Lecture handout including QS - Department of Materials Science ...

BH25 Course B: Materials for Devices BH25
Examples of dielectric materials:
Low dielectric constant, κ:
vacuum; dry air; many pure, dry gases
↓ many ceramics; paper; mica; polyethylene; glass; distilled water
High κ: some metal oxides, e.g. BaTiO 3
P
For a linear dielectric: (may get dielectric breakdown
at very high field, e.g. due to
E
ionisation & current flow)
Dipole formation & Symmetry
If a crystal structure has an inversion centre, or centre of symmetry, then it is centrosymmetric (see
AH62). Visualising centrosymmetric structures:
z
x
y
Every component must exist, reflected through the centre of symmetry.
A unique direction in a crystal structure is a lattice vector which is not repeated by the symmetry
present (see AH67). In centrosymmetric crystals there can be no unique direction (and hence no
possibility of a resultant electric dipole moment without the presence of an electric field).
Non-centrosymmetric structures do not necessarily include a unique direction, e.g.:
ZnS - sphalerite, or zinc blende
Cubic F, with 1 / 2 tetrahedral interstices filled
(see Data Book & AH31)
This is a non-centrosymmetric structure; but with
no unique direction.
1 / 2 1 / 3 4 / 4
1 / 1 2 / 2
3 / 1 4 / 4
1 / 2
o S
• Zn
Note, however, that the directions are asymmetric: [111] is not equivalent to ⎡⎡
⎣⎣111⎤⎤
⎦⎦ ; and
⎡⎡111 ⎣⎣
⎤⎤
⎦⎦ is not equivalent to ⎡⎡
⎣⎣111
⎤⎤
⎦⎦ ; etc.