Sample Preparation for TEM

mapping has been used in the work presented in this thesis and HAADF imaging will be discussed here while EDX mapping will be discussed in Section 3.4.

As stated previously, the scattering of electrons is highly sensitive to the atomic number (Z) of the atoms in the sample (approximately proportional to Z2). The forward scattered electrons that are scattered out to high angles (generally >3˚) [41] are collected using a ring-shaped detector while raster scan is being carried out in STEM mode on the sample in order to form the HAADF images. As the direct beam is avoided, this will be a form of dark-field imaging, but the contrast arising from difference in atomic numbers is much higher than other forms of imaging. Figure 3.7 shows a schematic of the STEM mode operation and HAADF detector.

STEM mode is available in the JEOL 2100F out of the two TEMs used in this work. HAADF imaging has been used for the InGaAs quantum well tube study (shape and interfaces) presented in Chapters 4 and 7. One such example is shown in Figure 4.1 (d). The STEM probe sizes used for these studies were 0.5 and 0.7 nm. The HAADF imaging related to this work was performed in collaboration with Dr. Yanan Guo.

3.3

Sample Preparation for TEM

As mentioned earlier, the electrons need to pass through the sample in order to be analysed by TEM. Hence, the samples should be thinner than 100-200 nm depending on the TEM operating conditions and material. Thickness should be even less for high resolution imaging, ideally less than about 70 nm. Special techniques are employed to prepare TEM samples so that they are thin, clean and free of sample preparation artefacts. Traditional

mechanical polishing techniques such as tripod polishing and dimple grinding, chemical etching, small angle cleavage, and focused ion beam (FIB) are some of them. The next three sub-sections discuss the techniques used to prepare TEM samples for the work presented in this dissertation.

3.3.1

Sample Preparation for Determination of Crystal Structure and

Defect Densities of Nanowires using the Side View

All nanowires discussed in this work were seeded by 30 or 50 nm Au particles and the increase in diameter due to tapering is such that the complete nanowire can be examined using the Philips CM 300 operated at 300 kV. In fact, large portion of the nanowire could even be imaged at high resolution, further assisted by the fact that the edge of the nanowire is thinner than the diameter itself. Hence, no special preparation technique was employed for the side view analysis. Nanowires were dispersed on Cu grids with lacy carbon membranes by gentle contact and swipe with the as-grown sample or sonicate and drop-cast method. The contact and swipe method is fast and easily results in a high density of nanowires on the Cu grid whereas the drop-cast method yields an even distributed population of isolated nanowires and exerts less mechanical stress (by twisting and bending) on the nanowires.

3.3.2

Cross-section Sample Preparation of Layers, Planar Nanowires,

Nano-Particles Embedded in the Substrate and Nanowires in the

Early Stages of Growth using Tripod Polishing

The presence of the underlying substrates is a common feature of all the above stated samples. Hence this needs to be thinned to electron transparency along with the rest of the sample. The main idea of tripod polishing is to create a wedge shape with the sample as shown in Figure 3.8 step (ii), so that the thin edge is thin enough to a level that is electron transparent.

Two pieces of the sample are glued face to face in between another two dummy substrates of the same material and the wedge shape is polished from the two sides of the substrate sandwich consecutively as shown in Figure 3.8 steps (i) and (ii). SiC lapping papers with decreasing roughness are used for this. The micrometers on the tripod are used to control the angle of the wedge and alignment. Once the sample is polished, it is glued to a Cu grid and lifted off in acetone (Figure 3.8 step (iii)).

A final step of ion beam polishing, using a precision ion polishing system (PIPS) is carried out in order to finely thin the edge and remove any damage left on the sample by mechanical polishing. Here, two Ar ion beams with an optimum energy and tilt angles are directed to the thinned area of the sample, so that any undesirable depositions (such as grit) from polishing and amorphised layers are removed. A Gatan PIPS (Model 691) was used to

§ 3.3 Sample Preparation for TEM 45

First polish Second polish Si 3.5 keV at ± 8˚ 1.5 keV at ± 8˚ InP 2.8 keV at ± 6˚ 1-1.5 keV at ± 6˚

Table 3.1: Typical ion beam energy values used in the PIPS. Figure 3.8: Process of TEM sample preparation using tripod polishing.

finish the tripod polished samples presented in this work. Ion beam polishing was carried out in two steps. First step was at a higher energy, while the second step, which is intended to remove any re-deposited material from the first stage of ion beam polishing, was done at a lower energy. The ion beam energies used for polishing depends on the sample material and very importantly on the individual sample. Therefore, the progress of polishing had to be carefully monitored and adjusted accordingly. However, the table below provides some typical values used in the ion beam polishing steps of the two materials prepared by tripod polishing in this work.

Figure 3.9: Process of nanowire cross-section preparation using microtome sectioning.

3.3.3

Lateral Cross-Section Preparation of Nanowires using Microtome

Sectioning

The nanowires need to be laterally sliced in order to view the growth of radial heterostructures as discussed in Chapters 4 and 7, or to have a clear look at the evolution of cross section shapes along the nanowire as in Chapter 6. As the nanowires are filamental structures, they first need to be reinforced in a resin as shown in Figure 3.9 steps (i) and (ii). Once they are embedded, the substrate is removed by immersing in liquid N2, and the resin

block with the nanowires is precision sliced using a microtome (Figure 3.9 steps (iii) and (iv)). The slices are usually 30-50 nm thick, and the length-wise region of the nanowire that the slices are collected from can be calculated using this thickness and the approximate number of slices. The microtomed slices that are collected on DI water are then picked up onto lacy carbon grids as shown in Figure 3.9 step (v). The microtomed cross-section samples used for the work presented in this dissertation was prepared in collaboration with Dr. Yanan Guo and Amira Ameruddin.