Transmission Electron Microscopy (TEM)

As the name implies, this technique passes a high energy electron beam through an ultra-thin sample in order to achieve its functionality (which is not just limited to imaging). The electrons interact with the sample in many ways as it passes through; hence, with the appropriate equipment configuration, diffraction, electron energy loss, electron phase change, elastic and inelastic scattering of electrons can be used to extract a wealth of information on crystal structure, morphology, defects, strain and composition. This discussion will be limited only to the functionality used for the work presented in this dissertation.

Figure 3.6: Ray diagrams of TEM in, (a) diffraction mode, (b) imaging mode. Adapted from [41].

§ 3.2 Electron Microscopy 41

As shown in Figure 3.4, the three main types of electrons produced by their transmission through the sample are, minimally altered direct electrons, and elastically and inelastically scattered electrons. Bragg scattering by the crystal planes parallel to the incident electron beam gives rise to diffraction spots at the back focal plane as shown in Figure 3.6 (a) and (b), which depicts simplified ray diagrams of TEM in two basic modes, namely diffraction and imaging. The patterns formed by these spots (diffraction patterns) are different depending on the type of planes that are parallel to the electron beam. Hence these, viewed on the phosphor screen in the diffraction mode (Figure 3.6 (a)) are used to tilt the sample and align it in the required angle before imaging.

3.2.2.1 Diffraction

In addition to being an aid in tilting the samples to the required viewing angle, the diffraction patterns contain other valuable information in reciprocal space about the sample. The pattern profile can indicate the crystal phase while the distance between the spots can be used to measure the lattice constant, hence the strain and composition of a known compound. Additional spots, satellite spots and streaking of spots could indicate extended planar defects such as stacking faults and mixed phase. The relative orientation of the indexed diffraction pattern compared to the image can be used to ascertain the crystal growth direction as in the case of nanowires.

When carried out with a converged beam on the sample, the diffraction pattern results in discs instead of spots. The patterns formed inside these discs can be used to gauge the thickness and the polarity with the help of simulations.

Two TEMs, a Philips CM 300 operated at 300 kV and JEOL 2100F operated at 200 kV have been used for the work presented in this dissertation. Diffraction functionality in both machines have been used for alignment of sample and determination of crystal phase in Chapters 4 to 7, confirming epitaxial growth of InP layers on Si in Chapter 5, and confirming the growth direction of <111> oriented nanowire in Chapter 6.

3.2.2.2 Bright-field and dark-field imaging

Bright-field imaging is the most commonly used functionality of TEM. An aperture called objective aperture can be inserted at back focal plane (see Figure 3.6 (b)) to select the set of beams one would like to be used to create the image in imaging mode. For example, if the direct (undiffracted) beam is selected, then the image would contain contribution from all electrons that went through the sample without diffracting. This would also include electrons that passed through areas that no sample was present, leading to a bright background and hence the name ‘bright-field’.

Scattering of the electrons will be different depending on the material (elements with higher atomic number will scatter more) giving rise to Z-contrast in bright-field images. Areas with higher thickness will also appear darker due to scattering and as fewer electrons could escape thick regions without getting absorbed compared to thin ones. Bright-field images can also be created without using any aperture since the proportion of diffracted electrons is much less than those pass directly. However the contrast of the image will be much lower. All TEM images presented in this work (nanowires, layer and their cross sections) are bright- field images.

Similarly if a spot or a collection of spots other than the direct beam is selected at the back focal plane, then the image is created only with the contribution from electrons diffracted from regions pertaining to those relevant spot/spots. In this case, the background will be dark, since the undiffracted beam is blocked. The area which contributed to the selected spot/spots will be bright, forming dark-field images. Spot/spots arising from a different material or phase can be used for this. Conversely, the areas in the sample that give rise to unknown diffraction spots can also be identified using dark-field imaging.

3.2.2.3 High Resolution TEM (HRTEM)

HRTEM uses the phase difference between the direct beam and the diffracted beams to form an interference image. This phase contrast image can translate down to individual columns of atoms. HRTEM images are obtained by tilting to exact zone axis, and imaging under high magnification and very fine focus with no or large objective aperture that allows as many beams (including the direct beam) to pass through. The fact that details down to the placement of individual atom columns can be seen makes this a very powerful technique. It can be used on its own or complementarily with diffraction patterns in order to study interfaces and grain boundaries, identify phase, planar defects, strain, lattice constant and dislocations.

In the work presented in this thesis, HRTEM has been routinely used in both the above mentioned machines for precision atomic level measurements such as quantum well thicknesses, InP-Si (in layers) and InP-InGaAs (in nanowire quantum wells) interface analysis, strain mapping, nanowire crystal phase, and planar defect studies.

3.2.2.4 Scanning Transmission Electron Microscope (STEM) and High Angle Annular Dark Field (HAADF) imaging

STEM is a mode available in some TEMs where the electron beam is focused to a small spot and raster scanned over the thin sample. The raster scanning functionality enables characterisation techniques like EDX mapping, HAADF imaging and electron energy loss spectroscopy given the appropriate detectors are available. HAADF imaging and EDX