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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.50 n.3 Concepción sep. 2005

http://dx.doi.org/10.4067/S0717-97072005000300003 

 

J. Chil. Chem. Soc., 50, N° 3 (2005), págs: 541-546

 

EVALUATION OF b-DIKETONATE TIN AND INDIUM COMPLEXES AS PRECURSORS FOR THE PHOTODEPOSITION OF SnO2 AND In2O3 THIN FILMS

 

G.E. BUONO-CORE,*1 G. CABELLO,1 JOSE LUIS CAYON,1 M. TEJOS2 AND R.H. HILL3

1 Instituto de Química, Pontificia Universidad Católica de Valparaíso, Valparaíso (Chile)
2 Facultad de Ciencias, Universidad de Valparaíso, Valparaíso (Chile)
3 Department of Chemistry, Simon Fraser University, Burnaby, B.C. V5A 1S6 (Canada)

E-mail address: gbuonoco@ucv.cl


ABSTRACT

SnO2 and In2O3 thin films have been succesfully prepared by direct UV irradiation of amorphous films of b-diketonate complexes on Si(100) substrates. The as-deposited films were analyzed by Auger electron spectroscopy (AES), whereas annealing was required in order to get X-ray diffraction spectra, indicating the amorphous nature of the films. The surface morphology of the films examined by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) revealed that In2O3 films are much smoother than SnO2 films, with average surface roughness (Ra) of 3 nm and 11 nm respectively. The low resistivity values determined for these metal oxide films (3-4 x 10-4 W cm) demonstrate the potential use of these deposited films in gas-sensing devices for detection of environmentally significant concentrations of oxidizing species such as NO2 and O3.

Keywords: Indium oxide; Tin oxide; photodeposition; metal b-diketonates; thin films


INTRODUCTION

Since the early 1980's, there has been a tremendous growth and progress in the development of a variety of semiconductors sensors [1]. These sensors, with ever improving performance cost ratio, will probably be the key components in the further penetration of microelectronics into new products and new applications [2,3].

Thin films are essential building materials in semiconductor microsensors [4,5]. These materials can approximately be divided in two main groups: the first include sensors that detect oxygen with variations of bulk or surface conductance; the second comprises all the materials that detect oxidizing and reducing gases in air at constant oxygen partial pressure by means of surface-conductance variations.

Indium oxide (In2O3) [6-10] and tin oxide (SnO2) [11-16] based films are among the most widely investigated semiconductor oxides as gas sensors due to their high sensitivity (i.e., normalized conductance variation) to several gases (e.g. CO, NO2, H2, CH4, H2S and ethanol).

Thin films may be deposited on a substrate by both physical and chemical means. Some of the most common deposition techniques used in microsensor technology are Spin Casting, Evaporation, Sputtering, which can be classified as physical methods, and Reactive Growth, Chemical Vapour Deposition (CVD) and Plasma Deposition (or PECVD, Plasma Enhanced CVD), which are all chemical methods [4].

We have developed in the last few years, a novel photochemical method by which we have been able to deposit copper and nickel oxides thin films, using b-diketonate complexes as precursors [17-19]. In this report, we present the synthesis and characterization of bis(b-diketonato)tin(II) and tris(b-diketonato)indium(III) complexes to be examined as precursors for the photochemical deposition of SnO2 and In2O3 thin films for possible use as gas sensors.

EXPERIMENTAL DETAILS

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 spectrophotometer. NMR spectra (400 MHz) were determined with a Brucker Model Avance Digital. Grazing indicence X-ray diffraction patterns were obtained using a D5000 X-ray diffractometer. The X-ray source was Cu 40 kV/30MA. SEM analysis was performed on a Jeol 5410 scanning electron microscope with an EDAX microanalysis attachment. Auger electronic spectra were obtained using a PHI double pass CMA at 0.85 eV resolution. Spectra were acquired using an electron beam energy of 5 keV and sputter depth profiling with a 3 keV Ar+ ion beam. Sensitivity factors provided by PHI were used to obtain normalized Auger intensities. Atomic Force Microscopy was performed on a Nanoscope IIIa (Digital Instruments, Santa Barbara, CA) in contact mode using silicon nitride tips. Resistivity of the films was determined by the 4-point probe method using a Signatone S-301 4-point probe, a Keithley 2000 Digital Multimeter, and a Keithley 224 programmable current source. Film thickness was determined by interferometry 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 UV absorption of the complexes.

The solid state photolysis was carried out at room temperature under a UVS-38 254 nm lamp equipped with two 8W tubes, in an air atmosphere.

The substrates for deposition of films were borosilicate glass microslides (Fischer, 2x2 cm) and p-type silicon(100) wafers (1x1 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 seconds and finally with deionized water. They were dried in an oven at 110oC and stored in glass containers.

Synthesis of b-diketonate complexes.

Tetrahydrofuran was dried and distilled from sodium benzophenone ketyl at atmospheric pressure before use. The reagents used for the synthesis of the ligand and the tin and indium complexes, were from Aldrich Chemical Co., and were used without further purification. The diketone 1-phenyl-1,3-nonanedione (L1) was prepared by the method reported by Adams and Hauser [20]. The sodium salt used in the synthesis of the tin complex, was prepared by the method previously described in the literature [21].

a) Bis(1-phenyl-1,3-nonanedionato)Sn(II)

Stannous chloride (1.0 g, 5.27 mmol) and Na(1-phenyl-1,3-nonanedionato) (2.42 g, 10.54 mmol) were loaded in a 250 mL flask under a nitrogen atmosphere. After addition of 80 mL of THF to the flask, the solution mixture was stirred at room temperature for 2 h. After filtration of the solution and removal of THF a pale yellow liquid was obtained. Purification of the crude product by distillation under reduced pressure afforded pale yellow Sn(L1)2 (70.3% yield). IR data (film): n CO 1597.2 (s) cm-1. Anal: Calcd. for C30H38O4Sn: C, 61.98; H, 6.59. Found: C, 61.86; H, 6.53.

b) Tris(1-phenyl-1,3-nonanedionato)In(III)

For the synthesis of the In complex a modified procedure reported by R.C. Young was used [22]. To 0.33 g (1.1 mmol) of In(NO3)3 dissolved in 3 mL of distilled water, a solution of 1g (4.3 mmol) of ligand, 4 mL of distilled water and 0.8 mL of 5M NH3 is added in small portions with magnetic stirring. The mixture is taken to pH 8-10 by adding 5M NH3, after which a precipitate is formed. After stirring for 1 hour, the solution is

filtered under reduced pressure. The crude product is recrystallized from ethanol to give a yellowish In(L1)3 powder (80.3% yield). IR data (film): n CO 1588.4 (s) cm-1. Anal: Calcd. for C45H57O6In : C, 66.83; H, 7.10. Found: C, 66.63; H, 6.99.

Preparation of amorphous thin films.

The thin films of the precursor complexes were prepared by the following procedure: A portion (0.5 ml) of a solution of the diketonate complex in CH2Cl2 was dispensed onto a silicon chip placed on a spin coater and then rotated at a speed of 1500 rpm 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 and in some cases by SEM.

Calibration of the FT-IR absorption on Si surfaces.

A solution of Sn or In complex in CHCl3 (0.2038 g in 5.0 ml) was prepared. From a microsyringe, 4 ml of this solution was placed on the silicon surface and allowed to dry. The area of the dried drop was 15.2 mm2 (1.52 x 1013 nm2) and correspond to a coverage of 1.132 x 103 molecules/nm2. The FT-IR of the surface was obtained. This process was repeated several times and the spectra collected with each additional drop. A linear plot of the absorbance at 1570 cm-1 vs coverage was used to determine coverage in films prepared for photodeposition.

Photolysis of complexes as films on Si surfaces.

All photolysis experiments were done following the same procedure. A typical experiment is described. A film of the diketonate complex was deposited on p-type Si(100) by spin-coating from a CHCl3 solution. This resulted in the formation of a smooth, uniform coating on the chip. The FT-IR spectrum of the starting film was first obtained. The chip was then placed under a UVS 254 nm lamp. After the IR 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.

RESULTS AND DISCUSSION

In recent years, we have developed a novel photochemical method for the deposition of a variety of metals and oxides [17-19, 23-27]. 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 several diketonate complexes of Sn and In of the type M(R1COCHCOR2)x (M=In or Sn) were investigated in order to determine if they formed good amorphous films upon spin coating. Although it was found that most of the above complexes form good quality films, the best results were observed with the complexes where R1 = phenyl, R2 = n-hexyl. These compounds can be spin coated from chloroform onto a suitable substrate such as borosilicate glass or a Si(100) chip, forming amorphous films showing no sign of crystallization on examination under an optical microscope up to 1000x magnification. These complexes also showed the highest photosensitivity in solution as well as a film, of all the b-diketonate derivatives. For further study we hence concentrated on Sn, Sn(L1)2, and In, In(L1)3, complexes with 1-phenyl-1,3-nonanedione (L1) as ligand.

Solution Photochemistry

Although the photochemistry of several transition metal 1,3-diketonates has been extensively studied [28-32], no reports can be found in the literature concerning Sn or In complexes. We therefore carried out experiments to evaluate the photosensitivity of the complexes Sn(L1)2 and In(L1)3. The UV spectra of both tin and indium complexes show similar absorption bands, with maxima at 234, 252 and 324 nm for Sn(L1)2 and 232, 254 and 324 nm for In(L1)3. When dichloromethane solutions of these complexes were photolyzed with 254 nm UV light, a rapid decrease in the absorption bands of the tin and indium complexes could be observed after 20 and 80 min of irradiation, respectively. Figs 1 and 2 show the UV profile of the photoreactions obtained by determining the UV spectra of samples taken at 1 and 5 min intervals, for Sn and In respectively. These results demonstrated that Sn and In diketonate complexes are highly photoreactive in solution when irradiated with 254 nm UV light.


 

Fig. 1. Changes in the UV spectrum of a solution of Bis(1-phenyl-1,3-nonanedionato)Sn(II) (1.13 x 10-4 M in CH2Cl2) upon 20 min irradiation with 254 nm light (1 min intervals).

  Fig. 2. Changes in the UV spectrum of a solution of Tris(1-phenyl-1,3-nonanedionato)In(III) (3.41 x 10 ­5 M in CH2Cl2) upon 80 min irradiation with 254 nm light (5 min intervals).

Solid State Photochemistry.

Thin films of Sn(L1)2 and In(L1)3 were prepared by spin-coating chloroform solutions of the complexes on Si(100) chips. The FT-IR spectra were similar to those obtained for crystalline samples.

Irradiation of thin films (~ 400 nm thickness) of Sn(L1)2 and In(L1)3 under air atmosphere, led to the disappearance of the absorptions associated with the ligand, as shown by the FT-IR monitoring of the reaction. At the end of the photolysis there are no detectable absorptions in the infrared spectrum. These results suggest that the diketonate groups on the precursors are photodissociated on the surface forming volatile products which are readily desorbed from the surface.

The resultant films were analyzed by Auger spectroscopy (Figs. 3 and 4). For Sn(L1)2, Auger electron spectroscopy analysis of the produced films after sputtering with Ar+ for 30 sec, showed that the film contained only Sn (33%) and O (64%), indicating also that the films are free from carbon contamination in the bulk (Fig. 3).


 
Fig. 3. Auger survey spectrum of SnO2 film (200 nm thick) photodeposited from Bis(1-phenyl-1,3-nonanedionato)Sn(II) after sputtering with Ar+ for 30 s.   Fig. 4. Auger survey spectrum of In2O3 film (120 nm thick) photodeposited from Tris(1-phenyl-1,3-nonanedionato)In(III) after sputtering with Ar+ for 5 min.

On the other hand, the results of Auger analysis for films obtained from In(L1)3 showed that the only peaks observed in the spectrum were indicative of carbon (12.5%), indium (34.5%) and oxygen (53.6%). Both the relative amounts of indium and oxygen and the position of the indium lines are indicative of In2O3 formation. The carbon content of the film is presumably a result of contamination rather than inefficient photochemistry. Sputtering of the surface with Ar+ for 5 min results in a decrease of the carbon signal to 4.6% (Fig 4).

In order to confirm the formation of SnO2 and In2O3, the resultant films were analyzed by XRD (not shown here). For this, films were annealed in an air atmosphere at 500 0C for 3 h in a Lindberg furnace and allowed to return to room temperature slowly. For In based films, the XRD pattern exhibit strong reflections associated with the (211), (222), (400) and (440) planes of cubic polycrystalline In2O3 at 2q angles of 21.5, 30.6, 35.5 and 51.2 degrees respectively [JCPDS 6-0416]. XRD spectrum of tin oxide films showed peaks at 2q angles of 26.61, 33.89 and 37.95 degrees associated with the (110), (101) and (200) planes of tetragonal SnO2 [JCPDS 21-1250].

AFM analysis of the films revealed that the SnO2 film (thickness = 200 nm) has a rougher surface morphology with an average roughness (Ra) of about 11 nm (Rmax = ± 108 nm) (Fig. 5), while the In2O3 film (thickness = 120 nm) showed a much smoother surface with an average roughness (Ra) of about 3 nm (Rmax = ± 39 nm) (Fig. 6). Despite the difference in surface roughness, AFM and SEM micrographs showed that in both, In and Sn films, grains are not well packed on the substrate, leaving relatively large mesopores in the films, and consequently having high porosity. These results are important since it has been reported that thickness and porosity of semiconductor films strongly influence their gas sensing properties [33].


 
Fig. 5. Atomic force micrograph of SnO2 film (200 nm thick) photodeposited from Bis(1-phenyl-1,3-nonanedionato)Sn(II) at room temperature under air atmosphere onto a Si(100) wafer. Image size 10 x 10 mm; z-scale 500 nm. RMS = 11 nm.   Fig. 6. Atomic force micrograph of In2O3 film (120 nm thick) photodeposited from Tris(1-phenyl-1,3-nonanedionato)In(III) at room temperature under air atmosphere onto a Si(100) wafer. Image size 1 x 1 mm; z-scale 100 nm. RMS = 3 nm.

To further characterize the photodeposited films, resistivity was determined by the 4-point probe method. Similar values of 3.18 x 10-4 and 3.95 x 10-4 W cm were obtained for as-deposited In2O3 and SnO2 films respectively, which are in agreement with previous values given in the literature [34, 35]. It is worth mentioning that the value found for the SnO2 film is a lower figure than those reported for undoped SnO2 films on silicon [36].

CONCLUSIONS

SnO2 and In2O3 thin films have been succesfully prepared by a purely photochemical method using b-diketonate complexes as precursors. Electrical and physical properties of the films demonstrated the potential use of these photochemically produced semiconductor oxides as gas sensors. Since the addition of metal clusters such as platinum and palladium can strongly improve the sensor sensitivity and selectivity [37], experiments are currently underway to study the applicability of this simple method for the preparation of oxide films with specific dopants such as Pt or Pd.

ACKNOWLEDGMENTS

This research was supported by FONDECYT, Chile (Project No. 1010390) and Pontificia Universidad Católica de Valparaíso (Project D.I. No. 125.735/01). 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.

 

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