Interdiffusion Region Research Articles

Record-Low thermal boundary resistance at bonded GaN/diamond interface by controlling ultrathin heterogeneous amorphous layer

Thermal boundary resistance (TBR) in semiconductor-on-diamond structure bottlenecks efficient heat dissipation in electronic devices. In this study, to reduce the TBR between GaN and diamond, surface-activated bonding with a hybrid SiOx-Ar ion source was initially applied to achieve an ultrathin interfacial layer. The simultaneous surface activation and slow deposition of the SiOx binder layer enabled precise control over layer thickness (2.5–5.3 nm) and formation of an amorphous heterogeneous nanostructure comprising a SiOx region between two inter-diffusion regions. Crucially, the 2.5-nm-thick interfacial layer achieved a TBR of 8.3 m2⋅K/GW, a record low for direct-bonded GaN/diamond interface. A remarkable feature is that the TBR is extremely sensitive to the interfacial thickness; Varying from 8.3 m2⋅K/GW to 34 m2⋅K/GW with thickness difference of only 2.8 nm. Theoretical analysis revealed the origin of this phenomena: a diamond/SiOx inter-diffusion layer extend the vibrational frequency, far exceeding that of crystalline diamond, which increases the lattice vibrational mismatch and suppresses phonon transmission.

What drives the heterogeneous interdiffusion in the Li-Si interfacial region of Si anodes: the Li flux or the Si flux?

The electrochemical reaction in silicon (Si) electrode, accompanying with tremendous volume expansion, causes rapid capacity fade of Li-ion batteries. The Li-ion concentration gradient and structural distribution uniformity influence the inhomogeneous expansion, and the kinetic mechanism of lithiation and interfacial morphology evolvement remains debated. The present study focuses on the dynamics of Li-Si interdiffusion at Si/Li interfaces with various Si-facet orientations and phases using a machine-learning potential. We find that the Si flux from bulk Si to Li-Si interface regions controls the length of Li-Si interdiffusion region. The lithiation length in different Si/Li interface systems exhibits the order of amorphous-Si > crystalline-Si(110) > crystalline-Si(100) > crystalline-Si(111), which agrees with the experimental trend. Our atomic simulations further reveal that the key factor determining the Li-Si interdiffusion is the difference of on-site Si atomic energies between the bulk Si and the Li-Si interface regions. We propose that the large interdiffusion extent is due to a low thermodynamics barrier. Our findings provide insights for the development of high-performance Si anode materials.

Se Inter-Diffusion Limits Absorber Layer Grain Growth in CdSe - CdTe Photovoltaics

Diffusion of Se from the CdSe window layer into the CdTe absorber improves the short circuit current density by narrowing the band gap and increasing the carrier lifetime. Thicker CdSe layers, however, show a dramatic loss in photocurrent collection due to Se over-alloying. Electron microscopy investigations show that this decrease in performance is due to the formation of small grains (∼783 nm average diameter), which exhibit grain boundary porosity in the Se inter-diffusion region. The larger grain boundary area and void free surfaces give rise to higher levels of nonradiative recombination, and therefore, a lower photocurrent. It is proposed that the small grain size is due to a drag force exerted by segregated Se solute atoms on a moving grain boundary, while faster Se diffusion along the grain boundaries results in vacancy build up and porosity due to the Kirkendall effect. The results indicate that the device processing conditions must be carefully controlled such that the negative effects of Se alloying (i.e., smaller grains, Kirkendall voids) do not undermine its benefits. Published by the American Physical Society 2024

The Preparation of Ti–Ta Impedance-Graded Coatings with Limited Diffusion by Cold Spraying Combined with Hot Isostatic Pressing

Ti–Ta impedance-graded coatings were prepared using cold spraying combined with hot isostatic pressing. Compared to the general Ti–Ta binary diffusion couple, the interdiffusion coefficient of as-sprayed Ti–Ta can be increased by approximately 25 times at 1100 °C due to grain refinement at the interface of the cold-sprayed particles. By the control of interdiffusion, pure Ta and pure Ti regions can remain in the materials after hot isostatic pressing at 900 °C. Hot isostatic pressing with capsulate reduced the porosity of the material efficiently to less than 0.02%. The strength of the as-sprayed Ti–Ta composite coating was significantly improved to 990.1 MPa, and the fracture strain reached 11.5%. The strengthening mechanism of Ti–Ta composite coatings relies primarily on the hindrance of dislocation slip by phase interfaces between α and β. Moreover, the macroscopic interfacial bonding strength of the graded material exceeds 881 MPa, which is comparable to that of bulk materials.

Interface analyses and mechanical properties of stainless steel/nickel alloy induced by multi-metal laser additive manufacturing

Multi-metal laser additive manufacturing is an emerging technology to fabricate complex components with excellent comprehensive performances. In this study, stainless steel/nickel alloy multi-metal has been induced by laser direct energy deposition. The microstructural evolution, compositional segregation, and crack formation at the interface of the two metals have been investigated by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, electron backscattering diffraction, and microhardness test. The results show that the microstructure morphologies are predominated by equiaxed dendrite and columnar dendrite for stainless steel and nickel alloy, respectively. The Marangoni convection effect promotes the serious compositional segregation of stainless steel on the nickel alloy side and serious segregation of 316L SS occurs locally in near the IN 718 side of the interdiffusion region. During the cooling process, the segregated stainless steel has a lower solidification temperature than the surrounding nickel alloy. The compositional segregation and different physical properties of the two metals are the main causes for the ductility dip cracking near the interface. During tensile deformation, the multi-metal fractures on the stainless steel side, and the ultimate tensile strength and elongation of the multi-metal have achieve 524 MPa and 24.8 %, respectively.

Density functional theory study of formation and diffusion of hydrogen, deuterium, and tritium in Pd-V intermetallic compounds

Permeation of hydrogen isotopes in palladium/vanadium bimetallic membranes is known to deteriorate over time because of Pd-V interdiffusion. Intermetallic compounds may form in the interdiffusion region. Density functional theory is employed to study how Pd-V compounds may affect the permeation. Three compounds Pd8V, α-Pd2V, and PdV4 are explored in this study. Formation and migration energies of hydrogen, deuterium, and tritium are calculated and subsequently compared to the data in pure Pd and V metals. The calculations show that both the formation and migration energies in the compounds are higher than in the pure metals. Thus, the permeation of these isotopes in the compounds is lower than in the pure metals. In addition, the least permeable compound is the one near the middle of the composition range, i.e. the α-Pd2V. The results provide atomistic insight for the permeation reduction in Pd/V membranes as interdiffusion progresses.

Gate-Bias Induced Threshold Voltage (V TH) Instability in P-N Junction/AlGaN/GaN HEMT

In this work, we studied the threshold voltage ( <formula> <tex>$V_{TH}$</tex> </formula> ) instability in E-mode p-n junction (PNJ)/AlGaN/GaN high-electron-mobility transistor (HEMT) using pulsed-I/V measurement and positive bias temperature instability (PBTI) test. <formula> <tex>$V_{TH}$</tex> </formula> shifts positively under forward gate bias, which is ascribed to electron trapping in the gate-stack region. Benefiting from the special p-GaN/n-GaN junction, reduced electric field suppresses electron trapping in the PNJ gate, resulting in more stable <formula> <tex>$V_{TH}$</tex> </formula> compared with the conventional Schottky-type p-GaN gate. Specifically, less positive threshold voltage shift ( <formula> <tex>$V_{TH}$</tex> </formula> ) is observed under higher temperatures, or after being stressed for a prolonged period with gate bias exceeding 6 V. The decrease in positive <formula> <tex>$V_{TH}$</tex> </formula> results from enhanced hole injection and especially hole trapping in the p-GaN/n-GaN interdiffusion region, which compensates for electron trapping and reduces positive <formula> <tex>$V_{TH}$</tex> </formula> .

Investigation of Electrical Parameters of CdTe Photovoltaic Devices by Computational Analysis

This article discusses the numerical analysis of heterojunction photovoltaic cells based on cadmium telluride (CdTe) using the SCAPS‐1D software. A novel glass/FTO/SnO2/CdS/interdiffusion/CdTe/SnS2/Se photovoltaic cell design is proposed for investigation. Additionally, the effect of thickness variation in the p‐type CdTe film, thickness variation in the interdiffusion layer, defect density in the CdTe film, defect density in the interdiffusion layer, and acceptor concentration in the CdTe layer is studied in order to improve the photovoltaic cell's efficiency. The study determines an optimal thickness of 1 and 0.6 μm for the CdTe and interdiffusion layers, respectively, a defect density of 1 × 1014 cm−3 for the CdTe film and interdiffusion region, an interface defect density of 1 × 1010 cm−3 between CdTe/interdiffusion, SnS2/CdTe, and CdS/interdiffusion, and an acceptor concentration value of 8.5 × 1015 cm−3. The optimized CdTe photovoltaic cell structure with current density (Jsc) = 30.003 mA cm−2, open‐circuit voltage (Voc) = 1.061 V, and fill factor (FF) = 88.10% achieves a 28.07% efficiency, compared to the baseline CdTe photovoltaic cell structure with a 23.01% efficiency.

Processing of IN718-SS316L bimetallic-structure using laser powder bed fusion technique

ABSTRACT A crack-free Inconel 718 – Stainless steel 316 L bimetallic structure has been fabricated using the laser powder bed fusion (LPBF) process. Firstly, an optimum process parameter set resulting in defect-free bonding of both materials has been predicted by carrying out a parametric study. Thereafter, microstructural and mechanical characterization such as optical microscopy, X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectroscopy, hardness and tensile testing have been carried out to investigate the characteristics of the bimetallic interface. The porosity quantification shows a density greater than 99.5% with no observable cracks or porosity at the transition zone. The secondary electron micrographs reveal a small inter-diffusion region at the interface. The interface and the nearby region consist of both cellular and columnar grains. The tensile strength of the bimetallic-structure approaches the strength of stainless steel 316 L. Even with the material discontinuity, the fracture happens at the weaker side (stainless steel 316 L) indicating the strong strength of the dissimilar interface. The fractography of the fractured surface displays a typical ductile cup and cone type of fracture. The results strongly suggest the suitability of the LPBF process for fabricating defect-free high-strength IN718-SS316L bimetallic structures.

Tailoring the CdS/CdSe/CdTe multilayer structure for optimization of photovoltaic device performance guided by mapping spectroscopic ellipsometry

Thin film CdTe superstrate solar cells have been fabricated by sputtering starting from CdS/CdSe front layers deposited on transparent conductor coated glass. The performance of such devices is sensitive to the fabrication details including the temperature-time profile, which leads to CdSe/CdTe interdiffusion and formation of a CdTe1-xSex bandgap-graded absorber. Mapping spectroscopic ellipsometry (M-SE) has been applied to the CdS and CdSe thin films for process calibration, which involves determining the deposition rate in terms of effective thickness (volume/area) versus spatial position on the sample. The goal is to optimize the performance of the devices by correlating cell parameters with these two effective thicknesses. Intended variations in the thicknesses along with unintended spatial non-uniformities enable coarse and fine-scale optimization, respectively. Using these methods, the highest performance solar cells from the CdS/CdSe/CdTe structure are obtained with 13 nm CdS and 100 nm CdSe. An increase in the CdS thickness above 13 nm leads to a decrease in open-circuit voltage and fill-factor attributed to the formation of a CdSe1-zSz interdiffusion region with z approaching 0.5, where the alloy electronic properties are likely to suffer. Our results demonstrate that M-SE, exploited in conjunction with deposition non-uniformities, serve as a viable approach for process optimization of complex solar cell structures.

Wire-arc additive manufacturing of nickel aluminum bronze/stainless steel hybrid parts – Interfacial characterization, prospects, and problems

Hybrid parts of nickel aluminum bronze (NAB) and 316L stainless steel were fabricated using a commercially available wire-arc additive manufacturing (WAAM) technology to evaluate the feasibility and cracking tendency. Focused Ion beam (FIB) based Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), Electron Backscatter Diffraction (EBSD), and Transmission Electron Microscopy (TEM) were used to characterize the built (NAB)-substrate (SS) interfacial characteristics. FIB extracted a selected region of the interface, and the spatial distribution of the interface across several sections was characterized by using the state-of-the-art technique for 3D EBSD mapping. A metallurgically bonded interface without any pores and cracks, with the inter-diffusion region in a thickness of 2 μm, was formed, which was further confirmed by a video with the results of 3D reconstructed EBSD maps. The interface did not exhibit any strong texture orientation owing to the control of the thermal gradient as NAB is more conductive than 316L. EDS elemental mapping confirmed that Fe3Al intermetallic was formed at the NAB/SS bimetallic-joint interface. Occasional liquation cracks on the grain boundaries in the heat-affected zone (HAZ) of 316L substrate were observed. Fe-Al based intermetallic formation, along with the penetration of copper along the HAZ cracks, was noticed. The problems associated were highlighted, and remedial measures were suggested to open up the possibilities of additive manufacturing to fabricate NAB-Stainless steel hybrid parts for industrial repair and maintenance applications.

Nanojoining and tailoring of current–voltage characteristics of metal-P type semiconductor nanowire heterojunction by femtosecond laser irradiation

Selective engineering of the interface between nanoscale components and the electrical properties of heterojunctions is key to the development of next-generation nanoscale circuit elements. In this paper, we show how laser processing of a metal-P type semiconductor nanoscale heterojunction between Ag and CuO nanowires can be used to control the nature of the electrical contact by reducing the Schottky barrier at the Ag–CuO interface to Ohmic contact. Elimination of the Schottky barriers occurs in response to lattice matching of Ag(111)∥CuO(111) planes at the interface induced by controlled irradiation with femtosecond (fs) laser pulses. An interdiffusion region with a mixed Ag/CuO composition is also present over a localized area of the interface between the Ag and CuO nanowires after fs laser processing, but both Ag and CuO nanowires remain crystalline away from the heterojunction. In addition, the Ag nanowire becomes totally embedded in the larger CuO nanowire after irradiation. Fabricated nanowire devices from Ag–CuO nanowire heterojunctions transition from a double-Schottky contact configuration prior to laser processing to a rectifying behavior as irradiation time increases. This study illustrates that fs laser processing can be highly effective in the engineering of electrical performance in metal–semiconductor nanoscale heterojunction devices.

Microstructure and Mechanical Property of Ti-5Al-2.5Sn/Ti-6Al-4V Dissimilar Titanium Alloys Integrally Fabricated by Selective Laser Melting

Metallic structural components made from dissimilar titanium alloys are playing important roles in high-end industries such as aerospace and aviation. In this paper, a Ti-5Al-2.5Sn/Ti-6Al-4V dissimilar titanium alloy material was additively manufactured by the selective laser melting (SLM) process. Interface characteristics, microstructure, and mechanical properties of the SLM-deposited samples before and after complete annealing treatment were researched to provide some technical basics for the integral fabrication of dissimilar titanium alloy components. The results demonstrated that a defect-free metallurgical bonded interface with a ~ 70-μm-wide element inter-diffusion region could be obtained between the Ti-5Al-2.5Sn and Ti-6Al-4V layers under both the as-deposited and the complete annealing states. The interfacial bonding strength between the dissimilar titanium alloys was always higher than the strength of the Ti-5Al-2.5Sn layers. Microstructure evolution and element inter-diffusion mechanisms of the SLM-produced Ti-5Al-2.5Sn/Ti-6Al-4V samples were also revealed and related to the change in mechanical properties.

Selective laser melting of tungsten-copper functionally graded material

A tungsten-copper functionally graded material was processed based on SLM additive manufacture despite encountering some difficulties from materials characters. The effect of laser parameter on the interfacial defects and bonding performance are evaluated. The SLM produced tungsten is in columnar structures with random orientation. Plenty of fine tungsten grains are present at the bonding region, owing to a high cooling rate incited by the underlying copper. A metallurgically bonded interface with a 50–80 μm inter-diffusion region is formed. The interfacial bonding mechanism, which associates with intense Marangoni convection at the interface, is revealed and discussed.

Effect of ZrO₂ Nanomaterials on Wettability and Interfacial Characteristics of Al-19Cu-11Si-2Sn Filler Metal for Low Temperature Al to Cu Dissimilar Brazing.

Dissimilar Al 3003 and Cu tubular components were successfully brazed without interface cracking using ZrO2 nanomaterials reinforced with Al-19Cu-11Si-2Sn filler. The filler was initially cast using an induction furnace and processed into ring form for brazing. Al-19Cu-11Si-2Sn filler with coarse CuAl2 and Si phases (43 and 20 μm) were refined to 8 and 4 μm, respectively, after the addition of 0.1 wt. % ZrO2 and shows significant improvement in the mechanical properties. ZrO2 nanomaterials’ induced diffusion controlled growth mechanism is found be the responsible for the refinement of CuAl2 intermetallic and Si particles. The wettability of Al-19Cu-11Si-2Sn-0.1ZrO2 increased to 78.17% on Cu side and 93.19% on the Al side compared from 74.8% and 89.9%, respectively. Increase in the yield strength, ultimate tensile strength, and percentage elongation were noted for the brazed joints. Microstructure of induction brazed joint with 40 kW for 6 seconds using Al-19Cu-11Si-2Sn-0.1ZrO2 filler shows thin interfacial CuAl2 intermetallic compound along the copper side and inter-diffusion region along the aluminum side and their respective mechanism is discussed. The tensile strength of the joints increased with increasing the nanomaterials addition and shows a base metal fracture. Analysis of fractured samples shows the effectiveness of ZrO2 reinforced filler in crack propagation through the filler.

Novel high strength titanium-titanium composites produced using field-assisted sintering technology (FAST)

To increase the strength of titanium alloys beyond that achievable with α-β microstructures, alternative reinforcing methods are necessary. Here, field-assisted sintering technology (FAST) has been used to produce a novel Ti-5Al-5Mo-5V-3Cr (Ti-5553) metal-matrix-composite (MMC) reinforced with 0-25 wt.% of a ∼2 GPa yield strength TiFeMo alloy strengthened by ordered body-centred cubic intermetallic and ω phases. The interdiffusion region between Ti-5553 and TiFeMo particles was studied by modelling, electron microscopy, and nanoindentation to examine the effect of graded composition on mechanical properties and formation of α, intermetallic, and ω phases, which resulted in a > 200 MPa strengthening benefit over unreinforced Ti-5553.

Interfacial characteristic and mechanical performance of maraging steel-copper functional bimetal produced by selective laser melting based hybrid manufacture

A combination of selective laser melting (SLM) additive manufacture and subtractive process was explored to produce maraging steel‑copper bimetal. Relationships among laser parameter, interfacial characteristic and mechanical performance are elucidated. A metallurgical bonded interface with a 30–40 μm inter-diffusion region is formed. Gradient submicro grains with strong 〈111〉 orientation exhibit at the interface, which owe to high cooling rate and temperature gradient caused by high thermal conductive copper. A selected region of the interface was extracted by focused ion beam (FIB) for interfacial bonding analysis. The bonding mechanism is revealed and illustrated in detail. Interfacial intense Marangoni flows pull the copper toward the molten pool of maraging steel and the liquid maraging steel penetrates into the melting copper, which contributes to interfacial bonding. The bonding strength of hybrid processed bimetals are evaluated. Fracture in tensile is not present at the interface but on the copper side. The highest flexural strength reaches 557 MPa, which is slightly higher than that of the copper. Effects of parameter on fracture behaviors are also elucidated. This hybrid manufacture increases the productivity and functionality of the direct SLM-produced part, and provides a new approach for producing high-performance functional dissimilar bimetals based on laser additive manufacture.

Interfacial bonding mechanism and annealing effect on Cu-Al joint produced by solid-liquid compound casting

Copper-aluminum (Cu-Al) based lamellar composites were prepared using a solid-liquid compound casting (SLCC) technology. Characterization results showed that the Cu-Al composites were fully-sintered at 700°C under an argon atmosphere using the SLCC technology. Cu-Al interfacial bonding was uniform with a well-defined transitional and inter-diffusion region. Intermetallic compounds and solid solutions of CuAl2, CuAl, Cu9Al4, CuAl3 and Cu3Al2 were detected at the interfacial region. With the increase of annealing temperature, the width of the Cu-Al interfacial region was increased, and the interfacial bonding strength was also increased, whereas the types of the intermediate phases were changed. With the increase of dwelling time at a given annealing temperature, the width of Cu-Al interfacial region was increased, the interfacial bonding strength was decreased and the mesophases were changed. The bonding strength of the as-prepared composite was 30MPa, whereas those of specimens annealed at 200°C for 2h, 300°C for 2h, 400°C for 2h, 300°C for 30min and 300°C for 1h were 59, 39, 74, 56, and 49MPa, respectively. The Cu-Al interfacial bonding mechanisms were identified to be rapid inter-diffusion of copper and aluminum and formation of interfacial and graded microstructures. The formation of copper-aluminum interface is a combined result of inter-atomic diffusion and interfacial chemical reactions, the latter of which is more dominant in the diffusion process.

The Microstructural Evolution of Vacuum Brazed 1Cr18Ni9Ti Using Various Filler Metals.

The microstructures and weldability of a brazed joint of 1Cr18Ni9Ti austenitic stainless steel with BNi-2, BNi82CrSiBFe and BMn50NiCuCrCo filler metals in vacuum were investigated. It can be observed that an interdiffusion region existed between the filler metal and the base metal for the brazed joint of Ni-based filler metals. The width of the interdiffusion region was about 10 μm, and the microstructure of the brazed joint of BNi-2 filler metal was dense and free of obvious defects. In the case of the brazed joint of BMn50NiCuCrCo filler metal, there were pits, pores and crack defects in the brazing joint due to insufficient wettability of the filler metal. Crack defects can also be observed in the brazed joint of BNi82CrSiBFe filler metal. Compared with BMn50NiCuCrCo and BNi82CrSiBFe filler metals, BNi-2 filler metal is the best material for 1Cr18Ni9Ti austenitic stainless steel vacuum brazing because of its distinct weldability.

Fundamental Concept of Interdiffusion Problems

In accordance with the definition of diffusivity, the origin of coordinate system of the original diffusion equation is set at a point in the solvent material. Kirkendall revealed that Cu atoms, Zn atoms and vacancies move simultaneously in the interdiffusion region. This indicates that the original diffusion equation is a moving coordinate system for the experimentation system outside the diffusion region. The diffusion region space which means vacancies and interstices among atoms plays an important role in the diffusion phenomena. The theoretical equation of the Kirkendall effect is reasonably obtained as a shift between coordinate systems of the diffusion equation. The situation is similar to the well-known Doppler effect in the wave equation. Boltzmann transformed the original diffusion equation of a binary system into the nonlinear ordinary differential equation in accordance with the parabolic law. In the previous works, the solutions of the diffusion equation transformed by Boltzmann were analytically obtained and we found that the well-known Darken equation is mathematically wrong. In the present study, we found that the so-called intrinsic diffusivity corresponds in appearance to the physical solution obtained previously. However, the intrinsic diffusivity itself conceived in the diffusion research history is essentially nonexistent.