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Interplay of impurities and solution flow as determinants of step pattern dynamics
Nicholas A Booth1, Alexander A Chernov, Peter G Vekilov
1Department of Chemical Engineering, University of Houston, Houston, Texas 77204-4004, USA.
Summary
Impurities influence crystal growth patterns by affecting step kinetics, leading to step bunching. Two distinct regimes of step bunching exist, controlled by solution flow and impurity concentration, especially at higher growth rates.
Area of Science:
- Crystallography
- Surface Science
- Materials Science
Background:
- Previous theories on step pattern evolution focused on impurity effects or solution flow independently.
- Recent studies linked step patterns to the orientation of solution flow and step motion in pure solutions.
Purpose of the Study:
- To investigate the combined effects of impurities and solution flow on the surface morphology dynamics of potassium dihydrogen phosphate (KDP) (101) face.
- To differentiate the impurity-induced step bunching mechanism on the (101) KDP face from that on the (100) KDP face.
Main Methods:
- In situ real-time monitoring using phase-shifting interferometry.
- Controlled variation of solution supersaturation and observation of step bunching phenomena.
Main Results:
- At low supersaturations (sigma ≤ 0.035), step bunching occurs irrespective of flow direction, driven by impurity adsorption that slows and destabilizes steps.
- At higher supersaturations (sigma > 0.040), impurities do not influence step bunching, which is then governed by solution flow direction, indicating two distinct regimes.
- The transition between regimes depends on terrace exposure times (tau), with shorter tau (higher growth rates) reducing impurity effects.
Conclusions:
- Impurity-step interactions on the KDP (101) face differ from the (100) face, with impurities destabilizing steps without causing growth cessation.
- Two distinct step bunching regimes exist: one dominated by impurities at low supersaturation and another by solution flow at high supersaturation.
- Growth rate and terrace width influence impurity adsorption and thus control the transition between these regimes.