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A5: Nanoscale and low dimensional effects

Effect of Period Thickness on the Cross-plane Thermal Transport of (Ti,W)N/(Al,Sc)N  Metal/Dielectric Superlattices

Bivas Saha1, Yee Rui Koh1, Joseph P. Feser2, Sridhar Sadasivam1, Timothy Fisher1, Ali Shakouri1 and Timothy D. Sands1
1Birck Nanotechnology Center, Purdue University, West Lafayette, IN-47906, USA.2Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA

Understanding the role of interfaces in thermal transport across nanoscale superlattices is important to the design of thermoelectric materials and devices with improved efficiency. Nitride metal/semiconductor superlattices are not only promising for achieving highly efficient thermoelectric devices, they also serve as a model system where fundamental physical processes involving electron-phonon interactions in thermal conduction can be tested and understood.

Epitaxial and coherent (Ti,W)N/(Al,Sc)N superlattices with period thickness (a) ranging from 0.8-30 nm were deposited on (001) MgO substrates by reactive dc magnetron sputtering. High-resolution XRD analysis along with reciprocal space mapping indicates that the superlattices are pseudomorphic and grow with 002 orientations. The x-ray reflectivity (XRR) analysis suggests that the superlattice interfaces are extremely sharp with interface roughness of the order of one to two atomic layers. High resolution transmission electron microscopy (HRTEM) along with HAADF-STEM indicates cube-on-cube epitaxial growth with a very low density of extended defects.

Time domain thermo-reflectance (TDTR) measurements are used to characterize thermal transport in the superlattices. The effective thermal conductivity of TiN/(Al,Sc)N superlattices exhibits a minimum of 4.5 W/m-K at a period thickness of 4 nm.  The corresponding thermal interface conductance are very high, G > 2 GW/m2-K for a > 4 nm at room temperature, and even higher for superlattices with a < 4 nm.  The measured conductance increases linearly as a function of temperature for a = 10 nm, and 20 nm and exceeds predictions of the diffuse mismatch model (DMM) by 5-6 fold even when full phonon dispersion relations are considered. For (Ti,W)N/(Al,Sc)N superlattices, we find that the thermal conductivity saturates for superlattices with a < 4 nm and is reduced to about 1.7 W/m-K due to additional alloy scattering from heavy tungsten (W) atoms. Such low thermal conductivity is ideal for developing highly efficient thermoelectric materials with increased figure-of-merit.