Effect of annealing temperature on structural and optical properties of Cr2O3 thin films by PLD

In the present work, pulsed laser deposition (PLD) technique was applied to a pellet of Chromium Oxide (99.999% pure) with 2.5 cm diameter and 3 mm thickness at a pressure of 5 Tons using a Hydraulic piston. The films were deposited using Nd: YAG laser λ= (4664) nm at 600 mJ and 400 number of shot on a glass substrate, The thickness of the film was (107 nm). Structural and morphological analysis showed that the films started to crystallize at annealing temperature greater than 400 oC. Absorbance and transmittance spectra were recorded in the wavelength range (300-4400) nm before and after annealing. The effects of annealing temperature on absorption coefficient, refractive index, extinction coefficient, real and imaginary parts of dielectric constant were also study. It was found that all these parameters decrease as the annealing temperature increased to 500 °C, while the energy gap after annealing increase from 3.4 eV to 3.85 eV.

antiferromagnetic which is the only solid chromium oxide that is stable under ambient conditions [1] and thermodynamically stable at temperatures higher than 500 °C [2]. This oxide has several applications in catalysis and solar thermal energy collectors [2] hydrogen storage, wear resistance materials, dye and pigment, advanced colorants, digital recording system, black matrix films, solar energy application, coating materials for thermal protection, and electrochromic material [3]. A wide variety of chemical and physical methods have been used for large area synthesis of Cr 2 O 3 films, e.g. chemical vapour deposition (CVD) at either atmospheric or low pressure, plasma enhanced CVD (PECVD), electrodeposition, metal oxidation, chemical spray pyrolysis, RF magnetron sputtering, molecular beam epitaxy and atomic layer deposition [4].
use this search method pulsed laser deposition for wide uses, and the first to use the technique of pulsed laser deposition (Smith and Turner) in 1965 where he prepares semiconductors and thin film of insulation [5].
In the present investigation, we report the effect of annealing temperature on Cr 2 O 3 thin films prepared by pulsed laser deposition in order to study the structural (XRD, AFM) and optical constants (including refractive index, extinction coefficient, real and imaginary part of dielectric constant).

Experimental
Chromium Oxide powder (99.999% pure) Table 1 shows material properties of Cr 2 O 3 , was compacted into a pellet of 2.5 cm diameter and 3 mm thickness at pressing it under 5 Tons using a Hydraulic piston. Films have been deposited onto glass substrate from Cr 2 O 3 target method using pulsed laser deposition (using Nd: YAG Laser at the wavelength of 4664 nm). Laser pulse frequency was 6 Hz, the distance between the target and the substrate (with dimensions of 2.5 cm×2.5 cm) was 6.5 cm at 600 mJ and 400 numbers of shot. Schematic diagram of a pulsed laser deposition system is shown in Fig.1.  The base pressure inside the chamber was typically low pressure (8x10 -2 mbar) and a vacuum chamber generally at (10 -3 Torr). The laser beam passed variable beam attenuator and was focused onto the target inside a chamber under 45 • angle. The glass substrates have been first cleaned in detergent and water to remove any oil or dust that might be attached to the surface of the substrate, and then they are rubbed gently under tap (15) minutes. After that, they are placed in a clean beaker containing distilled water and then rinsed in ultrasonic unit for (15) minutes with pure alcohol solution. The slides eventually are dried by air blowing and wiped with soft paper. The thickness of the films (107 nm) was measured by the Tolansky interference method. Structural analysis of Cr 2 O 3 thin films has been carried out in X-ray diffractrometer type SHIMADZU, power diffraction system with CuK α (λ = 1.54056 Ǻ). The X-ray scans are performed between 2θ values of 20° and 80°. The surface morphology, particle size distribution and root mean square of roughness of films prepared under various conditions were analyzed using atomic force microscope model (AA3000 Scanning

Probe
Microscope SPM, tip NSC35/AIBS from Angstrom Advance Inc). The optical absorbance spectra of Cr 2 O 3 films on glass substrates under different deposition condition (number of shots and annealing temperatures) were measured using spectrophotometer (SHIMADZU UV-Visible 1650 PC), covering the wavelength range 200 -1100 nm.
Results and discussion 1. Structural and morphological studies Fig. 2 shows the X-ray diffraction of Cr 2 O 3 thin films at different annealed temperature (300, 400, 500) o C prepared at 600 mJ and 400 number of shot at room temperature.
We have not found any other impurity peaks in comparison with the standard values of Cr 2 O 3 bulk crystal structure Fig. 3 (card no. 96-900-8085). From the figure, at 300 o C there is no noticeable peak in the entire range, it indicates that the films are amorphous in nature and this agree compatible with some researchers [5][6][7][8]. While at annealing 400 o C it is seen that there is only one peak in spectra. At more annealing temperature (500 ᴼ C) more peak seen in the spectra.
The size of the crystallite (D) was determined from the XRD data using Scherrer formula [6]: where k is a correction factor=0.9, β: is the full width at half maximum (FWHM) in radian and (λ) is the incident wavelength. The increase in the high peaks with the increase of the degree of crystallization material, as well as an increase in crystalline size are shown in Table 2.

Fig. 4: Atomic force microscopy for Cr 2 O 3 thin films at different annealing temperature at a-RT, b-300 o C, c-400 o C and d-500 o C.
The data collected for those samples from AFM images which are listed in Table 3  Atomic Force Microscope images of Cr 2 O 3 thin films which is in a good agreement with structural properties from XRD analysis Table 2 illustrates increase in average diameter with increasing annealing temperature from RT to (500) °C , while the annealing film by 500 °C have maximum values of Average Roughness and average r.m.s roughness value, (27 nm) and (32 nm), respectively.

Optical studies
Ultraviolet absorption is a process in which the outer electrons of atoms or molecules absorb radiant energy and undergo transitions to high energy levels. The optical properties of a material describe how the characteristics of light are affected when it passes through it. Study of optical properties of thin films generally focuses on the energy gap (E g ). A film is not just composed of perfect bulk material separated by grain boundaries; it also includes defects like unwanted impurities, stoichiometry deviations, point defects. The transmittance spectra for Cr 2 O 3 films on glass substrate with different annealing temperature were carried out in the wavelength range 300-1100 nm at room temperature and shown in Fig. 5.

RT 300 ᵒC 400 ᵒC 500 ᵒC
It is evident that the optical transmittance increases in the visible region with increasing annealing temperature.
The absorption coefficient of the annealed Cr 2 O 3 film can be calculated by a simple method from absorption spectra. The absorption coefficient, α (λ), is defined as [7]: α (λ) = 2.303A/t (2) where (A) is the absorbance and (t) is the film thickness. Results are shown in Fig. 6. The absorption coefficient decreases with the increasing of the wavelength, note that the absorption coefficient changes slightly towards high wavelengths and absorption coefficient begins to increase rapidly near the edge ( =380 nm) of the optical absorption reflecting a direct electronic transitions within this range of wavelengths. For a given wavelength, the absorption coefficient increases with decreasing the annealing temperature might be due to the increasing regularity of crystalline granules and increased its size and give the opportunity for small grain to grow by removing barriers or granular boundary. The figure show have two band, one in UV region ,and the other in visible region which is called Q-band at the range about (330-430) nm.

Fig. 6: Absorption coefficient as a function of the wavelength for the Cr 2 O 3 film at different annealing temperature.
The optical energy gap values (E g ) for Cr 2 O 3 thin films on glass have been determined by using equation [8].
The plot of (αhʋ) 2 against photon energy of thin film as shown in Fig. 7.
The observed values of the optical energy gap are increase with increasing annealing temperature. This means that annealing led to the absorption edge shift toward high energies near the levels of conduction band which are filled with electrons so the electrons need more energy and made energy gap seems to increase [9,10]. RT 300 ᵒC 400 ᵒC 500 ᵒC

Fig. 7: Variation of the (αhυ) 2 vs. photon energy (hν) for Cr 2 O 3 films.
The refractive index is an important parameter for optical materials and applications. The index of refraction for as deposited and annealed Cr 2 O 3 films was estimated from the reflectance (R) data using the relation [9,10]:  It can be seen that the refractive index decreases with the increasing of wavelength, i.e. anomalous dispersion. This behavior is due to the increasing in energy gap. The variation refractive indices in the visible and ultraviolet region with increasing annealing temperature can be attributed to the variation of optical absorption in the visible region and ultraviolet region after annealing. It is known that extinction coefficient and absorption coefficient can be related by [4]: From Fig. 9 one can easily obtain the extinction coefficient of Cr 2 O 3 films. The extinction coefficient depends mainly on absorption coefficient; for this reason, the behavior of it is similar for absorption coefficient. It can be seen that the extinction coefficient decreases as the annealing temperature increase.

Fig. 9: The extinction coefficient of Cr 2 O 3 films as a function of the wavelength prepared at different annealing temperature.
The dielectric constant can be defined as [11]: ε = ε r + iε i (6) The real (ε r ) and imaginary (ε i ) parts of the dielectric constant are related to the n (refractive index) and k (extinction coefficient) values. These values can be calculated using the following formulas [12]: Figs. 10 and 11 present the dependence of the dielectric constant of the Cr 2 O 3 films on the wavelength. The real and imaginary parts follow the same pattern and the values of real part are higher than imaginary part. It can be seen that the real and imaginary parts of the dielectric constant decreases with increasing wavelength. On the other hand both of them are decreases with increasing annealing temperature. The variation of the dielectric constant depends on the value of the refractive index. By contrast, the dielectric loss depends mainly on the extinction coefficient values which are related to the variation of absorption. In Table 4 the optical parameter of Cr 2 O 3 (T%, α, K, n, εr, εi, Eg) and different annealing temperature at λ=500 nm.

Conclusions
The deposited films of chromium oxide were thermodynamically stable at temperatures higher than 500 °C and this result is in a good agreement with previous results. The outcome of this investigation can be summarized as follows -X-ray diffraction (XRD) study shows that Cr 2 O 3 thin films are amorphous in nature.
-The transmittance increases with decreasing of annealing temperature -The energy gap increases from 3.4 to 3.85 eV with increasing annealing temperatures and found to increase with increasing crystallite size.
-The optical constants values (α, k, n, ε r and ε i ) decrease with increases annealing temperature -The value of refractive index lies within 1.9 to 2.3 in the studied spectral range. Appreciable order of integrated optical quantities suggests that this material is a potential candidate for the application in selective surface devices.