First-principles calculation of the TiN effective work function on SiO2 and on HfO2

L. R. C. Fonseca and A. A. Knizhnik
Phys. Rev. B 74, 195304 – Published 3 November 2006

Abstract

This paper reports density functional theory (DFT) electronic structure calculations of the valence band offsets (VBO) between TiN and cristobalyte SiO2 and between TiN and monoclinic HfO2 for various interface chemical compositions. To investigate the impact of species interdiffusion on the effective TiN work function, we considered modifications of the TiN and dielectrics composition within the first monolayer from the interface. We found that the calculated VBO’s depend on the stoichiometry of the interface: they are the smallest for oxygen/nitrogen rich interfaces and increase for reduced interfaces where metal-metal bonds are formed. The impact of the interface stoichiometry on the VBO for the assumed interface models can be as large as 0.3eV and 1.2eV for the TiNSiO2 and TiNmHfO2 interfaces, respectively. We also found that species interdiffusion within our model systems does not affect the VBO significantly. All the calculated VBO’s obtained from stoichiometric interfaces and their structural modifications are smaller than expected from available data. Finally we considered possible schemes for calculating the effective metal work function on a dielectric. We conclude that the rather poor accuracy of such work function calculations (stemming from the underestimated VBO’s) may be explained by the crudeness of our model interface, which is limited by the lack of experimental data on the interfaces atomistic structures. Our analysis indicates that with the introduction of a transition region between the metal and the dielectric instead of the sharp interfaces and their slight variations studied here, or by overoxidizing the interface, the calculated VBO’s may improve. On the other hand, the accuracy problem may have a more fundamental origin, namely the local density approximation (LDA) of DFT which leads to a severe band gap underestimation. Scaling our calculated VBO’s by the corresponding experimental band gaps yields better agreement with measured TiN work functions on HfO2 for stoichiometric or near-stoichiometric interfaces. However, for TiN on SiO2 the scaled VBO is still considerably smaller than experimental data suggests. The inclusion of GW corrections or self-interaction corrected pseudopotentials improves the agreement with experimental data significantly, which strongly suggests that the VBO is underestimated by LDA/DFT as well as the band gap.

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  • Received 23 June 2006

DOI:https://doi.org/10.1103/PhysRevB.74.195304

©2006 American Physical Society

Authors & Affiliations

L. R. C. Fonseca*

  • Wernher von Braun Center for Advanced Research, Avenida Perimetral Norte 301, Campinas, São Paulo 13098-381, Brazil

A. A. Knizhnik

  • Kinetic Technologies Ltd., Kurtchatov Square 1, Moscow 123182, Russia

  • *Corresponding author. Electronic address:fonseca@vonbraunlabs.com.br
  • Electronic address: knizhnik@kintech.ru

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Issue

Vol. 74, Iss. 19 — 15 November 2006

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