Electron distributions tunneling

A wavefunction ip and its eigenvalue E define an orbital. The orbital is therefore an energy level available for electrons and it implies the relevant electron distribution. In mathematical models, these distributions extend to infinity, but in a pictorial representation it is sufficient to draw the volume in which the probability of presence of the electron is rather arbitrarily around 90%. The spatial distribution of atomic and molecular orbitals have implications for processes of electron tunneling (section 4.2.1). 

At these temperatures the distribution of occupied levels in the conduction bands ( the Fermi distributions ) in the two metal electrodes ( Fig.l ) are quite sharp, with a boundary between filled and empty states ( the Fermi level ) of characteristic width k T ( k =0.08617 meV/K=0.69503 cm Vk ). An applied bias voltage V between the two electrodes separates the Fermi levels by an energy eV. If the barrier oxide is sufficiently thin electrons can tunnel from one electrode to the other. This process is called tunneling since the electrons go through a potential barrier, rather than being excited over it. The barrier must be thin for an appreciable barrier to flow. For a typical 2 eV barrier the junction resistance is proportional to, where s is the barrier width in Angstroms (17). The... 

Quantum mechanics offers a more realistic description of this behavior. At the surface of the metal, a potential barrier retains the electrons. However, there is a certain probability that electrons tunnel through the barrier and thus can exist in the immediate vicinity of the surface. The electron distribution outside the solid depends on, among other parameters, the electronic properties of the solid and the characteristics of the potential barrier. 

That the description of electrons as waves is not simply a mathematical construct but is visibly real can be demonstrated by using a device called a scanning tunneling microscope (STM). This instrument allows the mapping of electron distributions in molecules at the atomic level. The picture shows an orbital image of tetracyanoethene deposited on a Ag surface, taken at 7 K. [Picture courtesy of Dr. Daniel Wegner, University of Munster, and Professor Michael F. Crommie, University of California at Berkeley]... 

One important sem source that is not based on thermionic emission is the field emission (fe) source. Fe-sem systems typically give images of much higher resolution than conventional sems due to the much narrower energy distribution (on the order of 0.25 eV) of the primary electron beam. A fe source is a pointed W tip from which electrons tunnel under the influence of a large electric field. This different mechanism of electron generation also results in a brightness comparable to a conventional thermionic source with much less current. 

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