A photochemical method for the preparation of indium oxide

and indium–cobalt oxides thin films

G.E. Buono-Core a,*, G. Cabello a, B. Torrejon a, M. Tejos b, R.H. Hill c

a Instituto de Quımica, Universidad Catolica de Valparaıso, Valparaıso, ChilebFacultad de Ciencias, Universidad de Valparaıso, Valparaıso, Chile

cDepartment of Chemistry, Simon Fraser University, Burnaby, BC, Canada V5A 1S6

Received 16 March 2004; received in revised form 21 April 2005; accepted 17 May 2005

Abstract

Indium oxide and indium–cobalt oxide thin films have been successfully prepared by direct UV irradiation of

amorphous films of b-diketonate complexes on Si(1 0 0) substrates. Deposited films were characterized by X-ray

diffraction, Auger electron spectroscopy and X-ray photoelectron spectroscopy. The surface morphology of

the films, examined by atomic force microscopy and scanning electron microscopy, revealed that mixed indium–

cobalt oxide films are much smoother than In2O3 films, with rms surface roughness of 7.24 and 26.1 nm,

respectively.

# 2005 Elsevier Ltd. All rights reserved.

Keywords: A. semiconductors; A. oxides; A. thin films

1. Introduction

The electronic properties of In2O3 make it suitable for applications in various solid state devices such

as solar cells [1], contact material in III–V semiconductors [2,3] and gas sensors [4–7]. Many deposition

techniques have been applied for the production of In2O3 and InOx films such as dc and rf sputtering

[8,9], evaporation [10,11], thermal oxidation of indium films [12], chemical vapor deposition [13] and

the sol–gel method [14] in order to improve the properties of the film. The functional properties of the

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Materials Research Bulletin 40 (2005) 1765–1774

* Corresponding author.

E-mail address: gbuonoco@ucv.cl (G.E. Buono-Core).

0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2005.05.008

materials depend on their microstructure (grain and agglomerate size, porosity, roughness, etc.) and

composition (doping, deviation from stoichiometry). In turn, these parameters are controlled by

synthesis conditions [15].

Recently Yamura et al. [16–18] reported the preparation of indium oxide–cobalt oxide thin films

as chemical sensor for the detection of CO gas. They have shown that the incorporation of transi-

tion metal oxides, such as Co3O4, CuO, NiO and ZnO, can promote the CO sensing properties of

indium-based sensors. Among these, Co3O4 was the most effective in terms of improved sensitivity

and selectivity of the sensor. Similar findings were later reported [19], in which the sensitivity of an

In2O3-based sensor for detection of CO gas was increased through ultra thin Co adsorption and

annealing.

In previous articles we have reported a novel photochemical method for the deposition of a variety of

metals and metal oxides [20–22]. In this method, thin films of inorganic or organometallic precursors

upon irradiation are converted to amorphous films of metals or oxides, depending on the reactions

conditions. The development of this method requires that the precursor complexes form stable

amorphous thin films upon spin coating onto a suitable substrate and that photolysis of these films

results in the photoextrusion of the ligands leaving the inorganic products on the surface.

In this work, indium and cobalt diketonate complexes are proposed as source materials for the direct

photochemical deposition of indium–cobalt oxide thin films.

2. Experimental

2.1. General procedure

The FT-IR spectra were obtained with 4 cm�1 resolution in a Perkin Elmer Model 1605 FT-IR

spectrophotometer. UV spectra were obtained in a Hewlett-Packard 8452-A diode array spectrophot-

ometer. X-ray diffraction patterns were obtained using a D5000 X-ray diffractometer. The X-ray source

was Cu 40 kV/30 mA. Auger electronic spectra were obtained using a PHI double pass CMA at 0.85 eV

resolution at the Surface Physics Laboratory, Department of Physics, Simon Fraser University. X-ray

photoelectronic spectra were obtained in an XPS-Auger Model PHI 1257 at the Department of Physics,

Universidad de Chile. The atomic compositions were evaluated using the PHI sensitivity factors. Atomic

force microscopy was performed in a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) in contact

mode. Film thickness was determined using a Leica DMLB optical microscope with a Michelson

interference attachment.

Solution photochemistry was carried out in 1 cm quartz cells, which were placed in a Rayonet RPR-

100 photoreactor equipped with 254 nm lamps. Progress of the reactions was monitored by determining

the UV spectra at different time intervals, following the decrease in UVabsorption of the complexes. The

solid state photolysis was carried out at room temperature under a UVS-38 254 nm lamp equipped with

two 8 W tubes, in an air atmosphere.

The substrates for deposition of films were borosilicate glass microslides (Fischer, 2 cm � 2 cm) and

p-type silicon(1 0 0) wafers (1 cm � 1 cm) obtained from WaferNet, San Diego, CA. Prior to use the

wafers were cleaned successively with ether, methylene chloride, ethanol, aqueous HF (50:1) for 30 s

and finally with deionized water. They were dried in an oven at 110 8C and stored in glass

containers.

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2.2. Synthesis of b-diketonate complexes

2.2.1. Tris(1-phenyl-1,3-pentanedionato)In(III)

For the synthesis of the In complex, a modified procedure reported by Young was used [23]. To 0.33 g

(1.1 mmol) of In(NO3)3 dissolved in 3 mL of distilled water, is added in small portions with magnetic

stirring, a solution of 0.57 g (3.5 mmol) of ligand (1-benzoylacetone), 4 mL of distilled water and

0.8 mL of 5 M NH3. The mixture is taken to pH 8–10 by adding 5 M NH3, after which a precipitate is

formed. After stirring for 1 h, the solution is filtered under reduced pressure. The crude product is

recrystallized from ethanol to give a white powder (80.3% yield), m.p.: 208–212 8C; IR data (film): nCO

1515.4 (s); 1561 (s); 1592 (s) cm�1; UV data l (log e) in CH2Cl2: 320 nm (4.72), 254 nm (4.34), 232 nm

(4.19).

2.2.2. Bis(1-phenyl-1,3-pentanedionato)Co(II)

For the synthesis of the Co(II) complex, a method reported by Ellern and Ragsdale was used [24]. To

an aqueous solution of benzoylacetone (2 mmol in 20 mL of H2O), 5 M NH3 is added dropwise until a pH

between 8 and 10 is reached. CoCl2�6H2O (1 mmol in 10 mL of H2O) is then added and the mixture

stirred until a precipitate is formed. The crude product is filtered and dried under vacuum, and

recrystallized from EtOH/CHCl3 at 30 8C to obtain an orange powder (76% yield), m.p.: 125–

130 8C; IR data (film): nCO 1512.0 (s), 1592 (s) cm�1; UV data l (log e) in CH2Cl2: 322 nm (4.43),

250 nm (4.12), 232 nm (4.06).

2.3. Preparation of amorphous thin films

The thin films of the precursor complexes were prepared by the following procedure: a silicon chip

was placed on a spin coater and rotated at a speed of 1500 rpm. A portion (0.1 mL) of a solution of the

diketonate complex in CH2Cl2 was dispensed onto the silicon chip and allowed to spread. The motor was

then stopped and a thin film of the complex remained on the chip. The quality of the films was examined

by optical microscopy (1000� magnification) and in some cases by SEM.

2.4. Photolysis of complexes as films on Si(1 0 0) surfaces

All photolysis experiments were done following the same procedure. Here is the description of a

typical experiment. A film of the diketonate complex was deposited on p-type Si(1 0 0) by spin coating

from a CH2Cl2 solution. This resulted in the formation of a smooth, uniform coating on the chip. The

UV–vis spectrum of the starting film was first obtained. The chip was then placed under a UVS 254 nm

lamp. After the UV–vis spectrum showed no evidence of the starting material, the chip was rinsed several

times with dry acetone to remove any organic products remaining on the surface, prior to analysis.

3. Results and discussion

The electronic spectra of thin films of the In3+ and Co2+ complexes exhibited bands at 254 and 330 nm

approx. The observed absorption bands have been assigned to the various electronic transitions, the band

at 254 nm corresponding to LMCT transition (ligand-to-metal charge transfer) while the absorption at

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330 nm being assigned to an intraligand p! p* transition. Hence, the photochemistry of these

complexes is initiated through the irradiation of the LMCT band at 254 nm.

The photodeposition of indium oxide and cobalt oxide was achieved by UV (254 nm) irradiation of an

amorphous film of the precursor complexes M(C6H5COCHCOCH3)x (M = In3+ and Co2+) under air

atmosphere. The UV–vis spectrum of the precursors deposited on quartz cells was easily detected and

was used to monitor the reaction throughout the photochemical process (Fig. 1a and b). The loss of

starting material was clearly evident, and at the end of the photolysis, after a 2 h irradiation period, there

G.E. Buono-Core et al. /Materials Research Bulletin 40 (2005) 1765–17741768

Fig. 1. (a) UV–vis spectral changes associated with the irradiation (254 nm) for 45 min of a film of tris(1-phenyl-1,3-

pentanedionato)In(III). (b) UV–vis spectral changes associated with the irradiation (254 nm) for 90 min of a film of bis(1-

phenyl-1,3-pentanedionato)Co(II).

were no detectable absorptions associated with the diketonate ligand in the UV–vis spectrum. When

Si(1 0 0) substrates were used for deposition, the progress of the reactions were monitored by FT-IR

spectroscopy.

3.1. Characterization of indium oxide thin films

The as-deposited In oxide films were analyzed by Auger electronic spectroscopy (Fig. 2). The

spectrum shows the peaks associated to C (23.4%), In (31.1%) and O (45.6%). The signal position as well

as the relative amounts of each element are indicative of the formation of In2O3. It has been well

established that the signals associated to In as an oxide correspond to M4N4.5N4.5 and M5N4.5N4.5

transitions which appear, as a doublet, at 399 and 405 eV, respectively. On the other hand, oxygen signals

corresponding to KL2.3L2.3 transitions appeared at 510 eV [25].

In general, these Auger MNN levels are extremely sensitive to chemical changes in the surroundings.

In our case, there is a signal at 397.9 eV corresponding to MNN transitions and the oxygen signal at

502 eV. These small differences in peaks position can be attributed to the amorphous characteristics of

the obtained films, considering that most of the reported values correspond to polycrystalline samples

[25].

The presence of metallic In has been established as ‘‘plasmon peaks’’ at 381 and 392 eV, and the

signals corresponding to MNN transitions attributed to In appearing as less resolved doublet, ranging

from 399–404 to 405–410 eV for each doublet peak, respectively [25]. Although in our case only a single

signal is observed for the MNN transitions, the [O]/[In] ratio of 1.47 (1.5 for In2O3), is indicative of the

almost stoichiometric formation of the oxide. The high carbon content (25.4%) detected on the surface of

the as-deposited films is probably the result of contamination rather than an inefficient photolysis. It has

been argued that the presence of phenyl rings in the precursors often introduces carbon impurities in

G.E. Buono-Core et al. /Materials Research Bulletin 40 (2005) 1765–1774 1769

Fig. 2. Auger survey spectrum of an as-deposited In2Ox film (200 nm thick) photodeposited from tris(1-phenyl-1,3-pentane-

dionato)In(III) after sputtering with Ar+ for 30 s.

MOCVD of other metals [26]. However, after Ar+ ion sputtering of the films for 5 min, the intensity of the

C signal decreases to 5%.

In order to further confirm the formation of In2O3, the as-deposited films were annealed at 900 8Cunder air in a Lindberg oven for 2 h, and then analyzed by XRD (Fig. 3). In the XRD spectrum can be

observed peaks at 2u angles of 21.58, 30.68, 35.58 and 51.28 associated to (2 1 1), (2 2 2), (4 0 0) and

(4 4 0) planes, respectively, revealing the formation of cubic polycrystalline In2O3 [27].

A morphological analysis by AFM of amorphous films of In oxide, with a thickness of 200 � 20 nm,

showed a rough surface with rms values of 26.1 nm, revealing also the formation of craters of diverse

diameters (50–1500 nm) (Fig. 4).

3.2. Characterization of indium–cobalt oxide thin films

For the deposition of In–Co oxide films, solutions with different proportions of the In and Co

complexes were spin-coated on the appropriate substrate and the films irradiated until no IR absorptions

due to the precursors were observed.

The chemical composition of the In–Co oxide films (10, 30 and 50% of Co) was investigated by XPS

(Fig. 5) and XRD (Fig. 6). The XPS spectrum shows signals for In 3d5/2, In 3d3/2 and O 1s for oxidized

In amorphous films. The binding energy 444.7 eV in the In 3d5/2 signal corresponds to the In 3d5/2 peak

for In2O3, which was shifted to a slightly higher binding energy than the 443.56 eV of In metal [28,29].

Also shown in the spectrum are signals of In 3d3/2 at 452.9 eV which corresponds to the indium oxide

form [29]. The In 3d3/2 signal for In metal has been reported at 451.3 eV [28]. The oxygen signals show

two O 1s peaks, centered at 531.5 and 533.3 eV. The binding energy at 533.3 eV is due to an adsorbed

oxygen or hydroxide in the surface of the film. Another large peak, 531.5 eV, comes from the O 1s of

In2O3 films.

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Fig. 3. X-ray diffraction pattern of an annealed (900 8C, 2 h, oxygen atmosphere) indium oxide film produced by UV

irradiation.

The Co 2p3/2 and 2p1/2 peak positions at 780.9 and 795.8 eV, respectively (Fig. 5), are in agreement

with the presence of Co3O4 in the as-deposited films [30,31]. Moreover, this is also indicated by the

almost complete absence of satellite structures (shake-up peaks) (Fig. 5 inset), characteristic of Co(II)

high-spin compounds, such as CoO, at ca. 787.0 and 804.0 eV. It has been observed that the binding

energy (BE) values of the most intense Co photoelectronic peak (Co 2p) does not allow a clear distinction

between CoO (pure Co(II)) and Co3O4 (CoIICoIII2 O4). In the last case, a single asymmetrical peak is

usually evidenced since the contributions of the two oxidation states are not well resolved [32–34].

G.E. Buono-Core et al. /Materials Research Bulletin 40 (2005) 1765–1774 1771

Fig. 4. AFM image of indium oxide amorphous thin film deposited on Si(1 0 0). Image size 10 mm � 10 mm with z-scale of

200 nm.

Fig. 5. XPS survey spectrum of an as-deposited Co(50 mol%)–In2O3 thin film produced by UV irradiation at 254 nm. (Inset) Co

2p peaks in the 750–850 eV region.

XPS analysis revealed surface contamination of the as-deposited films as expected. A strong C 1s peak

appeared at �285 eV binding energy prior to sputtering of the films with Ar+, indicating that films have

carbon contamination in the surface more than in the bulk.

X-ray diffraction spectra (Fig. 6) of the annealed films clearly show polycrystalline cubic In2O3 with a

preferential (2 2 2) texture (2u angle 30.88) for the films loaded with 10 and 30 mol% of Co. However, for

the Co(50 mol%)–In film, there is a decrease in intensity of the peak at 30.88, preferentially a (2 2 2)

texture, and an increment in the (5 3 3) plane at 77.88 corresponding to a polycrystalline cubic Co3O4.

G.E. Buono-Core et al. /Materials Research Bulletin 40 (2005) 1765–17741772

Fig. 6. X-ray diffraction patterns of annealed (900 8C, 2 h, oxygen atmosphere) Co(10 mol%)–In2O3, Co(30 mol%)–In2O3 and

Co(50 mol%)–In2O3 thin films photodeposited on Si(1 0 0) substrates.

Fig. 7. AFM micrograph of a 30 mol% Co–indium oxide film, photodeposited on Si(1 0 0). Image size 10 mm � 10 mm with

z-scale of 500 nm.

Other peaks observed at 2u angles of 19.1, 49.3 and 65.28 are associated with the (1 1 1), (3 3 1) and

(4 4 0) planes of Co3O4.

The AFM images of indium oxide–cobalt oxide mixtures amorphous films (30 and 50% of cobalt) are

shown in Figs. 7 and 8. This image shows one surface roughness with crater formation of diameter diverse

for the film with 30% of cobalt, with an rms roughness of 30.4 nm. Although no correlation seems to exist

between the rms values and the amount of cobalt present in the films, it can clearly be seen from Fig. 8

that films loaded with 50 wt.% Co, show a very smooth surface, with an rms roughness of 7.24 nm, and

that the final deposit is uniform with a relative small number of visible craters or other imperfections.

4. Conclusion

High quality In2O3–Co3O4 thin films with compositions ranging from pure In2O3 to 10–50 mol% of

Co, were successfully prepared at room temperature by a simple photochemical method using metal b-

diketonate complexes as precursors. It has been shown that morphology of the films depend on the

composition, those with higher Co content having a smoother surface. Experiments are underway in

order to evaluate the gas sensor properties of these films.

Acknowledgments

The authors thank FONDECYT, Chile (Proyect No. 1010390) and Pontificia Universidad Catolica de

Valparaıso (Proyect D.I. No. 125.735/01) for financial support for this research. G. Cabello wish to thank

Program MECESUP (Chile) for Doctoral Fellowship. Support of Fundacion Andes (Convenio C-13672)

and FNDR V Region (BIP 20175666-0) for the acquisition of Brucker NMR instrument is also gratefully

acknowledged.

G.E. Buono-Core et al. /Materials Research Bulletin 40 (2005) 1765–1774 1773

Fig. 8. AFM micrograph of a 50 mol% Co–indium oxide film, photodeposited on Si(1 0 0). Image size 10 mm � 10 mm with z-

scale of 500 nm.

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