Al2O3 Barrier Research Articles

Unraveling Interdiffusion Phenomena and the Role of Nanoscale Diffusion Barriers in the Copper-Gold System.

Diffusion is one of the most fundamental concepts in materials science, playing a pivotal role in materials synthesis, forming, and degradation. Of particular importance is solid state interdiffusion of metals which defines the usable parameter space for material combinations in the form of alloys. This parameter space can be explored on the macroscopic scale by using diffusion couples. However, this method reaches its limit when going to low temperatures, small scales, and when testing ultrathin diffusion barriers. Therefore, this work transfers the principle of the diffusion couples to small scales by using core-shell nanowires and in situ heating. This allows us to delve into the interdiffusion dynamics of copper and gold, revealing the interplay between diffusion and the disorder-order phase transition. Our in situ TEM experiments in combination with chemical mapping reveal the interdiffusion coefficients of Cu and Au at low temperatures and highlight the impact of ordering processes on the diffusion behavior. The formation of ordered domains within the solid-solution is examined using high-resolution imaging and nanodiffraction including strain mapping. In addition, we examine the effectiveness of ultrathin Al2O3 barrier layers to control interdiffusion of the diffusion couple. Our findings indicate that a 5 nm thick layer serves as an efficient diffusion barrier. This research provides valuable insights into the interdiffusion behavior of Cu and Au on the nanoscale, offering potential applications in the development of miniaturized integrated circuits and nanodevices.

Microstructural Passivation of Patterned Al2O3 Back Contacts on CdS/CdTe Solar Cells

AbstractPatterned aluminum oxide (Al2O3) back contact on cadmium telluride (CdTe) solar cells can effectively mitigate the loss of the majority carriers while retaining the passivation benefits for minority carriers. This study investigates the impact of the point‐contact geometry on cell performance, where the spacing of the microholes is systematically varied at a constant microhole diameter (≈16 µm). Microhole arrays are created on an evaporated Al2O3 layer (≈30 nm) on CdTe using laser‐beam lithography and selective wet etching processes. Comparative analysis indicates a significant decrease in fill factor (FF) and short‐circuit current (ISC) when the contact fraction falls below 30%, likely due to the Al2O3 barrier impeding majority carriers. Above this fraction, FF and ISC are comparable to those of baseline CdTe cells without Al2O3. Open‐circuit voltage (VOC) shows a gradual decrease as Al2O3 coverage is reduced but exhibits a slight increase (<15 mV) when the contact fraction exceeds 30%. Cathodoluminescence (CL) characterization reveals that significantly improved radiative recombination occurs within the CdTe grain bulk with Al2O3, whereas this effect on grain boundaries is minimal. The findings imply complex local carrier dynamics in the patterned back‐contact region, closely tied to the microstructural properties of CdTe‐based solar cells.

Oxidation behavior and structural damage mechanism of LaMgAl11O19 dual-ceramic thermal barrier coating with varying thickness of Al plating layer

Previous results clearly confirmed that the presence of an Al plating layer promotes the development of a dense Al2O3 barrier layer when exposed to high temperatures. This enhances the structural stability and improves the oxidation resistance of the dual ceramic coating. In the work, a continuous study was conducted to investigate the effect of varying Al plating layer thickness on the high-temperature oxidation characteristics of a CoCrNiAlY–YSZ–LMA dual ceramic coating. The results indicate that, compared to samples lacking Al layers, all Al-coated samples exhibit dense surface morphology, effectively inhibiting the permeation of O2. This results in slower growth of the TGO and minimal elemental diffusion. However, significant differences are observed in structural damage. Al layers of 5 μm and 10 μm thickness show slight local fracture damage and good structural stability. In contrast, the 15 μm Al-coated samples experience more severe structural fracture damage within the YSZ layer, including transverse cracking. These potential oxidation and structural damage mechanisms of the coatings with varying Al plating layer thickness were systematically discussed.

Component regulation and high-entropy engineering for enhanced antioxidant and high-temperature mechanical properties of protective films

Transition metal nitrides (TMNs) have gained widespread application in protecting structural components due to their high strength and hardness. However, TMNs still have the challenge of structural instability and mechanical deterioration caused by oxidation under harsh high temperature conditions. Herein, we present a strategy combining component regulation with high-entropy engineering to develop an advanced high-temperature Al-containing high-entropy nitrides (HENs) material. To prevent the phase decomposition of AlN within the (NbMoTaWAl)N, theoretical simulations were employed to determine a critical atomic percent of 25.0% Al for maintaining the stability of the high-entropy structure. Ensuing experimental synthesis creates three NbMoTaWAlN films with varying Al content: a high-entropy film with 0.0% Al (HEN), a high-entropy film with 21.2% Al (HEN-Al), and an amorphous transition metal nitride film with 30.2% Al (a-TMN-Al), validating key high-entropy engineering benchmarks. It is found that the unique HEN-Al film exhibits excellent oxidation resistance and high-temperature hardness, attributed to the uniform distribution of Al atoms in the robust high-entropy structure, which creates conditions for forming a dense and continuous Al2O3 barrier layer, effectively hindering the diffusion of oxygen into the film interior. This study provides new insights to develop a new generation of high-temperature protective materials.

Small polaron hopping and tunneling transport in Maxwell–Wagner relaxation dominated Al2O3/TiO2 subnanometric laminates

Maxwell–Wagner relaxation dominated Al2O3/TiO2 nanolaminates (ATA NLs) have recently demonstrated their potential for high-density energy storage applications. In this report, we have unraveled the defect-mediated transport mechanisms prevailing in Al2O3/TiO2 sub-nanometric laminates. Temperature-dependent ac conductivity measurements revealed the signature of small polaron hopping in TiO2 active layers and trap-assisted tunneling transport through Al2O3 barrier layers, which was corroborated by resonant photoelectron spectroscopy and temperature-dependent current–voltage measurement. The polaronic defect states, found ∼1 eV below the Fermi level, served as the hopping centers and leakage paths for current. The signature of quantum tunneling transport and the negative differential conductance observed toward higher electric field was attributed to the splitting of delocalized minibands. These transport properties of Al2O3/TiO2 nanolaminates will help in tailoring these materials for next-generation storage capacitors.

Effect of oxide diffusion barrier and substrate on the reliability of stainless-steel-based CIGS solar cells

This paper investigates the effect of the oxide diffusion barrier and substrate on the residual stress and the delamination at the CIGS/Molybdenum (Mo) interface when stainless-steel-based CIGS solar cells cool down to room temperature from the CIGS deposition process (500 °C) and hotspot conditions (100, 200, 300, and 400 °C). The focus is on the performance of Al2O3 and SiO2 oxide diffusion barriers and SS 304, SS Duplex, and SS 430 substrates. Through Finite Element method (FEM) and Machine Learning (ML) simulations, we calculated the energy release rate (J integral) of the CIGS/Mo interface crack and residual stress of the CIGS layer while concurrently varying the thickness of the diffusion barrier layer from 0.1 to 3 μm and the SS substrate from 25 to 200 μm. ML algorithms were used to predict the J integral at the CIGS/Mo interface and the CIGS residual stress for arbitrary CIGS deposition temperatures. Our simulation method is validated by the agreement between some of our simulation results and experimental findings from other researchers. Based on the simulation results, the following key findings were observed 1) A thinner diffusion barrier (for both SiO2 and Al2O3) is more effective in preventing the delamination at the CIGS/Mo interface, but a thicker diffusion barrier is more effective in reducing the stress in the CIGS layer 2) SiO2 diffusion barrier is better suited for preventing the CIGS/Mo interface delamination, while an Al2O3 diffusion barrier is better suited for reducing the residual stress in the CIGS layer 3) The thickness of stainless steel substrate significantly alters the residual stress in the CIGS layer (up to ∼30 times) 4) The SS 430 substrate is better suited for minimizing the residual stress in the CIGS layer and preventing delamination at the CIGS/Mo interface. The proposed methodology can be applied to other flexible solar cells to enhance their reliability.

Tunable Mid‐Infrared Multi‐Resonant Graphene‐Metal Hybrid Metasurfaces

AbstractElectrically tunable graphene‐metal metasurfaces with controllable optical properties have attracted interest for straightforward manipulation of free space light. Their resonance tuning range depends on graphene's electrical transport characteristics, which are affected by its quality, operating conditions, and the device design. An important example of the latter is the direct contact of metallic antennas with the graphene layer that limits the extent to which a bias voltage can tune the metasurface's permittivity. In this work, this issue is resolved in a straightforward and fabrication‐efficient way for graphene‐metal hybrid metasurfaces with multiple plasmonic resonances. It is demonstrated that the incorporation of a 10 nm Al2O3 barrier layer enhances the tuning range of mid‐infrared resonances compared to metasurfaces without barrier layer, i.e., from 300 to 700 nm for a 7.3 µm resonance and from 110 to 140 nm for a 4.7 µm resonance. The improved tunability of the metal/dielectric/graphene metasurface can be attributed to the reduced electrical coupling between metal and graphene, as confirmed by an equivalent circuit model. These results bring closer the use of active metasurfaces based on two‐dimensional materials under ambient conditions, with possible applications as optical filters, modulators, and information processing devices that require dynamic control of light.

Preparation of a flexible photoanode of the photoelectrochemical ultraviolet photodetector based on rutile TiO2 nanowires and the suppressed charge recombination at solid/liquid interface

Flexible ultraviolet (UV) photodetector exhibits a promising application in portable electronic gadgets, display devices and biomedical imaging. In this study, a flexible photoanode of the photoelectrochemical (PEC)-type UV photodetector based on rutile TiO2 nanowires (TiO2 NWs) grown on carbon fiber cloth is realized. In photovoltaic applications, an insulating layer is usually introduced to suppress interfacial recombination and reduce the surface trap states. The interfacial recombination of semiconductor/electrolyte is suppressed by coating an Al2O3 barrier layer on the TiO2 NWs. The photodetectors of the TiO2@Al2O3 NWs show stable photocurrent, a high light/dark current ratio (I light/I dark) of 1170, a faster rise and decay response times of 0.09 and 0.09 s, and excellent spectral selectivity from 300 to 400 nm. The peak responsivity of the photodetectors reaches 2.8 mA W−1 at 360 nm. This flexible photoanode have a potential application in wearable PEC UV photodetector.

Healing Donor Defect States in CVD-Grown MoS2 Field-Effect Transistors Using Oxygen Plasma with a Channel-Protecting Barrier.

Molybdenum disulfide (MoS2 ), a metal dichalcogenide, is a promising channel material for highly integrated scalable transistors. However, intrinsic donor defect states, such as sulfur vacancies (Vs ), can degrade the channel properties and lead to undesired n-doping. A method for healing the donor defect states in monolayer MoS2 is proposed using oxygen plasma, with an aluminum oxide (Al2 O3 ) barrier layer that protects the MoS2 channel from damage by plasma treatment. Successful healing of donor defect states in MoS2 by oxygen atoms, even in the presence of an Al2 O3 barrier layer, is confirmed by X-ray photoelectron spectroscopy, photoluminescence, and Raman spectroscopy. Despite the decrease in 2D sheet carrier concentration (Δn2D = -3.82×1012 cm-2 ), the proposed approach increases the on-current and mobility by 18% and 44% under optimal conditions, respectively. Metal-insulator transition occurs at electron concentrations of 5.7×1012 cm-2 and reflects improved channel quality. Finally, the activation energy (Ea ) reduces at all the gate voltages (VG ) owing to a decrease in Vs , which act as a localized state after the oxygen plasma treatment. This study demonstrates the feasibility of plasma-assisted healing of defects in 2D materials and electrical property enhancement and paves the way for the development of next-generation electronic devices.

In-situ synthesis of high-quality carbon nanotubes on Cu powder by constructing an Al2O3 barrier layer

The synthesis of high-quality carbon nanotubes (CNTs)-Cu composite powder by chemical vapor deposition remains a persistent challenge due to catalyst deactivation resulting from its diffusion into Cu powder at elevated temperatures. To overcome this obstacle, a novel approach involving pre-coating Cu powder with an Al2O3 barrier layer was proposed to facilitate the in-situ growth of CNTs. Experimental results demonstrated that synthesized CNTs exhibited a desirable combination of uniform distribution, high aspect ratio and tunable structures achieved by utilizing different carbon source precursors. Notably, single/double-walled CNTs (S/DMCNTs) with exceptional structural integrity was achieved on Cu powder, which has not been reported previously. The Raman ID/IG ratio of present S/DMCNTs-Cu composite powder reached approximately 0.18, which was significantly superior compared to the relevant reported data.

Hot corrosion behavior of CoCrNiAlY–YSZ–RSZ dual-ceramic thermal barrier coating with Al plating in molten sulfate mixture

A novel Al plating layer was deposited on the CoCrNiAlY–YSZ–RSZ dual - ceramic thermal barrier coating. The hot corrosion behaviors of these coatings in molten sulfate mixture were studied by the detail characterizations. This coating without Al plating possessed a fast mass gain trend, severe elemental diffusion and rapid TGO growth during the hot sulfate exposure, resulting in a significant increase in mass gain from 6.12 mg/cm2 at 20 h to 13.67 mg/cm2 at 100 h and aggravated coating fracture. In comparison, a continuously dense Al2O3 barrier layer formed on the coating's surface, preventing or delaying the infiltration of the molten sulfate and oxygen, and eventually led to an enhanced corrosion resistance. This coating with Al plating showed a slight mass gain from 8.20 mg/cm2 at 20 h to 9.31 mg/cm2 at 100 h, but also exhibited very slow TGO growth and stable structure. The potential molten sulfate corrosion mechanisms of these coatings with and without Al plating were systematically discussed.

Al2O3 barrier layer for enhancing UV to visible rejection ratio in p-Si/n-MgZnO heterojunction visible blind UV photodetectors

The fabrication of visible blind ultraviolet photodetectors based on p-Si/n-MgxZn1-xO heterojunction is challenging due to the generation of visible photoresponse caused by the drift of visible light generated electrons from p-silicon to n-MgxZn1-xO across the interface. Here, it is shown that by incorporating a thin ∼30 nm Al2O3 interlayer at the heterojunction, the UV-to-visible rejection ratio in p-Si/n-Mg0.25Zn0·75O photodetectors can be significantly enhanced. The Al2O3 interlayer inserted between n-Mg0.25Zn0·75O and p-Si serves as an effective blocking layer against the transport of visible light-generated electrons from p-Si to n-Mg0.25Zn0·75O, thereby suppressing the visible photoresponse. As a result, the UV-to-visible rejection ratio was improved by approximately three orders of magnitude, with a peak responsivity of 0.17 mA/W at 270 nm and a reverse bias of 0.5 V. This high degree of visible blindness of p-Si/n-Mg0.25Zn0·75O heterojunction photodetectors with an Al2O3 barrier layer holds promise for next-generation visible-blind UV photodetection applications.

Growth of graphene from solid amorphous carbon: A new geometry for control carbon diffusion barrier

As an alternative strategy to fabricate high-quality graphene over a large area, metal catalysis has been attempted at elevated temperatures with solid carbon sources. However, graphene was generally fabricated on the surface of metal catalysis layer using amorphous carbon as solid carbon sources. In this study, a thin Al2O3 barrier (1 nm) deposited on an amorphous-C/Ni bilayer stack is demonstrated to enable direct growth of few layer graphene (FLG) identified as 3–4 layers on substrate surface at 700 °C and 800 °C. Moreover, the obtained graphene shows a good transmittance (93%) under the light source of 550 nm. The findings provide a way to directly synthesis FLG on the required substrate at low temperature, which may dramatically broaden the applications range of graphene.

Core-shell Cu@Al2O3 fillers for enhancing thermal conductivity and retaining electrical insulation of epoxy composites

Heat accumulation has emerged as a critical issue that affects the performance and reliability of modern electronic devices in their evolving toward miniaturization, high power density and high integration. The application of thermal interface materials (TIMs) as a thermal management strategy is commonly employed to tackle this problem. Herein, we report the development and characterization of highly thermally conductive but electrically insulating Cu@Al2O3/epoxy composites. Stemming from the excellent thermal conductivity of Cu, the Cu@Al2O3/epoxy composites exhibit thermal conductivity of 1.32 W/m·K at 10 vol% filler content, which is nearly 7 times higher than that of the epoxy resin. Simultaneously, the Cu@Al2O3/epoxy composites still retain a high electrical resistivity of 2.3 × 1013 Ω·cm, which is 6 orders of magnitude greater than that of Cu/epoxy composites without Al2O3 barrier layers, because the dense nanoscale insulating Al2O3 shell effectively inhibits the electron transfer. In addition, the Cu@Al2O3/epoxy composites possess lower dielectric constant and dielectric loss, and better mechanical properties than those of Cu/epoxy composites.

Characterization of Cu2ZnSnS4 thin films prepared with and without thin Al2O3 barrier layer

We investigated the effect of thin Al2O3 barrier layer (∼20 nm) deposited by radio frequency magnetron sputtering technique on the structural, morphological, optical, and electrical properties of Cu2ZnSnS4 (CZTS) films. Major differences in the crystalline qualities of the films were not observed with the addition of the Al2O3 layer except for a small decrease in crystallite size. Raman spectra of the film with the Al2O3 layer exhibited the peaks belonging to ZnS and Cu2-xS secondary phases. X-ray photoelectron spectroscopy analysis showed that SO42- and CuSO4 compounds were formed on the surfaces of the films. The formation of the CuSO4 phase was associated with the bonding between CuO and S in an oxidized CZTS surface. The presence of the ZnS, Cu2-xS secondary phases, and CuSO4 on the surfaces of the films were also confirmed by scanning electron microscopy images. The film with the Al2O3 layer differed from the other films with its surface that have randomly distributed porous-structured agglomerations and a decrease in the ZnS secondary phases on its surface. All films exhibited different Na distributions in secondary ion mass spectroscopy depth profiles. The Al2O3 layer caused a slower gradient in Na diffusion with decreasing the accumulation of Na element at a distinct region. The activation energy for the thermally activated band conduction was calculated as 0.030 eV and attributed to the thermal release of the holes from the vacancy of copper (VCu) defect states. The results of temperature-dependent conductivity measurements indicated that there were different conduction mechanisms in the films.

Sub-nanometer atomic layer deposited Al2O3 barrier layer for improving stability of nonfullerene organic solar cells

Organic solar cells (OSCs) is a promising next-generation photovoltaic technology, however, the device stability remains to be the main barrier for its future commercialization. Herein, we reported the application of a sub-nanometer Al2O3 barrier layer in nonfullerene OSCs via atomic layer deposition (ALD), for the purpose of preventing metal ion diffusion from indium tin oxide (ITO) into the polymer layer caused by the corrosion of Poly(3,4-ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS). The thickness of the ALD-Al2O3 barrier layer was precisely optimized by controlling the number of ALD cycles (n) to achieve simultaneously good photoelectric properties and conformal coverage. An average power conversion efficiency (PCE) of 15.02% was demonstrated for the optimal OSCs with ALD-Al2O3 barrier layer. The above mentioned suppression of metal ion diffusion was experimentally confirmed by the cross sectional observations of transmission electron microscopy (TEM) and chemical mapping from energy-dispersive X-ray spectroscopy (EDX), resulting in a significantly improved operational stability with a device lifetime 3-times longer than that without ALD-Al2O3 barrier layer. Such ALD-assisted interface modification provides an effective approach to realize high-performance and stable OSCs.

Enhancement of high-temperature oxidation resistance of CoCrNiAlY–yttria-stabilized zirconia–LaMgAl11O19 dual-ceramic coating by arc Al plating

A novel Al plating was developed to enhance the oxidation resistance of CoCrNiAlY–yttria-stabilized zirconia (YSZ)–LaMgAl11O19 dual-ceramic coating. The results indicated that the lanthanum magnesium hexaaluminate (LMA) layer without the Al plating sustained severe volume shrinkage, consequently initiating the formation and propagation of longitudinal micro-cracks. These micro-cracks behaved as channels that accelerated the inner diffusion of oxygen. The rapid growth of the thermally grown oxide (TGO) layer induced high interface stress, resulting in the severe fracturing of the interface between the CoCrNiAlY and YSZ layers. However, the Al plating not only effectively suppressed the volume shrinkage of the LMA layer and the initiation of large-size longitudinal micro-cracks but also induced the formation of an in-suit Al2O3 barrier layer. The strong synergistic effect of the dense ceramic layer and Al2O3 layer effectively inhibited the inner diffusion of oxygen, consequently reducing the interface stress and rapid TGO layer growth. Accordingly, a considerably effective anti-oxidation performance was achieved by the dual-ceramic coating with the Al plating.

A Benchmark Study of Burning Rate of Selected Thermites through an Original Gasless Theoretical Model

This paper describes a kinetic model dedicated to thermite nanopowder combustion, in which core equations are based on condensed phase mechanisms only. We explore all combinations of fuels/oxidizers, namely Al, Zr, B/CuO, Fe2O3, WO3, and Pb3O4, with 60 % of the theoretical maximum density packing, at which condensed phase mechanisms govern the reaction. Aluminothermites offer the best performances, with initiation delays in the range of a few tens of microseconds, and faster burn rates (60 cm s−1 for CuO). B and Zr based thermites are primarily limited by diffusion characteristics in their oxides that are more stringent than the common Al2O3 barrier layer. Combination of a poor thermal conductivity and efficient oxygen diffusion towards the fuel allows rapid initiation, while thermal conductivity is essential to increase the burn rate, as evidenced from iron oxide giving the fastest burn rates of all B- and Zr-based thermites (16 and 32 cm·s−1, respectively) despite poor mass transport properties in the condensed phase; almost at the level of Al/CuO (41 versus 61 cm·s−1). Finally, formulations of the effective thermal conduction coefficient are provided, from pure bulk, to nanoparticular structured material, giving light to the effects of the microstructure and its size distribution on thermite performances.

Improvement of in-plane uniformity of cathodoluminescence from ZnO luminescent layers for electron beam excitation assisted optical microscope

We fabricated flat and homogeneous Al2O3/ZnO/Al2O3 heterostructure luminescent layers by atomic layer deposition (ALD) to serve as a nanometer-scaled light source for high-spatial-resolution optical microscopy based on electron beam excitation (EXA). A smooth surface was obtained by inserting an Al2O3 buffer layer and an Al2O3 barrier layer resulting in brighter and more uniform cathodoluminescence (CL) compared with that from a directly deposited ZnO layer. The root mean square (rms) value determined by atomic force microscope drastically decreased from 2.4 nm (for typical ZnO film) to 0.5 nm (for the six-layer pairs of the Al2O3/ZnO/Al2O3 heterostructure). The CL brightness increased by two times of that in the Al2O3/ZnO/Al2O3 heterostructure due to a waveguide effect. However, the increase in the number of the layer pairs from one to six reduced the CL brightness by half. The CL emission variability was about 30% improved that is supposed to enable high-resolution using Al2O3/ZnO/Al2O3 luminescent layers for an EXA microscope.

Improved performance of transparent conductive Cu-based GZO multilayer thin films on flexible substrates via two Al2O3 layers and oxygen-containing atmosphere

High performance of transparent conductive Cu-based Ga: ZnO (GZO) multilayer thin films deposited on flexible substrates are achieved by two Al2O3 barrier layers and oxygen. The multilayers exhibit high figure of merit (FOM) of 3.03 × 10−2 Ω−1 with the resistivity of 4.24 × 10−5 Ω cm and sheet resistance of 6.06 Ω/sq., while the average optical transmittance is above 84% in the visible range. The FOM value is the highest among all the reported Cu-based transparent conductive thin films. The influence mechanisms of Al2O3 layers and oxygen on structural, morphological, electrical and optical properties of the multilayer films are also investigated. These results indicate that the insertion of two thin Al2O3 barrier layers and the filling of oxygen during the deposition of GZO layers is an effective way to improve the photoelectric performance of the Cu-based GZO multilayer films.