Electron Microscopes (EMs) use a focused beam of electrons to 'bombard' the sample and gain information about its structure and composition. The basic ideas of an EM are:
The 'products' resulting from the interaction of the electron beam with the sample, like X-rays, back-scattered, Auger, and secondary electrons, cathodoluminescence (see Figure 7.1), are detected and interpreted as an image. In what follows we will discuss in more detail secondary electrons and CL.
Generation of secondary electrons depends strongly on the topography. Due to their low energy of ~5 eV, only the secondary electrons that are created near to the surface (10 nm) can leave the sample and be collected. Any change in topography of the sample that is larger than this sampling depth will change the collection efficiency. Collection of these electrons is made by a collector in combination with a secondary electron detector. The collector is a mesh with a +100V potential applied to it which is placed in front of the detector, attracting the negatively charged secondary electrons. The electrons then pass through the grid-holes into the detector to be counted.
For each point on the studied area the microscope counts the number of secondary electrons reaching the detector and displays it as a pixel on a picture or display. The pixel intensity is determined by the number of the counted electrons. This process can be repeated in a scanning mode, e.g. 30 times per second, thus obtaining a Scanning/Secondary Electron Microscope (SEM) picture of the investigated area. Due to its high resolution compared to optical microscopes, electron microscopes are fascinating tools for investigating porous semiconductors.
Figure 7.1: Schematic representation of the energies produced from electron beam interaction with solid matter. The sample is bombarded by an electron beam. Due to the interaction of the beam with the sample different electromagnetic waves and electrons are generated: UV, IR, Visible, X-rays, secondary, Auger and back-scattered electrons. Scanning the sample with the incident beam will allow us to collect CL spectra from different regions of the sample, and thus 2D CL photographs can be obtained. More tan that, the penetration depth of the incident beam and thus the excited volume in the sample depends on the energy of the incident electrons. Thus, a 3D image of the sample is possible to be obtained if the scanning mode and electron energy variation are combined.
Photoluminescence and cathodoluminescence are said to be complementary. For example, a scanning electron microscope configured with cathodoluminescence can yield information on the spatial distribution of the luminescence. At the same time non-scanning PL methods can provide macroscopic data.
Quite often the CL systems are supplied with cryogenic cooling. However, this really depends on the application. Specimen cooling increases the spectral discrimination and provides more insight about the physics of the light emission processes. Some direct band gap semiconductors give adequate CL at room temperature. However, the information contained in the CL emission may be blurred by thermal processes. Also, certain transitions may not be activated at low injection conditions unless the specimen is cold. This can be critical in achieving the mix of spatial resolution and spectral resolution from a sample.