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The electronic structure at the atomic scale of ultrathin gate oxides


The narrowest feature on present-day integrated circuits is the gate oxide—the thin dielectric layer that forms the basis of field-effect device structures. Silicon dioxide is the dielectric of choice and, if present miniaturization trends continue, the projected oxide thickness by 2012 will be less than one nanometre, or about five silicon atoms across1. At least two of those five atoms will be at the silicon–oxide interfaces, and so will have very different electrical and optical properties from the desired bulk oxide, while constituting a significant fraction of the dielectric layer. Here we use electron-energy-loss spectroscopy in a scanning transmission electron microscope to measure the chemical composition and electronic structure, at the atomic scale, across gate oxides as thin as one nanometre. We are able to resolve the interfacial states that result from the spillover of the silicon conduction-band wavefunctions into the oxide. The spatial extent of these states places a fundamental limit of 0.7 nm (four silicon atoms across) on the thinnest usable silicon dioxide gate dielectric. And for present-day oxide growth techniques, interface roughness will raise this limit to 1.2 nm.

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Figure 1: The measured EELS O K edges of bulk a-SiO2 and for O atoms at an atomically smooth interface between [100]Si and native a-SiO2.
Figure 2: EELS spectra recorded point by point across a gate stack containing a thin gate oxide.
Figure 3: Oxygen bonding profiles from O K edge EELS.


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We thank D. R. Hammann, M. S. Hybertsen, P. Rez, J. Neaton and B. Batlogg for discussions, and J. Silcox and M. Thomas for access to the Cornell Center for Materials Research STEM. Funding for the operation and acquisition of the STEM was provided by the NSF. Upgrades were founded by the US Air Force Office of Scientific Research. The X-ray diffraction was performed on X16B at the National Synchrotron Light Source.

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Muller, D., Sorsch, T., Moccio, S. et al. The electronic structure at the atomic scale of ultrathin gate oxides. Nature 399, 758–761 (1999).

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