No abstract

Electronic excitation by lasers or electron beams can modify the properties of materials. The changes are not just due to heat, nor do they result from the well-known collision dynamics of much radiation damage. Everyday examples of modi®cation by electronic excitation include photography, and photochromics (such as sunglasses) which change colour. In the last few years it has become clear that excitation can offer novel types of modi®cation, with better-controlled changes. The ®eld has evolved through a mix of basic science, of new laser and electron beam tools, and of new needs from microelectronics, photonics and nanotechnology. Underlying this development are some common themes which integrate the basic science and its applications. These include especially the ideas of energy localisation and charge localisation. There are detailed comparisons of experiment and theory for halides, but there is a wealth of information for other materials. From this, we identify ways to connect understanding to technological needs, like selective removal of material, controlled changes, altering the balance between process steps, and possibilities of quantum control. The ®eld is reviewed in full in our recent book [N. Itoh, A.M. Stoneham, Materials Modi®cation

We have investigated visible photoluminescence excited by Ar-ion laser (488 nm, 2.54 eV) at room temperature from Si+-implanted silica glass, as-implanted and after subsequent annealing in vacuum. We found two visible luminescence bands: one peaked around 2.0 eV, observed in as-implanted specimens and annealed completely after heating to about 600 °C, the other peaked around 1.7 eV observed only after heating to about 1100 °C, the temperature at which Si segregates from SiOx. It was found that the 2.0 eV band anneals parallel to the E′ centers, as detected by electron spin resonance studies. It was also found that Raman lines around 520 cm−1, due to Si—Si bonds, grow and that interference patterns are induced by annealing Si+-implanted silica glass. Based on these studies, we ascribe the 2.0 eV band to the electron-hole recombination in Si-rich SiO2 and the 1.7 eV band to the electron-hole recombination in the interface between the Si nanocrystal and the SiO2 formed by segregation of crystalline Si from SiOx.

Swift heavy ions cause material modification along their tracks, changes primarily due to their very dense electronic excitation. The available data for threshold stopping powers indicate two main classes of materials. Group I, with threshold stopping powers above about 10 keV nm(-1), includes some metals, crystalline semiconductors and a few insulators. Group II, with lower thresholds, comprises many insulators, amorphous materials and high T(c) oxide superconductors. We show that the systematic differences in behaviour result from different coupling of the dense excited electrons, holes and excitons to atomic (ionic) motions, and the consequent lattice relaxation. The coupling strength of excitons and charge carriers with the lattice is crucial. For group II, the mechanism appears to be the self-trapped exciton model of Itoh and Stoneham (1998 Nucl. Instrum. Methods Phys. Res. B 146 362): the local structural changes occur roughly when the exciton concentration exceeds the number of lattice sites. In materials of group I, excitons are not self-trapped and structural change requires excitation of a substantial fraction of bonding electrons, which induces spontaneous lattice expansion within a few hundred femtoseconds, as recently observed by laser-induced time-resolved x-ray diffraction of semiconductors. Our analysis addresses a number of experimental results, such as track morphology, the efficiency of track registration and the ratios of the threshold stopping power of various materials.

Electronic excitation by lasers or electron beams can modify the properties of materials. The changes are not just due to heat, nor do they result from the well-known collision dynamics of much radiation damage. Everyday examples of modi®cation by electronic excitation include photography, and photochromics (such as sunglasses) which change colour. In the last few years it has become clear that excitation can offer novel types of modi®cation, with better-controlled changes. The ®eld has evolved through a mix of basic science, of new laser and electron beam tools, and of new needs from microelectronics, photonics and nanotechnology. Underlying this development are some common themes which integrate the basic science and its applications. These include especially the ideas of energy localisation and charge localisation. There are detailed comparisons of experiment and theory for halides, but there is a wealth of information for other materials. From this, we identify ways to connect understanding to technological needs, like selective removal of material, controlled changes, altering the balance between process steps, and possibilities of quantum control. The ®eld is reviewed in full in our recent book [N. Itoh, A.M. Stoneham, Materials Modi®cation