Films minces à base de Si nanostructuré pour des cellules ...

a direct bandgap semiconductor [Gnutzmann 74] and increases the probability of
radiative recombinations. Thus, QCE leads to strong variations in the optical and
electronic properties of a material.
tel-00916300, version 1 - 10 Dec 2013
Figure 1.9: (a) The consequence of Quantum Connement Eect, and (b) the density of
states of structures conned in 1D, 2D and 3D.
The connement of excitons in a semiconductor in one, two or three spatial dimensions
are termed as quantum wells, quantum wires or quantum dots respectively
(Fig. 1.9b).
1.4.2 Silicon nanostructures
The quantum connement eects (QCE) in three dimensionally conned semiconducting
crystals was identied in the early 1980s [Ekimov 81]. About the same
time investigations began on the Si nanostructures conned along one direction
(quantum wells) and QCE were witnessed [Abeles 83, Ibaraki 84, LeComber 85].
Quantum well refers to a structure in which a low bandgap material is sandwiched
between layers of higher bandgap which act as potential barriers. If the higher
bandgap layer is suciently thick, it provides a high potential barrier and each well
is electronically isolated without the overlap of electronic wavefunctions. Such alternated
wells and barriers are collectively referred as multiple quantum wells. On
the other hand, if the barrier thickness is suciently low, there is an overlap of
wavefunctions and ecient charge transport is possible normal to the layers. These
structures behave like direct bandgap material and are termed superlattices. In general,
it was reported if the barrier thickness is 4 nm or below, miniband formation
occurs leading to inter-well electronic coupling in a superlattice [Peterson 88].
A deep investigation on the QCE in Si and its applicability towards optoelectronic
devices began in the 1990s stimulated by the discovery of size dependent
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