Abstract: Two-dimensional (2D) materials are promising for use in lithium (Li) electrodes due to their high surface ratio. By using density functional theory (DFT) calculations, we investigate the adsorption and diffusion of Li on a newly predicted 2D GeP3 material [Nano Lett., 2016, 17, 1833]. The most favourable adsorption sites for Li are identified, and a semiconducting to metallic transition induced by Li adsorption is found, which indicates excellent electrical conductivity. The GeP3 monolayer has an estimated capacity of 648 mA h g−1, which is almost twice that of commercially used graphite (375 mA h g−1). During full Li intercalation, the GeP3 layer undergoes only 1.2% lattice parameter reduction. Moreover, GeP3 possesses the advantages of a small diffusion barrier (∼0.5 eV) and low average open-circuit voltages (∼0.4 V). Our results highlight a new class of promising anode materials, i.e. 2D phosphide, as potential rechargeable lithium batteries with ultrahigh-capacity, superior ionic conductivity, and low average open-circuit voltage.

Abstract: A method of processing a substrate to enable selective doping without a photolithography process is provided The method includes forming a diffusion barrier on the substrate having a patterned structure using plasma deposition method, removing the diffusion barrier except for part of the diffusion barrier using wet etching, forming a diffusion source layer on the patterned structure and the part of the diffusion barrier, and applying energy to the diffusion source layer

Abstract: Copper interconnects in modern integrated circuits require a barrier layer to prevent Cu diffusion into surrounding dielectrics. However, conventional barrier materials like TaN are highly resistive compared to Cu and will occupy a large fraction of the cross-section of ultra-scaled Cu interconnects due to their thickness scaling limits at 2–3 nm, which will significantly increase the Cu line resistance. It is well understood that ultrathin, effective diffusion barriers are required to continue the interconnect scaling. In this study, a new class of two-dimensional (2D) materials, hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS2), is explored as alternative Cu diffusion barriers. Based on time-dependent dielectric breakdown measurements and scanning transmission electron microscopy imaging coupled with energy dispersive X-ray spectroscopy and electron energy loss spectroscopy characterizations, these 2D materials are shown to be promising barrier solutions for Cu interconnect technology. The predicted lifetime of devices with directly deposited 2D barriers can achieve three orders of magnitude improvement compared to control devices without barriers. Atomically thin h-BN and MoS2 may provide a viable alternative to conventional barrier materials in Cu interconnects. A team led by Zhihong Chen at Purdue University utilized two-dimensional crystals to mitigate Cu diffusion into the dielectric, a known cause of chip failure. By means of time-dependent dielectric breakdown measurements to investigate the diffusion barrier properties of atomically thin h-BN and MoS2, they recorded a substantial improvement of the time-to-breakdown, owing to a reliability enhancement of the dielectric underneath Cu under normal operating conditions. A number of structural and electrical characterizations, including scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy confirmed that two-dimensional h-BN and MoS2 films effectively prevent Cu diffusion, highlighting their potential applicability as sub-nanometer barrier for interconnect technology.

Abstract: Academics and industry have sought after combining the exceptional properties of diamonds with the toughness of steel. Since the early 1990s several partial solutions have been found but chemical vapor deposition (CVD) diamond deposition on steel substrate continues to be a persistent problem. The main drawbacks are the high carbon diffusion from gas phase into substrate, the transition metals on the material surface that catalyze sp2 bond formation, instead of sp3 bonds, and the high thermal expansion coefficient (TEC) mismatch between diamond and steels. An intermediate layer has been found necessary to increase diamond adhesion. Literature has proposed many efficient intermediate layers as a diffusion barrier for both, carbon and iron, but most intermediate layers shown have not solved TEC mismatch. In this review, we briefly discuss the solutions that exclusively work as diffusion barrier and discuss in a broader way the ones that also solve, or may potentially solve, the TEC mismatch problem. We examine some multilayers, the iron borides, the chromium carbides, and vanadium carbides. We go through the most relevant results of the last two and a half decades, including recent advances in our group. Vanadium carbide looks promising since it has shown excellent diffusion barrier properties, its TEC is intermediary between diamond and steel and, it has been thickened to manage thermal stress relief. We also review a new deposition technique to set up intermediate layers: laser cladding. It is promising because of its versatility in mixing different materials and fusing and/or sintering them on a steel surface. We conclude by remarking on new perspectives.

Abstract: The deliberate introduction of K and Na into Cu(In, Ga)Se2 (CIGS) absorbers was investigated by varying a combination of an SiO2 diffusion barrier, coevaporation of KF with the CIGS absorber, and a KF postdeposition treatment (PDT). Devices made with no diffusion barrier and KF coevaporation treatment exhibited the highest photovoltaic conversion efficiency with the smallest overall distribution in key current density–voltage ( J-V ) performance metrics. Out-diffusion of Na and K from the substrate, KF coevaporation, and KF PDT all increased carrier concentration, open-circuit voltage, fill factor, and power conversion efficiency. Quantum-efficiency analysis of devices highlighted the greatest loss in the short-circuit current density due to incomplete absorption and collection. Secondary ion mass spectrometry illustrated the efficacy of the SiO2 film as a sodium and potassium diffusion barrier, as well as their relative concentration in the absorber. Introduction of KF appeared to enhance diffusion of Na from the substrate, in agreement with previous studies.

Abstract: The self-diffusion behavior of vanadium subcarbide (V2C) is investigated using density functional theory calculations, owing to its potential application as a diffusion barrier in nuclear applications Three ordered V2C structures, two of which correspond to experimentally observed phases, are characterized in terms of their equilibrium structural, electronic and elastic properties Our model for self-diffusion in V2C considers diffusion of carbon and vanadium to occur separately on each sublattice Two sets of self-diffusion coefficients are calculated for each structure: one for vacancy-mediated diffusion of vanadium and the other for interstitial diffusion of carbon Calculated activation energies and diffusion prefactors are compared to experimental data for the cubic transition metal carbides as there is no experimental self-diffusion data for any of the hexagonal subcarbides

Abstract: Growth of perovskite oxide thin films on Si in crystalline form has long been a critical obstacle for the integration of multifunctional oxides into Si-based technologies. In this study, we propose pulsed laser deposition of a crystalline SrTiO3 thin film on a Si using graphene substrate. The SrTiO3 thin film on graphene has a highly (00l)-oriented crystalline structure which results from the partial epitaxy. Moreover, graphene promotes a sharp interface by highly suppressing the chemical intermixing. The important role of graphene as a 2D substrate and diffusion barrier allows the expansion of device applications based on functional complex oxides.

Abstract: For double MgO-based p-MTJ spin-valves with a top Co2Fe6B2 free layer ex-situ annealed at 400 °C, the insertion of a nanoscale-thickness Fe diffusion barrier between the tungsten (W) capping layer and MgO capping layer improved the face-centered-cubic (f.c.c.) crystallinity of both the MgO capping layer and tunneling barrier by dramatically reducing diffusion of W atoms from the W capping layer into the MgO capping layer and tunneling barrier, thereby enhancing the TMR ratio and thermal stability (Δ). In particular, the TMR ratio was extremely sensitive to the thickness of the Fe barrier; it peaked (154%) at about 0.3 nm (the thickness of only two atomic Fe layers). The effect of the diffusion barrier originated from interface strain.

Abstract: In the quest for metal contacts for electronic devices handling high current densities, we report the results of Pd/Ti/Pd/Ag and Pd/Ge/Ti/Pd/Ag contacts to n-GaAs and compare them to Ti/ Pd/Ag and AuGe/Ni/Au. These metal systems have been designed with the goal of producing an electrical contact with (a) low metal–semiconductor specific contact resistance, (b) very high sheet conductance, (c) good bondability, (d) long-term durability and (e) cost-effectiveness. The structure of the contacts consists of an interfacial layer (either Pd or Pd/Ge) intended to produce a low metal–semiconductor specific contact resistance; a diffusion barrier (Ti/Pd) and a thick top layer of Ag to provide the desired high sheet conductance, limited cost and good bondability. The results show that both systems can achieve very low metal resistivity (ρM ∼ 2 × 10−6 Ω cm), reaching values close to that of pure bulk silver. This fact is attributed to the Ti/Pd bilayer acting as an efficient diffusion barrier, and thus the metal sheet resistance can be controlled by the thickness of the deposited silver layer. Moreover, the use of Pd as interfacial layer produces contacts with moderate specific contact resistance (ρC ∼ 10−4 Ω cm2 ) whilst the use of Pd/Ge decreases the specific contact resistance to ρC ∼ 1.5 × 10−7 Ω cm2 , as a result of the formation of a Pd4(GaAs, Ge2) compound at the GaAs interface.

Abstract: This work demonstrates a solution to the problem of increase in Schottky barrier height ( ϕ B) with thermal annealing (thermal instability) in unpinned (low ϕ B) Ti/Ti O 2/n-Ge metal-interfacial layer (IL)-semiconductor (MIS) contacts through plasma nitridation of the Ti O 2 layer. Unlike TiO2, unpinned ( ϕ B = 0.09 eV) Ti O x N y contacts are thermally stable for anneals up to 30 min at 400 °C. The thermal stability improves with increasing nitrogen concentration ([N], 2.5–9.5%) and is independent of thickness (2–5 nm) for [N] = 9.5%. Additionally, the plasma nitridation process is shown to increase the oxygen vacancy concentration (n-type doping) and reduce the ϕ B dependence on Ti O x N y thickness in unannealed Ti O x N y contacts. Enhanced thermal stability is attributed to the incorporated nitrogen acting as a diffusion barrier that prevents contact pinning through reduction of the TiO2 layer by contact metal during the anneal, as well as preserves the amorphous nature of the IL along with its fixed...

Abstract: The thermal stability of joints in thermoelectric (TE) modules, which are degraded during interdiffusion between the TE material and the contacting metal, needs to be addressed in order to utilize TE technology for competitive, sustainable energy applications. Herein, we deposit a 200 nm-thick Zr-based thin-film metallic glass (TFMG), which acts as an effective diffusion barrier layer with low electrical contact resistivity, on a high-zT Se-doped AgSbTe2 substrate. The reaction couples structured with TFMG/TE are annealed at 673 K for 8–360 hours and analyzed by electron microscopy. No observable IMCs (intermetallic compounds) are formed at the TFMG/TE interface, suggesting the effective inhibition of atomic diffusion that may be attributed to the grain-boundary-free structure of TFMG. The minor amount of Se acts as a tracer species, and a homogeneous Se-rich region is found nearing the TFMG/TE interface, which guarantees satisfactory bonding at the joint. The diffusion of Se, which has the smallest atomic volume of all the elements from the TE substrate, is found to follow Fick’s second law. The calculated diffusivity (D) of Se in TFMG falls in the range of D~10−20–10−23(m2/s), which is 106~107 and 1012~1013 times smaller than those of Ni [10−14–10−17(m2/s)] and Cu [10−8–10−11(m2/s)] in Bi2Te3, respectively.

Abstract: In recent years great efforts have been made in the development of γ-TiAl alloys for use in aerospace applications such as turbines, where low densities and high temperature strength are required. However, γ-TiAl alloys show poor oxidation resistance at temperatures T > 850 °C due to the formation of a non-protective oxide layer consisting of a mixture of TiO 2 + Al 2 O 3 in air, which could be easily spalled off, resulting in a shortened lifetime of the components. One promising way to overcome this problem is the deposition of an oxidation protective coating with low oxygen permeability at high temperatures. However, the interdiffusion between coating and substrate is still challenging even for advanced oxidation resistant coatings such as MCrAlY (M = Ni or Co) and aluminide. The present work focuses on the (Cr,Al)ON coating system, inspired from its outstanding diffusion barrier properties. Four (Cr,Al)ON coatings with different Cr:Al and N:O ratios were deposited onto γ-TiAl substrate by the innovative High-Speed Physical Vapor Deposition (HS-PVD) technology, which enables the deposition of oxygen-rich coatings in a stable plasma process without target poisoning. Basing on hollow cathode discharge (HCD) and gas flow sputtering (GFS), the HS-PVD made it possible to deposit (Cr,Al)ON coatings at a deposition rate ds/dt > 16 μm/h. The as-deposited coatings show an X-ray amorphous structure. Subsequently, the transmission electron microscopy (TEM) analysis confirmed the nanocomposite coating structure. The thermal stability and oxidation behavior of the coatings were evaluated by means of differential scanning calorimetry (DSC) and thermogravimetry (TGA) between T = 100 °C and T = 950 °C. It was confirmed that the X-ray amorphous structure even remained stable at temperatures up to T = 950 °C. Moreover, cross-sectional SEM images of the coated samples after the HT-XRD measurements showed neither the formation of oxides at the coating substrate interface nor the interdiffusion of Ti into the coating, indicating a promising performance of the diffusion barrier. The results of the conducted research reveal a high potential of the HS-PVD deposited (Cr,Al)ON coatings for the oxidation protection of γ-TiAl at T > 850 °C in turbine applications.

Abstract: To enable the development of next-generation solid oxide fuel cells (SOFCs), the fabrication of dense and defect-free diffusion barrier layers via constrained sintering has been a significant challenge. Here, we present a double layer technique that enables complete densification of a defect-free gadolinia-doped ceria diffusion barrier layer. In this approach, top and bottom layers were individually designed to perform unique functions based on systematic analysis of constrained sintering. The top layer, which contains 1 wt% CuO as a sintering aid, provides sufficient sintering driving force via liquid-phase sintering to allow complete densification of the film, while the bottom layer without a sintering aid prevents detrimental chemical reactions and regulates the global sintering rate to eliminate macro-defects. Such fabrication of dense diffusion barrier layers via a standard ceramic processing route would allow the use of novel cathode materials in practical SOFC manufacturing. Furthermore, the strategy presented in this study could be exploited in various multi-layer ceramic applications involving constrained sintering.

Abstract: Diffusion of species in icy dust grain mantles is a fundamental process that shapes the chemistry of interstellar regions; yet measurements of diffusion in interstellar ice analogs are scarce. Here we present measurements of CO diffusion into CO$_2$ ice at low temperatures (T=11--23~K) using CO$_2$ longitudinal optical (LO) phonon modes to monitor the level of mixing of initially layered ices. We model the diffusion kinetics using Fick's second law and find the temperature dependent diffusion coefficients are well fit by an Arrhenius equation giving a diffusion barrier of 300 $\pm$ 40 K. The low barrier along with the diffusion kinetics through isotopically labeled layers suggest that CO diffuses through CO$_2$ along pore surfaces rather than through bulk diffusion. In complementary experiments, we measure the desorption energy of CO from CO$_2$ ices deposited at 11-50 K by temperature-programmed desorption (TPD) and find that the desorption barrier ranges from 1240 $\pm$ 90 K to 1410 $\pm$ 70 K depending on the CO$_2$ deposition temperature and resultant ice porosity. The measured CO-CO$_2$ desorption barriers demonstrate that CO binds equally well to CO$_2$ and H$_2$O ices when both are compact. The CO-CO$_2$ diffusion-desorption barrier ratio ranges from 0.21-0.24 dependent on the binding environment during diffusion. The diffusion-desorption ratio is consistent with the above hypothesis that the observed diffusion is a surface process and adds to previous experimental evidence on diffusion in water ice that suggests surface diffusion is important to the mobility of molecules within interstellar ices.

Abstract: We show the suppression of nucleation density in chemical vapor deposited graphene through the use of a sputtered metal coating on the exterior of a copper catalyst enclosure, resulting in the growth of sub-centimeter scale single crystal graphene domains and complete elimination of multilayer growth. The sputtered coating suppresses nucleation density by acting as both a diffusion barrier and as a sink for excess carbon during the growth, reducing the carbon concentration in the interior of the enclosure. Field effect mobility of hBN-templated devices fabricated from graphene domains grown in this way show room temperature carrier mobilities of 12 000 cm2 V−1 s−1 and an absence of weak localization at low temperature. These results indicate a very low concentration of line and point defects in the grown films, which is further supported by Raman and transmission electron microscopic characterization.

Abstract: La0.8Sr0.2(Ga0.8Mg0.2)1-x Fe x O3-δ (LSGMF, x = 0.2–0.9) was prepared by a solid-state reaction method, and the characterization was investigated. Limiting current oxygen sensors were fabricated with La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) as solid electrolyte and La0.8Sr0.2(Ga0.8Mg0.2)0.1Fe0.9O3-δ (LSGMF9) as dense diffusion barrier. The influences of temperature, oxygen concentration, and the thickness of dense diffusion barrier (L) on sensing properties of oxygen sensors were investigated. The results show that the crystal structure of samples is perovskite. The electrical conductivity increases with increasing x. A transition from semiconductive to pseudometallic behavior which is associated with oxygen losses from the lattice is observed with an increase of temperature for x = 0.3, 0.5, and 0.7. The thermal expansion coefficient (TEC) in the temperature range 300–1000 °C increases to a maximum and then decreases with increasing x. TECs between LSGMF9 and LSGM are close. The limiting current oxygen sensor exhibits an excellent sensing performance, and the limiting current responses depend linearly on the oxygen concentration. As the L value increases, the limiting current decreases. The polarization resistance of the sensor decreases with increasing oxygen concentration.

Abstract: Poor electronic conductivity of bulk lithium sulfide (Li2S) is a critical challenge for the debilitating performance of the lithium–sulfur battery. This study focuses on investigating the thermodynamic and kinetic properties of native defects in Li2S based on a first-principles approach. It is found that the hole polaron p+ can form in Li2S by removing a 3p electron from an S2– anion. The p+ diffusion barrier is only 90 meV, which is much lower than the Li vacancy (VLi–) diffusion barrier. Hence p+ has the potential to serve as a charge carrier in the discharge product. Once the vacancy–polaron complex (VLi––2p+) forms, the charge transport will be hindered due to the relatively higher diffusion barrier of the complex. Heteroatom dopants, which can decrease the p+ formation energy and increase VLi– formation energy, are expected to be introduced to the discharge product to improve the electronic conductivity.