By combining a high precision translation stage, sensitive multichannel detectors, large magnification optical
microscope objectives and bright spectrometers high spatial resolution Raman
imaging becomes possible.
One understands by imaging the representation of
Raman spectral parameters over a certain area of which each pixel has an associated
spectrum; from those one can extract the Raman parameters using fitting routines;
the images are defined by three coordinates (x, y, z), where x and y are the surface
spatial coordinates, and z represents the corresponding Raman parameter which the
distribution is imaged.
The parameters of a Raman peak are the frequency
(wavenumber), intensity, polarization, and linewidth. When more than one Raman
band is present, the ratios between the different Raman bands can be also obtained.
Light ongoing onto a solid undergoes several interaction processes with the solid, as
described in previous chapters, among them light scattering, either elastic or inelastic. The Raman effect consists of the coupling of the electromagnetic field of
the incident light with the optical phonons through the induced electric dipole
moment.
The incident photons, characterized by their energy, wavevector, and
polarization, are inelastically scattered by the crystal. The inelastic component
(Raman) of the outgoing light is characterized by its corresponding intensity,
energy, wavevector, and polarization, which are determined by the structure and
nature of the solid.
The inelastic component (Raman) of the outgoing light is characterized by its corresponding intensity, energy, wavevector, and polarization, which are determined by the structure and nature of the solid.
The relation between the incident and the scattered light is governed by the corresponding Raman scattering selection rules, which are determined by the lattice symmetry and the nature of the crystal [38]. The energy
exchange between the incident light and the solid can only take the values of the phonon energies. When the lattice absorbs the energy necessary to excite a phonon
the Stokes (S) component of the spectrum is observed, whereas the release of a
phonon to the electromagnetic wave results in the anti-Stokes (AS) component of
the Raman spectrum.
Usually, Raman experiments deal with the S component,
because its intensity is several times that of the AS component. The AS-component takes a relevant role when dealing with temperature measurements. We will discuss about temperature later on, because contact-less temperature measurements are very
important for the device reliability analysis; on the other hand, the thermal aspects
are crucial in nanostructures, where the thermal conductivity, and heat dissipation are dramatically affected by the low dimensionality [39]; e.g. Si nanowires
(NWs) immersed in low heat dissipation media present evident signs of overheating in the presence of a laser beam for the acquisition of the Raman spectrum [40–42].
Strain. The bond lengths in strained materials are different from those of the
unstrained ones; therefore, strain shifts the Raman peaks; the sign of that shift
depends of the type of strain (compressive or tensile), Dx being proportional to
the strain [97]. One needs to establish the strain tensor to give accurate strain
estimations, which in the case of l-R experiments is limited by the scattering
geometry. This is a successful application of l-R spectroscopy applied to
semiconductor devices, for which an exhaustive literature is available [30–32,
58, 98–103]. Strain can influence the electronic properties of the semiconductor
structures; for example, the strain can reverse the order of heavy and hole
subbands [104], or may produce changes in the band offsets [105] of the
heterostructures.
