Ultrathin Layer Research Articles

P-GaN/n-IGZO self-powered ultraviolet photodetector with ultralow dark current and high sensitivity

Self-powered ultraviolet (UV) photodetector has attracted growing research interest for applications in biological and human related fields with a low power consumption. However, typical construction strategies of heterojunction require materials with high crystal quality to realize high device performance with low dark current, which normally necessitates costly thin film epitaxy process. In our study, a high sensitivity self-powered UV photodetector with an ultralow dark current has been proposed by constructing p-GaN/n-IGZO heterojunction via a facile room temperature sputtering of IGZO thin film on commercially available p-GaN/sapphire substrate. By implementing UV-Ozone surface treatment to introduce an ultrathin oxide layer in the junction interface, dark current of the photodetector can be as low as 0.45 pA, which leads to a high detectivity of 1.9 × 1013 Jones with a rise time of 0.92 ms and a decay time of 11.61 ms when operating in self-powered mode. Our work has put forward a facile and cost-effective route for developing highly sensitive UV detecting devices with a low dark current and a low power consumption, which paves the path to advances in green industry of UV optoelectronics.

Addressing sodium ion-related degradation in SHJ cells by the application of nano-scale barrier layers

Silicon heterojunction (SHJ) solar cells are renowned for their high efficiency. However, SHJ solar cells are susceptible to various contaminants, leading to significant performance degradation when exposed to damp-heat conditions (e.g., 85 °C and 85% relative humidity). Sodium (Na) has been identified as one of the main contributors to degradation in silicon solar modules subjected to damp-heat conditions. This work investigates the role of an ultra-thin AlOx capping layer (∼10 nm) in preventing the failure in SHJ cells caused by Na-related contaminants. NaCl is applied directly to the solar cell, and the unencapsulated cell undergoes a damp heat test at 85 °C and 85% relative humidity (DH85). It is found that without the AlOx barrier layer, the SHJ cells experience a relative reduction in power of ∼30%rel after only 20 h at DH85. Both the front and rear sides of the cell degrade when exposed to NaCl. This is primarily due to a deterioration of the Ag contact resulting in increased series resistance (Rs), and decrease in fill factor (FF), and an increase in recombination, leading to a significant drop in open-circuit voltage (Voc), particularly when NaCl is applied on the rear side. However, when an AlOx barrier layer is applied to the SHJ cells, the performance losses caused by NaCl are significantly reduced to only ∼3.3%rel. The loss in Voc on the rear side is completely suppressed, and there is only a slight increase in Rs of ∼50%rel compared to ∼300%rel increase in Rs for cells without the AlOx barrier layer. These findings indicate that the ultra-thin AlOx barrier layer provides effective protection for SHJ cells against Na ions, mitigating both Rs and recombination losses. This AlOx barrier layer depositing method is compatible with existing industrial mass-production ALD tools and thus presents a viable solution at the cell level for SHJ cells.

Transparent pn junction of Mg:NiO/ZnO/SnO2via potential regulation and carrier injection of ZnO transition layer

Transparent pn junction of Mg:NiO/ZnO/SnO2 is prepared via an approach of continuous co-sputtering-annealing method. The Mg:NiO/ZnO/SnO2 pn junction exhibits high transmittance of ∼70–85 % in visible light, obvious photovoltaic enhancement of ∼1.6 × 102 folds than that of unmodified Mg:NiO/SnO2, stable output during 9 weeks’ cycle. It’s mainly attributed to the ultrathin ZnO transition layer, obtaining appropriate Fermi level and higher intrinsic carrier concentration can optimize carrier equilibrium, meanwhile sustaining high transparency and providing interface optimization. Furthermore, the extra hole carrier caused by Ni vacancy and Ni2+/Ni3+ mixed state can take full advantage of the defects to optimize the kinetic equilibrium. Additionally, the intrinsic high physical-chemical properties of inorganic NiO, ZnO and SnO2 can increase the intrinsic stability.

Soft deposition of TCOs by pulsed laser for high-quality ultra-thin poly-Si passivating contacts

In this work, the applicability of pulsed laser deposition (PLD) of transparent conductive oxides (TCOs) on high-quality ultra-thin poly-Si based passivating contacts is explored. Parasitic absorption caused by poly-Si layers can be minimized by reducing the poly-Si layer thickness. However, TCO deposition on poly-Si contacts, commonly by sputtering, results in severe deposition-induced damage and further aggravates the surface passivation for thinner poly-Si layers (<20 nm). Although a thermal treatment at elevated temperature (∼350 °C) can be used to partially repair the surface passivation quality, the contact resistivity severely increases due to the formation of a parasitic oxide layer at the poly-Si/ITO interface. Alternatively, we show that PLD TCOs can be used to mitigate the damage on ultra-thin (∼10 nm) poly-Si layers. Further improvement in poly-Si contact passivation can be achieved by increasing the deposition pressure while low contact resistivities (∼45 mΩ cm2) and good thermal stability (up to 350 °C) are achieved with a PLD indium-doped tin oxide (ITO) layer on high-quality ultra-thin poly-Si(n+) contacts. This allows for the application of a highly transparent front side contact by combining the excellent opto-electrical properties of a PLD ITO film with a 10 nm thin poly-Si contact.

Enhanced Switching Reliability of Hf0.5Zr0.5O2 Ferroelectric Films Induced by Interface Engineering.

Ferroelectric materials have been widely researched for applications in memory and energy storage. Among these materials and benefiting from their excellent chemical compatibility with complementary metal-oxide-semiconductor (CMOS) devices, hafnia-based ferroelectric thin films hold great promise for highly scaled semiconductor memories, including nonvolatile ferroelectric capacitors and transistors. However, variation in the switched polarization of this material during field cycling and a limited understanding of the responsible mechanisms have impeded their implementation in technology. Here, we show that ferroelectric Hf0.5Zr0.5O2 (HZO) capacitors that are nearly free of polarization "wake-up"─a gradual increase in switched polarization as a function of the number of switching cycles─can be achieved by introducing ultrathin HfO2 buffer layers at the HZO/electrodes interface. High-resolution transmission electron microscopy (HRTEM) reveals crystallite sizes substantially greater than the film thickness for the buffer layer capacitors, indicating that the presence of the buffer layers influences the crystallization of the film (e.g., a lower ratio of nucleation rate to growth rate) during postdeposition annealing. This evidently promotes the formation of a polar orthorhombic (O) phase in the as-fabricated buffer layer samples. Synchrotron X-ray diffraction (XRD) reveals the conversion of the nonpolar tetragonal (T) phase to the polar orthorhombic (O) phase during electric field cycling in the control (no buffer) devices, consistent with the polarization wake-up observed for these capacitors. The extent of T-O transformation in the nonbuffer samples is directly dependent on the duration over which the field is applied. These results provide insight into the role of the HZO/electrodes interface in the performance of hafnia-based ferroelectrics and the mechanisms driving the polarization wake-up effect.

Zinc and graphene oxide composites as new protective coatings for oil and gas pipes

A method was developed to obtain a durable coating consisting of zinc and graphene oxide (Zn-GO) in order to reduce the mechanical wear and tear rate of oil and gas pipelines made of steel. Graphene oxide was obtained from graphite by wet chemical oxidation (unmodified and modified Hummers’ method) using potassium permanganate and sulfuric acid. The process was carried out at various temperatures. The steel was covered with an ultrathin layer of Zn-GO using the electrophoretic deposition method. The GO particle size (< 90 nm) was confirmed by XRD and laser analysis. For GO particles obtained by the modified Hummers’ method, a significant correlation was observed in the scratch (R2 = 0.87) and the Vickers microhardness tests (R2 = 0.93), which indicates a lower wear rate of Zn-GO-coated steel.

Ultrathin Cobalt-Based Prussian Blue Analogue Nanosheet-Assembled Nanoboxes Interpenetrated with Carbon Nanotubes as a Fast Electron/Potassium-Ion Conductor for Superior Potassium Storage.

Rechargeable potassium-ion batteries (PIBs) are regarded as potential substitutes for industrial lithium-ion batteries in large scale energy storage systems due to the world's abundant potassium supplies. Althogh cobalt hexacyanocobaltate (CoHCC) exhibits broad potential as a PIB anode material, its performance is currently unsatisfactory. Herein, novel 5 nm scale ultrathin CoHCC nanosheet-assembled nanoboxes with interspersed carbon nanotubes (CNTs/CoHCC nanoboxes) are fabricated to realize a highly reactive PIB anode. The ultrathin CoHCC layers substantially accelerate electron conduction and provide numerous active sites, while the connected CNTs provide fast axial electron transport. Consequently, the optimized anode exhibits a remarkable discharge capacity of 580.9 mAh g-1 at 0.1 A g-1 and long-term stability with 71.3% retention over 1000 cycles. In situ and ex situ characterizations and density functional theory calculations are further employed to elucidate the K+ storage process and the reason for the enhanced performance of the CNTs/CoHCC nanoboxes.

Mechanisms of linear control of exchange bias in Fe/FeO bilayer

The microscopic nature of pinning uncompensated moment interface, how it is pinned by an antiferromagnet, and how the mechanism with this coupling drives exchange bias remain outstanding challenges. In this work, the Fe/FeO ferromagnet/antiferromagnet bilayer with an ultrathin FeO layer is modeled and the correlation between exchange bias and microscopically uncompensated antiferromagnetic magnetization is studied, based on first-principles calculations and modified Monte-Carlo simulations, where spin energies are exactly calculated and energy barriers are detectable to avoid the occurrence of unphysical tunneling during spin reversals. The linear dependence of exchange bias on uncompensated antiferromagnetic magnetization in Fe/FeO bilayer experimentally is reproduced and interpreted by exploring the monolayer-resolved magnetization behavior in the FeO layer, which depends on antiferromagnetic anisotropy, cooling field, fraction of Fe atoms permeated in the FeO layer, and magnetic frustration effect. It is evidenced that this linear dependence requires plenty of controllable pinning uncompensated antiferromagnetic spins located in the layer depths close to but not in contact with the ferromagnetic layer.

Sb segregation in ultrathin GaAsSb layers: A quantitative analysis of soaking/desorption stages

We present a quantitative analysis of Sb segregation in ultrathin GaAsSb films (1–20 ML) grown with soaking/desorption methods. Sb soaking advances Sb incorporation almost to the beginning of the growth, increasing the compositional gradient for all thicknesses. Application of the desorption step alone increases the compositional gradient at the upper interface by removing a fixed amount of Sb but erases layers thinner than 10 ML. When both methods are combined, the Sb balance changes with layer thickness. Thinner layers lose Sb while thicker layers gain Sb. The amount of Sb desorbed is related to the floating layer content at the time of desorption, which varies with thickness. Segregation simulations show that the segregation energy changes during layer growth, being initially higher but stabilizing around the same value for all cases. Sb-soaked samples reach the segregation steady state at a distance of 5 ML from the origin, much earlier than non-soaked samples (10–15 ML). The effect of desorption produces a large perturbation of the segregation steady state, which is much greater in the desorption-only sample than when combined with soaking. The combined effect of soaking and desorption gives the best GaAsSb films with steeper interfaces and no delay in onset.

Field‐Free Switching and Enhanced Electrical Detection of Ferrimagnetic Insulators Through an Intermediate Ultrathin Ferromagnetic Metal Layer

AbstractPerpendicularly magnetized ferrimagnetic insulators offer great potential for the development of fast and energy‐efficient spintronic devices. However, a major challenge for these devices is the requirement of an auxiliary magnetic field to achieve spin‐orbit torque (SOT)‐driven magnetization switching, along with the extremely small electric read‐out signal from the adjacent heavy metal layer. In this work, an approach by introducing an ultrathin Co layer primarily with in‐plane magnetization at the interface of the Tm3Fe5O12/Pt bilayers, which enables field‐free deterministic switching of the perpendicular Tm3Fe5O12 layer is demonstrated. Meanwhile, it is observed that a large anomalous Hall resistance readout signal from the coupling‐induced perpendicular component of the interfacial Co, which is nearly two orders of magnitude larger than that observed in Tm3Fe5O12/Pt bilayers. The crucial role played by the Co layer in modifying the SOT is elucidated. This research represents a significant step toward the practical implementation of ferrimagnetic insulator devices.

De-Pinning Fermi Level and Accelerating Surface Kinetics with an ALD Finish Boost the Fill Factor of BiVO4 Photoanodes to 44.

With the rapid development of performance and long-term stability, bismuth vanadate (BiVO4 ) has emerged as the preferred photoanode in photoelectrochemical tandem devices. Although state-of-the-art BiVO4 photoanodes realize a saturated photocurrent density approaching the theoretical maximum, the fill factor (FF) is still inferior, pulling down the half-cell applied bias photon-to-current efficiency (HC-ABPE). Among the major fundamental limitations are the Fermi level pinning and sluggish surface kinetics at the low applied potentials. This work demonstrates that the plasma-assisted atomic layer deposition technique is capable of addressing these issues by seamlessly installing an angstrom-scale FeNi-layer between BiVO4 and electrolyte. Not only this ultrathin FeNi layer serves as an efficient OER cocatalyst, more importantly, it also effectively passivates the surface states of BiVO4 , de-pins the surface Fermi level, and enlarges the built-in voltage, allowing the photoanode to make optimal use of the photogenerated holes for achieving high FF up to 44% and HC-ABPE to 2.2%. This study offers a new approach for enhancing the FF of photoanodes and provides guidelines for designing efficient unassisted solar fuel devices.

Boosting Bifunctional Catalysis by Integrating Active Faceted Intermetallic Nanocrystals and Strained Pt-Ir Functional Shells.

PtIr-based nanostructures are fascinating materials for application in bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysis. However, the fabrication of PtIr nanocatalysts with clear geometric features and structural configurations, which are crucial for enhancing the bifunctionality, remains challenging. Herein, PtCo@PtIr nanoparticles are precisely designed and fabricated with a quasi-octahedral PtCo nanocrystal as a highly atomically ordered core and an ultrathin PtIr atomic layer as a compressively strained shell. Owing to their geometric and core-shell features, the PtCo@PtIr nanoparticles deliver approximately six and eight times higher mass and specific activities, respectively, as an ORR catalyst than a commercial Pt/C catalyst. The half-wave potential of PtCo@PtIr exhibits a negligible decrease by 9mV after 10 000 cycles, indicating extraordinary ORR durability because of the ordered arrangement of Pt and Co atoms. When evaluated using the ORR-OER dual reaction upon the introduction of Ir, PtCo@PtIr exhibits a small ORR-OER overpotential gap of 679mV, demonstrating its great potential as a bifunctional electrocatalyst for fabricating fuel cells. The findings pave the way for designing precise intermetallic core-shell nanocrystals as highly functional catalysts.

Layer-By-Layer Assembly of Atomically Precise Alloy Nanoclusters Photosystems for Solar Water Oxidation.

Atomically precise metal nanoclusters (NCs) have garnered tremendous attention as light-harvesting antennas in heterogeneous photocatalysis due to unique atomic stacking mode, quantum confinement effect, and enriched active sites. However, metal NCs as photosensitizers suffer from extremely short carrier lifetime, poor photostability, and difficulty in carrier migration, which hinder the wide-spread utilization of metal NCs in solar energy conversion. To solve these problems, herein, Ag-doped glutathione (GSH)-capped gold NCs, i.e., alloy Au1- x Agx @GSH NCs and non-conjugated insulating polymer of poly(diallyl-dimethylammonium chloride) (PDDA) are utilized as the building blocks for layer-by-layer assembly of spatially multilayered alloy NCs/metal oxide (MO) photosystems. The alternately deposited ultrathin PDDA layer in-between Au1- x Agx @GSH NCs on the MO substrate functions as an efficient charge flow mediator to relay the directional photoelectron transfer over Au1- x Agx @GSH NCs, giving rise to the cascade charge transfer chain. This peculiar carrier migration mode endowed by exquisite interface configuration design significantly boosts the unidirectional electron migration from the Au1- x Agx @GSH NCs to the MO substrate, substantially improving the visible-light-driven photoelectrochemical water oxidation performances of MO/(PDDA-Au1- x Agx )n multilayer heterostructured photoanodes. The work will inspire the rational construction of alloy metal NCs-based photosystems for modulating spatially controllable charge transfer pathway for solar energy conversion.

Semitransparent Perovskite Solar Cells with Ultrathin Protective Buffer Layers.

Semitransparent perovskite solar cells (ST-PSCs) are increasingly important in a range of applications, including top cells in tandem devices and see-through photovoltaics. Transparent conductive oxides (TCOs) are commonly used as transparent electrodes, with sputtering being the preferred deposition method. However, this process can damage exposed layers, affecting the electrical performance of the devices. In this study, an indium tin oxide (ITO) deposition process that effectively suppresses sputtering damage was developed using a transition metal oxides (TMOs)-based buffer layer. An ultrathin (<10 nm) layer of evaporated vanadium oxide or molybdenum oxide was found to be effective in protecting against sputtering damage in ST-PSCs for tandem applications, as well as in thin perovskite-based devices for building-integrated photovoltaics. The identification of minimal parasitic absorption, the high work function and the analysis of oxygen vacancies denoted that the TMO layers are suitable for use in ST-PSCs. The highest fill factor (FF) achieved was 76%, and the efficiency (16.4%) was reduced by less than 10% when compared with the efficiency of gold-based PSCs. Moreover, up-scaling to 1 cm2-large area ST-PSCs with the buffer layer was successfully demonstrated with an FF of ∼70% and an efficiency of 15.7%. Comparing the two TMOs, the ST-PSC with an ultrathin V2Ox layer was slightly less efficient than that with MoOx, but its superior transmittance in the near infrared and greater light-soaking stability (a T80 of 600 h for V2Ox compared to a T80 of 12 h for MoOx) make V2Ox a promising buffer layer for preventing ITO sputtering damage in ST-PSCs.

High-energy all-solid-state lithium batteries enabled by Co-free LiNiO2 cathodes with robust outside-in structures.

A critical current challenge in the development of all-solid-state lithium batteries (ASSLBs) is reducing the cost of fabrication without compromising the performance. Here we report a sulfide ASSLB based on a high-energy, Co-free LiNiO2 cathode with a robust outside-in structure. This promising cathode is enabled by the high-pressure O2 synthesis and subsequent atomic layer deposition of a unique ultrathin LixAlyZnzOδ protective layer comprising a LixAlyZnzOδ surface coating region and an Al and Zn near-surface doping region. This high-quality artificial interphase enhances the structural stability and interfacial dynamics of the cathode as it mitigates the contact loss and continuous side reactions at the cathode/solid electrolyte interface. As a result, our ASSLBs exhibit a high areal capacity (4.65 mAh cm-2), a high specific cathode capacity (203 mAh g-1), superior cycling stability (92% capacity retention after 200 cycles) and a good rate capability (93 mAh g-1 at 2C). This work also offers mechanistic insights into how to break through the limitation of using expensive cathodes (for example, Co-based) and coatings (for example, Nb-, Ta-, La- or Zr-based) while still achieving a high-energy ASSLB performance.

Pure Amorphous and Ultrathin Phosphate Layer with Superior Ionic Conduction for Zinc Anode Protection.

Fast and uniform ion transport within the solid electrolyte interphase (SEI) is considered a crucial factor for ensuring the long-term stability of metal electrodes. In this study, we present the fabrication of ultrathin artificial interphases consisting of a zinc phosphate nanofilm with pure amorphous characteristics and a surfactant overlayer. The thickness of the interphases can be precisely controlled within the range of a few tens of nanometers. We explore the impact of artificial SEI structure, including thickness and crystallinity, on its protective capabilities. The pure amorphous phosphate layer with optimized nanoscale thickness is found to provide an abundance of short and isotropic ion migration pathways and a low diffusion energy barrier. These features facilitate rapid and homogeneous Zn2+ transportation, resulting in compact and planar zinc deposition. Meanwhile, the hydrophobic alkyl moieties of the overlayer prevent disassociation of water at the interface. As a result, this nanofilm endures ultralong cycling stability with a low overpotential and enables high Zn plating/stripping reversibility. The Zn||MnO2 full cell shows a stable cycle life for 700 cycles under practical conditions of lean electrolyte, high areal capacity cathode, and limited Zn excess. These findings provide insights into the design and optimization of SEI layers for protection of metal anodes.

Overcoming Copper Substrate Thermodynamic Limitations in Anode-Free Lithium Pouch Cells via In Situ Seed Implantation.

Anode-free lithium metal batteries (AFLMBs) exhibit enhanced energy density and cost-effective manufacturing, albeit constrained by short lifespans due to inhomogeneous lithium nucleation and growth on the inherently lithiophobic Cu current collector. Although numerous attempts at Cu surface modifications aim to mitigate this thermodynamic limitation, they often result in substantial irreversible capacity loss and/or lack the stability required for practical applications. Here, we present an in situ seed implantation (ISI) strategy to address the aforementioned challenge. A 36 s ISI treatment created an ultrathin lithium metal layer, composed of uniform lithium nuclei with ∼100 nm in diameter, equating to 0.05 mAh cm-2, on the Cu substrate. This approach facilitates dense lithium deposition during cycles, effectively doubling the lifespan of an Ah-level 437 Wh kg-1 AFLMB. Our ISI strategy offers a straightforward and efficient solution that maintains battery energy density and manufacturing cost effectiveness, and its application extends beyond lithium metal.

High performance ultra-thin perovskite solar cell by surface plasmon polaritons and waveguide modes

Although perovskite solar cell has been widely studied due to their high-efficiency potential, the design of ultra-thin perovskite solar cells for flexible and less toxic device fabrication has received less attention. The use of ultra-thin perovskite thin layer leads to low-efficiency solar cells due to insufficient absorption of light. In this paper, the optical absorption of the ultra-thin PSC is improved via surface plasmon polaritons and waveguide modes through the using a silver back grating electrode into hole transport free-layer structure. The investigations are based on the FDTD numerical method and the effects of grating dimensions and periodicity on the absorption of the active layer are systematically investigated to achieve the high performance of the structure. Also, the value of the short circuit current density was investigated by changing the dimensions of the grating. In the optimum device with the thickness of the absorber layer of 150 nm, the absorption enhancement of 20% is obtained in the wavelength 300 nm-800 nm compared to the reference flat solar cell. In addition, the design of ultrathin solar cells based on the grating electrode with perovskite thicknesses of 50 nm, 100 nm, and 200 nm is studied. The results provide a way for the design and production ultra-thin, low toxicity, and high performance solar cells.

Highly Stable Short Channel Ultrathin Atomic Layer Deposited Indium Zinc Oxide Thin Film Transistors With Excellent Electrical Characteristics

Highly Stable Short Channel Ultrathin Atomic Layer Deposited Indium Zinc Oxide Thin Film Transistors With Excellent Electrical Characteristics

Defect engineering boosts the reliability of ultra-thin MLCCs

Modern electronics highly demand base metal electrode multilayer ceramic capacitors (BME MLCCs) with ultra-thin dielectric layers and high capacitances owing to the trend for miniaturization. The reliability of MLCCs is crucial for practical applications and significantly affected by the lamination process and material defects. However, the detailed relationships between these factors and the reliability of ultra-thin MLCCs remain elusive. In this study, we investigated the influence of different lamination pressures (denoted as S1, S2 and S3) on the microstructure and reliability of BaTiO3 (BT) based protocol MLCCs with hundreds of layers. Microstructural observations reveal that among the devices, S2 has the highest roughness for both the Ni internal electrode and dielectric layers. Dielectric property investigations show that S1 and S3 meet the X5R standard and the designed capacity of 10 μF while S2 fails to meet the X5R standard at high temperatures. The electron energy loss spectra investigation reveal that oxygen vacancies are asymmetrically distributed on both sides of the Ni-BT interface in the individual dielectric layers, especially for S2, which indicates that one-side pressure lamination from the bar bulk induces sintering differences on the two sides of the dielectric layer. The results from this study indicate that applying an appropriate critical pressure between BT and Ni (S1) is preferable for decreasing the asymmetry and improving the reliability of MLCCs with ultra-thin layers.