Simultaneous Bunching and Debunching of Surface Steps: Theory and Relation to Experiments

We study a model of two-dimensional step flow where the velocity of a step depends predominantly on the width of the terrace remaining behind it. While the uniform step train is unstable towards step bunching, the bunches themselves are unstable and tend to debunch. This leads to patterns where slow moving fairly straight bunches coexist with fast, strongly bent single steps, in qualitative agreement with experiments on electric current driven step motion on Si(111) surfaces. Analytical predictions of the shape and velocity of the single steps agree very well with Monte Carlo simulations.