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Details of Grant 

EPSRC Reference: EP/D03163X/1
Title: Dislocation Configurations in Strained Multilayered Semiconductor Structures
Principal Investigator: Cockayne, Professor D
Other Investigators:
Researcher Co-Investigators:
Project Partners:
La Trobe University
Department: Materials
Organisation: University of Oxford
Scheme: Standard Research (Pre-FEC)
Starts: 01 March 2006 Ends: 28 February 2007 Value (£): 18,298
EPSRC Research Topic Classifications:
Materials Characterisation
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Summary on Grant Application Form
Dislocations are elongated microscopic faults in crystals. If they are visualized, which is possible with high magnification electron microscopes, they would appear to be a tangle of filaments running through otherwise perfect crystalline regions. The way in which they are created has been studied for decades; however while there has been considerable success in understanding the behaviour of dislocations in simple, single strained-layer systems, the study of important multi-layered systems has been impeded by the absence of a theoretical basis upon which to base any experimental study. Since Professor Usher and coworkers have now developed such a theory it is necessary to test the theory in technologically significant, multi-layered strained material systems. This is the intention of the present study.The creation, movement and multiplication of dislocations is driven by internal and external stresses. In the case of the metals used in aircraft or automotive frames and components, external stresses acting on dislocations can lead to cracks that propagate through the structure and result in catastrophic failure. For semiconductor devices, stationary dislocations in relatively small numbers may not be a problem. However many devices include strained layers which are necessary for their function and the internal stresses associated with these layers can act to first move and then multiply to create new dislocations. As an example, the lasers found in the pickup of CD players include a thin quantum well (QW) layer within which electrons are confined before losing energy and emitting a photon. In a device made from high quality, dislocation-free materials, most of the electrons within the QW will emit a photon as they lose energy. However if a high density of dislocations is created in a QW, electrons will lose energy when they meet them without emitting light, and the electron energy is lost to vibrations of the crystal lattice rather than being converted into light. As the amount of light emitted reduces, more and more electrons must be injected into the QW to maintain the level of light output. This results in the device temperature increasing, which in turn accelerates the processes of dislocation movement, multiplication and creation and the device eventually fails. A further example of the disastrous consequences of high densities of dislocations can be found in fast transistors that employ a strained layer within which the electrons flow. The speed of the electrons depends mainly on being able to move freely through the device under the influence of an applied voltage. However electrons will collide with dislocations and momentarily slow, thus inhibiting current flow and slowing the response of the device. The higher the density of dislocations the slower the device becomes and so the strained layer must be created in such a way that dislocations are not formed during or after manufacture of the device.While single strained layers find application in many device structures, there are important classes of devices that depend on multiple strained layers for their functionality. These include surface emitting lasers, in which approximately 30 strained layers act as very effective optical mirrors to reflect light back into the lasing region allowing laser action to persist. Multiple strained layers such as these have also been used as dislocation filters, to confine dislocations below a surface, above which material with a lower dislocation density can be grown. Though poorly understood, this is extremely important technologically because there are many interesting materials that cannot be grown without including internal stresses and this inevitably results in the formation of dislocations. There are therefore significant benefits to being able to reduce the density of such dislocations through filtering.
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