Ultrathin Oxide Films Research Articles

Modelling of insulating potential in ultra-thin (42 å) silicon oxide film

Based on previously conducted measurements of the tunneling current-voltage characteristics of metal-SiO2-Si (MOS) structures, modeling of the insulating potential in an ultra-thin (4.2 nm) silicon oxide film was performed. The potential in the dielectric was defined in the shape of a trapezoid, with the lateral slopes simulating transition layers and the top base representing the bulk of SiO2. The model parameters – the barrier height and the coordinates of the trapezoid's corner points – were calculated to achieve the maximum match between the experimental and theoretical voltage derivatives of the current logarithm. Common features of the insulating potential, similar to those in thinner silicon oxide films (3.7 nm), were identified: the barrier occupies up to half of the nominal volume of the dielectric gap and is shifted towards the gate electrode, with its slope towards the semiconductor substrate being much more gradual compared to the slope adjacent to the gate.

Ultra-high NA graphene oxide flat lens on a fiber facet with near diffraction-limited focusing

The realization of a high numerical aperture (NA) fiber lens is critical for achieving high imaging resolution in endoscopes, enabling subwavelength operation in optical tweezers and high efficiency coupling between optical fibers and photonic chips. However, it remains challenging with conventional design and fabrication. Here we propose an ultrathin (400 nm) graphene oxide (GO) film lens fabricated in situ on a standard single-mode fiber facet using the femtosecond laser direct writing technique. An extremely high NA of 0.89 is achieved with a near diffraction-limited focal spot (FWHM=0.68λ), which is verified theoretically and experimentally. The diameter of the fabricated fiber GO lens is as small as 12 μm with no beam expansion structure. The proposed fiber GO lens is promising for applications such as super-resolution imaging, compact optical tweezers, medical endoscopes, and on-chip integration.

Vacancy Ordering in Ultrathin Copper Oxide Films on Cu(111).

Oxide thin films grown on metal surfaces have wide applications in catalysis and beyond owing to their unique surface structures compared to their bulk counterparts. Despite extensive studies, the atomic structures of copper surface oxides on Cu(111), commonly referred to as "44" and "29", have remained elusive. In this work, we demonstrated an approach for the structural determination of oxide surfaces using element-specific scanning tunneling microscopy (STM) imaging enhanced by functionalized tips. This approach enabled us to resolve the atomic structures of "44" and "29" surface oxides, which were further corroborated by noncontact atomic force microscopy (nc-AFM) measurements and Monte Carlo (MC) simulations. The stoichiometry of the "44" and "29" frameworks was identified as Cu23O16 and Cu16O11, respectively. Contrary to the conventional hypothesis, we observed ordered Cu vacancies within the "44" structure manifesting as peanut-shaped cavities in the hexagonal lattice. Similarly, a combination of Cu and O vacancies within the "29" structure leads to bean-shaped cavities within the pentagonal lattice. Our study has thus resolved the decades-long controversy on the atomic structures of "44" and "29" surface oxides, advancing our understanding of copper oxidation processes and introducing a robust framework for the analysis of complex oxide surfaces.

Microstructure and coloration mechanism of TC11 aerospace titanium alloy ultra-thin thermal oxide films

This study explores the microstructure and coloration mechanism of high-saturation structural colors in ultra-thin oxide films formed on TC11 aerospace titanium alloy near permissible engineering temperatures. Utilizing optical microscopy (OM), spectrophotometry, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB), transmission electron microscopy (TEM), and energy dispersive spectrometer (EDS), the optical properties, thickness, and microstructure of the oxide films were examined. The research posits a coloration theory grounded in zero-order Fabry-Pérot resonance-enhanced absorption. Key findings reveal a color transition in the oxide films from golden to light blue as oxidation conditions vary from 450 °C for 5 h to 550 °C for 60 h, with an average thickness ranging from 16.3 nm to 63.4 nm. Despite changes in oxidation conditions, the film's structure remains consistent and uniform. It predominantly consists of Brookite-type TiO2, TiO and Al2O3, with trace amounts of MoO2 and MoO3. The film's color is independent of its structural composition and instead related to its thickness, influencing the absorption wavelength per the Fabry-Pérot model, which closely matches the observed absorption wavelengths. Notably, oxidation acceleration between 30 and 60 h at 550 °C heralds the commencement of a deviation from the coloration mechanism of the oxide film based on the Fabry-Pérot cavity.

Photo-electric properties of ultra-thin cuprous oxide films prepared by electrodeposition

Photo-electric properties of ultra-thin cuprous oxide films prepared by electrodeposition

Growth of ultrathin cobalt oxide films on Pd(100): Refined structural model

We present an atomistic-level study on the growth of cobalt oxide ultra-thin films prepared on Pd(100) substrate performed by a combination of scanning tunneling microscopy (STM) and Low Energy Electron Diffraction (LEED). In this contribution, we report a new surface structure c(12×2)-CoO(111), which co-exist with the previously reported extended c(4 × 2)-CoO(100) domains at low Co deposition coverages. We demonstrate that the lattice parameters derived for the newly observed hexagonal c(12×2)-CoO(111) phase fit the three- or sixfold lattice parameter of the c(4 × 2)-CoO(100) layer. The c(12×2)-CoO(111) layer was shown to grow directly on top of the c(4 × 2)-CoO(100) structure, offering a transition mechanism from rectangularly to hexagonally terminated CoO surface. At growing Co coverages, HEX-CoO(111) domains exhibiting a Moiré pattern followed by the formation of well-defined Co3O4(111) islands, were prepared and structurally characterized. Both latter phases have been previously reported in the literature. Finally, we provide a refined model for the epitaxial growth of cobalt oxide thin films on Pd(100), including the formation of new c(12×2)-CoO(111) phase.

Percolation threshold in annealed ultrathin SiOx films by 2D Monte Carlo simulations

In this work, the kinetics of phase separation in ultrathin non-stoichiometric Si oxide (SiOx, x < 2) films during high-temperature annealing is studied by Monte Carlo simulations on the two-dimensional...

Unravelling the nanoscale mechanism of polarization reversal in a Hf0.5Zr0.5O2-based ferroelectric capacitor by vector piezoresponse force microscopy.

Ferroelectricity is in demand in many device concepts in electronics, energy and microsystem engineering. The performance of ferroelectrics-based devices is determined by either out-of-plane or in-plane polarization, or out-of-plane or in-plane piezoelectric strain. Real prospects for the practical implementation of innovative devices opened up after the discovery of ferroelectricity in ultrathin hafnium oxide films, due to their perfect compatibility with silicon technology. Ferroelectric properties of this material have been assigned to an orthorhombic structural phase with a single polar axis, but the spatial orientation of the polarization vector and the tensorial piezoelectric behaviour, which are inextricably coupled, still remain unknown. Herein, the rotation of the polarization vector in a Hf0.5Zr0.5O2 (10 nm) capacitor during polarization switching and the spatial distribution of longitudinal and shear piezoelectric coefficients are elucidated at the nanoscale using operando vector piezoresponse force microscopy. In most of the capacitor, a 180°-flipping of the polarization vector is observed, which is consistent with the orthorhombic phase structure. However, a rather large fraction of the capacitor is also occupied by nanoregions of ferroelastic (non-180°) switching, which is explained by the effect of the local mechanical stress. To quantify the three-dimensional piezoresponse, a novel approach exploiting the Poisson effect in artificially created non-ferroelectric regions is proposed and it shows that the shear piezoelectric coefficient is twice the longitudinal coefficient. The experimental insights entail an important step in fundamental understanding of the ferroelectric and piezoelectric properties of hafnium oxide and have great potential to trigger new versions of ferroelectric-based devices.

Rapid-response humidity sensors based on ultra-thin films stacked with single-layer graphene oxide

Monitoring and control of humidity is important for industry, agriculture and human activities. Currently, it is achallengeto develop highly sensitive humidity sensors with both the response time and recovery time less than 1 s.Therefore, it isdifficult to dynamically monitor rapid changes in humidity. Here, we report a resistive humidity sensor based on an ultrathin graphene oxide (GO) film, which is formed by stacked single-layered GO. Both the response and recovery time of the sensor is less than 1 s in the range of 10 %–95 % relative humidity (RH). The shortest response and recovery time of the reported GO-based resistive humidity sensor can be 0.33 s at 10 % RH, indicating the fastest response and recovery property to date. Moreover, the sensor is sensitive, and it can accurately detect the distance between the sensor and the human finger as well as the strength of the human breath. The simple preparation, microscale structure, and extremely fast response make the GO-based sensor a new alternative for integrated humidity sensors with real-time monitoring functions.

Growth of metastable 2H-CaSi2 films on Si(111) substrates with ultrathin SiO2 films by solid phase epitaxy

The Si-nano dot substrates formed using the ultrathin silicon oxide films were applied to fabricate CaSi2 films. The CaSi2 formed by this process was identified as the metastable phase 2H as the main component, and the 1H structure existed partially at the grains of the 2H phase. Although no experimental reports exist for the formation of 2H-CaSi2 crystal, the Si-nano dot substrates are considered as the high-entropy substrate to form the metastable phases. We experimentally determined the lattice parameter of the 2H phase by the annular dark field–scanning transmission electron microscopy observations using the Si as an internal standard sample.

Advanced two-objective optimization of thickness and large-area homogeneity of ZnO ultrathin films deposited by atomic layer deposition

Semiconductor thin films and coatings have become one of the most relevant research fields due to their significant applications in priority energy-related technologies such as solar cells, photocatalysts, and smart windows. Since all these fields are conceived as tools to fight against the effects of climate change, a real impact requires the successful deposition of semiconductor films on large-area substrates such as windows, panels, pipes, and containers, to give rise to photoactive components suitable for buildings, industries, cars, and parks. However, scalability remains one of the major issues in almost all methodologies known for the deposition of semiconductor films, irrespective of the phase approach used, i.e., either from vapor- or liquid-phase. Here, a mathematical metamodel was applied to simulate the atomic layer deposition (ALD) of zinc oxide (ZnO) ultrathin films (a versatile photoactive material in energy-related research) and optimized their thickness and homogeneity over the whole area of 8 in.-diameter Si wafers. Knowing all ALD parameters that define the quality and properties of the deposited films, we delimitated a set of four metamodel-inputs (zinc precursor dose, purge, and the inner and outer carrier gas flows) based on literature review, expertise, costs, and reactor design aspects specific to the deposition of ZnO. The average thickness and homogeneity of the films were established as the two outputs of the metamodel, which were the object of optimization. Using advanced iterative procedures, we carried out three rounds of experiments that lead us to a set of ALD parameters to deposit a ZnO ultrathin film with an average thickness of 11.38 nm that leads to a deposition rate of 1.9 Å/cycle, which represents 90% of the highest reported value for ZnO by ALD (2.1 Å/cycle). The homogeneity over the whole 8 in.-diameter wafer reached 2.61 nm, which represents the smoothest distribution of thickness values in the entire deposited area. Given the origin of the limits constraining this optimization procedure, our results hold promise in supporting the transition from the laboratory-level synthesis of thin-film-based optoelectronic devices to their large-scale production. This could ultimately help to circumvent the difficulties faced in scaling the ALD technology and enable alternative deposition methodologies such as thermal ALD, otherwise inaccessible to the production chain.

Breaking through the plasma wavelength barrier to extend the transparency range of ultrathin indium tin oxide films into the far infrared

Indium tin oxide (ITO) film, which is the most commonly used transparent conductive film (TCF), has traditionally been believed to be transparent in the visible spectrum but to reflect infrared (IR) light beyond the plasma wavelength (λp). However, our theoretical analysis challenges this notion by demonstrating that an ultrathin ITO TCF that is thinner than the light's penetration depth can overcome the transmission barrier at λp. To validate the theoretical modeling, we have successfully fabricated ultrathin ITO films that, despite having λp ≈ 1 μm, remain transparent from 400 nm to 20 μm. This represents the broadest transparency range ever reported for any In2O3-based TCF. The 10-nm-thick ITO TCFs have high visible transmittance (91.0% at 550 nm), low resistivity (5 × 10−4 Ω cm), and good IR transmittance (averaging 60% over 1.35–18.35 μm). Their IR transparency facilitates radiative cooling of the underlying circuitry. When an operational resistor is enclosed by commercial ITO TCFs that are 140 nm thick, its temperature increases. However, using 10-nm-thick ITO TCFs instead of the commercial ones can completely avoid this temperature rise. Moreover, attaching a silver grid to a 10-nm-thick ITO TCF can reduce the effective sheet resistance to ∼10 Ω/□ at the expense of only ∼3% transmittance. This development paves the way for large-scale applications that require low sheet resistance and far-IR transparency.

Thermal Scanning-Probe Lithography for Broad-Band On-Demand Plasmonic Nanostructures on Transparent Substrates.

Thermal scanning-probe lithography (t-SPL) is a high-resolution nanolithography technique that enables the nanopatterning of thermosensitive materials by means of a heated silicon tip. It does not require alignment markers and gives the possibility to assess the morphology of the sample in a noninvasive way before, during, and after the patterning. In order to exploit t-SPL at its peak performances, the writing process requires applying an electric bias between the scanning hot tip and the sample, thereby restricting its application to conductive, optically opaque, substrates. In this work, we show a t-SPL-based method, enabling the noninvasive high-resolution nanolithography of photonic nanostructures onto optically transparent substrates across a broad-band visible and near-infrared spectral range. This was possible by intercalating an ultrathin transparent conductive oxide film between the dielectric substrate and the sacrificial patterning layer. This way, nanolithography performances comparable with those typically observed on conventional semiconductor substrates are achieved without significant changes of the optical response of the final sample. We validated this innovative nanolithography approach by engineering periodic arrays of plasmonic nanoantennas and showing the capability to tune their plasmonic response over a broad-band visible and near-infrared spectral range. The optical properties of the obtained systems make them promising candidates for the fabrication of hybrid plasmonic metasurfaces supported onto fragile low-dimensional materials, thus enabling a variety of applications in nanophotonics, sensing, and thermoplasmonics.

(Invited) Toward Rigorous Modeling of Metal-Metal, Metal-Oxide and Oxide-Oxide Interfaces for the Electrochemical Oxygen Cycle

To achieve Net-Zero Emissions by 2050, hydrogen and hydrogen fuel cells will play a significant role in powering vehicles. In this presentation, I will show, through first-principles-based modeling, how to achieve rigorous modeling of solid-solid interface for the hydrogen and oxygen evolution reactions in electrolysis and the oxygen reduction reaction in fuel cells; and how the fundamental understanding paves the way toward developing both model electrocatalysts and industrial electrocatalysts with significantly improved performance. Specifically, I will introduce the following three topics.[1] metal-metal interfaces: strain evolution and strain tuning of epitaxial, stepped and free-standing platinum group metals for the oxygen reduction reaction in fuel cells.(1)[2] metal-oxide interfaces: stability and activity of monolayer oxide/Pt interface toward improving the hydrogen evolution reaction in alkaline conditions.(2)[3] oxide-oxide interfaces: self-consistent modeling of active phases, reaction centers, and catalytic mechanisms of Ni- and Co-based layered double hydroxides for the oxygen evolution reaction.(3, 4) L. Wang, Z. Zeng, W. Gao, T. Maxson, D. Raciti, M. Giroux, X. Pan, C. Wang, J. Greeley, Tunable intrinsic strain in two-dimensional transition metal electrocatalysts. Science 363, 870-874 (2019).Z. Zeng, K.-C. Chang, J. Kubal, N. M. Markovic, J. Greeley, Stabilization of ultrathin (hydroxy)oxide films on transition metal substrates for electrochemical energy conversion. Nature Energy 2, 17070 (2017).F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. F. d. Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. R. Cuenya, P. Strasser, In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution. Nature Communications 11, 2522 (2020).F. Dionigi, J. Zhu, Z. Zeng, T. Merzdorf, H. Sarodnik, M. Gliech, L. Pan, W.-X. Li, J. Greeley, P. Strasser, Intrinsic Electrocatalytic Activity for Oxygen Evolution of Crystalline 3d-Transition Metal Layered Double Hydroxides. Angew. Chem., Int. Ed. 60, 14446-14457 (2021). Figure 1

Transparent and Stretchable Piezoresistive Strain Sensors with Buckled Indium Tin Oxide Film

AbstractOwing to excellent performance and a simple mechanism, stretchable piezoresistive strain sensors have been applied to human skin for monitoring physical activities, physiological activities, etc. However, it is still a challenge to simultaneously realize highly sensitive and stretchable piezoresistive strain sensors with high optical transparency. This study reports a transparent and stretchable piezoresistive strain sensor with 2D surface buckling by fabricating ultrathin indium tin oxide (ITO) film on the biaxially pre‐stretched polyacrylate (VHB) elastomer followed by pre‐stretch releasing. To the authors' knowledge, semiconductors are applied for a stretchable piezoresistive strain sensor for the first time. Furthermore, this strain sensor exhibits a high sensitivity of 569, a high transparency of 88% and a high biaxial stretchability of 110% at the same time. This device demonstrates the good long‐term stability over 500 stretching–relaxing cycles. The high sensitivity can be mainly attributed to the piezoresistive effect of the semiconductor where carrier mobility and the resulting resistivity can be significantly changed by the strain. The strain sensors attached to human skin are used to monitor many human motions such as chewing, swallowing, breathing, and walking. ITO‐based strain sensors pave the way toward the development of highly stretchable and sensitive wearable electronics.

Ionic transport in Samarium doped Ceria free-standing single crystal membrane

We present the realization of Samarium Doped Ceria (SDC) free-standing membranes and their structural and ionic transport properties. An epitaxial layer of water-soluble Sr3Al2O6 (SAO) sacrificial layer was deposited onto SrTiO3 (100) oriented single crystal substrate, followed by the deposition of the SDC layer. The free-standing SDC membrane was produced by etching the SAO layer in water. The creation of a 2D free-standing membrane allowed us to determine its transport and structural properties without any possible contribution of the substrate. Moreover, the availability of a thin SDC single crystal free-standing membrane offers the opportunity to tune the electrical properties by applying, for instance, compressive or tensile strain much larger than any substrates can induce by the lattice mismatch. Furthermore, this procedure allows the transfer of the film onto any kind of substrate and complex integrated circuit, i.e. those based on silicon technologies, giving the possibility to realize a new class of complex devices based on oxide ultra-thin films.

Aluminum-doped zinc oxide (AZO) ultra-thin films deposited by radio frequency sputtering for flexible Cu(In,Ga)Se2 solar cells

Zinc oxide ultra-thin films doping with aluminum (AZO) were produced through radio frequency (rf) sputtering at a fixed pressure of 10 mTorr while varying the rf power between 80 and 140 W. The crystal structure of hexagonal Wurtzite was consistent throughout, with improved crystallinity observed at higher rf powers due to optimal diffusivity of the sputtered particles during nucleation and growth. The size of the crystallite was increased from 10.37 to 16.58 nm with increasing the rf power from 80 to 140 W. The Raman spectra provided evidence of the formation of ultra-thin AZO films, with discernable changes in morphology due to the influence of rf power. The value of optical band gap fluctuated between 3.49 and 3.58 eV as a function of rf power, a basis of the Burstein–Moss effect. The resistivity of the ultra-thin AZO films declined while augmenting rf power. A bilayer structure of intrinsic ZnO (i-ZnO) and AZO was fabricated and exhibited good transmittance, well-crystalline morphology, and excellent electrical conductivity. The optimized window layer (i-ZnO and AZO) was used to produce flexible Cu(In,Ga)Se2(CIGSe) solar cells with a photo conversion efficiency of 9.53%. Therefore, ultra-thin ZnO films exhibit potential as a favorable option for a window layer in the production of high-efficient flexible solar cells in cost effective way.

Cell Response on Laser-Patterned Ti/Zr/Ti and Ti/Cu/Ti Multilayer Systems

Arranged patterns obtained via ultrafast laser processing on the surface of Ti/Cu/Ti/Si and Ti/Zr/Ti/Si thin-film systems are reported. Two differently designed multilayer thin films Ti/Cu/Ti/Si and Ti/Zr/Ti/Si were deposited on silicon using the ion sputtering method. The bioactive surfaces of these systems involve the formation of laser-induced periodic surface structures (LIPSS) in each of the laser-written lines of mesh patterns on 5 × 5 mm areas. The formation of nano- and micro-patterns with an ultra-thin oxide film on the surfaces was used to observe the effects of morphology and proliferation of the MRC-5 cell culture line. To determine whether Ti-based thin films have a toxic effect on living cells, an MTT assay was performed. The relative cytotoxic effect, as a percentage of surviving cells, showed that there was no difference in cell number between the Ti-based thin films and the control cells. There was also no difference in the viability of the MRC-5 cells, except for the Ti/Cu/Ti/Si system, where there was a slight 10% decrease in cell viability.

On the CO2 photocatalytic reduction over indium tin oxide (ITO) ultra-thin films in water vapor: Experimental and theoretical study

Artificial photosynthesis that converts CO2 and H2O into compounds with added value is a viable method for reducing atmospheric CO2 concentration. In this study, we investigated the photocatalytic activity of ultrathin indium tin oxide (ITO) films on rigid and flexible substrates for CO2 reduction in water vapor and batch and flow setups. To explain a viable reaction mechanism for the 2760 ± 10 % μmol.gcat-1.hr-1 production rate achieved in a continuous reaction system, several theoretical models were developed. According to DFT simulation results, oxygen vacancy might be regarded as the main reaction site for CO2 to CO conversion. Moreover, a viable reaction pathway leading to CH4 formation is proposed. The catalyst’s lower rate of CO2 reduction in the batch reactor as compared with the flow reactor was justified via surface affinity simulation. Our findings highlight the significance of the catalyst’s structural design and reaction media configuration for stability and high activity.

PdRh-Sensitized Iron Oxide Ultrathin Film Sensors and Mechanistic Investigation by Operando TEM and DFT Calculation.

Metal oxide semiconductor (MOS) thin films are of critical importance to both fundamental research and practical applications of gas sensors. Herein, a high-performance H2 sensor based on palladium (Pd) and rhodium (Rh) co-functionalized Fe2 O3 films with an ultrathin thickness of 8.9nm deposited by using atomic layer deposition is reported. The sensor delivers an exceptional response of 105.9 toward 10ppm H2 at 230 °C, as well as high selectivity, immunity to humidity, and low detection limit (43 ppb), which are superior to the reported MOS sensors. Importantly, the Fe2 O3 film sensor under dynamic H2 detection is for the first time observed by operando transmission electron microscopy, which provides deterministic evidence for structure evolution of MOS during sensing reactions. To further reveal the sensing mechanism, density functional theory calculations are performed to elucidate the sensitization effect of PdRh catalysts. Mechanistic studies suggest that Pd promotes the adsorption and dissociation of H2 to generate PdHx , while Rh promotes the dissociation of oxygen adsorbed on the surface, thereby jointly promoting the redox reactions on the films. A wireless H2 detection system is also successfully demonstrated using the thin film sensors, certifying a great potential of the strategy to practical sensors.