Growth of step bunches formed by the drift of adatoms

We study power-law growth of step bunches produced by the drift of adatoms. When evaporation of the adatoms is neglected (conserved system), the terra ce width between the bunches increases as L proportional to t(beta) with be ta approximate to 1/2 through a hierarchical bunching. The time dependence of L is not affected by the form of step repulsion. When the adatoms evapor ate to the atmosphere (non-conserved system), the motion of the steps chang es drastically: isolated steps are always present in large terraces, and co llision of the steps with the bunches is repeated. When the drift of adatom s is fast, the terrace width grows by 'effective coalescence' of the bunche s as L proportional to t(1/2), Similarly to the bunching in the conserved s ystem. When the drift of adatoms is slow, the bunches grow by 'bunch size e xchange' with beta smaller than and the terrace width saturates in a late s tage. The exponent beta increases with increasing drift velocity and approa ches beta approximate to 1/2. (C) 1999 Elsevier Science B.V. All rights res erved.