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Molecular beam epitaxy is applied for the GaN/InGaN growth under In‐bilayer stabilized conditions, which require substrate temperatures above 580 °C for sufficient indium desorption. Under these conditions the indium content is limited to xIn≤0.3$x_{\text{In}} \leq 0.3$, as determined by a novel reflection high energy electron diffraction technique, which is based on indium adsorption and subsequent consumption at varying gallium flux. As‐grown structures show degraded surfaces after indium desorption. This effect is eliminated by applying excess gallium before and after InGaN growth, which results in morphologies and dislocation densities comparable to GaN/AlGaN structures. [ABSTRACT FROM AUTHOR]

Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage the membrane and limit the ability to make them ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler compound GdPtSb on graphene-terminated Al2O3 substrates. Despite the presence of the graphene interlayer, the Heusler films have epitaxial registry to the underlying sapphire, as revealed by x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy. The weak Van der Waals interactions of graphene enable mechanical exfoliation to yield free-standing GdPtSb membranes, which form ripples when transferred to a flexible polymer handle. Whereas unstrained GdPtSb is antiferromagnetic, measurements on rippled membranes show a spontaneous magnetic moment at room temperature, with a saturation magnetization of 5.2 bohr magneton per Gd. First-principles calculations show that the coupling to homogeneous strain is too small to induce ferromagnetism, suggesting a dominant role for strain gradients. Our membranes provide a novel platform for tuning the magnetic properties of intermetallic compounds via strain (piezomagnetism and magnetostriction) and strain gradients (flexomagnetism). Single crystalline membranes enable the tuning of materials properties via strain states that are not accessible to bulk crystals or epitaxially clamped films. Here, the authors demonstrate the synthesis and strain gradient-induced magnetism in membranes of the Heusler compound GdPtSb. [ABSTRACT FROM AUTHOR]

M n 2 Au is a bimetallic antiferromagnetic with an extraordinarily high Néel temperature of 1600 K. M n 2 Au has also been found to demonstrate Néel-order spin–orbit torque, allowing for electrical switching of the antiferromagnetic Néel vector. This has led to much interest in this material for spintronic applications. Here, we detail the growth of high quality M n 2 Au (110) thin films. The films were grown on a high quality Pt (111) buffer layer, which was grown on an A l 2 O 3 (0001) substrate. The M n 2 Au films have excellent crystalline quality as demonstrated by x-ray diffraction, x-ray reflection, and reflection high energy electron diffraction measurements. The M n 2 Au (110) films are found to grow with three equivalent in-plane rotation domains, caused by the Pt/ A l 2 O 3 substrate. [ABSTRACT FROM AUTHOR]

Reflection high energy electron diffraction (RHEED) information is critical for the growth of thin films; however, only a small percentage of the data from RHEED videos is typically used. The use of full videos in machine learning can require dimension reduction techniques. In this paper, three dimension reduction techniques, principal component analysis (PCA), non-negative matrix factorization (NMF), and kmeans clustering, are compared to investigate their benefits to the analysis of RHEED data. Three different heterostructures with different growth modes, all deposited on Ti-terminated strontium titanate by pulsed laser deposition, were used for the analysis: lanthanum aluminate with layer-by-layer growth, lithium cobalt oxide with island growth, and strontium ruthenate with a transition from layer-by-layer to step-flow growth. A phase shift in intensity fluctuations of different RHEED spots was discovered and discussed in terms of their sensitivity to the film growth characterization. The diffraction spots that were more sensitive to the growth were differentiated from the spots that are affected by the substrate as a function of film thickness. It was concluded that NMF provides the analysis that is easiest to interpret without the loss of detailed physical information due to its non-negativity constraint and lack of forced orthogonality such as in PCA. Analysis of the full RHEED videos enables a more detailed understanding of growth characteristics and control of growth processes as aided by dimension reduction. [ABSTRACT FROM AUTHOR]

We investigate the structural and magnetic properties of La 0.7 Sr 0.3 CrO 3 (LSCO) / La 0.7 Sr 0.3 MnO 3 (LSMO) heterostructures grown on (001)-oriented MgO by molecular beam epitaxy. Due to the large film-substrate lattice mismatch, strain relaxation is found to occur within the first 2–3 unit cells (uc) of the film as evidenced by reflection high energy electron diffraction and high-resolution synchrotron x-ray reciprocal space mapping. We find that the presence of the LSCO spacer and capping layers leads to ferromagnetism in ultra-thin LSMO layers with thicknesses of the order of 2 uc with the magnetic easy axis oriented in the film plane. Net magnetic moments of 1.4 and 2.4 μ B / Mn are measured for (2 uc LSCO/2 uc LSMO) and (2 uc LSCO/4 uc LSMO) superlattices, respectively, by superconducting quantum interference device magnetometry. The effective magnetic anisotropy of the relaxed (2 uc LSCO/4 uc LSMO) heterostructure is found to be an order of magnitude higher than bulk LSMO highlighting the critical role of interfacial interactions in tuning magnetic anisotropy at complex oxide interfaces. [ABSTRACT FROM AUTHOR]

We report on the optical characterization of a nitrogen plasma source based on radiofrequency (RF) used to grow III-nitride materials by molecular beam epitaxy (MBE). Optical emission spectroscopy (OES) was used to study the nitrogen plasma response as a function of the RF power applied and the flow rate of molecular nitrogen. Analysis of the intensities of spectral signals assigned to atomic and molecular species and the ratio of these intensities is performed in detail. The OES results show that the plasma source studied is sensitive to the RF power applied to produce an atomic nitrogen signal, while varying the incoming flow impacts the signal of metastable nitrogen molecules; this outcome allows for the determination of conditions under which certain types of nitrogen species are favored over others. InN films were grown on AlN-buffered Si(111) substrates by MBE under different plasma operational parameters, where, according to the OES studies, atomic nitrogen or excited molecular nitrogen is favored in the plasma. In situ reflection high-energy electron diffraction, scanning electron microscopy, and x-ray diffraction techniques were employed to characterize the InN samples. It is found that the surface morphology of the InN films is highly sensitive to the plasma conditions. A transition in the growth mode from smooth compact films to coalesced islands and columnar structures is observed when the dominant reactive species is atomic nitrogen or excited molecular nitrogen. The results of the characterization are discussed and correlated with the reactive nitrogen species present in the plasma. [ABSTRACT FROM AUTHOR]

Epitaxial Al/GaAs/Al structures having controlled thickness of high-quality GaAs and pristine interfaces have been fabricated using a wafer-bonding technique. III–V semiconductor/Al structures are grown by molecular beam epitaxy on III–V semiconductor substrates and bonded to silicon and sapphire. Selective etching is used to remove the III–V substrate followed by surface cleaning and superconductor regrowth, resulting in epitaxial Al/GaAs/Al tri-layers on sapphire or silicon substrates. Structures are characterized with reflection high energy electron diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, transmission electron microscopy, and x-ray diffraction. Applications of these structures to the field of quantum information processing are discussed. [ABSTRACT FROM AUTHOR]

We demonstrate the epitaxial growth of (111)-oriented Sr1 − xCaxTiO3 (SCTO) thin films on (0002) GaN using a thin (100) TiO2 buffer layer by RF-plasma-assisted oxide molecular beam epitaxy. We explore the growth window of SCTO for both x = 0 (i.e., SrTiO3) and x = 0.24 with various TiO2 buffer layer thicknesses and find that a substrate temperature of 650 °C and TiO2 thickness of 1 nm produce the highest quality films, as determined by reflection high energy electron diffraction, x-ray diffraction, and atomic force microscopy. SCTO films deposited without a buffer layer are polycrystalline, confirming the necessity of the TiO2 buffer layer to facilitate (111)-oriented epitaxial growth of SCTO on GaN. Transmission electron microscopy performed on the samples shows that the SCTO films grown with a TiO2 buffer layer are highly textured, and selected-area electron diffraction patterns indicate that some domains in the oxide exhibit cation ordering along the (111) direction. We also show that the growth mode changes from 2D to 3D as the thickness of SCTO increases past 10 nm, which leads to film relaxation and a rougher surface morphology. [ABSTRACT FROM AUTHOR]

The Kagome antiferromagnet Mn 3 Sn has garnered much attention due to the presence of exciting properties such as anomalous Hall and Nernst effects. This paper discusses the synthesis of crystalline Mn 3 Sn thin films, prepared on Al 2 O 3 (0001) substrates at 453 ± 5 ° C using molecular beam epitaxy. The growth is monitored in situ using reflection high energy electron diffraction and measured ex situ using x-ray diffraction, Rutherford back-scattering, and cross-sectional scanning transmission electron microscopy. Our analysis shows the in-plane lattice constants of a 1 , M = 4.117 ± 0.027 Å and a 2 , M = 4.943 ± 0.033 Å, which is a very unexpected result when compared to the bulk a -plane Mn 3 Sn. This indicates a strain in the film and makes it challenging to provide a straightforward explanation. In an effort to explain our results, we discuss two possible orientation relationships between the Mn 3 Sn films and the sapphire substrates. Samples prepared under these conditions appear to have smooth surfaces locally, but overall the film has a 3D island morphology. First-principles calculations provide atomic models of the Mn 3 Sn (11 2 ¯ 0) lattice on Al 2 O 3 (0001) high symmetry sites, indicating that the L3-R90 ° is the most stable configuration. A detailed discussion of the experimental data and theoretical results, as well as strain effects, is provided. [ABSTRACT FROM AUTHOR]

BaTiO3 is a technologically relevant material in the perovskite oxide class with above‐room‐temperature ferroelectricity and a very large electro‐optical coefficient, making it highly suitable for emerging electronic and photonic devices. An easy, robust, straightforward, and scalable growth method is required to synthesize epitaxial BaTiO3 thin films with sufficient control over the film's stoichiometry to achieve reproducible thin film properties. Here the growth of BaTiO3 thin films by hybrid molecular beam epitaxy is reported. A self‐regulated growth window is identified using complementary information obtained from reflection high energy electron diffraction, the intrinsic film lattice parameter, film surface morphology, and scanning transmission electron microscopy. Subsequent optical characterization of the BaTiO3 films by spectroscopic ellipsometry revealed refractive index and extinction coefficient values closely resembling those of stoichiometric bulk BaTiO3 crystals for films grown inside the growth window. Even in the absence of a lattice parameter change of BaTiO3 thin films, degradation of optical properties is observed, accompanied by the appearance of a wide optical absorption peak in the IR spectrum, attributed to optical transitions involving defect states present. Therefore, the optical properties of BaTiO3 can be utilized as a much finer and more straightforward probe to determine the stoichiometry level present in BaTiO3 films. [ABSTRACT FROM AUTHOR]

The droplet epitaxy of indium gallium nitride quantum dots (InGaN QDs), the formation of In–Ga alloy droplets in ultra-high vacuum and then surface nitridation by plasma treatment, is firstly investigated by using plasma-assisted molecular beam epitaxy. During the droplet epitaxy process, in-situ reflection high energy electron diffraction patterns performs the amorphous In–Ga alloy droplets transform to polycrystalline InGaN QDs, which are also confirmed by the characterizations of transmission electron microscopy and X-ray photoelectron spectroscopy. The substrate temperature, In–Ga droplet deposition time, and duration of nitridation are set as parameters to study the growth mechanism of InGaN QDs on Si. Self-assembled InGaN QDs with a density of 1.33 × 1011 cm−2 and an average size of 13.3 ± 3 nm can be obtained at the growth temperature of 350 °C. The photoluminescence emissions of uncapped InGaN QDs in wavelength of the visible red (715 nm) and infrared region (795 and 857 nm) are observed. The formation of high-indium composition of InGaN QDs via droplet epitaxy technique could be applied in long wavelength optoelectronic devices. [ABSTRACT FROM AUTHOR]

Bismuth incorporation and surface reconstruction have been studied simultaneously during GaAsBi growth by molecular beam epitaxy by means of in situ wafer curvature monitoring and reflection high energy electron diffraction, respectively. Growth temperature and flux ratio have been varied successively. As/Ga atomic ratio close to unity has been applied for the study of the growth temperature effect. During the growth regime under the (1 × 3) reconstruction, Bi incorporation is found to be independent of the growth temperature, for temperatures where Bi desorption is insignificant. On the contrary, Bi incorporation becomes highly dependent on the growth temperature as soon as the (2 × 1) reconstruction regime is reached. Only for the lowest temperatures, the Bi incorporation reaches the same level during the (2 × 1) reconstruction than for the (1 × 3) reconstruction. When the As/Ga flux ratio is increased, the bismuth incorporation is observed to decrease for GaAsBi growth in the (2 × 1) reconstruction regime. Our results indicate that the (1 × 3) and (2 × 1) surface reconstructions are always successively observed and that an energy barrier has to overcome to transit from the (1 × 3) to the (2 × 1) reconstruction, with this mechanism being temperature dependent. Finally, a difference in surface stress with reconstruction has been identified. [ABSTRACT FROM AUTHOR]

In this work, radio-frequency (RF) plasma-induced damage to III-nitride surfaces and bulk defects is observed and mitigated. It is shown that for InN films, the surface is more sensitive to plasma-induced damage than GaN films, as observed via atomic force microscopy and reflection high energy electron diffraction. In order to isolate any possible plasma-induced damage, a growth window for InN is established, and temperature ranges are determined for other damaging effects which include roughening due to low adatom mobility, InN decomposition, and indium desorption. In situ plasma monitoring and optimization are accomplished with a combination of optical emission spectroscopy as well as a remote Langmuir probe. It is shown that by increasing the plasma nitrogen flow, the positive ion content increases; however, the ion acceleration potential reduces. Additionally, a reduced RF plasma power results in a reduction of atomic nitrogen species. These plasma species and energetic variations result in variations in the bulk unintentional background electron concentrations observed by room temperature Hall effect measurements of ∼1 μm thick InN films. By increasing the nitrogen flow from 2.5 to 7.5 sccm for a constant RF power of 350 W, the background electron concentration decreases by 74% from 1.36 × 1019 cm−3 to 3.54 × 1018 cm−3, while maintaining a smooth surface morphology. Additionally, photoluminescence spectra indicate optical emission energies shift from ∼0.81 to 0.71 eV (closer to the fundamental bandgap of InN) by limiting the damaging plasma species. Finally, conditions are presented to further minimize plasma-induced damage in III-nitride devices. [ABSTRACT FROM AUTHOR]

We study in this work the growth and fabrication of top-down highly doped n + InAs(Si)/p + GaSb(Si) Esaki tunneling diodes on (001) GaAs substrates. A careful investigation on the highly mismatched GaSb/GaAs growth is first conducted by means of Reflection High-Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM), and X-Ray Diffraction (XRD) analyses. These results are expected to pave the way to methods for III-Sb buffer layer's integration with low threading dislocation (TD) densities. A comparison between AFM, XRD, defect revealing by chemical etching and transmission electron microscopy (TEM) is then presented to calculate the precise TD density and its influence on the device structure. In the last part, we report on first operating sub-30 nm III-V vertical NW tunneling devices on (001) commercial GaAs substrates. [ABSTRACT FROM AUTHOR]

We describe a UHV setup for grazing incidence fast atom diffraction (GIFAD) experiments. The overall geometry is simply a source of keV atoms facing an imaging detector. Therefore, it is very similar to the geometry of reflection high energy electron diffraction experiments used to monitor growth at surfaces. Several custom instrumental developments are described making GIFAD operation efficient and straightforward. The difficulties associated with accurately measuring the small scattering angle and the related calibration are carefully analyzed. [ABSTRACT FROM AUTHOR]

Trace amounts of Mg deposited on a diamond (100) substrate surface facilitate the growth of cubic boron nitride (c‐BN) by ion beam‐assisted molecular beam epitaxy. Fourier transform infrared spectroscopy indicates that films grown with Mg are cubic, while those without Mg are either hexagonal BN or lacking measurable cubic or hexagonal signatures. Initiating the growth with 0.005 monolayer equivalent of Mg is sufficient to yield epitaxial films with >99% c‐BN. Reflection high energy electron diffraction, electron energy loss spectroscopy, and X‐Ray photoelectron spectroscopy indicate the surface of the film to be sp2‐bonded BN, consistent with the results of other groups. High‐resolution scanning transmission electron microscopy reveals c‐BN with a high density of stacking faults and twinning. A model is proposed by which Mg locally diminishes the energy barrier to dissociation of the as‐deposited sp2‐bonded BN, facilitating the nucleation of c‐BN. [ABSTRACT FROM AUTHOR]

(Sc2O3)x(Ga2O3)1−x was grown by molecular beam epitaxy at low temperatures (100 °C) using a variety of growth sequences to avoid surface segregation of Ga. Continuous and digital growth techniques always produced Ga segregation. This surface segregation was attributed to the stronger bond between the Sc and O compared to the Ga and O. A digital growth technique (alternate opening of Sc and Ga shutters with the O shutter open continuously during the growth) was unsuccessful in eliminating this effect. The segregation was eliminated using a growth technique in which the Ga shutter was closed for a set amount of time toward the end of the growth while the O and Sc shutters remained open. Characterization with reflection high energy electron diffraction, x-ray diffraction, and transmission electron microscopy revealed the growth of a fine-grained polycrystalline film under these conditions. A third growth technique was used that involved closing the Ga shutter for a set amount of time toward the end of the growth while the O and Sc shutters were open continuously. This technique was successful in depositing a uniform film. However, the breakdown field was only 1.40 MV/cm (at 1 mA/cm2). The addition of Ga to Sc2O3 diminished the insulating properties of the film. These initial experiments indicate that phase segregation is likely to be a major issue with most growth techniques and that alloying Ga2O3 with elements other than Sc, such as Gd or Al, might be a more successful approach. [ABSTRACT FROM AUTHOR]

I. INTRODUCTION It is an honor to dedicate this special collection to Dr. Scott A. Chambers, who has had a rewarding and impactful career in surface science, spectroscopy, and thin film synthesis. C. Beyond environmental oxides to functional materials and international impact Although Scott's research in EMSL initially focused on the model surfaces and interfaces of environmentally relevant oxides, he quickly saw the possibilities of oxides as functional materials. Scott developed an initial oxide-focused MBE system as a prototype that provided both research results and a foundation for the development of a more complete system that was designed for the EMSL facility and available when EMSL opened in 1997. [Extracted from the article]

The thin films of c -plane dominated hexagonal ζ - phase Mn 2 N were successfully grown on a cubic MgO (001) substrate directly using plasma-assisted molecular beam epitaxy. The surface was comprehensively studied through experimental and theoretical approaches. Reflection high energy electron diffraction revealed two pseudo-cubic domains along [100] MgO and [110] MgO , and one hexagonal domain, which is 30° apart from the [100] MgO direction. Scanning tunneling microscopy was used to image and resolve the high quality surface, revealing a distorted hexagonal surface structure. Furthermore, atomic resolution of the hexagonal domain with a 2 × 2 reconstructed surface is presented. For the c -plane surface, theoretical investigations were carried out using first-principle studies to determine the surface formation energy for various reconstructed surfaces. The theoretical study shows that the nitrogen terminated 2 × 2 structure with a manganese adatom on the surface is the most stable reconstruction that reproduces the experimental results. A corresponding simulated scanning tunneling microscopy model is also presented, providing strong support for the experimentally observed in-plane lattice structures. The manganese:nitrogen stoichiometry within the bulk and surface is in good agreement with the expected ratio of 2:1. [Display omitted] • Heteroepitaxial growth of Mn 2 N on cubic substrate. • Film grown on cubic substrate isn't always constrained to maintain cubic symmetry. • Crystal structure accommodated on symmetry mismatched substrate in various ways. • STM images reveal 2 × 2 reconstructed pseudo-cubic and hexagonal domains. • N-terminated Mn ad-atom model corresponds to the most stable Mn 2 N surface. [ABSTRACT FROM AUTHOR]

This article, titled "Bismuth Containing III–V Quaternary Alloy InGaAsBi Grown by MBE," provides new information on the epitaxial growth of InGaAsBi, a quaternary alloy. It references a previous publication that was the first to create InGaAsBi and presents additional findings on the growth surfaces and X-ray diffraction patterns of InGaAsBi and InGaAs layers. The article aims to contribute to the understanding of molecular beam epitaxy (MBE) and provides valuable insights into the fabrication of Bi-containing InGaAs-based alloys. [Extracted from the article]

Contradictory reports about the electronic and magnetic behavior of bulk Nd0.5Ba0.5MnO3 with uniform disorder exist in the literature. In this work, we investigated single crystalline, A-site disordered thin films of Nd0.5Ba0.5MnO3, grown on the SrTiO3 substrate by pulsed laser deposition. The epitaxial growth of these films in the layer-by-layer fashion has been confirmed by in situ reflection high energy electron diffraction, atomic force microscopy, x-ray reflectivity, and x-ray diffraction measurements. Interestingly, these films are found to be electrically insulating and exhibit spin-glass behavior at low temperatures. This offers a distinct approach to study Ba based half-doped manganites and an opportunity to further manipulate competing charge/orbital ordering and ferromagnetism in such systems, through heterostructure engineering. [ABSTRACT FROM AUTHOR]

Contradictory reports about the electronic and magnetic behavior of bulk Nd0.5Ba0.5MnO3 with uniform disorder exist in the literature. In this work, we investigated single crystalline, A-site disordered thin films of Nd0.5Ba0.5MnO3, grown on the SrTiO3 substrate by pulsed laser deposition. The epitaxial growth of these films in the layer-by-layer fashion has been confirmed by in situ reflection high energy electron diffraction, atomic force microscopy, x-ray reflectivity, and x-ray diffraction measurements. Interestingly, these films are found to be electrically insulating and exhibit spin-glass behavior at low temperatures. This offers a distinct approach to study Ba based half-doped manganites and an opportunity to further manipulate competing charge/orbital ordering and ferromagnetism in such systems, through heterostructure engineering. [ABSTRACT FROM AUTHOR]

Secondary ion mass spectroscopy, Fourier transformed infrared spectroscopy, ellipsometry, reflection high energy diffraction and transmission electron microscopy are used to gain inside into the effect of Ge on the formation of ultrathin 3C‐SiC layers on Si(111) substrates. Accompanying the experimental investigations with simulations it is found that the ultrathin single crystalline 3C‐SiC layer is formed on top of a gradient Si1–x–yGexCy buffer layer due to a complex alloying and alloy decomposition processes promoted by carbon and germanium interdiffusion and SiC nucleation. This approach allows tuning residual stress at very early growth stages as well as the interface properties of the 3C‐SiC/Si heterostructure. Useful yields of secondary ions of Ge in Si matrix and Si dimer are estimated. [ABSTRACT FROM AUTHOR]

[Display omitted] • Strontium fluoride forms weakly-interacting interfaces with Ag(1 1 1). • Almost ideal and continuous dielectric layer is formed when growth is carried out at room temperature. • A valence band offset of 6.7 eV between Ag and SrF 2 is obtained for 5 nm of dielectric film. • High temperature growth causes the formation of 3D fluoride structures. Molecular beam epitaxy (MBE) is used to grow ultrathin SrF 2 layers at different temperatures on Ag(1 1 1) epitaxial films prepared on mica. The electronic properties, structure and morphology of the films are probed by ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS) supported by ab-initio calculations, reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM). The SrF 2 films are composed of crystallites with their [1 1 1] direction parallel to the substrate normal, thereby reproducing the symmetry of the substrate. Twinned domains are also observed. At the Ag/SrF 2 interface, fluoride molecules do not dissociate and the interaction with the substrate is weak. At room temperature, fluoride crystallites merge together, resulting in a continuous film covering the entire substrate when it reaches a nominal thickness of 5 nm. Growth at higher temperature induces the formation of triangular 3D islands, leaving sizable portions of the Ag substrate uncovered. At 400 °C, also a small fraction of SrF 2 (0 0 1)-oriented crystallites are observed, with their [1 −1 0] axis rotated by about 30° with respect to the [1 −1 0] substrate direction. [ABSTRACT FROM AUTHOR]

Terahertz (THz) emission increase is observed for GaAs thin films that exhibit structural defects. The GaAs epilayers are grown by molecular beam epitaxy on exactly oriented Si (100) substrates at three different temperatures (Ts = 320 °C, 520 °C and 630 °C). The growth method involves the deposition of two low-temperature-grown (LTG)-GaAs buffers with subsequent in-situ thermal annealing at Ts = 600 °C. Reflection high energy electron diffraction confirms the layer-by-layer growth mode of the GaAs on Si. X-ray diffraction shows the improvement in crystallinity as growth temperature is increased. The THz time-domain spectroscopy is performed in reflection and transmission excitation geometries. At Ts = 320 °C, the low crystallinity of GaAs on Si makes it an inferior THz emitter in reflection geometry, over a GaAs grown at the same temperature on a semi-insulating GaAs substrate. However, in transmission geometry, the GaAs on Si exhibits less absorption losses. At higher Ts, the GaAs on Si thin films emerge as promising THz emitters despite the presence of antiphase boundaries and threading dislocations as identified from scanning electron microscopy and Raman spectroscopy. An intense THz emission in reflection and transmission excitation geometries is observed for the GaAs on Si grown at Ts = 520 °C, suggesting the existence of an optimal growth temperature for GaAs on Si at which the THz emission is most efficient in both excitation geometries. The results are significant in the growth design and fabrication of GaAs on Si material system intended for future THz photoconductive antenna emitter devices. [ABSTRACT FROM AUTHOR]

Single-crystalline membranes of functional materials enable the tuning of properties via extreme strain states; however, conventional routes for producing membranes require the use of sacrificial layers and chemical etchants, which can both damage the membrane and limit the ability to make them ultrathin. Here we demonstrate the epitaxial growth of the cubic Heusler compound GdPtSb on graphene-terminated Al2O3 substrates. Despite the presence of the graphene interlayer, the Heusler films have epitaxial registry to the underlying sapphire, as revealed by x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy. The weak Van der Waals interactions of graphene enable mechanical exfoliation to yield free-standing GdPtSb membranes, which form ripples when transferred to a flexible polymer handle. Whereas unstrained GdPtSb is antiferromagnetic, measurements on rippled membranes show a spontaneous magnetic moment at room temperature, with a saturation magnetization of 5.2 bohr magneton per Gd. First-principles calculations show that the coupling to homogeneous strain is too small to induce ferromagnetism, suggesting a dominant role for strain gradients. Our membranes provide a novel platform for tuning the magnetic properties of intermetallic compounds via strain (piezomagnetism and magnetostriction) and strain gradients (flexomagnetism). Single crystalline membranes enable the tuning of materials properties via strain states that are not accessible to bulk crystals or epitaxially clamped films. Here, the authors demonstrate the synthesis and strain gradient-induced magnetism in membranes of the Heusler compound GdPtSb. [ABSTRACT FROM AUTHOR]

The accurate control of the crystal phase in III–V semiconductor nanowires (NWs) is an important milestone for device applications. Although cubic zinc-blende (ZB) GaAs is a well-established material in microelectronics, the controlled growth of hexagonal wurtzite (WZ) GaAs has thus far not been achieved successfully. Specifically, the prospect of growing defect-free and gold catalyst-free wurtzite GaAs would pave the way towards integration on silicon substrate and new device applications. In this article, we present a method to select and maintain the WZ crystal phase in self-assisted NWs by molecular beam epitaxy. By choosing a specific regime where the NW growth process is a self-regulated system, the main experimental parameter to select the ZB or WZ phase is the V/III flux ratio. Using an analytical growth model, we show that the V/III flux ratio can be finely tuned by changing the As flux, thus driving the system toward a stationary regime where the wetting angle of the Ga droplet can be maintained in the range of values allowing the formation of pure WZ phase. The analysis of the in situ reflection high energy electron diffraction evolution, combined with high-resolution scanning transmission electron microscopy (TEM), dark field TEM, and photoluminescence all confirm the control of an extended pure WZ segment, more than a micrometer long, obtained by molecular beam epitaxy growth of self- assisted GaAs NWs with a V/III flux ratio of 4.0. This successful controlled growth of WZ GaAs suggests potential benefits for electronics and opto-electronics applications. [ABSTRACT FROM AUTHOR]

Molecular beam epitaxy (MBE) is the best suited technique to engineer perovskite oxide properties, thanks to individual atom evaporation from elemental sources. Unfortunately, significant source drift often prevents exploiting this advantage, and improving the control of the composition of MBE grown oxide thin layers remains a challenge. In this context, in situ reflection high energy electron diffraction (RHEED) has long been identified as a useful tool, as surface reconstructions and RHEED oscillations depend on oxide cationic composition. We show here that monitoring the appearance of half-order streaks along the [210] RHEED azimuths of Ti-rich surfaces provides enhanced control of the cationic composition of SrTiO3 thin layers as compared to the more common strategy relying on [100] azimuths monitoring. We also provide quantitative evaluation of the uncertainty on composition control enabled by this method, namely ±6.7%. In the end, we describe an original procedure to control the composition of perovskite oxide quaternary solid solutions. [ABSTRACT FROM AUTHOR]

Epitaxial SrTiO3 thin films were grown by molecular beam epitaxy (MBE) using an oxygen plasma source. SrO and TiO2 layers were alternately deposited on 0.5 wt% (001) Nb doped SrTiO3 substrates whose surface were terminated with TiO2. A two-dimensional (2-D) layer-by-layer growth mode was confirmed by the reflection high energy electron diffraction (RHEED) oscillation during the growth and the well-terraced SrTiO3 surface was obtained after growth. X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) analysis showed that the obtained SrTiO3 thin films had epitaxially (001) oriented and stoichiometric crystal structures without any interfacial layer. With simple MIS (Metal-Insulator-Semiconductor) capacitors using Ir top electrodes, the dielectric constants of the SrTiO3 thin film with 10, 15, 20, 100 nm were measured at room temperature, 10 kHz and zero bias. [ABSTRACT FROM AUTHOR]

Atomically smooth (001) surfaces of SrTiO3 cut from the high-quality single crystals at two different miscut angles 0.9 and 7.0 deg between the real flat surfaces and crystallographic planes (001) were analyzed by means of the reflection high energy electron diffraction (RHEED) method from the room down to liquid helium temperatures. The diffraction patterns typical of the RHEED geometry close to ideal for a small miscut angle and those exhibiting distinct features of the specific periodicity associated with regular steps, which form due to the larger miscut angle, are presented. The surface symmetry and energetics were shown to impose differences in lattice parameters in parallel to a surface with respect to the bulk. This effect results in weak incommensurability between lattice parameters in surface layers and the bulk values relaxed due to step edges and misfit dislocations. The special surface nanostructures formed on the etched surfaces treated in chemically active solvents are also analyzed. The intrinsic nanostructures on single crystal surfaces open new prospects for applications when the ordered and controlled nanoscale architecture is required. [ABSTRACT FROM AUTHOR]

A heterostructure containing several sheets of CdSe fractional monolayer quantum dots with nominal coverages of 0.5 and close to 0.25 of a monolayer (ML) embedded in a ZnSe matrix was successfully grown by a combination of submonolayer epitaxy and molecular beam epitaxy. Due to surface reconstruction properties of cation- and anion-terminated surfaces, one complete Cd–Se alternate source exposure cycle produces a submonolayer saturation coverage of around 0.5 ML of CdSe. To obtain specific coverages lower than 0.5 ML, it is necessary to work with an unsaturated Cd surface, then the control of Cd exposure time becomes crucial. To determine the appropriate Cd exposure times, a careful study of the adsorption process and knowledge of the time to obtain the saturation coverage are required. Four separated sheets of CdSe fractional monolayer quantum dots with coverage Θ ≃ 0.5 ML were deposited in the first section of the heterostructure to analyze the surface reconstruction streaks in the reflection high energy electron diffraction (RHEED) patterns. The analysis performed during the adsorption process to reach a Cd saturated surface (nominal coverage Θ = 0.5 ML) suggests that the large Cd atoms produce strain-induced disorder in the underlying layers during the initial stages of the Cd chemisorption process. The results of the RHEED study were employed to estimate the Cd exposure time for the growth of the last sheet of CdSe fractional monolayer quantum dots with Θ ≃ 0.25 ML. The low temperature photoluminescence spectrum presented two intense and narrow excitonic peaks at energies of 2.750 eV and 2.778 eV, corresponding to the CdSe fractional monolayer quantum dots with nominal coverages of 0.5 ML and ∼0.25 ML, respectively, with narrower linewidth for lower CdSe coverage. [ABSTRACT FROM AUTHOR]

We report on the van der Waals epitaxy of high-quality single-crystalline AlN and the demonstration of AlGaN tunnel junction deep-ultraviolet light-emitting diodes directly on graphene, which were achieved by using plasma-assisted molecular beam epitaxy. It is observed that the substrate/template beneath graphene plays a critical role in governing the initial AlN nucleation. In situ reflection high energy electron diffraction and detailed scanning transmission electron microscopy studies confirm the epitaxial registry of the AlN epilayer with the underlying template. Detailed studies further suggest that the large-scale parallel epitaxial relationship for the AlN epilayer grown on graphene with the underlying template is driven by the strong surface electrostatic potential of AlN. The realization of high-quality AlN by van der Waals epitaxy is further confirmed through the demonstration of AlGaN deep-ultraviolet light-emitting diodes operating at ∼260 nm, which exhibit a maximum external quantum efficiency of 4% for an unpackaged device. This work provides a viable path for the van der Waals epitaxy of ultra-wide bandgap semiconductors, providing a path to achieve high performance deep-ultraviolet photonic and optoelectronic devices that were previously difficult. [ABSTRACT FROM AUTHOR]

We studied the thermal transport across a GaAs/AlGaAs interface using time-resolved Reflection High Energy Electron Diffraction. The lattice temperature change of the GaAs nanofilm was directly monitored and numerically simulated using diffusive heat equations based on Fourier's Law. The extracted thermal boundary resistances (TBRs) were found to decrease with increasing lattice temperature imbalance across the interface. The TBRs were found to agree well with the Diffuse Mismatch Model in the diffusive transport region, but showed evidence of further decrease at temperatures higher than Debye temperature, opening up questions about the mechanisms governing heat transfer at interfaces between very similar semiconductor nanoscale materials under highly non-equilibrium conditions. [ABSTRACT FROM AUTHOR]

High entropy oxides (HEOs) are a class of materials, containing equimolar portions of five or more transition metal and/or rare-earth elements. We report here about the layer-by-layer growth of HEO [( La 0.2 Pr 0.2 Nd 0.2 Sm 0.2 Eu 0.2 )NiO3] thin films on NdGaO3 substrates by pulsed laser deposition. The combined characterizations with in situ reflection high energy electron diffraction, atomic force microscopy, and x-ray diffraction affirm the single crystalline nature of the film with smooth surface morphology. The desired +3 oxidation of Ni has been confirmed by an element sensitive x-ray absorption spectroscopy measurement. Temperature dependent electrical transport measurements revealed a first order metal-insulator transition with the transition temperature very similar to the undoped NdNiO3. Since both these systems have a comparable tolerance factor, this work demonstrates that the electronic behaviors of A-site disordered perovskite-HEOs are primarily controlled by the average tolerance factor. [ABSTRACT FROM AUTHOR]

AlGaN alloys find important applications in UV emitters and detectors, as well as in high-power high-frequency electronics. While reflection high energy electron diffraction (RHEED) is a standard technique for in situ monitoring of the growth of AlGaN alloys by plasma assisted molecular beam epitaxy, this paper investigates a new mode of its application. During the growth of AlGaN alloys, the ratio of the group III (Al + Ga) to group V (active nitrogen) adatoms critically controls the materials property of AlGaN films and is optimal within a very narrow window of operation. Moreover, this ratio is dependent in a complex fashion on various growth parameters, including substrate temperature, and is difficult to determine quantitatively in real time. This paper provides a method to estimate that important parameter. This can be carried out through the capture of the RHEED image from the fluorescent screen using an inexpensive video camera setup and a simple analysis procedure. While most RHEED analyses focus on the diffraction pattern, e.g., the line spacing, this work quantifies diffused scattering of the electron beam from a metallic layer that forms on the top of the growth surface during deposition under excess group III conditions, which is typically employed during growth. Two alternate methods for data analysis have been explored and compared. The results indicate that this process can qualitatively trace the variation of the thickness of the thin metallic layer, and hence the group III to group V flux ratio, for different substrate temperatures. This technique being simple, fast, and cost-effective can be incorporated into standard MBE systems for real-time in situ characterization of AlGaN alloys. [ABSTRACT FROM AUTHOR]

We study the formation of nanoholes and quantum rings in GaAs and AlGaAs surfaces by local droplet etching (LDE) with Ga and In. The quantum rings are formed by the droplet etching process and surround the nanohole openings. Our data show that a low As pressure is essential for LDE and that process conditions with high Ga coverage yield formation of additional hillocks or large hills. With atomic force microscopy we establish that the amount of material removed from the nanoholes is equal to the amount of material stored in the quantum rings. Basing on the experimental observations, we propose a model of nanohole and quantum ring formation. [ABSTRACT FROM AUTHOR]

Thin GaAs films grown on lattice-matched In0.53Ga0.47As/InP(001) have been imaged by in vacuo scanning tunneling microscopy. We observe that the morphological evolution of these strained films depends on the deposition and diffusion of group III adatoms and the incorporation of As from the vapor, in addition to Asaro–Tiller–Grinfeld instabilities. We compare step edge densities to a model for the Ga adatom density and find that the absolute magnitude of the group V and group III fluxes has a strong effect on the number of adatoms on the surface and, thus, the morphology of the films. [ABSTRACT FROM AUTHOR]

A method to form epitaxial Ge1-xSnx quantum dots (QDs) on Si (111) substrates has been developed by codeposition of Ge and Sn on ultrathin SiO2 films with predeposited Ge nuclei. Hemispherical Ge1-xSnx QDs with an ultrahigh density (∼1012 cm-2) were epitaxially grown in the nanometer-size range. The QD size was controlled by changing the GeSn deposition amount. High-resolution transmission electron microscopy observations revealed that the main formed Ge1-xSnx QDs had less strain and no misfit dislocations. [ABSTRACT FROM AUTHOR]

The growth of indium on a vicinal Si(100)-(2×1) surface at room temperature by femtosecond pulsed laser deposition (fsPLD) was investigated by in situ reflection high-energy electron diffraction (RHEED). Recovery of the RHEED intensity was observed between laser pulses and when the growth was terminated. The surface diffusion coefficient of deposited In on initial two-dimensional (2D) In-(2×1) layer was determined. As growth proceeds, three-dimensional In islands grew on the 2D In-(2×1) layer. The RHEED specular profile was analyzed during film growth, while the grown In islands were examined by ex situ atomic force microscopy. The full width at half maximum of the specular peak decreased during the deposition, indicative of well-ordered growth and an increase of the island size. The In islands developed into elongated-polyhedral, circular, and triangular shapes. The elongated and triangular islands were highly oriented, parallel and perpendicular to the surface terrace edges, while the circular islands show a top flat surface. Deposition of In on Si(100)-(2×1) by fsPLD influenced the formation of the initial In-(2×1) layer and the morphology of the grown islands. [ABSTRACT FROM AUTHOR]

The heteroepitaxial growth of InAs on InP(001) by solid source molecular beam epitaxy has been studied for a range of different growth temperatures and annealing procedures. Atomic force microscopy images show that nanowires are formed for deposition in the temperature range of 400–480 °C, and also following high temperature annealing (480 °C) after deposition at 400 °C. The wires show preferential orientation along <110> and often exhibit pronounced serpentine behavior due to the presence of kinks, an effect that is reduced at increasing growth temperature. The results suggest that the serpentine behavior is related to the degree of initial surface order. Kinks in the wires appear to act as nucleation centers for In adatoms migrating along the wires during annealing, leading to the coexistence of large three-dimensional islands. [ABSTRACT FROM AUTHOR]

Significant surface pitting and a degraded surface roughness are almost always observed on GaAs (100) surface after conventional thermal oxide desorption. Here we report on the use of a Ga-triggered low temperature oxide desorption method that can be used to preserve the atomic monolayer (ML) steps. By providing an external supply of atomic Ga at a relatively low substrate temperature of ∼450 °C without an As4 overpressure, this technique resulted in an atomically smooth GaAs ML steps with a root mean square roughness of 0.25 nm, nearly identical to as-grown GaAs surface (0.2 nm). The demonstrated results show the potential for applications in optoelectronics such as regrowth and patterned substrate growth. [ABSTRACT FROM AUTHOR]