First-principles deformation potential extraction and computationally efficient mobility calculations

• A first-principles framework to extract deformation potentials in silicon based on density-functional theory (DFT) and density-functional perturbation theory (DFPT).
• By incorporating ionized impurity scattering as well, we calculate the $n$-type and $p$-type mobility versus carrier density and make comparisons to experiments, indicating excellent agreement.
• The fact that the method we present uses well-established theoretical tools and requires the extraction of only a limited number of matrix elements, makes it generally computationally very attractive, especially for semiconductors with a large unit cell and lower symmetry.

Journal [81]

Modelling thermoelectric performance in nanoporous nanocrystalline silicon

• Investigated the effects of embedded SL barriers and pores (for porosity < 5%) on ZT using equilibrium classical MD simulations combined with the fully quantum mechanical non-equilibriumGreen’s function method.
• Upon nanostructuring to reduce thermal conductivity, it is beneficial to place the Fermi level high into the bands, i.e., under highly degenerate conditions, to optimise the power factor.
• We found that there is an optimal porosity, situated at ~ 2% for our simulated geometries, beyond which point the degradation of the PF outweighs the degradation of the thermal conductivity.

Journal [79]

Theoretical model for the Seebeck coefficient in superlattice materials

• We have developed a simple analytical model for the Seebeck coefficient, S, of a channel with embedded superlattice (SL) barriers, which takes into account the energy relaxation due to electron-optical phonon scattering.
• We have compared the model against the non equilibrium Green's function (NEGF) method and also with experiment, and found very good agreement.
• The developed model provides an alternative and less time-consuming method for the description of the Seebeck coefficient in SLs and nanocomposite systems.

Journal[78]

Impact of the scattering physics on the power factor of complex thermoelectric materials

• Full numerical simulator that removes the constant relaxation time approximation.
• The accurate considerations of the scattering physics gives different best doping indications and materials rankings with respect to the commonly used constant relaxation time approximation.
• The temperature trends are different as well, but for the highest doping levels a similar trend can be recovered upon temperature scaling of the constant relaxation time.
• We deliver effective constant relaxation times τ* as a general indication for half Heuslers.

Journal [80]

Phonon transport in hierarchical structures

• Hierarchical material nanostructuring is a very promising direction for high performance thermoelectric materials. In this work we investigate thermal transport in hierarchically nanostructured silicon.
• We consider the combined presence of nanocrystallinity and nanopores, arranged under both ordered and randomized conditions.

Journal [70], Journal [74]

Phonon transport in nanoporous Si from atomistic simulations

• Introducing porosity into materials can drastically reduce their thermal conductivity. If the electronic properties of the materials are also preserved, such a decrease can improve the materials' thermoelectric performance.
• We have performed a systematic and detailed investigation of how incremental disorder in the geometry of porous materials affects thermal conductivity.
• This study aims to help design and engineer porosity for thermoelectric applications.

Journal[77]

Quantum transport simulations of nanocomposite materials

• In this work we have shown that, by careful choice of the band offset, nanoinclusions could also provide small improvements in the power factor.
• For a particular band offset the power factor becomes independent of the nanoinclusion density, allowing for thermal conductivity optimisation without impacting the power factor.

Journal [67]

Bipolar thermoelectric materials

• We show that unipolar models in use in the literature no longer hold in bipolar systems.
• This is important because the bipolar effect is often a limiting factor in the thermoelectric efficiency of narrow bandgap materials at high temperatures.
• Moreover, we show that the lower the thermal conductivity of a material is, the lower the doping required to optimise ZT also is.

Journal [72]

Hierarchically nanostructured 2D materials

• Investigation of the thermoelectric power factor (PF) of hierarchically nanostructured 2D materials with quantum transport simulations, including interactions of electrons with acoustic and optical phonons.
• Results show that the PF can be enhanced by ~20% under elastic scattering conditions irrespective of nanoinclusion number density and SL barrier heights, as long as the Fermi level is placed well into the bands and charge carrier relaxation is avoided.

Journal [73]

Nanostructured materials with randomized nanoinclusions

• We have shown that the thermoelectric power factor is robust to randomised variations in the positions, diameter, and height of the barriers introduced by nanoinclusions.
• This result is important since our other work has shown such variations can have a significant impact on the thermal conductivity.

Journal [69]

Improving power factor with band convergence

• Band convergence is an effective strategy to improve the thermoelectric performance in complex bandstructure materials.
• Theoretical calculations to identify the outcome of band convergence usually employ detailed density functional theory (DFT) for bandstructure calculations, but the transport calculations are kept simplistic using the constant relaxation time approximation due to the complications involved with detailed scattering calculations.
• We investigate the benefits of band aligning in improving the thermoelectric power factor under different scattering scenarios, and as a test case we consider the Co-based p-type half- Heuslers TiCoSb, NbCoSn and ZrCoSb.

Phonon transport Monte Carlo

• Established and tested a phonon transport simulator using the semi-classical Monte Carlo (MC) method.
• The MC simulator uses a single-phonon approach rather than the typical multi-phonon approach largely available in the literature.
• Results were verified using existing theoretical and experimental works in literature.

Journal [70]

Software development

Our aim is to develop a user-friendly computational platform, that can be used by theoreticians and experimentalists alike, as a predictive tool for assessing electro-thermal transport in disordered, nanostructured materials.

To date, we

• developed a phonon transport GUI-based software to estimate thermal properties of porous silicon material using semi-classical Monte Carlo
• are in the process of linking our group's Monte Carlo, non-equilibrium Green's function, and tight-binding codes.

Ferromagnetic semiconductor channel spin MOSFET

Use a ferromagnetic semiconductor as channel material instead of traditional semiconductor materials as silicon

• sustain the spin polarization across the channel between Source and Drain avoiding the loss of spin polarization in the channel due to spin flip scattering
• the transistor operation is used for computing, the magnetic functionality to store the bit, good for computing-in-memory and reconfigurable logic
• ballistic transport approach with point electrostatics, well validated
• simulation with real materials band structures give promising results
• encouraging results for Room Temperature operation spin MOSFET design

Journal[68]