Practical Electron Microscopy and Database

Threading dislocations (TDs) are a prevalent type of crystallographic defect found in semiconductor materials, particularly within heteroepitaxial layers where one material is grown atop another with a differing lattice structure or lattice constant. These defects are line dislocations that result from the disruption of the regular atomic arrangement in the crystal. TDs typically form during the growth of a heteroepitaxial layer due to lattice mismatch between the substrate and the epitaxial layer, which induces strain in the material. To accommodate this strain, dislocations may form, extending vertically through the thickness of the epitaxial layer along the growth direction. These dislocations can exhibit either edge, screw, or mixed character, depending on the Burgers vector. The presence of threading dislocations significantly impacts the material's properties, often serving as scattering centers for carriers, recombination sites for electron-hole pairs, and nucleation points for additional defects. This makes TDs particularly detrimental to the performance of semiconductor devices, including LEDs, lasers, and transistors. Observational techniques such as transmission electron microscopy (TEM) are commonly used to identify and characterize these dislocations, as the contrast they produce allows for detailed analysis of their structure and behavior.

In silicon (Si) technology, the (001) surface (refer to ICsAndMaterials at link) is the most common substrate orientation used in the semiconductor industry. When SiGe/Si heteroepitaxy (refer to ICsAndMaterials at link) is growing on a (001) substrate, the growth direction is typically [001], which is perpendicular to the substrate. TDs tend to be parallel to this growth direction (i.e., along [001]). The (001) substrate orientation supports the formation of well-defined TDs that can propagate vertically through the epitaxial layer.

In electron diffraction, the direction of the electron beam relative to the crystal lattice is crucial for observing and analyzing dislocations. The electron beam direction is typically aligned along a specific crystallographic axis to achieve a diffraction pattern that reveals the dislocation's characteristics. For the observation of dislocations, the electron beam is often aligned along a low-index zone axis of the crystal, such as:

• [001]: This is a common direction in cubic crystals like silicon, where the beam is aligned perpendicular to the (001) plane. This allows for diffraction patterns that highlight dislocations lying in planes like (110) or (111).
• [011] or [111]: These directions are also used depending on the nature of the dislocation and the desired contrast in the diffraction pattern. The [011] direction is particularly useful for observing edge dislocations in the (110) plane, as it lies in the zone axis that allows clear observation of such dislocations.
• In the edge dislocations on the (111) plane with a Burgers vector of 1/2[1-10]), we would typically choose an electron beam direction that is perpendicular to the plane containing the dislocation. For example, if we are observing dislocations in the (111) plane:
  • [110]: This beam direction is often used because it is perpendicular to the (111) plane and lies in the zone axis that is appropriate for visualizing dislocations in the (111) plane.
• To observe a threading edge dislocation on (110) plane, in Si crystal, with a Burgers vector a/2[1-10], the best electron beam direction would typically be along the [001] zone axis because:
  • Geometry of the Dislocation:
    • The dislocation lies in the (110) plane, and the Burgers vector is a/2[1-10]. This indicates that the dislocation is parallel to the [001] direction since threading dislocations are typically aligned along the growth direction, which in this case would be along [001].
      • Understanding the Dislocation:
        • A threading edge dislocation is typically a dislocation that has its line (the direction in which the dislocation extends) perpendicular to the Burgers vector.
        • The Burgers vector, b, is a vector that describes the magnitude and direction of the lattice distortion resulting from the dislocation.
      • Dislocation Line and Burgers Vector Relationship:
        • For an edge dislocation, the dislocation line direction, L, and the Burgers vector, b, are perpendicular:

          ------------------------------------------- [4605a]

      • Finding the Dislocation Line Direction:
        • Since the dislocation is an edge dislocation, and we know the formula 4605a, we can deduce that the dislocation line direction must lie in the (110) plane and be perpendicular to b.
        • The (110) plane contains the directions [1-10] (which is the direction of the Burgers vector) and [001]. Therefore, the direction of the dislocation line must be along [001], because it's the only direction in the ( 110 ) plane that is perpendicular to [1-10].
  • Diffraction Condition:
    • Aligning the electron beam along the [001] direction allows you to view the dislocation edge-on, providing the best contrast for observing the dislocation's characteristics.
    • The (110) plane will be visible, and you can see the dislocation line running parallel to the [001] axis, making it easier to analyze the dislocation structure and its interaction with the surrounding crystal.
  • Visibility of the Burgers Vector:
    • The [ 001 ] zone axis is perpendicular to the (110) plane, meaning the Burgers vector a/2[1-10] lies within the plane that is perpendicular to the beam direction. This orientation will maximize the diffraction contrast for the dislocation.