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

Paper DI-MoA7
Spectroscopic Studies of Valence Band States in HfO2, TiO2, ZrO2 and HfTiO4 by Soft X-Ray and Vacuum Ultra-Violet Photoemission Spectroscopy

Monday, November 15, 2004, 4:00 pm, Room 304B

Session: High-k Dielectrics: Electronic Properties
Presenter: C.C. Fulton, North Carolina State University
Authors: C.C. Fulton, North Carolina State University
G. Lucovsky, North Carolina State University
Correspondent: Click to Email
Valence bands of HfOsub@2@, TiO@sub 2@, ZrO@sub 2@ and HfTiO@sub 4@ films have been studied by soft x-ray photoemission spectroscopy (SXPS) at energies between 70 and 170 eV, and by He I and II ultra-violet photoemission spectroscopy (UPS) at energies of 21.2 and 40.8 eV. For HfO@sub 2@, SXPS studies reveal two valence bands as well as the Hf 4f doublet, whereas He I UPS studies have probed only the uppermost valence band. SXPS and UPS energies are referred to an extrapolated threshold energy at 8.1±0.2 below vacuum, or ~3 eV below the valence band edge of c-Si. The first SXPS valence band has features at 2.2±0.2 eV and 6.2±0.2 eV. The 4f doublet components are at 13.7±0.2 eV and 15.4±0.2 eV. The second valence band is centered at ~22 eV. The UPS valence band displays features at 1.5±0.2 eV and 5.2±0.2 eV. Energies of the symmetry determined valence band molecular orbitals have been obtained from ab intio calculations on relatively small clusters. The top of the valence band is assigned to O 2p @pi@ non-bonding states. With this as a reference, calculated MOs are: Hf 5d @pi@ + O 2p @pi@ at 0.5±0.2 eV and Hf 5d @sigma@ + O 2p @sigma@ at 5.4±0.2 eV, with an energy difference of 4.9±0.3 eV, and O 2s at 22.2±0.2 eV. Setting the valence band edge to the O 2p p non-bonding energy, and averaging SXPS and UPS spectra, the experimental MOs in the first valence band are at 1.8±0.2 eV for Hf 5d @pi@ + O 2p @pi@, and 5.7± eV for Hf 5d @sigma@ + O 2p @sigma@, with an energy difference of 3.9±0.3 eV. The O 2p @pi@ non-bonding and Hf 5d @pi@ + O 2p @pi@ bands overlap and contribute to a broad spectral feature, and this resulting agreement between theory and experiment is good. The average energy of the O 2s band is 22±0.3 eV, and is in excellent agreement with the calculated energy of 22.2 eV. The paper will also include SXPS and UPS valence band studies for TiO@sub 2@, ZrO@sub 2@ and HfTiO@sub 4@.