Cathodoluminescence imaging

SEM cathodoluminescence imaging

What is cathodoluminescence?

When an electron beam interacts with a material, a multitude of processes occur which can be employed for various kinds of microscopy. Besides electron signals, a broad spectrum of electromagnetic radiation ranging from x-rays to the mid-IR is generated through a variety of incoherent and coherent processes.

The radiation that is generated in the ultraviolet/visible/near-infrared regime of the electromagnetic spectrum is referred to as cathodoluminescence (CL) as the radiation is generated by cathode rays (fast electrons). The electron beam causes the material to flouresce as it returns to a ground state.

Cathodoluminescence imaging schematic

Figure 1: Schematic of the processes that occur when an energetic beam of electrons impinges on a sample. These processes are used for different characterization techniques as indicated in the schematic. CL is the electromagnetic radiation in the UV/VIS/IR spectral range.

Why cathodoluminescence?

Cathodoluminescence emission can be used to explore many fundamental properties of matter. It can be used to study light transport, scattering, electronic structure of a material (e.g. bandgap, defects), resonant phenomena and much more. It thus presents a valuable source of information for fundamental research as well as applied research with a direct link to industry (metrology, failure analysis).

Conventional optical microscopy is limited in resolution by the diffraction limit. Abbe’s law of diffraction states that two points that are spaced less than d = λ0/(2NA), where λ0 is the free space wavelength and NA is the numerical aperture of the microscope, cannot be resolved by the microscope. This makes conventional optical microscopy unsuited for studies at the true nanoscale.

Electron micrograph of a nanoantenna

Figure 2: Electron micrograph of a 125 nm long gold rod, which acts as nanoantenna and is resonant at λ0 = 750 nm. The scale bar is 50 nm. The overlaid red circle represents a diffraction-limited spot for λ0 = 750 nm and an NA = 1. The small blue dot represents a 5 nm electron beam (to scale). A magnified image of the area enclosed by the gray dashed circle is shown on the right. This image illustrates the importance of electron beam excitation for studying nanomaterials.

A solution to this problem is to use a beam of fast electrons to probe materials at the nanoscale. Using electrons as an optical excitation source has several advantages. First, the excitation resolution can be very high, as typical scanning electron microscopes can focus and position an electron beam with 1 – 10 nm precision. As such the beam acts as a very pure nanoscale broad band excitation source which is probeless and non-destructive. Furthermore, since the measurements are performed in an electron microscope environment the full electron microscopy toolbox can be used to correlate nanoscale geometrical features with the optical response.

The power of cathodoluminescence is that it combines functional optical information with the superior spatial resolution associated with electron microscopy. This makes the technique highly appealing for a large variety of applications and research, especially in the fields of optics research, materials science, and geology.