Impact of interfaces on phonon and electron transport in oxides and chalcogenides

Thermal boundary conductance (TBC or G) dictates the temperature rise in a wide range of nanoelectronics with a significant number of solid/solid interfaces, and more so as film thickness scales down to nanometer lengths. Thus, both accurate measurements and the control of TBC are important in the thermal design of nanoelectronics. For example, high TBCs are favorable for applications that require enhanced cooling performance such as dense logic gates and interconnect systems, quantum cascade lasers, and power electronics. In contrast, low TBCs are preferred for applications that require large temperature rises in devices, such as phase change memory, and thermoelectric energy conversion devices, and thermal barrier coatings. While TBC plays an important role in variety of applications, the measurement of TBC between metal and amorphous materials is very important since passivation layers to electrically insulate metallic structures in electronic devices are usually realized with thin amorphous oxide films. However, it is difficult to find the available TBC data between metal and these amorphous materials. This is due to the temperature signals from thermal measurements being dominated by the high thermal resistance of amorphous oxide films compared to their interfacial component. To reliably measure metal-amorphous oxide TBC, time-domain thermoreflectance (TDTR) is performed with using nanograting transducers rather than blanket film transducer. By using nanograting transducers, we have significantly improved the experimental sensitivity to reduce uncertainties of the measured TBC between metals and amorphous oxides. Another important area where TBC finds its application is phase change memory devices where the switching efficiency relies on the heat confinement. Phase change superlattice films are being considered as emerging candidates for energy-efficient phase change memory. In this study, the electro-thermal properties of these superlattice films are unveiled. These superlattices are shown to have significantly lower thermal conductivity than that of GST 225 (conventional phase change material) over high temperature ranges, displaying the impact of interfaces on thermal transport. Furthermore, a minimum thermal conductivity is observed with different period sizes of superlattices, which has been typically observed in superlattices with high quality interfaces. At the same time, strong electrical anisotropy is measured in such films. The resulting thermal and electrical transport properties shown in phase change superlattice films demonstrate their promise in achieving energy-efficient phase change memory. Finally, improving the TBC between the substrate and the metal film in atomic layer deposition (ALD) process is explored in this thesis. ALD is a well-developed technique to produce an ultra-thin film with a high quality and conformality. Contrary to typical physical vapor deposition processes such as evaporation and sputtering, ALD processes include the surface chemisorption of precursors onto the surface of a substrate to synthesize films on top of them, which could create strong covalent bonds at an interface between an ALD film and a substrate. The quality of an ALD film and adhesion properties can be finely controlled by using plasma as co-reactants in ALD cycle, called plasma-enhanced ALD (PEALD). Prior studies have shown that the stronger bonds at an interface could translate to improved TBC. In this study, we demonstrate how the adhesion energy of a Pt film can be significantly enhanced using a plasma-enhanced PEALD process. In addition, we find that the improved adhesion energy of the PEALD Pt on a dielectric substrate increases the TBC of the PEALD Pt film by showing a clear favorable correlation between thermal interfacial resistance and mechanical adhesion

Keywords: Mechanical Engineering.
Advisor: Goodson Kenneth E.
Pop Eric
Prinz Friedrich (Classicist)