Stöger-Pollach M, Franco H, Schattschneider P, Lazar S, Schaffer B, Grogger W, Zandbergen H W
University Service Center for Transmission Electron Microscopy, Technische Universität Wien, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria.
Micron. 2006;37(5):396-402. doi: 10.1016/j.micron.2006.01.001. Epub 2006 Feb 9.
Measuring low energy losses in semiconductors and insulators with high spatial resolution becomes attractive with the increasing availability of modern transmission electron microscopes (TEMs) equipped with monochromators, C(s) correctors and energy filters. In this paper, we demonstrate that Cerenkov losses pose a limit for the interpretation of low energy loss spectra (EELS) in terms of interband transistions and bandgap determination for many materials. If the velocity of a charged particle in a medium exceeds the velocity of light, photons are emitted leading to a corresponding energy loss of a few electronvolt. Since these losses are strong for energies below the onset of interband transitions, they change the apparent loss function of semiconductors and insulators, with the risk of erroneous interpretation of spectra. We measured low energy losses of Si and GaAs with a monochromated TEM demonstrating the effect of sample thickness on Cerenkov losses. Angle resolved EELS and energy filtered diffraction patterns (taken without a monochromator) show the extremely narrow angular distribution of Cerenkov losses. The latter experiment provides a method that allows to decide whether Cerenkov radiation masks the very low loss signal in EELS.
随着配备单色仪、C(s)校正器和能量过滤器的现代透射电子显微镜（TEM）越来越普及，以高空间分辨率测量半导体和绝缘体中的低能量损失变得颇具吸引力。在本文中，我们证明了切伦科夫损失对许多材料在带间跃迁和带隙确定方面解释低能量损失谱（EELS）构成了限制。如果介质中带电粒子的速度超过光速，就会发射光子，导致相应的几电子伏特能量损失。由于这些损失在低于带间跃迁起始能量时很强，它们会改变半导体和绝缘体的表观损失函数，存在光谱错误解释的风险。我们用单色TEM测量了Si和GaAs的低能量损失，证明了样品厚度对切伦科夫损失的影响。角分辨EELS和能量过滤衍射图案（在没有单色仪的情况下获取）显示了切伦科夫损失极其狭窄的角分布。后一个实验提供了一种方法，可以确定切伦科夫辐射是否掩盖了EELS中极低的损失信号。
