Email: i.mitchell@uwo.ca
Office:
Phone:
Fax:Room 100, Physics and Astronomy Bldg
(519) 661-3393
(519) 850-2422Professor Ian Mitchell
Department of Physics and Astronomy
Physics and Astronomy Building
University of Western Ontario
London, Ontario
N6A 3K7
Canada
Education:
BSc. Physics, University of Adelaide , 1959
PhD. Physics, Australian National University , 1964
Current Interests:
When fast particles from an accelerator are directed into any solid target, they individually slow down by energy losses to two major processes, one associated with interactions with electrons (excitation, ionization and collective phenomena) the other with billiard ball type collisions with the atoms of which the solid is formed. If enough energy is transferred in the latter type of collision, the struck atom may be ejected from its site. In a crystalline lattice, this process gives rise to a lattice site vacancy and an energetic recoiling atom which may in turn trigger a cascade of displacements, leading to a temporary population of the homeless.
Depending on the system under study and its temperature, many of the vacancies and interstitials formed in this manner may disappear by pairwise recombination but the norm in semiconductor crystals at room temperature is that a complex reservoir of point defects remains. The location and spatial extent of the reservoir, also the concentration of defects, are sensitive to the choice of incoming particle type and particle energy. We have great flexibility in this respect, since the UWO Tandetron accelerator can supply particle beams of almost any element in the periodic table and over an energy range from tens of keV to many MeV.
The defect residues are interesting objects in their own right but in our laboratories, as in a number of accelerator laboratories around the world, defect residues are proving to be extremely useful as a source of mobile point defects, released as a pulse i.e. a flux transient during a rapid heating of the sample following particle irradiation. For a very short time and for some distance from the defect source, the transient drives the instantaneous local concentration of point defects to values that exceed the equilibrium values by many orders of magnitude. In consequence, we have a powerful tool for exploring processes such as activation, diffusion and clustering of lattice impurities that are sensitive to the presence of defects.
Currently we are using this method to examine the anomalous movement and activation of boron, technologically the most important of the dopant species used for altering the conductivity type for silicon devices. In an analogous set of studies we are using fast particle irradiation to modify the compositional profiles across the boundaries between well and barrier components of extremely narrow (e.g. 10 nm and less) structures - so-called quantum well structures - with a view to both understanding and capitalizing on the associated changes in the wavelengths of optical emission. If such engineering of the quantum wells can be controlled, there is a direct route to integrated optoelectronic circuitry. A large and very successful collaborative research program with scientists at the Institute for Microstructural Sciences at NRCC , Ottawa is now entering its fifth year.
