haadf imaging

**HAADF** (High-Angle Annular Dark Field) is a **STEM imaging mode that collects electrons scattered to high angles** — producing images where contrast is approximately proportional to $Z^{1.7}$ (atomic number), providing directly interpretable "Z-contrast" images. **How Does HAADF Work?** - **Detector**: Annular detector collecting electrons scattered to high angles (typically > 50-80 mrad). - **Scattering**: High-angle scattering is dominated by Rutherford (nuclear) scattering, which depends on $Z$. - **Contrast**: Heavy atoms scatter more -> appear brighter. Light atoms scatter less -> appear dimmer. - **Incoherent**: HAADF imaging is largely incoherent, avoiding the complex contrast reversals of coherent TEM. **Why It Matters** - **Directly Interpretable**: Bright spots = heavy atoms. No contrast reversal with focus. The most intuitive electron microscopy mode. - **Interface Analysis**: Clearly reveals interdiffusion, segregation, and abrupt vs. graded interfaces. - **Single-Atom Detection**: Can detect individual heavy dopant atoms (e.g., single Bi atoms in Si). **HAADF** is **see-the-heavy-atoms imaging** — the most intuitive STEM mode where bright means heavy and dark means light.