Transmission Electron Microscopy

As the electrons travel through a very thin specimen, they are either scattered by a variety of processes or they may remain unaffected by the specimen. The end result is that a nonuniform distribution of electrons emerges from the exit surface of the specimen. It is this nonuniform distribution that contains all the structural and chemical information about the specimen. Electron scattering can be grouped in two ways, that are elastic and inelastic scattering. There terms are simply descriptions of scattering that result in no loss of energy and some measurable loss of energy, respectively. Elastically scattered electrons are the major source of contrast in TEM images and they also create the intensity in diffraction patterns

(DPs). Inelastic scatter transfers energy to the specimen, generating a lot of useful signals which form different images of the specimen or get spectroscopic information about its chemistry and electronic structure.

The uniform electron intensity in the incident beam is transformed into a nonuniform intensity after scattering by the specimen. The variable electron intensity hits the viewing screen or the electron detector, which translates into contrast on the screen. That is DP shows nonuniformity because it separates out the diffracted and direct beams. Therefore, a fundamental principle of imaging in the TEM is [28]: first view the DP, since this pattern tells about the scattering of the specimen. DPs are the basis of all image formation in the TEM as all crystallographic analysis and defect characterization. In order to obtain a DP with a parallel with beam of electrons, the stander way is to use a selecting aperture. This operation is called selected-area diffraction (SAD). By selecting the direct beam and the diffracted beam in SAD pattern to form bright field (BF) and dark field (DF) images, respectively. To get good strong contrast in both BF and DF images, it is necessary to tilt the specimen to two-beam conditions in which the diffracted beam and the direct beam, are only the two strong spots in the pattern.

TEM image contrast arises because of the scattering of the incident beam by the specimen. The electron wave can change both its amplitude and its phase as it traverses the specimen and both these kinds of change can give rise to image contrast. Thus a fundamental distinction in TEM is between amplitude contrast and phase contrast. The amplitude contrast that there are two principal types, namely mass-thickness contrast and diffraction contrast.

Mass-thickness contrast is most important in looking at amorphous materials such as polymers. Diffraction contrast from regions close to the defect would depend on the properties of the defect, the strain field especially. Phase contrast imaging is often thought to be synonymous with high-resolution TEM (HRTEM) [28]. In fact, phase contrast appears in

atomic structure of a thin specimen.

Buried dots have been analyzed principally by high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) of cross-sectional specimens. The size of QDs can be estimated from the contrast, whereas the composition is much more difficult to qualify, since the contrast is weak and the strain sensitivity in electron microscopy is insufficient for the level of precision required. Most TEM equipped with EDS and electron energy loss spectrometry (EELS), giving the chemical information and a lot of other detail about the samples. An estimate of the composition can be made by combining STEM using a very small probe with EDS, however EDS cannot provide an absolute composition value and since STEM creates a two-dimensional image of the dot assumptions are need regarding the dot shape [29]. Furthermore, HRTEM can be used to determine the morphology of QD, the local strain and structural peculiarities on atomic scale.

In this work, TEM (JEOL JEM-2010F) studies were performed on the magnification from 2000 to 1500000 at 200 kV with a filed emission gun in National Device Laboratories (NDL). It was also equipped with STEM, EDS, EELS and CCD cameras.

Electron microscopy imaging is the only direct method of structural investigation with sufficient resolution for capped QDs without destroying the buried dots [30]. Among these the cross-sectional and the plan-view TEM imaging techniques are suitable methods to characterize directly the shape, the size, and the strain field of nanometer-scaled structures and related defects, especially using electron diffraction contrast imaging with BF and DF modes. DF images using the 200 diffraction condition are frequently used [31], viewing the layers edge-on in a cross-sectional specimen. The contrast of bright and black regions in TEM image has its origin in the well-know composition sensitively of the 200 reflection [32]. It can use the structure factor to interpret the chemical sensitive for the 200 reflection and strain sensitive for the 400 reflection. The scattering from the unit cell by the expression

= ∑ ^⋅

What this equation says is that the atoms within the unit cell all scatter with a phase difference given by 2πiK⋅r_i where ri is a vector which defines the location of each atom within the

By considering only the case where K = g since this is an infinite, perfect cell

* So, the structure factor function can be written

=∑ ^{+ +}

For face-centered cubic (fcc), the coordinates of atoms are

2)

Substituting these values into ri the above equation gives

}

For GaAs materials, the Ga located on the fcc lattice and the As related to it by the basis vector [1/4, 1/4, 1/4]. The expression for F becomes

fcc

The intensity is equal to the square of amplitude (i.e., F²), so the intensity is present

)

The atomic scattering factor f(θ) of Ga and As are given 4.07

3.64 _As

Ga = f =

f

Therefore, the intensity of the 400 reflection is larger than the 200 reflection.

The 200 and 400 reflections are both sensitive to the chemical composition of InAs/GaAs QD structures. However the intensity of the {200} beam is smaller than that of the {400} beam, a typical DP of a QDs layer is shown in Figure 2.4. Consequently, the 400 diffraction condition is chosen to form the DF image in this study.

Figure 2.4 The schematic diagram (left panel) and the TEM image (right panel) of DP for InAs/GaAs QD structures.