Electronic structure Engineering for thermoelectricity
With ultra-scaled semiconductor devices being used increasingly for thermoelectric applications, it important to analyze theoretically what kind of nanostructures are good for thermoelectric applications. In this respect we apply the following procedure to evaluate materials and structures.
Electronic structure Calculation:
The electronic structure of different zinc-blende (Si, Ge, GaAs, etc), wurtzite (GaN, etc) and Chalcogenides (PbTe, PbSe, etc) are calculated using the tight-binding formulation. From the electronic structure of these materials for various geometries like nanowires, thin flims, bulk, etc the thermoelectric properties can be calculated using the Landauer’s principle.
Why DOS Engineering?
This is done to maximize the thermoelectric power factor which is defined below as:
Thermoelectric figure of merit and Power Factor used for the evaluation of thermoelectric properties of materials in the linear transport regime (copyright: Abhijeet Paul)
DOS Engineering, How?
DOS or Density of States engineering is possible by a variety of ways. Some of the methods which are utilized in our group are :
- Nanostructuring -> creating nanowires, thin flims etc
Power Factor (PF) , Seebeck coefficient (S) and Electronic conductance (G) in Silicon nanowires as a function of nanowire cross-section size. These calculations are based on Tight binding model and Landauer’s approach.
- Strain engineering à Application of biaxial and uniaxial strain can modify the DOS and hence the thermoelectric properties. See this page on how thermoelectricity depends on the material aspects.
Enhancement in thermoelectric power factor Enhancement in thermoelectric power factor with uniaxial tensile strain in GaAs nanowires due to the contribution from L valleys. (copyright: Abhijeet Paul)
- Creating superlattices and alloys à Allows to manipulate DOS and hence the thermoelectric properties.
- Hollow structures à Creating pores in crystalline nanostructures also provides attractive method for tuning the electronic part of the thermoelectricity.
Hollow SiNW and the calculation of ZT in such nanowires. There is not a big reduction in ZT but the overall thermal conductivity drops due to pores in the nanowire. (Copyright: Abhijeet Paul)
- New materials à Exploration of new materials from the periodic table in search of better thermoelectric performance. This involves investigating the electronic structure of these materials and calculation of thermoelectric properties.
(a-b) PbSe nanowires for two different channel orientations. (c) P-type thermoelectric PF for PbSe, PbTe and Si nanowires with cross-section size 1.2nm. The enhanced p-type PF in chalcogenides is attributed to the higher VB DOS which comes from the L-type symmetry. (copyright: Abhijeet Paul)
Group member involved:
- Abhijeet Paul: Application of new materials, nanostructuring, strain, alloying to improve and enhance solid state thermoelectric efficiency.
- Kai Miao: Electronic structure calculation in Si/Ge superlattice nanowires for thermoelectric applications