Thermoelectric properties of lattice matched InAlN on semi-insulating GaN templates

Abstract

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.

Calculated thermoelectric properties of InxGa1−xN, InxAl1−xN, and AlxGa1−xN

“…This error could be due to experimental error, which is particularly high for InAlN as a result of complicating interfacial effects. 13 The material constants for InAlN are another potential source of error given the small body of literature available on this material. Similar to InGaN, calculated thermal conductivities are higher than those measured on thin film samples.…”

“…Additionally, GaN is non-toxic, an important consideration for distributed applications such as automobile waste heat recovery. 8 The potential of III-nitride materials for thermoelectric applications has motivated increasing research on the experimental properties of GaN, 9,10 InGaN, 3,5,11,12 InAlN, [13][14][15] and AlInGaN 16,17 as well as III-nitride based thermoelectric devices. 18 There have also been a few attempts to calculate and predict the properties of these materials, most notably the thermoelectric properties of GaN, 4 AlGaN, 4,19 and InGaN.…”

Thermoelectric properties of In-rich InGaN and InN/InGaN superlattices

“…Due to the semi-insulating nature, the influence of the substrate on the thermoelectric properties of the as-grown thin film heterostructures is negligible. 16,26,31 The (0001)-oriented, metal-polar films and SLs were grown on 10×10 mm 2 substrate pieces under slightly In-rich conditions, using a N-limited equivalent growth rate of Φ N = 6.4 nm/min. For the In-rich InGaN films a series of ∼1.1 µm-thick films were grown with different alloy compositions (nominal values of 0 < x(Ga) < 0.2), employing variable Ga-fluxes (equivalent growth rate of Φ Ga = 0.5 − 1.3 nm/min), a fixed In-flux of Φ In = 6.0 nm/min and growth temperature T TC of 530-550 • C as measured by a thermocouple (TC) attached to the backside of the substrate.…”

“…Comparable reductions in the thermal conductivity are observed in other nitride alloys. 3,11,[13][14][15][16]42 As a result of alloying, high-energy phonons that contribute to most of heat transport in InN at higher temperature are strongly scattered by Rayleigh scattering, due to disorder in mass and strength of bonds of In and Ga atoms. 7,42 As known from other alloy systems, alloying is thereby an effective approach to reduce the thermal conductivity of crystals.…”

“…[11][12][13] and InAlN [Refs. 3,4, and [14][15][16] alloys. Clearly, while the thermoelectric prospects for binary group-III-nitrides are rather poor due to their very high thermal conductivities (> 90 W/m-K), 17 ternary nitride alloys exhibit much improved thermoelectric properties [10][11][12][13][14][15] since alloying significantly reduces thermal conductivity.…”