Laser Irradiation Effect on The Optical Properties of CoO2 Thin Films deposited via Semi-Computerized Spraying System

In this paper deals with the effect laser irradiation on the optical properties of cobalt oxide (CoO2) thin films and that was prepared using semi computerized spray pyrolysis technique. The films deposited on glass substrate using such as an ideal value concentration of (0.02)M with a total volume of 100 ml. With substrate temperature was (350 C), spray rate (15 ml/min).The XRD diffraction given polycrystalline nature with Crystal system trigonal (hexagonal axes). The obtained films were irradiated by continuous green laser (532.8 nm) with power 140 mW for different time periods is 10 min,20min and 30min. The result was that the optical properties of cobalt oxide thin films affected by laser irradiation where the absorbance, absorbance coefficient, extinction coefficient and the real ε 1 and imaginary ε 2 part of the dielectric constant of the films increases after laser irradiation. While the Transmittance and refractive index decrease with laser irradiation. The optical energy gap was decreased from (1.89 eV to 1.6 eV) after laser irradiation, and this is a good variation of bandgap values for photovoltaic applications.


Introduction
Spray pyrolysis technique (SPT) is one of the most promising techniques employed to prepare metal oxide thin films like CoO 2 , ZnO, CuO, and CeO 2, etc [1] [2]. The SPT technique achieves the required compatibility for the development of solar cells, P-N junction diode, heterojunction diodes, and electrochemical electrodes [3]. Spraying technique is the most common today because of its applicability to producing a variety of conducting and semiconducting materials. Spraying deposition is a notable method for synthesizing thin layers inexpensiveness, no vacuumed chamber required, and the ability to synthesize sub-micron thin films [4]. In the case of films, it can be sprayed over an area larger than a lab-scale that can be employed at industrial production processes [5]. The thermal treatment of metallic oxide thin films and nanostructures is conducted in a furnace in the temperature range of 300-1000 °C, depending on the monitoring system, thermodynamics, and the intended impact [6]. During the deposition, this technique has many problems such as excessive heat, long processing time, and a high degree of energy loss. Also, losses in the electricity used for increasing and decreasing the substrate temperature and the furnace itself. Besides, this process is incompatible with thermally touchy substrates (e.g., Si, glass, amorphous alloys, and polymers), where the excessive temperatures can reason microstructural changes and thermal-expansion mismatch, main to mechanical failures. Furthermore, thermal procedures based on traditional heating techniques cannot induce spatially resolved thermal effects, requiring the nanostructures to be physically separated from the digital circuitry. These issues limit the direct integration of steel oxide films or nanostructures into the complementary fabrication process of metal oxide semiconductors [7].
The laser irradiation process offers solutions to the above issues and allows rather well-matched metal oxide thin films and nanostructures. This technique is especially based on the thermal effect. That caused by the applied laser could confine the temperature discipline at the desired role without losing excessive energy [8]. Different parameters could be faced, such as laser intensity, spot width, and scanning rate. These parameters can be varied under controlling for reaching the desired thermal effect. This system is characterized by a quick drastically for localized thermal effects that allowing particular manage over the material properties. The heating process fees of laser irradiation offer a magnitude greater than the prices of the annealing, which provides an ability to fast fabrication of materials with minimal power losses.
The laser-induced warmness can be restricted to a unique area in each in-plane and thickness directions. It is feasible to selectively anneal the films by thermal interference on the underlying structures. Moreover, the employing of lasers with sufficient efficiency can result in a sizable reduction in energy required for thermal processing. Depends on the definition of the optical ratio to output laser and the input electrical power [9]. In the present work, we report results of investigations carried out to study the effect of optical properties (Absorbance, reflectance, refractive index, dielectric constant, and optical bandgap) of CoO 2 thin films before and after of laser irradiation at difference period time.

Experimental Part:
Cobalt oxide thin films are prepared by the spraying technique by taking Cobalt chloride hexahydrate (CoCl 2 .6H 2 O) as a precursor material. The precursor solution is prepared by dissolving (CoCl 2 .6H 2 O) in deionized water using (0.02) M concentrations with a total volume of 100 ml.
The CoO 2 films were deposited on the glass substrate. The glass substrates were cleaned with distiller water and ethanol. The substrate temperature was (350⁰ C), spray rate (15 ml/min), by using the air as a carrier for precursor solution, the distance between prayer nozzle and substrate was (35cm) and the films were prepared with the thickness (632nm). It's done by using a semi-computerized spray pyrolysis technique which was made specifically for this work to prepare and irradiation by laser to thin films then studies the optical properties of CoO 2 films using UV-vis-NIR before Laser irradiation.
Then the CoO 2 films were irradiated using the green laser (532.8 nm) with power 140 mW for different periods is 10 min,20min, and 30min and then its optical properties were studied using UV-vis after irradiation.

Calculations:
Reflectance can be calculated from the following formula: The absorption coefficient(D ܿ݉ ିଵ ) is calculated in the fundamental absorption region using Lambert law [10].
Where t is film thickness, I is the intensity of transmitted light. If ( I / I R ) = T then To measure the optical band gap for the thin films, we use Tauc's relation as follows [11]: Where A is constant, hν photon energy, Eg the optical energy gap, and an index (n) could take different values according to the type of electronic transition. The extinction coefficient ( K o ) can be evaluated by the following [12]: Where λ: is wavelength and α: The absorption coefficient. Refractive index one of the fundamental properties of an optical can be measured from the relation [13]: The real (ߝ ଵ ) dielectric constant and imaginary (ߝ ଶ ) dielectric constant is determined using the relation [14]:

4.1: X-ray diffraction:
The XRD diagram of the cobalt oxide thin films representing a polycrystalline structure.      Fig.5 shows the refractive index of the CoO 2 sample irradiated by laser with wavelength. The refractive index of the sample ranged between 2.1 before irradiation and 1.6 after irradiation. It is noted that the refractive index decreases with laser irradiation on the CoO 2 films. This is due to the major contribution of electronic transition for interval laser irradiation. This may lead to a significant change in the optical parameter and this result is the agreement with [14].   from (450nm to 550nm).To be the highest value of the increase at the wavelength (500nm to 525nm) with the increase in the time of irradiation, and this spectral variation is quite similar to those of the absorbance coefficient. This increase is due the sample possesses optically quality with lesser defects and this parameter is highly important to making optoelectronic materials, these results were in a good agreement with the reference [16].

4.3: Optical energy gap:
The optical band gap of (CoO 2 ) films before and after laser irradiation have been displayed via plotting the relation (4) of verses (eV) for direct energy gap as proven in " Fig.8 10 band hole values had been decided to employ extrapolating the linear parts of these graphs to the energy axis at\ \left(\alpha\ h\ v\right) two. The motion pictures exhibited a reduction in the optical energy gap, it is lowered from: (1.89 eV to 1.6 eV) after laser irradiation of at 30 min of CoO2 films. The shift of optical electricity can be defined in phrases of quantum-size impact in which the film with large crystallites [17], for this reason ensuing in enchantment in crystalline of CoO2 movies and so the density of localized states decreases, and this result is an agreement with [18].

5.Conclusions:
The optical analysis of CoO 2 films shows that using a semi-computerized spray pyrolysis technique a useful method for the deposition of CoO 2 films. The result was that the optical properties of cobalt oxide thin films affected by laser irradiation where the absorbance, absorbance coefficient, extinction coefficient and the realߝ ଵ and imaginary ߝ ଶ part of the dielectric constant of the films increases after laser irradiation.
While the Transmittance and refractive index decrease with laser irradiation and the optical energy gap was decreased from 1.89 eV to 1.6 eV after laser irradiation. And that good  11 variation of bandgap values has been observed for photovoltaic applications. This result due to laser irradiation worked like annealing temperature to enhance the crystallization of thin films or increased surface roughness of the deposited films.