With the constant miniaturization of electronics, the thermal management issue is becoming the main limiting factor in dictating device performance [1]. Therefore, the study of thermodynamics is of practical interest as well as fundamental scientific interest. However, measuring temperature at these dimensions is difficult due to the increasing influence of the employed measurement tools and therefore new techniques need to be developed. One such technique, Electron Thermal Microscopy (EThM), has been previously used to study heat dissipation in nanowires and carbon nanotubes supported on SiN substrate [3, 4].

        EThM is an in-situ thermal imaging technique using the Transmission Electron Microscope (TEM) and relies on the observation of the solid to liquid phase transition of indium islands and provides a binary temperature map with 50nm spatial resolution[2]. The indium islands, thermally evaporated on the back of the substrate, as seen in the bright-field TEM image in Figure 1a, melt once heated to 429K. This solid to liquid phase transition is easily observed in the TEM, when operating in the appropriate dark-field conditions (Figure 1b), at which the molten islands appear bright compared to the solid ones. In addition, the presence of a high melting point thin oxide layer on the indium preserves the structure of the islands and allows the thermometry technique to be used repeatedly over a large experimental range.

        Two factors contribute to the observed temperature of the system; the heat source and the efficiency of the heat transfer mechanism to the lower temperature reservoir. Here we present the work on joule heated Pd nanowires supported on SiN substrate and use the EThM technique to evaluate the thermal properties of the device. The measured temperature and the efficiency of the heat transfer mechanism can be quantified in terms of the thermal conductivity of the different materials within the device and their thermal boundary resistances. Conventionally, the thermal transport in dielectrics, such as SiN, is phonon mediated. However, as the dimensions approach the mean free path of the phonons, new modasses of heat dissipation may dominate. Combining our experimental thermal measurements (Figure 1c) with simulations (Figure 1d), based on finite element analysis, we have explored different modes of thermal transport and show that the conventional phonon mediated thermal transport is not sufficient to explain the observed temperature gradient across the SiN, indicating that an additional mode is active.   

1.         Pop, E., Energy Dissipation and Transport in Nanoscale Devices. Nano Research, 2010. 3(3): p. 147-169.

2.         Brintlinger, T., et al., Electron thermal microscopy. Nano Letters, 2008. 8(2): p. 582-585.

3.         Baloch, K.H., et al., Remote Joule heating by a carbon nanotube. Nature Nanotechnology, 2012. 7(5): p. 315-318.

4.         Baloch, K.H., N. Voskanian, and J. Cumings, Controlling the thermal contact resistance of a carbon nanotube heat spreader. Applied Physics Letters, 2010. 97(6).

Figures:

Figure 1a) BF TEM image of a Pd device with In islands. b) DF TEM image of the device demonstrating the contrast difference between solid and molten islands. c) Experimental thermal map with the colors indicating the applied voltage at which the designated islands melt. The temperature scale is shown to the far right. d) Simulation results from the finite elemental analysis.

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