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Mo2TiC2 is the Ordered Double Transitional Metal Layered Carbides (ODTMLC) derived from its parent MAX phases Mo2TiAlC2 by a wet chemical etching. Its oxidation was done by a new ablated plasma thrust method in which the MXenes were at 750 °C under an oxygen background in the pulsed laser deposition chamber. The reflective high electron energy diffraction technique assures the oxidation at the ambient gas pressure p = 0.1 mbar, which was described in the previous paper. The obtained Mo2TiC2O2 was transferred for their topological test under angle-resolved photoemission spectroscopy, circular dichroism test, and Chemical Potential (CP) analysis. An indirect energy bandgap of 125 meV was obtained. The sine function of α along with period π and β with period 2π shows that there is a possibility of helical spin textures in both α (electron-like pocket around Γ ̄ ) and β (elliptical electron-like pocket around M ̄ ). The CP analysis shows the possibility of at least 100 meV bandgap creation on a single surface so that the surface charges will flow without any effect of bulk. The Mo2TiC2O2 can be used as topological insulating material. [ABSTRACT FROM AUTHOR]

Ultrafast electron diffraction (UED) stands as a powerful technique for real-time observation of structural dynamics at the atomic level. In recent years, the use of MeV electrons from radio frequency guns has been widely adopted to take advantage of the relativistic suppression of the space charge effects that otherwise limit the temporal resolution of the technique. Nevertheless, there is not a clear choice for the optimal energy for a UED instrument. Scaling to beam energies higher than a few MeV does pose significant technical challenges, mainly related to the inherent increase in diffraction camera length associated with the smaller Bragg angles. In this study, we report a solution by using a compact post-sample magnetic optical system to magnify the diffraction pattern from a crystal Au sample illuminated by an 8.2 MeV electron beam. Our method employs, as one of the lenses of the optical system, a triplet of compact, high field gradients (>500 T/m), small-gap (3.5 mm) Halbach permanent magnet quadrupoles. Shifting the relative position of the quadrupoles, we demonstrate tuning the magnification by more than a factor of two, a 6× improvement in camera length, and reciprocal space resolution better than 0.1 Å−1 in agreement with beam transport simulations. [ABSTRACT FROM AUTHOR]

Mo2TiC2 is the Ordered Double Transitional Metal Layered Carbides (ODTMLC) derived from its parent MAX phases Mo2TiAlC2 by a wet chemical etching. Its oxidation was done by a new ablated plasma thrust method in which the MXenes were at 750 °C under an oxygen background in the pulsed laser deposition chamber. The reflective high electron energy diffraction technique assures the oxidation at the ambient gas pressure p = 0.1 mbar, which was described in the previous paper. The obtained Mo2TiC2O2 was transferred for their topological test under angle-resolved photoemission spectroscopy, circular dichroism test, and Chemical Potential (CP) analysis. An indirect energy bandgap of 125 meV was obtained. The sine function of α along with period π and β with period 2π shows that there is a possibility of helical spin textures in both α (electron-like pocket around Γ ̄ ) and β (elliptical electron-like pocket around M ̄ ). The CP analysis shows the possibility of at least 100 meV bandgap creation on a single surface so that the surface charges will flow without any effect of bulk. The Mo2TiC2O2 can be used as topological insulating material. [ABSTRACT FROM AUTHOR]

In this study, we present a significant advance in the growth of epitaxial GeTe-Sb2Te3 alloys on Si(111)-(√3 × √3)R30°-Sb reconstructed surface by means of Molecular Beam Epitaxy. By employing X-ray diffraction and Raman spectroscopy, we show how phase, composition, and vacancy ordering can be tailored by tuning the growth parameters such as Ge and Te elemental fluxes as well as the substrate temperature. The effect of each parameter on the GeSbTe composition and phase is also discussed. A very surprising finding is that GeSbTe alloys are more ordered when grown at lower substrate temperatures. In addition, it was possible to fabricate ordered GeSbTe even for substrate temperatures as low as 120 °C. In situ high-energy electron diffraction is employed to monitor the crystallinity and surface roughness of GeSbTe films at different stages of growth. Thus, we identify the deposition parameter ranges whereby high structural quality GeSbTe with flat surfaces can be obtained. [ABSTRACT FROM AUTHOR]

To facilitate future novel devices incorporating rare earth metal films and III-V semiconductors on Si substrates, this study investigates the mechanisms of growth via molecular beam epitaxy of gadolinium (Gd) on aluminum nitride (AlN) by determining the impact of substrate temperature on microstructure. The Gd films underwent extensive surface analysis via in situ reflective high energy electron diffraction (RHEED) and ex-situ SEM and AFM. Characterization of the surface features of rare earth metal films is important, as surface geometry has been shown to strongly impact magnetic properties. SEM and AFM imaging determined that Gd films grown on AlN (0001) from 80 °C to 400 °C transition from wetting, nodular films to island–trench growth mode to reduce in-plane lattice strain. XRD and Raman spectroscopy of the films revealed that they were primarily comprised of GdN, Gd and Gd2O3. The samples were also analyzed by a vibrating sample magnetometer (VSM) at room temperature. From the room temperature magnetic studies, the thick films showed superparamagnetic behavior, with samples grown between 240 °C and 270 °C showing high magnetic susceptibility. Increasing GdN (111) 2θ peak position and single-crystal growth modes correlated with increasing peak magnetization of the thin films, suggesting that lattice strain in single-crystal films was the primary driver of enhanced magnetic susceptibility. [ABSTRACT FROM AUTHOR]

In this work, we present a systematic study of the effect of the stoichiometry of BaTiO3 (BTO) films grown on the Ge(001) substrate by molecular-beam-epitaxy using different characterization methods relying on beam diffraction, including reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD), and selected-area electron diffraction in transmission electron microscopy. Surprisingly, over a wide range of [Ba]/[Ti] ratios, as measured by the Rutherford backscattering spectrometry, all the BTO layers exhibit the same epitaxial relationship h100iBTO(001)// 〈110〉Ge(001) with the substrate, describing a 45° lattice rotation of the BTO lattice with respect to the Ge lattice. However, varying the [Ba]/[Ti] ratio does change the diffraction behavior. From RHEED patterns, we can derive that excessive [Ba] and [Ti] generate twinning planes and a rougher surface in the non-stoichiometric BTO layers. XRD allows us to follow the evolution of the lattice constants as a function of the [Ba]/[Ti] ratio, providing an option for tuning the tetragonality of the BTO layer. In addition, we found that the intensity ratio of the 3 lowest-order Bragg peaks I(001)/ I(002), I(101)/I(002), and I(111)/I(002) derived from ω-2θ scans characteristically depend on the BTO stoichiometry. To explain the relation between observed diffraction patterns and the stoichiometry of the BTO films, we propose a model based on diffraction theory explaining how excess [Ba] or [Ti] in the layer influences the diffraction response. [ABSTRACT FROM AUTHOR]

A clear knowledge of structures is essential to the understanding and potential control of complex interfacial phenomena that involve multiple intermolecular and surface interactions of different strengths. Molecules with the ability to form hydrogen bonds are often of particular interest. Here, we report the observation of 2- and 3-dimensional ordered assemblies of methanol molecules on hydrophobic silicon surfaces, using reflection high-energy electron diffraction. Direct structure probing reveals that the crystallization temperatures and the structural transformations of the hydrogenbonded networks are far beyond a single-stage description and strongly depend on the thermal annealing procedures used. Such results elucidate the unique self-assembling behavior of interfacial methanol even without much guidance from the smooth substrate. [ABSTRACT FROM AUTHOR]

The odd parity nature of 4f states characterized by strong spin–orbit coupling and electronic correlations has led to a search for novel topological phases among rare earth compounds, such as Kondo systems, heavy Fermions, and homogeneous mixed-valent materials. Our target system is thulium telluride thin films whose bandgap is expected to be tuned as a function of lattice parameter. We systematically investigate the growth conditions of TmxTey thin films on SrF2 (111) substrates by molecular beam epitaxy. The ratio between Te and Tm supply was precisely tuned, resulting in two different crystalline phases, which were confirmed by x-ray diffraction and x-ray photoemission spectroscopy. By investigating the crystalline quality as a function of the substrate temperature, the optimal growth conditions were identified for the desired Tm1Te1 phase. Additional low energy electron diffraction and reflective high energy electron diffraction measurements confirm the epitaxial growth of TmTe layers. X-ray reflectivity measurements demonstrate that homogeneous samples with sharp interfaces can be obtained for varied thicknesses. Our results provide a reliable guidance to prepare homogeneous high-quality TmTe thin films and thus serve as a basis for further electronic investigations. [ABSTRACT FROM AUTHOR]

M′2M″xXyene (M′ and M″ are the early transitional metals and X is carbide with x = 1 for y = 2 and x = 2 for y = 3) are the ordered double transitional metal layered carbides derived from their parent MAX phases M′2M″xAlXyene by a wet chemical etching method. Their oxides are predicted to have topological properties for which they should be annealed at around 800 °C in an oxygen background. This paper includes the new ablated plasma thrust method for the ionization and adsorption of oxygen on the M′2M″xXyene substrate in the pulsed laser deposition chamber. We have found that the background pressure has a negative effect and the substrate temperature has a positive effect on plume expansion. The density profile of the background gas is highly affected by deposition temperature. Similarly, it is found that the density of plasma generated by longer wavelengths is not affected significantly due to the inverse bremsstrahlung process. A shorter wavelength produces the bremsstrahlung process as photoionization takes place. At a certain time (200 ns), the pressure of background gas and plasma pressure are equal (snow-plogh effect) so that all the wavelengths then produce electrons (highest for shorter wavelengths), thereby increasing its density. The energy necessary for the oxidation of the substrate is provided by the energy of the ablated species. The adsorption is assured by the reflective high electron energy diffraction technique, and it is found that the ambient gas pressures p = 0.1 mbar and 0.2 mbar are appropriate for the adsorption process. The obtained M′2M″xXyene oxides can be used for their topological test. [ABSTRACT FROM AUTHOR]

In this work, through a comparative study of self-assembled (SA) and selective area grown (SAG) (In)GaN nanocolumn (NC) ensembles, we first give a detailed insight into improved crystallographic uniformity (homogeneity of crystallographic tilts and twists) of the latter ones. The study, performed making use of: reflective high energy electron diffraction, X-ray diffraction and scanning electron microscopy, reveals that unlike their SA counterparts, the ensembles of SAG NCs show single epitaxial relationship to both sapphire(0001) and Si(111) underlying substrates. In the second part of the article, making use of X-ray diffraction, we directly show that the selective area growth leads to improved compositional uniformity of InGaN NC ensembles. This further leads to improved spectral purity of their luminescence, as confirmed by comparative macro-photoluminescence measurements performed on SA and SAG InGaN NC ensembles. An improved crystallographic uniformity of NC ensembles facilitates their integration into optoelectronic devices, whereas their improved compositional uniformity allows for their employment in single-color optoelectronic applications. [ABSTRACT FROM AUTHOR]

In recent experiments, clear transitions in velocity and trail width of an arc spot initiated on nanostructured tungsten were observed on the boundary of the thick and thin nanostructured layer regions. The velocity of arc spot was significantly decreased on the thick nanostructured region. It was suggested that the grouping decreased the velocity of arc spot. In this study, we try to explain the phenomena using a simple random walk model that has properties of directionality and self-avoidance. And grouping feature was added by installing an attractive force between spot cells with dealing with multi-spots. It was revealed that an entanglement of arc spot cells decreased the spot velocity, and spot cells tend to stamp at the same location many times. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials are among the most studied ones nowadays, because of their unique properties. They are made of single- or few-atom-thick layers whose variation in the stacking sequence may result in a variety of crystallographic structures, whether they are assembled by van der Waals forces or covalently bonded. Although identifying both the number of layers and the stacking sequence is of an utmost importance because of the driving role these parameters have on the properties, there is currently no technique available to do so. We demonstrate here that combining low energy (1–10 keV) electron diffraction with the usual high energy (> 50 keV) electron diffraction on the same 2D object is able to fill the gap. We illustrate this by taking the examples of a variety of 2D materials, built from either a single type of atom with low Z-number such as graphene (C), or two types of atoms with low Z-number such as diamane (C2H), or two types of atoms with high Z-number such as MoS2. Meanwhile, we propose a simplified method for comparing calculated patterns to experimental ones, and discuss the limitation of the technique. [ABSTRACT FROM AUTHOR]

Self-assembled AlN nanowires (NWs) are grown by plasma-assisted molecular beam epitaxy (PAMBE) on SiO2/Si (111) substrates. Using a combination of in situ reflective high energy electron diffraction and ex situ x-ray diffraction (XRD), we show that the NWs grow nearly strain-free, preferentially perpendicular to the amorphous SiO2 interlayer and without epitaxial relationship to Si(111) substrate, as expected. Scanning electron microscopy investigation reveals significant NWs coalescence, which results in their progressively increasing diameter and formation of columnar structures with non-hexagonal cross-section. Making use of scanning transmission electron microscopy (STEM), the NWs initial diameters are found in the 20–30 nm range. In addition, the formation of a thin (≈30 nm) polycrystalline AlN layer is observed on the substrate surface. Regarding the structural quality of the AlN NWs, STEM measurements reveal the formation of extended columnar regions, which grow with a virtually perfect metal-polarity wurtzite arrangement and with extended defects only sporadically observed. Combination of STEM and electron energy loss spectroscopy reveals the formation of continuous aluminum oxide (1–2 nm) on the NW surface. Low temperature photoluminescence measurements reveal a single near-band-edge (NBE) emission peak, positioned at 6.03 eV (at 2 K), a value consistent with nearly zero NW strain evidenced by XRD and in agreement with the values obtained on AlN bulk layers synthesized by other growth techniques. The significant full-width-at-half-maximum of NBE emission, found at ≈20 meV (at 2 K), suggests that free and bound excitons are mixed together within this single emission band. Finally, the optical properties of the hereby reported AlN NWs grown by PAMBE are comprehensively compared to optical properties of bulk, epitaxial and/or columnar AlN grown by various techniques such as: physical vapor transport, metal organic vapor phase epitaxy, metal organic chemical vapor deposition and molecular beam epitaxy. [ABSTRACT FROM AUTHOR]

We perform in situ synchrotron x-ray ray diffraction (SXRD)/reflection high energy electron diffraction (RHEED) studies on the growth of complex oxide thin films by molecular beam epitaxy. The unique deposition chamber, located at the Advanced Photon Source, allows the preparation of complex oxide samples with monolayer precision and facilitates the formation of direct correlations between in situ x-ray studies and the more prevalent RHEED investigations. Importantly, because SXRD and RHEED probe different atomic-scale processes during thin film synthesis, their concomitant use enables the extraction of details concerning growth behavior that one cannot determine from either probe alone. We describe the results of such in situ studies on the epitaxial growth of perovskite LaNiO3 on (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001). We find that during the earliest stages of growth, the RHEED and x-ray signals do not agree with each other, demonstrating that while regular RHEED oscillations may imply high quality growth, the film–substrate interface can undergo significant changes during deposition due to the occurrence of interdiffusion at the growth temperature. [ABSTRACT FROM AUTHOR]

In this work, the authors report the use of pulsed laser deposition to fabricate nickel oxide layers on silicon and assess the applicability of this structure as a hole selective contact. Films were deposited at temperatures of 300, 500, 700, and 900 °C. Results from reflective high energy electron diffraction patterns indicate increasing crystallinity with temperature up to 500 °C. However, layers deposited at 900 °C are amorphous. Nevertheless, results from atomic force microscopy, contact resistance measurements, and photoluminescence lifetime imaging indicate that films deposited at 900 °C have the best film smoothness, lowest specific contact resistivity, and highest lifetimes, and therefore, they are best suited for hole selective contact applications. Kelvin probe force microscopy and x-ray photoelectron spectroscopy were also carried out to study the effects of deposition temperature. [ABSTRACT FROM AUTHOR]

The structural evolution, epitaxy, and sublimation temperature of silver nanoclusters (NCs) on TiO2 (110) surfaces prepared in two different ways are reported here based on a combination of in situ reflection high energy electron diffraction characterization and ex situ electron imaging. It is shown that silver NCs deposited at room temperature on oxidized TiO2 (110) surfaces are unable to form a single epitaxy prior to sublimation. When heated close to sublimation, two particle orientations dominate: (111)Ag∥(110)TiO2, [-110]Ag∥[001]TiO2 and (112)Ag∥(110)TiO2, [-110]Ag∥[001]TiO2. Single twinned silver NCs are found to be stable even at temperatures close to sublimation. On the other hand, silver NCs prepared similarly on reduced TiO2 (110) surfaces behave very differently when heated to higher temperatures. On the reduced surface, the NCs are able to evolve into a single epitaxy-(111)Ag∥(110)TiO2, [-110]Ag∥[001]TiO2. The sublimation temperature for silver NCs on the reduced surface is found to be less than those on the oxidized surface by about 35°. The epitaxy formed by annealing is the same as the one formed by depositing silver onto reduced TiO2 (110) at 350 °C (>0.5 Tm of Ag). [ABSTRACT FROM AUTHOR]

The angular dependence of the x-ray fluorescence from Y and Mn inner K shell ionization induced by a grazing angle incident electron beam in several multilayered Y/Mn structures was analyzed using the distorted-wave approximation. Estimates of structural parameters, thickness, and interfacial roughness were obtained from the x-ray fluorescence measured in the total reflection regime and compared with ex situ x-ray reflectivity results. Although the sensitivity to the surface and interface roughness was relatively low, the thickness values for both single and buried layers were in good agreement with the x-ray reflectivity measurements. [ABSTRACT FROM AUTHOR]

We investigated the growth mechanisms of (110) SnO2 on (110) TiO2 by varying tin fluxes under a fixed oxygen pressure by plasma-assisted molecular beam epitaxy. A growth diagram was constructed that consists of two distinct growth regimes: an increase in growth rates in an oxygen-rich environment and a decrease in growth rates in a tin-rich environment. The excess tin played an important role for the unexpected decreasing growth rate in a tin-rich regime. Without a buildup of the tin adlayer coverage, the accumulation of macroscopic tin droplets on the SnO2 film surface was confirmed by the spotty high energy electron diffraction pattern and the absence of intensity oscillations. No SnO2 growth was observed when the impinging tin flux was larger than twice the stoichiometric tin flux, suggesting that all active oxygen atoms were consumed by evaporated tin atoms to form volatile tin suboxide (SnO). The formation of volatile SnO, causing the decreasing growth rate in a tin-rich condition, was monitored by in situ quadrupole mass spectrometry. [ABSTRACT FROM AUTHOR]

An in-depth model for the transient behavior of metal atoms adsorbed on the surface of GaN is developed. This model is developed by qualitatively analyzing transient reflection high energy electron diffraction (RHEED) signals, which were recorded for a variety of growth conditions of GaN grown by molecular-beam epitaxy (MBE) using metal-modulated epitaxy (MME). Details such as the initial desorption of a nitrogen adlayer and the formation of the Ga monolayer, bilayer, and droplets are monitored using RHEED and related to Ga flux and shutter cycles. The suggested model increases the understanding of the surface kinetics of GaN, provides an indirect method of monitoring the kinetic evolution of these surfaces, and introduces a novel method of in situ growth rate determination. [ABSTRACT FROM AUTHOR]

Radiation damage induced by high-energy (200 keV) electron irradiation in zircon has been studied thoroughly using imaging, diffraction, and electron energy-loss spectroscopy techniques in transmission electron microscopy. Both structural and compositional changes during the damage were measured using the above techniques in real time. It was found that the damage was mainly caused by the preferential sputtering of O. The loss of O occurred initially within small sporadic regions with dimension of several nanometers, resulting in the direct transformation of zircon into ZrxSiy. These isolated patches gradually connect each other and eventually cover the whole area of the electron beam. These differ from the previous observations either in the self-irradiated natural and synthetic zircon or in ion-beam irradiated thin zircon specimen. [ABSTRACT FROM AUTHOR]

We report an in situ study of epitaxial island formation and alloying of FePd nanoparticles using reflection high-energy electron diffraction (RHEED) followed by ex situ atomic structure characterization. We used aberration corrected (AC) scanning transmission electron microscopy (STEM) with a high-angle annular dark field (HAADF) detector for the study of chemical ordering. The FePd nanoparticles were formed by sequential deposition of Pd and Fe onto NaCl substrates, which was then followed by heating to high temperatures to promote chemical ordering during a short period of annealing. We show that the epitaxial island growth and subsequent alloying of Fe into Pd can be clearly detected by RHEED. The high resolution and the atomic number (Z) contrast of AC HAADF-STEM allow a clear observation of nanometer-sized local chemical order in very small nanoparticles. The results revealed complex chemical structures at an intermediate stage of the ordering process with ordered regions of both L10 and L12 phases. [ABSTRACT FROM AUTHOR]

The polarity of GaN grown by plasma-assisted molecular beam epitaxy was inverted from N-face to Ga-face by simultaneously exposing the surface to Mg and activated N fluxes during a growth interruption at a reduced substrate temperature. Growth studies suggested that a MgxNy compound was responsible for inverting the crystal. The change in polarity was verified in situ by reflection high energy electron diffraction via GaN surface reconstructions, and ex situ by convergent beam electron diffraction and KOH etch studies. The surface of the inverted material showed smooth step flow features. Ga-face high electron mobility transistors with good dc and small signal performance were fabricated on the inverted epilayers. A drain-source current of 0.84 A/mm was measured at a gate-source voltage of +1 V. Current-gain cutoff and maximum oscillation frequencies of 22 and 53 GHz, respectively, were measured in these devices. The device performance is similar to that of Ga-face transistors with comparable dimensions. [ABSTRACT FROM AUTHOR]

The near surface structural evolution in dehydrogenation process of air exposed Pd coated Mg nanoblades was characterized in situ from room temperature to ∼573 K using reflection high energy electron diffraction (RHEED). The evolved normalized diffraction intensity and the full width at half maximum of diffraction peaks have been correlated with the growth of crystal and the change in crystal size, respectively. With RHEED, we are able to detect crystal sizes smaller than ∼2 nm. At room temperature the dominant structures near surface were Pd and MgH2. With the substrate heating MgH2 started to gradually decompose at ∼380 K. When the temperature increased to ∼480 K MgH2 was nearly depleted and Mg6Pd alloys started to form. In addition, at high temperatures pure Mg reappeared and MgO was enhanced significantly even in high vacuum condition. We have discussed the effect of Mg oxide on the dehydrogenation process and the alloy formation between Pd and Mg as well as the accompanying migration of Mg to the surface at high temperatures. Based on our experimental results the structures and compositions that limit the hydrogenation/dehydrogenation cycle of Pd coated Mg nanoblades are suggested. Our findings can help the design of future recyclable hydrogen storage materials. [ABSTRACT FROM AUTHOR]

Thermal decomposition of wurtzite (0001)-oriented GaN was analyzed: in vacuum, under active N exposure, and during growth by rf plasma-assisted molecular beam epitaxy. The GaN decomposition rate was determined by measurements of the Ga desorption using in situ quadrupole mass spectrometry, which showed Arrhenius behavior with an apparent activation energy of 3.1 eV. Clear signatures of intensity oscillations during reflection high-energy electron diffraction measurements facilitated complementary evaluation of the decomposition rate and highlighted a layer-by-layer decomposition mode in vacuum. Exposure to active nitrogen, either under vacuum or during growth under N-rich growth conditions, strongly reduced the GaN losses due to GaN decomposition. [ABSTRACT FROM AUTHOR]

The nucleation and growth of indium on a vicinal Si(100)-(2×1) surface at high temperature by femtosecond pulsed laser deposition was investigated by in situ reflection high energy electron diffraction (RHEED). RHEED intensity relaxation was observed for the first ∼2 ML during the growth of In(4×3) by step flow. From the temperature dependence of the rate of relaxation, an activation energy of 1.4±0.2 eV of surface diffusion was determined. The results indicate that indium small clusters diffused to terrace step edges with a diffusion frequency constant of (1.0±0.1)×1011 s-1. The RHEED specular beam split peak spacing, which is characteristic of a vicinal surface, was analyzed with the growth temperature to obtain the average terrace width. Gradual reduction in the terrace width during growth of In(4×3) was observed with In coverage and is attributed to the detachment of In atoms from terrace edges. At a substrate temperature of 405 °C, the average terrace width decreased from 61±10 Å, which corresponds to the vicinal Si(100) surface, to an equilibrium value of 45±7 Å after deposition of ∼23 ML. Further In coverage showed a transition of the RHEED pattern from (4×3) to (1×1) and the growth of rounded In islands (average height of ∼1 nm and width of ∼25 nm), as examined by ex situ atomic force microscopy. [ABSTRACT FROM AUTHOR]

We report a systematic investigation on the structural and magnetic properties of molecular-beam epitaxial growth of CrTe thin films with different layer thicknesses and Cr/Te flux ratios. A phase diagram of the growth parameters is established based on the detailed analyses of the reflection high-energy electron diffraction patterns, atomic force microscopy, and magnetization. Our high-resolution transmission electron microscopy results show that under appropriate growth conditions, a metastable zinc-blende (ZB) phase of CrTe film can be achieved with a nominal thickness of 5 nm. The magnetic properties of ZB CrTe exhibit a strong in-plane anisotropy with an easy axis along the [001] direction and hard axes along the [011] and [011] directions. Correspondingly, the uniaxial (KU) and cubic (KC) anisotropy constants are obtained through the fitting of the [011] hard-axis direction. The temperature dependence of the remanent magnetization indicates the TC∼100 K of ZB CrTe is attained. [ABSTRACT FROM AUTHOR]

The reflection high energy electron diffraction (RHEED) surface pole figure technique has been applied to an in situ study of the biaxial texture evolution in the anisotropic film of Mg nanoblades. These nanoblades were grown by thermal vapor under the shadowing effect using oblique angle deposition. To compensate for the effects of the anisotropic morphology of the Mg film on the RHEED surface pole figure, a method of intensity normalization has been employed. From the normalized pole figures, we observed a development of [formula] biaxial texture in the film during the growth. When the film grows thicker the texture axes tilt more towards the incident vapor flux. The variation of the azimuthal angle orientation is mainly around the [0001] axis during growth and the azimuthal dispersion angle around the [0001] axis reduces. The change of texture axis tilting angle has been correlated to the change of the nanoblade tilting angle. The azimuthal angle alignment of the nanoblades is argued to be the result of maximizing the vapor flux capture cross section by nanoblades during the growth. [ABSTRACT FROM AUTHOR]

We have investigated the molecular-beam epitaxy (MBE) of InSb nanostructures on (100) GaSb substrates. We show that MBE leads to a low density (∼1-3×109 cm-2) of large islands even when varying the growth conditions on a wide range (substrate temperature ∼370-450 °C, growth rate ∼0.3-1.2 ML/s). Plastic relaxation takes place from the onset of island formation, regardless of the amount of InSb deposited after the two-dimensional to three-dimensional transition. These results show that In adatoms have a very long diffusion length on a Sb-terminated surface and that the energy for dislocation generation in InSb is low. This can be attributed to the low enthalpy of formation and low melting point of InSb. To circumvent this problem we have developed a MBE growth procedure based on the deposition of an amorphous InSb layer at low temperature followed by an annealing step to allow for reorganization to take place. This dramatic change of the growth conditions leads to the formation of small InSb quantum dots with a density in excess of 7×1010 cm-2. Uncapped quantum dots, however, are relaxed. In contrast, buried quantum dots are fully strained and emit near 3.5 μm at room temperature. Our results show that although formerly similar the InSb/GaSb materials system behaves completely differently from the InAs/GaAs case study system. [ABSTRACT FROM AUTHOR]

We have investigated the adsorption of dodecanethiol on zinc- and oxygen-terminated ZnO surfaces. Strong enthalpic adsorption is demonstrated by the stability of sulfur on both ZnO surfaces for temperatures up to 400 °C. The minimal presence of the S 2p3/2 170 eV peak suggests absorption of the sulfur as an unoxidized thiol. The results indicate a higher surface coverage of the thiol on the zinc-terminated surface. Evidence from reflection high energy electron diffraction measurements for the surface ordering after thiol treatment of the oxygen-terminated ZnO surface suggests that the dodecanethiol molecules can adsorb in a highly ordered manner. These results further open the possibility for biofunctionalization of ZnO for biosensing applications. [ABSTRACT FROM AUTHOR]

We report on high resolution x-ray diffraction studies of the crystalline perfection and the relaxation of elastic strain in AlN grown by MBE on sapphire(0001). Thin (200–300 nm thick) AlN layers were grown with a very low density of threading screw dislocations. A density of 1.75-8.5×105 cm-2, the lowest value ever reported for III-Nitride epitaxial layers, was observed in a surface layer formed over a defective nucleation layer. Residual elastic strain was found in investigated AlN layers. Stress was found to be close to that expected from thermal expansion mismatch between the AlN and sapphire(0001). A model for the structural transformation of crystalline defects accounts for these observations. [ABSTRACT FROM AUTHOR]

Oxide films of BaTiO3 and MgO were deposited on SrTiO3 single crystal substrates by laser molecular beam epitaxy. Reflection high-energy electron diffraction was performed in situ to investigate the change of growth mode and the lattice relaxation. Due to the different mismatches between epitaxial oxide films and substrates, two kinds of strain relaxation behavior during the growth process can be observed. For the BaTiO3/SrTiO3 system with the small mismatch of 2.18%, the coherent epitaxial growth can be maintained within the critical thickness. The experimental data were well in accord with the elastic strain theory in the small mismatch system. The coherent epitaxial growth is crucial to fabricate the functional oxide superlattice. However, strained island can form at the initial period to release strain energy in the case of MgO/SrTiO3 system with the large mismatch of 7.8%. It provides a useful practical method to realize self-organized nanostructures by introducing the moderate strain at interface. Our systematic study on the interface strain of functional oxide heteroepitaxial growth could give an instructive method to realize different heterostructures, e.g., superlattice and nanostructures. [ABSTRACT FROM AUTHOR]

We report a heat treatment of catalyst in air that drastically enhances a growth of carbon nanotubes (CNTs) by means of thermal chemical vapor deposition (CVD). An Fe catalyst film deposited on a Si substrate was heat treated at 700 °C in air before the acetylene CVD. The growth rate of the CNTs grown with the heat treatment was more than seven times higher than that of growth without the heat treatment. A scanning electron microscopy observation showed that the heat treatment in air promotes a granulation of the Fe catalyst. X-ray photoelectron spectroscopy and reflection high energy electron diffraction analyses showed that the heat treatment in air promotes an oxidation of the catalyst film and formation of Fe2O3 nanoparticles, suggesting that the heat treatment of Fe catalyst in air prevented the formation of Fe silicide that would deactivate the catalyst effect of Fe. The Fe2O3 nanoparticles do not agglomerate and can keep their original particles size. Thus the catalyst can maintain a catalyst activity during CNTs growth and, as a result, gives a growth enhancement of CNTs. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of the growth and fabrication of Bragg-spaced quantum wells, a type of resonant photonic band gap structure. To begin, we considered the impact of disorder and drift in the periodicity of the quantum wells on the formation of the resonant photonic band gap. We found that steady decrease in the periodicity greater than a few percent leads to collapse of the resonant photonic band gap, while random disorder in the quantum well periodicity of several percent leads to extra peaks in the resonant photonic band gap due to coupling to “intermediate band” states. Next, we optimized the growth of low x (x<=0.06) InxGa1-xAs/GaAs quantum wells, the building block of Bragg-spaced quantum well structures. Growth parameters optimized include growth rate, modulation of substrate temperature for barrier/quantum well, and V/III flux ratio. Fast growth of quantum wells was achieved with some of the narrowest heavy-hole exciton linewidths (0.37 meV) reported to date for quantum wells of these widths. Using the optimized InGaAs/GaAs quantum wells as a building block, we grew near-ideal N=210 Bragg-spaced quantum well structures. By monitoring growth rates during growth with reflection high energy electron diffraction and correcting drift by adjusting cell temperature, drift and disorder in periodicity were kept to less than 1%. We see no fundamental barriers to growing much longer structures such as N=1000 periods or longer. [ABSTRACT FROM AUTHOR]

We prepared highly aligned, biaxially textured BaxPb1-xTiO3 (PBT) on amorphous Si3N4 by using an ion-beam-assisted deposited MgO as a template layer. PBT was deposited on a biaxially textured MgO using sol-gel synthesis, metal-organic chemical-vapor deposition, and molecular beam epitaxy. The biaxial texture of the PBT was inherited from the MgO template. The reflection high-energy electron diffraction (RHEED) and cross-section transmission electron microscopy (TEM) experiments suggest that exposure of the MgO template to atmospheric moisture before PBT heteroepitaxy resulted in a significant narrowing of the PBT in-plane orientation distribution. The microstructures of the biaxially textured PBT films were analyzed by x-ray diffraction, RHEED, and TEM. The dynamic contact mode electrostatic force microscopy polarization hysteresis loops confirmed that these films are ferroelectric. [ABSTRACT FROM AUTHOR]

We prepared highly aligned, biaxially textured BaxPb1-xTiO3 (PBT) on amorphous Si3N4 by using an ion-beam-assisted deposited MgO as a template layer. PBT was deposited on a biaxially textured MgO using sol-gel synthesis, metal-organic chemical-vapor deposition, and molecular beam epitaxy. The biaxial texture of the PBT was inherited from the MgO template. The reflection high-energy electron diffraction (RHEED) and cross-section transmission electron microscopy (TEM) experiments suggest that exposure of the MgO template to atmospheric moisture before PBT heteroepitaxy resulted in a significant narrowing of the PBT in-plane orientation distribution. The microstructures of the biaxially textured PBT films were analyzed by x-ray diffraction, RHEED, and TEM. The dynamic contact mode electrostatic force microscopy polarization hysteresis loops confirmed that these films are ferroelectric. [ABSTRACT FROM AUTHOR]

Analysis of biaxial texture of MgO films grown by ion-beam-assisted deposition (IBAD) has been performed using a quantitative reflection high-energy electron diffraction (RHEED) based method. MgO biaxial texture is determined by analysis of diffraction spot shapes from single RHEED images, and by measuring the width of RHEED in-plane rocking curves for MgO films grown on amorphous Si[sub 3]N[sub 4] by IBAD using 750 eV Ar[sup +] ions, at 45° incidence angle, and MgO e-beam evaporation. RHEED-based biaxial texture measurement accuracy is verified by comparison with in-plane and out-of-plane orientation distribution measurements made using transmission electron microscopy and x-ray rocking curves. In situ RHEED measurements also enable the analysis of the evolution of the biaxial texture which narrows with increasing film thickness. RHEED-based measurements of IBAD MgO biaxial texture show that the minimum in-plane orientation distribution depends on the out-of-plane orientation distribution, and indicates that the minimum obtainable in-plane orientation on distribution is 2°. [ABSTRACT FROM AUTHOR]

Both the growth mechanism and material properties of ZnO epilayers are found to be strongly dependent on slight misorientation of GaN templates. The observation by in situ reflection high-energy electron diffraction and atomic force microscopy reveals that the growth mode changes from a three-dimensional to a two-dimensional growth mechanism, as the misorientation of GaN templates is varied from [0001] just to 0.2 degree toward the a axis. However, ZnO grows under a three-dimensional mode for misorientation toward the m axis. The observed changes in the growth mechanism are discussed in terms of the different surface migration length along the m and a axis, which is caused by the different surface atomic arrangement along the a and m axis. With an increase in the misorientation angle from 0 to 0.2° toward the a axis, the linewidth of (0002) x-ray rocking curve shows a dramatic narrowing from 1768 to 277 arcsec, while the photoluminescence intensity of exciton emission shows a remarkable enhancement by more than one order of magnitude. Such improvement in material properties can be ascribed to the observed change of the growth mode from three- to two-dimensional growth. [ABSTRACT FROM AUTHOR]

• First report of GaAs (1 1 1)A on Ge (1 1 1) using MBE. • No antiphase domain form during the growth of GaAs(1 1 1) A/Ge(1 1 1). • Twinned domains are the main defects observed in the GaAs epilayer. • The twin density can be minimized by tuning the As:Ga BEP ratio. We perform the growth of GaAs (1 1 1) epilayers on nominal Ge(1 1 1) wafers by molecular beam epitaxy (MBE). The polarity of GaAs is (1 1 1)A and homongeneous over the full area, as measured by transmission electron microscopy and high energy electron diffraction. This orientation conflicts with the common growth model for GaAs on Ge(1 1 1). Twinned domains are the main defects in our GaAs (1 1 1) epilayers. Using cathodoluminescence, we observe that some twin boundaries hold large number of non-radiative recombination centers. During growth, we find that only a narrow domain of As:Ga ratios lead to the growth of smooth and twin-free GaAs (1 1 1)A epilayers. At low As:Ga ratio, the surface is rough; while at high As:Ga ratio the epilayers present large densities of twinned domains. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Epitaxial D0 19 -Mn 3 Ga layers were grown on MgO(1 1 1), GaN(0 0 1) and Pt(1 1 1) substrates. • D0 19 -Mn 3 Ga is a non-collinear antiferromagnet with an hexagonal spin structure. • The ferromagnetic component along the c axis is manipulated due to induced strain. • The ferromagnetism of D0 19 -Mn 3 Ga has been tuned from 23.7 to 11.8 and 2.5 kAm−1. Abstract The hexagonal D0 19 -Mn 3 Ga phase is a non-collinear antiferromagnet with a triangular spin configuration. The Mn magnetic moments, lying in the (0 0 1) basal plane, are slightly canted towards the c axis producing a ferromagnetic component (FMC), which saturation magnetization in bulk samples is 7.7 kAm−1. Here, we report on the epitaxial growth of Mn 3 Ga epitaxial thin films on MgO(1 1 1), GaN(0 0 1) and MgO(0 0 1)/Pt(1 1 1) substrates with lattices mismatches of −4.77%, 2.1%, and 2.45%, respectively. Structural characterization with reflection high-energy electron diffraction was used to evaluate the critical thicknesses at which relaxation is reached. We provide evidence of the manipulation of the magnitude of the FMC of the Mn 3 Ga thin films grown under tensile and compressive deformations. The FMC has been tuned from 23.7 to 11.8 and 2.5 kAm−1 for the growth of Mn 3 Ga on MgO(1 1 1), GaN(0 0 1), and MgO(0 0 1)/Pt(1 1 1) substrates. The modulation of the FMC is explained in terms of the Dzyaloshinskii-Moriya interaction. [ABSTRACT FROM AUTHOR]

Abstract: It is shown that a source generating a slowly varying or quiet component of radio emission from ultracool stars such as the brown dwarf TVLM 513-46546 can be the gyrosynchroton radiation from the magnetic loops system distributed quasi-uniformly over the star's surface. Such a model explains the low modulation of the flux and makes it possible to understand the pumping mechanism of magnetic loops by high energy electrons. The main parameters of magnetic loops and which part of the star's surface is the emission source are determined using the information on microwave radiation from the brown dwarf TVLM 513-46546. [ABSTRACT FROM AUTHOR]

Examines the effect of space charge and the resulting electric fields on the trajectories of high energy electrons during irradiation of planar polymethylmethacrylate shields. Penetration of high energy electrons through slab shields; Decrease in flux of high energy electrons in some regions of materials; Enhanced shielding effectiveness of space charge and the resulting electric fields.

Provides information on a study that measured the surface mean-square atomic vibrational amplitude or equivalently the surface Debye temperature with reflection high-energy electron diffraction. Description on the Debye-Waller effect; Methodology of the study; Results and discussion on the study.

Presents a study which investigated the growth mechanisms of HgTe using a combination of reflection high energy electron diffraction and scanning tunneling microscopy. Description of the experimental setup; Interpretation of the findings; Conclusion.

Presents information on a study which described the surface point defects that were produced during low-energy ion bombardment of the germanium (001)-(2X1) using in situ reflection high-energy electron diffraction. Information on the reflection high-energy electron diffraction; Experimental details; Results and discussion; Conclusions.

We have used time-resolved reflection high energy electron diffraction (RHEED) measurements to study anion exchange reactions in molecular beam epitaxy (MBE) grown III-V semiconductors. In the experiment, InAs surfaces are exposed to Sbx fluxes and subsequent changes in the crystals RHEED patterns are examined. We find that when an InAs surface is initially exposed to an Sb flux the specular spot intensity first decreases, then recovers back toward its initial value. The shape of the intensity versus time curves is extremely reproducible if the absolute Sb flux and the Sb species are kept constant. The length of time required for the RHEED pattern to stabilize is much shorter for cracked Sb than for uncracked Sb. The RHEED dynamics are also faster if the total Sb flux increases. The behavior of the RHEED dynamics as a function of Sb flux and Sb species is consistent with the changes in the RHEED pattern being due to an Sb/As exchange reaction on the crystal’s surface. The RHEED data are compared to previously published x-ray photoelectron spectroscopy (XPS) data which studied exchange reactions on InAs surfaces exposed to Sb fluxes. The XPS study confirmed that the incident Sb did indeed exchange with As in the epilayer and estimated the exposure time needed to complete the Sb/As exchange reaction. The time scales for exchange associated with the RHEED and XPS data are in good agreement. This further indicates that RHEED could be used to indirectly probe anion exchange reactions, potentially opening up several avenues of research ranging from basic materials science to MBE process control in manufacturing. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated the growth mechanism of indium arsenide and gallium arsenide strained short-period superlattices grown on gallium arsenide and indium arsenide substrates by reflection high-energy electron diffraction and transmission electron microscopy. Methods; Results; Discussion.

Provides information on a study that examined phase formation in the interfacial reactions of ultrahigh vacuum deposited titanium thin films on (111)silicon by in situ reflected high energy electron diffraction and transmission electron microscopy. Methodology of the study; Results and discussion on the study.

Examines the epitaxial growth of barium fluoride on oriented gallium arsenide substrates as well as on thin intermediate oriented strontium-fluoride buffers. Use of in situ reflection high-energy electron diffraction and x-ray diffraction; Findings of thickness measurements of cleaved films using a scanning electron microscope; Means to determine directly the amount of misfit in the stacked barium-fluoride/strontium-fluoride layers.