Reaction pathways in atomistic models of thin film growth.

Adam L Lloyd,Chris Scott,Miao Yu,Steven D Kenny,Ying Zhou,Roger Smith

doi:10.1063/1.4986402

Abstract

The atomistic processes that form the basis of thin film growth often involve complex multi-atom movements of atoms or groups of atoms on or close to the surface of a substrate. These transitions and their pathways are often difficult to predict in advance. By using an adaptive kinetic Monte Carlo (AKMC) approach, many complex mechanisms can be identified so that the growth processes can be understood and ultimately controlled. Here the AKMC technique is briefly described along with some special adaptions that can speed up the simulations when, for example, the transition barriers are small. Examples are given of such complex processes that occur in different material systems especially for the growth of metals and metallic oxides.

Highlights

Thin solid films can be grown in many different ways but in all cases incident particles move towards a substrate and can either stick to the surface, reflect from it, or implant below the surface
The adaptive kinetic Monte Carlo (AKMC) technique is briefly described along with some special adaptions that can speed up the simulations when, for example, the transition barriers are small
It is these diffusive processes that in many cases are responsible for the final surface topography so the pathways by which atoms or groups of atoms move, i.e., the “reaction” pathways of the system, need to be determined

Summary

INTRODUCTION

Thin solid films can be grown in many different ways but in all cases incident particles move towards a substrate and can either stick to the surface, reflect from it, or implant below the surface. The search regions can become more complex and the time required to calculate a representative set of transitions increases This places a time limitation on the method as the saddle point searches in high dimensional systems are costly to compute and low energy pathways are more difficult to find. In AKMC, besides the way local searches are conducted, the method includes a transition re-use mechanism [similar to that used in the kinetic activation-relaxation technique (KART)10] and implements a mean rate method for dealing with the low barrier problem (discussed in Sec. IV) and an external driver that allows for the inclusion of additional events in the rate tables— such as surface deposition modeled using MD. The saddle point searches can be conducted by a variety of different methods as it was found that the efficiency of these methods was dependent on the system under investigation

THE NEED FOR PATHWAY DETERMINATION IN THIN FILM GROWTH

FILM GROWTH DEPENDENT ON COMPLEX REACTION PATHWAYS

FILM GROWTH WHEN SOME PATHWAYS HAVE SMALL ENERGY BARRIERS

CONCLUSIONS