Beam Epitaxy In Ultrahigh Vacuum Research Articles

Magnetic switching driven by spin–orbit torque in topological-insulator-based (Bi0.5Sb0.5)2Te3/Ta/CoFe/Cu/CoFe/IrMn heterostructure

The giant spin–orbit torque (SOT) generated by topological surface states in topological insulators (TIs) provides an energy-efficient writing method for magnetic memory. In this study, we demonstrate a topological insulator/spin valve (TI/SV) device that operates at room temperature. An ultrathin, high-quality TI (Bi0.5Sb0.5)2Te3 (BST) thin film is epitaxially grown as a functional layer on a (0001)-Al2O3 substrate via molecular beam epitaxy in ultrahigh vacuum. Subsequently, Ta/CoFe/Cu/CoFe/IrMn layers are grown on BST/Al2O3 thin films using magnetron sputtering to form TI/SV devices via a subsequent standard lithography process. The resulting TI/SV devices exhibit a giant magnetoresistance of up to ∼1.1% at room temperature. Additionally, a low switching current density of approximately 1.25 × 105 A cm−2 is achieved, which implies high potential for further reducing the energy consumption of SOT-based devices. The SOT conversion efficiency and charge-spin conversion efficiency of the TI layer are approximately 4.74 × 10−6 Oe A−1 cm2 and 1.33, respectively, as extracted from the SOT-induced shift of the magnetic switching field. Moreover, the switching current density reduces steadily with the device size scaling down. This study can facilitate the realization of energy-efficient magnetic memory devices in the future.

Electron transport property of epitaixial bilayer graphene on SiC substrate

Graphene can find great potential applications in the future electronic devices. In bilayer graphene, the relative rotation angle between graphene layers can modulate the interlayer interaction and hence induces rich physical phenomena. We systematically study the temperature dependent magnetoresistance (MR) properties in the epitaxial bilayer graphene (BLG) grown on the SiC substrate. High quality BLG is synthesized by molecular beam epitaxy in ultra-high vacuum. We observe the negative MR under a small magnetic field applied perpendicularly at temperature < 80 K, which is attributed to a weak localization effect. The weak localization effect in our epitaxial BLG is stronger than previously reported ones in epitaxial monolayer and multilayer graphene system, which is possibly because of the enhanced interlayer electron transition and thus the enhanced valley scattering in the BLG. As the magnetic field increases, the MR exhibits a classical Lorentz MR behavior. Moreover, we observe a linear magnetoresistance behavior in a large field, which shows no saturation for the magnetic field of up to 9 T. In order to further investigate the negative and linear magnetoresistance, we conduct angle-dependent magnetoresistance measurements, which indicates the two-dimensional magnetotransport phenomenon. We also find that the negative MR phenomenon occurs under a parallel magnetic field, which may correspond to the moiré pattern induced local lattice fluctuation as demonstrated by scanning tunneling microscopy measurement on an atomic scale. Our work paves the way for investigating the intrinsic properties of epitaxial BLG under various conditions.

Single-layer CrI3 grown by molecular beam epitaxy

Single- and few-layer chromium triiodide (CrI3), which has been intensively investigated as a promising platform for two-dimensional magnetism, is usually prepared by the mechanical exfoliation. Here, we report direct growth of single-layer CrI3 using molecular beam epitaxy in ultrahigh vacuum. Scanning tunneling microscopy (STM), together with density functional theory (DFT) calculation, revealed that the iodine trimers, each of which consists of three I atoms surrounding a three-fold Cr honeycomb center, are the basic units of the topmost I layer. Different superstructures of single-layer CrI3 with periodicity around 2–4 nm were obtained on Au(1 1 1), while only the 1 × 1 structure was observed on the graphite substrate. At an elevated temperature of 423 K, single-layer CrI3 began to decompose and transformed into single-layer chromium diiodide. Our bias-dependent STM images suggest that the unoccupied and occupied states are spatial-separately distributed, consistent with the results of our DFT calculation. We also discussed the role of charge distribution in the super-exchange interactions among Cr atoms in single-layer CrI3.

δ-Doping of oxygen vacancies dictated by thermodynamics in epitaxial SrTiO3 films

Homoepitaxial SrTiO3(110) film is grown by molecular beam epitaxy in ultra-high vacuum with oxygen diffusing from substrate as the only oxidant. The resulted oxygen vacancies (VOs) are found to be spatially confined within few subsurface layers only, forming a quasi-two-dimensional doped region with a tunable high concentration. Such a δ-function distribution of VOs is essentially determined by the thermodynamics associated with the surface reconstruction, and facilitated by the relatively high growth temperature. Our results demonstrate that it is feasible to tune VOs distribution at the atomic scale by controlling the lattice structure of oxide surfaces.

Oxygen Reduction Reaction Activity and Durability for Model Pt Shell Layers on Ir(111) Prepared by Molecular Beam Epitaxy

We fabricated well-defined Pt–Ir bimetallic surfaces through monolayer-thick Pt epitaxial growth on Ir(111) substrates (Pt n ML/Ir(111):n = 1–4) using molecular beam epitaxy in ultrahigh vacuum (UHV). The surface structures of Pt n ML/Ir(111) were verified using reflection high-energy electron diffraction in UHV. Electrochemical characterization tests, including evaluations of oxygen reduction reaction (ORR) activity and durability, were performed using cyclic voltammetry and rotating disk electrode techniques in 0.1 M HClO4. Regarding the initial ORR activity for Pt n ML/Ir(111) surfaces, Pt2ML/Ir(111) surfaces exhibited the highest ORR activity (24 times that of the clean Pt(111) surfaces). Initial ORR activities declined when square-wave potential cycles (PCs) were applied between 0.6 and 1.0 V vs. reversible hydrogen electrode. The Pt4ML/Ir(111) surface exhibited the highest ORR activity even after 5000 PC loadings (6 times that of the clean Pt(111) surfaces), suggesting that Pt-shell/Ir-core core–shell nanoparticles have potential for use as practical Pt-based bimetallic ORR catalysts.

Development of Effective Technique for Irradiating Samples of Thin Films Using the Focused Ion Beam

In this paper, the technique of irradiation of thin films by Ga+ ions inside of scanning electron microscope, by a focused ion beam is developed. The influence of irradiation on magnetic properties of thin epitaxial films of Co, grown by molecular beam epitaxy in ultrahigh vacuum on a silicon surface with a intermediate layer of copper atoms. It has been shown that low radiation doses lead to an increase of magnetic hardness of the samples. At higher doses, there is a decrease of coercive force, which is apparently due to a change of crystalline structure of the material. An interesting fact is that in the dose range of 1012-1015 the relative remanences is practically unchanged and only at higher doses the magnetic properties of the film worsen.

Invited: Electrochemical Properties for UHV-Prepared Pt/M Well-Defined Single Crystal Surfaces

Background Discussion on oxygen reduction reaction (ORR) mechanisms1) of alloy catalysts is complicated because of uncertainties in outermost surface structures. To clarify the effect of catalyst’s outermost surface structures on ORR, Markovic and co-workers2) evaluated ORR activities single-crystal planes of Pt3Ni. Their pioneering work clearly demonstrates that design of atomic arrangements of the catalyst surfaces directly links to developments in superior PEFC cathode electrode catalysts. To date, EC properties for the Pt-based single crystal alloy surfaces have been reported3-9): our approach is well-defined model-catalyst fabrications by using molecular beam epitaxy in ultra-high vacuum (UHV:~10-8Pa) condition. In this study, ORR activities and structural stabilities for the UHV-prepared well-defined Pt-based bimetallic (Pt/M) surfaces are discussed. Experimental The MBE apparatus and UHV-EC sample transfer system have been described earlier10). Pt-shell layers were prepared on fcc single crystal substrates through electron-beam vacuum depositions of another elements and thermal annealing in UHV. The resulting surface structures were verified with reflection high-energy electron diffraction (RHEED), IR reflection absorption (IRRAS) spectra for carbon monoxide (CO) adsorption, and scanning tunneling microsope (STM). The UHV-prepared samples were transferred to a N2-purged glove box without air exposure. CV curves are recorded in N2-purged 0.1 M HClO4. After that, LSV measurements were conducted in O2-saturated 0.1 M HClO4: RHE is used as a reference electrode. Results and discussion Pt/Au(111): On the basis of RHEED and CO-IRRAS measurements, epitaxial growth of Pt on Au(111) was confirmed. On the basis of the hydrogen-adsorption charges (QHad), we deduce that the 0.3nm-thick Pt almost covers the Au(111) substrate. The half-wave potential for the Pt0.3nm/Au(111) was ca. 10mV more positive than that of the Pt(111). In contrast, the potential for the Pt0.15nm/Au(111) shifted 60mV negatively. The results show that extended Pt(111) domains might correlate with the ORR activity enhancement. Ni/Pt(111)-skin: A UHV-STM image of the sample fabricated through 1ML-thick Ni deposition on ca. 800K-Pt(111) (Ni/Pt(111)-skin) showed 6-fold symmetric, honeycomb atomic arrangement. However, atomic-scale disorder with a corrugation of 0.06-0.12nm height is present at the topmost surface. XPS analysis showed positive shift of Pt 4f band. The LSV curve of the Ni/Pt(111)-skin shifted 70mV positively, suggesting that remarkable ORR activity enhancement is originated from the strains- and electronic-changes of the topmost Pt(111) lattice. Pt/Pd(111): Atomically flat PtmML/Pd(111) surfaces were prepared through Pt depositions on Pd(111) at 673 K. The LSV curve for the Pt2ML/Pd(111) showed 20mV positive shift relative to clean Pt(111). To evaluate EC stability of the PtmML/Pd(111), potential cycles (0.6~1.0V) were applied in O2 saturated 0.1M HClO4. Although the Pt layers less than 2MLs showed decrease in activity with increasing cycles, the activity of the Pt4ML/Pd(111) remained nearly unchanged during the cycles. EC Stability: Changes in ORR activities for the Pt-shell layers (Pt/Au(111), Ni/Pt(111)-skin, Pt/Pd(111)) during the potential cycles are summarized in Fig. 1. The results suggest that the Pt-shell layers thicker than 3MLs stabilize the topmost Pt(111) lattice in PEFC conditions. Acknowledgement This work was supported by NEDO. References 1)T.Toda et al. JES, 146, 3750 (1999).2)V.R.Stamenkovic et al. Science 315, 493 (2007).3)H. E. Hoster et al. Chemphyschem. 11, 1518 (2010).4)J.Baricuatro and M. Soriaga, Electrocatal. 1, 42, (2010).5)A.Bergbreiter et al. Chemphyschem. 11, 1505 (2010).6)S.Axnanda, et al. Chemphyschem. 11, 1468 (2010).7)M.Wakisaka, et al. Electrochem.Commun. 13, 317 (2011).8)I. E. L. Stephens et al. JACS 133, 5485 (2011).9)T.Rurigaki et al. JEAC, 716, 58 (2014).10)T.Wadayama et al. Electrochem.Commun. 12, 1112(2010). T.Wadayama et al. JPCC 115, 18589 (2011). Y.Iijima. et al., JEAC, 685, 79 (2012). Y.Yamada et al. Surf. Sci. 607, 54 (2013). N.Todoroki et al., PCCP, 15, 17771 (2013).

Preparation and characterization of Ni(111)/graphene/Y2O3(111) heterostructures

Integration of graphene with other materials by direct growth, i.e., not using mechanical transfer procedures, is investigated on the example of metal/graphene/dielectric heterostructures. Such structures may become useful in spintronics applications using graphene as a spin-filter. Here, we systematically discuss the optimization of synthesis procedures for every layer of the heterostructure and characterize the material by imaging and diffraction methods. 300 nm thick contiguous (111) Ni-films are grown by physical vapor deposition on YSZ(111) or Al2O3(0001) substrates. Subsequently, chemical vapor deposition growth of graphene in ultra-high vacuum (UHV) is compared to tube-furnace synthesis. Only under UHV conditions, monolayer graphene in registry with Ni(111) has been obtained. In the tube furnace, mono- and bilayer graphene is obtained at growth temperatures of ∼800 °C, while at 900 °C, non-uniform thick graphene multilayers are formed. Y2O3 films grown by reactive molecular beam epitaxy in UHV covers the graphene/Ni(111) surface uniformly. Annealing to 500 °C results in crystallization of the yttria with a (111) surface orientation.

Atomic structure and reactivity of ferromagnetic Fe deposited on Si(001)

This study presents a correlated study of structural, reactivity, and magnetic properties of ultrathin Fe layers grown on Si(001) by molecular beam epitaxy in ultrahigh vacuum. The interface reactivity is characterized by Auger electron spectroscopy. The surface structure is characterized by low electron energy diffraction with spot profile analysis. The magnetism of the synthesized layers is investigated by magneto-optical Kerr effect. At room temperature, metal Fe layers with poor long-range order are synthesized; these layers are ferromagnetic with an extremely low coercitive field (below 1 Oe). The reactivity with Si is low in this case, with formation of an interface layer of about 8 A Fe equivalent thickness with about 7 at.% Si diffused. Samples synthesized at higher temperatures (500 °C) exhibit better long-range order, though the Fe reactivity with Si is higher and leads to the formation of an interface compound whose approximate stoichiometry is very close to Fe3Si. Once this compound is formed (for an equivalent Fe thickness of about 14 monolayers), disordered metal Fe islands are developing with subsequent Fe deposition, which contain also about 8 at.% Si diffused. These structures exhibit a much lower ferrimagnetism, with saturation magnetization about one order of magnitude lower than in the case of the room temperature synthesis. In this case of high temperature synthesis, two phases are observed, a ferrimagnetic one and a superparamagnetic one.

Self-ordered electrochemical growth on single-crystal electrode surfaces

This paper is a brief review of self-ordered electrochemical growth on single-crystalelectrode surfaces on which arrays of nanostructures are created by replication of patternsmade by atomic steps or surface reconstruction. Whenever possible the parallel is madebetween electrochemical growth and molecular beam epitaxy in ultrahigh vacuum. Anatomistic view of electrodeposition is given first to help with identifying the similarities ofand specific differences between the two techniques of growth. Recent examples whereself-organized nanostructures are prepared on metals and silicon substrates are discussed.

Compact and transferable threefold evaporator for molecular beam epitaxy in ultrahigh vacuum

A very compact molecular beam epitaxy (MBE) source is presented. It contains three Knudsen-type crucibles, two resistively and one electron bombardment heated. An efficient water cooling is implemented to allow MBE deposition under ultrahigh-vacuum conditions. Due to its small size, the evaporator fits inside a DN 36 CF T-piece which makes a separate bakeout possible. Refills of the crucibles as well as changes of the source materials and even repairs of the source can be carried out within a few hours without breaking the vacuum of the main chamber. The design and the usage of the MBE source are described in detail and its functionality is demonstrated exemplarily with data of silver deposition on the silicon (111) surface.

An evaporation source for ion beam assisted deposition in ultrahigh vacuum

We describe the design, construction, and operation of an ion beam assisted deposition source for molecular beam epitaxy in ultrahigh vacuum. At a typical deposition rate of a monolayer per minute, the source may be operated in each of five modes: using self-ions from the vapor, self-ion plus noble gas ions from an additional gas inlet, both pulsed or continuous, or with complete suppression of ions. The source is based on electron bombardment heating of a metal rod or a crucible while the ions generated from the vapor are focused electrostatically onto the sample. Additional ions may be extracted from a noble gas stream injected into the ionization region. Examples for each of the different modes are given for Co deposition onto Cu(111), a system known to resist layer-by-layer growth.

Self-Assembled Magnetic Dots, Antidots, Dot Chains, and Stripes: Epitaxial Co on Ru(0001)

Lateral magnetic nanostructures have been grown via molecular beam epitaxy in ultrahigh vacuum and characterized ex-situ with atomic force and magnetic force microscopy. We observed that epitaxial growth of Co onto Ru(0001) at elevated temperature results in three-dimensional Co islands (dots) or a flat Co film network with deep holes (antidots) in truncated pyramidal shapes. The lateral size of these dots/antidots, in the order of 100 nm, tends to be uniform at each given coverage. We attribute the growth mode mainly to strain relaxation of Co epitaxy on Ru, which has a 8% lattice mismatch. In addition, we have explored the placement of these dots on a grooved Ru(0001) surface. The dots automatically align into linear chains along the asymmetric grooves to form either dot chains or continuous stripes, which would open new opportunities in creating either ordered magnetic arrays or arbitrary arrangements.

Structure and growth in metal epitaxy on low-index Au surfaces—a comparison between solid∣electrolyte and solid∣vacuum interfaces

On the basis of results of scanning tunnelling microscopy studies we compare the atomic scale mechanism for metal-on-metal epitaxial growth during electrodeposition, at the electrochemical interface and during molecular beam epitaxy in ultrahigh vacuum. Examples include the nucleation and (sub-)monolayer growth of Cu on Au(111), Ni on Au(100) and Ni on Au(111). The results show significant discrepancies in specific growth steps, which can lead also to rather different film morphologies, but they also reveal surprising agreement in other steps in the overall growth process and also in structural aspects. These findings will be discussed, aiming at a unified description of the growth process in both situations. Characteristic differences between growth in the different environments, such as strongly different effective interactions between metallic adspecies, will be elucidated.

In situ fabricated Ga2O3–GaAs structures with low interface recombination velocity

Amorphous Ga2O3 films have been deposited in situ on (100) GaAs layers grown by molecular beam epitaxy in ultrahigh vacuum. The Ga2O3–GaAs interface is stable during photoexcitation and the photoluminescence (PL) intensity, measured at 514.5 nm excitation wavelength, is enhanced drastically by a factor of 420 as compared to a corresponding bare GaAs surface. The Ga2O3–GaAs interface recombination velocity derived from a modified dead layer model is below 104 cm/s. Furthermore, the PL intensity of Ga2O3–GaAs structures approaches that of a very low interface state density (2×109 eV−1 cm−2) AlGaAs–GaAs reference structure.

Bls Studies of Exchange Coupling in The Iron Whisker/Cr/Fe System

ABSTRACTBrillouin light scattering (BLS) and magneto-optic Kerr effect have been carried out on a series of specimens consisting of a Fe(001) whisker substrate upon which thin Cr(001) and Fe(001) layers have been deposited by means of molecular beam epitaxy in ultrahigh vacuum. The Fe film was 20 monolayers (ML) thick, and the Cr(001) films were grown having various thicknesses. It is demonstrated that BLS thin film frequencies measured in the saturated magnetic state with the thin film magnetization parallel with the applied magnetic field can be used to obtain the exchange coupling strength between the thin film and whisker magnetizations both for antiferromagnetic coupling and for ferromagnetic coupling, provided that the ferromagnetic coupling is not too strong. It is also shown that the coupling strength is extremely sensitive to the quality of the chromium growth: a small deterioration in the growth conditions has been found to reduce the exchange coupling by nearly a factor of two.

Growth and luminescence spectroscopy of a CuCl quantum well structure

A structure of the type CaF2/CuCl/CaF2, in which the confined layer is 150 Å thick, has been grown on a Si(111) surface by molecular beam epitaxy in ultrahigh vacuum. The optical properties of the CuCl film have been characterized by low-temperature photoluminescence as a function of excitation density. Striking differences in the excitonic emissions are observed between the quantum well structure and a bulk single crystal of CuCl, and are discussed in terms of film purity and thickness.

Epitaxial growth of Ag films on InP (001) by atomic beam epitaxy in ultra-high vacuum

Epitaxial growth of Ag films on InP (001) by atomic beam epitaxy in ultra-high vacuum