Reflection High-energy Diffraction Research Articles

Topological signatures of Mo2TiC2O2

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 Γ̄) and β (elliptical electron-like pocket around M̄). 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.

Single-crystal-like germanium thin films on large-area, compliant, light-weight, flexible, single-crystal-like substrates.

Germanium (Ge) films were heteroepitaxially grown on flexible, large-area, single-crystal-like metallic substrates. Multiple, heteroepitaxial, buffer layers of nanoscale dimensions were deposited on the triaxially textured, single-crystal-like, thermo-mechanically processed Ni-W alloy substrates. Ge films were deposited on a CeO2-terminated, heteroepitaxial buffer stack on the metallic substrate using electron beam evaporation. X-ray diffraction θ-2θ scans showed a very strong Ge (400) peak and the full width at half-maximum (FWHM) of the Ge (400) rocking curve was 0.93°. The Ge (111) ϕ-scan showed a FWHM value ∼4°. Based on the X-ray ω-scan, ϕ-scan and (111), (110), and (001) X-ray pole-figures, the Ge film deposited on the flexible, metallic substrate had a cube-on-cube heteroepitaxial relationship with the single-crystal-like metallic substrate. Reflection-high-energy-diffraction (RHEED) patterns from the Ge layer was streaky indicative of a smooth and essentially single-crystal-like Ge film. Cross-section TEM examination revealed a sharp interface between the Ge film and the topmost buffer layer, CeO2, with a low defect density. The CeO2 layer serves as a highly compliant layer that modulates its lattice parameter to attain excellent lattice-matching to the heteroepitaxial Ge layer. Ge films grown on these flexible metal substrates exhibited electron mobilities in the range of 175-250 cm2V-1s-1. Such single-crystal-like semiconductor films on low-cost, flexible, large-area, scalable, single-crystal-like metallic substrates could potentially enable high-performance electronic devices for a range of applications.

RHEED analysis of the oxidized M′2M″xXyene sheets by ablated plasma thrust method in pulsed laser deposition chamber

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.

Chemoheteroepitaxy of 3C‐SiC(111) on Si(111): Influence of Predeposited Ge on Structure and Composition

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.

Simplified Determination of RHEED Patterns and Its Explanation Shown with the Use of 3D Computer Graphics.

The process of preparation of nanostructured thin films in high vacuum can be monitored with the help of reflection high energy diffraction (RHEED). However, RHEED patterns, both observed or recorded, need to be interpreted. The simplest approaches are based on carrying out the Ewald construction for a set of rods perpendicular to the crystal surface. This article describes how the utilization of computer graphics may be useful for realistic reproduction of experimental conditions, and then for carrying out the Ewald construction in a reciprocal 3D space. The computer software was prepared in the Java programing language. The software can be used to interpret real diffractions patterns for relatively flat surfaces, and thus it may be helpful in broad research practice.

Low temperature growth of epitaxial ferroelectric BaTiO3

BaTiO3 exhibits several functional properties, such as high dielectric constant, large Pockels coefficient, and strong ferroelectricity/piezoelectricity. These properties can be used for a variety of applications, such as ferroelectric tunnel junctions in non-volatile memory devices. To achieve large-scale integration of BaTiO3, however, one requires the synthesis of high-quality BaTiO3 films at low temperatures in order to be compatible with the thermal budget of electronic processes in use today. Here, we describe the synthesis of BaTiO3 thin films by molecular beam epitaxy and find that coherently strained and ferroelectric BaTiO3 can be grown at temperatures as low as 310 °C. Using reflection high energy diffraction, we demonstrate a surface mobility of BaO and TiO2 adatoms that is high enough to promote ferroelectric crystal growth at low temperatures. A clear ferroelectric polarization switching is observed using piezoresponse force microscopy. Our results pave the way toward large-scale integration of ferroelectric BaTiO3 with mainstream electronics platforms.

CoFeB/MgO/CoFeB magnetic tunnel junctions prepared by layer-by-layer growth of naturally oxidized MgO

We fabricated CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) by naturally oxidizing dc-sputtered Mg and investigated the formation process of MgO(001) by reflective high-energy diffraction. The 0.26 nm thick Mg layer on amorphous CoFe(B) had an amorphous structure in the as-deposited state and a monolayer (ML) of textured MgO(001) formed by natural oxidation. By repeating the deposition/oxidation of 0.26 nm thick Mg layers, a highly textured MgO(001) barrier layer was prepared. Surprisingly, (001)-oriented crystallization was confirmed even for 2 ML thick MgO. The MTJs prepared by layer-by-layer growth of naturally oxidized MgO exhibited high MR ratios (up to 200%) and low resistance-area products suited for device applications.

Atomic structure of Sr/Si(0 0 1)(1 × 2) surfaces prepared by Pulsed laser deposition

A buffer layer formed by depositing a ½ monolayer of Sr on Si(0 0 1) is known to passivate the Si surface, while its surface structure constitutes a suitable template for the integration of various functional oxides with the existing Si platform. We used Pulsed Laser Deposition (PLD) to prepare a Sr/Si(0 0 1)(1 × 2) surface and analysed it using in-situ Reflection High-Energy Diffraction (RHEED) in combination with low-temperature Scanning Tunneling Microscopy (STM). The STM images reveal an atomically ordered surface with terraces composed of one-dimensional (1D) chains running along perpendicular directions on neighbouring terraces. The 1D chains are separated by 0.78 nm and exhibit a low-amplitude corrugation with a period of 0.39 nm. The measured values agree well with the size of the (1 × 2) unit cell observed for similar MBE-grown surfaces, while the density of the surface defects is somewhat higher in the presented case. According to simulated STM images based on DFT calculations, two types of surface defects were identified and explored: arrays of Sr vacancies and Sr adatoms. These results show that PLD can offer precise control for the preparation of high-quality Sr-buffered Si(0 0 1) surfaces.

Self-formation of InAs quantum dots on SiOx/semiconductor substrates by molecular beam deposition

InAs quantum dots (QDs) were grown on SiO2/Si and SiOx/GaAs substrates by molecular beam deposition. InAs QDs were formed on SiOx films through a Volmer–Weber mode. The structural information obtained by atomic force microscopy, reflection high-energy electron-beam diffraction, and transmission electron microscopy revealed the formation of spatially separated InAs QDs having a single-crystalline structure on SiOx films. Photoluminescence (PL) was observed from InAs QDs on SiOx films and analyzed by theoretical calculation. PL peak of 1.22–1.23 eV at 15 K was attributed to the first excited state transition concerning a heavy hole.

Rapid deposition and thermoelectric properties of ytterbium boride thin films using hybrid physical chemical vapor deposition

Thin films of YbB4 and YbB6 have been deposited on sapphire substrates via hybrid physical chemical vapor deposition (HPCVD) using elemental ytterbium and decaborane, B10H14, as sources. The deposition condition dependence of the composition, purity, crystallinity and electrical properties has been investigated using x-ray diffraction (XRD), reflection high-energy diffraction (RHEED), profilometry and thermoelectric property measurements. A YbB6 thin film (∼2.5 µm) was produced with a power factor reaching ca. 200 µW m−1 K−2 at the highest investigated temperature, 563 K. Such performance slightly surpasses that of bulk material and offers a positive prospect for high-temperature thermoelectric application.

Magnetic tunnel junction with Fe(001)/Co phthalocyanine/MgO(001) single-crystal multilayer

A single-crystal multilayer consisting of Fe, cobalt phthalocyanine (CoPc), and MgO was grown on an fcc-MgO(001) substrate. The reflection high-energy diffraction observation showed that the ultrathin fcc-MgO(001) film could be grown on bcc-Fe(001)/CoPc when the thickness of CoPc was less than 0.7 nm (about two molecular layers). A tunnel junction with a bcc-Fe(001)/CoPc/fcc-MgO(001)/bcc-Fe(001) layer achieved a tunnel magnetoresistance of about 20% and an electric field endurance of more than 1 V/nm at room temperature. The novel multilayer in the present study will enable us to advance the frontiers of molecular spintronics.

Structural and electrical properties of K3Li2Nb5O15 thin film grown by pulsed laser deposition

Potassium lithium niobate K3Li2Nb5O15 (KLN) with tetragonal tungsten bronze-type structure (TTB) thin film was successfully deposited on MgO substrate by pulsed laser ablation method. The substrate was coated by a perovskite conductive layer of La0.5Sr0.5CoO3. X-ray diffraction revealed (001) orientation with low mosaicity. The oriented film had smooth and droplets-free surface as evidenced by reflection high electron energy diffraction and atomic force microscopy. Using Pt top electrode, dielectric measurements indicated stable dielectric permittivity of about 400 up to 105Hz. A dielectric relaxation was also observed. Moreover, the capacitance voltage measurements of the obtained (001)KLN film show ferroelectric behavior. Our results provide useful insights into the growth of tetragonal tungsten and other complex materials. These results will be useful for the growth and integration of epitaxial TTB thin films on perovskite and Si substrates.

Molecular beam epitaxy of 2D-layered gallium selenide on GaN substrates

Large area epitaxy of two-dimensional (2D) layered materials with high material quality is a crucial step in realizing novel device applications based on 2D materials. In this work, we report high-quality, crystalline, large-area gallium selenide (GaSe) films grown on bulk substrates such as c-plane sapphire and gallium nitride (GaN) using a valved cracker source for Se. (002)-Oriented GaSe with random in-plane orientation of domains was grown on sapphire and GaN substrates at a substrate temperature of 350–450 °C with complete surface coverage. Higher growth temperature (575 °C) resulted in the formation of single-crystalline ε-GaSe triangular domains with six-fold symmetry confirmed by in-situ reflection high electron energy diffraction and off-axis x-ray diffraction. A two-step growth method involving high temperature nucleation of single crystalline domains and low temperature growth to enhance coalescence was adopted to obtain continuous (002)-oriented GaSe with an epitaxial relationship with the substrate. While six-fold symmetry was maintained in the two step growth, β-GaSe phase was observed in addition to the dominant ε-GaSe in cross-sectional scanning transmission electron microscopy images. This work demonstrates the potential of growing high quality 2D-layered materials using molecular beam epitaxy and can be extended to the growth of other transition metal chalcogenides.

Reflection high-energy electron diffraction measurements of reciprocal space structure of 2D materials

Knowledge on the symmetry and perfection of a 2D material deposited or transferred to a surface is very important and valuable. We demonstrate a method to map the reciprocal space structure of 2D materials using reflection high energy diffraction (RHEED). RHEED from a 2D material gives rise to ‘streaks’ patterns. It is shown that from these streaks patterns at different azimuthal rotation angles that the reciprocal space intensity distribution can be constructed as a function of momentum transfer parallel to the surface. To illustrate the principle, we experimentally constructed the reciprocal space structure of a commercial graphene/SiO2/Si sample in which the graphene layer was transferred to the SiO2/Si substrate after it was deposited on a Cu foil by chemical vapor deposition. The result reveals a 12-fold symmetry of the graphene layer which is a result of two dominant orientation domains with 30° rotation relative to each other. We show that the graphene can serve as a template to grow other materials such as a SnS film that follows the symmetry of graphene.

The preparation and ferromagnetism of single crystal ε-Fe3N(111) film on SrTiO3(100) substrate

We report the growth of single crystal hexagonal ε-Fe3N(111) film on SrTiO3(100) substrate by ablating high-purity iron target in activated nitrogen. The measurements of reflection high-energy diffraction, x-ray diffraction, atomic force microscopy and vibrating sample magnetometer reveal that the ε-Fe3N(111) films have well crystallization, smooth surface, and strong ferromagnetism. The root mean square roughness of the ε-Fe3N film surface is 0.25 nm. The saturation magnetization values are ∼900 and ∼1050 emu/cm3 at 300 and 10 K, respectively. The in-plane and out-of-plane coercivity forces are 550 and 675 Oe, respectively. Meanwhile, the film has obvious magnetic anisotropy.

Correlation between morphology, electron band structure, and resistivity of Pb atomic chains on the Si(5 5 3)-Au surface

Structural and electron transport properties of multiple Pb atomic chains fabricated on the Si(5 5 3)-Au surface are investigated using scanning tunneling spectroscopy, reflection high electron energy diffraction, angular resolved photoemission electron spectroscopy and in situ electrical resistance. The study shows that Pb atomic chains growth modulates the electron band structure of pristine Si(5 5 3)-Au surface and hence changes its sheet resistivity. Strong correlation between chains morphology, electron band structure and electron transport properties is found. To explain experimental findings a theoretical tight-binding model of multiple atomic chains interacting on effective substrate is proposed.

Deposition of thermoelectric strontium hexaboride thin films by a low pressure CVD method

Thin films of SrB6 were deposited on sapphire substrates using a chemical vapor deposition method, with elemental strontium and decaborane, B10H14, used as the strontium and boron sources, respectively. The formation of highly crystalline, phase-pure SrB6 films was confirmed with X-ray diffraction and reflection high energy diffraction (RHEED) analysis, and the films’ thermoelectric transport properties were measured. A relatively high deposition temperature of 850–950°C was found to be optimal for obtaining well-crystallized films at an extremely high deposition rate. The thermoelectric transport properties of the SrB6 thin films were observed to be comparable to those reported for bulk materials, but an unexpectedly high electrical resistivity led to a reduced power factor value for the thin films.

P24] A thermodynamic assessment of the Al-Ti-V system

We report the growth of single crystal hexagonal ε-Fe3N(111) film on SrTiO3(100) substrate by ablating high-purity iron target in activated nitrogen. The measurements of reflection high-energy diffraction, x-ray diffraction, atomic force microscopy and vibrating sample magnetometer reveal that the ε-Fe3N(111) films have well crystallization, smooth surface, and strong ferromagnetism. The root mean square roughness of the ε-Fe3N film surface is 0.25 nm. The saturation magnetization values are ∼900 and ∼1050 emu/cm3 at 300 and 10 K, respectively. The in-plane and out-of-plane coercivity forces are 550 and 675 Oe, respectively. Meanwhile, the film has obvious magnetic anisotropy.

Impact of cation stoichiometry on the early stage of growth of SrTiO3 deposited by pulsed laser deposition

By performing in situ growth studies during pulsed laser deposition, we observed a strong reduction of the surface diffusion coefficients for slightly non-stoichiometric SrTiO3. Both, stoichiometric and non-stoichiometric thin films exhibit 2D layer by layer growth. However, in the non-stoichiometric case the 2D island coalescence is significantly delayed, which goes along with a shift of the reflection high electron energy diffraction (RHEED) minimum. We could explain this shift of the RHEED minimum by developing a model for the step density evolution taking into account finite surface diffusion.

GaAs nanowires grown by Ga-assisted chemical beam epitaxy: Substrate preparation and growth kinetics

Growth kinetics of GaAs nanowires (NWs) on Si(111) substrates by Ga-assisted chemical beam epitaxy is studied as a function of growth conditions such as substrate temperature (Ts), V/III flux ratio and catalyst dimension. The preparation method for Si(111) substrates is optimized in order to obtain a thin surface oxide with a thickness around 0.5nm, allowing both the decomposition of metalorganic precursors and GaAs nucleation at oxide pinholes. The use of thinner oxides enables the growth of a GaAs layer whereas the utilization of thicker oxides could even inhibit GaAs nucleation. The successful self-formation of Ga droplets over this slightly oxidized Si surface has been observed by scanning electron microscopy (SEM), whose initial size is demonstrated to affect both the NW growth rate and the resultant NW aspect ratio. The formation of these droplets is crucial to enable the catalytic growth of NWs whose morphology is thoroughly analyzed by SEM, showing a self-organized array of vertically aligned match shaped GaAs NWs with a hexagonal footprint. In addition, the crystalline structure of NWs is monitored in-situ by reflection high energy diffraction, showing pure zincblende phase along the whole NW stem.In terms of better NW aspect ratio, higher crystalline quality and faster growth rates, the best NW growth conditions are found at Ts=580°C, using an effective flux ratio V/III≈0.8. Moreover, NW growth kinetics is demonstrated to be improved when using a pre-deposited Ga coverage of 7.5 monolayers, stabilized for 90s prior to the NW growth.