Lateral thermoelectric devices were fabricated using c-plane GaN thin films grown on sapphire by MOCVD. The device design is appropriate for on-chip integration for power generation in the 1 V and tens of µA range. The fabricated devices were measured to have a maximum open circuit voltage of 0.3 V with a maximum output power of 2.1 µW (=0.15 V×14 µA) at a relatively small temperature difference (ΔT) of 30 K and an average temperature (Tavg) of 508 K. In addition, the suitability of GaN for high temperature thermoelectric applications was confirmed by measurements at 825 K.

The effects of carrier concentration, composition, and temperature on the thermoelectric properties of high quality n-type InGaN grown by metal organic chemical vapor deposition (MOCVD) were systematically investigated. The Seebeck coefficient was found to decrease and electrical conductivity increase with increasing carrier concentration, while both were found to decrease with increasing indium composition. Additionally, thermal conductivity was found to decrease by over an order of magnitude as indium composition was increased from 0 to 19%. These trends resulted in optimum carrier concentration and indium composition of 1.1×1019 cm−3 and 17%, respectively, with a room temperature ZT of 0.04. Increasing temperature resulted in a rapidly increasing ZT, reaching a maximum value of 0.34 at 875 K. This significantly improved ZT demonstrates the potential of InGaN and other III-Nitride materials for high temperature thermoelectric applications.

The thermoelectric properties of III-nitride materials are of interest due to their potential use for high temperature power generation applications and the increasing commercial importance of the material system; however, the very large parameter space of different alloy compositions, carrier densities, and range of operating temperatures makes a complete experimental exploration of this material system difficult. In order to predict thermoelectric performances and identify the most promising compositions and carrier densities, the thermoelectric properties of InxGa1−xN, InxAl1−xN, and AlxGa1−xN are modeled. The Boltzmann transport equation is used to calculate the Seebeck coefficient, electrical conductivity, and the electron component of thermal conductivity. Scattering mechanisms considered for electronic properties include ionized impurity, alloy potential, polar optical phonon, deformation potential, piezoelectric, and charged dislocation scattering. The Callaway model is used to calculate the phonon component of thermal conductivity with Normal, Umklapp, mass defect, and dislocation scattering mechanisms included. Thermal and electrical results are combined to calculate ZT values. InxGa1−xN is identified as the most promising of the three ternary alloys investigated, with a calculated ZT of 0.85 at 1200 K for In0.1Ga0.9N at an optimized carrier density. AlxGa1−xN is predicted to have a ZT of 0.57 at 1200 K under optimized composition and carrier density. InxAl1−xN is predicted to have a ZT of 0.33 at 1200 K at optimized composition and carrier density. Calculated Seebeck coefficients, electrical conductivities, thermal conductivities, and ZTs are compared with experimental data where such data are available.

A novel polarization field engineering based strategy to simultaneously achieve high electrical conductivity and low thermal conductivity in thermoelectric materials is demonstrated. Polarization based electric fields are used to confine electrons into two-dimensional electron gases in GaN/AlN/Al0.2Ga0.8N superlattices, resulting in improved electron mobilities as high as 1176 cm2/Vs and in-plane thermal conductivity as low as 8.9 W/mK. The resulting room temperature ZT values reach 0.08, a factor of four higher than InGaN and twelve higher than GaN, demonstrating the potential benefits of this polarization based engineering strategy for improving the ZT and efficiencies of thermoelectric materials.

The thermoelectric properties of nearly lattice matched n-type InxAl1−xN (x ≈ 0.18) grown by metal organic chemical vapor deposition (MOCVD) are investigated with particular attention to the potentially conductive GaN template and InAlN/GaN interfacial polarization charges. The thermoelectric properties of InAlN are measured over a range of carrier densities and through temperatures as high as 815 K. The maximum room temperature ZT was found to be 0.007 at a carrier density of 6.4 × 1019 cm−3. The ZT of InAlN at this carrier density increases to 0.05 at 815 K. It is also shown that the interfacial charge in InAlN/GaN structures and the resulting two dimensional electron gas (2DEG) lead to greatly improved electron mobility and power factor when 2DEG conduction is dominant. Using this strategy, a 250% improvement in power factor is realized as the thickness of InAlN is decreased from 290 nm to 34 nm. Methods for extending these power factor enhancements to thicker materials are discussed.