Structural , Morphological and Optical Properties of Aluminium Doped ZnO Thin Film by Dip-Coating Method

Dip coating method is used for depositing Aluminium (Al) doped Zinc Oxide (ZnO) thin films on glass substrate. Undoped, 1%, 3% and 5% Al doped ZnO thin films are prepared and their structural, morphological and optical properties are studied. X-ray diffraction study confirms that films are polycrystalline with hexagonal arrangement. Field Emission Scanning Electron Microscope (FESEM) is used to analyze the surface morphology and it shows the spherical like structure. Energy Dispersive X-ray Analysis (EDX) pattern shows the presence of Zn, O and Al. UV-Vis. Spectroscopy and Photoluminescence spectroscopy (PLS) are used to study optical properties. Optical study indicates that both undoped and Aluminium doped ZnO films are transparent in UV region and closer to visible region. The band gaps of the films are 3.55eV, 3.68eV, 3.82eV and 3.25eV for undoped, 1%, 3% and 5% Aluminium doped ZnO films correspondingly. keywords: Thin film, Aluminium doped ZnO, Dip coating, X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), UV-Vis. Spectroscopy (UV), Photoluminescence spectroscopy (PL).


INTRODUCTION
Transparent conductive oxide (TCO) materials have many applications in the area of displays, photovoltaic devices and LED's [1][2][3] .Zinc oxide has extensive band gap (3.37eV) at room temperature, good chemical stability, excellent thermal stability and high exciting binding energy (60meV) 4 .ZnO thin films have many advantages such as non-toxic, low cost, good electrical and optical properties with very high electrical resistivity 5 .The properties of ZnO films can be improved by doping any one of the group III element.Aluminium is one of the group III element which is mainly doped with ZnO to obtain good stability and low resistivity 6 .
Many methods were used to obtain ZnO thin films such as chemical bath deposition 7 , dip coating method 8 , thermal vacuum evaporation method 9 , sol-gel method 10 , hydrothermal method 11 , magnetron sputtering 12 , oxygen plasma assisted molecular beam epitaxy 13 , laser coating and several other methods.Dip coating method is simple, inexpensive and gives uniform thin films.
The aim of the present work is to study the effect of aluminium doping on structural, morphological and optical properties of ZnO thin films.

Cleaning of glass substrate
The glass substrates for depositing the films are initially cleaned with detergent, DI water and acetone.Nitric acid and DI water are used for secondary cleaning process and finally the glass substrates are cleaned in ultrasonic bath using acetone and dried for 10 minutes in hot air oven.

Deposition of undoped ZnO and Aluminium (Al) doped ZnO thin films.
Zinc acetate dehydrate (ZAD) is used as base material.Ethanol and Monoethanolamine were used as solvent and stabilizer respectively.The zincate bath was prepared by dissolving ZAD in 100ml of ethanol and stirred for 2 h, using magnetic stirrer and left covered for 24 hours.Molar ratio between ZAD and MEA is kept at 0.1M.The undoped ZnO film is prepared by alternatively dipping the precleaned glass substrate into zincate and hot DI water baths for 30 sec each and the dipping process is repeated for 30 cycles.
Aluminium acetate is used for doping aluminium and is added along with ZAD in 100 ml of ethanol and is stirred for 2 h, in magnetic stirrer and kept covered for 24 hours.The solutions are prepared for 1%, 3% and 5% Al content.Al doped ZnO films are formed by alternately dipping the pre-cleaned microscopic glass substrate in Al dissolved zincate bath and hot DI water bath for 30 sec each and the dipping process is repeated for 30 cycles.The ZnO films are prepared for undoped, 1%, 3% and 5% Al content and post annealed at 350 o C.

Structural Studies
Figure  Debye-Scherrer's formula 14 is used to calculate the crystalline size of the prepared films.the average crystalline size (D) for undoped ZnO is 31.32 nm and it decreases to 16.30 nm for 1% Al and 18.66 nm for 3% Al and 19.82 nm for 5% Al concentration.Micro strain and dislocation density increases with decrease in Al content.Lattice constants 'a' and 'c' well matches with the standard value.

Morphological studies
The FESEM images of different Al content ZnO thin films are represented in Fig. 2. The Spherical shaped particles are seen for undoped, 1% and 3% of Al doped ZnO thin films 17 .Higher concentration of 5% Al doped ZnO thin film shows the large variance in the grains and splitting of spherical shape to rod shape (Fig. 2d).The particulate grain almost covers the substrate surface more or less evenly.It is seen that the morphology of ZnO thin films is strongly influenced by aluminium doping. (1) The dislocation density (δ) is calculated using equation 2, and micro strain (ε) values is calculated using equation 3 15 (2) Where D indicates crystalline size, λ for the wavelength, β for the FWHM and θ is the diffraction angle in degrees 16 .Table : 1 indicates that   (αhu)2 and (hυ) for undoped and Aluminium doped ZnO thin films.The band gap is obtained from the straight line region of (αhu) 2 versus hυ and the band gap energy of undoped, 1%, 3% and 5% Aluminium doped ZnO thin films are 3.55eV, 3.68eV, 3.82eV and 3.25eV respectively.Fig. 4 (b) shows that the band gaps initially widened with increasing aluminium content up to 3% and then the absorption edge shifted again to longer wavelength range 18 .This red shift is clarified by stress relaxation mechanism.This is because impurity band merges into conduction band causing the reduction in the band gap 19 .
Figure 5 shows the undoped and Al doped ZnO films PL spectrum for wavelength ranging from 300nm to 550nm under the excitation of 325nm.With increase in the Al doping concentration, the intensity of peak increases with a decrease in peak broadening 20 .The emission spectrum shows two peaks.The strong band emission peak obtained near 381nm relates to near band edge . 1(a) represents XRD of undoped, 1%, 3%, and 5% Al concentrations of ZnO films, post annealed at 350 o C.The peaks appeared in the XRD pattern shows the Wurtzite hexagonal form of ZnO (JCPDS card: 89-1397).The orientation of peak at (002) plane shows the polycrystalline nature, other XRD peaks appear at (100), (101), (102), (110), (103) and (112) planes.Fig.1 (b) shows change in lower diffraction angle (33.5 o to 33.5 o ) for different peaks owing to the Al concentration.The intensity of the peak in the (002) plane of undoped and Al doped ZnO shows decreased crystal size.The XRD indicates only ZnO peaks and no peaks related to aluminium and its components.The reason may be presence of a small amount of aluminium content in the form of very small cluster in the ZnO lattice, so it may be very hard to detect by XRD instrument and it may present in the grain boundaries of the ZnO nano crystals or distributed atomically within the ZnO lattice.

Fig. 1 .
Fig. 1.(a) xRD of undoped and Al doped ZnO Thin Films (b) The shift position in the diffraction angles at (002) peaks

Figure 4 (
Figure 4 (b) shows the relation between