Cathodoluminescence workflow for SEM
To maximize signal collection by a cathodoluminescence (CL) system, and to avoid a topographic contribution to the data, it is best for samples to be macroscopically flat. Fortunately, many sample types already fulfill this criterion (e.g., semiconductor thin films, nanoparticles on substrates and geological thin sections). When sample preparation is required, a high quality mechanical polish is essential and can be further enhanced when you use a broad-beam argon ion polishing tool.
You can perform most analysis at room temperature when you attach the specimen to regular scanning transmission microscope (SEM) stubs with conductive tape/paint or adhesive carbon pads. Highly insulating specimens will benefit from a thin (2 – 3 nm) layer of carbon deposited on the surface as well as connected electrically to the ground.
However, semiconductor samples benefit from cooling to cryogenic temperatures. At cryogenic temperatures not only does the luminescence intensity usually increase, but you can gain additional insight when studying low temperature physics. Liquid nitrogen or liquid helium cooling stages are available and compatible to enable these studies on the electron microscope environment.
Simple light collection methods position a light sensitive detector, such as a photodiode or photomultiplier tube (PMT), close to a specimen. The system senses light that falls on the detector, then records the light intensity. Although simple, this method lacks the sensitivity and ability to characterize the emitted light.
More advanced methods use collection optics, usually a reflective mirror, to collect up to 50x the light emitted and allow efficient coupling to spectral analyzers situated outside of the microscope’s vacuum chamber. For optimal collection efficiency, it is critical that the specimen alignment with respect to the optical collection system and electron optical axis are correct.
Light emitted from the sample can vary from position to position in intensity, wavelength (energy), angular distribution, and lifetime (time it takes for the light to be emitted after a sample is excited by the electron beam). There are many different methods you can use to capture this information:
Imaging: Measures the variation in emitted light intensities as the electron beam is scanned in an X,Y pattern across a sample. Panchromatic or polychromatic imaging is the simplest method to quantify the total light emitted at each pixel, and is often used to measure the dislocation density in semiconductor samples or reveal zonation in zircons for geochronology studies. Alternatively, you can use monochromatic or bandpass imaging to spectrally filter the light to select only the luminescence associated with a particular spectral feature. This method can reveal the distribution of specific minerals, and crystal defects such as stacking faults or impurities.
In earth sciences, color cathodoluminescence imaging is a favorable method to reveal and interpret reservoir quartz texture as well as shale provenance. Here the collected light is dispersed (split) into red, green and blue (RGB) wavelengths then sensed separately before being recombined in software. Color images may be created through multiple passes of the electron beam with sequential measurement of the RGB signals or, preferentially in a single pass of the electron beam using simultaneous detection of the colors.
Spectroscopy: Analyzes the spectral distribution of the light emitted. The emitted light wavelength can be analyzed to determine the electronic process taking place, thus reveal the nature of the luminescence center (e.g., a particular (trace) impurity, material band gap or crystal defect).
Spectrum imaging: In an advanced acquisition mode, as the electron beam is scanned in an X,Y pattern, you can acquire a full spectrum at every pixel to form a data cube with X and Y spatial dimensions as well as a wavelength dimension. Therefore you can acquire all spatial and spectral information in a single scan of the electron beam.
Depth resolved cathodoluminescence (bulk specimens only): When high energy electrons hit a sample they lose energy through inelastic scattering, exciting the specimen, then generating a photon. During this process, one electron may undergo many scattering events until it loses all energy. Thus a high energy electron will, on average, travel further into a specimen than a lower energy electron that generates cathodoluminescence from deeper within a specimen. To isolate the cathodoluminescence signal generated at a specific depth below the sample surface users can select the initial electron energy through control of the SEM operating parameters.
Angular-resolved emission pattern: The angle that a photon is emitted varies and will depend on the sample structure and mechanism that leads to the photon creation. In some applications, this is useful to measure the angular emission pattern to characterize the viewing angles of LEDs or displays, as well as to understand the interaction of light and matter.
Time resolved: Measures how the cathodoluminescence signal evolves as a function of time. The magnitude of the sampling time can vary by many orders of magnitude from several minutes to picoseconds. This acquisition mode is useful to understand how the electron beam changes a luminescence signal or to measure the minority carrier lifetime in semiconductors.
In earth sciences cathodoluminescence is an extrinsic property, in that the luminescence is associated with an impurity or defect in the host crystal. In many cases, the cathodoluminescence intensity can relate directly to the concentration of a trace element impurity and, in some material systems, a quantitative analysis may be made. Although cathodoluminescence is not a ‘fingerprint’ analytical technique like atomic emission spectroscopy or electron dispersive x-ray spectroscopy (EDS), the measured emission wavelength does correspond to the electronic transition involved. By use of the literature and reference spectra, it is possible to assign which impurity is responsible for the emission. This allows the reconstruction of geological processes, such as the temperature and pressure of rock formation as well as geochemical alterations.
In semiconductors, cathodoluminescence is usually an intrinsic property whose luminescent features are associated with the material band gap, doping and crystal defects. However, the competitive nature of the cathodoluminescence generation process can complicate data interpretation. Specifically, recombination of electron-hole pairs and non-radiative recombination pathways (often via phonon-assisted processes) can perturb the measured intensity of spatially resolved cathodoluminescence profiles. Researchers frequently measures dislocation density in semiconductors where threading dislocations are centers of high non-radiative recombination rate and are revealed as dark spots in a cathodoluminescence image.
Gaussian or Lorentzian curves are typically fit the acquired spectral peaks (in eV) during spectral analysis. This enables determination of the intensity, central wavelength, and full width half maximum (FWHM). In three-dimensional (spectrum image) datasets, you can apply curve fitting to every pixel within the dataset to extract distribution maps of particular luminescence centers (even if there is spectral overlaps with other features), and to reveal subtle shifts in the central wavelength or FWHM. This allows you to quantify compositional variation or stress.