Electron Microscopy on Thin Films for Solar Cells

Abstract

Electrons used for imaging in a scanning electron microscope are either SEs or BSEs. Both types of electrons are emitted from the specimen upon electron irradiation (Figure 12.2). The emission spectrum also contains contributions from Auger electrons (used for surface analysis of samples, see Chapter 15 and Section 16.3), which are emitted after ionization of an inner-core shell, as alternative to characteristic X-rays (see Section 12.2.3). By convention, SEs exhibit energies between 0 and 50 eV, whereas BSEs cover the energy range between 50 eV and the energy of the primary beam E b .SEs are mostly collected outside of the microscope column by a positively biased collector grid and then accelerated on a scintillator layer in front of a photomultiplier (note that also low-energy BSEs are thus detected). Due to their very small exit depths of few nanometers, and also due to the fact that the SE yield depends on the tilt of a given surface element, SEs are used to image the surface topography.SEs are retarded by a positive bias and repelled by a negative one at the sample surface. Also, these electrons are affected by the electrical field present owing to differently biased regions. Thus, negatively charged regions appear bright and positively charged areas dark. Therefore, it is possible to obtain a voltage contrast and also information on the local doping [3].SEs may also be detected inside the microscope column, by use of annular detectors. As much as the electron beam is focused by the series of electrostatic and magnetic lenses when running from top to bottom, it is spread energetically when emitted from the specimen and traveling up the microscope column. SEs and BSEs exhibit substantially different trajectories, and therefore, two corresponding annular detectors at different vertical positions in the column may be used (Figure 12.2). Images recorded by the in-column SE detector rather contain information from the surface. Figure 12.2 (a) Energy distribution of scattered SEs and BSEs. The spectrum features also peaks related to Auger electrons. (b)The electron trajectories in a typical scanning electron microscope, equipped with SE and BSE detectors as well as with a gas-injection system. Scanning Electron Microscopy j301The in-column BSE detector collects mainly Rutherford-type BSEs, that is, multiple elastically scattered BSEs (by nucleus-electron interaction), emitted into rather small angles with respect to the impinging electron beam, and the energy of these BSEs depends on the atomic number Z. This is why the technique is often referred to as Z contrast imaging, that is, it allows for detecting phases-exhibiting differences in Z. Additional energy filtering of these BSEs may reveal even small variations in compositions [4].Apart from Rutherford-type BSEs, also Mott-type (single elastically scattered) BSEs are emitted, however, at much larger angles (with respect to the impinging beam). The exit volume is therefore also smaller, that is, the BSEs are emitted close to the surface, and in consequence, the ...

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