Optical and Electrical Properties of TiO2 Doped Fe2O3 Thin Film Prepared by Spray Paralysis Technique

In this work, iron oxide (Fe2O3) and titanium oxide (TiO2) doping (Fe2O3) thin films have been prepared by spray pyrolysis technique (SPT) on a glass substrate. We have studied the effects of various doping percentage of (TiO2) on (Fe2O3) optical and electrical properties. The optical parameters of the prepared films as absorbance, absorption coefficient, optical energy gap and refractive index have been calculated for all prepared films. D.C conductivity and Hall effect for all films are measured. The results showed that absorbance of prepared films decreases with increasing percentage of (TiO2) this lead to increase the energy gap and conductivity decreased with increasing of percentage of (TiO2) as well as the results showed throughout the study that all films have tow activation energy, its value increase with increasing of percentage of (TiO2). Hall effect measurement showed all films have n-type conductivity.


INTRODUCTION
There are as many as 15 phases formed by Fe and O, as oxides of iron [1]. Metal oxide thin films have unique characteristics such as good magnetic properties and conductivity, high optical transmittance over the visible wavelength region, excellent adhesion to substrates and chemical stability and photochemical properties. Among magnetic materials, iron oxides, such as (α -Fe2O3) and (Fe3O4), are the most popular materials and possess many advantages in technological applications [2]. Iron oxide thin film (Fe2O3) can be used in several fields. (α -Fe2O3) is the most stable iron oxide compound material and is widely used in photoelectrodes, gas sensing, catalysts, magnetic recording, and medical fields. Iron oxide films can be used in a wide range of applications. Properties, such as high refractive index wide band gap and chemical stability make them suitable for use as gas-sensors. These oxides have been widely used in several industrial processes, such as dehydration, oxidation and Fischer-Tropsch synthesis [3]. Hematite (α-Fe2O3) is an inexpensive, abundant, non-toxic material, stable in most alkaline electrolytes. It is a poor electronic material with low conductivity [4]. As hematite typically exhibits ntype indirect band gap semiconducting behavior introduction of tetravalent ions into trivalent iron sites provides additional n-type doping [5]. TiO2 is the most important material as a catalyst for photoelectrochemical purification of wastewater because of its high oxidation potential in its valence band and high chemical stability [6]. TiO2 material has been attracting a great deal of attention amongst researchers because of its unique properties such as high optical transparency, wide band gap energy, high refractive index, high dielectric constant, non-toxicity, abundance in nature and good chemical stability in undesirable environment conditions [7]. It has been found that TiO2 in anatase crystalline phase is most active photocatalyst. Nevertheless, it has wide band gap energy of 3.2 eV, normal anatase phase TiO2 needs a UV radiation to initiate its photoactivity. Many attempts have been made to extend the light absorption edge of the catalyst to the visible light region by doping gold nanoparticles into TiO2 and to improve its photocatalytic degradation efficiency [8]. It has attracted much attention in recent years due to its great potential for applications in optical elements, electrical insulation, capacitors or gates in microelectronic devices, photovoltaic solar cells, antireflection coatings, optical waveguides, photonic crystals and devices based on metal etc. [9].

II.
EXPERIMENTAL DETAILS The pure iron oxide (Fe2O3) and doped titanium oxide (TiO2) thin films were prepared by spraying aqueous solution of iron oxide. This solution was prepared by dissolving 6.488g of (FeCl3.9H2O) {which is a powder of black color, it's molecular weight (126.20 g/mol.)} in 200 ml distilled water and doped with different ratio of TiO2 {1, 3 and 5 at %}. The dissolving weight (Wt) of the materials was determined by using the following equation

III. RESULTS AND DISCUSSION A-Optical properties
The optical properties of deposited films on glass by using UV-visible transmittance spectrum type (UV spectrophotometer Shimadzu model UV-1800). The absorption spectra of the films recorded in the wavelength from (500 to 1100) nm are showed in Figure (1). In general, it was found that the absorbance decreases with increasing wavelength for all the prepared thin films. This physically means that an incident photon was not able to excite the electron and transfer it from valence band to the conduction band because the energy of incident photon less than the value of the energy gap value of the semiconductor this lead to the absorbance decrease with increasing of wavelength. Also we can observed that the absorbance decreases as doping percentage increases because the energy gap will increase as doping increase, this confirms (TiO2) atoms entry within the crystal structure of the prepared films, this result is in agree with [11]. The absorption coefficient (α) for the prepared thin films was calculated from the following equation [12]: α= 2.303×A / t ………… (2) Where t: is the thickness of thin film in (cm), A: absorption. Figure (2) shows the absorption coefficient verses wavelength for different TiO2 doping percentage. In general, the absorption coefficient decreases with increasing of wavelength. The absorption coefficient exhibits high values (α >10 4 ) which means that there is a large probability of the direct transition. It is observed that the absorption coefficient (α) decreases with the increasing of the percentage of TiO2. This is due to the increasing of energy gap with a percentage of doping. This result agrees with the result shown by [13]. The energy gap is calculated for all prepared thin films by drawing between (αhν) 2 as a function of (hν) as shown in figure (3), by extended straight line of the curve and intersection with x-axis that gives the value of the energy gap for prepared thin films. It shows the energy gap of pure Fe2O3 equal to 2.1 eV, therefore energy gap is direct and confirm with studies [15]. Figure (3) shows the energy gap that increases with increasing percentage of doping because of formed localized levels inside energy gap that contributed to an increase of energy gap and then decreases the electronic transfer, the values of energy gap of prepared thin films shown in Table (1

.17
The refractive index is the ratio between the speed of light in vacuum to its speed in material which doesn't absorb this light. The refractive index (n) was calculated from the following relation (4) [16].
The variation of the refractive index versus wavelength for all films at different percentage of TiO2 is shown in figure (4). It can be noticed from this figure the refractive index (n) value decreases when percentage of TiO2 increases, this results is in agreement with [11].This behavior can be explained on the basic of that increases percentage of doping leads to make prepared samples less dense (decreasing the packing density) and the change in crystalline structure, which in turn increases propagation velocity of light through the sample which results decrease of the refractive index (n) values [17].

B-Electrical properties
The electrical conductivity has been measured as a function of temperature for films in the range (R.T -200) o C by using the electrical circuit. The measurements have been done using sensitive digital electrometer type Keithly (2400) and electrical oven. The change of electrical conductivity with temperature for most cases of intrinsic semiconductors is given by the following equation [18]: Where kB : is the Boltzmann constant, o: is the minimum electrical conductivity at K Ea : is the thermal activation energy and T: is the absolute temperature, The electrical resistance of the prepared films was determined as a function of the substrate temperature and its value (Re) is given for a rectangular shaped sample by the following equation [19]: Where Re: is the resistance of the sample (thin film), b: the width of electrodes, t: the film thickness, l: the distance between two electrodes. The conductivity of the thin film (σ D.C ) which can be calculated using the equation (7): The activation energy (Ea) which can be calculated using the equation (5)  in the low temperature depends on the ionization impurity and at high temperature depends on the generation of electron-hole pairs [20][21]. The activation energy (Ea) calculated from the slope of lnσ vs. 1000/T according to equation (8) , which shown in Figure (6). Table (2) shows that the value of Ea1 is smaller than values of Ea2. This indicates that the conductivity depends on the temperature.   The Hall measurements of the deposited samples have been done using system (HMS-3000) were shown n-type conductivity, which give negative values of RH. The type of charge carriers, concentration (ne), Hall mobility (μH), conductivity (σ) and resistivity (ρ) have been estimated from Hall measurements in the table (3).  Table (3) the value of RH was increased with increasing of percentage of TiO2 and The type of charge carriers concentration (ne) was decreased because that the structure of thin film was make capture to the change carriers in the grain bounding and then decrease the scattering. Mobility increases with increasing of percentage of doping due to the decreasing of the carrier concentration.
IV. CONCLUSIONS Pure and doped Fe2O3 thin films have been successfully deposited on glass substrates at 450 °C using spray pyrolysis technique. The optical properties show that the optical energy gap increase with the increasing of TiO2 concentration, while the photon transition was allowed direct transition and absorption coefficient decreased with increasing TiO2 doping concentration. σD.C was decreased with increasing of percentage of TiO2 and had two activation energy also all thin films had n-type carriers. Mobility increases with increasing of doping.