Ultra-high Vacuum Molecular Beam Research Articles

Microstructure of Co(112̄0) epitaxial thin films, grown on MgO(100) single-crystal substrates

Co(112̄0) epitaxial thin films with hcp structure were prepared on MgO(100) single-crystal substrates heated at 300 °C by ultra high vacuum molecular beam epitaxy. The microstructure is investigated by employing X-ray diffraction and high-resolution transmission electron microscopy. The film consists of two types of domains whose c-axes are rotated around the film normal by 90° each other. Stacking faults are observed for the film along the Co[0001] direction. An atomically sharp boundary is recognized between the film and the substrate, where some misfit dislocations are introduced in the film at the Co/MgO interface. Dislocations are also observed in the film up to 15 nm thickness from the interface. Presence of such stacking faults and misfit dislocations seem to relieve the strain caused by the lattice mismatch between the film and the substrate. X-ray diffraction analysis indicates that the out-of-plane and the in-plane lattice spacings of the film are in agreement within 0.5% and 0.1%, respectively, with those of the bulk hcp–Co crystal, suggesting the strain in the film is very small.

Epitaxial Growth of FeCo Thin Films on SrTiO3 Single-Crystal Substrates

FeCo thin films were prepared on SrTiO3(100), SrTiO3(110), and SrTiO3(111) single-crystal substrates by ultra-high vacuum molecular beam epitaxy. The film growth structure and magnetic properties were investigated. FeCo epitaxial thin films are obtained on these SrTiO3 substrates at elevated temperatures with different crystal growth orientations. The nucleation and growth process of FeCo thin films on SrTiO3 substrates vary depending on the orientation of the substrate and its temperature. FeCo(100) single-crystal thin films with an A2 structure are obtained on SrTiO3(100) substrates at 500°C. FeCo epitaxial thin films with (110)A2 and (211)A2 planes parallel to the substrate surface are obtained on SrTiO3(110) substrates. FeCo thin films epitaxially grown on SrTiO3(111) substrates consist of two crystallographic phases having A2 and hexagonal structures with the (110)A2 and (0001)hex planes parallel to the substrate surface. The metastable FeCo(0001)hex plane is presumably formed in order to relax the strain caused by the lattice mismatch at the FeCo/SrTiO3(111) interface. The lattice constants of Fe50Co50 crystal with the hexagonal structure are estimated to be a = 0.251±0.004 nm and c = 0.457±0.005 nm. The magnetic properties of FeCo epitaxial thin films grown on SrTiO3 substrates are influenced by the magnetocrystalline anisotropy and shape anisotropy caused by the roughness of FeCo islands that form the FeCo thin films.

Structural and Magnetic Properties of hcp-Ni Epitaxial Thin Films

Ni epitaxial thin films were prepared on Au(100)fcc underlayers hetero-epitaxially grown on MgO(100)B1 single-crystal substrates by ultra-high vacuum molecular beam epitaxy. The effects of substrate temperature on the structural and the magnetic properties were investigated. hcp-Ni crystals are successfully obtained at temperatures lower than 300°C. With increasing the substrate temperature, the volume ratio of more stable fcc phase increases in the film. The hcp-Ni crystal consists of two types of (11-20) domains whose c-axes are rotated around the film normal by 90°each other. X-ray diffraction shows that the Ni film prepared at 100°C consists primarily of hcp crystal. An atomically sharp boundary is recognized at the Ni(11-20)hcp/Au(100)fcc interface for the Ni film. The in-plane magnetization properties of hcp-Ni film are interpreted to be reflecting the magnetocrystalline anisotropy of bulk hcp-Ni crystal which was predicted by a computer simulation study.

SrTiO<sub>3</sub>単結晶基板上におけるCo薄膜のエピタキシャル成長

Co thin films were prepared on SrTiO3(100), SrTiO3(110), and SrTiO3(111) single-crystal substrates by ultra-high vacuum molecular beam epitaxy. The film structure and the magnetic properties were investigated. Co epitaxial thin films are obtained on these SrTiO3 substrates at elevated temperatures with different film orientations. The nucleation of Co thin film on SrTiO3 substrate varies depending on the substrate orientation and the substrate temperature. Co(100)fcc single-crystal films are obtained on SrTiO3(100) substrates. On SrTiO3(110) substrates, Co epitaxial thin films with Co(110)fcc, Co(111)fcc and/or Co(0001)hcp, and Co(100)fcc planes parallel to the substrate surface are formed. Thin films consisting of Co(111)fcc and Co(0001)hcp crystals epitaxially grow on SrTiO3(111) substrates. 0.4-0.5% reductions in lattice spacing normal to the substrate surface are observed for Co thin films prepared at a substrate temperature of 300°C. The film strain decreases with increasing the substrate temperature. The in-plane magnetization properties of Co thin films are influenced by the magnetocrystalline anisotropy and the shape anisotropy caused by the roughness of Co islands which form the Co thin films.

Annealing Time Dependence of Morphology and Structure of Epitaxial Graphene on 6H-SiC(0001) Surface

在分子束外延（MBE）设备中,采用高温退火的方法在6H-SiC（0001）表面外延石墨烯,并研究了退火时间对外延石墨烯形貌和结构的影响.利用反射式高能电子衍射（RHEED）、原子力显微镜（AFM）、激光拉曼光谱（Raman）和近边X射线吸收精细结构（NEXAFS）等实验技术对制备的样品进行了表征.RHEED结果发现,不同退火时间的样品在SiC衍射条纹的外侧都出现了石墨烯的衍射条纹.AFM测试表明,外延石墨烯的厚度随退火时间增加而增大,且样品孔洞减少、表面更加平整.Raman测试表明,外延石墨烯拉曼谱中2D峰和G峰的位置相对高定向热解石墨（HOPG）中2D峰和G峰的位置蓝移,且退火时间增加,峰的蓝移量减小.对样品中碳原子K边NEXAFS谱测量显示,样品中存在sp2杂化的碳原子,退火时间增加使碳原子的1s→π以及1s→σ吸收的强度增大,且1s电子到π态的吸收峰相对HOPG的向高能偏移.

Epitaxial growth of fcc-CoxNi100−x thin films on MgO(110) single-crystal substrates

Co x Ni 100 − x (x=100, 80, 20, 0 at. %) epitaxial thin films were prepared on MgO(110) single-crystal substrates heated at 300 °C by ultrahigh vacuum molecular beam epitaxy. The growth mechanism is discussed based on lattice strain and crystallographic defects. CoNi(110) single-crystal films with a fcc structure are obtained for all compositions. CoxNi100−x film growth follows the Volmer–Weber mode. X-ray diffraction analysis indicates that the out-of-plane and the in-plane lattice spacings of the CoxNi100−x films are in agreement within ±0.5% with the values of the respective bulk CoxNi100−x crystals, suggesting that the strain in the film is very small. High-resolution cross-sectional transmission microscopy shows that an atomically sharp boundary is formed between a Co(110)fcc film and a MgO(110) substrate, where periodical misfit dislocations are preferentially introduced in the film at the Co/MgO interface. The presence of such periodical misfit dislocations relieves the strain caused by the lattice mismatch between the film and the substrate.

Epitaxial Growth of Graphene on 6H-SiC (0001) by Thermal Annealing

An epitaxial graphene (EG) layer is successfully grown on a Si-terminated 6H-SiC (0001) substrate by the method of thermal annealing in an ultrahigh vacuum molecular beam epitaxy chamber. The structure and morphology of the EG sample are characterized by reflection high energy diffraction (RHEED), Raman spectroscopy and atomic force microscopy (AFM). Graphene diffraction streaks can be seen in RHEED. The G and 2D peaks of graphene are clearly observed in the Raman spectrum. The AFM results show that the graphene nominal thickness is about 4−10 layers.

Structure and Magnetic Properties of Co Epitaxial Thin Films Grown on MgO Single-Crystal Substrates

Co epitaxial thin films were prepared on single-crystal MgO substrates of (100), (110), and (111) planes heated at 300degC by ultra high vacuum molecular beam epitaxy. The film crystal structure varies depending on the substrate orientation. On MgO(100) substrate, a Co(112macr0) epitaxial bi-crystalline film with an hcp structure is obtained. A Co(110) single-crystal film with an fcc structure is formed on MgO(110) substrate, whereas a Co(0001) single-crystal film with an hcp structure is grown on MgO(111) substrate. Co film growth on MgO substrates follows the Volmer-Weber mode. The in-plane magnetization properties reflect the magnetocrystalline anisotropy of bulk hcp-Co or fcc-Co crystal. The Gilbert's damping constants of these 40-nm-thick epitaxial Co films are determined.

Microstructure and magnetic properties of FeCo epitaxial thin films grown on MgO single-crystal substrates

FeCo epitaxial films were prepared on MgO(100), MgO(110), and MgO(111) substrates by ultrahigh vacuum molecular beam epitaxy. FeCo thin films with (100), (211), and (110) planes parallel to the substrate surface grow on respective MgO substrates. FeCo/MgO interface structures are studied by high-resolution cross-sectional transmission electron microscopy and the epitaxial growth mechanism is discussed. Atomically sharp boundaries are recognized between the FeCo thin films and the MgO substrates where misfit dislocations are introduced in the FeCo thin films presumably to decrease the lattice misfits. Misfit dislocations are observed approximately every 9 and 1.4 nm in FeCo thin film at the FeCo/MgO(100) and the FeCo/MgO(110) interfaces, respectively. X-ray diffraction analysis indicates that the lattice spacing measured parallel to the single-crystal substrate surfaces are in agreement within 0.1% with those of the respective bulk values of Fe50Co50 alloy crystal, showing that the FeCo film strain is very small. The magnetic anisotropies of these epitaxial films basically reflect the magnetocrystalline anisotropy of bulk FeCo alloy crystal.

Growth of Ni Thin Films on Al2O3 Single-Crystal Substrates

Ni thin films were prepared on Al2O3(1120) and Al2O3(0001) single-crystal substrates by ultrahigh vacuum molecular beam epitaxy (UHV-MBE). The film growth structure and the magnetic properties were investigated. Ni epitaxial thin films with Ni(111) and Ni(100) planes parallel to the substrate surface are obtained on Al2O3(1120) substrates, whereas Ni polycrystalline films grow on Al2O3(0001) substrates. The Ni(111) film formed on Al2O3(1120) substrate consists of two types of domains whose orientations are rotated around the film normal by 30° each other. The magnetic properties of Ni films grown on Al2O3 substrates are influenced by the magnetocrystalline anisotropy and the shape anisotropy caused by the roughness of Ni islands which form the Ni thin films. The relationships between the film structure and the magnetic properties are discussed.

Structure and peculiarities of the (8 × n)-type Si(001) surface prepared in a molecular-beam epitaxy chamber: A scanning tunneling microscopy study

A clean Si(001) surface thermally purified in an ultrahigh vacuum molecular-beam epitaxy chamber has been investigated by means of scanning tunneling microscopy. The morphological peculiarities of the Si(001) surface have been explored in detail. The classification of the surface structure elements has been carried out, the dimensions of the elements have been measured, and the relative heights of the surface relief have been determined. A reconstruction of the Si(001) surface prepared in the molecular-beam epitaxy chamber has been found to be (8 x n). A model of the Si(001)-(8 x n) surface structure is proposed.

Synthesis and characterization of compositionally graded Si1−xGex layers on Si substrate

Thin film of silicon germanium (Si1−xGex) with tailored composition was grown on Si(100) substrate at 650°C in an ultrahigh vacuum molecular beam epitaxy system. The quality of the film was investigated by Rutherford backscattering spectrometry (RBS) in random and channeling geometries, glancing angle X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and atomic force microscopy. The nominal x-value ranged from 0 to 0.14. RBS/channeling measurements indicate that the strain associated with lattice mismatch is compressive in the film. The film shows island growth, which is driven by interface misfit dislocation.

Hetero-epitaxial ZnO 버퍼층이 As-doped ZnO 박막의 증착조건에 미치는 영향

ZnO thin films prepared by PLD method exhibit an excellent optical property, but may have some problems such as incomplete surface roughness and crystallinity. In this study, undoped ZnO buffer layers were deposited on (0001) sapphire substrates by ultra high vacuum pulse laser deposition (UHV-PLD) and molecular beam epitaxy (MBE) methods, respectively. After post annealing of ZnO buffer layer, undoped ZnO thin films were deposited under different oxygen pressure (<TEX>$35{\sim}350$</TEX> mtorr) conditions. The Arsenic-doped (1, 3 wt%) ZnO thin layers were deposited on the buffer layer of undoped ZnO by UHV-PLD method. The optical property of the ZnO thin films was analyzed by photoluminescence (PL) measurement. The <TEX>${\theta}-2{\theta}$</TEX> XRD analysis exhibited a strong (002)-peak, which indicates c-axis preferred orientation. Field emission-scanning electron microscope (FE-SEM) revealed that microstructures of the ZnO thin films were varied by oxygen partial pressure, Arsenic doping concentration, and deposition method of the undoped ZnO buffer layer. The denser and smoother films were obtained when employing MBE-buffer layer under lower oxygen partial pressure. It was also found that higher Arsenic concentration gave the enhanced growing of columnar structure of the ZnO thin films.

A thermodynamically consistent surface reaction mechanism for CO oxidation on Pt

The thermodynamic inconsistency of catalytic combustion reaction mechanisms has been a long-standing, fundamental, and practical problem. Here, we develop a thermodynamically consistent catalytic reaction mechanism for CO oxidation on Pt. First, we propose a modification of the bond index of the unity bond index–quadratic exponential potential (UBI-QEP) semiempirical framework for calculation of the activation energy of the Langmuir–Hinshelwood-type bimolecular surface reaction between co-adsorbed CO ∗ and O ∗ . Thermodynamic consistency is then ensured in the reaction mechanism by combining the semiempirical UBI-QEP theory, statistical mechanics, and constraint-based optimization against experimental data. Atmospheric pressure ignition and ultrahigh vacuum molecular beam experiments are selected as targets for optimization. The optimized mechanism is validated against redundant experiments, including temperature-programmed desorption, temperature-programmed reaction, and molecular beam experiments. Our microkinetic model is able to capture multiple types of data while being thermodynamically consistent over a wide range of conditions.

3C-SiC/Si template growth and atomic layer epitaxy of cubic GaN by RF-MBE

Growth of 2 in cubic gallium nitride (c-GaN) film on a cubic silicon carbide on Si template (3C-SiC/Si) using Si (0 0 1) and Si(1 1 1) substrates was performed by using a radio frequency molecular beam epitaxy (RF-MBE) system, which was equipped with a jet nozzle. The 3C-SiC/Si template was prepared using alternating exposure of acetylene ( C 2 H 2 ) gas introduced by the jet nozzle under ultra high vacuum and Si molecular beam from a Si effusion cell. Very smooth 3C-SiC/Si templates with RMS roughness by atomic force microscopy of about 0.2–0.4 nm were grown here. Good crystal quality of low-temperature buffer (LTB) layer was obtained on a good 3C-SiC/Si template and confirmed through the observation of RHEED pattern. A grown c-GaN film's X-ray diffraction full-width at half-maximum of (0 0 2) peak was 13.2 arcmin ( 2 θ ) for a 544 nm-thick film. c-GaN was also grown using an atomic layer epitaxy (ALE) method. The ALE method proved to be necessary for a uniform and homogeneous atomic nitrogen flux under slightly Ga excess condition to grow the uniform c-GaN using rf-MBE.

Ｌ２１構造を有するＣｏ２Ｃｒ１−ｘＦｅｘＡｌ薄膜の結晶構造と磁気特性

We successfully grew Co2Cr1-xFexAl full Heusler films (0 ≤ x ≤ 1) with the L21 structure onto GaAs(001) substrates by achieving stoichiometry in an ultrahigh-vacuum molecular beam epitaxy chamber. The films develop epitaxial crystallinity with the relationship of Co2Cr1-xFexAl(001)‹110›||GaAs(001)‹110›, which induces very strong uniaxial magnetocrystalline anisotropy, except for x = 0. For x = 1, in particular, the film is almost a single phase and its magnetic moment per formula unit exhibits Slater-Pauling behavior. Films of this type were used to fabricate magnetic tunnel junctions with 8.8% and 4.9% tunnel magnetoresistance (TMR) ratios at room temperature for x = 1 with an Al-O tunnel barrier and x = 0.6 with a MgO barrier, respectively. The TMR ratio can be further enhanced by both realizing an L21 single phase and eliminating lattice distortion.

Design and performance of a molecular beam epitaxy system for metallic heterostructure deposition illustrated by a study of the controlled epitaxy of Cu(111)∕Al2O3(0001)

We describe the design and construction of an ultrahigh-vacuum molecular beam epitaxy (MBE) system for the growth of metallic heterostructures, particularly magnetic metals, and alloys. The system, which was specifically designed to be both cost-effective and compact, incorporates an “axial” design with a large source to substrate distance (&amp;gt;69cm) to meet demands for high uniformity, low deposition rate, and compatibility with nanolithographic masks and templates. The growth and in situ characterization capabilities are specifically tailored to metallic film growth allowing for greatly reduced costs in comparison to commercial MBE systems. We demonstrate the performance of the system via a study of the controlled epitaxy of Cu(111) on Al2O3(0001), a useful substrate/buffer layer combination for the growth of many magnetic transition metals and their alloys. Exploiting the three-dimensional nature of the growth at room temperature we are able to control the in-plane crystallite size, independent of the surface roughness, by varying the deposition rate.

Thermal stability of Ti and Pt nanowires manufactured by Ga+ focused ion beam.

Ti and Pt nanowires have been produced by ultra high-vacuum molecular beam epitaxy deposition of Ti thin films and focused ion beam (FIB) deposition of Pt thin films, followed by cross-sectional FIB sputtering to form electron-transparent nanowires. The thermal stability of the nanowires has been investigated by in situ thermal cycling in a transmission electron microscope. Epitaxial single crystal Ti nanowires on (0001)Al(2)O(3) substrates are microstructurally stable up to 550-600 degrees C, above which limited dislocation motion is activated shortly before the Ti-wires oxidize. The amorphous FIB-deposited Pt wires are stable up to 580-650 degrees C where partial crystallization is observed in vacuum. Faceted nanoparticles grow on the wire surface, growing into free space by surface diffusion and minimizing contact area with the underlying wire. The particles are face-centred cubic (fcc) Pt with some dissolved Ga. Continued heating results in particle spheroidization, coalescence and growth, retaining the fcc structure.

Characterization Investigation of β-FeSi2 Semiconductor by In Situ Ultrahigh-Vacuum Transmission Electron Microscopy

In an integrated ultrahigh-vacuum system, a thin Fe layer was deposited on a Si (111) 7×7 surface at room temperature by ultrahigh-vacuum molecular beam epitaxy (UHV-MBE) and annealed at 600°C. The characterization of the as-deposited and annealed specimen for microstructure and electronic structure was investigated using an analytical ultrahigh-vacuum transmission electron microscope (UHV-TEM) and an electron energy-loss spectroscope (EELS). After deposition of a 5 ML Fe layer, an epitaxial Fe layer formed on the Si surface, which suggests that the Si substrate may induce the arrangement of Fe atoms in the thin Fe layer. When annealed at 600°C, epitaxial β-FeSi2 islands with moiré fringes formed on the Si substrate. The Fe 3d occupancy change for as-deposited Fe and β-FeSi2 was calculated by the empirical method after background subtraction by the power law and deconvolution by the Fourier ratio. The calculated results show that there is a 0.95±0.06 electron/atom decrease for Fe 3d occupancy from as-deposited Fe to β-FeSi2. Fe white lines intensity ratios for the two states were calculated from the second derivative spectra, which are 2.89 and 2.51 for as-deposited Fe and β-FeSi2, respectively. The results for the intensity ratio show that the higher intensity ratio is related to stronger magnetism.

Mechanism of cleaning Si(100) surface using Sr or SrO for the growth of crystalline SrTiO3 films

A method for removing SiO2 and producing an ordered Si(100) surface using Sr or SrO has been developed. In this technique, a few monolayers of Sr or SrO are deposited onto the as-received Si(100) wafer in an ultrahigh vacuum molecular-beam epitaxy system. The substrate is then heated to ∼800 °C for about 5 min, the SiO2 is removed to leave behind a Sr- or SrO-terminated ordered Si(100) surface. This Sr- or SrO-terminated Si(100) surface is well suited for the growth of crystalline high-k dielectric SrTiO3 films. Temperature programmed desorption measurements were carried out to understand the mechanism of removing SiO2 from Si(100) using Sr or SrO. The species we observed coming off the surface during the temperature cycle were mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalytic reaction SiO2+Sr(or SrO)→SiO(g)+O+Sr(or SrO). The reaction SiO2+Si→2SiO(g) at the SiO2/Si interface is limited and the pit formation is suppressed. The main roles that Sr or SrO play during the oxide removal process are catalysts promoting SiO formation and passivating the newly exposed Si surface, preventing further etching and the formation of pits in the substrate.