Electrical field enhances crystallization of amorphous silicon
Physicists at Kyung Hee University and Han Yang University (Seoul, Korea) increased the crystallization speed of amorphous silicon (a-Si) while simultaneously limiting defects. To reduce the temperature at which a-Si crystallizes, manufacturers often add certain metals, such as nickel. The group showed that by applying a modest electrical field as well, the speed of the process can be significantly increased--a crystallization time of 25 hours at 500C was reduced to 10 min by adding an electrical field of 80 V cm-1. The researchers started with 400-nm-thick films of hydrogenated a-Si to which they added a thin layer of nickel by sputtering. The samples were placed between two electrodes in a vacuum and crystallized at 500C.
Longer crystallization times allow crystalline regions to meet, and the interfaces form clear grain boundary regions with defect states that can trap carriers--in the Korean method there are no grain-boundary states. The researchers suggest that the electric-field-enhanced technique could be applied widely to crystallize amorphous materials, which could be significant because thin films of polycrystalline silicon are important to large-area electronic applications, such as in the switching electronics of flat-panel displays.