Interplay of impurities and solution flow as determinants of step pattern dynamics.
The first theory of step pattern evolution, the kinematic wave theory, employed the assumption of impurity effects on step kinetics. On the other hand, recent results have been considered within a framework linking step patterns to the mutual orientation of the solution flow and step motion directions in arbitrarily pure solutions. We explore the consequences of combining impurity and solution flow effects on the dynamics of the surface morphology of the (101) face of potassium dihydrogen phosphate (KDP) crystals. We employ phase-shifting interferometry for real time in situ monitoring of these dynamics. We find that, at solution supersaturations sigma</=0.035, step bunches form on all three vicinals of the (101) face regardless of the mutual orientation of the step motion and solution flow directions. Testing the mechanism of impurity-step pattern interactions, we show that bunching is caused by impurity molecules that adsorb on the surface and slow down and destabilize step trains without inducing growth cessation, i.e., the mechanism is inherently different from the one established for the (100) KDP face. We show that at sigma>0.040 impurities do not affect step bunching, and it is controlled by the direction of the solution flow, i.e., two distinct regimes of step bunching exist. The transition between the two regimes is governed by the exposure times of the terraces between steps tau : shorter tau's at higher growth rates lead to lower surface concentration of impurities and suppress the impurity effects on step kinetics and bunching.
Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4004, USA.
Nicholas A
NA
Booth
Alexander A
AA
Chernov
Peter G
PG
Vekilov