| Abstract:
Impurities that impede the motion of surface steps typically produce step-bunching instabilities during vapor growth, and the slower-moving step bunches reduce the growth rate. However Land, et. al [1] showed that during solution growth of KDP crystals impurities can induce dramatically different behavior. In particular under appropriate conditions as the driving force is increased, single isolated steps seem to be blocked by impurities but larger coherent bunches of steps emerge that begin to move and dominate the subsequent crystal growth.
Building on previous work [2], we produce a new and general model of impurity effects during crystal growth that can explain these and other regimes. We use a effective description of main component crystal growth, where dynamics is driven by chemical potential differences arising from the nonequilibrium driving force and step line tension and step repulsion effects. Impurities at step edges reduce the local driving force and can induce step bunching, initially slowing down growth in all regimes. However, the subsequent behavior can be very different in vapor and solution growth because of the different mechanisms of mass transport. Unlike vapor growth, in solution growth the attachment rate at a step is essentially independent of the terrace width in front. This allows a coherent bunch to stay together and move at large driving forces and we argue this is the key feature needed to understand the experiments of Land et. al.
Joint work with Madhav Ranganathan,
Department of Chemistry,
Indian Institute of Technology,
Kampur, India.
This work was supported by the NSF-MRSEC at the University of Maryland, DMR 0520471.
References:
[1] T. N. Thomas,T. A. Land, W. H. Casey, and J. J. DeYoreo, "Emergence of supersteps on KH2PO4 crystal surfaces", Phys. Rev. Lett.
92, 216103/1-4 (2004)
[2] M. Ranganathan and J. D. Weeks, "Theory of impurity induced step pinning and recovery in crystal growth from solutions" in Recent advances in thermodynamic research including nonequilibrium
thermodynamics: Proceedings of the 3rd National conference on thermodynamics of chemical and biological systems, Nagpur India (2008) |
