Effect of Annealing on Some Properties of Zn 2 SnO 4 Thin Films Prepared by PLD Technique

In this paper, the investigation of structural and optical properties of Zn2SnO4 thin films were studied. The films are performed on glass substrates by pulsed laser technique (PLD) using laser Nd: YAG at wavelength of 1064 nm with 800 mj laser energy using repetition rate of 6 Hz. and average 400 laser pulses at room temperature and annealing by tubular quartz furnace at temperature (573,773) K for 2 hours with air. XRD measurements showed that the structure for all samples is polycrystalline with a cubic nanostructure. Surface morphology was studied using scanning electron microscopy SEM and atomic force microscopy AFM. After annealing, the roughness of the surface and the mean grain size were increased. Optical properties as a function to wavelength in the range (300-1100 nm) have been studied. Absorption spectra of Zn2SnO4 thin films showed that absorption decreases with increasing annealing temperature. Direct energy gap for a Zn2SnO4 thin film was increases with increasing temperature for all samples due to crystal growth. The optical properties such as extinction coefficient, refractive index, and dielectric constant were also studied.


Introduction:
Zinc stannate Zn 2 SnO 4 , commonly known as zinc tin oxide ZTO, has been confirmed for high electron mobility (10-15 cm 2 / V.s).High electrical conductivity, Zn 2 SnO 4 is a n-type transparent conductive oxide with a wide band gap of 3.7 eV [1].It has wonderful visual optical properties that can be used in multiple fields such as solar cells, sensors to detect moisture and combustible gases.Fig 1 .Crystalline structures of Zn 2 SnO 4 (cubic spinel) [2].
Pulsed laser deposition is defined as a promising method for depositing metal thin films.The thickness of the films can be controlled by controlling the number of pulses [3].The idea of PLD is simple.The laser pulse beam is concentrated on the outer surface of the target material.Where the laser is absorbed and leads to rapid evaporation of the target material [4].Evaporated materials include highly ionized species.If the discharge is performed in a vacuum, it shows itself as a glowing plasma column directly opposite the target surface.In our current research, we study the structural and optical properties of Zn 2 SnO 4 thin films grown on a glass slide produced by the PLD [5].

Theory:
Zinc oxide ZnO and tin oxide SnO 2 is the most promising candidate for the development of transparent conductive material.SnO 2 is an rutile tetragonal structure with oxygen deficient n-type degenerate semiconductor with wide band gap of 3.6 eV.Its high optical transparency and electrical conductivity leads to very appealing applications in spintronics device [6].ZnO in view of its high transmission over a wide spectral range including the useful UV-vis region and other interesting characteristics such as low toxicity, relatively low cost, and stability in reductive chemical environments [7].SnO 2 and ZnO thin films have attracted significant interest recently for use in optoelectronic application such as solar cells, flat panel displays, photonic devices, laser diodes and gas sensors.In the present study, Nanoparticle powders of zinc stannate in date spinel Zn 2 SnO 4 were prepared by mixing stoichiometric amounts of tin oxide SnO 2 and zinc oxide ZnO, in the same molar ratio, (ZnO:SnO 2 = 1:1).The zinc stannate Zn 2 SnO 4 films have the advantages of both ZnO and SnO 2 ,and are promising for solar cell and sensor applications [8].

Experimental Work:
Tin oxide Nanoparticles SnO 2 purity (99.98 %) powder by Sky Spring Nanoparticles, Inc. 2935 Westhollow Dr., Houston, Tx 77082.and zinc oxide ZnO Nano Powder, with purity (99.9%) by Nanjing Nano Technology.Prepared Zn 2 SnO 4 by mixing stoichiometric amounts of tin oxide SnO 2 and zinc oxide ZnO, in the molar ratio, (ZnO:SnO 2 = 1:1) and ball milled for 1 hrs. to use it to make target as a disk of (1.5) cm diameter and (0.3) cm thickness using hydraulic piston type (SPECAC), under pressure of (6) tons for (15) minutes.finally the pellets were sintered in air at (1000 C˚) for (2) hrs.

PLD and deposition of thin film:
We used glass slides (2.5 x 7.5 cm) that were cleaned with liquid soap.Then immersed in distilled water using an ultrasound device for 15 minutes to remove all dust and dirt.Dry it with a thin strip of cotton to be ready for deposition at room temperature.Use PLD technique using the laser Nd: YAG with (λ = 1064 nm) at power (800 mJ), under the vacuum (3 × 10 -3 mp) using the double-stage rotary pump.Repeating frequency (6Hz) for laser pulse (400) is incident on the target surface with an angle of (45 °).
A set of PLD system is given in Fig. 2. The distance between the target and the laser (10 cm) was determined ,and between the target and the glass slide (2 cm).

Annealing process:
The annealing process was performed by a tubular quartz furnace Fig. 3. at a temperature of (573,773) K˚ for 2 hours with air.Fig. 2: Schematic of the PLD system [9] Fig. 3: tubular quartz furnace.

X-Ray Diffraction:
The crystalline structure of the Zn 2 SnO 4 thin films, which were deposited on glass slides by PLD method at RT, and (573,773 K˚) temperatures, was studied using x-ray diffraction techniques using (Bruker-D2 phaser vorsicht rontgenstrahl ung caution x-rays) device.This device registers the intensity as a function of the Bragg's angle [10]: Where Ө is the diffraction angle and λ is the XRD wavelength used.The grain size of the granularity G.S of the crystals, which is important in the study of X-ray spectrum characteristics, can be estimated by full width at half-maximum (FWHM) calculated by Scherrer equation [11]: Where θ is the Bragg's angle of the XRD Peak.

Measuring optical properties:
The optical properties of thin films are affected by crystalline structure, thickness and type of materials used.The optical properties have been investigated using UV-visible spectral spectrometer (SP-8001) in the range (300-1100) nm.The output data from the absorption, wavelength, and transmitter are used in a computer program to measure all optical constants.The value of the energy gap (E g ) is graphically estimated by equation [12] ( ) ( )

Were
= is the optical energy gap ; Blank's constant ; is the frequency of light; constant depends on the type of material.
When measuring absorption and transmittance spectra, we can know the behavior of the refractive index spectra n and extinction k of the ZTO [13].
( ) where R is the reflectance.The real and imaginary part of dielectric constant can be calculated by using the following equations [14]: The FWHM decrease with increasing annealing temperature indicate on increasing particle size.The intensity of the peaks increases as the temperature of the annealing increases, which means that the crystallization of the film improves as the temperature of the annealing increases.
The annealing process reduces crystalline defects and reduce optical dispersion.be due to the bigger clusters formed by the coalescence of two or more grains and decrease or disappearance the grain bounders.This result is identical with [15].within forbidden energy gap and conduction band.This leads to an increase in the value of E g .In the near UV-visible light region, a strong photo-absorption at a wavelength of 330 nm is presented.This result is in agreement with [2].occurs in these regions.also we found that the α of the Zn 2 SnO 4 films has a strong absorption coefficient at the short wavelength region (high energies).On the other hand, it is found that the value of α decreases with increasing of annealing temperature due to increase energy gap E g .The α values are approximately equal to the reported values by [2].photon energy (hν) and the choice of the desired linear section.From Fig. 10, we can see that the energy gap was increased from (3.55-3.7)eV, as a result of the increasing of the annealing temperature, This is due to the growth of grain size and the reduction of the number of grain boundaries.Increase E g as a result of reducing the amount of absorption, this may lead to improved the crystal structures, which leads to the reduction of defects (tail state) in the forbidden gap and this leads to an increase in the E g .The values of the approximate absorption coefficient with the values reported by [17].decreases generally with increased temperatures.This result is consistent with [18].3 .These measurements correspond with [18].

4. 2 . 2 .
scanning electron microscopy (SEM): Fig.6a,7a shows the SEM images of Zn 2 SnO 4 films deposited on the glass substrates by the PLD at RT and its temperature 773 K˚.Fig.6b,7b shows the SEM analysis images with the IMAGEJ program.The surface of all thin films is known as a cluster of granules known as clusters, which represents homogeneous surface morphology of thin films.The Zn 2 SnO 4 thin film has a perfectly uniform surface with no holes.With a wide distribution of particles of about 20-50 nm, which agrees with the XRD result.With annealing, the average size of aggregated particles increases obviously.This result may be attributed to higher atom mobility with the increase in temperature which causes more effective recrystallization and grain growth of the films that result in larger grains.This result was in agreement with[16].4-2.3 EDAX Study: The EDAX pattern shown in Fig. (6-c), (7-c) confirms the presence of Zn and Sn.The observed Zn/Sn ratio for the films measured from the EDX measurement of the Zn 2 SnO 4 stoichiometry for both the sediment by PLD in RT and annealed at (773) K˚.
According to the formula.(4) The behavior of the extinction coefficient (K) is similar to the behavior of the absorption coefficient (α).Fig.11shows the extinction coefficient (K) as a function of the wavelength (λ) of Zn 2 SnO 4 films at RT and (573,773) K˚.We can see from Fig.11and Table.
3 that the extinction coefficient (K)

Fig. 12 :
Fig. 12: The Variation of The Refractive Index n With Wavelength for Zn 2 SnO 4 thin films deposited at RT and annealing at (573,773) K˚.

Fig. 13 :
Fig. 13: The Variation of The Dielectric Constants (a)-real part (ε r ) and (b)-imaginary (ε i ) parts With Wavelength of Zn 2 SnO 4 Films at RT and annealing at (573,773) K˚.

Volume 13 ,
Issue 4, December 2018 , pp. (96-112) ISSN: 1992-0849 (Print), 2616-6801 (Online) Web Site: www.uokirkuk.edu.iq/kujssE-mail: kujss@uokirkuk.edu.iq001 5. Conclusion: Structural and optical properties analysis show that pulsed laser deposition is a useful method for the deposition of Zn 2 SnO 4 thin films on glass substrates.Post-annealing process helps to improve the crystalline quality thin films.All the films show polycrystalline with cubic nanostructure.XRD, AFM and SEM indicate that the Average Diameter, roughness Average and RMS of the nanoparticles thin film increases with increasing the annealing temperature.direct energy band gap values increases with increasing annealing temperatures about 3.7 eV for the film annealed at 773 K for 2 hour.

Table 1 :
Structural parameters: Inter-planar spacing, crystalline size of Zn 2 SnO 4 films deposited at RT and annealing at (573,773) K˚.
2 SnO 4 thin films deposited on glass slides by PLD at (RT) and annealing at (573,773 K˚).This Figure and Table 2 illustrate that the Average Diameter, roughness Average and RMS are increased with increased annealing temperature.This may

Table 2 :
AFM parameters (Average Diameter, roughness Average and RMS) for Zn 2 SnO 4 films deposited at RT and annealing at (573,773) K˚.