Self-assembled Si quantum-ring structures on a Si(100) substrate are fabricated by using a PECVD technique based on a growth-etching competition mechanism. The as-grown Si ring structures have superior morphology, excellent rotational symmetry, and ultrathin edge width (down to 10 nm, see figure). This growth model also represents a general scheme for controlling and tailoring the shape, size, and complexity of self-assembled nanostructures.