High-energy Electron 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.

Oxide desorption process from InSb surface under Sb flux

In this work, the process of oxide removal from the InSb (001) surface was studied in situ by high-energy electron diffraction in vacuum and under an antimony flux. The dependence of the oxide thickness on the annealing temperature was obtained. It has been found that the antimony flux slows down the process of oxide removal due to the oxide formation reaction. The oxide removal process was described by a system of kinetic equations, the activation energy of oxide decomposition was determined Keywords: InSb, oxide, activation energy, desorption.

Процесс десорбции оксида с поверхности InSb в потоке сурьмы

In this work, the process of oxide removal from the InSb (001) surface was studied in-situ by high-energy electron diffraction in vacuum and under an antimony flux. The dependence of the oxide thickness on the annealing temperature was obtained. It has been found that the antimony flux slows down the process of oxide removal due to the oxide formation reaction. The oxide removal process was described by a system of kinetic equations, the activation energy of oxide decomposition was determined.

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.

Atomic resolution HOLZ-STEM imaging of atom position modulation in oxide heterostructures

It is shown that higher order Laue zone (HOLZ) rings in high energy electron diffraction are specific to individual columns of atoms, and show different strengths, structure and radii for different atom columns along the same projection in a structure. An atomic resolution 4-dimensional STEM dataset is recorded from a <110> direction in a perovskite trilayer, where only the central LaFeO3 layer should show a period doubling that gives rise to an extra HOLZ ring. Careful comparison between experiment and multislice simulations is used to understand the origins of all features in the patterns. A strong HOLZ ring is seen for the La-O columns, indicating strong La position modulation along this direction, whereas a weaker ring is seen along the O columns, and a very weak ring is seen along the Fe columns. This demonstrates that atomic resolution HOLZ-STEM is a feasible method for investigating the 3D periodicity of crystalline materials with atomic resolution.

Bloch-wave interpretations of Gjønnes-Moodie lines in convergent-beam electron diffraction for non-symmorphic space-group crystals.

The contrast of Gjønnes-Moodie (GM) lines which appear in convergent-beam electron diffraction patterns for non-symmorphic space-group crystals is explained using Bloch waves. In the two-dimensional space groups p2mg and pg the Bloch waves for electron diffraction are described. In both space groups along the Δ line, Bloch waves are arranged as two different types, and it is shown that the two types of Bloch waves do not contribute to the intensity of forbidden reflections. Along the position where the forbidden reflection satisfies the Bragg condition, degeneracies of two Bloch waves are found and it is shown that the degenerated pair of Bloch waves do not contribute to the intensity. These Bloch-wave results provide a new perspective in the understanding of the contrast mechanism of GM lines previously described using scattering polynomials. They also advance the understanding of Bloch-wave behaviour in high-energy electron diffraction.

EXTENDED DEFECTS IN GaAs/Ge/GaAs HETEROSTRUCTURES WITH TURNING GaAs LAYERS

The GaAs layer turn on Ge at a right angle in the substrate plane was studied by high-energy electron diffraction and transmission electron microscopy methods when growing GaAs/Ge/GaAs heterostructures using individual molecular-beam epitaxy systems for GaAs and Ge, with sample transfer through atmosphere.

Fabrication and Study of the Electronic Structure of МоO3/Мо Nanofilms

The composition, the surface morphology, and the crystalline and electronic structure of МоО3 nanofilms are studied using a set of methods, namely, scanning electron spectroscopy and high energy electron diffraction, photoelectron spectroscopy, and secondary electron emission. These nanofilms are obtained by implanting $${\text{O}}_{{\text{2}}}^{{\text{ + }}}$$ ions into a Mo single crystal heated to Т = 850 K. Films with different thicknesses (~30, 60, and 90 A) are obtained at an ion energy of 1–5 keV and a dose of D = (4–8) × 1017 cm–2. It is shown that a continuous and homogeneous polycrystalline МоО3 film with a surface roughness of at most 1.5 nm is formed. The band gaps of these films are ~3.4 eV, and the widths of the conduction bands are 4.5 eV. It is discovered that there are four maxima of the density of electronic states in the valence band; probably, they are due to hybridization of the N5, N45, N4 energy levels of Mo with the L2, L23, and L3 energy levels of oxygen.

Surface Degradation Mechanism on CH3NH3PbBr3 Hybrid Perovskite Single Crystal by a Grazing E-Beam Irradiation.

To start a step such as some realization of minimized and integrated devices, it requires simply understanding the surface status of hybrid perovskite on the e-beam irradiation because many commercial semiconductor devices are performed with a surface patterning process using e-beam or etching gas. The surface status of CH3NH3PbBr3 (MAPbBr3) single crystal was studied after a grazing e-beam irradiation in an ultra-high vacuum. The prepared hybrid perovskite single crystal was irradiated by the 3 degree-grazing e-beam with energy of 15 kV for 10 min using a reflection high-electron energy diffraction technique. The e-beam irradiation on the MAPbBr3 hybrid perovskite single crystal induced the deformation from MAPbBr3 into MABr, Br2, and Pb on the surface. The gas phases of MABr and Br2 are depleted from the surface and the Pb element has remained on the surface. As a result of the e-beam irradiation, it formed a polycrystalline-like phase and Pb metal particles on the surface, respectively.

GaAs (1 1 1) epilayers grown by MBE on Ge (1 1 1): Twin reduction and polarity

We perform the growth of GaAs (111) epilayers on nominal Ge(111) wafers by molecular beam epitaxy (MBE). The polarity of GaAs is (111)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(111). Twinned domains are the main defects in our GaAs (111) 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 (111)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.

GROWTH PROCESSES OF SUBGRAIN AND MORPHOLOGY EVOLUTION IN THE SYNTHESIS OF Β-SiC FILMS AT (111) Si IN THE ATMOSPHERE OF METHANE

By the methods of transmission electron microscopy, high energy electron diffraction, atomic force microscopy, and Auger electron spectroscopy, the article studies the phase composition, orientation, substructure, and morphology of the films formed during pulsed photon treatment (PPT) by radiation of xenon lamps of silicon (111) Si substrates in an atmosphere of methane. We have established that in the range of the energy density of radiation (Ep) supplied to the substrate with a thickness of 0.45 μm for 3 s from 269 to 284 J cm-2 the oriented nanocrystalline films are formed on both surfaces of the substrates both from the irradiated and non-irradiated side β-SiC thickness of about 150 nm. In this case, the synthesis of films on the irradiated side is carried out with the possible participation of photon activation of processes and on the reverse side – only by thermal activation (short-term heat treatment (SHT). With an increase in the energy density of radiation in β-SiC films, the average subgrain size on the irradiated side is shown to increase from 4.2 nm (Ep = 269 J ·cm-2) to 7.9 nm (Ep = 284 J ·cm-2) and on the non-irradiated side 3.9 to 7.0 nm respectively. The surface roughness of the β-SiC surface proceeds consequentially on the irradiated side from 19 nm (Ep = 269 J ·cm-2) to 60 nm (Ep = 284 J ·cm-2) and on the non-irradiated side from 11 nm to 56 nm respectively. Based on the temperature dependences of the average grain size and roughness, we have estimated the apparent activation energies of the processes. The activation energy of subgrain β-SiC growth is practically independent of the activation method and is 1.3 eV. The activation energy for the evolution of roughness is 2.5 eV at a PPT and 3.5 eV at a SHT.

Au-Assisted Substrate-Faceting for Inclined Nanowire Growth.

We study the role of gold droplets in the initial stage of nanowire growth via the vapor-liquid-solid method. Apart from serving as a collections center for growth species, the gold droplets carry an additional crucial role that necessarily precedes the nanowire emergence, that is, they assist the nucleation of nanocraters with strongly faceted {111}B side walls. Only once these facets become sufficiently large and regular, the gold droplets start nucleating and guiding the growth of nanowires. We show that this dual role of the gold droplets can be detected and monitored by high-energy electron diffraction during growth. Moreover, gold-induced formation of craters and the onset of nanowires growth on the {111}B facets inside the craters are confirmed by the results of Monte Carlo simulations. The detailed insight into the growth mechanism of inclined nanowires will help to engineer new and complex nanowire-based device architectures.

Short-Range order of TlIn1–x Sn x Te2 thin films

The formation of the short-range order structure of amorphous nanometer-thick TlIn1–x Sn x Te2 films (х = 0.02–0.09) obtained via vacuum deposition onto substrates of fresh KCl and KJ chips and celluloid at a temperature below Т = 213 K is studied by high-energy electron diffraction. Both freshly deposited films and films held in vacuum (10–2 Pa) at room temperature in darkness for several months are studied. The effect of the tin concentration on the interatomic distances, the coordination numbers, and the time of amorphous phase stability of TlIn1–x Sn x Te2 films due to a great spread in bond lengths and bond angles is established.

Thin and ultrathin films of palladium oxide for oxidizing gases detection

Thin and ultrathin films of palladium oxide are set forth as promising materials for oxidizing gas detection. Thin and ultra thin PdO films were prepared by thermal oxidation at dry oxygen of previously formed Pd layers on polished poly-Al2O3, SiO2/Si (100), optical quality quartz, and amorphous carbon/KCl substrates. By high-resolution transmission electron microscopy and high-resolution scanning electron microscopy it has been found that thickness was about 5–10nm and 30–35nm for ultrathin and thin PdO films, respectively. By high-energy electron diffraction and X-ray analysis it has been established that oxidation of initial Pd layers at T=770–1070K led to formation of homogenous polycrystalline PdO films. At ozone and nitrogen dioxide detection PdO films prepared by oxidation at T=870K have shown the good values of sensitivity, signal stability, operation speed, and reproducibility of sensor response. In comparison with other materials, palladium oxide thin and ultrathin films have some advantages at gas sensor fabrication. Firstly, at rather low operating temperatures PdO films have shown good values of functional parameters. Secondly, the synthesis procedure of binary PdO films is rather simple and is compatible with planar processes of microelectronic industry.

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.

(Invited) In-Vacuo Studies of Transition Metal Dichalcogenide Synthesis and Layered Material Integration

The last decade has seen fevered interest in 2D materials for nanoelectronics. Previous studies focused primarily on tribological and energy storage applications due to the weak van der Waals bonding that exists between the atomic layers. These materials, which include graphene, hexagonal boron nitride, and the plethora of transition metal dichalcogenide (TMD) combinations, have electronic structures exhibiting metallic, semiconducting, and insulating properties. This promises devices with scalability to the atomic limit combined with defect free interfaces. Realizing this promise has not proved trivial. Defects in the naturally occurring material can dominate their properties, and even synthesized materials can suffer from high impurity concentration. Process residues such as photo-resists can impact device performance. Metal depositions can result in the formation of unexpected interface compounds that can dominate the contact behavior. We study both the synthesis and integration of 2D materials for nano- and optoelectronic applications. Using an in-vacuo MBE-ARPES cluster tool, we grow TMDs from elemental sources using van der Waals epitaxy. The growth can be carried out on a range of substrates include insulating, semiconducting, and metallic materials. The layer-by-layer nature of these growths can be verified in-situ using reflection, high-energy electron diffraction. We use in-vacuo angle resolved photoemission spectroscopy to study the electronic structure of these materials as a function of composition, substrate, and processing conditions. The in-vacuo set-up is vital to determining the intrinsic properties of these materials, many of which readily oxidize in air. Detailed oxidation studies allow us to correlate our in-vacuo determined properties with the properties determined using ex-situ methods. Complementary ex-situ characterization, including photoluminescence, is used to determine the optical properties of novel ternary TMD alloys, and TMD/TMD heterostructures. In addition to studying the synthesis of these materials, we also investigate how these materials interface with metals that are commonly used as contacts in electronic devices. It is well known that low work-function metals can react with TMDs to form metal-chalcogen compounds at the metal-semiconductor interface. Recently it has been shown that the process conditions during the electron beam deposition of metals can control the interface reaction. In particular, the reactor base pressure and deposition rate can be used to tune the interface chemistry. For example, the partial pressure of oxidizing species, that are typically present in a high-vacuum reactor, is sufficiently high to completely oxidize titanium as it is deposited by e-beam. This results in a sharp interface between TiO2 and the semiconductor interfaces; however, it is the properties of TiO2 rather than Ti that are dominating electrical and thermal conduction across the interface. We will present a summary of our work on the synthesis of novel TMD alloys, the determination of their electronic and optical properties as a function of composition and process condition, and the interfaces formed between these materials and typical device contact metals.

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.

Argon Nanoclusters with Fivefold Symmetry in Supersonic Gas Jets and Superfluid Helium

In this study argon nanoclusters (800 to \(\sim \)6500 atoms) formed in supersonic gas jets are compared to the nanoclusters stabilized in superfluid helium. High-energy electron and X-ray diffraction methods are utilized. Both techniques allow investigation of isolated clusters. It is shown that the theoretical prediction of the so-called multiply twinned particles with fivefold symmetry, such as icosahedra (ico) and decahedra (dec) is valid in the investigated cluster size interval. Around the point of the expected ico-to-dec size-dependent transformation at a cluster size of \(\sim \)2000 atoms, hexagonal ico and the statistical distribution of structures with a tendency for dec to replace ico are observed. Kinetic reasons, as well as temperature-related effects, could be responsible for the latter observations.

Structure of heterosystems formed by a SnO2 film and island metal (Ag, Au, or Pd) condensate

We have studied the phase composition and microstructure of thin tin(IV) oxide films surfacemodified with silver, gold, and palladium nanoislands. Using high-energy electron diffraction, we have shown for the first time that the thermal oxidation of the Sn films leads to the formation of nanocrystalline multiphase SnO2 films in which the major phase is orthorhombic. Also present are (in order of decreasing content) tetragonal and cubic phases. Blocks of SnO2(O) subgrains with 〈101〉 texture contain dislocations and stacking faults, which are interpreted as layers of the tetragonal phase. It has been shown that vacuum condensation makes it possible to modify the surface of SnO2 films with noble metals and obtain homogeneous nanoisland coatings characterized by a unimodal, uniform island size distribution.

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.