Preparation and Properties of Zinc Oxide and Aluminum Doped Zinc Oxide Thin Films
|Course||Atomic and Molecular Physics|
|Keywords||ZnO thin films AZO thin films sol-gel reverse microemulsion preferredα-axis orientation photoelectric properties|
Zinc oxide (ZnO) has recently attracted wide attention from researchers as a new direct wide band gap oxide semiconductor. The wide band gap of 3.37eV at room temperature, leads to its excellent shortwave light emitting ability. A large excitation binding energy of 60meV and its room temperature stability makes ZnO an ideal material for ultraviolet laser diodes (LDs). ZnO has very useful properties of piezoelectricity, gas & pressure sensitivity etc. Transparent conducting semiconductor aluminum doped zinc oxide (AZO) thin films, among many doped ZnO materials, have attracted much attention for their wide application. There are many factors that can affect the quality and structure of the thin films, such as Zn2+ concentration, the ratio between MEA and zinc ion, aging time, and annealing temperature, etc. Many efforts have been made to study these factors, but rarely focused on the effect of film thickness. The oriented growth of ZnO is an important subject in the field of ZnO thin films research. In particular, the achievement in the preparing of a-axis oriented ZnO films is crucial to enhancing the LEDs’luminous intensity and decreasing the energy loss of surface acoustic wave devices during transmission.The film thickness effects on the properties of sol-gel derived AZO thin films were studied systematically in this thesis. It was found that AZO thin films prepared by sol-gel method on amorphous glass substrate have perfect preferred c-axis orientation and well crystallization after annealed at 500℃. AZO thin films’average grain size decreases with the increase in the film thickness, which is attributed to the surface effect and the pinning effect of aluminum. The surface morphology of AZO thin films has little changes with the film thickness, which implies that film thickness doesn’t have any effects on the growth pattern of sol-gel derived AZO thin films. When the number of spin-coating is less than 7, the transmittance of AZO films shows little changes with thickness, all above 95%. AZO thin films under 5 times spin-coating have the largest surface carrier concentration and bulk carrier concentration of 2.50×1013cm-2 and 5.55×1017cm-3, the lowest surface resistivity and bulk resistivity of 1.08×106ohm·cm and 48.5ohm·cm at the same time. AZO thin films under 2-3 times spin-coating, compared to those under 5 times spin-coating, change a little in carrier concentration and resistivity, with a magnitude at the same order. But for the AZO thin films under only one spin-coating, an order of magnitude decrease in carrier concentration and an order of magnitude increase in resistivity are detected. We attributed these changes to the increase in the film density and the decrease in the defects. However, the films under more than 6 times spin-coating show a decrease in carrier concentration with the increase in film thickness, which may result from the decrease in the average grain size. Based on the analyses of each property, the best AZO films were obtained when the number of spin-coating is 5.The reverse micelle microemulsion method was adopted to grow a-axis oriented ZnO thin films; many efforts had been made to gain the optimized parameters. Ultimately, a-axis oriented ZnO thin films were successfully prepared, and the mechanism for ZnO oriented growth was introduced. The ratio between isopropyl alcohol and hexamethylene were kept at 2:1 for all precursor sol. It is suggested that 300-370℃and 550℃are the optimized pyrolysis temperature and annealing temperature respectively. The lower the concentration of Zn2+ in precursor, the easier for us to grow a-axis oriented films. But the lower concentration needs much more times of spin-coating and annealing, hence the concentration of zinc acetate aqueous solution was 0.5mol·L-1 and the number of spin-coating was 15-20. If the number of spin-coating was too small it was a little hard to detect the x-ray peaks. Instead, if it was too big the orientation might change. A process of "nucleation first and growth second" was introduced, and it was verified by our own experiments. It is a very original work to fabricate a-axis oriented ZnO thin films with reverse micelle microemulsion method, because no reports have ever been seen about this.