Optical Properties of Radio Frequency- Sputtered CdSe Thin Films for Photoelectric Applications

INTRODUCTION
The fascination with metal chalcogenide semiconductor nanoparticles stems from the fact that their optical characteristics are markedly different from those of bulk material, i.e., Their absorption and luminescence spectra are influenced by their size. Cadmium Selenide nanocrystals with nano heterostructures show promise as optic and optoelectronic devices [1, 2], solar batteries [3, 4], and fluorescent markers in biophysical processes [5, 6]. Chemical techniques of template synthesis or controlled precipitation in solution employing surfactants as stabilizers are used to make Cadmium Selenide nanocrystals in the majority of situations [5]. However, such synthesis processes may not necessarily allow for high nanoparticle concentrations, which are frequently required in thin-film optical composites. The fundamental absorption edge of polycrystalline or crystalline semi-conductor is the regions in which the electrons are transferred from the valence band to the conduction band (excited) by absorbing the incident photons”. These photons should have an energy equal to or higher than the semiconductor’s energy gap. The absorption edge becomes wide”) for unideal, polycrystalline and amorphous semiconductor because the allowed localized levels are found in energy gap. The width of these localized levels can be calculated using Urbach Rule [7]:

α = α0 e (hυ/ΔE)                                                  

, where α is the coefficient of absorption in (1/cm), The photon energy is denoted by the letter h υ, and ΔE is the energy gap’s breadth of localized levels. The energy divide values of Cadmium Selenide depend on many parameters, such as sputtering power, vacuum pressure, density of impurities, substrate temperature, and annealing temperature [8]. Usually Cadmium Selenide thin films have been prepared using physical and chemical methods such as thermal evaporation, chemical path deposition , sputtering, electron beam evaporation ,spray pyrolysis ,electro-deposition ,photoelectron chemical ,SILAR  and photochemical deposition[ [9, 10].Among all the techniques of deposition, RF One of the greatest compositions is magnetron sputtering. The physical characteristics of Cadmium Selenide films are regulated by process factors such as the partial pressure of argon, the sputtering power, and the sputtering rate duration, as well as deposition processes. In this work, RF magnetron sputtering was used to create Cadmium Selenide thin films on a glass substrate, and the effect of sputtering power and corresponding film thickness on optical characteristics was examined. 

MATERIALS AND METHODS
Cadmium Selenide powder with a purity of 99.99% was squeezed in a mold by a hydraulic piston to produce a disk with a diameter of 5 cm and a thickness of 3 mm, sintered at 500 °C for 4 hours, and placed as a sputtering target in a magnetron gun. A stainless steel chamber with a compact sputtering Coater System (CRC-600; Torr International Services LLC) was employed. The films were deposited from Cadmium Selenide target using a radio frequency (RF) magnetron sputtering source with Ar gas with rotational substrate controller at 100 °C. The sputtering chamber was throttled and the sputtering gas pressure was 2×10 -2 mbar after it was evacuated to less than 5×10 -5 mbar. The RF power supplied at (50-125) W and deposition time for 1 hour. The films were deposited on substrates made of glass with diameters of (4×1.5×0.15) cm with optical transmission of >95 percent. The substrates were cleaned by using ultrasonic in ethanol 96%, then by 15 minutes in distilled water, then left to dry at room temperature. The spectrophotometer (UV-VIS-2601, Biotech Engineering Management CO.LTD) was used to measure the optical spectrum of absorption and transmission.

RESULT AND DISCUSSION
Sputtered Films Thickness
In Table 1, the thickness of R.F sputtered The thickness of thin Cadmium Selenide films was measured. using the optical interferometer method. Thin Cadmium Selenide films have different thicknesses. is presented as a function of sputtering power. Present results are in agreement with published work [14, 15, 16]. Poor sputtering power is likely to result in a low deposition rate because to the less energetic argon over the target species. In the current work, this had an impact on the sluggish nucleation and development of a few tiny grains on the surface. The number of Cadmium Selenide atoms created from the target by sputtering rose as RF power increased, and the chance of the Cadmium Selenide atoms arriving on the substrate without collisions increased, resulting in the maximum deposition rate.

Spectrums of Absorbance (A), Reflectance (R), and Transmittance (T)
Fig. 1 shows that, absorbance, transmittance, and reflectance of light spectrums respectively of deposited films with different thicknesses, which are prepared at various sputtering power (50, 75,100 and 125) W At normal temperature, the wavelength range is 300-1100 nm. The Absorption (A) and transmission (T) is used to compute reflectance (T) Energy communication is governed by the rule of entropy. [14].

T = (1-R) 2 * e-αt                                                

, where t is the deposited layer thickness and absorption coefficient (α). The figures show In the UV range (300–400 nm), the films absorb a lot of light, and, the optical absorbance, transmittance, and reflectance in the visible region increase and decrease respectively with an increase of film thickness. This impact might be explained by an increase in cadmium and oxygen atom diffusion into the bandgap as localized levels. [12]. The figures show that the absorbance increases, transmittance, and reflectance decrease respectively The photon falling on the prepared film’s surface will be absorbed by the crystals inside a single grain in a series of operations as the thickness of the prepared film increases, and as a result, Its capacity to reflect or permeabilize without being absorbed by the complex atoms’ electrons will be restricted, particularly as the thickness of the prepared film increases. As the number of crystals in it grows, so does the thickness, resulting in total absorption and a rise in the absorption coefficient. What has been said thus far corresponds to the findings of investigations. [10]. The Cadmium Selenide thin films deposited on the glass substrate showed a transmittance of ~60-80 % in the range 700-1000 nm and are found to decrease as sputtering power increases along with the film thickness which shows the improvement in crystallinity.

Optical Band Gab and Absorption Coefficient
The Absorption Coefficient is the ratio of decreasing radiation intensity as it passes through a substance using the following equation. [16, 17]:
α =1/t ln 1/T                                                             

It is clear from Fig. 2, that the region of absorption occurs at (λ = 300 — 750 nm). The absorption coefficient takes higher values (> 104 cm-1), originating from valence band electrical transitions and impurities, as well as energy band transitions With rising wavelengths, these values quickly decrease. The fluctuation in absorption coefficient with wavelengths becomes minor in the spectrum area (700-800 nm), and the values of all Cadmium Selenide films are fairly near at the cut-off wavelength.
The increase in absorption coefficients with the increasing the film thickness which is related to sputtering power is clear and reduced nearly at the region of the cut-off wavelength range. This increment is related to the Cd and Se atoms in the crystal structure of Cadmium Selenide films, which occupy an impurity energy level and increase the absorption of the photons. Utilize the fundamental absorption, which corresponds to the transition from the valance band to the conduction band, to calculate a material’s bandgap. The connection between and the incident photon energy (hv) is as follows [16, 17]:

αhυ = A* (hυ – Eg)r                                             

, where A is a constant, Eg is the material’s bandgap, and r is a variable depending on the kind of transition. The authorized direct, allowed indirect, banned direct, and indirect transitions are prohibited are represented by the parameters 1/2, 2, 3/2, and 3, respectively. The direct energy gap of produced films was calculated using a plot of the absorption coefficient as a function of photon energy Fig. 3. Extrapolating the straight-line section of the (hv) 1/r versus h plot with r =1/2 yields the value of Eg. In Fig. 3, the plot of (h) 2 vs. h is displayed.  The direct badgap energy value was discovered to be (1.8, 1.75, 1.75, 1.62) eV for Cadmium Selenide thin films deposited using sputtering power of a range of 50, 75,100 and 125 W respectively. The direct bandgap energy was found to decrease as sputtering power increases along with film thickness [18]. The crystallite size-dependent features of the bandgap energy are responsible for these variations. Cadmium and oxygen diffusion occupy interstitial critical sites between Cadmium Selenide lattices, resulting in a modest decrease in direct bandgap. As a result of this process, new localized states of impurities emerge, causing the optical bandgap to shrink.
The refractive index and extinction coefficient follow the same pattern as the transmittance and absorbance spectra. Fig. 4 shows that it is found that, at (λ = 750-1000 nm) for different sputtering power (increasing the thicknesses) causes increasing of refractive index values. This behavior may be due to an increase in packing density because of increases in the degree of crystallization. The values of refractive index become relatively lower at wavelength about (1 µm), where films are transparent and the reflectance of Cadmium Selenide films estimated are very low. The refractive index of deposited Cadmium Selenide films ranges between these values, as seen in this diagram. ~2 and 3, The extinction coefficient, on the other hand, fluctuates between 5 × 10−2 and ~1 at the wavelength of λ = 750-1000 nm. Refractive index values in the present work are in agreement with the values that have been published (2.73–2.76) [19]. The high refractive index value makes these films a good material for photovoltaic NIR photodiode application. 
This coefficient relies on the absorption coefficient, for that, its behavior can be resembling absorption coefficient [20,21]. The behavior explanation of extinction coefficient with the sputtering power and related films thickness can be related in terms of induced impurities i.e for high absorption coefficients it is possible the films would contain enough impurities to give rise to a high extinction coefficient. Between the valence and conduction bands, the density of local levels generated by impurity atoms in the faulty material functions as a ladder for the transfer of electrons absorbing photons with energy less than the value of the op, as shown in the figure. This is owing to the density of the local levels generated by impurity atoms between the valence and conduction bands in the faulty material, since these levels function as a ladder for the transfer of electrons absorbing photons [21].
The tangible and intangible elements of the deposited dielectric constants Cadmium Selenide films for different sputtering power are calculated by using the following equations [8, 14].

ε1= n2 -k2                                                            

ε2= n2.k2                                                                     

The dependence of ε1 and ε2 on wavelength is shown in Fig. 4, and it is concluded, that the variations of ε1 and ε2, Because the smaller values of K2 in contrast to n2, the imaginary component of the dielectric constant 2 is mostly dependent on the K values, which are connected to fluctuations in absorption coefficient. Due to the intense interaction between the highly energy photons and the charge carriers of the material, the values of the actual dielectric increase significantly 

ONCLUSION
Sputtered Cadmium Selenide thin films were effectively obtained. The optical characteristics of Cadmium Selenide films are impacted by film thickness, which is linked to the sputtering power, according to spectrum examination of transmittance and absorbance at (300-1100) nm for the produced films. The ideal preparation conditions for producing Cadmium Selenide thin films may be inferred. is sputtering power of 100Watt with the thickness of ~ 210nm .These sputtered films recorded the greatest absorbance, the direct bandgap was confirmed and found to be around 1.7eV. This was the permeability with the lowest value. With this value of thickness in the manufacture buffer layers of solar cells and in the production of contemporary electronic devices, such as photodiodes (an optical signal detector in a communications system) that operate in the visible-near infrared range of the electromagnetic spectrum.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.