AVS 51st International Symposium
    Electronic Materials and Processing Monday Sessions
       Session DI-MoA

Paper DI-MoA2
d-states Coupling in Mixed High-k Oxides: Transition Metal Oxide Binary Alloys, and Transition Metal-rare Earth Complex Oxides

Monday, November 15, 2004, 2:20 pm, Room 304B

Session: High-k Dielectrics: Electronic Properties
Presenter: G. Lucovsky, NC State University
Authors: G. Lucovsky, NC State University
C.C. Fulton, NC State University
Y. Zhang, NC State University
J.L. Whitten, NC State University
R.J. Nemanich, NC State University
H. Ade, NC State University
D.G. Schlom, Penn State University
J.L. Freeouf, Oregon Research Institute
Y. Zou, NC State University
Correspondent: Click to Email
The lowest conduction band anti-bonding d*-states in (HfO@sub 2@)@sub x@(TiO@sub 2@)@sub 1-x@ pesudo-binary alloys, and in TiO@sub 2@ and HfO@sub 2@ have been studied by soft x-ray absorption spectroscopy (XAS). These studies distinguish between intra-atomic transitions within the Ti- and Hf-atoms and inter-atomic transitions that involve mixing of the Ti and Hf d*- and s*-states with O 2p*-states. Comparisons between the energies of the lowest d*-states in the O K@sub 1@ spectra of TiO@sub 2@ and HfO@sub 2@ indicate that the energy difference between these features is essentially the same as the energy difference between d*-state features at the respective band edges. There are systematic changes in the d*-state features of the alloys that are consistent with the coupling of valence band d-states of Ti- and Hf-atoms through bonding a common O-atom. This mixing has confirmed through complementary changes in the valence band electronic structure as deduced from ultra-violet photoelectron spectroscopy (UPS). Ab initio quantum chemistry molecular orbital calculations indicate that d-state coupling is maximized when the respective d-states of the Ti- and Hf-atoms have the same symmetry, and the Ti-O-Hf bonding group is collinear. This results in two coupled modes. The O K@sub 1@ edge spectrum of a HfTiO@sub 4@ film (x = 0.5), crystallized by annealing at 800°C, shows three d*-features, indicating that the coupling of Hf 5d*- and Ti 3d*-states is not optimized. Non-optimized coupling of d*-states associated with a dynamic Jahn-Teller effect has also been found in the complex lanthanide rare earth/transition metal oxides, Gd(Dy)ScO@sub 3@. Non-optimized d-state coupling is also evident in band edge spectra. The relationships between d-state coupling and band gap engineering of high-k dielectrics will be discussed.