Selective Energy Contacts
The property of a narrow selected energy transmission
range, dE, is a key requirement in choosing a suitable contact
structure. dE should be kept small to approach the entropic ideal,
but remain large enough to support the appropriate rate of carrier
extraction. Other properties of possibly critical interest are
the ease of manufacture of the structures and, for the ‘hot
lattice’ approach (see Hot Lattice), the thermal insulation.
Figure 1: Resonant tunnelling through a single
discrete level in a double barrier at close to zero bias.
Quantum mechanical tunnelling structures seem most
likely to satisfy the first of these requirements with resonant
tunnelling as the most appropriate. Resonance, in this context,
refers to a high level of electron transmission within the range
of resonant energies with low transmission outside this range.
The quality of resonance is evidenced by the narrowness of the
transmission range. This offers the possibility of very good energy
selection.
The simplest tunnelling structure is a single barrier.
This does not have any energy selection properties as the tunnelling
probability increases monotonically with carrier energy. Figure
1 shows a structure which does have the desired resonant property.
The device is shown at close to zero bias, as this is the optimal
condition for operation of a selective energy contact. A strong
resonance in transmission occurs at an energy corresponding to
that of the discrete level, Es , within the barrier
region. The allowed discrete level can be created as a quantised
level in a quantum well (QW), a quantum dot (QD) or, most simply,
as a defect level.
However, any QW structure suffers from an inherent
problem. QW energy levels are only confined in the longitudinal
direction, that perpendicular to the plane of the QW. In the plane
of the well, the normal band structure of the bulk material dominates
and there are allowed states at all energies above the first confined
level. This means that hot carriers at the resonant energy entering
the structure at a perfect right angle to this plane will be transmitted
effectively. However, other carriers with longitudinal energy at
the resonant energy but also with any additional component of momentum
in either transverse direction can also be transmitted, even though
total energy falls outside the selective energy window. Thus good
energy selection is lost and furthermore the energy of hot carriers
is squandered in phonon interactions in the plane of the contacting
structure.
Figure 2: Impurity defects randomly spaced
in a barrier material. The defect potential is the sum of the
atomic and barrier potentials. Tunnelling probability is strongly
peaked for states that happen to be in a resonant position.
Total energy confinement can be achieved in the
quantum dot version of Fig.1, with the QW replaced by a plane
of quantum dots, QDs. Such structures are being fabricated as
part of the Centre's work on silicon nanostructures. These are
being characterised with respect to QD size, uniformity and density
with TEM, SPM, Raman and photo-luminescence. Further experiments
are being carried out on electronic transport with respect to
carrier energy. These will lead to optimisation of the structures
for narrow selective energy ranges.
An alternative method for achieving total energy
confinement in a structure such as that in Fig.1 is to have multiple
defects or impurities in a barrier oxide, Fig. 2. The tunnelling
probability is then strongly peaked for states in a resonant position.
Hence resonance will automatically be greatest for the defects
that happen to be in this resonant position. (For the symmetrical
case this will be such that the barrier widths are equal, i.e.
for states physically near the centre of the barrier.) This makes
such a device conceptually easy to fabricate with an MIM or MOS
type of structure with defects randomly spaced in the insulator,
providing defect density is high enough and there is even dispersion.
1D tunnelling calculations for the structure in Fig.
2 have been carried out. These will be extended to model the 3D
situation of a real device and matched to empirical data. This
will yield a simple predictive model for defect potentials.
return to Hot Carrier
Cells
Hot Carrier
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