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首页> 外文期刊>Ultramicroscopy >The impact of surface and retardation losses on valence electron energy-loss spectroscopy
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The impact of surface and retardation losses on valence electron energy-loss spectroscopy

机译：表面和延迟损耗对价电子能量损失谱的影响

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摘要

The inelastic scattering of fast electrons transmitting thin foils of silicon (Si), silicon nitride (Si_3N_4), gallium arsenide (GaAs), gallium nitride (GaN) and cadmium selenide (CdSe) was analyzed using dielectric theory. In particular, the impact of surface and bulk retardation losses on valence electron energy-loss spectroscopy (VEELS) was studied as a function of the foil thickness. It is shown that for the materials analyzed, surface and retardation losses can cause a systematic, thickness-dependent modulation of the dielectric volume losses, which can hamper the determination of the bulk dielectric data as well as the identification of band-gap and interband transition energies by VEELS. For Si and GaAs, where the dielectric function is strongly peaked with high absolute values, retardation losses lead to additional intensity maxima in the spectrum. For thin films of these materials (below approx 100mn), the additional intensity maxima are related to retardation effects due to the finite size of the sample leading to the excitation of guided light modes. For thicker films, exceeding about 200 nm, the intensity maxima are caused by bulk retardation losses, i.e., Cerenkov losses. Although thickness-dependent modulations were observed for Si_3N_4, GaN and CdSe, the form of the dielectric functions and their lower maxima, means that for TEM samples < 100 nm thick, the band-gap energies of these materials can be accurately identified by VEELS. Guidelines are given that allow for forecasting the impact of surface and retardation losses on VEELS.
机译：利用介电理论分析了硅，氮化硅（Si_3N_4），砷化镓（GaAs），氮化镓（GaN）和硒化镉（CdSe）的快速电子传输薄膜的非弹性散射。特别地，研究了表面和整体延迟损失对价电子能量损失谱（VEELS）的影响，该影响是箔厚度的函数。结果表明，对于所分析的材料，表面损耗和延迟损耗会导致介电体损耗的系统性，厚度依赖性调制，从而可能会妨碍确定整体介电数据以及识别带隙和带间跃迁VEELS提供的能量。对于Si和GaAs，其介电函数以很高的绝对值强烈达到峰值，延迟损耗导致光谱中的其他强度最大值。对于这些材料的薄膜（约1亿以下），由于样本的有限大小导致了导光模式的激发，附加的强度最大值与延迟效应有关。对于超过约200nm的较厚的膜，最大强度是由体积延迟损失，即切伦科夫损失引起的。尽管对于Si_3N_4，GaN和CdSe观察到厚度依赖的调制，但是介电函数的形式及其较低的最大值表示，对于厚度小于100 nm的TEM样品，可以通过VEELS准确识别这些材料的带隙能量。给出了指导原则，可以预测地表和减速损失对VEELS的影响。

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