Single-crystal Islands Research Articles

Advancements in single-crystal silicon with elevated-laser-liquid-phase-epitaxy (ELLPE) for monolithic 3D ICs

In this study, we present a low thermal budget elevated-laser-liquid-phase-epitaxy technique designed for the precise fabrication of single-crystal islands (SCIs) intended for use in middle-end-of-line (MEOL) FinFETs. Each of these SCIs features a (100) orientation tended from Si seeding structure and is successfully integrated as channel materials in the MEOL circuit of a monolithic 3D IC (3DIC). This technique effectively mitigates the typical performance disparities associated with poly-Si channel materials in upper tiers, addressing a significant challenge in advanced electronic device fabrication and potentially enhancing the performance and reliability of MEOL FinFETs in monolithic 3DIC.

Single-Crystal Islands (SCI) for Monolithic 3-D and Back-End-of-Line FinFET Circuits

A single-crystal islands (SCI) technique using low thermal budget pulse laser process is proposed and demonstrated to fabricate single-crystal silicon islands over amorphous dielectric for monolithic 3-D and back-end-of-line (BEOL) FinFET circuits. By laser recrystallizing mask-defined a-Si islands encapsulated with conformal silicon nitride film, designed single-crystal Si islands can be obtained. The single crystallinity of the island are verified with SECCO Etch, high-resolution electron microscopy (HREM), transmission electron microscopy (TEM), and electron backside scattering (EBSD). About 40 nm FinFETs were successfully fabricated in the SCI Si islands and shown to exhibit excellent electrical performance and low variability that are compatible with the FinFETs fabricated on commercial silicon-on-insulator (SOI) wafer.

Metal Microsubstrates on Si(111): Crystallography, Morphology, and Interactions with a Molecular Adsorbate

We describe the growth and characterization of 10 nm to 10 micron-scale islands created by depositing Ag onto Si(111) at elevated temperatures. By varying the surface coverage of Ag, we are able to produce and control the distribution of Ag(111) and/or Ag(100) single-crystal islands. These were examined using scanning tunneling microscopy (STM), X-ray diffraction (XRD), helium ion microscopy (HIM), and atomic force microscopy. The formation of 3D islands with orientations of either (111) or (100) was confirmed by STM and XRD analysis. As these islands manifest as tiny single crystals of elemental Ag, they can be ideally used as low-cost or disposable substrates to host molecular architectures; we demonstrate this by comparing absorption and thermal behavior of 1,3,5-benzenetricarboxylic acid molecules on the islands and noting identical behavior and reactivity to experiments performed on Ag single-crystal counterparts. These islands provide an economical and practical method for facilitating molecular adsorption on two distinct microsubstrates at the same time while under identical experimental conditions.

Large area chemical vapour deposition grown transition metal dichalcogenide monolayers automatically characterized through photoluminescence imaging

Chemical vapour deposition (CVD) growth is capable of producing multiple single-crystal islands of atomically thin transition metal dichalcogenides (TMDs) over large areas. Subsequent merging of perfectly epitaxial domains can lead to single-crystal monolayer sheets, a step towards scalable production of high quality TMDs. For CVD growth to be effectively harnessed for such production it is necessary to be able to rapidly assess the quality of material across entire large area substrates. To date, characterisation has been limited to sub-0.1-mm2 areas, where the properties measured are not necessarily representative of an entire sample. Here, we apply photoluminescence (PL) imaging and computer vision techniques to create an automated analysis for large area samples of monolayer TMDs, measuring the properties of island size, density of islands, relative PL intensity and homogeneity, and orientation of triangular domains. The analysis is applied to ×20 magnification optical microscopy images that completely map samples of WSe2 on hBN, 5.0 mm × 5.0 mm in size, and MoSe2–WS2 on SiO2/Si, 11.2 mm × 5.8 mm in size. Two prevailing orientations of epitaxial growth were observed in WSe2 grown on hBN and four predominant orientations were observed in MoSe2, initially grown on c-plane sapphire. The proposed analysis will greatly reduce the time needed to study freshly synthesised material over large area substrates and provide feedback to optimise growth conditions, advancing techniques to produce high quality TMD monolayer sheets for commercial applications.

A method to detect triplet exciton transfer from singlet fission materials into silicon solar cells: Comparing different surface treatments.

Singlet fission is one of the most promising routes to overcome the single-junction efficiency limit for solar cells. Singlet fission-enhanced silicon solar cells are the most desirable implementation, but transfer of triplet excitons, the product of singlet fission, into silicon solar cells has proved to be very challenging. Here, we report on an all optical measurement technique for the detection of triplet exciton quenching at semiconductor interfaces, a necessary requirement for triplet exciton or charge transfer. The method relies on the growth of individual, single-crystal islands of the singlet fission material on the silicon surface. The islands have different heights, and we correlate these heights to the quenching efficiency of triplet excitons. The quenching efficiency is measured by spatially resolved delayed fluorescence and compared to a diffusion-quenching model. Using silicon capped with a blocking thermal oxide and aromatic monolayers, we demonstrate that this technique can quickly screen different silicon surface treatments for triplet exciton quenching.

Formation of Twinned Graphene Polycrystals.

Liquid metals have been widely used as substrates to grow graphene and other 2D materials. On a homogeneous and isotropic liquid surface, a polycrystalline 2D material is formed by coalescence of many randomly nucleated single-crystal islands, and as a result, the domains in a polycrystal are expected to be randomly aligned. Here, we report the unexpected finding that only 30°-twinned graphene polycrystals are grown on a liquid Cu surface. Atomic simulations confirm that the unique domain alignment in graphene polycrystals is due to the free rotation of graphene islands on the liquid Cu surface and the highly stable 30°-grain boundaries in graphene. In-depth analysis predicts 30 types of possible 30°-twinned graphene polycrystals and 27 of them are observed. The revealed formation mechanism of graphene polycrystals on a liquid Cu surface deepens our fundamental understanding on polycrystal growth and could serve as a guideline for the controlled synthesis of 2D materials.

Structure of bismuth films obtained using an array of identically oriented single-crystal bismuth islands preliminarily grown on a substrate

An array of identically oriented single-crystal bismuth islands formed on mica plates is used as a substrate to improve the structure of bismuth thin films grown by vacuum thermal evaporation. The array of islands is formed by the chemical etching of a single-crystal bismuth film 1 μm thick grown by floating-zone recrystallization under a coating. The structure of the obtained films is studied by X-ray diffraction, atomicforce microscopy, and electron backscatter diffraction scanning electron microscopy.

On the formation of arrays of single-crystal silicon islands with small angular dispersion

We investigate the possibility of the generation of arrays of single-crystal islands with small angular dispersion in order to use them as active media for solving the problems of controlling the parameters of X-ray beams. Using the example of a three-layer structure of substrate–adhesive–single-crystal wafer, we experimentally demonstrate the possibility of controlling the profile of diffractive elements by changing its temperature. It is revealed that the main reason for island misorientation is shrinkage of the adhesive material upon its solidification. Another reason for misorientation is softening the adhesive, which occurs upon plasma-chemical etching of the wafer. These effects can be eliminated by changing the temperature of attaching the wafer to the substrate surface and by using a nonsoftening adhesive. The temperature of attaching a single-crystal wafer to the substrate is calculated from the shrinkage coefficients of the adhesive and the thermal- expansion coefficients of the components of the structure.

Layered transition metal dichalcogenides: promising near-lattice-matched substrates for GaN growth.

Most III-nitride semiconductors are grown on non-lattice-matched substrates like sapphire or silicon due to the extreme difficulty of obtaining a native GaN substrate. We show that several layered transition-metal dichalcogenides are closely lattice-matched to GaN and report the growth of GaN on a range of such layered materials. We report detailed studies of the growth of GaN on mechanically-exfoliated flakes WS2 and MoS2 by metalorganic vapour phase epitaxy. Structural and optical characterization show that strain-free, single-crystal islands of GaN are obtained on the underlying chalcogenide flakes. We obtain strong near-band-edge emission from these layers, and analyse their temperature-dependent photoluminescence properties. We also report a proof-of-concept demonstration of large-area growth of GaN on CVD MoS2. Our results show that the transition-metal dichalcogenides can serve as novel near-lattice-matched substrates for nitride growth.

Reversed crystal growth of rhombohedral calcite in the presence of chitosan and gum arabic

Calcite crystals with a rhombohedral morphology were synthesized using a biomimetic method in the presence of organic structure-directing agents (chitosan and gum arabic). Analysis of the microstructures of these crystals by scanning electron microscopy, high resolution transmission electron microscopy and powder X-ray diffraction revealed a non-classical growth mechanism. Initially, the biomaterials and inorganic precursor molecules aggregated into large particles. Multiple nucleation of CaCO3 took place either on the surface of the aggregates (in the chitosan system) or inside the aggregates (in the gum arabic system). These nanocrystallites aggregated again to form some large polycrystalline rhombohedral or spherical particles. Surface recrystallization then occurred, forming small single-crystal islands, which joined together as the reaction time increased leading to a core–shell structure where a polycrystalline core was encased in a thin single-crystal shell. The crystallization then extended from the surface to the core, ultimately resulting in true single crystals.

In situ X-ray diffraction monitoring during metalorganic vapor phase epitaxy growth of low-temperature-GaN buffer layer

We investigated in situ X-ray diffraction (XRD) monitoring during the growth of low-temperature (LT)-GaN buffer layers on the (0001) c-plane sapphire substrates. The in situ XRD monitoring made it possible to observe the crystalline structures during their growth. We investigated the temperature dependence of LT-GaN buffer layers by in situ XRD monitoring during the thermal annealing of the LT-GaN layers. We clearly observed the evolution process, in which an LT-GaN buffer layer grown at 535°C was crystallized into a hexagonal structure by thermal annealing at temperatures of up to 1090°C. We also found that the LT-GaN buffer layer was transformed into nano size hexagonal single-crystal islands upon annealing by atomic force microscopy. The crystalline quality of the subsequent GaN layer strongly depended on the growth temperature of the LT-GaN buffer layer.

Mechanism of Twin Formation in Excimer-laser-induced Lateral Solidification of Si Films

ABSTRACTIn this paper, we present experimental findings pertaining to the formation of twins in isolated single-crystal islands obtained via the dot-SLS method. Systematic characterization of the islands using EBSD reveals that Σ = 3 CSL twins constitute the predominant extended defect. Given this, the surface orientation of the seed and twinned regions are shown to be related by a 180° rotation about a <111> axis, and the orientation of the twinned area is apparently manifested in the inverse pole figure as a reflection across the {112} zone. By considering (1) the overall pattern of the twin boundaries within the islands, (2) that solidification proceeds via a facetted mode of growth and, (3) that intragrain-defect-free regions were obtained when the dot-SLS process was performed on a {100} surface-oriented seed, we suggest that these twins are most likely generated heterogeneously at the Si/SiO2interface during the ledge nucleation stage of rapid lateral solidification of the films.

Nanostructured MnGa films on Si/SiO2 with 20.5 kOe room temperature coercivity

Nanostructured Mn67Ga33 films exhibiting high room temperature coercivity (HC = 20.5 kOe) have been prepared by sputtering onto thermally oxidized Si substrates. Both the morphology and the coercivity of the films can be tuned by varying the growth parameters. The low deposition rate film, sputtered at a reduced power and working pressure, demonstrates a discontinuous island-like growth and the highest HC. The large HC is linked to the presence of the high anisotropy DO22 Mn3Ga phase and the single domain character of the exchange isolated, dipolar interacting, single crystal islands.

Pulsed laser techniques for nanographoepitaxy

Graphoepitaxy is a technique that has been demonstrated as a means for fashioning regions of single crystal semiconductors on amorphous substrates. In that earlier work the heating period was long (1s or more) and the substrate needed to be held close to the melting point of the semiconductor. With a view to achieving three-dimensional integrated circuits, the authors have investigated the possibility using graphoepitaxy to yield device quality single crystal islands of silicon on an amorphous substrate without excessive heating of the underlying layers by using transient (<1ms) heating. A stationary laser pulse of 532nm wavelength, 100μs duration, and 22μm diameter impinged on a 50nm thick amorphous silicon film deposited on 12nm thick SiO2 inverted pyramid template. We demonstrated successful graphoepitaxial orientation of crystallized silicon but only under conditions that gave excessive heating (>400°C) of lower layers, thus indicating that this technique is impractical for three-dimensional integrated circuits.

Fabrication of Complex Architectures Using Electrodeposition into Patterned Self-Assembled Monolayers

Patterned self-assembled monolayers (SAMs) on solid surfaces can be used to direct electrodeposition to occur at specific locations. In previous work, single deposition steps have been used to create islands or lines in patterned SAMs. In this paper we present strategies for creating more complex structures, including via-like structures and multicomponent lines. We also show how finite size effects can be exploited to produce arrays of single-crystal islands.

Effects of vicinal steps on the island growth and orientation of epitaxially grown perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) thin film crystals on a hydrogen-terminated Si(1 1 1) substrate

On flat and off-cut Si(1 1 1) substrates terminated with monohydride, thin film crystals of organic semiconductor perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) were grown by molecular beam epitaxy (MBE) and were observed by atomic force microscopy (AFM). The thin film crystals on the hydrogen-terminated Si(1 1 1) surface (H–Si(1 1 1)) have a mesa-type island shape. On the flat H–Si(1 1 1) (average step distance >500 nm, off-angle <0.04°), with an increase in a thin film thickness, the size and number of the PTCDA islands were increased and decreased, respectively. These results show that the islands coalesced during the growth. After the coalescence of the islands, some islands became single crystals and the others became polycrystals. The ratio of the single crystal islands was estimated from the shapes of the islands. We found that the ratio of the single crystal islands after the coalescence was 48% and this value was much larger than a probability (1/6) estimated from an epitaxial relation. This result suggests that Ostwald ripening occurred in the PTCDA/H–Si(1 1 1) system. The PTCDA thin film crystals were also grown on the off-cut H–Si(1 1 1) (average step distance=10 nm, off-angle=1.8°). The number of the islands was much larger than that on the flat H–Si(1 1 1) and this result shows that the vicinal steps on the surface can work as effective nucleation sites. Coalescence of the islands was also observed on the off-cut H–Si(1 1 1). After the coalescence, the ratio of the single crystal islands on the off-cut H–Si(1 1 1) was 10% larger than that on the flat H–Si(1 1 1). This result shows that the orientation of the PTCDA islands can be controlled using the vicinal steps on the substrate surface.

Epitaxial growth and structure of highly mismatched oxides with rock-salt structure on MgO

Thin films of oxides with rock-salt structure were grown on (0 0 1)MgO single crystal substrates to study the influence of a high lattice mismatch ε s on growth phenomena via the chemical solution deposition method using toluene solutions of different 2-ethylhexanoates and neodecanoates. The films were pyrolyzed to crystallize the desired oxide and heat treated up to 1400°C to promote epitaxial growth. X-ray texture analyses, SEM and TEM investigations of CaO on MgO ( ε s≈14.2%) showed that the films broke into single crystal islands. The preferred epitaxial orientation relationships are [ 1 ̄ 1 0] (1 1 1)CaO∥〈1 1 0〉(0 0 1)MgO (four variants for reasons of symmetry). A two-dimensional near-coincidence site lattice (NCSL) model ( Σ * CaO=7/ Σ * MgO=8) is presented for the interface. For the higher mismatched system SrO on MgO ( ε s≈22.5%), SEM investigations and X-ray ( θ−2 θ) measurements suggest that the growth orientation has changed to (0 0 1)SrO∥(0 0 1)MgO with a few discrete in-plane rotations. The measured orientations correlate well with a two-dimensional NCSL model of rotated cubic SrO crystallites. The mechanisms leading to the experimentally observed structures and morphologies are discussed with respect to the influence of lattice mismatch.

Carbon Nanotube Modification Using BaF2 Vapor in Ultra-High Vacuum Environment

ABSTRACTCarbon nanotubes have attracted significant attention in the scientific community due to their unique properties and potential applications. One of the most promising applications is a carbon-nanotube transistor. The motivation of this work is to find ways to connect carbon nanotubes directly to silicon using Ba as a chemical link. We studied chemical interactions between carbon nanotubes and BaF2 vapors using x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Surfaces of silicon wafers were chemically modified to allow the epitaxial growth of BaF2 using molecular beam epitaxy (MBE). Samples containing 2D single crystal islands of BaF2 were covered with carbon nanotubes with an average coverage of 10 nanotubes per um2. The samples were transferred to an outgasing station inside the MBE system and heated to 900°C for two hours in a pressure of 10-9 mbar. XPS C1s data before and after heat show a major change in the nature of the carbon nanotube electronic states. In addition XPS shows formation of a Ba-C “carbide like” bond and no presence of fluorine. AFM images of the same region taken before and after heat exposure show remarkable changes in the surface morphology of the carbon-nanotube wall.

A Study of Fabrication and Characterization of Ultra-Small Polycrystalline Silicon Islands for Advanced Display and Microsensor Applications

AbstractPolycrystalline silicon (polysilicon) is a key component of integrated circuit technology, micromechanical systems and displays. High-mobility polysilicon thin-film transistors are replacing amorphous-silicon transistors in advanced active-matrix displays, and are also vital to the emerging fields of microsensors and microactuators [1]. In this study, the deposition processes (LPCVD) of polysilicon thin films were carefully studied and the basic kinetics were analyzed. The films were patterned into submicron islands and the relation of the deposition temperature versus the crystallinity of polysilicon islands was observed. A “threshold” deposition temperature for polysilicon crystals larger than 0.35 μm was found. The relationships of polysilicon crystal sizes vs. annealing temperature and vs. annealing duration were also systematically studied. Polysilicon islands of ∼ 0.1 μm lateral dimensions were successfully fabricated by ebeam lithography and several types of annealings were performed to increase micro-crystal size, with the goal of creating single-crystal high-mobility islands. Theoretically, transistors built from such islands should match the performance of state-of-the-art bulk CMOS devices. The crystal size was measured using TEM and AFM analysis. Moreover, EXAFS (Extended X-ray Absorption Fine Structure) measurements at SSRL (Stanford Synchrotron Radiation Lab) were employed to study the crystallization processes of polysilicon films and the correlation between the absorption curves, the thin-film electrical properties and the polysilicon grain sizes.

Preparation and characteristics of seeded epitaxial (Sr,Ba)Nb2O6 optical waveguide thin films using sol-gel method

Highly c-axis-oriented (Sr,Ba)Nb2O6 (SBN) films were grown on a seeded MgO(100) substrate via sol-gel method. The substrate was preseeded with epitaxial islands of SBN made by breaking up a continuous film into single-crystal islands by pores. Since the number of epitaxial nuclei was increased at the interface between the film and the substrate, the film on a seeded substrate had better highly orientation than that on unseeded substrate. The film having low Sr content exhibited better epitaxial growth because of the distorted unit-cell network and the change of lattice parameters of SBN thin film. For obtaining excellent optical properties, SBN:75 film was prepared on MgO substrate with SBN:25 composition seed layer. Because of low birefringence of refractive indices in the film having high Sr content, the optical scattering loss by the anisotropy of refractive indices was suppressed.