Applied Materials and Surface Science Laboratory
Silicon remains the dominant material system in many technologies, including micro/nanoelectroics and micro/nanoelectromechanical systems (M/NEMS). However, our capabilities are enhanced if we can effectively expand the materials base beyond that of silicon. Some of the material systems we are pursuing include silicon carbide (SiC), diamond, graphene and one- and zero-dimensional nanostructures. These are briefly described below.
Silicon carbide is an exciting semiconductor material with a wide range of outstanding properties, such as wide energy band gap, high thermal conductivity and extremely stable physicochemical properties. These properties make SiC an ideal choice for high power and high voltage electrical or electromechanical devices that operate at high temperatures and/or in harsh environments. Our group has developed a lower temperature (750°C) process for depositing polycrystalline SiC with control of doping and film stress, along with dry etching techniques that are highly selective to SiC versus SiO2 and silicon nitride.
Graphene is a single layer of carbon arranged in a honeycomb lattice. Common graphite is made of a staking of individual graphene layers. As a pure 2D material, graphene has sparked wide ranging interest since its first isolation few years ago, due to its zero effective mass, extremely high electron and hole mobilities, high chemical and thermal stability, high thermal conductivity, extreme sensitivity to single-molecule adsorption and other unique characteristics. However, reliable production of single- or few-layers graphene films with accurate thickness control remains quite difficult, and under many conditions it completely lacks an energy band gap that is crucial to many device functions. In our effort, we focus on the growth of epitaxial graphene on SiC surfaces by Si sublimation during high temperature annealings. Although epitaxial graphene shares many important features with suspended graphene obtained by exfolation, the presence of the SiC substrate induces new and exciting properties, many of which still not completely understood. Among these a possible opening an energy bandgap. We are currently investigating the mechanical and thermal properties of epitaxial graphene (using Raman spectroscopy, low-energy electron diffraction and microscopy, Auger electron spectroscopy, atomic force microscopy, scanning electron microscopy). We recently discovered that mechanical strain in epigraphene can be controlled by the annealing time, with important consequences on the surface morphology which may lead to important consequences in the electronic and atomic structure of the graphene film.
Materials with nanoscale structure such as nanowires offer unprecedented applications and device capabilities, ranging from ultrasensitive sensors to improved energy conversion processes. However, these nanostructures bring about unprecedented challenges in processing. Silicon remains the dominant material for MEMS and will likely continue into the realm of NEMS where the extremely small feature sizes enable development of ultra-high frequency (~1 GHz) resonators with extremely small effective masses, giving incredible force or mass sensitivity (possibly single molecule). Fabricating such structures with existing technology is very difficult, so to circumvent this issue we have utilized a combined top down/bottom up approach of integrating VLS grown silicon nanowires on lithographically patterned substrates. To date we have shown selective area nanowire growth by controlled deposition of Au catalyst clusters by galvanic displacement, and bridging nanowires in microfabricated trenches that exhibit excellent electrical and mechanical contact.
Beyond silicon nanowires we have also collaborated with the Fearing lab to create polymeric nanohair dry adhesives inspired by mechanism responsible the amazing adhesion of geckos onto various surfaces. Thorough study of the mechanism allows us to tailor the process of nanohair array fabrication in order to design novel adhesives which are self-cleaning, reusable, long-lived, intrinsically biocompatible, and stick to just about any surface with controlled attach/detach forces.
