Hua Xia

Phonon mode study of Si nanocrystals using micro‐Raman spectroscopy

First‐order Raman spectra of hydrogenated nanocrystalline silicon (nc:Si:H) films show unexpected features in their optical vibrational modes for crystallites with sizes ranging from 2 to 6 nm. Two size‐dependent spectral regions, one with the stronger intensity peaking at 505–509 cm−1 and another a shoulder‐like band between 512 and 517 cm−1, are clearly identified using a detailed line‐shape analysis and the strong phonon confinement model. The strong size dependence of the relative integrated intensities of the two bands suggests that the modification of the vibrational spectra can be attributed to an effect induced by the atomic vibrations from the near‐surface region of the nanocrystals.

RAMAN-SCATTERING STUDY OF THE LONGITUDINAL OPTICAL PHONONS IN ZNSE-ZNS STRAINED-LAYER SUPERLATTICES

Raman scattering studies were performed on ZnSe–ZnS strained‐layer superlattices with different strains. In the optical phonon regime, the Raman spectra of the ZnSe–ZnS superlattices with a superlattice axis along [001] show optical phonons confined in the ZnSe well and ZnS barrier layers, and display a two‐mode behavior corresponding to ZnSe‐like and ZnS‐like modes. We report the experimental results, by means of Raman scattering of confined longitudinal optical phonons in ZnSe–ZnS strained‐layer superlattices. The Raman frequency shift dependence on the layer thickness, superlattice period, and interface strain is presented and discussed.

Thermal analysis of the ferroelastic phase transitions of LaxPr1−xP5O14

The specific heat capacities of LaxPr1-xP5O14 crystals with x=0.9 and 0.7 were measured with a differential scanning calorimeter over the temperature range 350-450 K. A jump in the specific heat capacity at the phase transition temperature of 402 K and one thermal anomaly at about 385 K were found within experimental error. Factors contributing to the specific heat capacities were considered.

Elastic properties of amorphous hydrogenated silicon films

Abstract A Brillouin scattering study of elastic properties of a -Si: H film deposited onto a glass substrate is presented. From observed sound velocity of surface (Rayleigh) waves we have extracted the shear elastic constant C 44 to be (5.64 ± 0.05) × 10 11 day cm −2 . Due to the existence of the hydrogen atoms, the sound velocity of the surface (Rayleigh) waves is found to be 18% greater than the value of the amorphous silicon film.

ENHANCEMENT OF THE SHEAR ELASTIC CONSTANT IN PERIODIC AND QUASIPERIODIC TA/AL SUPERLATTICES

The Brillouin light‐scattering technique has been exploited in order to reveal surface acoustic phonons in both periodic and Fibonacci quasiperiodic Ta/Al superlattices. In periodic specimens, about 0.5 μm thick, it is found that, as the superlattice period is reduced from 10 to 4 nm, the phase velocity of the Rayleigh acoustic mode exhibits an anomalous increase, corresponding to a 20% enhancement of the effective shear elastic constant c44. In 2k‐component and 3k‐component Fibonacci quasiperiodic superlattices, with total thickness of about 1.5 μm, an enhancement of c44 similar to that of periodic specimens has been evidenced, which is connected to the average density of the interfaces, rather than to the quasiperiodicity. This enhancement can be attributed to the presence of interfaces extended over a number of atomic planes, due to the mutual interdiffusion between Ta and Al, as inferred by x‐ray and electron microscopy experiments.

STUDY OF THE RAYLEIGH ACOUSTIC MODE IN A-SI:H/A-SINX:H SUPERLATTICES

The Brillouin light‐scattering technique has been exploited for studying the Rayleigh acoustic mode in superlattice films consisting of alternate layers of a‐Si:H and a‐SiNx:H. The phase velocity of the Rayleigh mode has been measured on superlattices with different thicknesses of the a‐Si:H layer. The results obtained have been interpreted in terms of the effective medium model within the elastic continuum approximation; no anomalous behavior is observed, in contrast with previous ultrasonic measurements in superlattice with a minor nitrogen content.

BRILLOUIN-SCATTERING FROM SURFACE ACOUSTIC PHONONS IN A ZNSE-ZNSE1-XSX STRAINED-LAYER SUPERLATTICE

Brillouin scattering studies were performed on ZnSe‐ZnSe1−xSx strained‐layer superlattices with the composition x∼0.2. We have observed scattering from longitudinal guided modes (LGM) whose velocities are determined primarily by the C11 elastic constant. From the measured velocity of the first‐order LGM of the quasitransverse acoustic mode and of the quasilongitudinal acoustic mode, the C11 and C44 elastic constants in ZnSe‐ZnSe1−xSx superlattice film have been observed.

Raman scattering by acoustic phonons in Fibonacci Nb-Cu metallic superlattices

Abstract Raman scattering experiments have been performed on NbCu Fibonacci metallic superlattices. The Raman spectra are explained in terms of zone-folding effects and their frequencies are predicted by an elastic continuum model. The scattering intensities of folded doublets, however, are not accounted for by the standard photoelastic model.

Brillouin Light Scattering Investigation of the Elastic Properties of Ta/Al Metallic Superlattices

The Brillouin light scattering technique has been exploited for investigating the elastic properties of periodic superlattices made by alternating layers of Ta and Al. These multilayers, deposited by d.c. sputtering on glass and Si substrates, present a polycrystalline structure with (110) and (111) texture for the Ta and Al layers, respectively. They have total thicknesses of about 0.5 μm and periods ranging from 4 to 10 nm. Measurement of the phase velocities of the Rayleigh and Sezawa acoustic modes from the frequency position of the corresponding Brillouin peaks, yielded informaton on the effective elastic constants of the superlattices. for large periods (8-10 nm) the values determined experimentally are in good agreement with those calculated from the elastic constants of the bulk materials, while for lower periods (4-6 nm) the estimated elastic constants exhibit a marked increase. This anomalous behavior has been attributed to the presence of a transition layer at each interface, where Ta and Al interdiffuse, as observed by x-ray and electron microscopy experiments.

Optical observations of the birefringence and the domains of LaxPr1-xP5O14 near the ferroelastic phase transitions

The changes in the optical properties of five samples of LaxPr1-xP5O14 with different La contents were measured for spontaneous birefringence and rotation angle of the ellipsoid indicatrix axes induced by the ferroelastic phase transitions. Two types of domain and their movement and reproduction were observed in a temperature range from Tc to Tc-100 degrees C. The transition temperature is found to vary linearly with La content. The experimental data are explained and fitted to an approximation considering high-order terms relating to the order parameter.