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

BH34 Course B: Materials for Devices BH34
Applications of Ferroelectrics
Ferroelectrics are used in many commercial devices simply for their pyro- or piezo- electrical
properties, or just as dielectrics: perovskite ferroelectrics have high κ values ⇒ BaTiO 3
capacitors,
e.g. for camera flashes (> 50% of the ceramic capacitor market).
FE was discovered in 1921, and it’s always been obvious that the +P and –P states could be used to
encode 0 and 1 for computing, but only relatively recently (~1995) has this become a reality.
⇒ memory devices: FE-RAM - non-volatile, low voltage, small size & cost, fast, radiation hard.
Materials requirements: high κ ; high P sat & P r ; (relatively) small E C ; high T C
e.g. LiNbO 3
, PbTiO 3
, Pb(Zr x
Ti 1-x
)O 3
(PZT), SrBi 2
Ta 2
O 9
(SBT), fabricated as thin films
(~ a few hundred nm) in order to minimise switching voltage.
Wide-scale applications (cell phones, smart cards, video games) depend upon materials science:
optimisation of composition, microstructure, interface quality with electrodes, control of nano-scale
defects that pin domain walls.
FE switching in a thin film memory device element:
Thin Film, e.g. ~10 – 300 nm
Start with a single-domain polarisation state [0]
Reverse field ↓
Nucleation of the opposite polarisation
initiated at the surfaces
Forward growth of needle-like domains
parallel to the applied field
Sideways growth of domains
Reversed polarisation state [1]
(switching takes ~ 50 ns)