T. Yagi

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.

Structural and vibrational properties of GaN

Structural and vibrational properties of device quality pure GaN substrate grown using a lateral epitaxial overgrowth (LEO) technique were studied using x-ray diffraction, Brillouin, Raman, and infrared spectroscopy. Lattice constants were found to be a=3.1896±0.0002 A and c=5.1855±0.0002 A. Comparing the results with those on GaN epilayer directly grown on sapphire substrate, it is shown that the GaN substrate is indeed of high quality, i.e., the lattice is relaxed. However the GaN substrate has a small enough but finite residual strain arising from the pileup of the lateral growth front on SiO2 masks in the course of LEO. It was also found that the elastic stiffness constants C13 and C44, are more sensitive to the residual strain than the optical phonon frequencies. The high frequency and static dielectric constants were found to be 5.14 and 9.04. The Born and Callen effective charges were found to be 2.56 and 0.50.

Brillouin scattering study of bulk GaN

High-resolution Brillouin scattering experiments have been performed for a high-quality free-standing gallium nitride (GaN) substrate. Elastic stiffness constants are reported. A comparison is made with the results of an earlier study for a GaN thin film on sapphire substrate.

Brillouin scattering study of ZnO

Polarized Brillouin scattering measurements were carried out for a bulk ZnO single crystal. The whole set of elastic stiffness constants was determined to be c11=19.0, c12=11.0, c13=9.0, c33=19.6, c44=3.9, and c66=4.0 in units of 1011 dyn/cm2. The relationships between photoelastic constants at wavelength of 514.5 nm were also obtained: p11, p13, p44, and p66=1.8, 2.3, 0.50, and 0.38, respectively, relative to p12.

Effect of oxygen deficiency on the normal and superconducting properties of CaLaBaCu3Oy

Abstract Measurements of superconducting transition temperature T c and lattice parameters of CaLaBaCu 3 O y (CLBCO) indicate that CLBCO is a tetragonal bulk superconductor in the oxygen-content range from y =6.7 to 7. The T c decreases monotonously with decreasing oxygen content and no superconducting transition is observed below y =6.7. It is concluded that disappearance of the superconductivity for y y

Photoinduced Phenomena in Quantum Paraelectric Oxides by Ultraviolet Laser Irradiation

A photo-induced effect in the quantum dielectric system, such as SrTiO3, KTaO3, and CaTiO3, has been investigated under rather small external DC electric field. The dielectric measurement was performed under ultraviolet (UV) laser excitation combined with DC electric field. The gigantic photo-induced effects of dielectric properties of the quantum paraelectric systems have been firstly observed in the perovskite-type materials. The photoinduced dielectric effect was obtained as a function of DC electric field under constant UV irradiation.

Effects of high pressure on the Bose peak in a-GeS2 studied by light scattering

Abstract High-resolution Brillouin and Raman scattering measurements have been carried out for amorphous germanium disulfide (a-GeS 2 ) under high pressures (up to 2.5 GPa) using diamond anvil cell (DAC). The pressure dependence of the Bose peak is discussed in comparison with that of the longitudinal acoustic (LA) mode. Third-order elastic stiffness constants for these modes are obtained, which show a large anharmonicity with respect to the Bose peak modes.

Brillouin and Raman scattering study of the isotopically induced ferroelectric phase transition of SrTi18O3

The ferroelectric phase transition of the isotope 18 O exchanged strontium titanate SrTi 18 O 3 has been studied by both Brillouin- and Raman scattering near T m where the dielectric constant takes a maximum value. The Brillouin scattering study elucidates anomalous temperature dependence of the acoustic modes; the c 44 mode shows a drastic softening though the c 33 mode is almost independent of temperature. The Raman scattering study below T m shows a weak softening behavior of the ferroelectric mode. Existence of the ferroelectric micro-region is suggested in the isotopically induced ferroelectricity of SrTi 18 O 3 .

A single exponential time decay of the ferroelectric soft B2 mode of KDP studied by impulsive stimulated Raman scattering

Abstract The ferroelectric soft modes of KDP (KH2PO4) and the 90% deuterated compound (K(H0.1D0.9)2PO4) have been excited coherently by impulsive stimulated Raman scattering. The observed time dependence of the excited modes shows a complete single exponential time decay after an electronic response at its time origin. The temperature dependence of the relaxation time τ of the modes shows clearly a critical slowing down in paraelectric phase, τ = τ 0 T 0 (T−T 0 ) , where T0 is the phase transition point of the clamped crystal. The isotope effect on τ is well confirmed as τ0 is 0.11×10−12s for KDP and τ0 is 0.82×10−12s for K(H0.1D0.9)2PO4.

Successive phase transitions of hexagonal barium titanate: soft acoustic mode and coupled acoustic-soft optic mode

Abstract The successive phase transitions at T 0 (222 K) and T C (74 K) of hexagonal BaTiO 3 have been studied by Brillouin scattering. An intense central peak was found near T 0 and was identified as a result of scattering from the coupled soft optic and acoustic mode. The coupled mode model was employed in order to explain the observed spectral changes. The ω 0 2 Γ 0 ratio of the soft optic mode frequency ω 0 and the damping constant Γ 0 , and the square of bilinear coupling constant A show linear dependence on temperature difference T − T 0 , indicating the mean field behavior. The clear softening of c 66 shear acoustic mode was found near T C . The square of frequency of c 66 mode depends linearly on T − T C around the transition temperature. The results conclusively show that the phase transition at T C is the proper ferroelastic transition with the order parameter strain s 6 .