Experimental Characterization of Thermal Transport in Two-Dimensional Materials and Thin Films

By Brown, David Brandon

Georgia Institute of Technology

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Abstract

Advisors: Satish Kumar, Baratunde A Cola, Samuel Graham, Matthew McDowell, Eric M Vogel

Increased power density in modern microelectronics has led to thermal management challenges which degrade performance and reliability. The power consumption and heat removal are limited by the thermal boundary conductance (TBC) at the interfaces of dissimilar materials and the change in properties such as thermal conductivity and heat capacity for thin films at the nanoscale. New materials such as two-dimensional (2D) graphene and transition metal dichalcogenides (TMDs) are being investigated for applications in next generation devices, but the interfaces will play a critical role in the overall performance of these materials. In addition, high dielectric constant insulators such as hafnia (HfO2) are promising for many future applications, and the impact of the thermal properties cannot be overlooked. A fundamental understanding and precise characterization of the thermal transport properties at the interfaces and in the bulk of these materials is of the utmost importance to ensure energy efficient operation and long lifetime in future electronic devices. In this work, time-domain thermoreflectance (TDTR) is used to explore the TBC at metal-graphene interfaces. Transition metals Ti and Ni have been categorized as having a strong interaction with graphene and are expected to exhibit high TBC, but this was not observed due to formation of native oxide layer on the surface. The native oxide also reduced the TBC well below what is observed for the electron dominated metal-metal interfaces. In addition, the insertion of single-layer graphene significantly diminished the electronic contribution at Au-Au interfaces. The results highlight important design considerations for metal-graphene-metal interfaces in future devices. The interfaces of 2D hexagonal boron nitride (h-BN) and graphene were investigated using TDTR. The phonon transmission and TBC were calculated using two forms of the diffuse mismatch model for highly anisotropic materials. The findings of this investigation include experimental estimation of TBC and contributions of different phonon modes to transmission at h-BN-graphene interface. The spatial variation of TBC at interface of the 2D semiconducting TMD MoSe2 and metal was demonstrated using a modified TDTR technique. The results indicate enhanced TBC at Ti-MoSe2 interface compared to Al-MoSe2. Additionally, image clustering revealed increased TBC in single-layer regions compared to bilayer. Both findings are crucial to the design and performance of next generation devices featuring MoSe2. Finally, the thermal conductivity and heat capacity of HfO2 films of varying thickness was estimated using TDTR. A 20% reduction in bulk heat capacity observed for a 215 nm layer compared to thinner films is attributed to density differences originating from combined amorphous and crystalline film composition. The thickness-independent thermal conductivity of HfO2 layers from 12 to 215 nm was observed and the measured conductivity was close to the bulk value, a vital observation for the design and performance of electronic devices in the future.

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Researchers should cite this work as follows:

  • Brown, David Brandon (2020), "Experimental Characterization of Thermal Transport in Two-Dimensional Materials and Thin Films," https://matin.gatech.edu/resources/3819.

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