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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Engineering Ferroelectric Hf 0.5 Zr 0.5 O 2 Thin Films by Epitaxial Stress

The critical impact of epitaxial stress on the stabilization of the ferroelectric orthorhombic phase of hafnia is proved. Epitaxial bilayers of Hf 0.5 Zr 0.5 O 2 (HZO) and La 0.67 Sr 0.33 MnO 3 (LSMO) electrodes were grown on a set of single crystalline oxide (001)-oriented (cubic or pseudocubic setting) substrates with a lattice parameter in the 3.71–4.21 Å range. The lattice strain of the LSMO electrode, determined by the lattice mismatch with the substrate, is critical in the stabilization of the orthorhombic phase of HZO. On tensilely strained LSMO electrodes, most of the HZO film is orthorhombic, whereas the monoclinic phase is favored when LSMO is relaxed or compressively strained. Therefore, the HZO films on TbScO 3 and GdScO 3 substrates present substantially enhanced ferroelectric polarization in comparison to films on other substrates, including the commonly used SrTiO 3 . In conclusion, the capability of having epitaxial doped HfO 2 films with controlled phase and polarization is of major interest for a better understanding of the ferroelectric properties and paves the way for fabrication of ferroelectric devices based on nanometric HfO 2 films.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Strain Effect on Oxygen Evolution Reaction Activity of Epitaxial NdNiO3 Thin Films

Epitaxial strain can cause both lattice distortion and oxygen non-stoichiometry, effects that are strongly coupled at the heterojunctions of complex nickelate oxides. In this paper, we decouple these structural and chemical effects on the oxygen evolution reaction (OER) using a set of well-defined, coherently-strained epitaxial NdNiO3 (NNO) films. We show that within the regime where oxygen vacancies (VOs) are negligible, compressive strain is favorable for the OER while tensile strain is unfavorable, as the former induces orbital splitting, resulting in a higher occupancy in the orbital and weaker Ni-O chemisorption. However, when the tensile strain is large enough to promote the formation of VOs, an increase in the OER is also observed. The partial reduction of Ni3+ to Ni2+ due to VOs increases the average occupancy of the eg antibonding orbital to more than one, which is believed to account for this increased OER activity. Our work highlights that epitaxial-strain-induced lattice distortion and VO generation can be simultaneously exploited to tune OER activities, of importance for the predictive synthesis of high-performance electrocatalysts.

strain, oxygen vacancy, orbital polarization, meta↗
DOE OSTI2017

Flexible Quasi-Two-Dimensional CoFe 2 O 4 Epitaxial Thin Films for Continuous Strain Tuning of Magnetic Properties

Epitaxial thin films of CoFe 2 O 4 (CFO) have successfully been transferred from a SrTiO 3 substrate onto a flexible polyimide substrate. By bending the flexible polyimide, different levels of uniaxial strain are continuously introduced into the CFO epitaxial thin films. Unlike traditional epitaxial strain induced by substrates, the strain from bending will not suffer from critical thickness limitation, crystalline quality variation, and substrate clamping, and more importantly, it provides a more intrinsic and reliable way to study strain-controlled behaviors in functional oxide systems. It is found that both the saturation magnetization and coercivity of the transferred films can be changed over the bending status and show a high accord with the movement of the curvature bending radius of the polyimide substrate. Lastly, this reveals that the mechanical strain plays a critical role in tuning the magnetic properties of CFO thin films parallel and perpendicular to the film plane direction.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Surface-Induced Orientation Control of CuPc Molecules for the Epitaxial Growth of Highly Ordered Organic Crystals on Graphene

The epitaxial growth and preferred molecular orientation of copper phthalocyanine (CuPc) molecules on graphene has been systematically investigated and compared with growth on Si substrates, demonstrating the role of surface-mediated interactions in determining molecular orientation. X-ray scattering and diffraction, scanning tunneling microscopy, scanning electron microscopy, and first-principles theoretical calculations were used to show that the nucleation, orientation and packing of CuPc molecules on films of graphene are fundamentally different compared to those grown on Si substrates. Interfacial dipole interactions induced by charge transfer between CuPc molecules and graphene are shown to epitaxially align the CuPc mole-cules in a face-on orientation in a series of ordered superstructures. At high temperatures, CuPc molecules lie flat with respect to the graphene substrate to form strip-like CuPc crystals with micron sizes containing monocrystalline grains. Such large epitaxial crystals may potentially enable bulk-like properties to improve the device properties in organic electronics, which charge transport, exciton diffusion and dissociation are currently limited by grain size effects and molecular orientation.

Xiao, Kai↗
DOE OSTI2018

Three-dimensional atomic scale electron density reconstruction of octahedral tilt epitaxy in functional perovskites

Octahedral tilts are the most ubiquitous distortions in perovskite-related structures that can dramatically influence ferroelectric, magnetic, and electronic properties; yet the paradigm of tilt epitaxy in thin films is barely explored. Non-destructively characterizing such epitaxy in three-dimensions for low symmetry complex tilt systems composed of light anions is a formidable challenge. Here we demonstrate that the interfacial tilt epitaxy can transform ultrathin calcium titanate, a non-polar earth-abundant mineral, into high-temperature polar oxides that last above 900 K. The comprehensive picture of octahedral tilts and polar distortions is revealed by reconstructing the three-dimensional electron density maps across film-substrate interfaces with atomic resolution using coherent Bragg rod analysis. The results are complemented with aberration-corrected transmission electron microscopy,film superstructure reflections, and are in excellent agreement with density functional theory. Conclusively, the study could serve as a broader template for non-destructive, three-dimensional atomic resolution probing of complex low symmetry functional interfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Unusual layer-by-layer growth of epitaxial oxide islands during Cu oxidation

Elucidating metal oxide growth mechanisms is essential for precisely designing and fabricating nanostructured oxides with broad applications in energy and electronics. However, current epitaxial oxide growth methods are based on macroscopic empirical knowledge, lacking fundamental guidance at the nanoscale. Using correlated in situ environmental transmission electron microscopy, statistically-validated quantitative analysis, and density functional theory calculations, we show epitaxial Cu 2 O nano-island growth on Cu is layer-by-layer along Cu 2 O(110) planes, regardless of substrate orientation, contradicting classical models that predict multi-layer growth parallel to substrate surfaces. Growth kinetics show cubic relationships with time, indicating individual oxide monolayers follow Frank-van der Merwe growth whereas oxide islands follow Stranski-Krastanov growth. Cu sources for island growth transition from step edges to bulk substrates during oxidation, contrasting with classical corrosion theories which assume subsurface sources predominate. Our results resolve alternative epitaxial island growth mechanisms, improving the understanding of oxidation dynamics critical for advanced manufacturing at the nanoscale.

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

ZrTe2/CrTe2: an epitaxial van der Waals platform for spintronics

Abstract The rapid discovery of two-dimensional (2D) van der Waals (vdW) quantum materials has led to heterostructures that integrate diverse quantum functionalities such as topological phases, magnetism, and superconductivity. In this context, the epitaxial synthesis of vdW heterostructures with well-controlled interfaces is an attractive route towards wafer-scale platforms for systematically exploring fundamental properties and fashioning proof-of-concept devices. Here, we use molecular beam epitaxy to synthesize a vdW heterostructure that interfaces two material systems of contemporary interest: a 2D ferromagnet (1T-CrTe 2 ) and a topological semimetal (ZrTe 2 ). We find that one unit-cell (u.c.) thick 1T-CrTe 2 grown epitaxially on ZrTe 2 is a 2D ferromagnet with a clear anomalous Hall effect. In thicker samples (12 u.c. thick CrTe 2 ), the anomalous Hall effect has characteristics that may arise from real-space Berry curvature. Finally, in ultrathin CrTe 2 (3 u.c. thickness), we demonstrate current-driven magnetization switching in a full vdW topological semimetal/2D ferromagnet heterostructure device.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Low-temperature grapho-epitaxial La-substituted BiFeO 3 on metallic perovskite

Bismuth ferrite has garnered considerable attention as a promising candidate for magnetoelectric spin-orbit coupled logic-in-memory. As model systems, epitaxial BiFeO 3 thin films have typically been deposited at relatively high temperatures (650–800 °C), higher than allowed for direct integration with silicon-CMOS platforms. Here, we circumvent this problem by growing lanthanum-substituted BiFeO 3 at 450 °C (which is reasonably compatible with silicon-CMOS integration) on epitaxial BaPb 0.75 Bi 0.25 O 3 electrodes. Notwithstanding the large lattice mismatch between the La-BiFeO 3 , BaPb 0.75 Bi 0.25 O 3 , and SrTiO 3 (001) substrates, all the layers in the heterostructures are well ordered with a [001] texture. Polarization mapping using atomic resolution STEM imaging and vector mapping established the short-range polarization ordering in the low temperature grown La-BiFeO 3 . Current-voltage, pulsed-switching, fatigue, and retention measurements follow the characteristic behavior of high-temperature grown La-BiFeO 3 , where SrRuO 3 typically serves as the metallic electrode. These results provide a possible route for realizing epitaxial multiferroics on complex-oxide buffer layers at low temperatures and opens the door for potential silicon-CMOS integration.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Facet-selective morphology-controlled remote epitaxy of ZnO microcrystals via wet chemical synthesis

We report on morphology-controlled remote epitaxy via hydrothermal growth of ZnO micro- and nanostructure crystals on graphene-coated GaN substrate. The morphology control is achieved to grow diverse morphologies of ZnO from nanowire to microdisk by changing additives of wet chemical solution at a fixed nutrient concentration. Although the growth of ZnO is carried out on poly-domain graphene-coated GaN substrate, the direction of hexagonal sidewall facet of ZnO is homogeneous over the whole ZnO-grown area on graphene/GaN because of strong remote epitaxial relation between ZnO and GaN across graphene. Atomic-resolution transmission electron microscopy corroborates the remote epitaxial relation. The non-covalent interface is applied to mechanically lift off the overlayer of ZnO crystals via a thermal release tape. The mechanism of facet-selective morphology control of ZnO is discussed in terms of electrostatic interaction between nutrient solution and facet surface passivated with functional groups derived from the chemical additives.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Hybrid molecular beam epitaxy of germanium-based oxides

Abstract Germanium-based oxides such as rutile GeO 2 are garnering attention owing to their wide band gaps and the prospects of ambipolar doping for application in high-power devices. Here, we present the use of germanium tetraisopropoxide (GTIP), a metal-organic chemical precursor, as a source of germanium for the demonstration of hybrid molecular beam epitaxy for germanium-containing compounds. We use Sn 1- x Ge x O 2 and SrSn 1- x Ge x O 3 as model systems to demonstrate our synthesis method. A combination of high-resolution X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy confirms the successful growth of epitaxial rutile Sn 1- x Ge x O 2 on TiO 2 (001) substrates up to x = 0.54 and coherent perovskite SrSn 1- x Ge x O 3 on GdScO 3 (110) substrates up to x = 0.16. Characterization and first-principles calculations corroborate that germanium occupies the tin site, as opposed to the strontium site. These findings confirm the viability of the GTIP precursor for the growth of germanium-containing oxides by hybrid molecular beam epitaxy, thus providing a promising route to high-quality perovskite germanate films.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Epitaxial growth of highly-crystalline spinel ferrite thin films on perovskite substrates for all-oxide devices

The potential growth modes for epitaxial growth of Fe 3 O 4 on SrTiO 3 (001) are investigated through control of the energetics of the pulsed-laser deposition growth process (via substrate temperature and laser fluence). We find that Fe 3 O 4 grows epitaxially in three distinct growth modes: 2D-like, island and 3D-to-2D, the last of which is characterized by films that begin growth in an island growth mode before progressing to a 2D growth mode. Films grown in the 2D-like and 3D-to-2D growth modes are atomically flat and partially strained, while films grown in the island growth mode are terminated in islands and fully relaxed. We find that the optimal structural, transport and magnetic properties are obtained for films grown on the 2D-like/3D-to-2D growth regime boundary. The viability for including such thin films in perovskite-based all-oxide devices is demonstrated by growing a Fe 3 O 4 /La 0.7 Sr 0.3 MnO 3 spin valve epitaxially on SrTiO 3 .

Science & Technology - Other Topics↗
DOE OSTI2016

Epitaxial growth of iridate pyrochlore Nd 2 Ir 2 O 7 films

Epitaxial films of the pyrochlore Nd 2 Ir 2 O 7 have been grown on (111)-oriented yttria-stabilized zirconia (YSZ) substrates by off-axis sputtering followed by post-growth annealing. X-ray diffraction (XRD) results demonstrate phase-pure epitaxial growth of the pyrochlore films on YSZ. Scanning transmission electron microscopy (STEM) investigation of an Nd 2 Ir 2 O 7 film with a short post-annealing provides insight into the mechanism for crystallization of Nd 2 Ir 2 O 7 during the post-annealing process. STEM images reveal clear pyrochlore ordering of Nd and Ir in the films. As a result, the epitaxial relationship between the YSZ and Nd 2 Ir 2 O 7 is observed clearly while some interfacial regions show a thin region with polycrystalline Ir nanocrystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Nanoscale self-templating for oxide epitaxy with large symmetry mismatch

Direct observations using scanning transmission electron microscopy unveil an intriguing interfacial bi-layer that enables epitaxial growth of a strain-free, monoclinic, bronze-phase VO 2 (B) thin film on a perovskite SrTiO 3 (STO) substrate. For this study, we observe an ultrathin (2–3 unit cells) interlayer best described as highly strained VO 2 (B) nanodomains combined with an extra (Ti,V)O 2 layer on the TiO 2 terminated STO (001) surface. By forming a fully coherent interface with the STO substrate and a semi-coherent interface with the strain-free epitaxial VO 2 (B) film above, the interfacial bi-layer enables the epitaxial connection of the two materials despite their large symmetry and lattice mismatch.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Defect reduction by liquid phase epitaxy of germanium on single-crystalline-like germanium templates on flexible, low-cost metal substrates

Single-crystalline-like germanium (Ge) templates were demonstrated on low-cost, flexible metal substrates and have been used to fabricate III–V materials and devices for photovoltaics and flexible electronics applications. However, these Ge templates, fabricated by magnetron sputtering or plasma enhanced chemical vapor deposition, contain a high density of defects. In order to improve the performance of optoelectronic devices made using the Ge templates, a homo-epitaxial Ge layer has been additionally grown by a liquid phase epitaxy (LPE) method. The LPE Ge layer is composed of significantly larger grains (~26 μm) compared to that of the Ge template (~200 nm). This large size of the LPE Ge grains effectively reduces the volume fraction of grain boundaries. The LPE Ge shows an out-of-plane texture Δω of ~0.63° and an in-plane texture ΔΦ of ~1.26°, which signify improvements of ~39.3% and ~76.6% compared to that of the vapor-deposited Ge template, respectively. Further, the LPE Ge is strain free, compared to the strained Ge template hetero-epitaxially grown on cerium oxide. Defects caused by low-angle grain boundaries, impurities and strain relaxation in the Ge template are found to be suppressed in the LPE Ge. The threading dislocation density is roughly estimated to be ~5 × 10 6 cm -2 in the LPE Ge compared to ~1 × 10 10 cm -2 in the Ge template.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Electrodeposition of nanometer-thick epitaxial films of silver onto single-crystal silicon wafers

Silver films were deposited epitaxially for the first time onto low-index, single-crystal silicon wafers through an electrochemical method in an aqueous silver acetate bath. A negative potential of -2.34 V vs. Hg/Hg 2 SO 4 was used for both pre-polarization and during the deposition to avoid Si oxidation. The epitaxy of Ag films on Si(111), (110) and (100) was characterized by X-ray diffraction symmetric scans and pole figures. The Ag films showed [111], [110] and [100] out-of-plane orientations, respectively, with in-plane order determined by the Si substrates. Interface models consistent with the observed orientations invoke coincident site lattices (CSLs), in which four unit meshes of Ag coincide with three unit meshes of Si. These CSLs reduce the lattice mismatch from -24.9% to +0.13%. A thickness of about 10 nm was obtained for Ag deposited for ten minutes. A comparison of silver acetate electrolyte and cyanide electrolyte was also performed, showing advantages of the acetate bath over the cyanide bath for growth of epitaxial films of Ag on Si surfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Unusual electrical conductivity driven by localized stoichiometry modification at vertical epitaxial interfaces

Precise control of lattice mismatch accommodation and interfacial cation diffusion is critical to modulate correlated functionalities in epitaxial heterostructures, particularly when the interface composition is positioned near a compositional phase transition boundary. Here we select La 1-x Sr x MnO 3 (LSMO) as a prototype phase transition oxide and establish vertical epitaxial interfaces with NiO for exploring the strong interplay between strain accommodation, stoichiometry modification, and localized electron transport across the interface. It is found that localized stoichiometry modification overcomes the dead layer problem plaguing LSMO and leads to strongly directional conductivity, as manifested by a more than three orders of magnitude difference between out-of-plane and in-plane conductivity. Comprehensive atomic scale structural characterization and transport measurements reveal that this emerging behavior is created by a compositional change produced by preferential cation diffusion that pushes the LSMO phase transition from insulating into metallic within an ultrathin interface region. This study explores the nature of unusual electrical conductivity at vertical epitaxial interfaces and highlights the beneficial role of controllable cation diffusion that enables emerging functionalities for a broad range of potential applications such as memristors, spintronic devices, and novel nanoelectronic devices using strongly correlated materials.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Probing crystallographic orientation-specific carrier lifetimes in epitaxial Ge/AlAs and InGaAs/InP heterostructures

Current silicon (Si) fin transistors rely on (100) and (110) crystallographically oriented surfaces, and the proposed alternate channel transistor technology comprises materials with higher mobility than Si. Crystallographically oriented epitaxial germanium (Ge) and indium–gallium arsenide (InGaAs) have the potential to replace Si in ultra-low power transistor applications. The higher carrier lifetime is an indication of superior material quality, which relates to the leakage current of a fin transistor. To gain insights into the carrier recombination dynamics in these crystallographically oriented epitaxial Ge and InGaAs layers, the contactless microwave photoconductive decay (μ-PCD) technique at an excitation wavelength of 1500 nm was employed to probe the orientation-specific carrier lifetimes. Highly effective carrier lifetimes >200 ns for (100)Ge/AlAs and (110)Ge/AlAs, and ~80 ns for (111)Ge/AlAs heterostructures, were extracted at room temperature. The measured carrier lifetime has a strong dependence on the surface orientation, which could be related to orientation-specific bulk trap states present within the bandgap of Ge. The (111)Ge orientation has 3 times lower carrier lifetime compared to the (100)Ge and (110)Ge surface orientations. On the other hand, the carrier lifetimes of 125 μs and 10 ns were determined from (100)InGaAs/InP and (110)InGaAs/InP heterostructures, respectively. The reduction in carrier lifetimes in both (111)Ge and (110)InGaAs was due to high electrical conductivity or higher bulk trap states present within the bandgap as well as the facet-dependent growth of the (110)InGaAs layer on InP. A surface passivating layer is indispensable for these orientation-specific epitaxial layers to improve the carrier lifetime. Therefore, the higher carrier lifetimes from technologically interesting (100)Ge and (110)Ge surfaces would offer a path for the development of Ge-based ultra-low power electronics, and optoelectronic devices based on the (100)InGaAs layer.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Structure and magnetotransport properties of epitaxial nanocomposite La{sub 0.67}Ca{sub 0.33}MnO{sub 3}:SrTiO{sub 3} thin films grown by a chemical solution approach.

Epitaxial La{sub 0.67}Ca{sub 0.33}MnO{sub 3}:SrTiO{sub 3} (LCMO:STO) composite thin films have been grown on single crystal LaAlO{sub 3}(001) substrates by a cost effective polymer-assisted deposition method. Both x-ray diffraction and high-resolution transmission electron microscopy confirm the growth of epitaxial films with an epitaxial relationship between the films and the substrates as (002){sub film}||(002){sub sub} and [202]{sub film}||[202]{sub sub}. The transport property measurement shows that the STO phase significantly increases the resistivity and enhances the magnetoresistance (MR) effect of LCMO and moves the metal-insulator transition to lower temperatures. For example, the MR values measured at magnetic fields of 0 and 3 T are {approx}44.6% at 255 K for LCMO, {approx}94.2% at 125 K for LCMO:3% STO, and {approx}99.4% at 100 K for LCMO:5% STO, respectively.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗