First-principles calculations of the electrical properties of LaAlO3 and its interface with Si

LaAlO3 is one of the potential candidates to replace SiO2 as a high permittivity dielectric for future generations of metal-oxide-semiconductor field effect transistors. Using first-principles plane-wave calculations within density functional theory, its bulk and surface electronic properties and the relative stability of cubic c-LaAlO3(001)/Si(001) interfaces are investigated. In agreement with experiment, our study shows that the dielectric constant of crystalline LaAlO3 (similar to 30) is comparable to that of hexagonal La2O3. To accurately calculate the c-LaAlO3(001) surface energy, several ways of eliminating the surface dipole moment of the polar surface are presented, with the transfer of an oxygen anion from one boundary surface to the other being identified as the energetically most favorable mechanism. We have found that lanthanum-terminated c-LaAlO3(001)/Si(001) interfaces are in general more stable than aluminum-terminated interfaces for both the oxidized and nonoxidized Si(001) surfaces. We have also identified a significant reduction of the c-LaAlO3(001)/Si(001) valence band offset due to the creation of interface dipoles for O-rich interfaces. Analysis of the density of interface states shows that La-Si bonds at the c-LaAlO3(001)/Si(001) interface do not create interface states in the silicon band gap, in contrast to Hf-Si bonds in m-HfO2(001)/Si(001) interfaces studied previously.722

Segregation trends of the metal alloys Mo-Re and Mo-Pt on HfO2: A first-principles study

Using first-principles calculations, we compared the segregation trends at the surface of metal alloys with those at an interface with HfO2. The choice of this oxide was motivated by its significance as a potential replacement for SiO2 in advanced transistors. We considered Mo-Re and Mo-Pt alloys as typical examples of disordered and ordered alloys, respectively. The segregation to the surface/interface was analyzed in terms of metal and oxygen adsorption energies. It is shown that chemical bonding at the metal/oxide interface strongly influences segregation both in Mo-Re and Mo-Pt alloys. In particular, bonding with oxygen atoms at the oxide/Mo-Re alloy interface depletes the Re content of the interfacial layer. In the case of Mo-Pt on HfO2 an oxygen-rich interface promotes the formation of one monolayer (but not two monolayers) of Mo separating PtMox from HfO2, while a stoichiometric interface favors an abrupt PtMox/HfO2 interface. This study also shows that the presence of Mo in the alloy stabilizes Pt which can potentially decrease the tendency of Pt to diffuse into the oxide matrix. The individual constituents of these intermetallic compounds exhibit high vacuum work functions, and therefore these alloys are also likely to have sufficiently high work functions to be considered as promising candidates for p-type gate electrodes in future generations of transistors. (c) 2006 American Institute of Physics.100

First-principles investigation of the WC/HfO2 interface properties

The thermodynamic and electronic properties of tungsten carbide surfaces and interfaces with monoclinic hafnia (WC/m-HfO2) are investigated through first-principles calculations. We show that oxidation of the WC surface and of the WC/m-HfO2 interface is energetically favorable. An oxygen monolayer on the W-terminated WC(0001) surface gives rise to a larger vacuum work function than that for the C-terminated WC(0001) surface, while the opposite result is obtained for the WC(0001) effective work function on hafnia: a carbon intermediate layer results in larger work function than an oxygen intermediate layer. This result is explained by the atomic structure of the intermediate layers neighboring the interface which differ if the interface is O or C rich. (C) 2006 American Institute of Physics.99