Atomically defined tips in scanning probe microscopy - McGill Physics

2Theoretical background2.1 Growth of ultra-thin insulating filmsGrowth of well-ordered ultra-thin 1 insulating films on crystalline substratesis of key importance for electronics, and is crucial to numerous areas of fundamentalresearch, such as friction and catalysis. On crystalline substrates, wellorderedultra-thin films of high quality can be fabricated by epitaxial growth,where the structure of the substrate determines the orientation of the depositedfilm. This section describes the fundamental physical processes taking place duringthe epitaxial growth of ultra-thin films in ultra-high vacuum (UHV).2.1.1 Nanoscale processes during growthFrom the experimental point of view, in ultra-high vacuum materials arecommonly deposited onto surfaces by thermal evaporation. Heating of materialsin UHV increases their vapor pressure, creating a flux of particles that reach asurface. Usually special evaporators, so-called Knudsen effusion cells, are used toform a homogeneous flux of evaporated material over a sufficiently large area. TheKnudsen design consists of a uniformly heated crucible acting as an isothermalenclosure with a small aperture opening. When the free path of particles is largerthan the size of the aperture, the Knudsen cell generates a flux with cosine angularintensity distribution, which can be collimated to form a uniform beam. Dependingon the deposited material, the particles arrive at the surface in a form of either1. A precise definition of an ultra-thin film is not easy, because there is no simple way todistinguish this category from the large class of thin films. From a practical point of view, onetends to classify films of thickness below 100 nm as ultra-thin films, as in this range structures andproperties of thin films start to differ from the corresponding bulk counterparts.7