E. Anastassakis

Polycrystalline Si under strain: Elastic and lattice‐dynamical considerations

Expressions are derived for the effective phonon deformation potentials of polycrystalline materials, based on the properties of tensor invariants and the Voigt–Reuss–Hill averages [Proc. Phys. Soc. London Sect. A 65, 349 (1952)]. These results, and similar ones for the elastic constants, are used to determine the effect of strains on the long‐wavelength optical phonons exhibited by such materials. Application to polycrystalline Si shows that under a uniaxial or biaxial stress, the triply degenerate phonons split into singlet and doublet components that shift at different rates and appear indistinguishably in the Raman spectrum. The implications of such effects on the Raman band shape are discussed.

Strain characterization of polycrystalline diamond and silicon systems

The phonon deformation potentials of polycrystalline diamond and silicon, derived through the Voigt–Reuss–Hill averaging technique, are used to establish the systematics of strain characterization of polycrystalline films, considering all possible stress configurations. The results are compared to Raman data from the literature.

Strained superlattices and heterostructures: Elastic considerations

The properties of tensor invariants are used to obtain the complete component array of the strain/stress field and the elastic energy density in strained heterostructures and superlattices of cubic materials. An arbitrary direction of growth is assumed. In‐plane isotropic strains or stresses are considered independently. Normal‐to‐the‐plane uniaxial strains are also treated, in connection with pulsed laser annealing of bulk materials. The results are expressed in terms of the elastic constants and simple geometrical functions; they are readily applicable and require no extensive computation. Specific orientations of current interest are treated explicitly.

Selection rules of Raman scattering by optical phonons in strained cubic crystals

Strain characterization of materials through Raman spectroscopy often requires the optical phonon wave vector to be in directions other than those of the strain-modified phonon eigenvectors. As a result, the observed mode frequencies are mixtures of the strain-modified eigenfrequencies. The selection rules for such generalized scattering configurations are derived here for the strained zincblende-diamond family crystals. The formulation is based on the relative magnitude of the LO-TO and the strain-induced splittings. The results are important for the analysis of strain-modified Raman lineshapes and of “forbidden” lines. Specific examples are worked out in detail for externally stressed bulk crystals (90° scattering geometry), and for [001], [111], and [110] strained heterojunctions (backscattering under oblique incidence and/or detection).

Polyimide thin-film waveguides : optical and Raman spectroscopic studies

Polyimide films deposited on fused quartz, silicon, and ceramic substrates are examined. Waveguide techniques are used to determine the guide modes from which the index of refraction is inferred. Film thicknesses are determined from transmission interferences and, independently, from a luminescence‐induced reflection interference pattern. Using a waveguide configuration we also observed and analyzed Raman scattering features in the region above 1000 cm−1.

Stabilized cubic zirconia: A Raman study under uniaxial stress

Yttria‐stabilized cubic zirconia has been studied at 300 K by use of Raman spectroscopy under uniaxial stress along the [001] and [111] directions. The main effects observed are the frequency shifts of the F2g‐type Raman band at ∼610 cm−1 which vary linearly with the applied stress and tend to increase with Y2O3 concentration. Effective deformation potentials are determined for the F2g band. These potentials are necessary for strain characterization of stabilized cubic zirconia, in buffer or epitaxial film configurations. Polycrystalline yttria‐stabilized tetragonal zirconia is likewise studied.

Elastic properties of textured diamond and silicon

We have used invariance properties of fourth-rank tensors within the Voigt–Reuss–Hill averaging procedure, to calculate the elastic stiffness and compliance of cubic polycrystalline materials exhibiting texture. The computation is extended to the bulk and shear modulus as well as to Young’s modulus, volume compressibility and Poisson’s ratios. Numerical values and polar plots are given for all texture directions in the plane (110) of diamond and silicon.

Surface-related phonon mode in porous GaP

Abstract Porous GaP layers prepared by electrochemical anodization of (1 0 0) and (1 1 1) A-oriented n -GaP crystalline substrates in HF solution have been studied by Raman spectroscopy. A surface vibrational mode at 397 cm −1 was observed in porous GaP. The process of anodization results in downward shifts of the TO and LO phonon frequencies, which are attributed to phonon confinement in crystalline GaP particles of nanometer size.

Angular dispersion of ‘‘backward’’ Raman scattering: Weakly absorbing cubic materials (Si)

We have shown that the angular distribution of Raman scattering power from the surface of diamond‐type opaque materials can be adequately described by use of classical optics. It turns out that within the commonly‐used nearly backward scattering geometries there are significant variations of the scattered power due to purely geometrical factors. Only crystals with small extinction coefficients are considered. Families of curves for the scattered power are derived in terms of the angles of incidence and detection and of the index of refraction for three of the most widely‐used crystal orientations. The analysis reveals the optimum scattering configuration for a given geometrical situation and provides a quantitative measure for the deviation from the selection rules when a ‘‘backward’’ geometry for forbidden scattering is less than exact. The importance of incidence and/or detection at the Brewster angle is examined. The experimental data obtained in Si are consistent with the theoretical predictions. The ...

Effect of temperature gradients on the first‐order Raman spectrum of Si

We report the results of measurements of the first‐order Raman spectrum of Si under conditions of large temperature inhomogeneities generated by a critically focused cw laser beam. We find that the Stokes and anti‐Stokes bands are extensively deformed due to large asymmetric broadenings which develop on the side of the low‐frequency shifts. The broadenings are thus attributed to temperature gradients and are shown to be important in determining temperature distributions.