Ultrahigh Vacuum Molecular Beam Epitaxy Research Articles

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.

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.

Structural, electronic, and magnetic properties of pseudomorphic CrFe nanostripes on W(110)

We have grown pseudomorphic binary ${\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}$ alloy monolayers and sequences of Cr and Fe nanostripes on W(110) by molecular-beam epitaxy in ultrahigh vacuum. By coadsorption of Cr and Fe a pseudomorphic random CrFe alloy grows on the W(110) substrate. At a substrate temperature of $700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ the CrFe alloy forms monolayer stripes in the step flow growth mode. We have measured magnetic properties of the monolayer alloy for $0.75\ensuremath{\leqslant}x\ensuremath{\leqslant}1$ using Kerr magnetometry. At a constant relative temperature $t=T∕{T}_{C}$ the saturation value of the Kerr rotation shows a maximum at $x=0.95$ and then decreases with decreasing Fe concentration vanishing at $x=0.75$. The Curie temperature ${T}_{C}$ shows a similar dependence on the composition. For $x\ensuremath{\geqslant}0.75$ scanning tunneling spectroscopy reveals conductivity maxima in the unoccupied local density of states near the Fermi level at constant energies independent of the Cr concentration. This behavior can be explained by a simple model of $d$-band filling, similar to the explanation of the Slater-Pauling curve for binary alloys. We demonstrate the growth of artificially heterogeneous structures using sequential deposition of Fe and Cr. The temperature dependence of the multistripe growth and interdiffusion is investigated. Appropriate deposition parameters allow the growth of a two-dimensional analogon to an Fe-Cr-Fe spin valve.

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.

Effect of nitridation on the growth of GaN on ZrB2(0001)∕Si(111) by molecular-beam epitaxy

The effect of nitridation on the epitaxial growth of GaN on lattice-matched ZrB2(0001) films prepared ex situ and in situ was studied using an ultrahigh-vacuum molecular-beam epitaxy (MBE)-scanning probe microscopy system. The growth of GaN was carried out by rf-plasma-assisted MBE, and epitaxy of wurtzite GaN was observed on both ex situ and in situ prepared ZrB2 samples. The polarity was found to be consistently N-polar regardless of the samples, based on the observation of a series of N-polar Ga-rich reconstructions: (3×3), (6×6), and c(6×12). The nitridation of ZrB2 film was conducted by exposing it to active nitrogen and well-ordered hexagonal-BN (h-BN) formation was observed when the annealing temperature was above 900°C. The partially formed BN layer affected neither the epitaxy nor the polarity of GaN, but when the surface was fully covered with well-ordered h-BN, GaN growth did not occur.

Structure-induced magnetic anisotropy in the Fe(110)∕Mo(110)∕Al2O3(112¯0) system

Fe(110) films were epitaxially grown on sapphire substrates using a Mo(110) buffer layer in an ultrahigh-vacuum molecular-beam epitaxy system. The magnetic properties were examined ex situ by Brillouin light scattering and superconducting quantum interference device magnetometry. To determine the magnetic anisotropy constants the frequency of the Damon-Eshbach [J. Phys. Chem. Solids 19, 308 (1961)] surface spin-wave mode was measured as a function of the in-plane angle between the external magnetic field and the Fe[001] crystal axis. The angle-dependent frequency was fitted by a spin-wave model. We found that the easy axis of the cubic magnetocrystalline anisotropy K1 and an additional uniaxial in-plane anisotropy K‖(2) are aligned parallel to the in-plane Fe[001] axis for Fe-layer thicknesses from 0.8to37nm, with K1 increasing and K‖(2) decreasing with increasing Fe thickness. Possible origins of the observed uniaxial anisotropy are discussed.

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 (>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.

Magnetic and electronic properties of binary alloy monolayers:FexMn1−xandCo1−xFexon W(110)

Pseudomorphic binary alloy monolayers Fe x Mn 1 - x and Co 1 - x Fe x were grown on W(110) by molecular-beam epitaxy in ultrahigh vacuum. Fe atoms are randomly replaced by Mn or Co atoms. The pseudomorphic monolayers form extended areas of monolayer coverages at elevated temperatures in the step flow growth mode. For Fe x Mn 1 - x /W(1100) monolayers the Kerr rotation decreases with decreasing Fe concentration, corresponding to a decrease of the magnetization of Δμ/Δx=-(8′2)μ B /atom. The Curie temperature decreases with a rate of [ΔT C (x)/Δx]T C (Fe)=-(1.8′0.2)×10 - 2 /at. %. Scanning tunneling spectroscopy reveals characteristic maxima in the unoccupied local density of states near the Fermi level, which shift towards E F with decreasing Fe concentration for both alloys investigated here. The reduction of magnetization with decreasing Fe concentration can be explained by a simple model of d-band filling, similar to the explanation of the Slater-Pauling curve for binary alloys. We observe a maximum value for T C and for the Kerr rotation for the pure Fe/W(110) monolayer, in contrast to the behavior of bulk alloys where maximum values are obtained for Co-Fe alloys.

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.

Si(100) Surface Cleaning Using Sr and SrO

AbstractA 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 a ultrahigh vacuum molecular beam epitaxy system. The substrate is then heated to ∼800°C for about 5 minutes, the SiO2 is removed to leave behind a Sr terminated Si(100) surface. This Sr 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 was mainly SiO and O, no significant amount of Sr containing species was observed. We conclude that the SiO2 removal is due to the catalyst reaction SiO2 + Sr (or SrO) → SiO (g) + O + Sr (or SrO). The reaction happened through several intermediate steps. 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 preventing further etching and the formation of pits in the substrate.

Structural observation of Mn silicide islands on Si(1 1 1) 7×7 surface with UHV-TEM

Atomic structure and crystallography of Mn silicide islands formed on an Si(1 1 1) surface was investigated. Mn with 8–10 MLs was deposited on an Si(1 1 1) 7×7 surface at room temperature by means of an ultrahigh vacuum molecular beam epitaxy, and then transferred to an ultrahigh vacuum transmission electron microscope (UHV-TEM) without exposure to air. Annealing at 400°C caused the round shaped silicide islands composed of MnSi with a size of several tens of nanometers to grow epitaxially. From transmission electron microscopy analysis, it was found that the crystallography of the islands matches with that of the Si(1 1 1) substrate with the relation (1 1 1) Si∥(1 1 1) MnSi and [ 1 ̄ 0 1] Si ∥ [ 1 ̄ 2 1 ̄ ] MnSi , but their interface was incommensurate.

High Resolution Transmission Electron Microscopy Study on the Structure of Ge Nanoparticles by Using an Ultrahigh Vacuum-Molecular Beam Epitaxy-Transmission Electron Microscope System

Ge nanoparticles were deposited on thinned Si substrates in an ultrahigh vacuum molecular beam epitaxy instrument (UHV-MBE) and transferred into an UHV-field emission transmission electron microscope (UHV-FE-TEM) through an UHV-transferring system. TEM observation indicated that the Ge nanoparticles had a round shape and were 2 to 5 nm in diameter. The crystalline Ge nanoparticles exhibited single crystals, single-twinned crystals, and decahedral multi-twinned particles (MTPs). It was found that the crystal structure of some nanoparticles changes from a diamond structure to a novel structure, which is proposed to be a β-Sn structure, with a=0.38 nm and c=0.71 nm. The particles with the new structure are not formed when the substrate temperature is above 370°C. The structure of MTPs has been discussed. Some Ge nanocrystals even became amorphous and then fluctuated continually during HRTEM observation.

Surface phases of GaAs and InAs (001) found in the metalorganic vapor-phase epitaxy environment

We have characterized the (2×4) and (4×2) reconstructions of GaAs and InAs (001) that are present in a metalorganic vapor-phase epitaxy (MOVPE) reactor. Scanning tunneling micrographs show that these surfaces are terminated with arsenic and gallium (or indium) dimers. The (2×4) dimer row exhibits a mottled appearance, which is ascribed to the adsorption of alkyl groups on some of the sites. On the (4×2), <10% of the surface is covered with small (2×4) islands. These results show that, in the MOVPE environment, the GaAs and InAs surface structures are nearly the same as those found in ultrahigh vacuum molecular beam epitaxy.

Structural properties of Ga2O3(Gd2O3)–GaAs interfaces

Ga 2 O 3 (Gd 2 O 3 )–GaAs heterostructures in situ fabricated using a multichamber ultrahigh vacuum (molecular beam epitaxy) system were studied by x-ray reflectivity measurement and high-resolution transmission electron microscopy. The oxide–GaAs interfaces were found to be very smooth with the roughness no more than 1 nm. Moreover, an interfacial roughness as small as one atomic layer of GaAs (0.33 nm) was observed using x-ray reflectivity.

Structure and Magnetism of MBE-Grown Co/Cu Multilayers

Structural and magnetic properties of Co/Cu multilayers deposited in the ultra-high vacuum molecular beam epitaxy system on glass substrates with different modulations periods were investigated. A structural characterization was performed by means of RHEED and Auger spectroscopy (in situ), small angle X-ray reflectivity and scanning tunneling microscopy. The samples obtained have a textured, polycrystalline layered structure for deposition at room temperature. Magnetization and in-plane magnetoresistance measurements were performed as a function of Cu and Co layer thicknesses. An influence of different buffers and of interface quality on magnetic properties was investigated.