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A New Cathodoluminescence System for Nanoscale Optics, Materials Science, and Geology


T. Coenen , 1 * S. V. den Hoedt , 1 and A. Polman 2 1 DELMIC B.V. , T ijsseweg 11 , 2629 JA Delſt , T e Netherlands 2 Center for Nanophotonics , FOM Institute AMOLF , Science Park 104 , 1098 XG Amsterdam , T e Netherlands


* coenen@delmic.com


Abstract: Cathodoluminescence (CL) spectroscopy is a technique that can be used to unravel optical and material properties at the nanoscale. This article describes a novel CL detection system. The system sensitivity, the modularity, and the capability to perform angle-resolved CL imaging enable many new research directions. We introduce the technique and give a description of the setup as well as the different CL emission processes. Subsequently, we show a number of practical examples where the technique is used in the fi elds of nanophotonics, materials science, and geology.


Introduction


In modern-day technology there is an ongoing eff ort to miniaturize functional elements in integrated electronics and photonics, light-emitting diodes, solar cells, and sensors for enhanced performance. Associated with this trend is a strong need to perform various forms of microscopy and spectroscopy down to the nanoscale to gain a fundamental understanding about the materials involved, and also for quality control and failure analysis of devices.


Electron microscopy has proven to be an excellent method for obtaining structural information on small-length scales. Traditionally, electron microscopes were designed for imaging transmitted, secondary, and backscattered electrons to provide spatial information down to the atomic scale, and it was generally not possible to characterize the optical properties of a material in this way. Conversely, (light) optical microscopy and spectroscopy provide a means to directly determine optical properties in detail, but these methods are limited in spatial resolution by diff raction.


In CL spectroscopy visible/near-infrared light generated by a fast electron beam is collected and analyzed. T e key benefi t of this technique is that the high spatial resolution of the electron microscope is combined with the functional optical information that is contained within CL emission. CL spectroscopy has been used for decades in studies of bulk and thin-fi lm materials and now has reemerged as an important tool for nanoscale optical characterization of (nanostructured) materials, largely as the result of recent advances in electron microscopy and light detection. CL Emission Processes . CL is light in the UV/VIS/ NIR regime emitted from a material under fast free electron bombardment. Interestingly, the discovery and analysis of CL emission in vacuum tubes generated by “cathode rays” played a key role in the discovery of the electron [ 1 ]. T ere are two distinct physical processes that can lead to CL emission, each of which can serve a purpose in particular applications. Incoherent CL. T e fi rst type is categorized as incoherent CL emission where no phase relationship exists between the incoming electron fi eld and the CL emission. T is type of CL emission bears a strong resemblance to photoluminescence, as it involves quantum mechanical transitions in a material (see


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Figure 1a ). Such CL is dominant in phosphors (rare-earth doped materials), direct bandgap semiconductors (GaAs, GaN, InP, etc.), and most dielectrics (geological materials, glasses, ceramics, etc.) [ 2 – 4 ]. Typically, this emission is most effi ciently excited by slower electrons, and hence this CL is generated in an interaction volume below the impact position, where the size of the volume is determined by the energy of the primary electrons. As a result one primary electron can generate many photons resulting in a bright CL signal, which forms the basis for effi cient electron scintillators and cathode ray tube (CRT) screens. Because of its sensitivity to intrinsic material properties (electronic band gap, defect centers, etc.), this type of CL is very useful for material fi ngerprinting, quality assurance, and failure analysis of materials. Coherent CL. The second type of CL is categorized by coherent interactions between the electron and a material. In this case, the evanescent electromagnetic fi eld of the electron coherently polarizes the material (see Figure 1b ). Even though these coherent processes are physically distinct from conven- tional “luminescence” processes like photoluminescence or incoherent CL, they also fall under the general umbrella term “cathodoluminescence.” Electron-induced radiation emission (EIRE) is sometimes used as an alternative term for these coherent processes to avoid confusion with the incoherent form of CL. Examples of coherent excitations include transition radiation (TR), which is generated when an electron traverses an interface between two media; bulk plasmon and surface plasmon polariton (SPP) excitation in plasmonic metals; and Cherenkov radiation, which is generated when the speed of the electron exceeds the phase velocity of light in the material. T e advantage of this type of process is that it is very localized (< 20 nm) and fast (~1 fs); as a result it is broadband both


Figure 1 : (a) Schematic representation of incoherent CL emission. The electron beam has an interaction volume within the material (indicated in red) in which excitations are generated. Valence electrons are promoted into an excited state after which they can decay with the emission of a photon, as indicated by the schematic Jablonski diagram. (b) Schematic representation of coherent CL emission. The evanescent fi elds of the electron can locally polarize a material, which leads to direct light emission (TR) and excitation of other supported electro- magnetic modes such as SPPs.


doi: 10.1017/S1551929516000377 www.microscopy-today.com • 2016 May


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