Ion scattering study of the surface structure of epitaxial NiSi2 on Si(001)

Ion Scattering Spectroscopy was used to study the surface atomic structure of epitaxial NiSi₂ grown on the (001) surface of Si. The ion scattering data does not agree with the model recently proposed by Jona et al. [Solid State Commun. 57, 687 (1986)], but supports a model in which the NiSi₂ (001) surface consists of a bulk-like Si termination of the fluorite structure with a 0.3 Å ± 0.05 Å inward relaxation and a 20%–30% concentration of vacancies in the Si terminated plane.     

Structure analysis of thin iron-silicide film from φ-scan RHEED Patterson function

The atomic structure of thin iron silicide film, grown epitaxially on the Si(111) surface, has been analyzed by means of the three-dimensional RHEED Patterson function analysis. The iron-silicide-terminated surface with (2 × 2) periodicity has been prepared by a solid-phase epitaxy method. 2 ML of Fe were deposited on the Si(111)-(7 × 7) surface and annealed at 500°C. Three-dimensional Patterson function was calculated from series of φ-scanned RHEED intensity distributions converted to the k-space. The resulting model of γ-FeSi₂ structure consists of two silicide layers faulted to each other with three relaxed Si adatoms above the H₃ site.     

Supraleitung und spezifische Wärme von Titan-Rhodium-Legierungen

The specific heats of Ti-Rh alloys have been measured between 0.9 and 8 °K for atomic percent Rh concentration between 0 and 10. The transition temperatures of the hexagonal (0–2 At-% Rh) and the cubic (3–10 At-% Rh) phases are given by the calorimetric measurements. The transition temperature and the electronic specific heat increase with Rh-concentration. The influence of the rate of cooling on the transition temperature has been studied for some alloys. The electrically, magnetically, and calorimetrically determined transition temperatures are compared. Contrary to Hf₂Rh and Zr₂Rh down to 1.2 °K Ti₂Rh has not been observed to become superconducting.     

Surface structure of GaAs(211)

The atomic structures of the two inequivalent (211) surfaces of GaAs have been investigated by LEED. Both surfaces, prepared by etching and heat-cleaning or ion-sputtering and annealing, are unstable and develop large (110) facets which exhibit the atomic geometry of the (110) GaAs surface. These facets entirely cover the surface. Three sets of facets, making 30°, 30° and 54° angles with the (211) plane, are detected on one surface. Only two sets, making 30° angles with the (211) plane, are detected on the other. The reasons for this difference are not understood at this time. The LEED study of Si(211) and Ge(211) shows that the Si surface is flat whereas the Ge surface exhibits reconstructed (311) facets. The structural difference between the (211) surfaces of GaAs and Ge and the facetting of the compound are invoked to explain the problems encountered in the MBE growth of GaAs on Ge(211).     

Diffraction de neutrons dans CoCl2, 6D2O determination de ses structures cristallographique et magnetique

We have performed neutron diffraction experiments on a CoCl₂, 6D₂O monocrystal. We show that a crystallographic transition from the C 2/m monoclinic structure to the P $\text{1}$ triclinic structure occurs at T = Tmt = 73.5K., together with a twinning of the monocrystal. We describe the two possible twins (four types of domains) related to the loss of the initial monoclinic symmetry, say of the elements 2 and m : either m becomes twin plane (Albite twinning) or 2 becomes twin axis (Pericline twinning). Each domain has an antiferromagnetic structure P 2a T, with an easy magnetization axis tilted from the ac plane by an angle φ = (30 ± 4)°, its component on this plane making an angle φ = (3.5 ± 2)° with the c axis.     

用低能电子衍射谱计算过渡金属吸附CO的表面结构

本文用低能电子衍射谱研究了CO分子吸附在过渡金属上的表面结构。     

Structural and optical properties of metastable SiGe/Si films with a low germanium concentration

The properties of metastable Si_{1 ? x} Ge _x /Si (10% < x < 16%) layers grown by molecular beam epitaxy on Si(100) substrates have been investigated using atomic force microscopy, X-ray diffraction, and low-temperature luminescence spectroscopy. It has been shown that ring-like aggregates are formed on the surface of layers grown at temperatures of 500–700°C. The size and shape of these aggregates suggest that their formation is associated with the diffusion instability arising due to the existence of a relationship between the surface diffusion, stresses, and the wetting potential during the growth of the epitaxial film. The existence of deviations from the homogeneous germanium distribution in the layer plane has been confirmed by a detailed analysis of the X-ray rocking curves and two-dimensional diffraction patterns. The structures with severe surface disturbances are characterized by an abnormal change in the decay times of the emission lines of bulk silicon, which indicate the presence of local electric and/or strain fields in subsurface regions. The perturbations of the flat crystallization front are suppressed as the growth temperature of layers decreases to 350°C. Despite the absence of a coating layer of silicon, the photoluminescence spectra of the layers themselves depend weakly on their thickness and growth temperature and remain sensitive only to the technological concentration of germanium. A slowly decaying luminescence associated presumably with the localization of excitons near the SiGe-Si interface has been observed in one of the samples grown at a temperature of 700°C and containing a dense array of ring-like aggregates.     

β-SiC(001)-(2×1)表面结构的第一性原理研究

利用缀加平面波加局域轨道(APW+LO)的第一性原理方法计算了β-SiC(001)-(2×1)表面的原子及电子结构. 原子结构的计算结果表明，与Si(001)-(2×1) 表面的非对称性Si二聚体模型不同，β-SiC(001)-(2×1)表面为对称性的Si二聚体模型，其二聚体的Si原子间键长也较大，为0.269nm. 电子结构的计算结果表明，在费米能级处有明显的态密度，因此β-SiC(001)-(2×1)表面呈金属性. 在带隙附近存在四个表面态带,其中的两个占有表面态带已由价带的同步辐射光电子能谱实验得到证实. 关键词： 碳化硅 缀加平面波加局域轨道方法 原子结构 电子结构     

The growth and domain structure of YBa2Cu3Ox films on neodymium gallate substrates with a deviation of the normal to the surface from the [110] direction in NdGaO3

The growth, crystal structure, and electrophysical properties of YBa₂Cu₃O_x (YBCO) epitaxial films grown with and without a CeO₂ epitaxial sublayer on NdGaO₃ (NGO) substrates with the normal to the surface deviating from the [110] axis by 5° to 26.6° around the [001] axis are investigated. It is shown that the orientation of YBCO epitaxial films grown on such substrates is determined by the existence of symmetry-equivalent directions in the substrate and in the CeO₂ layer, as well as by the rate of film deposition. For a high deposition rate, YBCO films grow on the CeO₂ sublayer in the [001] orientation irrespective of the orientation of the substrate and the sublayer. It was found that when the angle of deviation of the substrate plane is from the (110) NGO plane, twinning of one or both twinning complexes in YBCO may be suppressed.     

Parameter variation in low-quartz between 94 and 298K

The structure of low quartz has been refined at 94, 115, 150, 190, 240 and 298K using Mo Kα intensities up to 2θ = 90° with residuals around 1.25%. The major structural change from 94 to 298K is an increase of the SiOSi angle from 142.69(4) to 143.65(5) corresponding to a rotation by ～0.8° of a rigid SiO₄ tetrahedron around the crystallographic 2-axis through its central Si. The orientations of the principal axes of vibration of Si and O do not change significantly with temperature. The temperature dependence of the mean-square displacement of O along its principal axes follows the Debye formula of thermal motion quite well, while for Si the experimental slopes are significantly less than their calculated values.     

Ferromagnetism in laser-deposited GaMnAs layers

The properties of the GaMnAs layers grown by laser deposition on semi-insulating GaAs(100) substrates at temperatures from 300 to 650°C have been investigated. A strong anisotropy of the hysteretic curve of the angle of rotation of the plane of polarization with a change in the magnetic field direction in the sample plane was found during investigation of the magneto-optical Kerr effect (300 K). The domain structure in GaMnAs layers has been observed for the first time at room temperature by magnetic-force microscopy.     

SiC based Si/SiC heterojunction and its rectifying characteristics

The Si on SiC heterojunction is still poorly understood, although it has a number of potential applications in electronic and optoelectronic devices, for example, light-activated SiC power switches where Si may play the role of an light absorbing layer. This paper reports on Si films heteroepitaxially grown on the Si face of (0001) n-type 6H-SiC substrates and the use of B₂H_6 as a dopant for p-Si grown at temperatures in a range of 700--950~\du. X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM) tests have demonstrated that the samples prepared at the temperatures ranged from 850~℃ to 900~℃ are characterized as monocrystalline silicon. The rocking XRD curves show a well symmetry with FWHM of 0.4339° Omega. Twin crystals and stacking faults observed in the epitaxial layers might be responsible for widening of the rocking curves. Dependence of the crystal structure and surface topography on growth temperature is discussed based on the experimental results. The energy band structure and rectifying characteristics of the Si/SiC heterojunctions are also preliminarily tested.     

Excimer laser-assisted chemical process for formation of hydrophobic surface of Si (001)

Silicon (Si) wettability is one of the important parameters in the development of Si-based biosensing and lab-on-chip devices. We report on UV laser induced hydrophobicity of Si (001) wafers immersed in methanol during the irradiation with an ArF excimer laser. The irradiation with 800 pulses of the laser operating at 65 mJ/cm² allowed to significantly increase the hydrophobicity of investigated samples as characterized by the static contact angle change from 77° to 103°. Owing to the irradiation with relatively low laser fluence, no measurable change in surface morphology of the irradiated samples has been observed with atomic force microscopy measurements. The nature of the hydrophobic surface of investigated samples is consistent with X-ray photoelectron spectroscopy analysis that indicates formation of Si–O–CH₃ bonds on the surface of the laser-irradiated material.     

Crystallization of sputter deposited amorphous metal thin films

We have studied the crystallization of DC sputter deposited amorphous W-Si alloys under three separate conditions: free standing films, amorphous films deposited on Si substrates, and films with one of four different overlayers. In the case of the free standing films the crystallization temperature versus composition was measured by differential thermal analysis (DTA) for films with Si contents from 5 at% to 38 at%. Films with Si concentrations of 38% to 22% were amorphous as deposited, while those with less Si were crystalline. The crystallization temperature was a strong function of composition with a maximum T_c of 915°C (at a heating rate of 20 K/min) in alloys with 28% Si. Overlayers of W, Cu, Au and Al were investigated. Both the W and Cu overlayers had little effect on the stability of the underlying W-Si, while both the Au and Al reduced the crystallization temperature by at least 100°C. The results reported here reinforce the observation that the choice of overlayer plays a critical role in determining the overall stability of metallization systems that include amorphous layers as diffusion barriers.     

Nucleation and growth of Si on Pb monolayer covered Si(111) surfaces

With a scanning tunneling microscope (STM), we study the initial stage of nucleation and growth of Si on Pb monolayer covered Si(111) surfaces. The Pb monolayer can work as a good surfactant for growth of smooth Si thin films on the Si(111) substrate. We have found that nucleation of two-dimensional (2D) Pb-covered Si islands occurs only when the substrate temperature is high enough and the Si deposition coverage is above a certain coverage. At low deposition coverages or low substrate temperatures, deposited Si atoms tend to self-assemble into a certain type of Si atomic wires, which are immobile and stable against annealing to ~ 200 °C. The Si atomic wires always appear as a double bright-line structure with a separation of ~ 9 Å between the two lines. After annealing to ~ 200 °C for a period of time, some sections of Si atomic wires may decompose, meanwhile the existing 2D Pb-covered Si islands grow laterally in size. The self-assembly of Si atomic wires indicate that single Si adatoms are mobile at the Pb-covered Si(111) surface even at room temperature. Further study of this system may reveal the detailed atomic mechanism in surfactant-mediated epitaxy.     

In situ three‐dimensional reciprocal‐space mapping during mechanical deformation

Mechanical deformation of a SiGe island epitaxically grown on Si(001) was studied by a specially adapted atomic force microscope and nanofocused X‐ray diffraction. The deformation was monitored during in situ mechanical loading by recording three‐dimensional reciprocal‐space maps around a selected Bragg peak. Scanning the energy of the incident beam instead of rocking the sample allowed the safe and reliable measurement of the reciprocal‐space maps without removal of the mechanical load. The crystal truncation rods originating from the island side facets rotate to steeper angles with increasing mechanical load. Simulations of the displacement field and the intensity distribution, based on the finite‐element method, reveal that the change in orientation of the side facets of about 25° corresponds to an applied pressure of 2–3 GPa on the island top plane.     

Influence of surface oxidation on the photoelectron diffraction intensities from InP single crystals

The Monte Carlo method was used to calculate the influence of amorphous overlayers on the angle-dependent intensity distribution due to photoelectron diffraction (XPD) in monocrystalline substrates. The influences of the overlayer thickness, the elastic and inelastic mean free paths in the amorphous overlayer, the shape of the initial intensity distribution, as well as the overlayer chemical composition, on XPD is discussed and some trends are formulated. The simulated P 2p intensities of oxidized InP agree with the experimental data. The effect of elastic scattering on the intensity minimum around the detection angle 18° is stronger than that of inelastic scattering. A new method for the thickness determination of thin overlayers on single ecrystals is discussed.     

Angle resolved photoemission measurements on AgSi(111) 7 × 7 interfaces

The AgSi(111) interface is investigated by LEED, AES and angle resolved photoemission spectroscopy using 50 eV synchrotron radiation in p-polarization. Results on room temperature (RT) silver growth on Si(111) 7 × 7 are characterized by an evolution of the LEED pattern and of the d band shape which is consistent with 2D island formation in the submonolayer range. When the Ag coverage (Θ) is increased, a progressive build-up of Ag layers occurs with a possible interdiffusion of the atomic constituents. The ordered $\text{Si(111)}\text{3}\text{×}\text{3}\text{R(30°)Ag}$ structure (R₃) obtained by annealing a 1 ML RT deposit gives rise to new interface states near E_F. In contrast to the RT deposit at the same Θ, two well defined d band peaks are present while the bulk Si emission near 3.4 eV is clearly seen. The R₃ data would favour recent crystallographic models which conclude to an embedment of the Ag atoms in a threefold hollow adsorption site.     

Edge misfit dislocations in GexSi1 ? x/Si(001) (x ～ 1) heterostructures: role of buffer GeySi1 ? y (y < x) interlayer in their formation

The structure of dislocations in Ge _x Si_{1 − x} (x ∼ 0.4–0.8) films grown by molecular beam epitaxy on Si(001) substrates tilted by 6° toward the nearest (111) plane has been studied. The epitaxy of GeSi films on substrates deviating from the exact (001) orientation has allowed us to establish the main mechanism of formation of edge misfit dislocations (MDs), which most effectively (for heterostructures of the given composition) relieve stresses caused by the mismatch between lattice parameters of the film and substrate. Despite the edge MDs being defined as immobile (sessile) dislocations, their formation proceeds according to the gliding mechanism proposed by Kvam et al. [J. Mater. Res. 5, 1900 (1990)]. A comparative estimation of the propagation velocities of the primary and induced 60° dislocations, as well as the resulting 90° MDs, has been performed. It has been established that the condition providing for the most effective edge MD formation by the induced nucleation mechanism is the appearance of 60° MDs in a stressed film immediately after it reached a critical thickness. A source of these dislocations can be provided by a preliminarily grown buffer GeSi layer that occurs in a metastable state at the initial stage of plastic relaxation.     

Magnetic linear dichroism in photoemission from an ultrathin iron silicide film

The effect of magnetic linear dichroism in photoemission of Fe 3p electrons was used to investigate the magnetic properties of the Si(100)2 × 1 surface on which iron films up to 10 monolayers thick were deposited at room temperature under ultrahigh vacuum. The experiments were performed with linearly polarized light (at a photon energy of 135 eV) incident at an angle of 30° to the surface. The photoelectron spectra were measured in a narrow solid angle oriented along the normal to the sample surface for two opposite magnetization directions which were parallel to the surface plane and perpendicular to the polarization vector of the light wave. An analysis of the data obtained showed that the effect has a threshold character and appears after deposition of eight Fe monolayers, when the ferromagnetic silicide Fe₃Si is formed on the surface.