Exploration of Strain Fields in Crystalline Nanowires

Band-gap engineering is one of the leading promises of nanotechnology. If new semiconductor materials are constructed in a low-dimensional format their electronic properties are predicted to change profoundly. Twenty years ago the technology was limited to quantum well structures, in which the electrons are confined in sheets. Today even greater improvements are expected by making nanowires and quantum dots , with the electrons confined in two and three dimensions respectively. Nanowires, which can be grown quite easily using a suitable catalyst, are believed to show enhanced electronic and optical behaviour because of (at least) three physical mechanisms: the quantum confinement mentioned above, alloying and strain. Understanding the strain fields that necessarily accompany the formation of these wires is the main goal of the proposal.Nanowires must be highly crystalline for the electrons to move freely inside them. The structure of crystals can be studied very effectively with X-rays which diffract from the parallel planes of atoms within them. The diffraction occurs when the spacing between the planes matches the wavelength of the X-rays used, which happens to be exactly in the range of accessibility of the powerful new generation of synchrotron radiation facilities like the new Diamond Light Source, currently under construction in the UK. In the interim, one of the currently operating facilities, the Advanced Photon Source in Chicago, will be used instead and the technology will be transferred. These sources provide X-ray beams that are thousands of time more coherent than previous generations of machine, sufficiently coherent that the diffraction from a single silicon nanowire should be measurable. An X-ray detector measures the intensity distribution of the diffraction, but completely loses the information about the relative phase shift between different parts of the pattern. However, as the mathematician R.H.T. Bates showed, if the coherent diffraction pattern is measured on a sufficiently fine scale, the missing phase information can be recovered from the self-consistency of the pattern. This oversampling technique is the key to the success of the coherent X-ray diffraction method, which the Principal Investigator of this proposal has developed over the past few years and which will be employed to map out the strain distributions within individual silicon nanowires.Attention will be focussed initially on nanowires made of silicon, familiarly known as SiNWs, on which there is considerable activity today. Later on in the project, the scope will be broadened to other semiconductors. The technological applications of nanowires are exciting, ranging from logic switches, optical cavities and multicolour light-emitting diodes. Strain has not yet been reported in SiNWs, but is expected to make a significant contribution to the electronic properties. Surface strain fields in silicon are known to extend over tens of atomic layers. The nanowire format, in which inclined surfaces meet along the prism edges, should strongly enhance the magnitude and length scale of these effects. To date, the structures have been investigated by transmission electron microscopy (TEM), but these methods have been so far insufficiently quantitative to identify strain fields. A TEM image is fundamentally a real quantity, while the new coherent X-ray diffraction methods can produce three dimensional images with both real and imaginary parts. The latter is a direct image of one component of the strain field. Looking for strain fields in freestanding SiNWs is the primary objective of the research proposed here. Once this is achieved, the nanowires will be manipulated using some of the new tools of nanotechnology, such as a Focussed Ion Beam, or by chemical and thermal treatments to explore the effect on the internal strain fields.