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At least 991 records · Page 55
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Crystal Growth and Characterization of Epitaxial Layers of Laser and Nonlinear Optical Materials for Thin-Disk and Waveguide laser applications

This paper summarizes the main results we obtained in our laboratories in relation with crystalline layers obtained by liquid phase epitaxial growth of lanthanide doped KLu(WO{sub 4}){sub 2} and Nb:RbTiOPO{sub 4} grown on KLu(WO{sub 4}){sub 2} and RbTiOPO{sub 4} substrates, respectively. Macroscopic defect free epitaxial layers were grown and characterized in terms of their compositional homogeneity, structural stress in the layer/substrate interface and laser and waveguiding performances.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Dynamic Hydride Vapor Phase Epitaxy (D-HVPE) as a Route to Inexpensive, High-Performance III-V Materials and Devices

This work describes the recent development of Dynamic Hydride Vapor Phase Epitaxy (D-HVPE), which extends the standard HVPE process to allow for the deposition of relatively complex heterostructures and devices at growth rates in excess of 300 um/h. This process uses low-cost elemental metal precursors, and uses all of the reactants efficiently to create high-performance devices. D-HVPE also lends itself to potential in-line growth, which when combined with the low-cost precursors and extremely fast growth rates for high-quality III-V materials, can lead to significantly lower epitaxial growth costs. Here, we focus on the growth of high-efficiency photovoltaics as an example device to show the capabilities of the D-HVPE growth process.

14 SOLAR ENERGY↗
DOE OSTI2021

Crystal Symmetry Engineering in Epitaxial Perovskite Superlattices

Interface plays a critical role in determining the physical properties and device performance of heterostructures. Traditionally, lattice mismatch, resulting from the different lattice constants of the heterostructure, can induce epitaxial strain. Over past decades, strain engineering has been demonstrated as a useful strategy to manipulate the functionalities of the interface. However, mismatch of crystal symmetry at the interface is relatively less studied due to the difficulty of atomically structural characterization, particularly for the epitaxy of low symmetry correlated materials on the high symmetry substrates. Overlooking those phenomena restrict the understanding of the intrinsic properties of the as- determined heterostructure, resulting in some long-standing debates including the origin of magnetic and ferroelectric dead layers. Here, perovskite LaCoO 3 -SrTiO 3 superlattice (SL) is used as a model system to show that the crystal symmetry effect can be isolated by the existing interface strain. Combining the state-of-art diffraction and electron microscopy, it is found that the symmetry mismatch of LaCoO 3 -SrTiO 3 SL can be tuned by manipulating the SrTiO 3 layer thickness to artificially control the magnetic properties. Furthermore, the work suggests that crystal symmetry mismatch can also be designed and engineered to act as an effective strategy to generate functional properties of perovskite oxides.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Dominant Energy Carrier Transitions and Thermal Anisotropy in Epitaxial Iridium Thin Films

High aspect ratio metal nanostructures are commonly found in a broad range of applications such as electronic compute structures and sensing. The self-heating and elevated temperatures in these structures, however, pose a significant bottleneck to both the reliability and clock frequencies of modern electronic devices. Any notable progress in energy efficiency and speed requires fundamental and tunable thermal transport mechanisms in nanostructured metals. Here, in this work, time-domain thermoreflectance is used to expose cross-plane quasi-ballistic transport in epitaxially grown metallic Ir(001) interposed between Al and MgO(001). Thermal conductivities ranges from roughly 65 (96 in-plane) to 119 (122 in-plane) W m -1 K -1 for 25.5–133.0 nm films, respectively. Further, low defects afforded by epitaxial growth are suspected to allow the observation of electron–phonon coupling effects in sub-20 nm metals with traditionally electron-mediated thermal transport. Via combined electro-thermal measurements and phenomenological modeling, the transition is revealed between three modes of cross-plane heat conduction across different thicknesses and an interplay among them: electron dominant, phonon dominant, and electron–phonon energy conversion dominant. The results substantiate unexplored modes of heat transport in nanostructured metals, the insights of which can be used to develop electro-thermal solutions for a host of modern microelectronic devices and sensing structures.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Ionic‐Liquid‐Gating‐Induced Hydrogenation in Epitaxial Strontium Ferrite

Abstract Incorporating hydrogen into complex oxides holds promise for discovering exotic phenomena and novel functionalities by promoting couplings between ion and lattice/charge/spin/orbital degrees of freedom. Here, electrolyte gating‐driven hydrogenation is investigated in epitaxial brownmillerite SrFeO 2.5 thin films in which the hydrogenation‐induced lattice distortion is qualitatively different from its counterpart SrCoO 2.5 . The achievable lattice expansion in hydrogenated SrFeO 2.5 is weaker than in SrCoO 2.5 and primarily occurs along the normal of the stacked octahedral FeO 6 and tetrahedral FeO 4 layers. Upon the substitution of Fe with Co, the lattice expansion monotonically increases with increase of Co/Fe ratio, implying an intrinsic difference in accommodating hydrogen between Fe‐ and Co‐based brownmillerites. Moreover, a net magnetic moment in hydrogenated SrFeO 2.5 films is observed above room temperature, which gradually weakens with the increase of Co substitution, suggesting a stronger canted magnetism in Fe‐based hydrogenated brownmillerites. This work clarifies the electrolyte gating‐driven hydrogenation mechanisms in brownmillerite SrFeO 2.5 epitaxial thin films and those with Co substitution, particularly the deterministic role of Co/Fe ratio in the evolution of structure and properties upon hydrogenation.

Chemistry↗
DOE OSTI2019

Anomalous Defect Dependence of Thermal Conductivity in Epitaxial WO 3 Thin Films

Lattice defects typically reduce lattice thermal conductivity, which has been widely exploited in applications such as thermoelectric energy conversion. Here, an anomalous dependence of the lattice thermal conductivity on point defects is demonstrated in epitaxial WO 3 thin films. Depending on the substrate, the lattice of epitaxial WO 3 expands or contracts as protons are intercalated by electrolyte gating or oxygen vacancies are introduced by adjusting growth conditions. Surprisingly, the observed lattice volume, instead of the defect concentration, plays the dominant role in determining the thermal conductivity. In particular, the thermal conductivity increases significantly with proton intercalation, which is contrary to the expectation that point defects typically lower the lattice thermal conductivity. The thermal conductivity can be dynamically varied by a factor of ≈1.7 via electrolyte gating, and tuned over a larger range, from 7.8 to 1.1 W m -1 K -1 , by adjusting the oxygen pressure during film growth. The electrolyte-gating-induced changes in thermal conductivity and lattice dimensions are reversible through multiple cycles. These findings not only expand the basic understanding of thermal transport in complex oxides, but also provide a path to dynamically control the thermal conductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Altermagnetic Band Splitting in 10 nm Epitaxial CrSb Thin Films

Altermagnets are a newly identified family of collinear antiferromagnets with a momentum-dependent spin-split band structure of non-relativistic origin, derived from spin-group symmetry-protected crystal structures. Among candidate altermagnets, CrSb is attractive for potential applications because of a large spin-splitting near the Fermi level and a high Néel transition temperature of around 700 K. Molecular beam epitaxy is used to synthesize CrSb (0001) thin films with thicknesses ranging from 10 to 100 nm. Structural characterization, using reflection high energy electron diffraction, scanning transmission electron microscopy, and X-ray diffraction, demonstrates the growth of epitaxial films with good crystallinity. Polarized neutron reflectometry shows the absence of any net magnetization, consistent with antiferromagnetic order. In vacuo angle resolved photoemission spectroscopy (ARPES) measurements probe the band structure in a previously unexplored regime of film thickness, down to 10 nm. These ARPES measurements show a bulk-type, 3D momentum-dependent band splitting of up to 0.7 eV with g-wave symmetry, consistent with that seen in prior studies of bulk single crystals. The distinct altermagnetic band structure required for potential spin-transport applications survives down to the ∼10 nm thin film limit at room temperature.

Santhosh, Sandra↗
DOE OSTI2021

Digital Tuning of the Transition Temperature of Epitaxial VO 2 Thin Films on MgF 2 Substrates by Strain Engineering

Abstract Straining the vanadium dimers along the rutile c ‐axis can be used to tune the metal‐to‐insulator transition (MIT) of VO 2 but has thus far been limited to TiO 2 substrates. In this work VO 2 /MgF 2 epitaxial films are grown via molecular beam epitaxy (MBE) to strain engineer the transition temperature ( T MIT ). First, growth parameters are optimized by varying the synthesis temperature of the MgF 2 (001) substrate ( T S ) using a combination of X‐ray diffraction techniques, temperature dependent transport, and soft X‐ray photoelectron spectroscopy. It is determined that T S values greater than 350 °C induce Mg and F interdiffusion and ultimately the relaxation of the VO 2 layer. Using the optimized growth temperature, VO 2 /MgF 2 (101) and (110) films are then synthesized. The three film orientations display MITs with transition temperatures in the range of 15–60 °C through precise strain engineering.

Howard, Sebastian A.↗
DOE OSTI2016

Exploring Polarization Rotation Phase Instabilities in Super-Tetragonal BiFeO3 Epitaxial Thin Films and Their Technological Implications

Many functional properties of ferroelectrics are underlain by structural instabilities, which render these materials very susceptible to small alternating applied fields (electric, mechanical, etc.) through certain constitutive coupling relations, e.g., elastic compliance and piezoelectric response, and often such instabilities can be shifted by static applied fields thus meaning tunable dynamic properties. Structural instabilities are naturally accommodated on the brink of morphotropic phase boundaries (MPB s) where multiple phases of small energy difference coexist in different crystallographic forms. Canonical MPB is realized through compositional mixture, as is typically exemplified by Pb(Zr1-xTix)O3 solid solutions and relaxor ferroelectrics of (1-x)PbMg1/3Nb2/3O3-xPbTiO3. More recently, a strain-driven MPB has been discovered in BiFeO3 (BFO) thin films epitaxially grown on LaAlO3 (LAO) crystal substrates (which imposes about -4.5% in-plane strains). Such an MPB is in between a rhombohedral (R) phase that bulk BFO exhibits and a so-called super-tetragonal (T) phase, which name hints at its giant lattice axial ratio (c/a ~ 1.23) and accordingly high electric polarization (~1.5 C m-2). The discovery of an MPB in BFO has revealed another facet of this multiferroic system, further adding opportunities to its many exotic functionalities such as domain wall conduction, magnetoelectric and photovoltaic effects As with other MPB s, large electric-field induced strains as well as more underlying lattice softening effects are observed near this MPB promising piezoelectric-based applications. In addition, T-phase BFO itself shows distinct properties, e.g., electronic band gap and optical absorption, from the R-phase and the resultant switching effects between them may also be exploitable. However, unlike conventional ferroelectric oxides where the phases across an MPB usually have subtle difference caused by rotations of an ion off-centering polarization, the BFO system bears multiple structural degrees of freedom, in particular antiferrodistortive modes of oxygen octahedral tilt, and a multitude of structural transition paths are thereby facilitated. Moreover, since the MPB of BFO is driven by epitaxial strain, it is sensitive to the strain relaxation related to film thickness and growth conditions, and above some critical thickness the films appear in the form of a hierarchical mixed-phase microstructure involving several coexistent polymorphs with distinct lattice distortion (monoclinic phases) and tilts. Elastic and electrical heterogeneities are necessary consequences of such complex microstructure, which couple to the intrinsic order parameters and expectedly have profound influence on the structural dynamics and material properties. All these make it a demanding task to obtain a deep understanding of this MPB system on par with its application prospect. The thin-film material form also brings in experimental restrictions to the MPB phase transition studies of BFO since many pertinent techniques fail to operate at the nanoscale or suffer from formidably weak signals. Therefore, static structural characterizations using e.g. X-ray diffraction, electron microscopy and Raman scattering have prevailed thus far. Recently, we adapted band-excitation piezoresponse spectroscopy (BEPS) to probe the bias-induced R/T phase transition dynamics of BFO/SrTiO3 (STO) thin films, and revealed the soft-mode elastic behavior of the transition. The efficacy of our method, along with standard piezoresponse force microscopy (PFM), can be significantly leveraged by integrating a variety of local and/or global excitations, e.g., tip pressure (10 s GPa attainable), heating, photo-irradiation, available to modern scanning probe platforms. This can thus afford unique opportunities to survey the structural dynamics of ferroelectric materials coupled to those field variables, enabling rapid discovery or validation of their functional properties. In this study, we focus on the T-phase BFO/LAO system; we have examined its phase transition behavior not only due to local coaction of tip bias and loading force, but also under device-level global electric fields in a coplanar capacitor. The intrinsic elastic softening phenomena near the structural transitions are comprehensively revealed by BEPS and corroborated by phase-field modeling. Our findings may open a new pathway for technological utilization of the MPB phase instabilities of BFO.

BiFeO3↗
DOE OSTI2022

Epitaxially crystallized polyethylene exhibiting near‐equilibrium melting temperatures*

Abstract The morphology and orientation of polymer crystals are important factors which determine the performance of thin‐film, polymer‐based technologies such as organic electronic devices and gas separation membranes. Here, we utilize polymer‐substrate epitaxy to achieve a highly oriented crystalline morphology during thin‐film processing. To accomplish this, we employ matrix‐assisted pulsed laser evaporation (MAPLE), a slow physical vapor deposition process, to deposit linear polyethylene epitaxially atop a graphene substrate. Via MAPLE, we demonstrate the ability to achieve a film morphology comprised of well‐aligned, edge‐on crystalline lamellae. Furthermore, we show that MAPLE can be exploited to grow crystalline lamellae composed entirely of extended polymer chains which exhibit a near‐equilibrium melting temperature. Our study demonstrates that MAPLE, as a bottom‐up approach, can deposit polymer thin films with improved control over crystalline morphology.

Wang, Yucheng↗
DOE OSTI2021

Electrically Pumped Epitaxially Regrown GaSb-Based Type-I Quantum-Well Surface-Emitting Lasers with Buried High-Index-Contrast Photonic Crystal Layer

Epitaxially regrown electrically pumped photonic crystal surface-emitting lasers (PCSELs) emitting near 2 and 2.6 μm are designed, fabricated, and characterized. A high-index-contrast photonic crystal layer is incorporated into the GaSb-based laser heterostructure by air-hole-retaining epitaxial regrowth. A square lattice of triangular holes is etched in the top waveguide core layer of the incomplete laser heterostructure. The nanopatterned surface is subsequently cleaned and regrown with AlGaAsSb p-cladding material. Transmission electron microscopy studies demonstrate uniform regrowth over the nanopatterned GaSb surface. The selected regrowth regimes yield a buried 2D array of elongated air-holes. The diode PCSELs based on moderately etched nanopatterns demonstrate band-edge lasing near 2 μm up to room temperatures. The cascade diode PCSELs operate near 2.6 μm with minimum threshold current densities of about 500 A cm -2 achieved at 180 K. The devices generate mW level output in narrow divergence beam emitted from the window in substrate contact. The angle-resolved electroluminescence measurements reveal a four-sub-band band structure with an apparent photonic bandgap corresponding to the buried high-index-contrast square photonic crystal layer. Finally, the PCSELs made of heterostructures supporting two modes in the vertical direction demonstrate two sets of sub-bands showing anti-crossing-like interaction.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Epitaxial Synthesis of Blue Phosphorene

Phosphorene is a new 2D material composed of a single or few atomic layers of black phosphorus. Phosphorene has both an intrinsic tunable direct bandgap and high carrier mobility values, which make it suitable for a large variety of optical and electronic devices. However, the synthesis of single-layer phosphorene is a major challenge. The standard procedure to obtain phosphorene is by exfoliation. More recently, the epitaxial growth of single-layer phosphorene on Au(111) was investigated by molecular beam epitaxy and the obtained structure described as a blue phosphorene sheet. In the present study, large areas of high-quality monolayer phosphorene, with a bandgap value equal to at least 0.8 eV, are synthesized on Au(111). The experimental investigations, coupled with density functional theory calculations, give evidence of two distinct phases of blue phosphorene on Au(111), instead of one as previously reported, and their atomic structures are determined.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Quantifying Equiaxed vs Epitaxial Solidification in Laser Melting of CMSX-4 Single Crystal Superalloy

The competition between epitaxial vs. equiaxed solidification has been investigated in CMSX-4 single crystal superalloy during laser melting as practiced in additive manufacturing. Single-track laser scans were performed on a powder-free surface of directionally solidified CMSX-4 alloy with several combinations of laser power and scanning velocity. Electron backscattered diffraction (EBSD) mapping facilitated identification of new orientations, i.e., “stray grains” that nucleated within the fusion zone along with their area fraction and spatial distribution. Using high-fidelity computational fluid dynamics simulations, both the temperature and fluid velocity fields within the melt pool were estimated. This information was combined with a nucleation model to determine locations where nucleation has the highest probability to occur in melt pools. In conformance with general experience in metals additive manufacturing, the as-solidified microstructure of the laser-melted tracks is dominated by epitaxial grain growth; nevertheless, stray grains were evident in elongated melt pools. It was found that, though a higher laser scanning velocity and lower power are generally helpful in the reduction of stray grains, the combination of a stable keyhole and minimal fluid velocity further mitigates stray grains in laser single tracks.

36 MATERIALS SCIENCE↗
DOE OSTI2020

A Discrete Dendrite Dynamics Model for Epitaxial Columnar Grain Growth in Metal Additive Manufacturing with Application to Inconel

Epitaxial columnar grain growth is a prevalent microstructural feature in the additive manufacturing (AM) of metal components such as Inconel, with cubic unit cell crystal lattice structure (face centered cubic (FCC) or body centered cubic (BCC)). These columnar grains evolve from the partly molten grains in the substrate or the solidified metal. This work proposes an efficient model to simulate the competitive growth of epitaxial columnar dendritic grains. The proposed model tracks the dynamic changes in the dendrites emanating from discrete points along the solid/liquid interface of a quasi-steady melt pool (MP). These dynamic changes include convergence and divergence of growing dendrites. The model is extended to predict the microstructure of large 3D parts and experimentally validated by comparing the simulation results for laser powder bed fusion (L-PBF) and wire-arc additive manufacturing (WAAM) processes. The microstructure and pole figures are predicted for Inconel 718 samples produced by L-PBF and Inconel 740H samples produced by WAAM processes. Finally, the model predictions compare well with the observed microstructure and pole figures results for both the L-PBF and WAAM processes.

20 FOSSIL-FUELED POWER PLANTS↗
DOE OSTI2019

Photocatalytic behaviors of epitaxial BiVO4 (010) thin films

Photocatalytic studies performed on well-defined epitaxial thin films and heterostructures allow the direct investigation of crystallographic-dependent reactivity, and can provide fundamental insights into the reaction mechanisms. In this work, we show that phase-pure, monoclinic BiVO4 nanoislands with a truncated pyramid shape displaying {110} sidewalls can be grown on (001)-oriented yttria-stabilized zirconia (YSZ) substrates with an epitaxial relationship of BiVO4(010)[001] // YSZ(001)[100]. The resulting nanostructures show a strong size-dependent mass-normalized photocatalytic activity, with highly dispersed nanoislands at low coverage being the most active for rhodamine B degradation. Different from studies performed on faceted nanocrystals, when BiVO4 nanostructured films react with Pb(NO3)2 in aqueous solution with and without NaIO3 under full arc irradiation of Xe lamp, nanoparticles were found on the (110) and (010) surface of BiVO4, respectively, indicating that the cocatalyst could be selectively deposition on different facets of the same photocatalayst.

Li, Guoqiang↗
DOE OSTI2013

Structure And Radiation Damage Behavior Of Epitaxial CrxMo1-x Alloy Thin Films On MgO

Phenomena related to the interaction of point defects and dopants with grain boundaries and interfaces have been very well documented. However, a quantitative understanding of such an interaction is still missing. In this paper we explore the correlation between radiation damage and interface structure. In doing so, CrxMo1-x(001) films of thickness ~100 nm were epitaxially grown on MgO (001) using molecular beam epitaxy. The interface dislocation density can be systematically varied by controlling the composition of the film. This system allows us to probe the response of the defects generated during the irradiation to the interface dislocation density. The microstructural features of these films before and after irradiation are carefully studied using high resolution scanning/transmission electron microscopy and electron diffraction. It has been found that the film/substrate system is very resistant to ion-induced irradiation damage. No visible point defect clusters, dislocation network or amorphization has been identified, which is contrasted with other materials of either metallic or ionic bonding. We conclude that the defects in the present system appear to be either sub-microscopic and highly dispersed or are annihilated during the irradiation process. Further detailed work is needed to identify the spatial distribution of the defects.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2019

Effect of epitaxial strain on the optical properties of NaOsO 3

We study the electronic and optical properties of NaOsO 3 under epitaxial strain using first principles methods. Structural optimization shows important changes with strain in the structural parameters such as the Os O bond lengths and the octahedral rotation angles. The key change in the electronic structure is the band gap, which increases with compressive epitaxial strain and decreases with tensile strain. The computed optical absorption coefficient is more or less isotropic for the unstrained structure for light polarization along the three crystallographic directions, but for the strained structure, it is enhanced if the light polarization is along a compressed crystallographic direction due to strain. This is explained in terms of the admixture of Os ( p ) orbitals with the Os ( d ) states that form the valence and the conduction bands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

Selective Transfer of Rotationally Commensurate MoS 2 from an Epitaxially Grown van der Waals Heterostructure

We report that large-scale synthesis of high-quality two-dimensional (2D) semiconductors is critical for their incorporation in emerging electronic and optoelectronic technologies. In particular, chemical vapor deposition (CVD) of transition-metal dichalcogenides (TMDs) via van der Waals epitaxy on epitaxial graphene (EG) leads to rotationally commensurate TMDs in contrast to randomly aligned TMDs grown on amorphous oxide substrates. However, the interlayer coupling between TMDs and EG hinders the investigation and utilization of the intrinsic electronic properties of the resulting TMDs, thus requiring their isolation from the EG growth substrate. To address this issue, we report here a technique for selectively transferring monolayer molybdenum disulfide (MoS 2 ) from CVD-grown MoS 2 -EG van der Waals heterojunctions using copper (Cu) adhesion layers. The choice of Cu as the adhesion layer is motivated by density functional theory calculations that predict the preferential binding of monolayer MoS 2 to Cu in contrast to graphene. Atomic force microscopy and optical spectroscopy confirm the large-scale transfer of rotationally commensurate MoS 2 onto SiO 2 /Si substrates without cracks, wrinkles, or residues. Furthermore, the transferred MoS 2 shows high performance in field-effect transistors with mobilities of up to 30 cm 2 /V s and on/off ratios of up to 10 6 at room temperature. Lastly, this transfer technique can likely be generalized to other TMDs and related 2D materials grown on EG, thus offering a broad range of benefits in nanoelectronic, optoelectronic, and photonic applications.

36 MATERIALS SCIENCE↗