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Local Structure, Dielectric Properties, and Phase Stability of Doped “High-k” Materials

As integrated transistors in microelectronic devices continue to shrink in size (scaling according to Moore’s Law), new materials are required for the advanced gate stack (the gate dielectric and the gate electrode layers) that control transistor operation.  SiO2 has traditionally been used as the gate dielectric that mediates the gate voltage operating on the transistor’s channel region, but over time with scaling this layer has been continuously reduced in thickness to increase its capacitance. SiO2 gate dielectric layers are now so thin (~ 1.5 nm) that they are no longer effective insulators, resulting in unacceptably large leakage currents. High dielectric constant, “high-k”, insulating materials, such as HfO2, Hf-silicates and Hf-oxynitrides, are currently being introduced as replacements for SiO2.  While they exhibit much smaller leakage currents than SiO2, their use raises materials science issues that must be resolved before optimum transistor performance can be achieved.

For example, unstable flatband voltages are observed after electrical stressing in devices with HfO2 gate dielectrics, and this effect is believed to be associated with oxygen vacancies in the HfO2 layer.  Improved properties have been reported as a result of the addition of other cations into HfO2.  In some cases, cation substitution also results in a favorable increase of the relative dielectric constant, k, from about 15 to 30.  Using first principles electronic structure calculations based on density functional theory, a Functional Properties Group researcher has been able to determine the local cation structure, i.e., the lattice position of the substitutional cation, as well as its interactions with other substituted cations. For example, when Al+3  is  substituted for Hf+4, the model predicts that two Al atoms prefer to sit next to an oxygen vacancy, thus creating a defect center that will affect flatband voltage stability during electrical stress (operation). In order to increase the dielectric constant of the stable monoclinic phase of HfO2 a highly-polarizable cation such as Ta must be used as a dopant. Otherwise, increasing the dielectric constant requires that the tetragonal or cubic phase of HfO2 be stabilized. However, the model predicts that these alternative phases of HfO2 are almost always metastable: the controlling factor is the size of the dopant ion, with small dopant ions favoring the stabilization of the alternative phases. Such information will assist in the commercialization of high-k gate dielectric materials by predicting the degree of metastability of alternative phases, which will determine their potential usefulness in the gate stack. Finally, the local atomic structure model was used to generate infrared and Raman spectra as a function of HfO2 crystal structure and substitutional cation content. These predicted spectra enabled the correct interpretation of experimental spectra of Zr-doped HfO2 published by Freescale Semiconductor Corporation.

Contact

Martin L. Green
301-975-8496 Telephone
301-975-5334 Facsimile
martin.green@nist.gov 

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