A change in microstructure, including dislocation Burgers vector, length, and behavior, has been observed to occur when the epilayer mismatch is varied in GexSi1−x layers grown on (100) Si. At low mismatches (<1.5%), there is an orthogonal array of very long 60° misfit dislocations. At higher mismatches (>2.3%) there is an orthogonal array of short edge dislocations. At intermediate mismatches (1.5 to 2.3%) there is a mixture of 60° and edge dislocations. The nature of the microstructure has a pronounced effect on the density of threading dislocations in the epilayer, which increase by a factor of ∼60× through a relatively small range of mismatch (1.7 to 2.1%, corresponding to x ranging from 0.4 to 0.5). These morphologies are discussed in the light of recent work on the sources of misfit dislocations. While mechanisms for the introduction and propagation of dislocations at low mismatch have recently been observed and explained, the high misfit case is clearly very different; i.e., surface nucleation seems to be likely in the latter case as opposed to operation of an internal source in the former. A mechanism for edge dislocation formation is proposed.
Flexible organic crystals enabled by cooperative phase transitions attract enormous interest in solid-state chemistry to produce light, biocompatible, and environmentally benign devices. The recently unveiled super-and ferroelastic organic semiconductor crystals provide a pathway to achieve ultraflexible single-crystal electronics. However, the mechanistic understanding of cooperative transitions in organic crystals is rather at the nascent stage, and most of such studies rely on the trial-anderror approach in molecular design. Compared to the well-studied phase transition in metallic alloys, the key challenge in understanding the organic phase transitions is the elusive crystallography involving intricate molecular dynamics and defects. Here, we leverage the phase transformation theory, genetic algorithm refined molecular modeling, and experimental validation to study the versatile cooperative transitions in bis(triisopropylsilylethynyl)-pentacene semiconductor crystals. The molecular rotation governed thermoelasticity, interconvertible super-and ferroelastic transitions, and molecular twinning are systematically studied by integrating the lattice crystallography and molecular motions. We illustrate the molecular defects of disclination dipoles and molecular stacking faults associated with the molecular twinning process. The fundamental understanding underpins the molecular mechanism of cooperative transitions in a variety of organic solids to promote a new avenue of environmentally responsive organic devices.
It has been observed that partial dislocation glide and stacking fault introduction occur in diamond cubic materials during tensile mismatched growth on (001) and compressive mismatched growth on (110) and (111). For reversed sense of mismatch, however, only full lattice dislocations are observed for strain relief. The general criteria are presented for when a partial misfit dislocation is possible as a function of growth surface orientation. It is shown that, for zero stacking fault energy, the slip regime (dislocation type) expected during (001) growth will hold for any growth orientation (hkl) for which 0≤h≤k≤l/2, and the opposite regime should occur for (hkl) when l≤k/2≤l. Effects of heterointerfacial line tension and stacking fault energy are also considered.
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