Built in polarization effect on thermoelectric efficiency of InGaN/GaN heterostructure

The superior thermoelectric (TE) efficiency of GaN/InxGa1−xN /GaN superlattice (SLs) requires maximum values of Seebeck coefficient (S) and electrical conductivity (σ); but minimum value of thermal conductivity (k). A smaller k would lead to even further increase of TE figure of merit (ZT). The built in polarization (BIP) of GaN/InxGa1−xN /GaN SLs enhances S and σ of the SLs, however role of BIP on k has not been explored. In this work the effect of BIP on k of GaN/InxGa1−xN /GaN SLs has been computed theoretically. The BIP of SL enhances Debye temperature, phonon mean free path and thermal conductivity of InxGa1−xN at room temperature. The variation of thermal conductivities (kp: including polarization mechanism and k: without polarization mechanism) with temperature predicts the existence of a transition temperature (Tp) where both thermal conductivities show crossover. Below Tp, kp is lower than k; while above Tp, kp is significantly contributed from BIP mechanism due to thermal expansion. This signifies poyroelectric behavior of SL. It has been shown that Tp depends on x, strain and boundary resistance due to interfaces. Thus, tailoring the In composition, strain and interface resistance, thermal conductivity of the SL can be modified as per requirement. This study will be useful for optimization of module geometry (no. of interfaces, thickness of well and barriers, length of thermo-elements and no. of periods in SLs) for maximum power generation.