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Conduction band-edge States associated with the removal of d-state degeneracies by the Jahn-Teller effect

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15 Author(s)
G. Lucovsky ; Dept. of Phys., North Carolina State Univ., Raleigh, NC, USA ; C. C. Fulton ; Y. Zhang ; Y. Zou
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X-ray absorption spectroscopy (XAS) is used to study band edge electronic structure of high-κ transition metal (TM) and trivalent lanthanide rare earth (RE) oxide gate dielectrics. The lowest conduction band d*-states in TiO2, ZrO2 and HfO2 are correlated with: 1) features in the O K1 edge, and 2) transitions from occupied Ti 2p, Zr 3p and Hf 4p states to empty Ti 3d-, Zr 4d-, and Hf 5d-states, respectively. The relative energies of d-state features indicate that the respective optical bandgaps, Eopt (or equivalently, Eg), and conduction band offset energy with respect to Si, EB, scale monotonically with the d-state energies of the TM/RE atoms. The multiplicity of d-state features in the Ti L2,3 spectrum of TiO2, and in the derivative of the O K1 spectra for ZrO2 and HfO2 indicate a removal of d-state degeneracies that results from a static Jahn-Teller effect in these nanocrystalline thin film oxides. Similar removals of d-state degeneracies are demonstrated for complex TM/RE oxides including Zr and Hf titanates, and La, Gd and Dy scandates. Analysis of XAS and band edge spectra indicate an additional band edge state that is assigned Jahn-Teller distortions at internal grain boundaries. These band edges defect states are electronically active in photoconductivity (PC), internal photoemission (IPE), and act as bulk traps in metal oxide semiconductor (MOS) devices, contributing to asymmetries in tunneling and Frenkel-Poole transport that have important consequences for performance and reliability in advanced Si devices.

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IEEE Transactions on Device and Materials Reliability  (Volume:5 ,  Issue: 1 )