Advancements in iii-nitride materials for thermoelectric applications

Thermoelectrics have attracted significant attention in recent years due to their ability to directly convert heat into electricity using a completely solid state device. Additionally, thermoelectrics are capable of operating as a heat pump and are promising for small scale cooling applications. The lack of moving parts in solid state devices imparts the important advantages of being highly scalable, noise and vibration free, and extremely reliable. These advantages make thermoelectrics desirable for a diverse range of applications such as distributed waste heat recovery and rapid small-scale cooling. The most commonly cited metric for thermoelectric materials is the thermoelectric figure of merit, ZT, which is directly related to device efficiencies. Traditional commercial thermoelectric materials have maximum ZT values near one. In order to break through this threshold and make significant improvements in thermoelectric device efficiencies new material systems and novel thermoelectric strategies must be pursued. III-Nitride materials (InN, AlN, and GaN) and their alloys have seen recent commercial success in visible spectrum optoelectronic devices as well as in power electronics. These materials also exhibit very good temperature stability, wide band gaps, and are non-toxic, which makes them highly desirable for thermoelectric electricity generation applications. The current work investigates the high temperature thermoelectric properties of ternary III-Nitride alloys (InGaN, InAlN, and AlGaN) both experimentally and theoretically. InGaN shows the highest ZT values experimentally reaching 0.34 at 875 K and theoretically reaching 0.85 at 1200 K. This high ZT makes InGaN competitive with commercial materials for high temperature electricity generation applications. Further improvements in ZT are demonstrated in metal organic chemical vapor deposition (MOCVD) grown GaN/AlN/AlGaN superlattices. These structures demonstrate a novel strategy for breaking through traditional limitations on thermoelectric efficiencies by using the exceptionally strong internal polarization fields present in III-Nitride materials to concentrate electrons into high mobility channels, thus improving electrical conductivity and ZT past conventional bulk limits. Using this strategy GaN/AlN/AlGaN superlattices demonstrate vastly improved electron mobilities and a four fold increase in room temperature ZT.