Optical Constants of Fe2O3 and TiO2:Fe2O3 Thin Films Prepared by Spray Pyrolysis Technique

In the present work, optical constants have been studied for pure and titanium dioxide (TiO2) doped iron oxide (Fe2O3) thin films with different titanium dioxide dopant concentrations (1, 3 and 5 %) prepared by chemical spray pyrolysis technique on glass substrates. The optical properties of the films, which were prepared with thickness of (260±10) nm, have been determined by using Shimadzu spectrophotometer, the optical transmittance measurements in the spectral region from (5001100) nm. Direct energy gap for (Fe2O3) equal (2.01) eV, it increases with increasing of TiO2%. The optical constants such as refractive index, extinction coefficient and dielectric constants have been calculated for all prepared films. The results show that all optical constants decreases with increasing of dopant ratios."


Introduction:
"Over the last decades, semiconductor thin films have attracted considerable attention from the research community because of their wide use in the fabrication of solar cells and other optoelectronic devices. The most spectacular examples are found in microelectronics, which modern electronic devices are based on different thin film technologies combined with methods of microstructuring [1]. Iron oxides are compounds widespread in nature. This class of compounds has a rich palette of possible oxygen to iron ratios, as well as many different phases for each composition [2]."Iron compounds are widely used as pigment, in catalysts, in thermite, and one biological example is hemoglobin. The stability and non-toxic nature of the iron oxides makes them suitable to use as pigments and coatings. Iron oxide can exhibit several crystal structures and compositions but the most important ones are hematite (α-Fe 2 O 3 ), maghemite (γ-Fe 2 O 3 ) and magnetite (Fe 3 O 4 ) [3]."Hematite, a-Fe 2 O 3 has a corundum structure, with a hexagonal unit cell. The structure can be described as: hexagonal close packed (hcp) anion (O -2 ) arrays with two thirds of the octahedral sites filled with a Fe +3 cation. It is the most stable iron oxide and often the end transformation of the oxide when annealed in air [2]. Thin films of iron oxide have attracted considerable attention in last few years due to their interesting magnetic properties. As far as hematite (α-Fe 2 O 3 ) is concerned, it is one of the most stable iron oxide under ambient conditions. Owing to its various interesting properties such as n-type semiconducting behavior, band gap (E g = 2.1 eV), low-cost synthesis, non-toxic nature, high resistant to corrosion. Because of the high electrical conductivity and high-sensitivity to gaseous environment, α-Fe 2 O 3 also used as an efficient material for the fabrication of effective gas sensors [4]." "Titanium dioxide TiO 2 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 [5]. TiO 2 (titania) is a cheap and one of the most efficient semiconductor photocatalysts for extensive environmental applications because of its strong oxidizing power, high photochemical corrosive resistance and cost effectiveness. Due to these inherent properties, TiO 2 is the most suitable candidate for degradation and complete mineralization of toxic organic pollutants in water [6]. Titanium oxide is well known that TiO 2 exists in three crystalline structures: rutile, anatase and brookite. It has one stable phase, rutile and two metastable polymorph phases, brookite and anatase [7]. In this paper, the effect of doping with TiO 2 has been studied on optical constants of Fe 2 O 3 thin films have been deposited by chemical spray pyrolysis before and after doping."

Experimental Details:
"Fe 2 O 3 solution is prepared by dissolving of (FeCl 3 .9H 2 O) [purity 97.0%, Sinopharm Chemical Reagent Co., Ltd], which is a powder of brown color, it's molecular weight is (162.20) g.mol -1 in (200) mL distilled water at room temperature , a magnetic stirrer is incorporated for this purpose for about (25) minutes to facilitate the complete dissolution of the solute in the solvent. Also, TiO 2 be can prepared of titanium chloride (TiCl 3 ) [AR grade, 99.9% pure, Merck made, Germany], TiCl 3 is a liquid material, it's molecular weight is (154.26) g.mol -1 ." The dissolving weight (W t ) of the above materials respectively were determined by using the following equation [8]:"

Results and Discussion:
"Optical absorption spectra depends on the chemical composition, crystal structure, energy of the incident photon, film thickness, and film surface morphology. Figure (1) displays the variation of absorbance spectra with wavelength from (500 to 1100) nm of the pure Fe 2 O 3 and TiO 2 :Fe 2 O 3 thin films. It appears that α-Fe 2 O 3 thin film has high absorbance in the visible region, indicating its applicability as an absorbing material [9]. From the plots, it can be justified that all films have a strong absorption at wavelength less than (640) nm, then the absorbance decreases with the increasing of wavelength. This attributed to the fact that at high wavelength the incident photons don't have enough energy to interact with atoms, the photon will transmit. When the wavelength decreases, the interaction between incident photon and material will occur, and then the absorbance will increase [10]. It is also noticeable that the absorbance decreases with increasing of TiO 2 ratio. This confirms titanium atoms entry within the crystal structure of the prepared film."  Figure (3), the absorption coefficient exhibits high values (α >10 4 cm -1 ) which means that there is a large probability of the direct transition [13], and then (α) decreases with the increasing of wavelength. It is observed that the absorption coefficient (α) decrease with increasing the concentration of TiO 2 , and this is due to the increasing of energy gap with increasing of doping ratio."

Figure (3):"Absorption coefficients as a function of wavelength for pure and doped Fe 2 O 3 thin films
"The values of optical energy gap (Eg opt ) for pure and doped Fe 2 O 3 thin films have been determined by drawing between (αhν) 2 as a function of (hν) as shown in Figure (4), by extended straight line of the curve and intersection with xaxis that gives the value of the energy gap for prepared thin films. Figure (4) shows the energy gap of pure Fe 2 O 3 is (2.1) eV. This result agrees with the result shown by [14]. Also, the energy gap is increased with increasing TiO 2 % because the decreasing of the density of state inside the optical gap, the increasing concentration of TiO 2 leads to decrease from the secondary levels, which lead to the contract tails region and this leads to expand in the optical 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 ( [15]."
"The variation of the refractive index versus wavelength in the range of (500-1100) nm for pure and doped Fe 2 O 3 thin films at different TiO 2 % are shown in Figure (5). It can be noticed from this figure the refractive index (n) value decreases when concentration of TiO 2 increases. This behavior can be explained on the basic of that increases concentration of TiO 2 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 decreasing of the refractive index (n) values [16]." "The imaginary part of the complex refractive index N is called the extinction coefficient. The extinction coefficient (k) for pure and doped Fe 2 O 3 thin films deposited with different concentration of TiO 2 is calculated by using equation (5) [15]." αλ/4π k  ………………….. (5) "The extinction coefficient (k) represents the amount of absorption energy in the thin film material, "which means the attenuation of an electromagnetic wave that is traveling in a material."The values of (k) depend on the density of free electrons in the material and on the structure defects. The relationship between extinction coefficient and wavelength of deposited pure and doped Fe 2 O 3 thin films is shown in Figure (6). In general, it is clear that the extinction coefficient (k) decreases with the increasing of wavelength (λ) for all prepared samples. The extinction coefficient (k) decreases with the increasing of doping ratio with TiO 2 for all prepared samples. In general, the behavior of (k) similar to the behavior of α. This is attributed to the same reason mentioned previously, since the increasing of TiO 2 concentration increases the optical energy gap as a result of absorbance decrement. This result agrees with the result shown by [17]." "The dielectric constant represents the ability of a matter for polarization. The matter can respond to different frequencies in a complex manner, at optical frequencies represented by light waves the electronic polarity is dominating above other remaining types of polarization. The real and imaginary dielectric constant can be calculated by the following equation [18]:" ε r = n 2 -k 2 ………………………… (6) ε i = 2nk …………………………. (7) "The variation of the real and imaginary parts of the dielectric constant values versus wavelength in the range of (500-1100) nm are drown in Figure (7) and (8) for pure and doped Fe 2 O 3 thin films. The behavior of ( r ) is similar to that of the refractive index (n) because of the smaller value of (k 2 ) compared with (n 2 ), while ( i ) is mainly depended on the (k) values. It is found that ( r ) and ( i ) decrease with increasing ratio of TiO 2 . It is seen that the value of real part is higher than the imaginary part. From the above note that the increase in the proportion of concentration of doping leads to rearrange the localized state causing decrease in conductivity film on the account that increases in fixed real isolation [19]. The values of optical constants of prepared thin films at λ cut off shown in Table (2)."