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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Chemistry and Interfacial Structure Promoting Quasi-van der Waals Epitaxial Growth of WS 2 Nanosheets on Sapphire for Prospective Application in Field-Effect Transistors

How do chemical and structural modifications to the supporting crystal surface affect the subsequent van der Waals (vdW) or quasi(Q)-vdW epitaxial growth of 2D nanocrystals? Developing an atomic-scale picture of such an interfacial system is crucial for understanding its impact on the physical and chemical properties of the supported 2D materials. The elucidation of the interfacial structure and chemistry needed to promote the Q-vdW epitaxial growth of 2D tungsten disulfide (WS 2 ) nanocrystals contributes to the growth mechanism understanding, thus pushing forward the integration of such atomically thin semiconductors toward real field-effect transistor applications. In addition to an atomic-force microscopy top view, we showcase a combination of X-ray techniques for a top-to-bottom investigation of the complexities of the buried interface structures. Furthermore, this approach uses X-ray photoelectron spectroscopy, X-ray standing wave excited X-ray fluorescence, and crystal truncation rod scattering to produce a highly resolved chemical-state-specific 3D atomic map for the extended interface structure of WS 2 /α-Al 2 O 3 (001). Employing these detailed analysis methods, along with density functional theory to further refine the picoscale structure, we demonstrate how two different types of interface engineering during the pregrowth stage lead to significant differences in the chemical and structural modifications to the terminal surface of c-face sapphire, which in turn leads to substantial differences in the submonolayer growth of supported WS 2 2D nanocrystals in terms of lateral domain sizes, epitaxial registry, vdW gaps, and stability.

36 MATERIALS SCIENCE↗
DOE OSTI2026

GeSn alloys with ∼21% Sn grown by effusion cell molecular beam epitaxy

We report the epitaxial growth of high quality GeSn alloys with Sn compositions up to 21.25 ± 1% by effusion-cell molecular beam epitaxy (MBE). Achieving such a high Sn content with effusion cells is particularly significant, since these sources typically impose stronger radiative heating on the substrate, which is a factor long considered a major obstacle to high-Sn GeSn epitaxy. Contrary to the prevailing assumption that high growth rates are good for suppressing Sn segregation, we demonstrate that carefully controlled low fluxes, combined with stable ultra-low substrate temperatures, enable significant Sn incorporation even under effusion-cell conditions. Structural analyses by high-resolution x-ray diffraction, reciprocal space mapping, atomic force microscopy, and transmission electron microscopy confirm sharp interfaces, high crystallinity, and smooth surfaces at high Sn contents. Furthermore, these findings establish the feasibility of MBE-grown high-Sn GeSn even under diffusion cells, thereby broadening opportunities for Si-compatible photonic and quantum devices.

Alloys↗
DOE OSTI2023

Molecular Beam Epitaxy of Monocrystalline GaAs on Water-Soluble NaCl Thin Films

The goal of this project was to demonstrate the feasibility of employing an epitaxial NaCl thin film as a water-soluble release layer for III-V photovoltaic devices and GaAs substrate reuse. Over the course of this project efforts were focused on: 1) achieving crystalline NaCl thin films on GaAs (100) substrate, 2) exploring the growth parameters for subsequent GaAs on NaCl thin films in effort to improve crystallinity of the semiconductor layer, 3) deposition and removal of single crystalline solar cell devices from the parent substrate, and 4) improving morphology and reducing large scale defects in the removed layers to fabricate a working a solar cell device. There was no previous work on direct integration of GaAs/NaCl/GaAs heterostructures at the time of this study. We used molecular beam epitaxy (MBE) to deposit both the alkali halide salt and subsequent III-V material in the same chamber, with no vacuum break. Single crystalline GaAs films were achieved on NaCl layers through careful tuning of the growth parameters and exposure to the reflection high energy electron diffraction (RHEED) beam. Dissolution of the NaCl layer in water provided rapid release of the semiconductor layer from the substrate. Monocrystalline solar cells were grown on the III-V templates using MBE and dynamic hydride vapor phase epitaxy (HVPE). However, the extended time at elevated temperatures required for the cell growth resulted in large area defects from fusion of the semiconductor overlayer to the substrate, causing shorts in fabricated devices. By changing the way that the template layer was grown, the density of these defects could be reduced. Unfortunately, the defects were not reduced to a level allowing for fabrication of a working device.

14 SOLAR ENERGY↗
DOE OSTI2020

Constructing Polymorphic Nanodomains in BaTiO 3 Films via Epitaxial Symmetry Engineering

Ferroelectric materials owning a polymorphic nanodomain structure usually exhibit colossal susceptibilities to external mechanical, electrical, and thermal stimuli, thus holding huge potential for relevant applications. Despite the success of traditional strategies by means of complex composition design, alternative simple methods such as strain engineering have been intensively sought to achieve a polymorphic nanodomain state in lead-free, simple-composition ferroelectric oxides in recent years. Here, a nanodomain configuration with morphed structural phases is realized in an epitaxial BaTiO3 film grown on a (111)-oriented SrTiO3 substrate. Using a combination of experimental and theoretical approaches, it is revealed that a threefold rotational symmetry element enforced by the epitaxial constraint along the [111] direction of BaTiO3 introduces considerable instability among intrinsic tetragonal, orthorhombic, and rhombohedral phases. Such phase degeneracy induces ultrafine ferroelectric nanodomains (1-10 nm) with low-angle domain walls, which exhibit significantly enhanced dielectric and piezoelectric responses compared to the (001)-oriented BaTiO3 film with uniaxial ferroelectricity. Therefore, the finding highlights the important role of epitaxial symmetry in domain engineering of oxide ferroelectrics and facilitates the development of dielectric capacitors and piezoelectric devices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Epitaxial Ferroelectric Hf 0.5 Zr 0.5 O 2 with Metallic Pyrochlore Oxide Electrodes

The synthesis of fully epitaxial ferroelectric Hf 0.5 Zr 0.5 O 2 (HZO) thin films through the use of a conducting pyrochlore oxide electrode that acts as a structural and chemical template is reported. Such pyrochlores, exemplified by Pb 2 Ir 2 O 7 (PIO) and Bi 2 Ru 2 O 7 (BRO), exhibit metallic conductivity with room-temperature resistivity of <1 mΩ cm and are closely lattice matched to yttria-stabilized zirconia substrates as well as the HZO layers grown on top of them. Evidence for epitaxy and domain formation is established with X-ray diffraction and scanning transmission electron microscopy, which show that the c-axis of the HZO film is normal to the substrate surface. The emergence of the non-polar-monoclinic phase from the polar-orthorhombic phase is observed when the HZO film thickness is ≥≈30 nm. Furthermore, thermodynamic analyses reveal the role of epitaxial strain and surface energy in stabilizing the polar phase as well as its coexistence with the non-polar-monoclinic phase as a function of film thickness.

36 MATERIALS SCIENCE↗
DOE OSTI2022

AlInP-passivated III–V solar cells grown by dynamic hydride vapor-phase epitaxy

We report the development of AlInP-passivated solar cells grown by dynamic hydride vapor-phase epitaxy (D-HVPE) with AM1.5G efficiencies of 26.0% for single-junction (1J) GaAs cells and 28.0% for GaInP/GaAs (2J) tandems. We compare the device performance of solar cells passivated with AlInP versus control cells passivated with GaInP, which has already enabled near-unity carrier collection in GaAs solar cells. 1J devices passivated with either AlInP or GaInP have an identical open-circuit voltage (V OC ) of 1.06 V and long-wavelength current collection near 95%, indicating that both window materials provide a similar degree of passivation. Adding AlInP passivation to each solar cell structure improves the current collection by 1.3 and 1 mA/cm 2 for the 1J and 2J, respectively. The AlInP also results in a top cell V OC boost of ~40 mV relative to a tandem device passivated only by a thin, highly doped GaInP emitter. Secondary-ion mass spectrometry measurements indicate that although O and Si both incorporate in the AlInP window, they do not appear in the subsequently grown absorber layers and do not impact its ability to passivate the front surface. We expect that these achievements, along with continued optimization, will enable parity of hydride vapor-phase epitaxy (HVPE)-grown device efficiencies with state-of-the-art devices grown by other epitaxial methods in the near future.

14 SOLAR ENERGY↗
DOE OSTI2025

V‐Groove Si Nanopatterning for the Direct Epitaxy of Orientation‐Patterned III–V Nonlinear Optical Crystals

Orientation‐patterned (OP) III–V semiconductors—used as quasi‐phase‐matched crystals for nonlinear optics applications—are typically epitaxially grown on expensive III–V substrates using a complex process requiring three separate epitaxy steps. In this work, a method is demonstrated for growing orientation‐patterned III–V crystals on Si substrates through V‐groove nanopatterning and a single epitaxial growth. V‐groove Si allows for suppression of random antiphase domain formation that is typical of III–V growth on (001)‐oriented Si through the use of (111)‐faceted trenches patterned on (001)‐oriented Si substrates. By alternating the directions of the V‐groove trenches between [110] and [10], antiphase boundaries can be selectively induced at the boundaries between the two directions of trenches due to the difference in symmetry between the III–V material and Si. This approach allows for a greatly simplified process for growing OP‐III‐Vs and more broadly is a new, robust approach for precisely patterning antiphase boundaries of arbitrary shape and length scales down to 50 nm.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Investigation of GaAs surface treatments for ZnSe growth by molecular beam epitaxy without a buffer layer

Atomically clean and smooth surfaces are critical prerequisite for the epitaxial regrowth of dissimilar semiconductors. Using ZnSe/GaAs as a model system, epitaxial regrowth without a buffer layer after thermal cleaning with various As-fluxes, atomic Ga flux, and atomic hydrogen treatments to the GaAs substrates and the surfaces were investigated in this work via in-situ Auger electron spectroscopy (AES). The ZnSe epilayers grown on the pre-treated GaAs surfaces without a buffer layer were characterized by X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) to evaluate the effectiveness of the surface treatment methods. It was found that a high quality ZnSe layer can be achieved by atomic hydrogen surface treatment at 300°C without requiring GaAs buffer layer growth, which reveals a path for epitaxy growth avoiding high temperature treatments.

42 ENGINEERING↗
DOE OSTI2020

Epitaxial Regrowth and Hole Shape Engineering for Photonic Crystal Surface Emitting Lasers (PCSELs)

In the present research, epitaxial regrowth by molecular beam epitaxy (MBE) is investigated as a fabrication process for void-semiconductor photonic crystal (PhC) surface emitting lasers (PCSELs). The PhC is patterned by electron beam lithography (EBL) and inductively coupled plasma (ICP) etch and is subsequently regrown by molecular beam epitaxy to embed a series of voids in bulk semiconductor. Experiments are conducted to investigate the effects of regrowth on air-hole morphology. The resulting voids have a distinct teardrop shape with the radius and depth of the etched hole playing a very critical role in the final regrown void’s dimensions. We demonstrate that specific hole diameters can encourage deposition to the bottom of the voids or to their sidewalls, allowing us to engineer the shape of the void more precisely as is required by the PCSEL design. A 980 nm InGaAs quantum well laser structure is optimized for low threshold lasing at the design wavelength and full device structures are patterned and regrown. An optically pumped PCSEL is demonstrated from this process.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Scanning transmission election microscopy observations of twisted epitaxial gold nanodiscs in twisted molybdenum disulfide bilayers

Atomic scale, scanning transmission electron microscopy (STEM) analysis of the moiré structures in twisted epitaxial gold nanodiscs encapsulated in twisted bilayer molybdenum disulfide is presented. High angle annular dark field STEM imaging reveals that the period of the moiré patterns between gold and molybdenum disulfide varies with different twist angles of the bilayer molybdenum disulfide, ranging from 1.80 nm (epitaxial alignment of gold) to 1.53 nm (twisted epitaxial alignment of gold). Additionally, bright field STEM imaging reveals a faint, larger "moiré of moiré" structure in cases where the bilayer molybdenum disulfide twist angle is small (~6°), arising from the overlapping three-layers, which is not visible in conventional transmission electron microscopy images. In conclusion, our experiments indicate that scanning transmission electron microscopy as a suitable tool for moiré analysis of twisted multilayer planar heterostructures, complementary to information provided by conventional transmission electron microscopy and diffraction.

Au↗
DOE OSTI2021

Epitaxial Thin Films of a Chalcogenide Perovskite

Chalcogenide perovskites have emerged as a new class of optoelectronic materials, especially for photovoltaic applications, but fundamental properties and applications of chalcogenide perovskites remain limited due to the lack of high-quality thin films. In this paper, we report direct epitaxial thin film growth of BaZrS 3 , a prototypical chalcogenide, by pulsed laser deposition. X-ray diffraction studies show that the films are strongly textured out-of-plane and have a clear in-plane epitaxial relationship with the substrate. Electron microscopy studies confirm the presence of epitaxy for the first few layers of the film at the interface, even though away from the interface, the films are polycrystalline with many extended defects, suggesting the potential for further improvement in growth. X-ray reflectivity and atomic force microscopy show smooth film surfaces and interfaces between the substrate and the film. The films show strong light absorption near the band edge and photoluminescence in the visible region, validating BaZrS 3 as a suitable candidate for ultrathin front absorbers in tandem solar cells. The photodetector devices show fast and efficient photo response with the highest ON/OFF ratio reported for BaZrS 3 films thus far. Our study opens up opportunities to use high quality thin films of chalcogenide perovskites to probe fundamental physical phenomena in thin films and heterostructures and also in photovoltaic and optoelectronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Hybrid Symmetry Epitaxy of the Superconducting Fe(Te,Se) Film on a Topological Insulator

It is challenging to grow an epitaxial 4-fold compound superconductor (SC) on a 6-fold topological insulator (TI) platform due to the stringent lattice-matching requirement. Here, we demonstrate that Fe(Te,Se) can grow epitaxially on a TI (Bi 2 Te 3 ) layer due to accidental, uniaxial lattice match, which is dubbed as “hybrid symmetry epitaxy”. This new growth mode is critical to stabilizing robust superconductivity with TC as high as 13 K. Furthermore, the superconductivity in this FeTe 1–x Se x /Bi 2 Te 3 system survives in the Te-rich phase with Se content as low as x = 0.03 but vanishes at Se content above x = 0.56, exhibiting a phase diagram that is quite different from that of the conventional Fe(Te,Se) systems. This unique heterostructure platform that can be formed in both TI-on-SC and SC-on-TI sequences opens a route to unprecedented topological heterostructures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Superconducting Fourfold Fe(Te,Se) Film on Sixfold Magnetic MnTe via Hybrid Symmetry Epitaxy

Epitaxial Fe(Te,Se) thin films have been grown on various substrates but never been grown on magnetic layers. In this work, we report the epitaxial growth of fourfold Fe(Te,Se) film on a sixfold antiferromagnetic insulator, MnTe. The Fe(Te,Se)/MnTe heterostructure shows a clear superconducting transition at around 11 K, and the critical magnetic field measurement suggests the origin of the superconductivity to be bulk-like. Structural characterizations suggest that the uniaxial lattice match between Fe(Te,Se) and MnTe allows a hybrid symmetry epitaxy mode, which was recently discovered between Fe(Te,Se) and Bi 2 Te 3 . Furthermore, the Te/Fe flux ratio during deposition of the Fe(Te,Se) layer is found to be critical for its superconductivity. Now that superconducting Fe(Te,Se) can be grown on two related hexagonal platforms, Bi 2 Te 3 and MnTe, this result opens a new possibility of combining topological superconductivity of Fe(Te,Se) with the rich physics in the intrinsic magnetic topological materials (MnTe) n (Bi 2 Te 3 ) m family.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Epitaxial Stabilization and Oxygen Evolution Reaction Activity of Metastable Columbite Iridium Oxide

Non-rutile polymorphs of binary iridium oxide such as columbite IrO 2 (α-IrO 2 ) are promising candidates for highly active acid-stable oxygen evolution reaction (OER) catalysts, yet their synthesis has been challenging due to the dominant thermodynamic stability of rutile IrO 2 (R-IrO 2 ). Here, we report the growth of α-IrO 2 via epitaxial thin films using pulsed laser deposition. We observe that, in competition with R-IrO 2 (100), the films can be optimized to be predominantly (100)-oriented α-IrO 2 . Surprisingly, the activity of α-IrO 2 shows a large discrepancy of ~0.2 V in the overpotential compared to its predicted activity, which is resolved via theoretical calculations to be a crystal orientation effect. This demonstrates that the electrocatalytic activity can be significantly varied upon crystal orientation, a parameter that is difficult to control in conventional polycrystalline systems but accessible in epitaxial thin films. In total, this study demonstrates epitaxial thin film growth as a powerful technique, which can overcome large energetic instabilities on the order of ~300 meV to stabilize metastable material structures inaccessible by bulk synthesis. As a result, this provides unique opportunities to effectively identify the atomic structure of active catalysts by combining investigations of metastable materials with theoretical predictions.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Scalable ultrafast epitaxy of large-grain and single-crystal II-VI semiconductors

A general problem for semiconductor applications is that very slow deposition on expensive single-crystal substrates yields high crystalline quality with excellent electro-optical properties, but at prohibitive costs and throughput for many applications. In contrast, rapid deposition on inexpensive substrates or nanocrystalline films yields low costs, but comparatively inferior crystallinity, carrier transport, and recombination. Here, we present methods to deposit single-crystal material at rates 2-3 orders of magnitude faster than state-of-the-art epitaxy with low-cost methods without compromising crystalline or electro-optical quality. For example, single-crystal CdTe and CdZnTe films that would take several days to grow by molecular-beam epitaxy are deposited in 8 minutes by close-spaced sublimation, yet retain the same crystalline quality measured by X-ray diffraction rocking curves. The fast deposition is coupled with effective n- and p-type in-situ doping by In, P, and As. The epitaxy can be extended to nanocrystalline substrates. For example, we recrystallize thin CdTe films on glass to deposit large grains with low defect density. The results provide new research paths for photovoltaics, detectors, infrared imaging, flexible electronics, and other applications.

14 SOLAR ENERGY↗
DOE OSTI2020

Epitaxial growth of rock salt MgZrN 2 semiconductors on MgO and GaN

Ternary nitride compound semiconductors have attracted recent attention as electronic materials since their properties can be tuned by cation stoichiometry and ordering. A recently discovered example is MgZrN 2 , a ternary analog to the rock salt semiconductor ScN. MgZrN 2 has a larger bandgap and stronger dielectric response than the binary compound. Polycrystalline thin films of MgZrN 2 have been studied, but demonstration of high-quality growth is still required to establish its suitability for technological applications. Here, we report on epitaxial growth of MgZrN 2 thin films on (100) and (111) MgO substrates and (001) GaN templates. The MgZrN 2 composition is confirmed by Rutherford backscattering spectrometry, showing no oxygen in the film except for a thin surface oxide layer. Epitaxial growth results in MgZrN 2 with x-ray diffraction rocking curves with a full-width at half-maximum in the range of 0.3–3.0°, depending on the substrate. Transmission electron microscopy analysis of the MgZrN 2 film grown on a (111) MgO substrate confirms epitaxial growth and shows a sharp film/substrate interface. In-plane temperature-dependent Hall effect measurements show that the material is an n-type semiconductor with a relatively high concentration ( n 300K ≈ 10 19 –10 20 cm -3 ) of thermally activated electrons. Room-temperature transport measurements show a conductivity of 25 S cm -1 and a Seebeck coefficient of -80 μ V K -1 . Overall, these results provide an important step toward integration of rock salt MgZrN 2 with other technological nitrides for device applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

All-epitaxial long-range surface plasmon polariton structures with integrated active materials

We demonstrate all-epitaxial structures capable of supporting short- and long-range surface plasmon polariton (SRSPP and LRSPP) modes in the long-wave infrared region of the electromagnetic spectrum. The SRSPP and LRSPP modes are bound to the interfaces of a buried heavily doped (n ++ ) semiconductor layer and surrounding quantum-engineered type-II superlattice (T2SL) materials. The surrounding T2SLs are designed to allow optical transitions across the frequency dispersion of the SPP modes. We map the SPP dispersion in our structure using grating-coupled angle- and polarization-dependent reflection and photoluminescence spectroscopy. The epitaxial structures are analytically described using a simplified three-layer system (T2SL/n ++ /T2SL) and modeled using rigorous coupled wave analysis with excellent agreement to our experimental results. The presented structures offer the potential to serve as long-range interconnects or waveguides in all-epitaxial plasmonic/optoelectronic systems operating in the long-wave infrared.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Defect characterization and charge transport measurements in high-resolution Ni/n-4H-SiC Schottky barrier radiation detectors fabricated on 250 μ m epitaxial layers

Advances in the growth processes of 4H-SiC epitaxial layers have led to the continued expansion of epilayer thickness, allowing for the detection of more penetrative radioactive particles. In this work, we report the fabrication and characterization of high-resolution Schottky barrier radiation detectors on 250 μm thick n-type 4H-SiC epitaxial layers, the highest reported thickness to date. Several 8 × 8 mm 2 detectors were fabricated from a diced 100 mm diameter 4H-SiC epitaxial wafer grown on a conductive 4H-SiC substrate with a mean micropipe density of 0.11 cm -2 From the Mott–Schottky plots, the effective doping concentration was found to be in the range (0.95–1.85) × 10 14 cm -3 , implying that full depletion could be achieved at ~5.7 kV (0.5 MV/cm at the interface). The current-voltage characteristics demonstrated consistently low leakage current densities of 1–3 nA/cm 2 at a reverse bias of -800 V. This resulted in the pulse-height spectra generated using a 241Am alpha source (5486 keV) manifesting an energy resolution of less than 0.5% full width at half maximum (FWHM) for all the detectors at -200 V. The charge collection efficiencies (CCEs) were measured to be 98–99% with no discernable correlation to the energy resolution. A drift-diffusion model fit to the variation of CCE as a function of bias voltage, revealed a minority carrier diffusion length of ~10 μm. Deep level transient spectroscopy measurements on the best resolution detector revealed that the excellent performance was the result of having ultralow concentrations of the order of 1011 cm-3 lifetime limiting defects—Z 1/2 and EH 6/7 .

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗