Materials World Network: Interplay Between Quantum Size Effect and Strain Effect on Growth of Nanoscle Metal Thin Films

在异质外延生长中，当一种薄膜材料生长在不同晶格尺寸的衬底材料上时，应变效应是普遍存在的。另一方面，在半导体衬底上的金属薄膜的生长中，当金属膜非常薄（几纳米厚）时，量子尺寸效应（QSE）效应成为主导。然而，在过去，这两个重要的影响被单独或分开研究，尽管事实上它们在许多情况下都存在。该材料世界网络（MWN）项目启动了美国和中国之间的国际合作，首次对这两种效应进行系统研究，以揭示它们在影响半导体衬底上金属薄膜生长热力学和动力学方面的基本关系和相互作用。该研究将极大地扩展我们的知识基础，以控制金属纳米薄膜和纳米结构的生长形态。该MWN项目包括一个理论研究，探讨量子尺寸效应（QSE）和应变效应对纳米厚金属薄膜生长的影响，并通过清华大学在中国北京的国际合作进行了相同主题的协同实验研究。本文主要以Si衬底上Pb薄膜的生长为模型系统，试图回答三个基本问题：（1）QSE与应变效应之间的关系是什么？(2)这两种效应是如何相互作用影响金属薄膜的热力学稳定性，特别是改变表面能及其厚度依赖性的？(3)这两种效应将如何共同作用以影响金属薄膜的生长动力学，例如通过共同改变表面扩散能垒？该研究将为QSE和应变效应的理解带来新的见解，不仅是单独的，而且是整体的，并为一个新的研究方向奠定了基础，预计将产生广泛的兴趣。该项目的一个特殊成果是通过中美之间的国际学生交流项目，不仅在基础研究方面，而且在全球视野方面对研究生进行教育和培训。

The strain effect is ubiquitous in heteroepitaxial growth when one kind of thin film material is grown on a substrate material of different lattice size. On the other hand, in the growth of metal thin films on semiconductor substrates, the quantum size effect (QSE) effect becomes dominant when the metal film is very thin of a few nanometers thick. In the past, however, these two important effects have been studied alone or separately despite the fact they are both present in many cases. This Materials World Network (MWN) project launches an international collaboration between US and China to carry out the first systematic study of both these effects to reveal their fundamental relationship and interplay on affecting the growth thermodynamics and kinetics of metal thin films on semiconductor substrates. The study will greatly expand our knowledge base to control the growth morphology of metal nanofilms and nanostructures.This MWN project encompasses a theoretical study to investigate the quantum size effect (QSE) and strain effect on growth of nanometer-thick metal thin films, augmented by an international collaboration of synergistic experimental study on the same topic at Tsinghua University in Beijing, China. Using primarily the growth of Pb thin films on Si substrate as a model system, the proposed research aims to answer three fundamental questions: (1) What is the relationship between the QSE and strain effect? (2) How these two effects will interplay with each other to affect the thermodynamic stability of metal thin films, particularly changing the surface energy and its thickness dependence? (3) How these two effects will act together to affect the growth kinetics of metal thin films, e.g. by changing together the surface diffusion energy barrier? The study will bring out new insights to the understanding of the QSE and strain effect not only individually but also as a whole, and lay the groundwork for a new research direction that is expected to generate broad interest. A special outcome of this project is the education and training of graduate students not only in fundamental research but also with a global perspective through international student exchange program between US and China.