1. Introduction

Nanocrystalline transition metal oxide thin films are fascinating materials because their electrical and magnetic properties which have great importance in the field of micro-electronics and nano-electronics, mainly for the development of optoelectronic devices. Materials in nanometer range are found to exhibit new modified physical and chemical properties for wide range of applications. The researchers are extensively investigating simple and economic deposition techniques to grow nanocrystalline metal oxide materials which are suitable substitute for the existing semiconductors. Amongst various metal oxides, iron oxide (α-Fe₂O₃) is a thermodynamically stable oxide of hexagonal close packed crystal structure with indirect and direct band gap energies around 1.9 and 2.7 eV, respectively [1]. In recent years, much attention of scientific community has been focused on iron oxides due to their potential applications such as magnetic devices [2], gas sensor [3-6], humidity sensor [7], solid state lithium ion battery [8], supercapacitor [9], water splitting for hydrogen production [10], solar cell [11, 12], solar filters [13] etc. Apart from this iron oxide nanostructures are technologically important due to their possible applications in many fields including high density magnetic storage devices, magnetic refrigeration systems, catalysis and chemical and biological sensors [14, 15].

In addition to this recently iron oxide has became a novel material for its potential applications in medical science such as drug delivery system, cancer therapy and magnetic resonance imaging due to its biocompatibility, catalytic activity and low toxicity [16–18].

A variety of chemical techniques have been used to fabricate iron oxide thin films such as spray pyrolysis [19, 20], thermal decomposition [21], atomic layer deposition [6], ion-beam assisted deposition [22], electrodeposition [23], RF magnetron sputtering process [24], chemical vapor deposition [25, 26] and SILAR method [27] etc. Among them, SILAR is simple, practical, nonhazardous and low cost soft chemical method. In SILAR technique, nanocrystalline thin films are grown by immersing substrate into separately placed cationic and anionic precursors. In between each immersion substrates are rinsed with purified water, so that only tightly adsorbed layer of species stays on the substrate [28].

In the present work, SILAR method has been developed to grow device quality nanocrystalline iron oxide thin films onto glass substrates as no significant data was published on optical, structural and electrical properties of α-Fe₂O₃ films. The prepared iron oxide thin films were characterized using different analytical techniques such as X-ray diffraction (XRD), FTIR spectroscopy, Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM). Optical absorption and electrical resistivity measurements were carried out to find optical band gap and activation energies of α-Fe₂O₃.

2. Experimental

2.1. Deposition of α-Fe₂O₃ Thin Film

Nanocrystalline α-Fe₂O₃ thin films were deposited by Successive Ionic Layer Adsorption and Reaction method onto glass substrates at room temperature. For the deposition of α-Fe₂O₃ thin films, 0.05 M ferric chloride of pH 1 was used as a cationic precursor and 0.001M sodium hydroxide of pH 11 was used as an anionic precursor. Before actual deposition glass substrates were boiled in chromic acid, washed with soap solution and finally cleaned with HCl and de-ionized water to provide clean substrate surface to achieve uniform deposition. The well cleaned glass substrate was then immersed in a cationic precursor solution for 20s for the adsorption of iron species on the substrate surface. The substrate was then rinsed in de-ionized water for 20s to remove weakly bound species of Fe³⁺ ions from its surface. Once the weakly bound species of Fe³⁺ ions was removed, the substrate was then immersed in an anionic precursor NaOH for 20 s to form a layer of iron oxide material. To remove un-reacted and excess species from substrate, it was rinsed in deionized water for 20 s. This completes one SILAR cycle. By repeating such sixty SILAR cycles, brown coloured, uniform thin films of thickness 251nm were obtained. The films deposited were annealed at 573K for 2h to remove any hydroxide phase formed during the synthesis process and post annealed at 773 K temperature for 3h to obtain reddish coloured pure phase of α-Fe₂O₃.

2.2. Characterization of α-Fe₂O₃ Thin Film

In the present work, thickness of the film was measured by gravimetric weight difference method using the relation [29],

(1)

where m is the mass of the film deposited on the substrate in gram; A is the area of the deposited film in cm² and ρ is the density of the deposited material in bulk form (Fe₂O₃ = 5.242 g/cm³) [9]. The crystal structure of the film material was identified by X-ray diffraction analysis with D8 Advance, Bruker, Germany diffractometer with monochromatic CuKα radiation of wavelength 0.154 nm. The crystallite size of the deposited sample was estimated using the Debye-Scherrer method. The Fourier Transform Infrared (FTIR) spectrum of the sample was collected from Shimadzu make FTIR unit IR Affinity-I. The film morphology was studied by using Field Emission Scanning Electron Microscope (Model: SUPRA 40) and Atomic Force Microscope (Model: Nanonics Multiview 2000^TM, Israel). The optical absorption studies were carried out using ELICO ® Double Beam SL 210 UV-VIS Spectrophotometer in the wavelength range 350 to750 nm. Electrical resistivity measurement of sample with temperature was performed using a two-point probe technique with digital electrometer and a stabilized power supply.

3. Results and Discussion

Several deposition trials were performed to optimize concentration of FeCl₃ precursor, pH of NaOH solution and number of SILAR deposition cycles. To optimize the concentration of FeCl₃ precursor, α-Fe₂O₃ films were prepared at fix pH of NaOH for 50 SILAR cycles by changing its concentration from 0.03 to 0.3M. Figure 1 shows the variation of α-Fe₂O₃ film thickness with concentration of FeCl₃ precursor. Initially the film thickness rapidly increases from 75 to 192 nm as concentration of FeCl₃ increases from 0.03 to 0.05M and above this it decreases slowly. At lower concentration of FeCl₃ optimum number of Fe³⁺ ions are available in the solution to form adhesive layer of α-Fe₂O₃ that gives maximum terminal thickness. But at higher concentration, more Fe³⁺ ions are available in the solution which increases the reaction rate producing rapid precipitate on the substrate surface, which was washed out after rinsing the substrate in water as a result lower terminal thickness is obtained. The optimized concentration of FeCl₃ precursor was found to be 0.05M.

Figure 1. Variation of α-Fe2O3 film thickness with concentration of FeCl3 solution for 50 SILAR deposition cycles

To optimize pH of NaOH solution, the optimized concentration of FeCl₃ (0.05M) and 50 SILAR cycles are kept constant. The variation of α-Fe₂O₃ film thickness with pH of NaOH is shown in Figure 2. It is observed that at lower pH values (pH < 8) the growth rate is very low due to less availability of OH^- ions and it increases up to 11 and it becomes optimum. For pH > 11, film thickness decreases due to high growth rate produces less adhesive film which was easily washed out while rinsing in water.

Figure 2. Variation of α-Fe2O3 film thickness with pH of NaOH solution for 50 SILAR deposition cycles

Figure 3 shows variation of α-Fe₂O₃ film thickness with number of deposition cycles at the optimized pH and concentration of FeCl₃ solution. Initially, film thickness increases up to 60 cycles and then decreases due to peeling of the powdery layer from the substrate surface [30, 31]. Also, by making several trials, the immersion and rinsing time periods were optimized and the values are listed in Table 1.

Figure 3. Variation of α-Fe2O3 film thickness with number of SILAR deposition cycles

Table 1. Optimized deposition parameters for the preparation of α-Fe2O3 film

3.1. Film Formation Mechanism

Thin films of α-Fe₂O₃ were prepared by SILAR method onto glass substrates by immersing glass substrates alternately into separately placed cationic and anionic precursors. The deposition mechanism follows the ion-by-ion growth process at nucleation sites on the immersed surfaces. In aqueous solution FeCl₃ dissociates as,

(2)

When the glass substrate was immersed in the cationic precursor, Fe³⁺ ions get adsorbed on the substrate due to attractive forces between ions in the solution and that of the surface of the substrate. These forces may be cohesive or vander Waals or chemical attractive [32]. After rinsing in deionized water for 20s, the glass substrate was then immersed in NaOH precursor where Fe³⁺ ions react with OH^- ions to give Fe(OH)₃ as,

(3)

The Fe(OH)₃ deposited on the substrate thermally decomposed to FeO(OH) as,

(4)

The further annealing of FeO(OH) for 3h at 773K gives α-Fe₂O₃ [33] as,

(5)

3.2. Structural Studies

To determine crystal structure and identification of phases of iron oxide thin films synthesized by SILAR method, X-ray diffraction analysis was carried out in the range of angle 2θ between 20 to 80 degree. A typical XRD peak pattern of iron oxide film of thickness 251 nm deposited onto glass substrate is shown in Figure 4. The (0 1 2), (1 0 4), (1 1 6) and (0 1 8) XRD peaks in the pattern confirms rhombohedral α-Fe₂O₃ (hematite) structure in accordance with JCPDS card 79-0007.

Figure 4. X-ray diffraction pattern of α-Fe2O3 thin film of thickness 251 nm

Comparison of observed XRD data of α-Fe₂O₃ thin film with the JCPDS card 79-0007 is shown in Table 2. The average crystallite size of the film was determined by using Scherrer formula,

(6)

where λ is the wavelength (0.154 nm); β is the angular line width at half maximum intensity in radians; θ is the Bragg’s angle. The average particle size calculated from the prominent peaks (0 1 2) and (1 0 4) is 33 nm.

Table 2. Comparison of observed crystallographic data of α-Fe2O3 thin film with standard JCPDS data card no. 79-0007

3.3. FTIR Study

For the FTIR measurements, a Shimadzu FTIR spectrometer working in the mid-infrared range from 400 to 4000 cm⁻¹ was used. The spectrum was acquired in transmittance mode with a resolution of 1 cm⁻¹. Figure 5 shows FTIR spectra of α-Fe₂O₃ thin film deposited at temperature 303 K. In the region below 700cm⁻¹ two absorption peaks at 617.22 and 455.20 cm⁻¹ corresponding to the metal-oxygen (Fe-O) vibration modes of the spinel compound are observed. These two Fe-O peaks are sharp and prominent and are in good agreement with literature [9, 34]. The absorption peaks at 717.52 and 952.84 cm^-1 may be attributed to Fe-OH vibration modes and the peak at 1992.47cm^-1 is attributed due to O-H stretching [32, 35]. The absorption peaks around 1357.89 and 1492.90 cm⁻¹ may be attributed to O-H bending vibrations combined with Fe atoms.

Figure 5. FTIR spectra of the α-Fe2O3 thin film in the range 400-4000cm-1

3.4. Surface Morphology

Morphology of the iron oxide thin film deposited onto glass substrate by SILAR method was examined by using FE-SEM image at magnification 44.92 KX and 77.93 KX (fig.6). The images show random distribution of α-Fe₂O₃ that covers whole substrate surface. Figure 6(A) and 6(B) shows porous granular structures of α-Fe₂O₃ with agglomeration at some places. The clear separation between the grains in magnified figure indicates vertical growth of α-Fe₂O₃. The agglomeration of grains may be due to overgrowth of α-Fe₂O₃, as the smaller primary particles have large surface free energy and would tend to agglomerate faster and grow into larger grains. The average grain size of these deposited particles is about 34 nm which is in good agreement with XRD results.

Figure 6. FESEM spectra of α-Fe2O3 thin film at magnification: (A) 84.11 KX and (B) 190.57 KX

Elemental analysis of the α-Fe₂O₃ thin film was achieved from the Energy Dispersive X-ray analysis (EDX) spectra presented in Figure 7. The line observed at 6.398 keV is due to K line of Fe element and the line at 0.525 keV is due to K line of oxygen. The additional peaks in the spectra are due to glass substrate especially because of Si and O. The calculated atomic ratio of Fe and O is approximately equal to 2:3 which agrees well with the stoichiometric composition of α-Fe₂O₃.

Figure 7. EDAX spectra of α-Fe2O3 thin film of thickness 251 nm

The surface morphology of α-Fe₂O₃ film was further analyzed using atomic force microscope having optical fiber tip coated with gold and chromium (Au, Cr) metal with φ = 20 nm at response frequency 52.38 kHz in tapped mode. Figure 8 shows the AFM image of α-Fe₂O₃ film deposited onto glass substrate. The 2D and 3D micrographs are analyzed to understand the roughness, height and grain orientation of α-Fe₂O₃. The 2D micrograph shows rms roughness of the film is 92.32 nm while average roughness is 79.43 nm. The maximum height of the film is 610.02 nm whereas average height is 259 nm which is in agreement with film thickness measured by gravimetric weight difference method. From the 3D micrograph, the grain orientation of α-Fe₂O₃ estimated is found to be 0.07pi.

Figure 8. AFM images of α-Fe2O3 thin film. (A) 2D image (B) 3D image

3.5. Optical Studies

The optical absorption spectrum of α-Fe₂O₃ thin film was studied in the range of 350-750 nm at room temperature (Figure 9). It appears that α-Fe₂O₃ thin film has high absorbance in the visible region, indicating its applicability as an absorbing material. The optical band gap (Eg) of α-Fe₂O₃ film was calculated using equation [36],

(7)

where, α is absorption coefficient, Eg is optical band gap energy, A is constant and n is equal to 2 for direct and 1/2 for indirect transition. The plot of versus of α-Fe₂O₃ film is shown in Figure 10.

Figure 9. Optical absorption spectra of α-Fe2O3 thin films deposited on to glass substrate

Figure 10. Plot of versus of α-Fe2O3 thin film deposited on to glass substrate

From linear nature of the plot, the indirect optical transition in α-Fe₂O₃ is confirmed. The band gap energy Eg is estimated by extrapolating the linear portion of the plot to the energy axis and is found to be 1.92 eV. Figure 11 shows plot of versus for α-Fe₂O₃ film which confirms the presence of direct transition. The optical direct band gap energy of α-Fe₂O₃ is found to be 2.97 eV. Both the values of band gap energies are quite more than literature which may be due to quantum confinement in the nanocrystalline α-Fe₂O₃ [1].

Figure 11. Plot of versus of α-Fe2O3 thin film deposited on to glass substrate

3.6. Electrical Resistivity

The dc two point probe method was employed to understand the variation of electrical resistivity of α-Fe₂O₃ with temperature. Figure 12 shows the variation of with reciprocal of temperature. It is clearly seen that the resistivity decreases with increase in temperature indicating semiconducting behavior of α-Fe₂O₃ . The room temperature electrical resistivity of α-Fe₂O₃ thin film deposited onto glass substrate by SILAR method is of the order of 9.46 × 10² Ω cm, which is very close to reported value (3 × 10² Ω cm) by A. A. Akl [20] and much less as compared to earlier reports (10⁴ Ω cm) [24]. The activation energy was calculated using relation,

(8)

where, ρ is the resistivity at temperature T, is constant, K is Boltzmann constant and is activation energy. The activation energy of α-Fe₂O₃ thin film deposited onto glass substrate was found 0.1 eV which is in good agreement with the earlier reports [37].

Figure 12. Variation of log of resistivity with reciprocal of temperature of α-Fe2O3 thin film

3.7. Thermoemf Measurement

To find the type of conductivity of α-Fe₂O₃, thermo-emf developed across hot-cold junction was measured as a function of applied temperature dependence (Figure 13). The polarity of thermo-emf was found to be positive towards the hot end with respect to the cold end which confirms that the α-Fe₂O₃ exhibits n-type semiconducting nature; similar type of conductivity was reported by Balouria et al [38]. The thermo-emf developed across the film increases with applied temperature difference, which may be because of increased carrier concentration and mobility of charge carriers in the film.

Figure 13. Variation of thermoemf with temperature difference of α-Fe2O3 thin film

4. Conclusions

In this work, SILAR method has been successfully developed to grow nanostructured iron oxide thin films onto glass substrates. The XRD, FTIR, FESEM, EDX and AFM characterization confirms nanocrystallinity of α-Fe₂O₃ . The random distribution of α-Fe₂O₃ nano-grains with agglomeration is observed on the substrate surface. The XRD studies revealed that SILAR grown α-Fe₂O₃ exhibits rhombohedral lattice. The indirect and direct band gap energies are found to be 1.92 and 2.97eV respectively. The porous n-type α-Fe₂O₃ films are semiconducting in nature and have activation energy of the order of 0.1 eV.