Phonons can scatter through several mechanisms as they travel through the material. These scattering mechanisms are: Umklapp phonon-phonon scattering, phonon-impurity scattering, phonon-electron scattering, and phonon-boundary scattering. Each scattering mechanism can be characterised by a relaxation rate 1/
Contents
- Phonon phonon scattering
- Mass difference impurity scattering
- Boundary scattering
- Phonon electron scattering
- References
All scattering processes can be taken into account using Matthiessen's rule. Then the combined relaxation time
The parameters
Phonon-phonon scattering
For phonon-phonon scattering, effects by normal processes (processes which conserve the phonon wave vector - N processes) are ignored in favor of Umklapp processes (U processes). Since normal processes vary linearly with
where
Mass-difference impurity scattering
Mass-difference impurity scattering is given by:
where
Boundary scattering
Boundary scattering is particularly important for low-dimensional nanostructures and its relaxation time is given by:
where D is the dimension of the system and p represents the surface roughness parameter. The value p=1 means a smooth perfect surface that the scattering is purely specular and the relaxation time goes to ∞; hence, boundary scattering does not affect thermal transport. The value p=0 represents a very rough surface that the scattering is then purely diffusive which gives:
This equation is also known as Casimir limit.
Phonon-electron scattering
Phonon-electron scattering can also contribute when the material is heavily doped. The corresponding relaxation time is given as:
The parameter