SEMATECH, UK researchers eye charge-trapping polarons

October 15, 2007 – Researchers at SEMATECH and University College London (UCL) say they may have figured out why novel dielectric materials capture electrons and holes, and that the phenomenon which degrades device performance may be more common than thought.

New high-k materials are getting a lot of attention for future (45nm and below) semiconductor architectures, but these materials also tend to capture electrons and holes, thought to be due to structural imperfections in the dielectrics that could be solved by better material stoichiometry.

In a paper published in Physical Review Letters, the researchers describe theoretical modeling that provides a window inside the structures of polarons, electrons that polarize surrounding atomic lattice to create potential wells, in a transition metal oxides. What they found indicates that the existence of polarons may be more common than currently assumed, that electron and hole localization as polarons can be a defining characteristic of such materials, and that charge trapping also can occur in structurally perfect materials.

Both electrons and holes may experience self-trapping by forming polarons in the highly polarizable high-k dielectric, such as HfO2, “just as the deformation of a thin rubber film would trap a billiard ball,” explained UCL Prof. Alexander Shluger. Further, at low temperatures, electrons and holes can hop around between trapping sites instead of propagating as a wave.

“This new understanding of the polaron-formation properties of the transition metal oxides opens interesting possibilities for mitigating these undesirable material characteristics and hopefully will stimulate further research on the polaronic features in high-k dielectrics,” added SEMATECH Fellow Gennadi Bersuker, one of the paper’s authors.


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