Ultrathin Layer Thickness Research Articles

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.

Tunable photochemical deposition of silver nanostructures on layered ferroelectric CuInP2S6

Two-dimensional layered ferroelectric materials, such as CuInP2S6 (CIPS), are promising candidates for novel and high-performance photocatalysts, owing to their ultrathin layer thickness, strong interlayer coupling, and intrinsic spontaneous polarization, while how to control the photocatalytic activity in layered CIPS remains unexplored. In this work, we report for the first time, the photocatalytic activity of ferroelectric CIPS for the chemical deposition of silver nanostructures (AgNSs). The results show that the shape and spatial distribution of AgNSs on CIPS are tunable by controlling layer thickness, environmental temperature, and light wavelength. The ferroelectric polarization in CIPS plays a critical role in tunable AgNS photodeposition, as evidenced by layer thickness and temperature dependence experiments. We further reveal that AgNS photodeposition process starts from active site creation, selective nanoparticle nucleation/aggregation, to continuous film formation. Moreover, AgNS/CIPS heterostructures prepared by photodeposition exhibit excellent resistance switching behavior and good surface enhancement Raman Scattering activity. Our findings provide new insight into the photocatalytic activity of layered ferroelectrics and offer a new material platform for advanced functional device applications in smart memristors and enhanced chemical sensors.

Vertically FeNi layered double hydroxide/TiO2 composite for synergistically enhanced photoelectrochemical water splitting

Non-precious oxygen evolution electrocatalysts have been attracting considerable attention because of their promising potentials in improving the efficiency of water splitting. Among all the candidates, FeNi electrocatalysts show the relatively high electrocatalytic performances, however, it still remains lack for the optimization of their structure and component, and in particular the heterogeneous synergistic effect by integrating with TiO2 photocatalyst. Here, we prepared FeNi electrocatalysts with tunable Fe/Ni composition on TiO2 film by a simple electrodeposition method and studied their microstructure and electrochemical performance. It is found that FeNi3/TiO2 composite composed of vertically layered double hydroxide (LDH), presents ultrathin layer thickness of 280 nm and open-channeled pore architecture, thus contributing to a high density of active sites, promoting the exchange of ions and electrons, and synergistically suppressing the electron-hole recombination. The optimized FeNi3/TiO2 composite exhibits an overpotential of 280 mV at current density of 10 mA/cm2 and a small Tafel slope of 34 mV/dec. Moreover, the optimized FeNi3/TiO2 composite shows no obvious current drop during the stability test for 8000 s. First-principles calculations demonstrate a synergistic effect of FeNi3/TiO2 composite with the smallest theoretical overpotential based on the density functional theory (DFT) simulations.

Quasistatic antiferromagnetism in the quantum wells of SmTiO3/SrTiO3 heterostructures

High carrier density quantum wells embedded within a Mott insulating matrix present a rich arena for exploring unconventional electronic phase behavior ranging from non-Fermi-liquid transport and signatures of quantum criticality to pseudogap formation. Probing the proposed connection between unconventional magnetotransport and incipient electronic order within these quantum wells has however remained an enduring challenge due to the ultra-thin layer thicknesses required. Here we address this challenge by exploring the magnetic properties of high-density SrTiO3 quantum wells embedded within the antiferromagnetic Mott insulator SmTiO3 via muon spin relaxation and polarized neutron reflectometry measurements. The one electron per planar unit cell acquired by the nominal d0 band insulator SrTiO3 when embedded within a d1 Mott SmTiO3 matrix exhibits slow magnetic fluctuations that begin to freeze into a quasistatic spin state below a critical temperature T*. The appearance of this quasistatic well magnetism coincides with the previously reported opening of a pseudogap in the tunneling spectra of high carrier density wells inside this film architecture. Our data suggest a common origin of the pseudogap phase behavior in this quantum critical oxide heterostructure with those observed in bulk Mott materials close to an antiferromagnetic instability.

Pseudocapacitive layered iron vanadate nanosheets cathode for ultrahigh-rate lithium ion storage

Pseudocapacitive charge storage has been regarded as a promising mechanism to achieve both high specific energy and power energy storage devices. Some pseudocapacitive anode materials show great high-rate performance, however, it remains a significant challenge to develop the cathode ones. Herein, for the first time, we report a layered iron vanadate (Fe5V15O39(OH)9⋅9H2O, named as kazakhstanite) nanosheets (FeVO NSs) with featuring ultrathin layer thickness (< 10 nm). The FeVO NSs are synthesized by a facile wet-chemical approach with a high yield. Compared to the FeVO nanoparticles, the crystalline layered FeVO NSs have additional interlayered Li+ storage sites, leading to the enhanced capacity. Ex-situ X-ray diffraction results demonstrate a non-phase change process and there is only ~ 1.1% layer expansion/shrinkage during lithiation and delithiation process. Based on detail kinetics analysis and ex-situ X-ray photoelectron spectroscopy results, it is found that over 70% of total capacity is pseudocapacitive contribution, which contributes to the ultrahigh-rate capability (a high capacity of 350, 273 and 90 mAh g−1 is achieved at 0.1, 1 and 20 A g−1, respectively) and excellent cycling stability over thousands of cycles. This work presents the high performance vanadate material that delivers highly pseudocapacitive behavior, and provides a promising direction to realize both high energy and high power lithium storage.

Crystal structure of Co/Cu multilayers prepared by pulse potential electrodeposition with precisely controlled ultrathin layer thickness

Co/Cu multilayers were electrodeposited in a single electrolyte using the pulse potential method and the layer thickness was precisely controlled in accordance with Faraday's law. X-ray diffraction revealed that multilayers with layer thicknesses in the range of 25–100 nm consisted of fcc-Co and fcc-Cu phases. For layers thinner than 10 nm, the fcc-Co and fcc-Cu phases merged to form a single crystal phase. When the layers were &amp;lt;1 nm, one diffraction peak of the single crystal phase became proportionally higher as the layer became thinner. The surface structure of multilayers also varied with the layer thickness.

Interfacial Chemistry in Al/CuO Reactive Nanomaterial and Its Role in Exothermic Reaction

Interface layers between reactive and energetic materials in nanolaminates or nanoenergetic materials are believed to play a crucial role in the properties of nanoenergetic systems. Typically, in the case of Metastable Interstitial Composite nanolaminates, the interface layer between the metal and oxide controls the onset reaction temperature, reaction kinetics, and stability at low temperature. So far, the formation of these interfacial layers is not well understood for lack of in situ characterization, leading to a poor control of important properties. We have combined in situ infrared spectroscopy and ex situ X-ray photoelectron spectroscopy, differential scanning calorimetry, and high resolution transmission electron microscopy, in conjunction with first-principles calculations to identify the stable configurations that can occur at the interface and determine the kinetic barriers for their formation. We find that (i) an interface layer formed during physical deposition of aluminum is composed of a mixture of Cu, O, and Al through Al penetration into CuO and constitutes a poor diffusion barrier (i.e., with spurious exothermic reactions at lower temperature), and in contrast, (ii) atomic layer deposition (ALD) of alumina layers using trimethylaluminum (TMA) produces a conformal coating that effectively prevents Al diffusion even for ultrathin layer thicknesses (∼0.5 nm), resulting in better stability at low temperature and reduced reactivity. Importantly, the initial reaction of TMA with CuO leads to the extraction of oxygen from CuO to form an amorphous interfacial layer that is an important component for superior protection properties of the interface and is responsible for the high system stability. Thus, while Al e-beam evaporation and ALD growth of an alumina layer on CuO both lead to CuO reduction, the mechanism for oxygen removal is different, directly affecting the resistance to Al diffusion. This work reveals that it is the nature of the monolayer interface between CuO and alumina/Al rather than the thickness of the alumina layer that controls the kinetics of Al diffusion, underscoring the importance of the chemical bonding at the interface in these energetic materials.

Film thickness influence of dual iridium complex ultrathin layers on the performance of nondoped white organic light-emitting diodes

Using a blue iridium complex phosphorescent dye of bis[(4,6-diflourophenyl)-pyridinato-N,C 2′)](picolinato) iridium(III) (FIrpic) and a yellow iridium complex phosphorescent dye of bis[2-(4-tertbutylphenyl)benzothiazolato- N,C 2,]iridium (acetylacetonate) [(t-bt) 2Ir(acac)] as dual ultrathin layers, nondoped white organic light-emitting diodes (WOLEDs) were fabricated. By optimizing the thickness of ultrathin layers, the influence of FIrpic and (t-bt) 2Ir(acac) dual ultrathin layers on the performance of WOLEDs was studied. The results showed that a stable pure white light emission was obtained with a maximum current efficiency of 11.08 cd/A and a power efficiency of 6.21 lm/W. The Commissions Internationale De L’ Eclairage (CIE) coordinates at (0.33, 0.33) were observed at 9 V bias and showed a very slight variation of (±0.01, ±0.01) in a wide range of voltages. The high power efficiency and stable electroluminescence spectra were attributed to the excitons confined effectively in the recombination zone by the charge trapping effect of dual phosphorescent ultrathin layers.

High-performance white organic light-emitting device using non-doped-type structure

White organic electroluminescent devices based on non-doped-type multiplayer that included [2-methyl-6-[2-(2,3,6,7-tetrahydro-1H, 5H-benzo[ij] quinolizin-9-yl) ethenyl]-4H-pyran-4-ylidene] propane-dinitrile (DCM2) and 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H, 5 H, 11 H-(1)-benzopyro yran –o (6,7-8-i, j) quinolizin-11-one (C545T) ultrathin layer have been fabricated. By appropriately controlling the ultrathin layer thickness, the device shows a maximum efficiency of 9.37 cd A −1 at 8 V, maximum power efficiency of 4.20 lm W −1 at 6 V, and maximum luminance of 23,840 cd m −2 at 18 V, respectively. The emission spectrum of the device covers a wide range of the visible region and can be sensitively adjusted by the thickness of DCM2 and C545T. The white emission with CIE coordinates ranging from (0.47, 0.35) at 7 V to (0.35, 0.34) at 15 V was obtained.

Micro/nanomechanical resonators for distributed mass sensing with capacitive detection

Micro/nanomechanical resonators have been designed and fabricated with the aim to be used as distributed mass sensors for in situ measurement of the thickness of ultra-thin layers. First, we present a comparative study of three kinds of oscillating devices (cantilever, bridge and quad beam). The quad beam design has been selected for fabrication because it combines high sensitivity and good electrical response. The complete fabrication process of the device is based on electron beam lithography, lift-off and reactive ion etching. The frequency response has been characterized by means of electrical excitation and capacitive read-out.

Precise Characterization of Silicon on Insulator (SOI) and Strained Silicon on Si1−xGex on Insulator (SSOI) Stacks with Spectroscopic Ellipsometry

ABSTRACTFurther improvements in CMOS circuit performance such as switching speed and power reduction will rely on the use of silicon on insulator (SOI) substrates with decreased functional layer thicknesses. According to the International Technology Roadmap for Semiconductors (ITRS), the silicon and buried SiO2 (BOX) layer thicknesses for a fully depleted device should be in the ranges of 10–16nm and 24 – 40nm by 2005, respectively. A key issue for fully-depleted CMOS transistors is control of such ultra-thin layer thicknesses and their uniformity along with other parameters such as surface and interface roughness. This poses a challenge to metrology, especially to conventional reflectometry technique because the layer thicknesses must be determined with angstrom precision for both silicon cap and SiO2 box layer.Spectroscopic ellipsometry (SE) is an optical and non destructive technique for determining thin film thickness and material optical properties. Because ellipsometry measures the change in the polarization state for both the amplitude ratio of the p to s polarizations, and in the phase retardation, it provides a precise way to characterize such ultra thin SOI stacks. Comprehensive characterization results for a number of thin and ultra thin SOI stacks with different thickness ranges will be presented together with measurement repeatability results relevant to the film thickness process tolerances. In addition, characterization results for advanced device applications such as strained silicon-on- Si1−xGex-on-insulator (SSOI) will also be shown, demonstrating the use and capability of spectroscopic ellipsometry for precise determination of layer thickness, material composition, interfacial layers, etc. Principles and advantages of the technique will also be discussed in the presentation.

Solid State Amorphisation in Magnetic Multilayers: the Interface Structure and the Electrical Transport Properties

The structure and electrical conductivity properties of the R.F. sputtered Fe⧸Zr and Fe⧸Ti multilayers with ultrathin layer thicknesses, in as-deposited states, have been studied using X-ray diffraction, low-angle X-ray and neutron reflectometry, conversion electron Mössbauer spectroscopy (CEMS), resistivity and magnetoresistivity measurements. The thickness ratio (β=dFe⧸dZr and dFe⧸dTi) of analysed multilayers was 0.5 and 1, the values of the bilayer thickness (λ=dFe+dTi,Zr) was varied from 9 Å to 600 Å, maintaining constant the total thickness of the samples by controlling the number of bilayers. The results obtained from CEM-spectroscopy and X-ray diffraction show that Fe layers of the thickness below 20 Å are alloyed forming an amorphous phase during deposition. This amorphous phase is distributed in the plane between the crystalline sublayers as well as in the grain boundaries according with the proposed model of the interpretation of the electrical conductivity as a function of the bilayer thickness (λ).

Quantum size effects and magnetoresistance in spin-valved Co/Cu/Co trilayer structures

Electron transport and the magnetoresistance of magnetron sputtered ultrathin Co(M1)/Cu/Co(M2) trilayer structures that are of comparable structural perfection are presented as a function of magnetic and nonmagnetic layer thicknesses. We apply the quantum well states model to the interpretation of the electron transport data, while also considering the shunting effects or classic diffuse bulk scattering effect on electron transport in these metallic trilayer structures. This approach represents a beyond free-electron approximation that takes into model calculations details of the electronic band structure of the trilayers and spin-dependent electron scattering by impurities and/or at interfaces. A concurrent description of both the resistivity and magnetoresistance data can be achieved, as distinguished from such general semiclassical ones as Camley–Barnas’s and its deviations that account for the magnetoresistance well, but fail to describe electron transport of these layered structures in the ultrathin layer thickness limit.

Structural and magnetic properties of amorphous Fe/Zr multilayers

The structure and magnetic properties of rf-sputtered Fe/Zr multilayers with ultrathin layer thicknesses, in as-deposited and annealed states, have been studied using x-ray diffraction, low- angle x-ray reflectometry, conversion electron Mössbauer spectroscopy and low-temperature magnetometry. The thickness ratio of the analysed multilayers was 0.5 and the values of the bilayer thickness was varied from 9 Å to 75 Å, maintaining constant the total thickness of the samples by controlling the number of bilayers. The results obtained show that, during sputter deposition, Fe layers with thicknesses below 20 Å are alloyed forming an amorphous phase. Those multilayers with nominal Fe layer thickness below 20 Å are composed of layers of this amorphous phase, behave as a ferromagnet below the Curie temperature , and the spin structure shows a low-temperature asperomagnetic ground state with a reduced canting of the spin structure as compared with homogeneous amorphous thin films. The structural relaxation during annealing indicates that an homogeneous Fe - Zr amorphous alloy can be produced after annealing these Fe/Zr multilayer structures.

Surface Plasmon Investigations of Light-Induced Modulation in the Optical Thickness of Molecularly Thin Photochromic Layers

We report evidence of photoisomerization-induced changes in the optical thickness of molecularly thin (9 Å) photochromic layers which consist of azosilane molecules immobilized on silicon oxide substrates. Further, this light-induced change in the optical thickness of these films can be reversed upon irradiation with light of an appropriate wavelength. This has been demonstrated by means of surface plasmon spectroscopy that allows the observation of very small changes in the optical thickness of ultrathin layers. The study of the thermal back isomerization reaction of the azobenzene molecules in these films reveals steric hindrance at the molecular level, as shown by using UV−vis spectroscopy.

The role of spin-dependent impurity scattering in Fe/Cr giant magnetoresistance multilayers

To probe the mechanism of giant magnetoresistance (GMR) observed in Fe/Cr multilayers, we have sputter deposited at the interfaces of Fe(15 Å)/Cr(12 Å) multilayers an additional, ultrathin (0–4 Å) layer of a variety of third elements (V, Mn, Ge, Ir, and Al). When alloyed with Fe in dilute concentrations, the elements chosen have known resistivities for spin-up (ρ↑) and spin-down (ρ↓) currents arising from spin-dependent impurity scattering. The results show a clear correlation between α=ρ↓/ρ↑ of the respective element and the way in which GMR varies with the ultrathin layer thickness. In addition, little difference in GMR is observed between multilayers where the ultrathin layer thickness t/2 is deposited on every Fe/Cr interface and those with a thickness t deposited on alternate interfaces. This investigation demonstrates the importance of the type and total number of scattering centers per multilayer period to the GMR effect.

Comparison of ultrathin W-Si and W-C multilayers for x-ray optics

New magnetic materials and x-ray mirrors have lead to the preparation of multilayered structures (MLS) with ultrathin periodic layers. However, physical limitations appear for periods below some tens of A. In particular, fabrication tolerances relative to these ultrathin layer thicknesses are critical for the realization of effective MLS. In sputtering, the deposition rate is easily reproducible and can be maintained at a constant and low value for several hours; for this reason we are able to perform a large number (more than 100) of periodic ultrathin layers by this system (3 to 1.5 nm period). High Resolution Transmission Electron Microscopy (HRTEM) and x-ray reflectivity measurements are used to compare interfaces and stacking regularities for different samples with 140 layers. The W/Si MLS present a better quality of interfaces and structures compared to the W/C MLS.

(100) and (111) Ni films epitaxially grown on Cu by the metal–metal epitaxy on silicon technique near room temperatures

A growth technique is described which uses the Cu films epitaxially grown on Si as seed layers for the further epitaxial growth of other metals. The technique, metal–metal epitaxy on silicon (MMES), is illustrated with the growth of both (100) and (111) Ni films. Ni, being a face-centered cubic (fcc) metal, tends to grow in the (111) orientation on various substrates, including (100) Si. The (100) Cu/Si structure, however, allows the growth of Ni in the (100) orientation near room temperatures using electron-beam evaporation in a vacuum of low 10−7 Torr. A minimal thickness of the Cu seed layer needed for the growth of (100) Ni is found to be between 20 and 50 Å. Using the (111) Cu/Si substrates, (111) Ni is observed. Both the single layer, Ni/Cu/Si, and the double layer, Cu/Ni/Cu/Si structures have been grown with controlled orientations. The latter allows the growth of periodic Cu–Ni structures with ultrathin layer thicknesses. The periodic structures of a Cu(50 Å)–Ni(25 Å) superlattice is well resolved for the films deposited on (100) and (111) Cu/Si, and on Cu/glass. The last structure shows both the (100) and (111) diffraction patterns of the superlattice, with well resolved satellite peaks. Heating the Cu–Ni structures shows an interesting orientation dependence for the interactions, with a higher outdiffusion rate of Cu through Ni for the (111) orientation than the (100) one. This is opposite of that observed for either the outdiffusion of Si through Au using single crystal Si substrates, or the reaction rate of Cu–Si for the Cu films epitaxially grown on Si. An interface model is used to explain the results observed. Possible applications of the epitaxial Ni films thus grown are suggested.