Dec 21, 2011
Examining hidden defects in silicon nanowires
Nanowires are able to grow in more than one crystal phase, but like any other crystalline material they can also contain imperfections and crystalline defects. Planar crystalline defects can be easily identified in a transmission electron microscope (TEM) if the defect is oriented in an edge-on position. But, if multiple planar defects cross the nanowire volume in different directions and do not cross the entire nanowire volume it can become impossible to orient a single defect in the edge on position. These defects are referred to as "hidden". In this situation different crystal grains in the nanowire, separated by defects, will superpose in the image or diffraction pattern obtained in the TEM. Such a superposition creates a Moiré pattern.
The Moiré patterns created by the superposition of different crystal grains can resemble silicon hexagonal crystal phases, as shown by recent work published in the journal Nanotechnology. In the studied nanowires no hexagonal phase with long-range order was found and the "odd" images and diffraction patterns were mostly due to planar defects causing superposition of different crystal grains.
The study of a slice of a nanowire prepared by focused ion beam provides the twinning relations of the different crystal grains present in the nanowire volume. This information is then used to explain the Moiré patterns that are observed if the nanowire is studied from the side and the different crystal grains superpose.
In the report, the team describes a strategy to unambiguously distinguish a hexagonal phase from a cubic phase with defects – the nanowire should be observed from different directions and dark field images should be obtained – and points out image features that indicate the presence of hidden defects.
Full details can be found in the journal Nanotechnology.
About the author
Martien den Hertog performed this work during her PhD at CEA Grenoble. She is now a researcher at the CNRS Institute Néel in Grenoble and is investigating the correlation between the structural properties of semiconducting nanowires and their optical and electrical behaviour.