Structures obtained from resolidification of flame-melted single-crystal germanium

When single-crystal germanium is partially melted with a gas flame in air, resolidification produces novel structures which may be useful for nanoscale fabrication. These features are dominated by a single, sharp cone, up to 5 mm long with a typical aspect ratio of 1.6. If impurity concentrations are sufficiently high, one or more micron-sized, impurity-rich spheres are formed at the tip of the cone. The surface displays a distinctive morphology which depends strongly on the chemical environment. After slow resolidification in air, the cone body and spheres are coated with fractal-like aggregates of spherical oxide particles. More rapid resolidification in laser-melted Ge can produce sharp (100 nm radius of curvature), clean cones. The observed features are attributed to dynamic resolidification, impurity segregation, and diffusion-limited aggregation processes.