We have performed a first-principles study of the structural and vibrational properties of the three low-pressure (cubic, tetragonal, and especially monoclinic) phases of ZrO2, with special attention to the computation of the zone-center phonon modes and related dielectric properties. The calculations have been carried out within the local-density approximation using ultrasoft pseudopotentials and a plane-wave basis. The fully relaxed structural parameters are found to be in excellent agreement with experimental data and with previous theoretical work. The total-energy calculations correctly reproduce the energetics of the ZrO2 phases, and the calculated zone-center phonon frequencies yield good agreement with the infrared and Raman experimental frequencies in the monoclinic phase. The Born effective charge tensors are computed and, together with the mode eigenvectors, used to decompose the lattice dielectric susceptibility tensor into contributions arising from individual infrared-active phonon modes. This work has been partially motivated by the potential for ZrO2 to replace SiO2 as the gate-dielectric material in modern integrated-circuit technology.

We investigate the structural, electronic, and optical properties of hydrogen-passivated silicon nanowires along [110] and [111] directions with diameter d up to 4.2 nm from first principles. The size and orientation dependence of the band gap is investigated and the local-density gap is corrected with the GW approximation. Quantum confinement becomes significant for d<2.2 nm, where the dielectric function exhibits strong anisotropy and new low-energy absorption peaks start to appear in the imaginary part of the dielectric function for polarization along the wire axis.

Crystalline structures, zone-center phonon modes, and the related dielectric response of the three low-pressure phases of HfO2 have been investigated in density-functional theory using ultrasoft pseudopotentials and a plane-wave basis. The structures of low-pressure HfO2 polymorphs are carefully studied with both the local-density approximation (LDA) and the generalized gradient approximation (GGA). The fully relaxed structures obtained with either exchange-correlation scheme agree reasonably well with experiment, although LDA yields better overall agreement. After calculating the Born effective charge tensors and the force-constant matrices by finite-difference methods, the lattice dielectric susceptibility tensors for the three HfO2 phases are computed by decomposing the tensors into the contributions from individual infrared-active phonon modes.PACS numbers: 77.84.Bw Hafnia (HfO 2 ) is technologically important because of its extraordinary high bulk modulus, high melting point, and high chemical stability, as well as its high neutron absorption cross section. HfO 2 resembles its twin oxide, zirconia (ZrO 2 ), in many physical and chemical properties. The resemblance is attributable to the structural similarity between the two oxides, which can in turn be explained by the chemical similarity of Hf and Zr, which have similar atomic and ionic radii (i.e., ionic radii for Hf 4+ and Zr 4+ of 0.78 and 0.79Å, respectively [1]) as a result of the so-called lanthanide contraction. Under ambient pressure, both oxides are monoclinic (m, space group P 2 1 /c) at low temperature, and transform to a tetragonal structure (t, space group P 4 2 /nmc) and then to a cubic structure (c, space group F m3m) as the temperature increases, as illustrated in Fig. 1.High-K metal-oxide dielectrics have recently been the focus of substantial ongoing efforts directed toward finding a replacement for SiO 2 as the gate dielectric in complementary metal-oxide-semiconductor (CMOS) devices. HfO 2 , ZrO 2 and their SiO 2 mixtures show promise for this purpose [2,3]. Thus, a systematic theoretical investigation of the structural and dielectric properties of these dielectrics, in both bulk and thin-film form, is clearly desirable. As a first step in this direction, we have, in a previous paper [4], investigated the bulk structures and lattice dielectric response of ZrO 2 polymorphs. We found that the dielectric responses vary dramatically with the crystal phase. Specifically, we found that the monoclinic phase has a strongly anisotropic lattice dielectric tensor and a rather small orientationally-averaged dielectric constant owing to the fact that the mode effective charges associated with the softest modes are relatively weak.This report presents the corresponding work on HfO 2

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