Imaging and Analysis of Crystal Defects Using Transmission Channeling

Extract

The passage of ions through a crystalline material can be affected by the regular and repeating arrangement of atoms within the crystal structure; this is called ion channeling. Channeling occurs when an ion beam is aligned with a planar or axial direction of a crystal lattice. The particles are shepherded through the crystal by the gentle steering effects of the atomic rows and planes so that they travel in the regions of reduced electron density between the atoms. This leads to a significantly reduced energy-loss rate compared with nonchanneled (or “random”) ions: MeV protons channeled in the {111} and {110} planes of silicon suffer an average energy-loss rate of 45% and 60% of the nonchanneled value, respectively. Channeling also lowers the close-encounter probability between the channeled ions and the crystal's atoms, reducing the rate of ion backscattering and ion-induced x-ray production. Comparison of the yields of these signals between channeled and nonchanneled alignment gives information on crystal quality, epitaxy, and the lattice position of interstitial elements.

Table I gives some useful parameters for 3-MeV protons channeled along axial and planar directions of silicon. The channeling critical angle ψ is a measure of how close in angle a channeled ion must be to an axial or planar direction in order for it to become channeled. This is of the order of 0.3° for ⟨110⟩ directions and 0.1° for the planar directions. Even when an ion beam is aligned to within the critical angle of a channeling direction, not every ion becomes channeled, as some are scattered by close interactions with the first few monolayers of the crystal. The fraction of non-channeled ions is given by the minimum yield X_min and this is ~30% for planar directions and a few percent for axes. Those ions that become channeled eventually leave their privileged trajectories and revert to nonchanneled paths through the crystal. This dechanneling occurs naturally, due to scattering from electrons within the channel and perturbations from the atoms of the channel walls. For planar channeling directions, the fraction of ions remaining channeled decreases exponentially with depth into the crystal and is described by a half-thickness z_1/2, the depth within the crystal at which 50% of the initially channeled ions have become dechanneled. This is of the order of 5 μm for 3-MeV protons channeled in the major planes of silicon. Finally, the energy-loss ratio shows the reduction in energy loss for a channeled ion compared with that of a random ion.