Exploration of Strain Fields in Crystalline Nanowires

带隙工程是纳米技术的主要承诺之一。如果新的半导体材料是以低维形式建造的，它们的电子性质预计将发生深刻的变化。20年前，这项技术仅限于量子井结构，即电子被限制在片状结构中。今天，通过制造纳米线和量子点，人们预计会有更大的改进，电子分别被限制在二维和三维空间。纳米线可以很容易地用合适的催化剂生长，据信由于(至少)三种物理机制：上述量子限制、合金化和应变，纳米线表现出增强的电子和光学行为。了解这些导线形成过程中必然伴随的应变场是该提议的主要目标。纳米线必须是高度结晶的，电子才能在其中自由移动。用从晶体内部原子的平行平面绕射的X射线可以非常有效地研究晶体的结构。当平面之间的间距与所用X射线的波长匹配时，就会发生衍射，而X射线的波长恰好处于强大的新一代同步辐射设施的可及范围内，比如英国目前正在建设的新钻石光源。在此期间，目前正在运营的设施之一--芝加哥的先进光子源将被取代，技术将被转让。这些光源提供的X射线束比前几代机器的相干性强数千倍，足够相干性，以至于单根硅纳米线的衍射应该是可以测量的。X射线探测器测量衍射的强度分布，但完全丢失关于图案不同部分之间的相对相移的信息。然而，正如数学家R.H.T.Bates指出的那样，如果在足够精细的尺度上测量相干衍射图，丢失的位相信息可以从图案的自洽性中恢复出来。这种过采样技术是相干X射线衍射法成功的关键，这项建议的主要研究人员在过去几年中开发了这种方法，并将用于绘制单个硅纳米线内的应变分布。人们首先将关注由硅制成的纳米线，即众所周知的SiNW，目前在这方面有相当大的活动。在该项目的后期，范围将扩大到其他半导体。纳米线的技术应用令人兴奋，从逻辑开关、光腔到多色发光二极管。应变尚未在SiNW中被报道，但有望对电子性质做出重大贡献。众所周知，硅中的表面应变场可以跨越数十个原子层。在纳米线形式中，倾斜的表面沿着棱镜边缘相遇，应该会大大增强这些效应的大小和长度范围。到目前为止，已经用透射电子显微镜(TEM)对这些结构进行了研究，但这些方法到目前为止还不足以定量地识别应变场。透射电子显微镜图像基本上是实量，而新的相干X射线衍射法可以产生实部和虚部都有的三维图像。后者是应变场的一个分量的直接图像。寻找独立式SiNW中的应变场是本文提出的研究的主要目标。一旦实现了这一点，将使用一些新的纳米技术工具来操纵纳米线，如聚焦离子束，或者通过化学和热处理来探索对内部应变场的影响。

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.