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In this research ZnO thin film will be doped with aluminium using sol-gel spin-coating method. Then the Al-doped ZnO thin film (as seed catalyst) on a glass substrate will be then layered bottom of an aligned ZnO nanorod structure which was prepared using sonicated sol-gel immersion technique. The nanorod thin film parameters will be characterizing through its I-V curve characteristics and also the optical and absorbance of aligned ZnO nanorod thin film.
1.2.1 Background study
UV sensor or stands for ultraviolet sensor are important devices that can be used in commercial and military applications. The applications are including gas sensing, space research, high temperature flame detection, air quality monitoring and many more. UV detectors currently used silicon-based detectors and photomultiplier tubes. These materials require costly filters and attenuators. However by replacing those technologies with wide band-gap semiconductors such as zinc oxide (ZnO) have been suggested . UV light emitting or receiving devices, solar cells, gas sensor and transparent electrodes are some of the zinc oxide applications. Various deposition methods such as chemical vapor deposition, sputtering and molecular beam epitaxy were used to fabricate ZnO thin films. However, these deposition methods are high production cost because required high temperature processing and vacuum technology .
In this paper, the purpose of this research is to fabricate ultraviolet sensor using zinc oxide thin film and characterize the sensor performances by its electrical properties. Furthermore, this research is to characterize aluminum doping concentration and the nanostructure of zinc oxide thin film. In this research, sol-gel spin coating method was used to prepare Al doped zinc oxide thin film and an aligned ZnO nanorod structured thin films on a glass substrate by the sonicated sol-gel immersion method.
1.2.2 Literature Review
ZnO is an n-type semiconductor with a direct band gap of 3.42eV and because of the oxygen vacancies and intrinsic defects such as interstitial zinc atoms, the electrical conductivity of the films is large . It is very good material for electronic device application due to its wide band gap and large exciton binding energy of 60meV . The properties of ZnO thin film can be improve by a doping process and annealing process . There are that focus on the doping process. To improved electrical or optical properties of ZnO, ZnO were frequently doped with group 3, 4 and 5 elements such as Gallium (Ga), Sn and Aluminium (Al) . Al doping is most suitable because it’s cheap, abundant and non-toxic material and will produce Al doped ZnO thin film with higher optical transmittance and low resistivity in infrared region .
From the journal , radio-frequency (RF) reactive magnetron sputtering method was used to deposit Al-doped ZnO on the smooth nucleation side of FSD films by. The electrical properties were discussed. The first electron concentration increased and next decreased with the increase of Al doping concentration. The maximum carrier concentration achieve when the film is doped with 2at% Al. Hall mobility is inversely proportional to the Al doping concentration. The high temperature annealing process is use to enhance the Hall mobility of the films .
From the journal , ZnO thin films with various weight percentages of aluminium and the electrical properties were discussed using sol-gel dip-coating method. The conductivity shows higher response of the doped ZnO thin films under UV compared with dark condition. Both condition proof that the conductivity of ZnO is directly proportional to doping concentration. The optical band gap energy is also directly proportional with doping concentration. The transmittance of ZnO thin films is about 75% when wavelength from 390nm to 850nm.
From the journal , at different dopant concentration the electrical properties of Al-doped ZnO thin films were discussed using sol-gel method. The resistance first decrease with increase aluminium ion concentration. Then the resistance of doped thin films increased with increasing dopant concentration and it decreases at higher Al concentration. The transmittance of ZnO thin films is about 80% when wavelength from 370nm to 850nm.
The advantage of gallium is an excellent dopant for increasing the conductivity and this source is less reactive and more resistive oxidation. Based on the journal , Ga-doped ZnO nanowires were growth using thermal deposition method. The ZnO nanowires have grown uniformly with high yield and average length of each nanowire is about 1.3um. From the XRD pattern, it can be deduced that the Ga element doped into the nanowires creates a clear broadening. The Ga-doped ZnO nanowires have a greater field-enhancement factor than the undoped ZnO nanowires .
Aqueous solution process with post-growth rapid photothermal processing (RPP) was used to prepare Sn-doped ZnO nanorods to develop aligned Sn-doped ZnO nanostructure. Aqueous solution method was chosen in preparation of metal oxide nanoparticles structure due to its cost and environment friendly. Rapid photothermal processing (RPP) as an alternatives of thermal annealing was combine with aqueous solution due to short cycle time, reduced exposure and flexibility .
Based on journals ,  and , the optimum doping concentration of aluminium is 2at%, 5at% and 1.5at% respectively. Optimal aluminium doping concentration will gives highly semiconducting properties of Al-doped ZnO thin film for application UV sensor . A research had done on various Al doping concentration and the optimum doping concentration is at 1at% Al. This research will used 1at% Al concentration based on journal . Aligned ZnO nanorods that were prepared using sonication method are a simple way and very low cost method compared to other techniques. It also will growth high quality ZnO nanorod with a small diameter size .
1.2.3 Problem Statement, Scope and Limitation
Most people are aware of the effects of UV through the painful condition of sunburn, but the UV spectrum has many other effects whether its benefits or damaging to human health. Too much exposure to UV radiation can affect human health. UV application for optical sensor used 230 nm to 400nm wavelength which is ultraviolet B (UVB) which is harmful to human skin. Since it is harmful to human skin, metal oxide semiconductor films have been considered due to its excellent chemical and physical properties. One of them is zinc oxide material. Zinc oxide is not only good in optoelectronic but also in electrical properties. Zinc oxide is then will be doped with aluminium to enhanced the zinc oxide thin film properties by using sol-gel spin-coating method. It is important to choose the right method for preparing aligned ZnO nanorod. The scope of this research is to investigate the electrical properties (I-V characteristic) and the optical properties of the ZnO nanorod thin film at different immersion time. The limitations of this research are on the solution preparation and deposition condition which are different for different groups and on the fix parameter.
1.2.4 Significant of the research
The nanomaterials based sensors are raising the advantage of size reduction and enhanced functionality . This research will used a simple method to prepare an aligned ZnO nanorod thin film which is sol-gel immersion method which is this method has not been reported by any research group. Moreover it is very low-cost method.
To prepare Al-doped ZnO thin film as seed catalyst layer.
To prepare aligned ZnO nanorod thin film at different immersion time.
To fabricate aligned ZnO nanorod thin film based UV photoconductive sensor.
To characterize fabricated UV sensor.
1.2.6 Research Questions
To achieve the objective of this research, some research questions would need to focus on which are:
What is the I-V curve characteristic of aligned ZnO nanorod thin films before and after exposing to the UV illumination?
What is the optical and absorbance characteristic of aligned ZnO nanorod thin films?
How to fabricate ZnO thin film?
What is the factor affecting the nanostructure ZnO thin film before and after exposing to the UV illumination?
2.1 Research Methodology
2.1.1 Al doped ZnO thin films
Sol-gel spin-coating method was used to prepare Al doped ZnO thin films. The solution include zinc acetate dihydrate (Zn(CH3COO)2 2H2O) as precursor, aluminium nitrate nonahydrate (Al(NO3)3 9H2O) as a dopant source,monoethanolamine (MEA, C2H7N14) as a stabilizer and 2-methoxyethanol as a solvent were prepared. The concentration of aluminium nitrate was 1.0at% Al doping and the molar ratio of MEA to zinc acetate was fixed at 1.0. At 3 hours before aged at room temperature for 24hours to yield clear and homogeneous solutions the solution was stirred at 80°C. The solutions were then used for deposition process by spin-coating technique. Substrates used were microscope on the glass substrates. At room temperature, the spin-coating was performed using 10 drops of solution per deposition with a speed of 3000rpm for 60s. The films were preheated in atmosphere ambient at 150°C for 10 minutes to evaporate the solvent each time after deposition process. The films were annealed at 500°C for 1 hour in air ambient using a furnace (Protherm) after repeating the coating procedure 10 times. The crystal structure and orientation of ZnO thin films were investigated by X-ray diffractometer (XRD). The cross-section of the film thickness is observed by using scanning electron microscope (SEM). By using UV-vis-NIR spectrophotometer, the optical properties of ZnO thin films were measured in the wavelength range from 200 nm to 1500nm. The electrical properties were measured by dc probing system .
Prepare the solution of zinc oxide
Precursor : zinc acetate dehydrate
Stabilizer : monoethanolamine
Dopant source : aluminium nitrate nonahydrate
Solvent : 2-methoxyethanol
Heat and stir the solution
Temperature : 80°C
Time : 3hr
Solution stirring and aging
Temperature : room temperature
Time : 24hr
Thin film deposition using spin-coating tech.
Speed : 3000rpm
Time : 60s
Temperature : room temperature
Solution : 10 drops of solution per deposition
Thin film drying
Temperature : 150°C
Time : 10min
Repeat 10 times
Thin film annealing
Temperature : 500°C
Time : 1hr
2.1.2 Aligned ZnO nanorod structure
An aligned ZnO nanorod structure was prepared using the sonicated sol-gel immersion method on a glass substrate coated with a ZnO thin film that had been doped with 1at% Al. The sol-gel coating method was used to prepare the Al-doped ZnO thin film as a seeded catalyst that was prepared on the glass substrate. A solution was prepared of 0.1M zinc nitrate hexahydrate (Zn (NO3)2 .6H2O) and 0.1M hexamethylenetetramine (HMT, H2NCH2CH2OH) in deionized water (DI). At 50°C for 30 minutes using an ultrasonic water bath, this solution was sonicated. The solution was then aged and stirred for 3 hours at room temperature. The sonicated and aged solution was poured into a Schott bottle, at the bottom which had been placed the horizontal Al-doped ZnO thin film coated glass substrate. The capped bottle was then immersed in a 95°C water bath for 4 hours. After the immersion process, the sample was cleaned with DI water and allowed to dry in ambient air. By using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD), the resulting ZnO nanorod structure was characterized for surface morphology and crystallinity. The optical transmittance and absorbance properties of the sample were characterized by UV-vis-NIR spectrophotometer .
Prepare the solution in deionized water
0.1M zinc nitrate hexahydrate
Sonicated solution using an ultrasonic water bath
Temperature : 50°C
Time : 30min
Solution stirring and aging
Temperature : room temperature
Time : 3hr
Placed the horizontal Al-doped ZnO thin film coated glass substrate at the bottom Schott bottle. Poured sonicated and aged solution
Temperature : 95°C water bath
Time : 4hr
Clean with DI water and dry in ambient air
Nanorod structure characterization
The electrical properties of ZnO thin film improved when ZnO thin film was doped with aluminium. Optimum aluminium doping concentration will gives highly semiconducting properties of Al-doped ZnO thin film for application UV sensor. It is important to find low cost method. The ZnO nanorods have an average diameter of 100nm. An average optical transmittance of 78% was observed. The electrical properties and optical properties of an aligned ZnO nanorod thin film were affected when varying the immersion time. High degree of crystalline, large surface area availability, a visible wavelength transparency and high UV absorption properties, this method show it’s suitable for UV sensor application.
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