616 cm-1, 1596 cm-1
(CdO stretching vibrations), and 1536 cm-1 (CdC stretching
vibrations) are typical of the â-diketonate ligand. The other
bands, observed at 1300-1000 cm-1, represent C-N bending
and/or stretching of the tmeda ligand overlapped with the
C-F and C-S stretchings. In particular, peaks between 1230
and 1030 cm-1 are associated with C-N stretching vibrations
of the tmeda ligand. Peaks at 2923, 1460, and 1377 cm-1
are due to the Nujol used to prepare the mull.
1
The H NMR spectrum (Table 3) of the Zn(tta)2‚tmeda
adduct shows a singlet at δ ) 6.20 whose integration
accounts for the two protons, one for each tta ring. The
multiplet at δ ) 7.14 represents the resonance of the Hb
protons of the thiophene ring, whereas the multiplets between
δ ) 7.73 and 7.79 are associated with resonances of the Ha
and Hc protons (see Scheme 1). In addition, multiplets at δ
) 2.50 and 2.83 represent resonances of the six and four
protons, respectively, of the tmeda methyl and methylene
groups.
Inorganic Chemistry, Vol. 44, No. 26, 2005 9687

Malandrino et al.
Figure 6. XRD pattern of a ZnO film deposited on a quartz substrate
from a Zn(tta)2‚tmeda precursor at 650 °C.
Figure 3. TG/DTG profiles of the Zn(tta)2‚tmeda adduct under a N2 flow
in the 25-450 °C temperature range.
Figure 7. EDX spectrum of a ZnO film deposited at 650 °C.
Figure 4. Weight changes with time for vaporization of Zn(tta)2‚tmeda
at various temperatures and a reduced pressure of 20 Torr under a N2 flow.
Figure 8. UV-visible absorption spectrum of a ZnO film deposited at
650 °C.
MOCVD Depositions from the Zn(tta)2‚tmeda Precur-
sor. The final assessment of a potential precursor for
MOCVD processes may only be obtained by applying it to
the fabrication of films containing the specific metallic center.
In the present case, the Zn(tta)2‚tmeda precursor has been
validated through its successful application to the fabrication
of ZnO films. Preliminary experiments have been made on
Si (100) and quartz substrates at 650 °C, while maintaining
the precursor temperature at 170 °C. Note that at this
temperature, the zinc adduct is molten, thus representing a
liquid precursor. Films deposited are uniform with smooth
surfaces, and grazing incidence XRD patterns (Figure 6)
exhibit peaks at 2θ of 31.90, 34.55, 36.40, 47.75, and 56.75°.
These peaks may be associated with the 100, 002, 101, 102,
and 110 reflections, respectively, of the hexagonal wurtzite
ZnO phase.30
Figure 5. Arrhenius plot for the evaporation of Zn(tta)2‚tmeda.
be used as a liquid precursor under processing conditions.
For each investigated temperature, the vaporization rate
remains constant during the 60 min experiment.
Figure 5 shows the Arrhenius relationship obtained by
plotting the vaporization rate as a function of the reciprocal
temperature. A linear relationship is evident, as expected for
vaporization processes without any side decomposition. The
apparent molar enthalpies of the vaporization process
(calculated from the slope of the Arrhenius plot) have been
evaluated to be 90 ( 1 kJ mol-1.
All these investigations clearly indicate that the Zn(tta)2‚
tmeda compound is thermally stable and volatile, and
therefore it is expected to be a good precursor for the
deposition of zinc-containing films.
9688 Inorganic Chemistry, Vol. 44, No. 26, 2005

NoWel Adduct of Zinc Bis(2-thenoyl-trifluoroacetonate)
nm. The absorbance in the UV region at about 370 nm can
be related to the band-to-band transition.
Finally, the surface morphology of this ZnO film has been
investigated by scanning electron microscopy. The sample
(Figure 9) presents a very uniform morphology, with
coalesced grains of about 100 nm. The surface of each grain
is highly nanostructured, and domains smaller that 10 nm
are visible on the grain surface.
Conclusions
The present synthetic strategy has proven to be an efficient
route for the preparation of a thermally stable and volatile
zinc adduct from commercially available products. The novel
Zn(tta)2‚tmeda represents, to the best of our knowledge, the
first reported example of adducts of zinc with tta â-diketo-
nate. The dynamic and static TG measurements indicate that
this adduct possesses mass-transport properties suitable for
MOCVD applications. In addition, the Zn(tta)2‚tmeda may
be used in the molten phase, thus representing a liquid
precursor under processing conditions, as isothermogravi-
metric measurements indicate the adduct is also thermally
stable in the liquid phase. In fact, MOCVD experiments on
quartz and (100) Si substrates yielded high-quality ZnO films
in terms of phase purity, morphology, and transparency.
These data clearly point to the efficacy and great potential
of this precursor for depositing ZnO nanostructured films.
Figure 9. SEM image of a ZnO film deposited at 650 °C.
The compositional purity of the ZnO films has been
confirmed by energy-dispersive X-ray analysis (EDX). The
EDX spectrum, reported in Figure 7, shows the presence of
the ZnL peaks at about 1.010 keV. The peak at 1.730 keV
is due to the Si KR peak of the quartz substrates, whereas
the peak at 2.120 keV is due to the Au sputtered on the
sample. In addition, note that the use of the windowless EDX
detector allowed for the detection of the O KR peak at 0.560
keV and the exclusion of any S and/or F contamination, thus
ruling out any sulfide and/or fluoride phase. In this context,
it is worth noting that dry oxygen is used as the reaction
gas, and therefore no fluoride phases form during deposition.
The UV-visible absorption spectrum of a ZnO film grown
on quartz at 650 °C is shown in Figure 8. This spectrum
shows that the ZnO film is highly transparent in the visible
region with nearly 90% transmission between 400 and 800
Acknowledgment. The authors thank the MIUR for the
financial support. CRIST (Centro Interdipartimentale di
Cristallografia Strutturale) University of Florence is gratefully
acknowledged.
Supporting Information Available: Crystallographic informa-
tion files (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
(30) ICDD Nï. 36-1451 in Powder Diffraction File; International Centre
of Diffraction Data: Newton Square, PA.
IC051175I
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