Qualitative link between work of adhesion and thermal conductance of metal/diamond interfaces

We report Time-Domain ThermoReflectance experiments measuring the Thermal Boundary Conductance (TBC) of interfaces between diamond and metal surfaces, based on samples consisting of [111] diamond substrates with hydrogen or with sp2 carbon surface terminations created using plasma treat- ments. In a concurrent theoretical study, we calculate the work of adhesion between Ni, Cu and diamond interfaces with [111] surface orientation, with or without hydrogen termination of the diamond surface, using first-principles electronic structure calculations based on density functional theory (DFT). We find a positive correlation between the calculated work of adhesion and the mea- sured conductance of these interfaces, suggesting that DFT could be used as a screening tool to identify metal/dielectric systems with high TBC We also explain the negative effect of hydrogen on the thermal conductance of metal/diamond interfaces.

Thermal Boundary Conductance between metals and dielectrics

Christian Monachon

This work consists in an experimental study of the main parameters governing the transport of heat across an interface between two solids, when it is dominated by phonons. The main experimental tool used is Time-Domain ThermoReflectance (TDTR), which is an optical technique able to measure the Thermal Boundary Conductance (TBC) of single interfaces between metallic layers about 100 nm thick and mm-sized, flat dielectric substrates. Emphasis is put on diamond as a dielectric, because the diamond/metal TBC is among the important parameters governing the thermal conductivity of diamond-based Metal Matrix Composites (MMC) with potential applications as heat sink materials. In a first part, TDTR as a technique is reviewed and its main advantages are identified. Experimental parameters that influence it the most strongly are the size of the laser spots used and the thickness of the metallic layer investigated. An experimental procedure is presented, that provides a reliable way of measuring these quantities. In a second part, the effect on the TBC of surface treatments of the dielectric substrate prior to the metallic layer deposition is investigated. On an AlN substrate, it is found that the formation of a native oxide on its surface decreases the thermal transport across its interface with an Al layer. Etching away this oxide improves the measured TBC from 43 to 241 MWm−2K−1. On a diamond substrate, the results are more nuanced. The as-received surface is found to be hydrogen-terminated and has traces of organic contaminants. Ridding the surface of these contaminants and terminating it with hydrogen using an Ar:H plasma treatment improves TBC in a way that depends on the diamond surface orientation, from 23 to 32 ([111] orientation) or 23 to 54 ([100] orientation) MWm−2K−1. Doing so using a mixture of H2SO4 and HNO3 (1:1) at 200°C partly terminates the surface with oxygen and yields a TBC of 125 MWm−2K−1. Forcing a graphitic sp2 termination of the surface by sputtering Ar ions on it further improves TBC, up to 150 MWm−2K−1. Finally, forcing oxygen termination using an Ar:O plasma yields a high TBC of 180 MWm−2K−1. Still on the oxygenated surface, the deposition of the Al layer by DC sputtering instead of evaporation results in the highest TBC value measured between Al and diamond, of 230 MWm−2K−1, an order of magnitude higher than the TBC of 23 MWm−2K−1 between Al and as-received diamond, and thrice as large as the highest value of 90 MWm−2K−1 reported in the literature. All the treatments applied except for the Ar:H plasma are found to yield TBCs independent of the diamond surface orientation. In a third part, some attempts are made to explain the origin of the observed increase in TBC. By comparing the temperature dependence of the TBC between Al on oxygenated and sp2-terminated diamond, we find that the latter case leads to a distinctly different temperature-dependent behavior. This suggests that the presence of oxygen atoms, presumably forming an interfacial oxide, is the main factor influencing TBC in this system. To confirm this hypothesis, the thickness of this presumed oxide is increased by depositing Al2O3 using Atomic Layer Deposition (ALD) of 1.6, 4.5, 6.2 and 10 nm on the substrate before the Al layer. It is found that the difference in temperature dependence of the TBC between the case where these layers are present and the case when the diamond surface is only oxygen-terminated can be explained simply by the resistive contribution of the deposited layer. This suggests that interfacial states created by the reaction of Al and O are a crucial parameter explaining the high TBC found. In another experiment, the work of adhesion of Cu and Ni on clean diamond is calculated using Density Functional Theory. Using known values for the work of adhesion between Al and Ti on diamond, the TBC between diamond and Al, Cu, Ni or Ti is found to scale roughly with the work of adhesion of each of these systems. Furthermore, the presence of hydrogen at the diamond surface is found to dramatically decreases both the work of adhesion and the thermal transfer across its interface with these metals, confirming the previous results obtained between Al and diamond. Still in the same approach, the TBC between diamond and Cr, Mo, Nb and W is measured before and after a high vacuum heat treatment at 800°C. After careful characterization of the samples both before and after heat treatment, it is found that carbides have formed in the metal layers after heat treatment. Despite the presence of these carbides, the TBC between the diamond substrate and the layers stays the same, suggesting that even before the heat treatment, an interfacial carbide is present between the metal and the diamond and governs heat transfer through the interface. In a final experiment, very thin layers of Ni (of 1.8, 3.2 and 7 nm) are inserted between Al or Ag and diamond. Once these layers have been carefully characterized, it is found that these interfacial layers increase the measured TBC between the top layer and diamond to values that, within error and accounting for electron-phonon coupling, equal 260 MWm−2K−1 measured for a layer of pure Ni on diamond, up from TBCs of 47 and 170 MWm−2K−1 measured in the same study for Ag/diamond and Al/diamond interfaces respectively. Finally, throughout this whole thesis, values for TBC between several metals (Ag, Al, Au, Cr, Cu, Mo, Nb, Pt, Ti, W) as well as some carbides (Cr, Mo, Nb, W) and diamond are provided. They are measured in clean conditions and thus are expected to set a maximum value to the TBC in non-ideal systems such as diamond reinforced metals with potential applications as heat sinks. Based on the results presented, solutions to improve thermal transport in diamond-based MMCs are proposed.

EPFL2013

Adsorbate-localized states at water-covered (100) SrTiO[sub 3] surfaces

Florian Le Formal, Santosh Raghavan, Alexandra Sarabando de Carvalho, Nava Setter

The electronic structure of hydrated SrTiO3 (001) surfaces is investigated using density-functional models. It is shown that adsorbed water molecules give rise to unoccupied electron states similar in localization and shape to wet-electron states recently reported for other oxide-water interfaces, and believed to serve as a preferred path for transfer of conduction electrons to the surface water molecules. Additionally, we found that chemisorbed water and hydrogen have donor levels in the band gap, and that chemisorbed hydrogen is oxidized and released in the presence of free holes. These gap states can serve as surface recombination centers in photoelectrochemical cells. (C) 2011 American Institute of Physics. [doi:10.1063/1.3529473]

American Institute of Physics2011

Alignment of energy levels in amorphous oxides and at semiconductor-water interfaces

Zhendong Guo

We conduct a detailed investigation of defects in two representative amorphous oxides: amorphous Al2O3 (am-Al2O3) and TiO2 (am-TiO2), by combining ab initio molecular dynamics (MD) simulations and hybrid functional calculations. Our results indicate that oxygen vacancies and interstitials occur neither in am-Al2O3 nor in am-TiO2 due to structural rearrangements which assimilate the defect structure, and that the injection of extra holes can lead to the formation of O-O peroxy linkages. In am-Al2O3, hydrogen is found to be amphoteric. Based on localized Wannier functions, we identify the nominal charge state and the composition of the defect core units related to C and N impurities in am-Al2O3, which are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively, indicating that none of them gives charge transition levels in am-Al2O3. In addition, those defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy is dependent on the oxygen chemical potential. In addition, we propose an exchange mechanism for hole transport in am-TiO2 that relies on the simultaneous breaking and forming of O-O peroxy linkages that share one O atom. Through the use of nudged-elastic-band calculations, a hopping path as long as 1.2 nm with barriers of 0.3-0.5 eV is demonstrated, suggesting that hole diffusion through O-O peroxy linkages is viable in am-TiO2. In this work, we also determine the band alignment between various semiconductors and liquid water by combining MD simulations of atomistic interface models, electronic-structure calculations at the hybrid-functional and GW levels, and a computational standard hydrogen electrode (SHE). Our study comprises GaAs, GaP, GaN, CdS, ZnO, SnO2, rutile and anatase TiO2. For each semiconductor, we generate atomistic interface models with liquid water at the pH corresponding to the point of zero charge. The MD simulations are started from initial configurations, in which the water molecules are either molecularly (m) or dissociatively (d) adsorbed on the semiconductor surface. The calculated band offsets are found to be strongly influenced by the adsorption mode at the semiconductor-water interface, leading to differences larger than 1 eV between m and d models of the same semiconductor. We then assess the accuracy of various ab initio electronic-structure schemes in determining the band alignment. In the last part, we try to evaluate photocatalysts for water-splitting by considering the surface coverage and the energy alignment. We determine surface concentrations of water molecules, protons, and hydroxyl ions adsorbed at the semiconductor-water interfaces mentioned above as a function of pH. This is achieved through the calculation of the acidity constants at the surface sites, which are derived from ab initio MD simulations and a grand-canonical formulation of adsorbates. It is finally shown how the potential of a semiconductor material as photocatalysts for water splitting can be inferred by combining the nature of the surface coverage and the alignment of the band edges to the relevant redox levels.