Influence of interfacial stress on the stability of epitaxial films

Washington University, Saint Louis MO

Investigators

Abstract

Fried 0706460 Epitaxially-grown thin alloy films are essential components in many nano- and microscale devices used in the electronics and other industries. Unfortunately, the growth of planar strained films is susceptible to instabilities. These instabilities are often manifested by the formation of surface ripples, surface cusps, and island-like mounds such as quantum dots and quantum wires. The demand for films with precisely tailored micro- and nanostructures has led to an extensive literature concerned with morphological instabilities during the epitaxy of thin alloy films. To date, attention has focused primarily on instabilities that arise from a mismatch in lattice parameters between the film and the substrate and surface diffusion. However, interfacial stress, which may strongly influence the formation of surface patterns, has received almost no attention. Moreover, because of the geometric complications associated with the description of evolving surfaces in three space dimensions, studies of growth-stress interactions have thus far confined attention to two-dimensional theories, wherein the interface is a curve. Additionally, no theory yet accounts for the influence of surface stress on phase segregation and related pattern formation, processes that occur during epitaxy. In this project, the investigator and colleagues: 1. Perform stability analyses and numerical simulations designed to explore the role of surface stress in the stability of epitaxially growing films. 2. Extend the existing two-dimensional theory to three space dimensions. 3. Develop a theory that describes the interaction of growth and phase segregation during the epitaxy of alloys. The investigator and his colleagues develop and analyze mathematical models, and carry out numerical simulations, of the behavior and characteristics of epitaxially grown thin films. An increased understanding of the morphological stability of these films allows for progress and innovation in a variety of strategically important applications. For instance, in the electronics industry, epitaxially grown multi-layers have been used to increase the storage capacity of hard drives and are now being studied for possible improvements. Progress rests directly on the ability to control small-scale features that form during growth. Another compelling example of the importance of epitaxial films is provided by recently proposed hybrid organic/inorganic devices capable of detecting airborne contaminants. These devices combine a monolayer of sensing molecules on a semiconductor transducer grown via molecular beam epitaxy. A precisely tailored surface structure is necessary to ensure that the sensing molecules adhere correctly to the transducer and perform reliably. Epitaxial films also have potential in the development of optical semiconductors for applications such as variable wavelength lasers. The devices have a wide range of potential applications, including the detection of toxic gases from vehicle exhaust, factories, accidents, fires, natural disasters, etc. With a variable wavelength laser, it is possible to shorten measurement times from 1-2 minutes to 1 second, allowing for more rapid containment and evacuation during emergencies.