7003全讯入口“博约学术论坛”系列报告
第 95 期     ( 2016年第 23期)

Title：Electromigration on Stepped Surfaces
报告人：Dr. Cormac Ó Coileáin (Trinity College Dublin)
时  间：2016年12月5日(周一)上午10:00
地  点：良乡理学楼C楼二楼报告厅
ABSTRACT
The influence of electric fields on the dynamics of steps, on vicinal surfaces, during high temperature annealing in ultra-high vacuum is examined. An innovative experimental method using custom setup was used to explicitly isolate the electromigration and thermal effects in the dynamics of the step-bunching process on the vicinal Si(111) surface.
Using atomic force microscopy to characterise and analyse the morphologies of stepped Si(111) annealed at 1130 °C and 1270 °C, it was found a reduction of the electric field results in a significant expansion of step-bunch width and an elongation of the crossing steps running along the terraces. The morphologies of step-bunches produced by annealing at 1130 °C (step-up electric field) and 1270 °C (step-down electric field), are usually described by the generalised BCF and transparent step models, respectively. These models predict different scaling relationships of the form ym ∝ hαEq, between the maximum slope of a step-bunch ym, step bunch height h and electromigration field E.  Experimentally extracted scaling exponents, α ≈2/3 and q ≈1/3, for Regime III (1270 °C) show good agreement with theoretical predictions. However, the scaling exponents α ≈ 3/5 and q ≈1/3 extracted from the morphologies created in the Regime II (1130 °C), were found to differ from those deduced from the transparent step model.
We also examine other morphological differences, such as the distinct difference observed in how the (1×1) to (7×7) phase transition manifests itself on vicinal Si(111) surfaces off-cut in the [11-2] and [-1-12] directions.  This method also allows for the first time the value of the critical electric field (Ecr), required to induce the step-bunching instability, to be probed. The dependence of Ecr on the mean initial inter-step distance (l) is investigated and discussed. Extended annealing times results in the anti-band surface instability. Using the initial or onset stage of antiband formation on step-bunched surfaces was examined at 1270 ◦C while systematically varying the E-field it was possible to estimate the adatom effective charge (qeff).  The effect of varying the initial inter-step distance on the onset of the antiband instability is also discussed and experimentally examined.
Finally we consider electromigration induced step-bunching on other surfaces, the behaviour was observed experimentally for the first time on an insulating oxide surface on vicinal C-plane α-Al2O3. Surface faceting cannot account for the surface structures created by annealing in the presence of an applied electric field at 1500 °C. The effect of electric field direction and strength on the surface morphology are investigated and comparisons are drawn with step-bunching on Si(111)..

Curriculum Vitae
Dr. Cormac Ó Coileáin received his bachelor, master and PhD degree from Trinity college, Queen’s University Belfast, and Trinity College Dublin, respectively. He is now the postdoc research fellow in Trinity College Dublin and King Saud University.  Despite beginning in Theoretical Physics, he has become an avid and flexible experimentalist with particular interests in surface science and nanotechnology, with extensive experience in ultra-high vacuum design and characterisation.  Since my initial work during his PhD, the dynamics of step flow and step bunching, particularly on silicon has been a long standing topic interest.  He has published more than 20 scientific research articles in peer reviewed international high impact journals.

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