Cathodoluminescence (CL) microscopy can be performed in the scanning (SEM) or transmission electron microscope (TEM), and—as with other forms of microscopy—the requirements for sample preparation vary significantly. Experiments in the SEM allow analysis for a broader range of sample types, including bulk specimens, 6" wafers, packaged devices, thin geological sections, epoxy mounts, and nanoparticles dispersed on a supportive substrate. Meanwhile, the TEM requires thinning specimens to electron transparency to attain a potentially higher spatial resolution.
SEM
CL analysis often requires little or no sample preparation beyond that needed for general SEM imaging and analysis. Samples should be macroscopically flat and free from sample preparation-induced defects, e.g., scratches from mechanical polishing or ion-induced defects from focused ion beam (FIB) sample preparation methods. We recommend a clean-up step for FIB-prepared specimens using broad argon beam milling, such as the PECS™ II or Ilion™ II system.
Many samples of interest are insulators or wide band gap semiconductors. For these materials, it may be necessary to coat the sample's surface with a conductive layer to prevent charge accumulation. For this purpose, a thin 1 – 3 nm carbon layer is preferred over metal evaporation due to the low and uniform absorption of the light escaping from the specimen. Analysis in an environmental or low-vacuum SEM chamber may be possible. However, some gases (e.g., water vapor) provide a background luminescence signature that may obscure the luminescence signal.
CL in the SEM does allow the analysis of sub-surface features by providing the freedom to control the penetration depth of the incident electron beam. The accelerating voltage of the SEM determines the depth below the surface and, depending on the sample, extends the analysis range up to 5 – 10 μm. For this analysis, the interaction volume created by the electron beam must be able to access the region of interest. Some semiconductor devices may limit analysis to specific sample geometries or require a small degree of sample processing.
When using detection systems that use a light-collection mirror, the sample's region of interest must be located at the focal point of the collection mirror. The Monarc® and ChromaCL™ systems allow for 1 mm clearance between the sample and mirror and accommodate 6" and 3" diameter samples, respectively. Additionally, the Monarc supports sample tilts of 25° (in a single axis).
In many nanophotonic applications, surface plasmon polaritons are only supported at metal-dielectric interfaces (e.g., metal-vacuum surface); any hydrocarbon contamination can dampen the optical response and provide misleading results. We strongly recommend using a plasma cleaner (e.g., Solarus® II) to prepare contamination-free surfaces for optimum results.
TEM
CL requires little or no sample preparation beyond what is necessary for general TEM imaging and analysis. We recommend using an argon-milling system like the PIPS™ II to clean up FIB-prepared specimens and remove gallium-induced defects. We also recommend plasma cleaning using a Solarus II plasma cleaner or similar system. In many specimens, the low intensity of the CL signal necessitates large probe currents, consequently leading to a higher rate of hydrocarbon contamination.
When preparing semiconductor devices and materials, note that as the sample becomes thinner, the effect of surface recombination becomes increasingly important. In cases where the sample is thinner than the minority carrier diffusion length, surface recombination may quench the emission entirely. The effect may be less severe for semiconductor samples that contain heterostructures and other nanoscale features, as the band gap alignment is often designed to prevent surface recombination.
In many nanophotonic applications, surface plasmon polaritons are only supported at metal-dielectric interfaces (e.g., metal-vacuum surface); any hydrocarbon contamination can dampen the optical response and provide misleading results. We strongly recommend using a plasma cleaner (e.g., Solarus® II) to prepare contamination-free surfaces for optimum results.
Sample's contribution to image drift
Common environmental factors can cause your sample to drift during an experiment, including vibration, thermal variations, and stray electromagnetic fields. This drift may cause image blurring that impacts the resolution and quality of your data. Below are frequently encountered sources of drift related to the sample itself:
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Charging
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Symptoms – Sample drift or jumping will increase as higher probe currents are applied
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Solution – Coat your sample with carbon to make the sample conductive
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Improper sample mounting
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Symptoms – Sample vibrates
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Solution – Verify how securely fastened the sample is to the grid or holder
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