In the constant quest for miniaturization, transistors and all their components continue to decrease in size. With them so has the thickness of a component material known as the gate dielectric - typically a thin layer of silicon dioxide, which has now been in use for decades. Unfortunately as its thickness decreases further, silicon dioxide starts to leak current, leading to unwieldy power consumption and reduced reliability. Scientists hope that this material can be replaced with others, known as high-dielectric constant (or high-k) dielectrics, which mitigate the leakage effects at these tiny scales. Metal oxides with high-k have attracted tremendous interest due to their application as novel materials in the latest generation devices. The impetus for their practical introduction would be further helped if their ability to capture and trap charges and subsequent impact on instability of device performance was better understood. It has long been believed that these charge trapping properties originate from structural imperfections in materials themselves. However, as is theoretically demonstrated in this publication, even if the structure of the high-k dielectric material is perfect, the charges (either electrons or the absence of electrons - known as holes) may experience self-trapping. They do so by forming polarons - a polarizing interaction of an electron or hole with the perfect surrounding lattice. This creates an energy well that traps the charge, just like a deformation of a thin rubber film traps a billiard ball.
Image 1: Illustration of the displacement of atoms in the structure to accommodate the presence of the self-trapped hole in the oxygen atom (red). Right – quantum mechanics view of the probability of finding a hole near certain atoms (larger blue structures represent higher probability)
The resulting prediction is that at low temperatures electrons and holes in these materials can move by hopping between trapping sites rather than propagating more conventionally as a wave. This can have important practical implications for the materials’ electrical properties. In summary, this new understanding of the polaron-formation properties of the transition metal oxides may open the way to suppressing undesirable characteristics in these materials.
Electron trapping at point defects on hydroxylated silica surfaces
L. Giordano, P. V. Sushko, G. Pacchioni and A. L. Shluger
Physical Review Letters, v. 99, pp. 136801(1-4), (2007)
We have considered two prototype defects (oxygen hole centre and E' centre) at several hydroxylated silica surfaces and calculated their structure and electronic properties using ab initio embedded cluster approach. We found that both of these defects are deep electron traps with electron affinities ranging between 4.0 and 5.8 eV, which can explain the origin of the negative charging of silica surfaces observed experimentally.
Effect of protons on the optical properties of oxide nanostructures
M. Muller, S. Stankic, O. Diwald, E. Knozinger, P. V. Sushko, P. E. Trevisanutto, and A. L. Shluger
Journal of the American Chemical Society, v. 129, pp. 12491-12496 (2007)
We demonstrated that photo-luminescence properties of oxide nanostructures can be controlled via selective chemical modifications of their surface sites. The oxygen-terminated corner sites of 3-10 nm large MgO nanocubes have been selectively protonated using UV irradiation under an H2 atmosphere followed by anneal at ~900 K. This changes the luminescence energy from 3.2 eV to 2.9 eV but does not affect the luminescence excitation spectrum. The nanocubes can be fully deprotonated using anneal at ~1200 K.
From insulator to electride: a theoretical model of nanoporous oxide 12CaO 7Al2O3
P. V. Sushko, A. L. Shluger, M. Hirano, and H. Hosono
Journal of the American Chemical Society, v. 129, pp. 942-951, (2007)
We developed a theoretical model of a complex oxide [Ca24 Al28 O64]4+ . (O2-)2-x(e-)2x, where [...]4+ forms a framework made of cages and compensated by extra-framework anions. The extra-framework of this material can be gradually modified so that C12A7 is converted from a wide-band-gap insulator (x=0) to a transparent polaron conductor (x=1) and then to a metal (x=2). The electron gas formed at x > 0 is confined to the lattice cages.
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