I have synthesized ZnO thin films at four different substrate temperature say, 300, 350, 400 and 450C. and the samples were annealed at 450C for half an hour. the xrd result shown better crystallinity at low temperature and the crystallinity decreases as the temperature increased. why?
Mr. S. Visweswaran, thank you to put your question in research-gate.
First of all, you deposit ZnO thin film at different temperature ranges from 300 to 450oC but annealed all the samples at 450oC for half an hour. Did you check the XRD of all films before annealing? Because, after annealing all films may show the same crystallinity due to same annealing temperature. But, in your case crystallinity decreases, which is strange. I think all of your film thickness is different and you are not getting the exact trend of crystallinity. With deposition temperature, the compactness of the film increases as well as the film thickness reduces. So, i think for your case, the film thickness reduces and you are not getting the exact change in crystallinity with temperature. Additionally, if the thickness of ZnO film increases, the grain size of the film increases as well as crystallinity increases and vice-versa. So, please try to make a thick film and measure the XRD.
Dear s. Visweswaran ,
This may be related to the breaking of Zn-O bonds with annealing in specific time/temperature, resulting in disassembling of the lattice arrangement as well as producing of the defects and compressive stress that can lead to increase in strain and dislocation density.
As deposited (magnetron sputtering) ZnO films (at high power densities) on glass are normally already crystalline when deposited at temperatures of 300°C and above. Even RT deposited thin films are already partially crystalline. A post-annealing normally yields further crystallization (of remaining amorphous fraction) and grain growth. It is impossible that the film 'becomes amorphous again'.
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