Abstract
ZnO is an II-VI semiconductor material with wide band-gap, which has hexagonal wurtzite structure. ZnO thin films were widely applied in solar cell, UV detector, SAW device, gas sensor and transparent electrodes et al for their excellent properties. In recent years, Al-doped ZnO (ZAO)thin films has become a hot issue of transparent conductive thin films field and preferred materials instead of ITO films not only because of their comparable optical and electrical properties(high optical transparency in the visible range,low electrical resistance)to ITO films, but also because of their lower price and higher thermal and chemical stability under the exposure to hydrogen plasma than ITO. ZAO film generally was deposited by magnetron sputtering ZAO ceramic target. However,due to the technology blockade and the high threshold of funds and technology, domestic company can not produce high-quality ZAO target yet. And foreign enterprises, which have enough accumulation of technology and funds, do many studies on ZAO target, and we have been left behind far from them.
In this paper, the studies are focused on reunion during the preparation process of nano-ZnO powder, the relationship between the performance of ZAO target with the particle size of powder, and the impact of O2/Ar flow ratio to ZAO film.The main conclusions are as follows:
The best precipitation technology with precipitant ammonia: the concentration of ammonia and ZnO is fixed at 1 mol / l, pH = 5, and reaction temperature is 70 ℃,reaction time is 2 hours, after centrifugal washing by distilled water, freeze-drying, the precursor orthogonal structure of the Zn (OH)2 was deposited, after calcined at 400 ℃ for two hours, a narrow particle size distribution, good dispersion of the hexagonal structure of ZnO powder can be prepared, the particle size is about 50 nm, the second particle size of 200 to 300 nm. pH value has a great impact of the nano-ZnO particle size,and when the end pH value is 5.0, the smallest size of nano-ZnO will be synthesised. The use of dispersant can greatly reduce the reunion pHenomenon, and at best when the concentration of polyethylene glycol-400 is 0.05 mol / l, but the introduction of new impurities is not conductive to synthesize high-purity ZnO. Use of ethanol washing and drying the powder can reduce the reunion phenomenon, and distilled water washing before the freeze-drying behave better to synthesize narrow size distribution of powder,and more suitable for large-scale industrial production.
The best sintering technology of ZAO target contains: (Ⅰ) skim: to slowly increase the temperature during 20 h from room temperature to 500 ℃. (Ⅱ) the first warming step:warming up to 800 ℃ at 5 ℃ / min. (Ⅲ) the second warming step: warming up to 1200 ℃ at 100 ℃ / h. (Ⅳ) keep the temperature at 1400 ℃ for two hours. (Ⅴ) the first cooling step: cooling to 800 ℃ at100 ℃/h. (Ⅵ) the second cooling step: furnace cooling to room temperature. When the ZAO target was kept at 1400 ℃ for two hours, it would attain higher density and lower resistance. During the process of sintering at 1000 ~ 1200℃, ZnO pHase and ZnAl2O4 pHase formed a solid solution, and the smaller Al3+ replace Zn2+ ions. The formation of oxygen vacancy is the main reason for the low resistance rate of the target. The more narrow distribution of powder, the smaller the reunion of small particles, and the less and smaller porosity in the process of sintering, and the smaller of the grain, so the target got the higher density, the fewer defects, and the lower resistance rate. The comparative test supported that the spherical nano-ZnO prepared by direct precipitation, then washed by distilled water and dried by vacuum freeze method was most suitable for the preparation of high density, low- resistance ZAO target.
The AZO thin film with a uniform thickness and 10-4Ω • cm resistance rate was prepared by DC magnetron sputtering method. The XRD diagram of ZAO film has a trong (002) diffraction peaks and the weaker (101) diffraction peak, and the diffraction angle of (002) diffraction peak was shift to lower diffraction angle. The transmittance of AZO film deposition in the UV areas was less than 20 percent, while the average transmittance of visible light district was nearly 90 percent, and that means the film has a good optical performance. The difference of Ar/O2 has no significant impact on the optical properties of the film for the average transmission in the visible light district are more than 85 percent, but the electrical properties of the films was greatly influenced by the Ar/O2 ratio .the conductivity of the film was best with the resistance rate of 5.3 × 10-4Ω • cm when theAr/O2 ratio was 0.1%.
Keywords: Reunion; nano-ZnO; ZAO(Al-doped ZnO)target; sinter; ZAO film;O2/Ar flow ratio
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