The electrical characteristics of aluminum (Al) doped zinc oxide (ZnO) thin film for high sensitivity humidity sensors are presented. The effects of Al doping concentration at
ZnO is n-type semiconductor with wide band gap 3.37 eV posses wurtzite family structure. It has many potentials in electronics devices such as sensors. Humidity sensor is essential to sense the moisture of the environment to control the human comfort, industry packaging [
ZnO thin film have been prepared using various method such as sol-gel [
In this study, we investigated characteristic of humidity sensor for Al-doped ZnO using sol-gel immersion method on the glass substrate. The doping concentration were vary for undoped ZnO, 0.4 at% and 0.6 at% Al-doped ZnO. The objective of this study is to study the effect of Al-doped ZnO toward humidity sensitivity.
At first the glass substrate were cleaned using standard cleaning method to remove all the contamination. For the template preparation, the glass substrate were sputtered with platinum for 30 nm at first and followed by gold for 30 nm. ZnO solution were prepared using zinc acetate dehydrate (Zn(CH3COO)2·2H2O) as precursor, monoethanolamine (MEA) as stabilizer, 2-methoxyethanol as solvent. The molar ratio of MEA to zinc acetate dehydrate were fixed at 1 : 1. All the material were mixed together using sol-gel method and were stirred at 80°C for 3 hour to yield a clear and homogeneous solution. The solution were aged for 24 hour at room temperature. Then, the substrate that have been coated were deposited by spin-coating technique using sol-gel ZnO solution. The coating solution will be dropped onto the substrates, which were rotated at 3000 rpm for 60 s. After the spin coating the film were dried at 150°C. This coating and drying process were repeated for 10 times to increase thickness for every sample. Then a trench was made for resistance action. The cross section of the thin film was made as shown in Figure
The cross-section of the sample.
Next, preparation for second sol-gel solution for the catalyst. In this solution, hexamethylenetetramine (HMT) act as the stabilizer, zinc acetate dehydrate (Zn(CH3COO)2·2H2O) as starting material and Aluminum Nitrate nanohydrate (Al(NO3)3·9H2O) as dopant source were mixed into the deionized water that act as the solvent. The quantities of aluminum nitrate will be varying to achieve
The samples characterization stage involved determination of electrical properties by 2 probe
A homemade humidity box that made from polystyrene box with size about 40 cm × 30 cm × 25 cm. A diagram of and photo the humidity box is shown in Figures
(a) The humidity box diagram. (b) The photo of humidity box.
The sensor measurement for
The graph for
Figures
Figure
Resistance versus relative humidity, RH%.
Sensitivity in this project is referred as the response of the resistance due to the changes of relative humidity. The value of the sensitivity was measured by taking the slope of the resistance versus relative humidity graph. Figure
Sensitivity versus doping concentration (at%).
From that graph shows the sample with 0.6 at% Al-doped ZnO has a higher sensitivity than other thin film. All the sensors roughly exhibit the region of sensitivity, as for example, when the value of relative humidity is higher the sensitivity will increase.
This explained the current measured in this project are always increasing as the relative humidity increase. The 0.6 at% Al-doped ZnO thin films having more sensitivity than the 0 at% and 0.4 at% are because of the adsorption of the moisture molecules by the Al-doped ZnO at the surface of both samples. There for the high sensitivity is 2.32 M (Ω/RH) at 0.6% doping Al-doped ZnO.
The PL spectra of undoped ZnO and Al-doped ZnO at room temperature are plotted in Figure
PL measurement at room temperature.
SEM image in Figure
SEM image of the Al doped ZnO thin films deposited at.
0 at%
0.4 at%
0.6 at%
Figure
XRD pattern of the Al doped ZnO thin films deposited at 0.4 at% and 0.6 at%.
The particles of zinc oxide had been grown successfully on the glass substrate by sol gel method. The electrical properties of Al-doped ZnO are investigated by using the
The author likes to express her thanks to Institute Of Science, Faculty of Pharmacy UiTM and Faculty of Mechanical UiTm for providing the laboratory facilities.