Ultrathin Layer Research Articles

Dual-Mode Semiconductor Device Enabling Optoelectronic Detection and Neuromorphic Processing with Extended Spectral Responsivity.

High-performance semiconductor devices capable of multiple functions are pivotal in meeting the challenges of miniaturization and integration in advanced technologies. Despite the inherent difficulties of incorporating dual functionality within a single device, a high-performance, dual-mode device is reported. This device integrates an ultra-thin Al2O3 passivation layer with a PbS/Si hybrid heterojunction, which can simultaneously enable optoelectronic detection and neuromorphic operation. In mode 1, the device efficiently separates photo-generated electron-hole pairs, exhibiting an ultra-wide spectral response from ultraviolet (265nm) to near-infrared (1650nm) wavelengths. It also reproduces high-quality images of 256×256 pixels, achieving a Q-value as low as 0.00437µWcm- 2 at a light intensity of 8.58µWcm- 2. Meanwhile, when in mode 2, the as-assembled device with typical persistent photoconductivity (PPC) behavior can act as a neuromorphic device, which can achieve 96.5% accuracy in classifying standard digits underscoring its efficacy in temporal information processing. It is believed that the present dual-function devices potentially advance the multifunctionality and miniaturization of chips for intelligence applications.

Ultrathin metal-dielectric planar interface for high-performance photonic spin Hall effect

Abstract This study explores the photonic spin Hall effect (PSHE) with a focus on its sensitivity to incident wave polarization and physical parameter variations. Traditionally surface plasmonic resonance (SPR) systems mainly enhance H-polarized PSHE. However, our proposed method involves an ultrathin metal layer, capped with a glass dielectric layer on a glass substrate, to achieve high-performance PSHE for both H- and V-polarization waves. This high-performance PSHE in terms of enhancement in PSHE magnitudes arises from the simultaneous presence of SPR and waveguiding effects, resulting in the emergence of hybrid TE and TM modes. Investigating Au and Ag ultrathin metal layers, we find that the highest H-polarized PSHE occurs at ~1.52 × 10^6 nm and ~2.05 × 10^5 nm for Au and Ag, respectively. Similarly, for Vpolarized incident light, maximum enhanced PSHE is observed at ~1.58 × 10^5 nm and ~9.48 × 10^4 nm for Au and Ag, respectively. By adjusting the thickness of the metal and/or glass cap layer, precise control over PSHE amplitude and active polarization response is achievable. This research unveils unique capabilities for generating hybrid modes that enhance PSHE for both polarizations, offering a potential platform for developing tunable polarizationfavorable spin optics optoelectronic devices.

Vertical GaN Schottky barrier diodes with ohmic contact on N-polar by the atomic layer deposition of aluminum oxide interfacial layer

In this paper, high-performance vertical GaN on GaN Schottky Barrier Diodes (SBDs) on 2-inch free-standing (FS)-GaN is presented with the specific on-state resistance (Ron) of 1.34 mΩ·cm2, breakdown voltage (Vbr) of 1150 V and figure of merit (FOM) of 0.98 GW/cm2. He-implanted edge termination (ET) structure is introduced to alleviate the peak electric field concentration effect at the anode edge of GaN SBDs. Furthermore, A new ohmic contact structure is explored by atomic layer deposition (ALD)-grown AlOx interfacial layer. Based on the self-cleaning effect of ALD and Ultra-thin AlOx interfacial layer, the Fermi-level Pinning (FLP) effect is alleviated effectively, the specific contact resistivity (ρc) on N-polar GaN was reduced from 1.63 × 10−3 to 7.14 × 10−5 Ω·cm2.The temperature variation test demonstrate the devices has potential of temperature sensors under high voltage and high temperature.

Impact of Al profile in high-Al content AlGaN/GaN HEMTs on the 2DEG properties

Ultra-thin high-Al content barrier layers can enable improved gate control and high-frequency operation of AlGaN/GaN high electron mobility transistors (HEMTs) but the precise composition control is very challenging. In this work, we investigate the compositional profiles of AlxGa1−xN/GaN HEMT structures with targeted Al content in the barrier layer, x = 0.50, 0.70, and 1, and thickness in the sub-10 nm range in correlation with the two-dimensional electron gas (2DEG) properties. The HEMT structures are grown by metal-organic chemical vapor deposition on SiC. The maximum Al content in the barrier layer, experimentally determined by scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, is found to be lower than that intended and the deviations from the designed structures increase progressively with increasing x. Compositionally sharp interface between GaN and Al0.46Ga0.54N and box-like Al profile is achieved for intended x∼0.50 while pronounced Al grading is found in the samples with intended x of 0.70 and 1, with a maximum Al content of 0.78 reached for the HEMT structure with intended AlN barrier layer. The impact of the experimentally determined Al profiles on the 2DEG properties, obtained by contactless and electrical Hall effect measurements and coupled with self-consistent solution of the Poisson–Schrödinger equation, is evaluated and discussed. It is shown that the observed deviations from the intended Al profiles have a negative effect on the 2DEG confinement and result in reduced mobility parameters, which have significant implications for the implementation of high-Al content AlGaN/GaN structures in high-frequency devices.

Modulating low-temperature interfacial polymerization with NaHCO3 for high-performance ultrathin nanofiltration membranes

Conventional interfacial polymerization (IP) encounters significant challenges in achieving the desired nanofiltration (NF) membrane structure, owing to uncontrolled diffusion and ultrafast polymerization. Our study introduced carbonates into the low-temperature interfacial polymerization (LTIP) process to precisely regulate the diffusion of amine monomers and polymerization kinetics. Carbonates in the aqueous phase restrict the diffusion of amine monomers while promoting the generation of nanobubbles. Further utilization of the low-temperature oil phase not only retards polymerization but also facilitates the formation of foam nanostructures in the polyamide layer. Density functional theory calculations and molecular dynamics simulations revealed the mechanisms underlying the regulation of amine monomer diffusion and gas-bubble release by carbonates and LTIP. The fabricated membrane has a smoother, ultrathin separation layer while maintaining a high permeability of 35.0 L·m−2·h−1·bar−1 (nearly doubled compared with the pristine membrane) and high Na2SO4 rejection of 99.5 %. This study confirms the practicality of the carbonate-modulated LTIP strategy.

Microscopic characterizations for 2D material-based advanced electronics

Two-dimensional (2D) materials have gained significant attention as potential candidates for next-generation electronics, owing to their unique properties such as ultrathin layer thickness, mechanical flexibility, and tunable bandgaps. The distinctive characteristics of 2D materials necessitate the development of nanoscale advanced characterization methods. In this review, we explore the role of microscopy techniques in developing 2D materials-based electronics, from material synthesis and characterization to device performance and reliability. We address the applications of microscopies by delving into the perspectives of channel materials, metal contacts, dielectric materials, and device architectures. Additionally, we provide an outlook on the future directions and potential utilization of microscopy techniques in future 2D semiconductor industry.

Magnetic Domain and Structural Defects Size in Ultrathin Films

Herein, a model is proposed for measuring the structural defects size r0 in an ultrathin magnetic layer with perpendicular magnetic anisotropy. Based on the observations of magnetic domains in Ta/Pt/Co/Pt ultrathin films, using polar magneto‐optical Kerr effect microscopy and measurements of their magnetic anisotropies, the correlation between magnetic domains size D and structural defects size r0, as well as the defects concentration parameter αK, which designates the degree of pinning, has been modeled. The average r0 value found is high in the sample with unannealed buffer layers and considerably decreases with annealing. It is 6.17 nm with unannealed Ta/Pt buffer layers, 1.06 nm in sample with Ta/Pt buffer layers annealed at 423 K, and 0.49 nm in that with buffer layers annealed at 573 K. The significant drop of r0 is in good agreement with the high depinning noted with buffer layers annealing in recent work.

Ferroelectric tunnel junctions based on a HfO2/dielectric composite barrier

The ferroelectric tunnel junction (FTJ), which possesses a simple structure, low power consumption, high operation speed, and nondestructive reading, has attracted great attention for the application of next-generation nonvolatile memory. The complementary metal–oxide–semiconductor-compatible hafnium oxide (HfO2) ferroelectric thin film found in the recent decade is promising for the scalability and industrialization of FTJs. However, the electric performance, such as the tunneling electroresistance (TER) effect, of the current HfO2-based FTJs is not very satisfactory. In this work, we propose a type of high-performance HfO2-based FTJ by utilizing a ferroelectric/dielectric composite barrier strategy. Using density functional theory calculations, we study the electronic and transport properties of the designed Ni/HfO2/MgS/Ni (001) FTJ and demonstrate that the introduction of an ultra-thin non-polar MgS layer facilitates the ferroelectric control of effective potential barrier thickness and leads to a significant TER effect. The OFF/ON resistance ratio of the designed FTJ is found to exceed 4 × 103 based on the transmission calculation. Such an enhanced performance is driven by the resonant tunneling effect of the ON state, which significantly increases transmission across the FTJ when the ferroelectric polarization of HfO2 is pointing to the non-polar layer due to the aroused electron accumulation at the HfO2/MgS interface. Our results provide significant insight for the understanding and development of the FTJs based on the HfO2 ferroelectric/non-polar composite barrier.

Perovskite/silicon tandem solar cells with bilayer interface passivation.

Two-terminal monolithic perovskite/silicon tandem solar cells demonstrate huge advantages in power conversion efficiency compared with their respective single-junction counterparts1,2. However, suppressing interfacial recombination at the wide-bandgap perovskite/electron transport layer interface, without compromising its superior charge transport performance, remains a substantial challenge for perovskite/silicon tandem cells3,4. By exploiting the nanoscale discretely distributed lithium fluoride ultrathin layer followed by an additional deposition of diammonium diiodide molecule, we have devised a bilayer-intertwined passivation strategy that combines efficient electron extraction with further suppression of non-radiative recombination. We constructed perovskite/silicon tandem devices on a double-textured Czochralski-based silicon heterojunction cell, which featured a mildly textured front surface and a heavily textured rear surface, leading to simultaneously enhanced photocurrent and uncompromised rear passivation. The resulting perovskite/silicon tandem achieved an independently certified stabilized power conversion efficiency of 33.89%, accompanied by an impressive fill factor of 83.0% and an open-circuit voltage of nearly 1.97 V. To the best of our knowledge, this represents the first reported certified efficiency of a two-junction tandem solar cell exceeding the single-junction Shockley-Queisser limit of 33.7%.

Enhancing light absorption in ultra-thin perovskite solar cells using plasmonic gold nanoparticle clusters

Perovskite solar cells with ultra-thin absorber layers offer potential cost savings in manufacturing, but their reduced thickness can limit light absorption and efficiency. This work explores using plasmonic gold nanoparticles as a light-trapping strategy to compensate for lower absorption in ultra-thin perovskite devices. Numerical simulations investigate embedding 25 nm radius gold nanoparticles within the 200 nm thick perovskite active layer to boost optical absorption through near-field enhancement and light scattering effects. The solar cell structure incorporating these plasmonic nanoparticles achieves a substantially higher short-circuit current density of 23.10 mA cm−2 compared to 18.70 mA cm−2 for a reference cell without nanoparticles. This study provides design approaches for realizing high-efficiency yet cost-effective ultra-thin perovskite photovoltaics by harnessing plasmonic light-trapping techniques. The results display methodologies to improve photon absorption and power conversion in thin-film perovskite devices through strategic nanoparticle integration.

Atomic Layer-Deposited Silane Coupling Agent for Interface Passivation of Quantum Dot Light-Emitting Diodes.

Inserting an insulating layer between the charge transport layer (CTL) and quantum dot emitting layer (QDL) is widely used in improving the performance of quantum dot light-emitting diodes (QLEDs). However, the additional layer inevitably leads to energy loss and joule heat. Herein, a monolayer silane coupling agent is used to modify the said interfaces via the self-limiting adsorption effect. Because the ultrathin layers induce negligible series resistance to the device, they can partially passivate the interfacial defects on the electron transport side and help confine the electrons within the QDL on the hole transport side. These interfacial modifications can not only suppress the nonradiative recombination but also slow down the aging of the hole transport layer. The findings here underline a low-temperature adsorption-based strategy for effective interfacial modification which can be used in any layer-by-layer device structures.

Hybrid Nanogel-Wrapped Anisotropic Gold Nanoparticles Feature Enhanced Photothermal Stability.

Anisotropic gold nanoparticles (AuNPs) are renowned for their unique properties - including localized surface plasmon resonance (LSPR) and adjustable optical responses to light exposure - that enable the conversion of light into heat and make them a promising tool in cancer therapy. Nonetheless, their tendency to aggregate and consequently lose their photothermal conversion capacity during prolonged irradiation periods represents a central challenge in developing anisotropic AuNPs for clinical use. To overcome this issue, an innovative approach that facilitates the encapsulation of individual anisotropic AuNPs within thin nanogels, forming hybrid nanomaterials that mirror the inorganic core's morphology while introducing a negligible (2-8nm) increase in overall diameter is proposed. The encapsulation of rod- and star-shaped anisotropic AuNPs within poly-acrylamide (pAA) or poly-(N-isopropylacrylamide) (pNIPAM) nanogels is successfully demonstrated. The ultrathin polymeric layers display remarkable durability, significantly enhancing the photothermal stability of anisotropic AuNPs during their interaction with near-infrared light and effectively boosting their photothermal capacities for extended irradiation periods. The outcomes of the research thus support the development of more stable and reliable AuNPs as hybrid nanomaterials, positioning them as promising nanomedicinal platforms.

Identifying Root Origin of Insulating Polymer Mediated Solar Water Oxidation.

Rational construction of high-efficiency photoelectrodes with optimized carrier migration to the ideal active sites, is crucial for enhancing solar water oxidation. However, complexity in precisely modulating interface configuration and directional charge transfer pathways retards the design of robust and stable artificial photosystems. Herein, a straightforward yet effective strategy is developed for compact encapsulation of metal oxides (MOs) with an ultrathin non-conjugated polymer layer to modulate interfacial charge migration and separation. By periodically coating highly ordered TiO2 nanoarrays with oppositely charged polyelectrolyte of poly(dimethyl diallyl ammonium chloride) (PDDA), MOs/polymer composite photoanodes are readily fabricated under ambient conditions. It is verified that electrons photogenerated from the MOs substrate can be efficiently extracted by the ultrathin solid insulating PDDA layer, significantly boosting the carrier transport kinetics and enhancing charge separation of MOs, and thus triggering a remarkable enhancement in the solar water oxidation performance. The origins of the unexpected electron-withdrawing capability of such non-conjugated insulating polymer are unambiguously uncovered, and the scenario occurring at the interface of hybrid photoelectrodes is elucidated. The work would reinforce the fundamental understanding on the origins of generic charge transport capability of insulating polymer and benefit potential wide-spread utilization of insulating polymers as co-catalysts for solar energy conversion.

Engineering of Co/MgO interface with combination of ultrathin heavy metal insertion and post-oxidation for voltage-controlled magnetic anisotropy effect

The voltage-controlled magnetic anisotropy (VCMA) effect in ferromagnet/insulator junctions provides an effective way to manipulate electron spins, which can form the basis of future magnetic memory technologies. Recent studies have revealed that the VCMA effect can be strongly tuned by a process of “interface engineering” exploiting ultrathin heavy metal layers and an electron depletion effect. To further decrease the numbers of electrons, chemical reactions, such as surface oxidation of ferromagnets, may also be an effective way to achieve this depletion. However, the knowledge of combined effect of heavy metal layers and oxidation is still lacking. Here, we demonstrate that dual interfacial engineering using an insertion of heavy metals (Pt or Re) and a post-oxidation process can have a remarkable effect on the perpendicular magnetic anisotropy and the VCMA effect. Interestingly, a strong enhancement of the perpendicular magnetic anisotropy is observed by dual interfacial engineering with Pt insertion, although it does not occur with Pt insertion or surface oxidation alone. Furthermore, even a sign reversal of the additional VCMA effect due to the ultrathin heavy metal layers is observed by utilizing dual interfacial engineering. These findings provide another degree of freedom for designing voltage-controlled spintronic devices and pave the way to interfacial spin–orbit engineering for the VCMA effect.

On the Topotactic Phase Transition Achieving Superconducting Infinite-Layer Nickelates.

Topotactic reduction is critical to a wealth of phase transitions of current interest, including synthesis of the superconducting nickelate Nd0.8Sr0.2NiO2, reduced from the initial Nd0.8Sr0.2NiO3/SrTiO3 heterostructure. Due to the highly sensitive and often damaging nature of the topotactic reduction, however, only a handful of research groups have been able to reproduce the superconductivity results. A series of in situ synchrotron-based investigations reveal that this is due to the necessary formation of an initial, ultrathin layer at the Nd0.8Sr0.2NiO3 surface that helps to mediate the introduction of hydrogen into the film such that apical oxygens are first removed from the Nd0.8Sr0.2NiO3 / SrTiO3 (001) interface and delivered into the reducing environment. This allows the square-planar / perovskite interface to stabilize and propagate from the bottom to the top of the film without the formation of interphase defects. Importantly, neither geometric rotations in the square planar structure nor significant incorporation of hydrogen within the films is detected, obviating its need for superconductivity. These findings unveil the structural basis underlying the transformation pathway and provide important guidance on achieving the superconducting phase in reduced nickelatesystems.

Plasma surface treatment of amorphous Ga2O3 thin films for solar-blind ultraviolet photodetectors

Recently, amorphous gallium oxide (a-Ga2O3) thin films are gaining more and more attention due to their advantages of low cost and low temperature growth. However, a-Ga2O3 solar-blind ultraviolet photodetectors are plagued by issues such as slow response speed and high dark current. In this work, radio frequency magnetron sputtering was used for a-Ga2O3 deposition, and a novel approach of plasma treatment (including oxygen, argon and hydrogen plasmas) was employed to tailor the device performance using a plasma-enhanced chemical vapor deposition system. It is demonstrated that: (i) the oxygen atoms and ions in the oxygen plasma effectively reduce the oxygen vacancy concentration and improve the respond speed of a-Ga2O3 photodetectors; (ii) the argon ions in the argon plasma create additional defects (such as dangling bonds and oxygen vacancies) and improve the responsivity of a-Ga2O3 photodetectors; (iii) the hydrogen atoms in the hydrogen plasma may incorporate into the film and form an ultra-thin passivation layer near the surface, which gives rise to a balance between responsivity and response speed. Finally, through the detailed analysis of the optical emission spectra and photoelectric performance, the underlying physical mechanism based on the energy band diagram has been proposed to interpret the obtained experimental results.

Improvement of voltage-controlled magnetic anisotropy effect by inserting an ultrathin metal capping layer

Improvement of voltage-controlled magnetic anisotropy effect by inserting an ultrathin metal capping layer

Mid-infrared photodetector spectrometer concept based on ultrathin all-dielectric metamaterial with azimuth-incidence-angle tuned perfect optical absorption: Design and analysis

Novel concepts for efficient compact spectroscopy are extensively researched due to their fundamental applications in prominent fields such as chemistry, biology, and physics. Here, with an unprecedented spectral-azimuthal resolution, such a concept is introduced and exemplified in the mid-infrared, in which its advantages are paramount and have yet to be established industrially. The concept is based on the design and instrumentation of optical absorption spectral tuning (or sensitivity) to the relative azimuthal component of light impinging on specifically designed metamaterials (MMs). The inversely-designed MMs offer perfect photo-absorption inside λ0/200 ultra-thin layer of lead telluride. Two small-footprint system designs are proposed to instrument the spectral-azimuth-angle tuning for spectrometry. The first is based on a single or few spinning MM layout elements, and the second, to avoid spinning, utilizes a fixed focal-plane-array approach. The latter exploits the inherent variations in the local azimuthal-incidence angle. While low absorption is the Achilles heel of conventional mid-infrared photodetector spectrometers, the optimized MMs, besides their unique spectral-azimuth-angle tuning functionality, provide giant absorption enhancement, facilitating higher resolution and even smaller in-plane form factor. The highlighted concept opens an additional dimension to encode-decode spectral information, yielding profound advantages over conventional designs, such as those based on diffraction gratings.

Substrate‐Free Terahertz Metamaterial Sensors With Customizable Configuration and High Performance

AbstractMetamaterials based on quasi‐bound states in the continuum (qBICs) with manipulable resonance quality (Q) factors have provided a standout platform for cutting‐edge terahertz (THz) sensing applications. However, most so far have been implemented as conventional metal patch structures with adjacent substrate layers, incurring the limitation of insufficient light‐matter interaction due to substrate effects. Here, qBIC‐driven metamaterials with substrate‐free metallic aperture structures for tailoring light‐matter interactions and exhibiting near‐ideal sensing performance is introduced. Specifically, it is incorporated ultrafast femtosecond laser processing technology to fabricate H‐type metallic aperture metamaterials with accessible high‐contrast Q factor resonances allowed by in‐plane symmetry breaking. Correspondingly, stronger light field energies are applied to the interactions due to completely eliminating the confinement of the substrate effect, enabling experimental sensitivity of up to 0.86 THz RIU−1 for the qBIC resonance, 1.9 times that of the conventional dipole resonance. Moreover, a high Q qBIC resonance achieved by optimized asymmetry parameter is exploited for detecting ultrathin layers of L‐proline molecules as low as 0.87 nmol. It is envisioned that this approach will deliver insights for real‐time, precise, and high‐performance detection of trace biomolecules, and open new perspectives for realizing ideal performance metadevices.

Enhanced Antifouling Capability of In Situ-Grown Hydrophilic-Hydrophobic Nanodomains on Membrane Surface in the Ultralow Pressurized Ultrafiltration Process.

Although hydrophilic modification of the membrane surface is widely adopted, polymeric membranes still suffer from irreversible fouling caused by hydrophilic components in surface water. Here, an ultrathin hydrogel layer (40 nm) with hydrophilic-hydrophobic textures was in situ grown onto the polysulfone ultrafiltration membrane surface using an organic-radical-initiated interfacial polymerization technique. The interfacial polymerization of hydrophilic and hydrophobic monomers ensured the molecular-scale distribution of hydrophilic and hydrophobic nanodomains on the membrane surface. These nanodomains, with their molecular lengths, facilitated dynamic repulsion interactions between the uniformly textured surface and foulant components with different degrees of hydrophilicity. Chemical force characterization confirmed that the adhesion force between the hydrophilic-hydrophobic textured membrane surface and foulants (dodecane, bovine serum albumin, and humic acid) was greatly reduced. Dynamic filtration experiments showed that a hydrophilic-hydrophobic textured membrane always possessed the largest water flux and the best antifouling performance. Furthermore, the foulant coverage ratio on the membrane surface was first evaluated by measuring changes in surface streaming potentials, which demonstrated a 69% reduction in the amount of foulant adhering to the hydrophilic-hydrophobic textured membrane surface. Therefore, the construction of hydrophilic-hydrophobic nanodomains on the membrane surface provides a promising strategy for alleviating membrane fouling caused by both hydrophobic and hydrophilic components during ultralow pressurized ultrafiltration processes.