Wavy and rough cracks in silicon

Single-crystal silicon exhibits a strong preference to cleave along a few certain planes, but in experiments we observe wavy cracks with almost no evidence of a preferred fracture direction. Furthermore, we find that the fracture surface is an anisotropic and self-affine fractal over five decades in length scale in the direction of the crack with a roughness exponent of 0.78. In our experiments a 1–4 cm wide strip of single-crystal silicon is heated to $378°\mathrm{C}$ and lowered into a 20 °C water bath at speeds of 0.2–5 cm/s. The thermal gradient produces cracks that, depending on the speed, are straight, wavy with amplitude 0.1–0.5 cm and wavelength 0.3–1 cm, or multibranched. The transition from one mode of fracture to another is discontinuous and hysteretic.