Atomic Layer Precision Research Articles

Single Layer Control of Nanoscale Metal-Insulator Transition at the LaAlO3/SrTiO3 Interface.

Modern quantum device fabrication often requires precisely adding and removing materials in situ at nanoscales, which is challenging for high-quality correlated oxide devices. In this work, we present a novel nanofabrication method that remotely controls the interfacial metal-insulator transition at the LaAlO3/SrTiO3 interface by selectively removing an LaAlO3 overlayer using a diamond tip. Remarkably, we observe a large force window within which single atomic layer precision of control is achievable. Our results confirm the critical thickness and charge transfer mechanism through a layer-by-layer removal process at the interface. Additionally, high-quality nanodevices, including nanochannels and single electron transistors, are successfully fabricated using this method. This nonvolatile and high-precision nanofabrication method provides a promising oxide platform for quantum engineering by harnessing the rich electron correlations at the nanoscale.

Ni doping induced property enhancement in laser ablated BaSnO3 films suitable for optoelectronic applications

Pulsed laser deposition is a straightforward approach for preparing films with superconducting to dielectric properties with atomic layer precision. The deep-seated mechanisms involved in the particle transport from target to substrate and subsequent film formation still need to be fully comprehended. This manuscript reports the property enhancement observed in laser ablated perovskite BaSnO3 films with Ni doping. Films' crystallinity improvement is observed, and an intensity enhancement of 1150% is observed on 3 mol% Ni-doping. The optimum Ni-doping concentration in BaSnO3 is found to be 3 mol%. Herein, Ni-doped BaSnO3 films deposited by PLD showed an unusual increase in film thickness (i.e., from 615 nm in the pure film to 1317 nm in the film with 7 mol% Ni-doping as revealed by lateral SEM analysis and spectroscopic ellipsometry). We propose an “Induced Magnetic field-assisted Particle Convergence (IMPC)” effect for this superficial growth enhancement. The film's optical properties are modified with an increased nickel doping level, and the bandgap energy shows renormalization. All the films show excellent transmittance (80–90%) in the Vis.-NIR region. Hall-effect measurement reveals the increased carrier concentration by three orders (2.98 × 1011 to 3.50 × 1014 cm−3). In addition, the enhancement in mobility from 3.13 to 20.93 cm2V−1s−1 and a decrease in electrical resistivity by six orders (i.e., from 4.05 × 109 to 1.13 × 103 Ω cm) are observed on 7 mol% Ni doping. XPS measurements reveals that the Ba, Sn and Ni ions are at 2+, 4+ and 2+ oxidation states. Using spectroscopic ellipsometric method, we estimated the optical constants of the films, the refractive index, dielectric constant, and extinction coefficient show a normal dispersion behavior. The high crystallinity, high transmittance, suitable surface topography, and improved electrical performances of the Ni-doped BaSnO3 films make them excellent candidates for optoelectronic devices and solar cells.

(Invited) Electrochemistry Happens at the Interface

Earth-abundant, active, selective and stable electrocatalysts are the cornerstone in our transition to defossilisation of the chemical industry, sector coupling, and sustainable energy. The catalyst surface is known to transform at the interface to the electrolyte, especially under operation conditions. Only the properties of the transformed interface coupled to the underlying layer gives rise to observed metrics like activity and stability. This complicates predictive electrocatalyst materials design – a challenge which can only be tackled by collaborative and interdisciplinary efforts, at the Interface between Chemistry, Physics, Materials Science, and Engineering. I believe this presents one of the main tasks for the young electrochemistry researchers in Europe and around the world, especially since the same challenges also arise in related technologies like Li-ion batteries.In my talk I will discuss how the old wisdom from semiconductor research “the Interface is the device” can be translated into the current challenge “Electrochemistry happens at the Interface”. We need new approaches in designing and studying these electrochemical interfaces for fundamental insights that may in a next step yield better performance in future applications. The talk will be split in two parts.Firstly, I will introduce how single crystalline surfaces can be obtained using epitaxial thin films, and discuss how these can be used to derive atomic-level structure-property relations by synergetic experimental and theoretical investigation. Such films can be fabricated with unit-cell or even atomic-layer precision and enable direct comparison to single facets typically investigated in density functional theory.Secondly, I will discuss opportunities and challenges in surface-sensitive operando characterization in a liquid medium.1 Information from the outermost surface of a catalyst can be obtained through a standing-wave approach2,3 or extraction of a surface-only signal from careful thickness-dependent studies.4 In my ERC project “Interfaces at work”, we are also developing interface-sensitive, laboratory-based operando X-ray photoelectron spectroscopy approaches based on the new XPS user facility at the University of Twente.Throughout the talk, I will refer to examples from LaNiO3 thin films, which are atomically flat both before and after application as electrocatalysts for the OER during water electrolysis. We selectively tuned the surface cationic composition in epitaxial growth. The Ni-termination is approximately twice as active for the OER as the La-termination.2 Our ex situ and operando characterization confirmed that the surface transformation pathways – and therefore the electrochemical functionality – depend on a single atomic layer at the surface. A second example will be the introduction of multi-cation compositions in so-called high entropy perovskite oxides (HEO), which can maximize the catalytic activity. The HEO LaCr0.2Mn0.2Fe0.2Co0.2Ni0.2O3-δ outperforms all of its parent compounds (single TM-site element in the LaTMO3 perovskite) by orders of magnitude.5 X-ray photoemission studies reveal a synergistic effect of simultaneous oxidation and reduction of different transition metal cations during adsorption of reaction intermediates. Rao, R. R., van den Bosch, I. C. G. & Baeumer, C. Operando X-ray characterization of interfacial charge transfer and structural rearrangements. in Reference Module in Chemistry, Molecular Sciences and Chemical Engineering 1–24 (Elsevier, 2023). doi:10.1016/B978-0-323-85669-0.00068-4.Baeumer, C. et al. Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis. Nat Mater 20, 674–682 (2021).Martins, H. P. et al. Near total reflection x-ray photoelectron spectroscopy: quantifying chemistry at solid/liquid and solid/solid interfaces. J Phys D Appl Phys 54, 464002 (2021).Baeumer, C. Operando characterization of interfacial charge transfer processes. J Appl Phys 129, 170901 (2021).Kante, M. V et al. A High-Entropy Oxide as High-Activity Electrocatalyst for Water Oxidation. ACS Nano (2023) doi:10.1021/acsnano.2c08096. Figure 1

Platinum Catalysts via Fluidized Bed Atomic Layer Deposition for PEM Fuel Cells

One of the major challenges for viable and durable proton exchange membrane fuel cells (PEMFC) in heavy duty applications is the development of stable and high-performing catalysts. The highest mass activities and stabilities can be achieved with platinum nanoparticles supported on a porous carbon support. However, these systems still suffer from degradation processes during cell operation, such as carbon corrosion and Ostwald ripening. [1] Atomic layer deposition (ALD) is an attractive alternative to the conventional wet chemical deposition of the platinum catalyst onto the carbon, promising to counteract these problems. [2,3]ALD is a surface coating method, which employs self-limiting half-reactions between gaseous precursor molecules and a solid substrate surface. Usually, two half-reactions separated by purging or vacuuming steps represent one ALD cycle. By performing a defined number of cyclic repetitions, thin films or nanoparticles can be grown on the substrate surface with atomic-layer precision. [4]Typically, ALD is used to coat flat surfaces. To achieve homogeneous coatings on powders, so-called fluidized bed ALD reactors fluidize the nanoparticle-powders in a gas flow, which also contains the gaseous precursor materials. This method ensures a high utilization of the precursor molecules while providing a good mixing of the sample. Thin films and nanoparticles can be deposited in a homogeneous manner onto the powder surface and into its pores. [4]Carbon corrosion of PEMFCs can be suppressed by applying conformal ALD coatings onto the support structure. Additionally, a porous overcoating can fixate the nanoparticles against detachment and dissolution while still leaving access to the reactive sites. [5] Applying ALD to deposit fuel cell catalysts with monodisperse and highly dispersed platinum nanoparticles can counteract Ostwald ripening. [2,6]In this study we show platinum particles deposited via ALD, which are highly dispersed and show a monodisperse size distribution. We further show TiO2-overcoatings to protect the carbon.Literature:[1] J. C. Meier, Beilstein J. Nanotechnol., 2014, 5, 55-67. [2] W-J. Lee, Adv. Mater. Interfaces, 2019, 6 1901210. [3] B. J. O’Neill, ACS Catal., 2015, 5, 1804. [4] W. W. McNeary, Appl. Catal. B, 2019, 254, 587-593. [5] Q. Liu, Chem. Soc. Rev., 2022, 51, 188-236. [6] Z. Xu et al., Appl. Catal. B, 2012, 111-112, 264.

Laser‐Assisted Thermomechanical Thinning of MoTe2 in Nanoscale Lateral Resolution

AbstractA key feature of 2D transition metal dichalcogenides (2D‐TMDCs) is that their properties are strongly dependent on their thickness, typically appearing in ultrathin mono‐ or few‐ layers. Thus, precise control of functional nanostructure is critical for fundamental research and applications in 2D‐TMDCs. Here, atomic layer precision thinning of molybdenum ditelluride (MoTe2) at nanoscale lateral resolution by introducing laser irradiated hot tip is demonstrated. The contact of the hot tip on MoTe2 surface promotes oxidation at nanoscale resolution which is simultaneously removed by thermomechanical scribing. This process completes the atomic layer precision thinning of MoTe2 while maintaining the high crystallinity of thinned MoTe2 flake. Further, the electrical properties of the MoTe2 flake are intact after thinning, which proves that the thinned MoTe2 flake obtained by these methods can potentially be utilized for device fabrication. It is believed that the work will enable applications of 2D‐TMDCs that require nanoscale resolution with controlled thickness.

Isostructural metal-insulator transition driven by dimensional-crossover in SrIrO3 heterostructures

Dimensionality reduction induced metal-insulator transitions in oxide heterostructures are usually coupled with structural and magnetic phase transitions, which complicate the interpretation of the underlying physics. Therefore, achieving isostructural MIT is of great importance for fundamental physics and even more for applications. Here, we report an isostructural metal-insulator transition driven by dimensional-crossover in spin-orbital coupled ${\mathrm{SrIrO}}_{3}$ films. By using in situ pulsed laser deposition and angle-resolved photoemission spectroscopy, we synthesized and investigated the electronic structure of ${\mathrm{SrIrO}}_{3}$ ultrathin films with atomic-layer precision. Through inserting orthorhombic ${\mathrm{CaTiO}}_{3}$ buffer layers, combined low-energy electron diffraction measurements and first-principles density functional theory calculations show that the crystal structure of ${\mathrm{SrIrO}}_{3}$ films remains bulklike with similar oxygen octahedra rotation and tilting when approaching the ultrathin limit. We observe that a dimensional-crossover metal-insulator transition occurs in isostructural ${\mathrm{SrIrO}}_{3}$ films. Intriguingly, we find the bandwidth of ${J}_{\mathrm{eff}}=3/2$ states reduces with lowering the dimensionality and drives the metal-insulator transition. Our results establish a bandwidth controlled metal-insulator transition in the isostructural ${\mathrm{SrIrO}}_{3}$ thin films.

Modeling the initial monolayer formation in thermally localized surface deposition

In atomic layer deposition (ALD), thin layers of materials are deposited on a substrate with atomic layer precision in the vertical direction. The ability to control layer growth in the lateral direction as well is expected to greatly increase the potential of ALD as a path to the bottom-up additive fabrication of electronic devices like solar panels and organic light-emitting diode displays. We explore the possibility of controlling the lateral growth by modifying the temperature profile on the substrate using, for instance, pulsed lasers. This maskless technique keeps the majority of the substrate at a low temperature suppressing one of the chemical half-reactions, while in a small, localized area, the substrate is heated, which allows the reaction to proceed at a higher rate. We test this idea with course-grained computational models that model the control of the temperature by various illumination protocols and simulate the nucleation and growth of the initial monolayer within this inhomogeneous temperature distribution. Our results suggest that the location and the extent of deposition can, in principle, be localized and controlled and address operational regimes in which a thin conducting line may be obtained.

Enhanced Photoluminescence of Multiple Two-Dimensional van der Waals Heterostructures Fabricated by Layer-by-Layer Oxidation of MoS2.

Monolayer transition metal dichalcogenides (TMDs) are promising for optoelectronics because of their high optical quantum yield and strong light-matter interaction. In particular, the van der Waals (vdW) heterostructures consisting of monolayer TMDs sandwiched by large gap hexagonal boron nitride have shown great potential for novel optoelectronic devices. However, a complicated stacking process limits scalability and practical applications. Furthermore, even though lots of efforts, such as fabrication of vdW heterointerfaces, modification of the surface, and structural phase transition, have been devoted to preserve or modulate the properties of TMDs, high environmental sensitivity and damage-prone characteristics of TMDs make it difficult to achieve a controllable technique for surface/interface engineering. Here, we demonstrate a novel way to fabricate multiple two-dimensional (2D) vdW heterostructures consisting of alternately stacked MoS2 and MoOx with enhanced photoluminescence (PL). We directly oxidized multilayer MoS2 to a MoOx/1 L-MoS2 heterostructure with atomic layer precision through a customized oxygen plasma system. The monolayer MoS2 covered by MoOx showed an enhanced PL intensity 3.2 and 6.5 times higher in average than the as-exfoliated 1 L- and 2 L-MoS2 because of preserved crystallinity and compensated dedoping by MoOx. By using layer-by-layer oxidation and transfer processes, we fabricated the heterostructures of MoOx/MoS2/MoOx/MoS2, where the MoS2 monolayers are separated by MoOx. The heterostructures showed the multiplied PL intensity as the number of embedded MoS2 layers increases because of suppression of the nonradiative trion formation and interlayer decoupling between stacked MoS2 layers. Our work shows a novel way toward the fabrication of 2D material-based multiple vdW heterostructures and our layer-by-layer oxidation process is beneficial for the fabrication of high performance 2D optoelectronic devices.

Mechanisms of Thermal Atomic Layer Etching.

ConspectusAtomic layer control of semiconductor processing is needed as critical dimensions are progressively reduced below the 10 nm scale. Atomic layer deposition (ALD) methods are meeting this challenge and produce conformal thin film growth on high aspect ratio features. Atomic layer etching (ALE) techniques are also required that can remove material with atomic layer precision. ALE processes are defined using sequential, self-limiting reactions based on surface modification and volatile release. Plasma ALE methods employ energetic ion or neutral species to release the modified material anisotropically using sputtering. In contrast, thermal ALE processes utilize gas species to release the modified material isotropically using thermal reactions. Thermal ALE can be viewed as the "reverse of ALD".There are a number of mechanisms for thermal ALE that have developed over the last five years. This Account will first examine the fluorination and ligand-exchange mechanism for thermal ALE. This mechanism is applicable for many metal oxide and metal nitride materials. Subsequently, the "conversion etch" mechanisms will be explored that are derived from the conversion of the surface of the substrate to a new material. The "conversion etch" mechanisms are needed when the initial material does not have a viable etching pathway via fluorination and ligand-exchange or when the material has a volatile fluoride. The thermal ALE mechanisms founded on either oxidation or halogenation of the initial substrate will then be examined with an emphasis on metal thermal ALE. Lastly, thermal ALE mechanisms will be considered that are based on self-limiting surface ligands or temperature modulation mechanisms. These various mechanisms offer a wide range of pathways to remove material isotropically with atomic layer control.Thermal ALE will be required to fabricate advanced semiconductor devices. This fabrication will increasingly occur beyond the limits of lithography and will extend into the third dimension. The situation is like Manhattan during the advent of skyscrapers. When there was no more room on the ground, building started to move to the third dimension. Three-dimensional devices require a sequential series of deposition and etching steps to build the skyscraper structures. Some etching needs to be vertical and anisotropic to make the elevator shafts. Other etching needs to be horizontal and isotropic to form the hallways. The mechanisms of thermal ALE will be critical for the definition of isotropic ALE processes.Reaching beyond the limits of lithography will also increase the need for maskless processing. The mechanisms of thermal ALE lead to strategies for selective etching of one material in the presence of many materials. In addition, area-selective deposition can benefit from the ability of thermal ALE to enhance deposition on the desired growth surfaces by removing deposition from other surrounding surfaces. Looking ahead, thermal ALE will continue to provide unique capabilities and will grow in importance as a nanofabrication processing technique.

Ferromagnetic MnSn Monolayer Epitaxially Grown on Silicon SubstrateThis work was supported by the National Natural Science Foundation of China (Grant Nos. 11774149, 11790311, 11574133, and 11834006) and the National Key R&D Program of China (Grant Nos. 2016YFA0300404, 2015CB921202, and 2014CB921103).

Two-dimensional (2D) ferromagnetic materials have been exhibiting promising potential in applications, such as spintronics devices. To grow epitaxial magnetic films on silicon substrate, in the single-layer limit, is practically important but challenging. In this study, we realized the epitaxial growth of MnSn monolayer on Si(111) substrate, with an atomically thin Sn/Si(111)--buffer layer, and controlled the MnSn thickness with atomic-layer precision. We discovered the ferromagnetism in MnSn monolayer with the Curie temperature (Tc) of ∼ 54 K. As the MnSn film is grown to 4 monolayers, Tc increases accordingly to ∼ 235 K. The lattice of the epitaxial MnSn monolayer as well as the Sn/Si(111)- is perfectly compatible with silicon, and thus an sharp interface is formed between MnSn, Sn and Si. This system provides a new platform for exploring the 2D ferromagnetism, integrating magnetic monolayers into silicon-based technology, and engineering the spintronics heterostructures.

On the mechanism of dislocation-dominated chip formation in cutting-based single atomic layer removal of monocrystalline copper

For mechanical cutting, when cutting depth is decreased to the atomic scale, minimum chip thickness could be down to a single atomic layer. The material removal is conducted in a new dislocation-dominated chip formation mechanism. It is different from the extrusion-dominated nano-cutting and concentrated shearing-dominated chip formation in micromachining. Serials of molecular dynamics simulations indicate that there are three deformation zones generated within the contact zone between the cutting tool and workpiece materials, including a dislocation slip zone, a chip formation zone and an elastic deformation zone. Under their cooperation, the final material removal could be achieved with a single atomic layer precision. The processed surfaces with a single atomic layer precision can be achieved through this new chip formation mechanism.

Tunable Current Transport in PdSe2 via Layer‐by‐Layer Thickness Modulation by Mild Plasma

AbstractThe thickness‐modulated phase transition from semi‐metallic (bulk) to semiconductor (a few layers) is the most unique property of pentagonal palladium diselenide (PdSe2). Thus, precise thickness tailoring is essential to fully utilize its unique thickness‐dependent property for exotic device applications. Here, tunable current transport in PdSe2 based field‐effect transistors (FETs) enabled by layer‐by‐layer thinning of PdSe2 using mild SF6:N2 plasma is presented. With this top‐down plasma‐etching method, the PdSe2 layer thickness can be precisely modulated without structural degradation, which paves the way to realize the complete potential of PdSe2‐based devices. By modifying the plasma power and exposure time, an atomic layer precision etching rate of 0.4 nm min−1 can be achieved. Atomic‐force microscopy, Raman spectroscopy, and secondary ion mass spectrometry confirm the uniform and complete removal of top layers of PdSe2 flake over a large area without affecting remaining bottom layers. Electrical characterization of current transport in plasma‐thinned PdSe2 FETs reveals excellent layer‐dependent conductivity similar to pristine PdSe2 FETs. This simple but highly scalable and controllable plasma‐etching technique provides a promising way to fabricate PdSe2 devices based on lateral heterostructures composed of different thicknesses PdSe2 flakes to exploit strongly thickness‐dependent electronic structures.

A Scalable Method for Thickness and Lateral Engineering of 2D Materials.

The physical properties of two-dimensional (2D) materials depend strongly on the number of layers. Hence, methods for controlling their thickness with atomic layer precision are highly desirable, yet still too rare, and demonstrated for only a limited number of 2D materials. Here, we present a simple and scalable method for the continuous layer-by-layer thinning that works for a large class of 2D materials, notably layered germanium pnictides and chalcogenides. It is based on a simple oxidation/etching process, which selectively occurs on the topmost layers. Through a combination of atomic force microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction experiments we demonstrate the thinning method on germanium arsenide (GeAs), germanium sulfide (GeS), and germanium disulfide (GeS2). We use first-principles simulation to provide insights into the oxidation mechanism. Our strategy, which could be applied to other classes of 2D materials upon proper choice of the oxidation/etching reagent, supports 2D material-based device applications, e.g., in electronics or optoelectronics, where a precise control over the number of layers (hence over the material's physical properties) is needed. Finally, we also show that when used in combination with lithography, our method can be used to make precise patterns in the 2D materials.

Atomically-smooth single-crystalline VO2 (101) thin films with sharp metal-insulator transition

Atomically-abrupt interfaces in transition metal oxide (TMO) heterostructures could host a variety of exotic condensed matter phases that may not be found in the bulk materials at equilibrium. A critical step in the development of such atomically-sharp interfaces is the deposition of atomically-smooth TMO thin films. Optimized deposition conditions exist for the growth of perovskite oxides. However, the deposition of rutile oxides, such as VO2, with atomic-layer precision has been challenging. In this work, we used pulsed laser deposition to grow atomically-smooth VO2 thin films on rutile TiO2 (101) substrates. We show that an optimal substrate preparation procedure followed by the deposition of VO2 films at a temperature conducive for step-flow growth mode is essential for achieving atomically-smooth VO2 films. The films deposited at optimal substrate temperatures show a step and terrace structure of the underlying TiO2 substrate. At lower deposition temperatures, there is a transition to a mixed growth mode comprised of island growth and layer-by-layer growth modes. VO2 films deposited at optimal substrate temperatures undergo a sharp metal to insulator transition, similar to that observed in bulk VO2, but at a transition temperature of ∼325K with ∼103 times increase in resistance.

Site-Selective Atomic Layer Precision Thinning of MoS2 via Laser-Assisted Anisotropic Chemical Etching.

Various exotic optoelectronic properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs) strongly depend on their number of layers, and typically manifest in ultrathin few-layer or monolayer formats. Thus, precise manipulation of thickness and shape is essential to fully access their potential in optoelectronic applications. Here, we demonstrate site-selective atomic layer precision thinning of exfoliated MoS2 flake by laser. The oxidation mediated anisotropic chemical etching initiated from edge defects and progressed by controlled scanning of the laser beam. Thereby, the topmost layer can be preferentially removed in designed patterns without damaging the bottom flake. In addition, we could monitor the deceleration of the thinning by in situ reflectance measurement. The apparent slow down of the thinning rate is attributed to the sharp reduction in the temperature of the flake due to thickness dependent optical properties. Fabrication of monolayer stripes by laser thinning suggests potential applications in nonlinear optical gratings. The proposed thinning method would offer a unique and rather straightforward way to obtain arbitrary shape and thickness of a TMDCs flake for various optoelectronic applications.

Phase transitions in few-monolayer spin ice films

Vertex models are an important class of statistical mechanical system that admit exact solutions and exotic physics. Applications include water ice, ferro- and antiferro-electrics, spin ice and artificial spin ice. Here we show that it is possible to engineer spin ice films with atomic-layer precision down to the monolayer limit. Specific heat measurements show that these films, which have a fundamentally different symmetry to bulk spin ice, realise systems close to the two-dimensional F-model, with exotic phase transitions on topologically-constrained configurational manifolds. Our results show how spin ice thin films can release the celebrated Pauling entropy of spin ice without an anomaly in the specific heat. They also significantly expand the class of vertex models available to experiment.

A Peeling Approach for Integrated Manufacturing of Large Monolayer h-BN Crystals.

Hexagonal boron nitride (h-BN) is the only known material aside from graphite with a structure composed of simple, stable, noncorrugated atomically thin layers. While historically used as a lubricant in powder form, h-BN layers have become particularly attractive as an ultimately thin insulator, barrier, or encapsulant. Practically all emerging electronic and photonic device concepts currently rely on h-BN exfoliated from small bulk crystallites, which limits device dimensions and process scalability. We here focus on a systematic understanding of Pt-catalyzed h-BN crystal formation, in order to address this integration challenge for monolayer h-BN via an integrated chemical vapor deposition (CVD) process that enables h-BN crystal domain sizes exceeding 0.5 mm and a merged, continuous layer in a growth time of less than 45 min. The process makes use of commercial, reusable Pt foils and allows a delamination process for easy and clean h-BN layer transfer. We demonstrate sequential pick-up for the assembly of graphene/h-BN heterostructures with atomic layer precision, while minimizing interfacial contamination. The approach can be readily combined with other layered materials and enables the integration of CVD h-BN into high-quality, reliable 2D material device layer stacks.

Precise control of ion and radical production using electron beam generated plasmas

Precise control over both the relative and absolute flux of plasma species as well as their energy deposition at surfaces is critical to enabling the plasma processing of materials with atomic layer precision. In addition to the need for low damage, the complex device structures proposed for next generation nanoelectronics will also require control over radical to ion ratio. Electron beam generated plasmas are capable of generating a wide range of charged particle densities (109–1012 cm−3), while maintaining low electron temperatures (0.1–1.0 eV) and in reactive gas backgrounds, a relatively low radical production rate compared to discharges. These characteristics provide the ability to precisely control both the ion energy at adjacent surfaces and the ion to radical flux ratio. The latter capability is realized by coupling an electron beam generated plasma with an auxiliary plasma source designed to promote radical production. In this work, optical emission spectroscopy and Langmuir probe measurements are combined to measure the positive ion and fluorine atom densities in electron beam generated Ar/SF6 plasmas coupled to a remote, inductively coupled plasma source operating in the same environment. The results indicate that the approach can be used to independently control the positive ion and F radical densities such that the ion to radical density ratio can be varied over a range of 2–3 orders of magnitude.

Resolving interfacial charge transfer in titanate superlattices using resonant x-ray reflectometry

Charge transfer in oxide heterostructures can be tuned to promote emergent interfacial states, and accordingly, has been the subject of intense study in recent years. However, accessing the physics at these interfaces, which are often buried deep below the sample surface, remains difficult. Addressing this challenge requires techniques capable of measuring the local electronic structure with high-resolution depth dependence. Here, we have used linearly-polarized resonant X-ray reflectometry (RXR) as a means to visualize charge transfer in oxide superlattices with atomic layer precision. From our RXR measurements, we extract valence depth profiles of SmTiO$_3$ (SmTO)/SrTiO$_3$ (STO) heterostructures with STO quantum wells varying in thickness from 5 SrO planes down to a single, atomically thin SrO plane. At the polar-nonpolar SmTO/STO interface, an electrostatic discontinuity leads to approximately half an electron per areal unit cell transferred from the interfacial SmO layer into the neighboring STO quantum well. We observe this charge transfer as a suppression of the t$_{2g}$ absorption peaks that minimizes contrast with the neighboring SmTO layers at those energies and leads to a pronounced absence of superlattice peaks in the reflectivity data. Our results demonstrate the sensitivity of RXR to electronic reconstruction at the atomic scale, and establish RXR as a powerful means of characterizing charge transfer at buried oxide interfaces.

Nanomanufacturing of silicon surface with a single atomic layer precision via mechanochemical reactions

Topographic nanomanufacturing with a depth precision down to atomic dimension is of importance for advancement of nanoelectronics with new functionalities. Here we demonstrate a mask-less and chemical-free nanolithography process for regio-specific removal of atomic layers on a single crystalline silicon surface via shear-induced mechanochemical reactions. Since chemical reactions involve only the topmost atomic layer exposed at the interface, the removal of a single atomic layer is possible and the crystalline lattice beneath the processed area remains intact without subsurface structural damages. Molecular dynamics simulations depict the atom-by-atom removal process, where the first atomic layer is removed preferentially through the formation and dissociation of interfacial bridge bonds. Based on the parametric thresholds needed for single atomic layer removal, the critical energy barrier for water-assisted mechanochemical dissociation of Si–Si bonds was determined. The mechanochemical nanolithography method demonstrated here could be extended to nanofabrication of other crystalline materials.