Preparation and Characterization of SiO 2 Thin Films as an Antireflective Layer

Uniform layers of SiO2 were prepared using thermal evaporation technique under high vacuum (10 mbar). Many characterizations were investigated using these films as antireflective layers. The morphological, crystal structural and optical properties of the layers were investigated by using SEM, XRD, and UV-Vis instruments.


Introduction
Anti-reflection coatings are often used to reduce surface reflection of desirable wavelengths from the cell, and allow more light to reach the semiconductor film layer, leading to enhancement of performance of the solar cell efficiency.One of the several inorganic materials, SiO2 films are the more favorite materials for various applications, such as solid state electronics and optoelectronic devices [1].In addition to the electronic applications, SiO2 can be used as a dispersion barrier on various plastic packaging materials [2].Thermal vacuum evaporation is very suitable method for the deposition of SiO2 thin films; for the production of thin films on various substrates and can be applied on a many kind of solar cells and electronic applications.Silica (SiO2) thin film has been used for surface passivation and diffusion masking during fabrication processes of planar silicon (Si) devices [3].Therefore, such insulating films have been applied to many types of electronic devices such as the static randomly memory devices as a metal-oxide-semiconductor in field effect transistors, insulated gates thin films transistors and as metal-insulator-semiconductors structure [4].
Silica thin film has been vastly used for protective layers and optical coatings as antireflection films for solar cell applications [5].Furthermore, SiO2 can be used for various microelectronic structures, as diffusion barriers and insulating layers with additional planarization capability [6].The physicochemical properties of silica thin coatings were applied in different methods to enhance photovoltaic devices, as encapsulate coatings or to insulate electrical connections in the cells [7][8].
In order to increase the transmittance of incident light and prevent disorders of external light, antireflective technologies are vastly utilized in optical components such as displays, solar cells, automotive glass, thermo-chromic windows, and so on [9].In present work, silica thin films were applied on glass substrates using thermal vacuum evaporation under 10 -5 mbar.

Experimental
In this study, SiO2 powder was evaporated by thermal evaporation system (homemade) under 10 -5 mbar to be deposited on glass substrate.The setup was used in this paper contains vacuum system consists of rotary pump and diffusion pump to create high vacuum in the chamber by controlling valves as shown in figure 1.A little amount of SiO2 powder was weighed (0.4 gm) and placed in a tungsten boat in the vacuum chamber and then heated to the Sublime degree by high current system to evaporate and then deposit on the glass substrate which has been cleaned by acetone and distilled water and after dried placed a (15 cm) over the tungsten boat.The structural studies of the prepared SiO2 films were discussed by X-ray diffraction method by using Shimadzu diffractometer (XRD -6000).The optical properties for SiO2 thin films deposited on glass substrates were analyzed using the Metertech SP-8001 UV-Visible Spectrophotometer; the scanning electron microscope SEM used for morphology studies is (Tescan Vega 3B).From figures, it can be seen that the silicon dioxide was uniformly deposited on glass, and it is clearly observed that the films were dense and compact in nature.The average particle size of synthesized SiO2 was found to be 90 nm.
Figure 5 shows the absorbance (A) and reflectance spectrum (R) of the coated silica, the figure revealed low absorbance and low reflectivity almost (8%) for the synthesized silica film, due to difference in refractive indices for the film and substrate, In order to achieve a zero reflectance or complete transmittance, the film must meet Eq. ( 2), [13]: Where no, n1 and ns are refractive indices of air, thin film and substrate, respectively.
For light reflected off of a material in air [14]: The extinction coefficient (k) is frequency dependence.We may assume a very small (k) for a single weakly absorbing media.Also, (k) vanishes at a very high frequency i.e k → 0, [14].

Calculation of film thickness:
The thin film thickness was calculated by the mass of the coated film if the area and density of the deposited SiO2 thin film were known, it is often called weight difference method [16], Where, t is the thickness, m is the weight of the thin film in gm, A is the area of the film in cm 2 , and ρ is the materials density of the coated film in gm cm −3 .The film thickness has been calculated 192 nm.
The absorption coefficient (α) was calculated using the Lambert law as shown in below [17]: Where (A) is the absorbance and (t) is known above for SiO2 thin film, respectively, and the band gap of the deposited SiO2 thin film was evaluated using Eq. 8 [18]: This formula for direct transitions, where, A is proportionality constant, and Eg is the energy band gap.The energy gap of the deposited SiO2 can be suggested as it is shown in the Figure 7, which was found to be (3.25 eV), While the energy gap of the bulk silica is (11 eV) [17].
This value of band gap makes the synthesized silica applicable for substrates of computer chips, electronic industries, and solar cells [19].

Figure 4 Figure 4 :
Figure 4 shows the UV-visible transmittance spectrum of the deposited films.As shown in the figure, the transmittance (T) of the Silica (SiO2) film was about 85% was achieved at almost 600 nm wavelength, can we know from transmittance the material can act as antireflectance or window layer.

Figure 5 :Figure 6 :
Figure 5: The absorbance and reflectance spectrum of the sample

Figure 7 :
Figure 7: Energy band gap of the deposited SiO2