ON THE ELECTRON-MICROSCOPE CONTRAST OF DOPED SEMICONDUCTOR LAYERS

The origin of the electron microscope contrast of doped semiconductor layers viewed in cross-section has been investigated. It has been demonstrated that the built-in elastic displacements associated with misfitting dopant atoms is the principal source of electron scattering contrast when imaging layers of relatively low dopant concentration. In the absence of atomic misfit, any observed contrast must be associated with the difference in atomic scattering factors of the dopant and matrix atoms. Diffraction contrast experiments on substitutionally doped B-Si multilayers indicate an increase in the two-beam normal and anomalous absorption coefficients and extinction distance which are quantitatively shown to be a manifestation of the relatively large atomic misfit between B and Si atoms. Furthermore, the image contrast is a sensitive function of the configuration of the dopant atoms; a reduction in the dopant atom misfit strain via clustering or precipitation will result in lower contrast levels as shown for B-doped Si layers grown at higher temperatures. Similar diffraction contrast experiments were carried out on substitutionally doped As-Si layers which failed to reveal any significant diffraction contrast owing to the minimal atomic misfit between As and Si atoms; any resultant contrast in this case is due to the larger atomic number of As atoms in the Si matrix.