Optical Properties of Mixed ZnO: Fe2O3 Grown via Pulsed laser deposition

In this study, mixing of zinc oxide (ZnO) nanoparticles with iron oxide(Fe2O3) at (0, 0.1, 0.3, 0.5 and 1)%wt., are deposited on glass substrates by pulsed laser deposition (PLD) technique for study characterization ZnO:Fe2O3 as solar cell electrode. The profound effect of mixed film on the structural and optical of ZnO: Fe2O3 thin films was observed. Meanwhile, the films have polycrystalline Hexagonal structures for ZnO, Rhombohedra and cubic structure for Fe2O3, and as indicated by the X-ray diffraction patterns of the films. The mean crystallite size of ZnO increase with increasing mixed ratio. The direct energy gap (Eg) of ZnO is 3.36 eV and decreasing with increasing mixed ratio.


Introduction
Recently, crystalline nanoparticle oxides have taken the researchers attention because of the increasing the surface area of the active and important in absorbing the largest amount of radiation falling on the material, which in turn is converted into an electric current, as in solar cells. Among them zinc oxide (ZnO) and iron oxide (Fe 2 O 3 ), regarded one of semiconductor materials which have many physical properties that make them suitable for thin-film applications. such as, good electrical conductivity, inexpensive material, and chemical stability [1][2]. ZnO semiconductors were widely used as photoelectrode in solar cell [3]. -type II-VI ZnO is a thermally and chemically stable compound semiconductor with large exaction binding energy (60 meV), and wide band energy about (3.37 eV) [4]. ZnO is promising material for application to window layers of hetero junction solar cells, UV light emitting/receiving devices, transparent electrodes, piezoelectric nanogenerators photocatalysts and antibacterial activities [5][6][7]. ZnO doped with transition metals such as Fe, Co, and

ISSN: 0067-2904
Ni have many advantages for their appropriate optical and electronic properties required for optoelectronic devices [8]. Fe 2 O 3 is one of the most materials used as photoelectrochemical and photocatalytic applications due to its have narrow band gap (2.0-2.2 eV), which could collect up to 40 % of the solar spectrum energy. Fe 2 O 3 nanoparticles exhibit great stability in aqueous solutions. The crystal structure of Fe 2 O 3 is rhombohedra , and has attracted biggest interest because of their potential applications in solar cell, catalysis, gas sensors, pigments, medicine applications, optical devices and it used as an anode material [9][10]. The technique of preparation of the semiconductor electrode plays an active role in the performance of such solar cells [1]. Many methods can be utilized to prepare ZnO and Fe 2 O 3 electrodes such as pulse laser deposition, chemical vapor deposition, spray pyrolysis, sol-gel method, hydrothermal [11][12][13][14]. In this work we focused attention on iron doped ZnO, Iron is also the most common in recent years.

Experimental
Nano powder ZnO from (US research nanomatrials, Inc, USA) mixed with nano powder Fe 2 O 3 were deposited on glass substrate using PLD technique. ZnO powders doping with 0, 0.1, 0.3, 0.5, 1(Fe 2 O 3 ) were mixed using a Mixar(METUCHEN, N.J. Q8B40 U.S.A) and then pressed under 6ton to form targets with 2cm diameter and 0.3 cm thickness. The target were ensured to be homogeneous and dense as possible to prepared good quality ZnO:Fe 2 O 3 billet, which would be deposited by the PLD technique. Film deposition was carried out inside a 10 −3 Torr evacuated chamber. The focused Qswitched Nd:YAG laser beam was incident at an angle of 45° on the target surface, and the energy of the laser was 900 mJ. the glass was cut into square-shape pieces (2.5 cm 2 and were ultrasonically cleaned in distilled water. The structure of the films were examined by X-ray diffraction (XRD-6000shemadzu with CuKα, wavelength =1.54 A˚ ). The UV-Visible optical transmission spectra of the thin films were recorded by (Shemadzu UV-160/UV-Visible recorder spectrophotometer).

Structure
The result of the XRD scan of ZnO thin films mixed with Fe 2 O 3 deposited on the glass substrate at RT temperature illustrated in Figure-

Atomic Force Microscope (AFM)
The three and two-dimensional (3D,2D) topographic image has been used to study the surface morphologies of the ZnO:Fe 2 O 3 thin film with (0, 0.1, 0.3 , 0.5 and 1) % wt are shown in Figure-2. The average diameter, roughness, surface thickness and root mean square (R.M.S) are deduced from AFM images. The AFM images appear and all samples are granular structure and good uniformity revealing a uniform growth of the films. The average diameter increased with increasing of mixed ratio with Fe 2 O 3 . Also increasing in roughness with increasing in mixed ratio, this may be attributed to grain growth and some structure densification of the deposition processes. The average diameter, roughness, peak-peak and root mean square (R.M.S) are shown in Table-1.

Optical properties
The optical absorption coefficient (α) is important parameter in the design of calculations of the electronic and optoelectronic device. where this factor changes according to the semiconductor materials and affects the amount of light absorbed by the films and is a function of the wavelength of the light incident. The absorption coefficient changed of ZnO and doped with Fe 2 O 3 at (0, 0.1, 0.3 , 0.5 and 1)%wt., concentration as shown in Figure-3 from the relation between absorption coefficient(α) and wave length (λ) . High absorption coefficient values for all films, were found to be ( > 104 cm -1 ), this is good evidence that all films had a direct energy gap. The spectral in the region about (500-1100) nm showed the high stability of absorption coefficient with increasing wavelength, this is because of the relative stability of the rate of increase in transmittance within this spectral. The absorption coefficient increase with increased of Fe 2 O 3 concentration, as Table-2

Hanfoosh and Hassan
Iraqi Journal of Science, 2019, Vol.60, No.9, pp: 2009-2014 0202 Optical energy gap can be used to estimate the difference between the valence and conduction bands, which can help determine the thermoelectric and electronic properties of the materials [17]. The photon energy(hν) versus the variation (αhν 2 ) for ZnO thin films which mixed at different concentration of Fe 2 O 3 (0, 0.1, 0.3,0.5 and 1) %wt. is shown in Figure-5. the optical energy gap is evaluated via extrapolating the linear portion to zero absorption coefficient (α=0 ) [18].The energy band gap of ZnO at a thickness of 200 nm is found to be 3.36 eV, this good agreement with A. Kaphle1 and P. Hari [19]. in which at mixed with Iron Oxide the band gap energy decrease as shown in Table-2 to reach to 2.78eV. This trend shifted toward low energy (blue shift) as a result of the increasing of grain size [20].

. Conclusion
The effects of mixed ratio Fe 2 O 3 on the performance of ZnO/glass as electrode are examined. The results of structure, morphological, and optical properties have been achieved. The X-ray pattern of ZnO demonstrates that increasing the mixed ratio leads to broadening in diffraction peaks and increasing in crystal size. The AFM images appear that all samples are granular structure and good uniformity revealing a uniform growth of the films. The band gap energy decrease with increasing Fe 2 O 3 , this trend shifted toward low energy (blue shift) as a result of the increasing of grain size.