nanosurface 2008-08-12 08:24
PhD studentship: Physical modelling of storage in quantum dot Flash memories
[b]PhD studentship: Physical modelling of storage in quantum dot Flash memories[/b]
At InESS, University Louis Pasteur and CNRS, Strasbourg (France), Transport at the nanometric scale
Field(s): computational physics, condensed matter, electronics, nanotechnology, quantum mechanics, semiconductors
Application deadline: Sep 15 (Mon), 2008
Submitted: Jul 14, 2008
Contact: Anne-Sophie CORDAN
E-mail: [email]anne-sophie.cordan@ensps.u-strasbg.fr[/email]
Phone: 33 3 90 24 44 25
Address: ENSPS - InESS Bd Sébastien Brant BP 10413 F 67400 ILLKIRCH Cedex (STRASBOURG)
Job description: Quantum Flash memories consist of a floating gate, made of a layer of nanocrystals embedded in the oxide. The advantages of such devices are for example a low heat dissipation, because of the limited number of electrons involved in the storage in the nanocrystals, as well as an improved endurance of the write/erase cycles. The basic physical phenomenon taking place in the storage process is the tunnel effect through an oxide thickness.
However in spite of the studies already undertaken on this topic, these memories are far from being perfectly controlled, both experimentally and theoretically. This is why the object of this thesis is to answer – by modeling and simulation – certain still unknown and yet fundamental points of physics, in close cooperation with teams of experimentalists. The team "Modeling of transport on a nanoscopic scale" currently has a model assuming a perfect oxide and the storage of only one electron per nanocrystal.
Nevertheless since it is not impossible that a second electron can be stored, the excited states with two electrons need to be introduced, first of all for one nanocrystal only and then for the whole layer of the floating gate. Indeed, the charging of an electron is more probable towards an excited level of the nanocrystal rather than towards the fundamental state. In addition, the charging process is perhaps not only due to the direct tunnel effect between the channel semiconductor and the nanocrystal: the most probable additional process would imply the defects found in the oxide. The latter will have to be included in the model in order to evaluate their influence on the process of charging/discharging of the memory.
The new model, like the existing one, will have of course to take into account the dispersion of the nanocrystal sizes and positions. In addition, various materials will be tested (Si, Ge or SiGe for nanocrystals, various oxides with strong permittivities for the insulator).
This work will be led, in addition to certain analytical stages, using a finite element software supplemented with a script language. In order to model situations as realistic as possible and then to compare the theoretical and experimental results, it will be carried out in permanent relation with teams of experimentalists within InESS, and in other laboratories (CEMES and LPCNO in Toulouse).
Electrical characteristics (write time, retention time) will be obtained for various combinations of geometries and materials in order to evaluate the efficiency of the memory and to help to optimize the experimental devices. The model thus built will allow a better comprehension of the working of the quantum flash memories for possible industrial applications.
Necessary knowledge and competences :
The candidate must have a good grounding in Quantum Mechanics as well as in Condensed Matter and Semiconductor Physics. Competence in numerical modelling techniques would also be desirable.
CONDITIONS: this grant concerns European candidates or coming from the other following countries (Albania, the principality of Andorra, Armenia, Azerbaijan, Bosnia and Herzegovina, Croatia, the Russian Federation, Georgia, Iceland, Liechtenstein, the Macedonia, Moldavia, Norway, Serbia-Montenegro, Switzerland, Turkey, Ukraine, the State of Vatican)