Applications of Laser Spectroscopy to Materials Science

For the production of materials for electronic circuits, such as chips, the demands regarding purity of material, its composition and the quality of the production processes become more and more stringent. With decreasing size of the chips and with increasing complexity of the electronic-circuits, measurements of the absolute concentration of impurities and dopants becomes important. The following two examples illustrate how laser spectroscopy can be successfully applied to the solution of problems in this field. 

Irradiating the surface of a solid with a laser, material can be ablated in a controlled way by optimizing intensity and pulse duration of the laser 

Whereas laser ablation of graphite yields thermalized C2 molecules, with a rotational-vibrational energy distribution following a Boltzmann distribution at the temperature T of the solid, ablation of electrical isolators, such as AlO, produces AlO molecules with a large kinetic energy of 1 eV but a rotational temperature of only 500 K [15.89]. 

For the production of thin amorphous silicon layers (e.g., for solar photovoltaic energy converters) often the condensation of gaseous silane (SiH ) or Si2Hg, which is formed in a gas discharge, is utilized. During the formation of Si(H) layers the radical SiH2 plays an important role, which 

Of particular interest for in situ determinations of the composition of alloys is the technique of laser micro-spectral analysis [15.91] where a micro-spot of the material surface is evaporated by a laser pulse and the fluorescence spectrum of the evaporated plume serves as monitor for the composition. 

Resonant two-photon ionization in combination with a time-of-flight mass spectrometer gives the mass spectrum. In many cases, one observes a broad mass range of clusters. The question is whether these clusters were emitted from the solid or whether they were formed by collisions in the evaporated cloud just after emission. Measurements of the vibrational-energy distributions can answer this question. If the mean vibrational energy is much higher than the temperature of the solid, the molecules were formed in the gas phase, where an insufficient number of collisions 

Surface science is a rapidly developing field that has gained a lot from applications of laser spectroscopy [1499, 1500]. The sensitive technique of surface-enhanced Raman spectroscopy, which gives information of molecules adsorbed on surfaces, was discussed in Sect. 3.4.2. 

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