Amorphous Interfacial Regions Research Articles

Thermal transport and structural improvements due to annealing of wafer bonded β-Ga2O3|4H-SiC

The impact of postbond annealing on the structural and thermal characteristics of 130 nm thick exfoliated (201) β-Ga2O3 (via H+ ion implantation) wafer bonded to (0001) 4H-SiC was studied. Thirty nanometer amorphous-Al2O3 was grown on the β-Ga2O3 substrates prior to bonding as an interlayer between β-Ga2O3 and 4H-SiC. The surface activated bonding technique was utilized for bonding, which induces a thin nanometer amorphous interfacial region at the bonded interface (Al2O3|4H-SiC). We demonstrate annealing the bonded structure at 800 °C up to 1 h is beneficial: (1) the removal of residual strain in the exfoliated β-Ga2O3 layer that was due to the exfoliation implant, (2) reduction of lattice mosaicity in the β-Ga2O3 layer, (3) nearly complete recrystallization of the amorphous bonded interfacial region, and (4) partial recrystallization of the initially amorphous-Al2O3 interlayer. The thermal characteristics correspondingly improve with the improvement in structural characteristics. The thermal conductivity of the as-bonded β-Ga2O3 layer was 2.9 W/m K, and the thermal boundary conductance of the bonded interface was 66 MW/m2 K. After annealing at 800 °C for 1 h, triple-axis x-ray diffraction ω:2θ measurements showed a reduction in strain for the β-Ga2O3 layer and the symmetric (201) rocking curve widths. We simultaneously observe a doubling of the β-Ga2O3 thermal conductivity to 6.0 W/m K and a 20% increase in the thermal boundary conductance. However, upon further annealing up to 10 h and fully recrystallizing both the Al2O3 interlayer and bonded interface, the thermal boundary conductance dropped by ∼30%. This preliminary result suggests that crystalline heterointerfaces may not necessarily be the most optimal interfacial structure for thermal transport.

Ion Species Dependence on the Interfacial Properties of Silicon Homojunctions Bonded Using an Ion Bombardment Treatment

Surfaces of (001) p-type silicon substrates were bombarded with either Ar or Kr ions prior to direct wafer bonding using an EVG© ComBond© high-vacuum wafer-bonding system.1 Previously, we showed that this ion bombardment treatment induces a thin ~nm amorphous region at the bonded interface.2 In this study, we extend upon this previous effort towards a fundamental understanding of this amorphous interfacial region by examining the dependence of the bonded interface’s properties with two different ion species. The ion energies used spanned from a factor of 0.5 to 2.5 times of a baseline energy E0. X-ray reflectivity measurements were performed on single unbonded wafers that were ion bombarded with either Ar or Kr to determine the thickness of the amorphous region as well as its mass density compared to bulk Si. Wafers treated with Ar showed no surface mass density dependence with ion energy, while wafers treated with Kr did show an ion energy dependence. While the Ar treatment reduces the surface density to ~80% of bulk Si density regardless of the energy used, the Kr treatment reduces the surface density to as low as ~70% of bulk Si density at 2.5⋅E0. Furthermore, for a given energy, the Ar treatment resulted in thicker layers than the Kr treatments, which is consistent with SRIM3 simulations because Kr is a heavier species than Ar. Current-voltage measurements were also used to probe the electrical properties of the bonded interfaces. Band structure modeling of the current-voltage measurements revealed that the amorphous region can be modeled with a step barrier that spans the thickness of the amorphous region.References Flötgen, et al., ECS Trans., 64(5), 103 (2014).E. Liao, et al., ECS Trans., 86(5), 55 (2018).F. Ziegler, et al., The Stopping Range of Ions in Solids vol. 1, (1985). Figure 1

Ion Species Dependence on the Interfacial Properties of Silicon Homojunctions Bonded Using an Ion Bombardment Treatment

Direct wafer bonded p-type (001) silicon homojunction structures were prepared with an ion bombardment surface treatment using either Ar or Kr ions prior to bonding in a high-vacuum direct wafer bonding system. The ion energies used spanned from a factor of 0.5 to 5 times of a baseline energy E0 for Ar and 0.5 to 2.5 times for Kr. The ion bombardment surface treatment induces a thin ~nm amorphous region at the Si|Si bonded interface. X-ray reflectivity measurements were performed on ion-bombarded single unbonded wafers. It was found that the Ar treated wafers showed a weak surface mass density dependence with ion energy, while wafers treated with Kr showed a strong ion energy dependence. While the Ar treatment reduces the surface density to ~80% of bulk Si density, the Kr treatment reduces the surface density to as low as ~70% of bulk Si density using an ion energy of 2.5⋅E0. Current-voltage measurements were also used to probe the electrical resistance across the amorphous interfacial region of the bonded structures prepared with Ar ions.

Formation and properties of ZrO2–Cu composite nanoglass films

Unlike conventional amorphous materials without grain boundaries and dislocations, nanoglasses consist of amorphous nanoparticles and glass-glass interfaces, which results in unique properties. In this work, we have developed a ZrO2–Cu composite nanoglass by multi-target magnetron co-sputtering, and its microstructure, optical and electrical properties have been studied by using aberration-corrected transmission electron microscope, spectroscopic ellipsometry, UV–Visible spectrophotometer and Hall measurements. It is found that Cu is enriched into the amorphous interfacial region. The ZrO2–Cu composite nanoglass shows high visible light transmittance (over 70%), high refractive index (1.98–2.08) and large optical band gap (5.0–5.7 eV). As the Cu content increases, the electrical properties are also improved. The enrichment of Cu at the interface further provides a new route for the preparation of 2D nano-conducting networks.

Molecular Dynamics Study on Wettability of Poly(vinylidene fluoride) Crystalline and Amorphous Surfaces

The present study investigates the effect of microscopic structure on the wettability of poly(vinylidene fluoride) (PVDF) surfaces using all-atom molecular dynamics simulations of water droplets brought into contact with both crystal and amorphous PVDF surfaces. For each case, computations were performed using five different droplet diameters, and the corresponding water droplet contact angles θ were obtained. Using the fact that the cosine of these contact angles for both surfaces are inversely proportional to the radius of the droplet contact surface ( rdr( Z0)), the contact angle θ∞ of the macroscopic water droplet was obtained by extrapolating cos θ to 1/ rdr( Z0) = 0. The estimated values of θ∞ on the crystal and amorphous surfaces were 96° and 86°, respectively, showing that the amorphous surface is less hydrophobic than the crystal surface. The contact angle of the crystalline/amorphous mixed surface was estimated using the Cassie equation to be 91°. This value agrees well with experimental measurement of the water contact angle on the PVDF film. Furthermore, the interaction energy, interface structure, and electrostatic potential were analyzed to clarify the reason for the lower hydrophobicity of the amorphous surface. This surface interacts more favorably with water than the crystal surface. Such an interaction reduces the excess free energy (interfacial tension) at the PVDF and water interface and makes the amorphous surface less hydrophobic. The amorphous interfacial region contains more water molecules than the crystal one, and water molecules are oriented toward the PVDF. This interface structure makes water strongly interact with the PVDF.

Thermal stability of the Ti-Zr-Cu-Pd nano-glassy thin films

Nano-glassy thin films consist of nanometer-sized glassy regions (clusters) with a structure corresponding to melt-quenched glasses and amorphous interfacial regions characterized by a reduced density, or locally enhanced free volume, and in many cases a modified chemical composition. Ti-Zr-Cu-Pd nano-glass thin films were synthesized by using direct current (DC) magnetron sputtering on Si substrates. The influence of the sputtering conditions (sputtering power and Ar gas pressure) on the nano- and microstructure of deposited films and on their thermal behavior was analyzed. The thin films were noted to exhibit a homogeneous amorphous structure when sputtered at low Ar pressure (0.2 Pa) and a nano-glass structure with various sizes of the glassy cluster at higher Ar pressures (0.5–0.8 Pa). By raising the Ar pressure, the average size of the glassy clusters was noted to decrease corresponding to an increase of the volume fraction of the interfacial regions. The glass transition and crystallization temperatures were determined by using differential scanning calorimetry (DSC) at heating rates up to 40 Kmin−1. The thermal stability of the thin films was studied by annealing at temperatures above the glass transition and crystallization temperatures. The microstructure of the thin films was studied by means of X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy. The results obtained indicate that the nano-glassy thin films exhibit higher thermal stability than homogeneous glassy thin films. The fact that an increase of the volume fraction of the interfacial component in nano-glassy thin films results in an ultra-stable thermal behavior is indicating that the presence of nano-glassy interfaces impedes crystallization.

Biodegradable polymers for marine antibiofouling: Poly(ε-caprolactone)/poly(butylene succinate) blend as controlled release system of organic antifoulant

Polymer carrier is critical for the controlled release of antifoulant in marine anti-biofouling. We have prepared biodegradable polymer blend consisting of poly(ε-caprolactone) (PCL) and poly(butylene succinate) (PBS) and used it as carrier of organic antifoulant (4,5-dichloro-2-octyl-isothiazolone). Polarizing optical microscope (POM) and differential scanning calorimetry (DSC) measurements show that the spherulite size of PCL decreases and its crystallinity slightly varies as PBS fraction increases leading the interfacial regions between PCL and PBS spherulites to increase in the total area. Our studies reveal that such amorphous interfacial regions allow the blend to degrade at a constant rate and the antifoulants to be uniformly dispersed in the blend. The antifoulants are released at a constant rate as the blend degrades in marine environment, which can be modulated by the blend composition. Marine field tests demonstrate that such a system has good marine anti-biofouling performance.

The atomic behavior during heating in the amorphous diffusion nanolayer between Cu and solder

The formation of an amorphous solid has been generally investigated through simulations due to the difficulties related to experimental observation. There has been little experimental observation of the dynamic behavior of atoms in amorphous solids especially in the case of the amorphous interfacial diffusion region, which governs the interaction between two contact metals. Here, we presented an in situ observation of the dynamic behavior of atoms in an amorphous interfacial region existing between solder and Cu. The layer was produced via rapid quenching in liquid nitrogen with a Cu specimen soldered with Sn3.0Ag0.5Cu at above the melting temperature of the solder. The investigation of the diffusion layer of the solidified specimen was conducted in situ with a high resolution transmission electron microscope (HRTEM) under heating at 100°C. The atoms aggregated in the amorphous layer in terms of different unit structures, which gave rise to an entirely disordered structure. The study showed that the heating caused dynamic interactions among atoms that resulted in the transformation of these unit structures in the amorphous nanolayer.

Factors influencing properties of interfacial regions in semicrystalline polyethylene: A molecular dynamics simulation study

Molecular dynamics simulations have been used to prepare semicrystalline samples of linear polyethylene (PE) using the anisotropic united atom (AUA4) model. The initial configurations of the simulations were obtained by controlled melting of the pure crystalline system and a unique re-connection method between chain segments in the intercrystalline regions. Systems containing two crystalline slabs and two large intercrystalline regions are built with different defects, introduced to mimic the structure of real PE systems, namely: bridging chains connecting two different crystalline slabs, re-entrant chains emerging and re-entering into the same crystalline slab, and free chains. We observe large structural differences between the different systems. We show that average and local density depend on defect types which in turn affect the dynamical properties. This explicit simulation of a crystalline–amorphous interfacial region thus gives a possible picture of intercrystalline regions that can serve to understand and predict penetrant gases transport in such materials.

Synthesis and microstructure of bulk nanocrystalline copper

The synthesis of bulk nanocrystalline copper (NC-Cu) by powder metallurgy is presented, from compaction of nanocrystalline powders to sintering and differential extrusion. At each step of the process, the microstructure is characterized using X-ray diffraction (XRD) analysis, field-emission gun scanning electron microscopy (FEG-SEM), and transmission electron microscopy (TEM). Particular attention is given to the concept of grain size in nanostructured materials and the comparison of results from the different characterization techniques. The fully dense material has a grain size of 100 nm with a microstructure best described in terms of the distribution of high-angle grain boundaries (GBs), twin boundaries, and low-angle GBs. Dislocations occur in half the grains and at most of the twin boundaries. The GBs are shown to be crystalline, and no evidence is found for amorphous interfacial regions. It is proposed that the grain size be defined only in terms of high-angle GBs, excluding low-angle GBs, for the discussion of mechanical properties. In this respect, the microstructure is compared with the NC-Cu material produced by other synthesis techniques. Powder metallurgy (P/M) processing is revealed as an alternative for the production of large-size submicrocrystalline and NC materials.

On eliminating deposition-induced amorphization of interfaces in refractory metal multilayer systems

Multilayer thin films with refractory metal components are susceptible to solid state amorphization. It is an objective to minimize this interfacial reaction in order to ensure smooth layering and compositionally abrupt interfaces. These features are desirable for maximum reflectivity in X-ray optic applications such as projection lithography as well as neutron supermirror applications. This goal is achievable when the deposition process is optimized to thermalize the sputtered neutrals. Low working gas pressures (below 5 m Torr) and large source-to-substrate distances (more than 12 cm) typically provide the proper conditions to form dense sputter deposits without adatom-induced, interfacial amorphization. The interfaces of Mo/Si and Ni/Ti multilayers are examined with high resolution electron microscopy for samples sputter deposited under thermalized conditions. It is shown to be possible to produce crystalline, refractory metal layers in the as-deposited structure without amorphous interfacial regions.

X-Ray-Optical Multilayer Structures Studied Using High Resolution Electron Microscopy.

ABSTRACTThe technique of high resolution electron microscopy has been used to examine the structure of several multilayer systems (MLS) on an atomic scale. Mo/Si multilayers, in use in a number of x-ray optical element applications, and Mo/Si multilayers, of interest because of their magnetic properties, have been imaged in cross-section. Layer thicknesses, flatness and smoothness have been analysed: the layer width can vary by up to 0.6nm from the average value, and the layer flatness depends on the quality of the substrate surface for amorphous MLS, and on the details of the crystalline growth for the crystalline materials. The degree of crystallinity and the crystal orientation within the layers have also been investigated. In both cases, the high-Z layers are predominantly crystalline and the Si layers appear amorphous. Amorphous interfacial regions are visible between the Mo and Si layers, and crystalline cobalt suicide interfacial regions between the Co and Si layers. Using the structural measurements obtained from the HREM results, theoretical x-ray reflectivity behaviour has been calculated. It fits the experimental data very well.

The poly-single crystalline silicon interface

Cross sectional transmission electron microscopy (TEM) reveals an amorphous interfacial region of the order of 2 nm thick between chemical vapor deposition-(CVD) deposited polycrystalline silicon films and the single-crystal silicon substrate. The continuity of this region varies from sample to sample and plays an important role in the effects produced by subsequent heat treatment. In cases where this interfacial layer is continuous, the deposited layer remains polycrystalline. When the region is discontinuous, complete epitaxial realignment of the poly is possible. The speed of realignment depends on the implanted arsenic dose and is much greater than reported for undoped films. Various impurities are also observed at the interface and correlate with the character of the interface.