Katsuo Suzuki

First-principles study on electronic and elastic properties of BN, AlN, and GaN

We have carried out first-principles total energy calculations to investigate electronic and elastic properties of both zinc-blende and wurtzite BN, AlN, and GaN. We have calculated lattice parameters, elastic constants, deformation potential constants, phonon frequencies at Γ point, Born effective charges, and piezoelectric constants. Lattice parameters are fully relaxed by using the first-principles molecular dynamics method with variable cell shape. The internal strain in a strained crystal is also relaxed by the first-principles molecular dynamics method. The internal strain influences the elastic constants, the deformation potential constants, and the piezoelectric constants effectively. We have calculated the wurtzite deformation potential constants D1–D5 considering the internal strain correction. The piezoelectric constants of wurtzite and also zinc-blende crystals have been calculated using the Berry phase approach and we have found from first principles that those of BN have an inverse sign in c...

Polarized Raman spectra in GaN

We have measured polarized Raman spectra in a 2.0 mu m GaN epitaxial layer of high quality, grown on a sapphire substrate. All symmetry-allowed optical phonons in GaN have been assigned as follows: A1(LO), 735 cm-1; A1(TO), 533 cm-1; E1(LO), 743 cm-1; E1(TO), 561 cm-1; E2, 144 and 569 cm-1. Using the Lyddane-Sachs-Teller relation, the static dielectric constants of GaN for the ordinary and extraordinary directions have been estimated as epsilon perpendicular to 0=9.28 and E/sub //0/=10.1. We have also observed quasi-LO phonons in GaN. A brief discussion on these will be given.

Brillouin scattering study of gallium nitride: elastic stiffness constants

High-resolution Brillouin scattering measurements on a high-quality wurtzite gallium nitride (GaN) single crystal were carried out and elastic stiffness constants were determined. A comparison is given with the results of a recently reported model for calculation of the elastic constants of III-V semiconductors based on the modified version of Keyess relations. A good agreement is found between the experimental and theoretical elastic constants for GaN.

Brillouin scattering study in the GaN epitaxial layer

Abstract We have investigated the elastic properties of a GaN epitaxial layer on a sapphire substrate by Brillouin scattering in the backward and 90° scattering geometries. A sample of high optical quality grown by the two-flow MOCVD method with a complex structure of In 0.05 Ga 0.95 N(0.05 μm)/GaN(2 μm)/sapphire was used. The weak spectra were not simple due to the boundaries and the InGaN thin film. The elastic constants of GaN were estimated and compared with those obtained from X-ray diffraction.

Optical phonons in GaN

Abstract We report on all optically active phonon frequencies of wurtzite GaN determined using Raman, infrared reflection, and attenuated total reflection spectra of a high quality GaN film of 2 μm thickness. We also show results of lattice dynamical calculations using a rigid ion model. Force constants and charge as fitting parameters were determined so as to reproduce all zone-center phonon frequencies and sound velocities for the high symmetry lines. We present phonon dispersion curves and phonon density of states of wurtzite GaN.

Defect structures in LiNbO3

We have re-examined the infrared absorption bands due to the O-H bond-stretching vibration and the polarization characteristics in undoped and MgO-doped LiNbO3 using well-characterized crystals. It has been found that the O-H bond stretching vibrational frequency nu (OH) has a strong correlation with Nb concentration in the crystals. We have also determined the position where hydrogen enters using Novaks empirical relationship between the values of nu (OH) and the length of the hydrogen bond and the structure analysis data for the undoped crystals. On the basis of those results and the polarization characteristics, we have examined the intrinsic and the extrinsic defect structure models in LiNbO3. It has been clarified that the behaviour of nu (OH) reflects the defect structures. The behaviour of nu (OH) supports the Li-site vacancy model as the intrinsic defect structure model, and the corresponding extrinsic defect model. A brief discussion is also given of the behaviour of nu (OH) in crystals simultaneously doped with two kinds of impurity.

First-Principles Study on Piezoelectric Constants in Strained BN, AlN, and GaN

Piezoelectric constants at the states suffering volume-conserving strains have been calculated by using geometric quantum Berry phase approach for bulk BN, AlN, and GaN with wurtzite structure. Two step calculations have been carried out: for given strains the first-principles molecular dynamics method has been applied to relax the electronic ground state and the ionic position in the unit cell, after then the piezoelectric constants have been calculated by applying the other probing strains. Very large changes have been found in piezoelectric constants at the states distorted differently. This nonlinear piezoelectricity is shown to come from the structural parameters in the distorted unit cell for each strained state.

A defect structure model of

We have systematically studied the behaviour of the absorption bands due to the O - H bond-stretching vibration and its polarization characteristics in with the use of well characterized crystals. It has been found that the O - H bond-stretching vibrational frequencies, , have a strong correlation with Nb concentration in the crystals as in the case of MgO-doped . The values of shift to the higher-frequency region when the Sc concentration in the crystal exceeds about 2.5 mol%. The magnitude of the frequency shift is smaller and the polarization dependence of the absorption bands due to is weaker in -doped than in MgO-doped . These features are attributed to the difference of the charge between and . We have also proposed an ideal defect structure model for -doped , which is based on the Li-site vacancy model as an intrinsic defect structure model. The observed behaviour of is consistently explained by the proposed defect structure model. This supports the justification of the extrinsic defect structure model based on the Li-site vacancy model for -doped as well as for MgO-doped .

On the Low-Temperature Thermal Conductivity of n -Ge

The contributions of inelastic and elastic phonon scattering by neutral donors to the low-temperature thermal resistance of Ge are calculated in detail. It is shown that the inelastic and elastic scattering mechanisms make almost the same contribution to the thermal resistance in Sb-doped Ge. The sum of the both contributions gives a good agreement with experiments.

Molecular Dynamics Study of α-Ag2S

Superionic conductor α-Ag 2 S is simulated by the molecular dynamics with a simple ionic potential. The simulations are performed for two systems, one with 384 ions (128 S ions and 256 Ag ions) and the other with 1296 ions (432 S ions and 864 Ag ions). The simulated results show a characteristic feature of α-AgI type superionic conductor: anions form a bcc lattice and cations are mobile among interstitial sites. The temperature dependence of the tracer diffusion constant of Ag ions and the Debye-Wallar factor of S ions are in good agreement with experimentally observed ones. The velocity autocorrelation functions and the electrical conductivity are also obtained.