Stress-induced defect processes

Sometimes crystallographers consider that measuring a crystal at very low temperature is a kind of panacea, able to solve all defects of the sample, all kinds of experimental errors, and enhance the response indefinitely. Young students might be disappointed to leam that these miracles do not take place. A bad crystal sample remains as such even at 10 K, and sometimes it becomes even worse because the cooling process and the residual stress induced by a temperature gradient may produce further damage to the sample. Many other kinds of experimental problems and sources of error (for example absorption, extinctions, disorder, etc.) are not attenuated by the low temperature. 

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

A correlation between the total momentum of impinging ions per deposited boron atom and the c-BN deposition has been observed. In this model, c-BN formation is correlated with the momentum-drive process, such as the formation of point defects in conjunction with the stress-induced phase transformation. 

It is interesting to note that not all defects in product crystals arise from the crystallization process itself. Thermal stresses induced in crystals are sometimes greater during the cooling and drying than during the crystallization period, and can be so large as to crack the crystalline material. These thermal stresses, which can produce defect concentrations on the order of 10 -10 /cm, ... 

Etching at nanopores becomes a three-dimensional process whereas the non- or less-defected oxides etch predominantly in a two-dimensional process that is much slower. In Figure 2.58b, the successive oxidation process is indicated by the areas located between the oxide 1 islands where the arrows symbolize the stress forces. It is known from fractal photocorrosion of Si, for example, that oxidation and dissolution are prevalent at sites with stress-induced distortion of bonds [235]. 

A periodic arrangement of many epitaxially grown thin layers with lattice mismatch constitutes a strained-layer superlattice. An example of such a superlattice structure can be found in the vertical-cavity surface-emitting laser (VCSEL). As discussed by Choquette (2002) and Nurmikko and Han (2002), the control of layer thickness, elastic strain due to LAN to us mismatch, stress-driven crack formation and processing induced defects in the superlattice presents major scientific and technological challenges in the development of these devices. 

Despite such limitations, plasma-deposited a-C(N) H films were found to be used in a number of applications. The stress reduction induced by nitrogen incorporation [12] and consequent adhesion improvement, allowed the development of a-C(N) H antireflective coatings for Ge-based infrared detectors [13]. It was also found that N can electronically dope a-C H films, and can strongly decrease the defect density, which gives prospects on its use as a semiconductor material [14]. Nitrogen incorporation was also found to decrease the threshold electric field in electron-field emission process [15], making possible the use of a-C(N) H films as an overcoat on emission tips in flat-panel display devices [16]. 

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