Effect of Alumina-Doping on Structural and Optical Properties of Zno Thin Films by Pulsed Laser Deposition

Alumina-doped Zi nc Oxide (AZO) thin films on quarts gl ass substrates have been deposited by pulsed laser deposition technique using a pulsed Nd-YAG laser with the wavelength o f ( λ = 532 nm) and duration (7ns). The structural and optical properties of these films were c haracterized as a function of Al 2 O 3 content (0-5w.t%)in the target at substrate tem peratures (400 ° C) and energy fluence (0.4 J/cm 2 ). The X-ray diffracti on patterns of the films showed that the undo ped and Al 2 O 3 -doped Z nO films exhibit wurtzite crystal stru cture and high crystalline quality. The optical properties were characterized by transmittance, absorption spectroscopy me asurements. For all films the average transmission in the wavelength r ange (330-900) nm was over 90% and the absorption edge shifted toward a shorter wavelength as Al 2 O 3 concentration increased. The optical energy gap of Al 2 O 3 doped ZnO thin films, measured from transmittance sp ectra could be controlled between (3.32eV and 3.59eV) by adjusting alumina concentration. AFM re sults sh ow that the samples with increasing concentration of Al 2 O 3 , the surface roughness increases.

In recent years, there has been a great deal of interest in wide band gap semiconductors for use in optical devices near-ultraviolet (UV) region.ZnO is one of the most attractive materials because of its good optical, electrical, and piezoelectric properties.Also it is a II-VI compound semiconductor with a wide direct band gap of 3.3 eV at room temperature [1].It has been used for several applications, such as transparent conductive oxide (TCO) films, solar cell windows, bulk acoustic wave devices and gas sensor [2][3][4][5][6].ZnO is promising candidate for such application due to its high transparency in the visible wavelength range and low electric resistance [7,8].The similarity of the properties between ZnO and GaN makes ZnO one of the most promising materials for the photonic devices in the ultraviolet range.Also the binding energy of the exciton of ZnO (60 meV) is larger than that of GaN (25 meV) at room temperature, giving it an advantage over GaN for exciton-related device applications [9].Among them alumina-doped ZnO films is a wide band gap semiconductor, which shows good optical transmission in the visible wavelength region (330-900 nm) [7].Furthermore there were various methods produce ZnO film, such as the metal-organic chemical vapor deposition (MOCVD) [8] and pulsed laser deposition (PLD) [10][11][12][13][14][15].In comparison with other techniques, PLD provides several advantages.For example, PLD films can crystallize at relatively low substrate temperatures due to the higher energy of the ablated particles in the laserproduced plume and relatively high deposition rates [16].In some reports [17],ceramic targets prepared by sintering the mixture ZnO powder and Al 2 O 3 powder was used.However, the expensive ceramic targets are usually brittle and would cause the cracking during deposition [18].The aim of this work is to produce high-quality Al 2 O 3 -doped ZnO thin films for optical gas sensor application by PLD.Special attention was paid to the influence of the processing parameters, such as dopant concentration into the targets on the structure, morphology, and optical properties of the films during the deposition.

Experimental procedure 2.1 Preparation of material powder and thin films.
Al 2 O 3 -doped ZnO thin films were synthesized by pulsed laser deposition system using a second harmonic Nd:YAG laser.Thin films were grown in a vacuum chamber generally in (10 -3 Torr) vacuum conditions.The Nd:YAG laser was operated at the wavelength of (λ=532 nm) with the repetition rate of (10Hz) and pulse duration of (7 ns).The target to substrate distance was (3cm).Targets of pure ZnO and Al 2 O 3 -doped ZnO films (0-5wt.%)porcelain materials were prepared by sintering at the temperature of 10°C.Amorphous fused silica (commercial available from Alfa Aesar) was used as substrates thin films were grown in Oxygen environment with O 2 partial pressure of of laser fluence focused on the target was about (0.4 J/cm 2 ).The deposition thin films were grown typically 10 min after the deposition thin films were cooled to room temperature.

Characterization of thin films
The absorption spectra of undoped and Al 2 O 3 -doped ZnO thin films were studied by UV-visible (Perkin Elemer Company) spectrophotometer in spectral range of (330-900) nm.
The data from transmission spectrum could use in the calculation of the absorption coefficient ( α ) by using the following equation [19]: Where d: is the thickness of thin film, and T is the transmission.The absorption coefficient ( α ) and optical energy gap (Eg) are related by [20]: Where A: is constant depending on transmission probability, h: is Plank , s constant , ν: is the frequency of the incident photon, Eg: is the energy gap of the material and n has different values depending on the nature of absorption process.The microstructures of the films were analyzed using Atomic Force Microscopy (AFM-Digital Instruments Nan Scope) working in tapping mode.Thickness measurements were carried out using a He-Ne laser at λ=362nm .The thicknesses of the films were calculate in the range from (X=330-900) nm.The polycrystalline structure of the films was analyzed with X-Ray Diffraction (XRD, Rigakn DMAX 2800), power diffraction system with Cu-Kα x-ray tube (λ = 1.54056Ǻ) was used.The x-ray scans were performed between 2θ values of 30° and 38°.

3-Results and discussion
The XRD spectra of Al The energy gap values depends in general on the film crystal structure, the arrangement and distribution of atoms in the crystal lattice, also it is affected by crystal regularity.The energy gap (Eg) value is calculated by extrapolation of the straight line of the plot of ( αhν) 2 versus photon energy for different Al 2 O 3 concentration in the target (0-5 wt.%) as shown in Fig. ( 4).The linear dependence of ( α h ‫ט‬ ) 2 with (hν) indicates direct band gap.ZnO is naturally an n-type marital and the Fermi level will fall in the conduction band when it is heavily doped.Since the states below are filled, the optical band gap should be increased to the higher energies .This means that the absorption edge of the Al 2 O 3 doped films will be shifted to the shorter wavelengths compared to that of pure ZnO films [7].
Figures (5)  The density and the size of the droplets increase with the dopant concentration into the targets.This result is probably due to the density of the sintered targets.[21] .The 3D AFM images taken from an area of (10 µm x 10 µm) were used for estimation of the grain size on the film surface.
Concerning the morphology characteristics, the ZnO target with the highest dopant concentration (3 wt% Al 2 O 3 ) and (5 wt% Al 2 O 3 ) is suitable for preparation of thin films for optical sensor application.Our previous investigation of the influence of the processing parameters on the morphology of the undoped ZnO films have shown that the increase of the substrate temperature leads to deposition of smoother films which is associated with the mobility of the atoms on the surface [22].The morphology of the sensing films is an important characteristic for optical gas detection.On the one hand, a porous surface with small grain size is recommended for better gas sensitivity [23].laser fluency energy 0.4 J/cm 2 .It was found that doped ZnO films with good structural and morphological properties can be deposited at x=0.05.The film deposited at an optimum alumina concentration 0.05 exhibited more intensity and FHWM with average grain size of (33-70) nm maintaining average RMS roughness of (51-189) nm.

Thin films of Al
The band gap was increased with increase of Al 2 O 3 concentration from 3.32eV to 3.59eV as well as with the change of thickness.The absorption edge was found to shift toward lower wavelength with increase in Al 2 O 3 (0-5wt.%) value.The increase of the doping concentration increase the roughness of the films.

1 10 −
mbar at substrate temperature of 400 C 0 and PDF created with pdfFactory Pro trial version www.pdffactory.comEng.& Tech.Journal, Vol.28, No.14, 2010 Effect of alumina-doping on Structural and Optical Properties of ZnO thin films by pulsed laser deposition 4679 2 O 3 have been deposited on to the glass substrate by PLD in 10 -3 mbar O 2 ambient at different alumina concentration with PDF created with pdfFactory Pro trial version www.pdffactory.comEng.& Tech.Journal, Vol.28, No.14, 2010 Effect of alumina-doping on Structural and Optical Properties of ZnO thin films by pulsed laser deposition 4681

Figures 5 .
Figures 5. Atomic force microscopy (AFM) of pure ZnO and Al 2 O 3 doped ZnO films grown on a glass substrate.

& Tech. Journal, Vol.28, No.14, 2010 Effect of alumina-doping on Structural and Optical Properties of ZnO thin films by pulsed laser deposition 4680
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