David Cherns

My interests lie in the application of novel electron microscopy techniques to the analysis of defects and interfaces, particularly in semiconductors, to clarify microstructure/property relationships. My present research includes the development of new materials for field emission displays, and studies of the role of defects in GaN light-emitting structures. In the latter, we have used transmission electron microscopy to analyse threading defects including dislocations, nanopipes and inversion domains. We have demonstrated, for the first time, that electron holography can be used to profile piezoelectric fields across InGaN quantum wells and around dislocations, and have used cathodoluminescence in a field-emission scanning electron microscope to examine the effects of individual threading defects on light emission.

Some images from recently published work

Images from a GaN (0001) sample with a 1.5 nm In0.28Ga0.72N quantum well buried 50 nm below the top surface. The left hand micrograph is a tranmission electron micrscope image from a plan view sample which has been slightly tilted to reveal a funnel-shaped pit and some threading edge dislocations.  The right hand pair of images are from a different area, (a) shows a TEM image and (b) shows a corresponding cathodoluminescence map in the quantum well emission, taken at 8K in a scanning electron microscope.  The CL map shows that pits and threading dislocations (positions marked) act as nonradiative recombination centres.
Electron holography results: the micrograph on the left shows a holographic image from an edge dislocation in GaN viewed near end-on (in the centre of the outlined box).  The graph on the right shows that the crystal potential is reduced within a few nm of the core consistent with the core being negatively charged.