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