Theory 008/Physics & Matter /Thin films
Ferroelectricity Dies at a Screening Length
There is a minimum thickness for ferroelectricity. The number may be a property of the electrodes, not the material.
The critical thickness below which ferroelectricity disappears is set by the depolarisation screening length rather than by strain, so a film can be ferroelectric below the accepted limit if the electrodes screen well enough.
01The evidence
Ferroelectricity has been reported to survive below the thickness at which it was thought impossible, with the survival depending on the electrode material.
02The tension
The critical thickness is treated as a material constant and attributed to depolarisation or to strain. The electrode dependence is noted but not incorporated as the controlling variable.
03The mechanism
In a thin film, bound charge at the surfaces creates a depolarising field that opposes polarisation. Electrodes screen that field over a length set by their electronic properties.
If screening length sets the limit, then the critical thickness should scale with the electrode's screening length rather than with strain, and better electrodes should push the limit down.
04What this is not
Strain models make the limit a property of the film. This makes it a property of the interface, which predicts electrode-dependent limits.
05Predictions
- P1Critical thickness should scale with electrode screening length.
- P2Better screening should lower the limit.
- P3The limit should shift with electrode chemistry at fixed strain.
06Refutation
Critical thickness invariant to electrode at fixed strain would restore the material-constant view.
07What it changes
It would make electrode choice a route to ultrathin ferroelectrics.