Spray Pyrolysis Deposition of α-Fe2O3 Thin Film

Transparent thin films of α-Fe2O3 have been deposited on glass substrates by chemical spray pyrolysis (CSP) technique using solution of aqueous iron (III) nitrate at 300-450◦C. Their structural, optical, and electrical properties were investigated without post-deposition annealing. XRD data confirm that the grown film at 450◦C was polycrystalline in nature and has preferred orientation in (110) direction while films grown at lower temperatures were amorphous. The film exhibits high average optical transmission (80%). The optical data revealed that the optical indirect and direct band gap energies of films were 1.9 eV and 2.68 eV respectively. Electrical measurements show that the deposited α-Fe2O3 film is n-type and the electrical resistivity of the film was about 2.5× 10 Ωcm. [DOI: 10.1380/ejssnt.2008.96]


I. INTRODUCTION
α-Fe 2 O 3 is n-type semiconductor having hexagonal closed-pack crystal structure.Iron oxide is widely used for direct water splitting under solar illumination due to their band gap, high resistivity toward corrosion, stability in solution, ease of manufacturing and material availability, and low cost [1].Furthermore, the iron oxide is essential material for humidity and gas sensors, catalyst, magnetic recording and medical fields [2][3][4].Many methods have been adopted to grow α-Fe 2 O 3 film such as solgel [5], metal organic [6], pulsed laser deposition [7], chemical spray pyrolysis (CSP) [8], filtered arc deposition [9] and MBE [10].The CSP technique is one of promising methods to prepare α-Fe 2 O 3 film and here we adopt this method to prepare this film.The reported results confirmed that the properties of α-Fe 2 O 3 were strongly dependent on deposition method.Recently Luo et al. [5] prepared α-Fe 2 O 3 by sol-gel method and they confirmed that the optical and electrical properties of photoanode α-Fe 2 O 3 (PEC junction) play major role in improvement of generated photocurrent.
In this paper we reported the structural, optical, thermoelectric and electrical properties of α-Fe 2 O 3 thin film prepared by spray pyrolysis technique.The CSP has many advantages over the other methods such as; simple, inexpensive, possibility of large area deposition does not need high vacuum apparatus, ease doping and stoichiometery preservation.Furthermore, the properties of grown film can be varied and controlled by optimization of spraying conditions.

II. MATERIALS AND METHODS
α-Fe 2 O 3 thin film has been deposited by spraying aqueous solution of 0.1M of Fe(NO 3 ) 3 .9H 2 O onto cleaned glass substrate at 350, 400 and 450 • C and the substrate * Corresponding author: raidismail@yahoo.comtemperature was kept to within ±5 • C. The spraying flow rate was adjusted to be around 3 ml/min.Air was used as gas carrier.The schematic diagram of home made spray pyrolysis system is presented in Fig 1 .The nozzle to glass substrate distance was around 30 cm.The thickness of film was measured using weighing method and found to be about 250 nm.The structural characterization of the film was carried out using x-ray diffraction (XRD) using Cu Kα source in the range of 2θ between 20 • and 50 • .The transmission spectrum of Iron oxide film at room temperature in the wavelength range of 400-800 nm has been investigated using Shimaddzu double-beam spectrophotometer.Ohmic contact was made on film by deposition of Al thick film through special mask which showed linear I-V characteristics over a wide range of applied voltage.The DC conductivity of film in the temperature range 300-450 K was performed using Keithly electrometer.Thermoelectric properties of iron oxide film were investigated.

III. RESULTS AND DISCUSSION
XRD results revealed that the films deposited at temperatures lower than 450 • C were amorphous in nature.104) directions.The reported results of α-Fe 2 O 3 prepared by spray method from iron chloride solution confirmed that the film is polycrystalline and has preferred orientation in (104) direction [8].No other phases have been observed in XRD spectrum e.g.magnetite and maghemite.Figure 3 demonstrated transmittance (T ) versus wavelength plot of the film deposited at 450 • C. The average transmittance of film in visible and NIR was about 80%.On the other hand, the film has red brownish color and showed good adherence to the substrate surface while other films exhibited not good enough adherence properties.
The absorption α of film was calculated from the following relationship α = (2.303/t)log(1/T ), (1)   Where t is film thickness.The optical absorption coefficient was calculated and found to be 7 × 10 4 cm −1 .This result was comparable with that results reported by Desai et al. [11].The optical band gap E g for film was determined from plot of α 2 versus hν as shown in Fig. 4 and according to the following equation Where A is a constant and n is equal to 2 and 1/2 for direct and indirect transitions.The indirect band gap and direct band gap energies at 300K were about 1.9 and 2.68 eV respectively and in fact these results agree well with those reported in ref. [9].The published data confirmed that the band gap of α-Fe 2 O 3 is ranging from 2.4-2.7 eV [8,9,11] depending on deposition methods and conditions.The quality, purity and structure of deposited film may affect the band gap.The electrical measurements revealed that the deposited film has n-type conductivity.The origin of n-type conductivity of film is surface adsorption, which causes a depletion layer on the surface, but the major carrier were still electrons [12].Figure 5 shows an Arrhenius plot.The activation energy E a calculated from the slope of linear fit of Fig.   is 0.7 eV and this result agrees with that for crystalline α-Fe 2 O 3 film prepared by filtered arc deposition FAD [9] and activation energies of films prepared at other temperatures presented at Table I.The high value of this acti-vation energy can be attributed to the formation of conduction barrier at grain boundary as reported by Grander et al. [13].The electrical resistivity at 300 K of film deposited at 450 • C was about 2.5 × 10 4 Ωcm.Films deposited at lower temperatures showed high resistivity as depicted in table I. Thermoelectric power measurements revealed that the film has a negative Seebeck coefficient of about 95 µV/K as shown in Fig. 6, the negative sign indicating n-type conduction of deposited α-Fe 2 O 3 film.On the other hand, all deposited films were n-type.

IV. CONCLUSIONS
To summarize, iron oxide thin film was deposited with aid of low cost spray pyrolysis technique using aqueous iron (III) nitrate as basic salt.The film deposited at 450 • C was polycrystalline and having rhomboheadral (hexagonal closed pack hematite α-Fe 2 O 3 ) crystal structure with preferred orientation in (110) direction.Neither other iron oxide nor impurity phases were detected.The films deposited at temperatures lower than 450 • C were amorphous.The film deposited at optimum temperature (450 • C) exhibited high optical transmittance.The indirect and direct band gap energies were found to be approximately 1.9 eV and 2.68 eV respectively.On the light of obtained results the α-Fe 2 O 3 film prepared by CSP can be used for photochemical applications.The n-type conductivity was confirmed by negative value of Seebeck coefficient.The modification of hematite film properties by introduction of dopants is under way.

Figure 2
Figure2shows the XRD spectra of Fe 2 O 3 films deposited at 400 • C and 450 • C. It is obvious that the film deposited at 400 • C was amorphous while the film deposited at 450 • C is polycrystalline in nature and crystallized in rhomboheadral (hexagonal closed pack hematite α-Fe 2 O 3 ) structure.The film has preferred orientation in (110) direction and other small diffracted peaks were in (102) and (104) directions.The reported results of α-Fe 2 O 3 prepared by spray method from iron chloride solution confirmed that the film is polycrystalline and has preferred orientation in (104) direction[8].No other phases have been observed in XRD spectrum e.g.magnetite and maghemite.Figure3demonstrated transmittance (T ) versus wavelength plot of the film deposited at 450 • C. The average transmittance of film in visible and NIR was about 80%.On the other hand, the film has red brownish color and showed good adherence to the substrate surface while other films exhibited not good enough adherence properties.The absorption α of film was calculated from the following relationship

FIG. 4 :
FIG. 4: Direct band gap transition of film.Inset: Determination of indirect band gap.

TABLE I :
Properties of iron oxide films deposited at different temperatures.