(Color online) O K 1-edge XAS spectra of 2 nm thick HfO2 films grown on GaAs (100) substrates as a function of post annealing temperatures. MO structures were assigned by considering seven-fold oxygen coordination on the basis of cubic symmetry. Second derivative spectra of O K 1 peak edge was used to determine the energy level of the MO states.
(Color online) (a) Fitted XAS O K 1-edge spectra for HfO2/GaAs after PDA at 700 °C. (b) XAS O K 1-edge spectra over a narrow energy region below the Hf 5d eg conduction band edge states for HfO2/GaAs as a function of PDA temperature. The two distinct defect states were located at about 1.6 ± 0.2 eV (D1) and 3.0 ± 0.2 eV (D2) below the conduction band edge of Hf 5d. (c) HRTEM images show the formation of epitaxial HfO2 films on GaAs through an annealing process. The different thickness of HfO2 with the annealing process is due to atomic transport with respect to Ga and As.
(Color online) XPS core-level spectra of Hf 4f/Ga 3d and As 3d in HfO2/GaAs as a function of PDA temperature. In the Hf 4f/Ga 3d spectrum for PDA at 700 °C, the Ga 3d spectra are deconvoluted with three bonding states; the Ga0, Ga1+, Ga3+, and Ga* bonding states correspond to GaAs substrate, Ga2O, Ga2O3, and GaAsO4, respectively. As 3d spectra are deconvoluted with As0, As*, As1+, As3+, and As5+ bonding states corresponding to GaAs substrate, elemental As, AsOx, As2O3, and As2O5, respectively. The intensity changes in excess elemental As (As0) were correlated with the thermal reaction of As–O bonding states. In order to be consistent in fitting results of As 3d bonding states, Gaussian-Lorentzian contributions of Hf 5p states are fixed for all samples due to overlapping of As 3d and Hf 5p states.
(Color online) Calculated defect formation energies of Ga or As impurities in HfO2 were plotted as a function of Fermi level with oxygen-deficient conditions: (a) Formation energy vs Fermi level for Ga or As interstitial in m-HfO2 and Ga or As substitution at O4 sites and (b) their total density of states for Ga- and As-related defects.
(Color online) The local density of states for Ga- and As-related defects corresponding to the most stable charge states.
The structural parameters obtained from the m-HfO2 supercell are a = 5.101 Å, b = 5.1.95 Å, c = 5.330 Å, and β = 99.69°, which are in good agreement not only with previous GGA calculations of DFT, but also with experimental results.
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