SEM is a powerful technique widely used for characterisation of materials. Electron microscopes use a beam of electrons generated either by tungsten or a field emission gun. This allows the SEM to reach magnifications up to 500,000 times and deeper depth of field as compared to the optical microscopes. The electrons are accelerated through a high voltage and a series of electromagnetic lenses and aperture to produce a fine beam of electrons. SEM images are obtained by scanning the fine beam onto a sample in a raster pattern and collecting the signal as it scans. The intensity of the signal is translated as the brightness and contrast in the images. The brightness and the contrast represent certain characteristics of the sample depending on which signals are collected for that particular scan. When an electron beam hits a sample, several interactions can occur, which generate a number of signals for detection. Various signals provide different types of information of the sample like topography, surface morphology, composition, crystal and electronic structure. Figure 3.5 shows the various interactions that can occur when an electron beam hits the sample.
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Figure 3.5: Electron-sample interaction
In SEM, two main signals that are commonly collected are the secondary electrons and back-scattered electrons. The secondary electrons are electrons that are ejected by the material near the surface and have low energies. Hence, the image retrieved from the secondary electrons signal can provide more topographic details. Backscattered electrons, on the other hand, have higher energy and are more sensitive to the atomic mass (Z) of the sample. Hence, the contrast obtained by the signals corresponds to the elements in the sample. Heavier elements would appear brighter due to the higher amount of backscattered electrons than the lighter elements.
3.3.2 Components of SEM
Figure 3.6 shows the schematic of an SEM comprising of the main components such as the electron gun, electromagnetic lenses, apertures, detectors and the vacuum chamber. Each of these components has their own detailed principle of operation. However in this thesis, they are briefly explained in the following sub-sections.
3. Experimental techniques
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Figure 3.6: Schematic of a typical SEM showing the main components
3.3.2.1 Electron guns
Electron guns can either be thermionic guns or field emission guns. Thermionic guns (eg. tungsten filament) produce electrons by applying thermal energy onto the filament to emit electrons. The electrons are then accelerated towards an anode. Field emission guns, on the other hand, use a very strong electric field to extract electrons from the filament. This in turn gives a brighter source as compared to thermionic guns. However, it requires a higher vacuum condition to operate.
3.3.2.2 Electromagnetic coil lenses
Since the principle of SEM uses electrons instead of light, the lenses are magnetic instead of the conventional glass used for the light microscopy. Similar to glass lenses, the role of the electromagnetic lenses is to form a smaller and finely focused probe incident on the sample. Two main lenses are used in SEM, the condenser lenses and the objective lenses as shown in figure 3.7. The condenser lenses control the number of electrons in the beam for a given aperture size while the objective lenses focus the electrons on the sample at the working distance. The lenses are controlled by adjusting the amount of current flowing through the coils around the iron core of the magnet.
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3.3.2.3 Detectors
SEM uses the Everhart-Thornley detector to collect signals from secondary and back- scattered electrons. The detector mainly consists of a scintillator surrounded by a Faraday cage. The schematic of the Everhart-Thornley detector is shown in figure 3.7. The Faraday cage is maintained at low positive potential (~+200 V) to attract low energy electron and the rest of the electrons are then attracted by the higher positive potential (~+10 kV) scintillator. The electrons are then converted into light photons and guided to a photomultiplier tube for amplification ready to be processed as the output signal.
Figure 3.7: Schematic of Everhart-Thornley detector for collecting secondary electron signals.
By applying a negative voltage to the Faraday cage, the detector can act as a back-scattered electron detector. However, a dedicated back-scattered electron detector is often used for this purpose.
3.3.2.4 Vacuum system
A standard SEM operates in high vacuum (10-6 Torr) to allow electrons to travel freely
between the sample and the detector. In addition the high vacuum also protects the electron guns of the system. Commonly the SEM comprises of a combination of pump system to reach the high vacuum. This usually includes a rotary mechanical pump pumping on the excess gas pumped by a diffusion pump or a turbo-molecular pump.
3. Experimental techniques
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3.3.2 Determining the parameters of nanowires using SEM images
In this thesis, SEM images using secondary electron mode are either obtained with a Zeiss Ultra FESEM, FEI Helios 600 Nanolab or FEI Verios system. The SEMs are normally operated with low voltage (1-3 kV) and small apertures to obtain high resolution details of the surface. The SEM images are used to determine the basic morphological detail of the nanowires such as the facets, roughness and shape of the nanowires. Such information generally gives more insight on what to expect when performing other characterization such as TEM. Nanowire parameters such as their height, tapering factor and facets are
extracted from the SEM images. This is basically done using software like Image J. In figure
3.8, a standard evaluation of a nanowire is shown. Measurements are done on more than 20 nanowires for each sample and the average values with its standard deviations are used for plots in this thesis.
Figure 3.8: Evaluation of nanowire dimensions from tilted SEM image and facet
determination from top view SEM image using calculated angles from the known cleaved edge.