THE NATURES OF POINT DEFECTS IN AMORPHOUS SILICON DIOXIDE

摘要

Amorphous silicon dioxide (a-SiO_2) plays a central role in many of today's technologies, including fiber optics for communications and satellite data bus applications, as the gate and field oxides in 90% of all contemporary metal-oxide-semiconductor (MOS) devices (e.g., computer chips), as windows, photomasks, and transmissive optics for ultraviolet-laser microchip lithography, and as thin films for highly reflective (or highly transmissive) coatings for laser optics. Point defects in a-SiO_2 introduced in the manufacturing process or induced by ionizing or particle irradiations (including ultraviolet photons) can degrade the otherwise excellent properties of this material, potentially leading to device failure in the field. In principal, a single fundamental defect type, or class of defects, may at the same time cause massive attenuation in optical fibers and lead to equally fatal threshold voltage shifts in MOS transistors. It is easy to imagine how improvements in identification and control of these defects could result in billions of dollars in cost savings to photonics and semiconductor industries over the next decade. In the various applications mentioned, point defects in a-SiO_2 may manifest their presence, e.g., by exhibiting luminescence and/or optical absorption bands or they may be detected electrically as trapped charges. As described in other papers in this volume, carefully obtained optical spectra can serve as a partial basis for formulating structural models for the defects which give rise to them. These models, usually presented as specific local atomic and/or electronic arrangements differing from those of the bulk, are built upon all available information, i.e., all forms of spectroscopic and "engineering" data. The most reliable models knit together the best available experimental data from all sources with solid-state-physics theory, often aided by high quality ab initio calculations. The results are unified constructs which readily facilitate predictions of the wider consequences of engineering changes in material fabrication or device processing. Only at their peril may manufacturers of electronic or photonic devices ignore the grand body of accumulated knowledge concerning the natures of point defects in a-SiO_2. The most detailed atomic-scale models for point defects in insulating materials generally derive from probing them by electron spin resonance (ESR) techniques. The (equally important) results of other spectroscopic and engineering measurements are tied to these models by a combination of experimentally established correlations and physical theory. The present paper reviews the application of continuous wave (cw) ESR methods to glassy materials and overviews what has been learned thereby of the natures of point defects in a-SiO_2. The bases of the current models for the E' centers, the nonbridging-oxygen hole centers (NBOHCs), and the peroxy radicals (PORs), as well as for a few hydrogen-related defects, will be recounted. However, the longest section will be devoted to the self-trapped holes (STHs), which are perhaps the most fundamental defects in a-SiO_2 but to date remain the least well understood.