Local Modification to Phononic Properties at Solid-Solid Interfaces: Effects on Thermal Transport

The development of nano-devices relies on a combination of internal solid-solid interfaces between materials with different elemental and crystallographic structures to provide the functional operation of the device. Traditionally, these devices have been designed with the operational performance and efficiency in mind, with consideration of thermal energy management and heat dissipation only as a design afterthought. Therefore, the current methodology in dealing with thermal management issues begins with external device structures and scales upwards. However, as the feature size of most nano-devices continues to diminish, the impact of thermal transport across solid-solid interfaces on device performance, reliability, and lifetime becomes increasingly important.

Developed starting in the 1980's, the transient thermoreflectance/time-domain thermoreflectance technique has been a primary tool in the measurement of nanoscale thermophysical properties and in particular thermal boundary conductance. Over the past several decades there have been a number of improvements made to refine and extend the applicability of the technique to nanoscale systems. In this work, the theory for modeling the measured results is extended from a sinusoidal to a pulsed waveform analysis which includes the effects of higher harmonics in the measured signal and allows for variations in the duty cycle of the modulation waveform. The inputs to the thermal model were critiqued and convergence criteria for the numerical analysis were established for high and low repetition rate laser systems to ensure accurate modeling. In addition, methods to quantify the sensitivity of input parameters in the thermal model were defined and the characterization of noise in the raw data and statistical outliers in the deduced parameter presented. To assist in the statistical interpretation of experimental results, a series of simulated data sets were analyzed to develop a set of empirical relations for the anticipated standard deviation in the collected results, based on the sensitivity of the model to the parameter being deduced and the amount of noise in the data.

Finally, an experimental study to test the theory of vibrational bridging through the use of an intermediate layer between two solids as a means to enhance thermal boundary conductance is presented and discussed. The thermal boundary conductance between Pt/Si and Pt/Ge is measured using the transient thermoreflectance technique and the results compared to measurements made with a Ni intermediate layer of varying thickness between the Pt and Si (or Ge). The measured results are compared to theoretical calculations of the contributions of interface bonding and electron transport across the metal-semiconductor interface. It is found that the contributions of electron transport are minimal, and that while it is assumed that bonding plays some role in the increased boundary conductance observed with the addition of the intermediate layer, it alone cannot explain the trend in the data. Overall, the results in the measured data support the hypothesis of vibrational bridging via a metallic intermediate layer and support the concept as a possible means to phononically engineer thermal conductance across solid-solid boundaries in nanodevices.