Introduction in Raman Spectroscopy
Optical methods provide unique access to intrinsic and extrinsic material properties.
Linear techniques like reflectivity, transmission, ellipsometry or the different kinds of modulation spectroscopy allow one to study the electronic structure of solids via its influence on the optical constants. Changes of the electronic band structure induced by external parameters can also be investigated. Although modern instrumentation covers a wide spectral range, the visible region with laser sources readily available is most interesting for most semiconductor studies. Low-energy elementary excitations, such as phonons, may also have an influence on the dielectric function and can be studied using infrared radiation. In inelastic light scattering experiments, i. e., Raman spectroscopy, elementary excitations of electronic, lattice dynamical or magnetic origin can be probed directly. Very generally, Raman scattering occurs due to the modulation of the electronic structure by an elementary excitation, e. g., the strain field associated with a phonon periodically changes a semiconductor band gap. Such variations cause mixing processes with electronic states created by absorption of laser photons and are responsible for Raman sidebands. Scattered electronic states recombine and photons with different frequencies can be detected. Their energy loss or gain with respect to the exciting laser corresponds to the elementary excitation energy.
Raman spectra show characteristic selection rules for different scattering geometries using polarized light. They reflect symmetry properties of elementary excitations and the electronic structure. The connection between microscopic models of the effect, based on electron-photon and electron-elementary excitation interaction, and a macroscopic picture of Raman scattering, derived from group theory, is made in the Raman tensor. Relative scattering intensities and selection rules are obtained by contracting this second-rank tensor with the polarization vectors of incident and scattered photons. The tensor elements can be calculated microscopically. Choosing suitable scattering geometries, the wave vector difference between incident and scattered photons in a Raman process is transferred to the elementary excitation and its dispersion can be determined.
Raman intensities may vary with laser energy. In semiconductors resonance enhancement of Raman intensities is observed around critical points of the electronic structure. Resonances of different elementary excitations at different energies allow a detailed study of their coupling to the electronic states and their composition of various atomic orbitals. Due to resonances, Raman scattering can oftentimes be observed from excitations which are otherwise too weak to study.
The small energies of most elementary excitations measured by Raman spectroscopy compared to visible laser frequencies as well as small scattering cross sections require spectrometers with superb stray light rejection, resolution and detection efficiency. Double grating monochromators provide excellent resolution. However, spectra are recorded step by step using photomultipliers and single-photon counting techniques. Multichannel systems usually consist of a subtractive double monochromator operating as a bandpass filter and a dispersive third stage. A wide spectral range is imaged onto CCD or other position-sensitive detectors. This parallel detection scheme is very efficient. Indeed, much recent progress in solid state physics to which Raman spectroscopy has contributed would not have been made without them.
contributed by Tobias Ruf