Brian F. Donovan

Spectral phonon scattering effects on the thermal conductivity of nano-grained barium titanate

We study the effect of grain size on thermal conductivity of thin film barium titanate over temperatures ranging from 200 to 500 K. We show that the thermal conductivity of Barium Titanate (BaTiO3) decreases with decreasing grain size as a result of increased phonon scattering from grain boundaries. We analyze our results with a model for thermal conductivity that incorporates a spectrum of mean free paths in BaTiO3. In contrast to the common gray mean free path assumption, our findings suggest that the thermal conductivity of complex oxide perovskites is driven by a spectrum of phonons with varying mean free paths.

Mechanisms of nonequilibrium electron-phonon coupling and thermal conductance at interfaces

We study the electron and phonon thermal coupling mechanisms at interfaces between gold films with and without Ti adhesion layers on various substrates via pump-probe time-domain thermoreflectance. The coupling between the electronic and the vibrational states is increased by more than a factor of five with the inclusion of an ∼3 nm Ti adhesion layer between the Au film and the non-metal substrate. Furthermore, we show an increase in the rate of relaxation of the electron system with increasing electron and lattice temperatures induced by the laser power and attribute this to enhanced electron-electron scattering, a transport channel that becomes more pronounced with increased electron temperatures. The inclusion of the Ti layer also results in a linear dependence of the electron-phonon relaxation rate with temperature, which we attribute to the coupling of electrons at and near the Ti/substrate interface. This enhanced electron-phonon coupling due to electron-interface scattering is shown to have negligi...

Thermal boundary conductance across metal-gallium nitride interfaces from 80 to 450 K

Thermal boundary conductance is of critical importance to gallium nitride (GaN)-based device performance. While the GaN-substrate interface has been well studied, insufficient attention has been paid to the metal contacts in the device. In this work, we measure the thermal boundary conductance across interfaces of Au, Al, and Au-Ti contact layers and GaN. We show that in these basic systems, metal-GaN interfaces can impose a thermal resistance similar to that of GaN-substrate interfaces. We also show that these thermal resistances decrease with increasing operating temperature and can be greatly affected by inclusion of a thin adhesion layers.

Size effects in the thermal conductivity of gallium oxide (β-Ga2O3) films grown via open-atmosphere annealing of gallium nitride

Gallium nitride (GaN) is a widely used semiconductor for high frequency and high power devices due to of its unique electrical properties: a wide band gap, high breakdown field, and high electron mobility. However, thermal management has become a limiting factor regarding efficiency, lifetime, and advancement of GaN devices and GaN-based applications. In this work, we study the thermal conductivity of beta-phase gallium oxide (β-Ga2O3) thin films, a component of typical gate oxides used in such devices. We use time domain thermoreflectance to measure the thermal conductivity of a variety of polycrystalline β-Ga2O3 films of different thicknesses grown via open atmosphere annealing of the surfaces of GaN films on sapphire substrates. We show that the measured effective thermal conductivity of these β-Ga2O3 films can span 1.5 orders of magnitude, increasing with an increased film thickness, which is indicative of the relatively large intrinsic thermal conductivity of the β-Ga2O3 grown via this technique (8.8...

Using Laser-Induced Thermal Voxels to Pattern Diverse Materials at the Solid-Liquid Interface.

We describe a high-resolution patterning approach that combines the spatial control inherent to laser direct writing with the versatility of benchtop chemical synthesis. By taking advantage of the steep thermal gradient that occurs while laser heating a metal edge in contact with solution, diverse materials comprising transition metals are patterned with feature size resolution nearing 1 μm. We demonstrate fabrication of reduced metallic nickel in one step and examine electrical properties and air stability through direct-write integration onto a device platform. This strategy expands the chemistries and materials that can be used in combination with laser direct writing.

Molecular Tuning of the Vibrational Thermal Transport Mechanisms in Fullerene Derivative Solutions

Control over the thermal conductance from excited molecules into an external environment is essential for the development of customized photothermal therapies and chemical processes. This control could be achieved through molecule tuning of the chemical moieties in fullerene derivatives. For example, the thermal transport properties in the fullerene derivatives indene-C60 monoadduct (ICMA), indene-C60 bisadduct (ICBA), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), [6,6]-phenyl C61 butyric acid butyl ester (PCBB), and [6,6]-phenyl C61 butyric acid octyl ester (PCBO) could be tuned by choosing a functional group such that its intrinsic vibrational density of states bridge that of the parent molecule and a liquid. However, this effect has never been experimentally realized for molecular interfaces in liquid suspensions. Using the pump-probe technique time domain thermotransmittance, we measure the vibrational relaxation times of photoexcited fullerene derivatives in solutions and calculate an effective thermal boundary conductance from the opto-thermally excited molecule into the liquid. We relate the thermal boundary conductance to the vibrational modes of the functional groups using density of states calculations from molecular dynamics. Our findings indicate that the attachment of an ester group to a C60 molecule, such as in PCBM, PCBB, and PCBO, provides low-frequency modes which facilitate thermal coupling with the liquid. This offers a channel for heat flow in addition to direct coupling between the buckyball and the liquid. In contrast, the attachment of indene rings to C60 does not supply the same low-frequency modes and, thus, does not generate the same enhancement in thermal boundary conductance. Understanding how chemical functionalization of C60 affects the vibrational thermal transport in molecule/liquid systems allows the thermal boundary conductance to be manipulated and adapted for medical and chemical applications.

Ballistic transport of long wavelength phonons and thermal conductivity accumulation in nanograined silicon-germanium alloys

Computationally efficient modeling of the thermal conductivity of materials is crucial to thorough experimental planning and theoretical understanding of thermal properties. We present a modeling approach in this work that utilizes frequency-dependent effective medium to calculate lattice thermal conductivity of nanostructured solids. The method accurately predicts a significant reduction in the thermal conductivity of nanostructured Si80Ge20 systems, along with previous reported thermal conductivities in nanowires and nanoparticles-in-matrix materials. We use our model to gain insight into the role of long wavelength phonons on the thermal conductivity of nanograined silicon-germanium alloys. Through thermal conductivity accumulation calculations with our modified effective medium model, we show that phonons with wavelengths much greater than the average grain size will not be impacted by grain boundary scattering, counter to the traditionally assumed notion that grain boundaries in solids will act as diffusive interfaces that will limit long wavelength phonon transport. This is further supported through a modulation frequency dependent thermal conductivity as measured with time-domain thermoreflectance.

Interplay between mass-impurity and vacancy phonon scattering effects on the thermal conductivity of doped cadmium oxide

Understanding the impact and complex interaction of thermal carrier scattering centers in functional oxide systems is critical to their progress and application. In this work, we study the interplay among electron and phonon thermal transport, mass-impurity scattering, and phonon-vacancy interactions on the thermal conductivity of cadmium oxide. We use time domain thermoreflectance to measure the thermal conductivity of a set of CdO thin films doped with Dy up to the saturation limit. Using measurements at room temperature and 80 K, our results suggest that the enhancement in thermal conductivity at low Dy concentrations is dominated by an increase in the electron mobility due to a decrease in oxygen vacancy concentration. Furthermore, we find that at intermediate doping concentrations, the subsequent decrease in thermal conductivity can be ascribed to a large reduction in phononic thermal transport due to both point defect and cation-vacancy scattering. With these results, we gain insight into the comple...

High resolution steady-state measurements of thermal contact resistance across thermal interface material junctions

Thermal interface materials (TIMs) are meant to reduce the interfacial thermal resistance (RT) across bare metal contacts in commercial electronics packaging systems. However, there is little scientific consensus governing material design for optimized thermal performance. This is principally due to the inability to separate the effects of the intrinsic material thermal properties from the magnitude of heat flow crossing the TIM-substrate junction (RC). To date, efforts to isolate these effects using standard thermal interface material characterization techniques have not been successful. In this work, we develop an infrared thermography-based steady-state heat meter bar apparatus with a novel in situ thickness measurement system having 0.5 nm sensitivity. These in situ thickness measurements allow us to simultaneously determine RT and RC independently across current state-of-the-art TIMs with ±5% uncertainty. In this work, thermal pastes with bond line thicknesses ranging between 5 and 50 μm are used to illustrate the capability of the apparatus to measure extremely thin materials that are expected to achieve relatively low values of RT. Results suggest that the contribution of the thermal contact resistance to the total thermal resistance can range from 5% to 80% for these materials. This finding highlights the need for appropriate metrology and independent measurements of RC and RT to better optimize thermal interface materials for a number of important electronics applications.

Localized thin film damage sourced and monitored via pump-probe modulated thermoreflectance

Damage in the form of dewetting and delamination of thin films is a major concern in applications requiring micro- or nano-fabrication. In non-contact nanoscale characterization, optical interrogation must be kept to energies below damage thresholds in order to conduct measurements such as pump-probe spectroscopy. In this study, we show that the thermoreflectance of thin films can indicate the degree of film damage induced by a modulated optical heating source. By adjusting the absorbed power of the pump heating event, we identify the characteristics of the change in the thermoreflectance signal when leading up to and exceeding the damage threshold of gold films of varying thicknesses on glass substrates.