SI Direction Research Articles

Effects of growth temperature on the surface morphology of silicon thin films on (111) silicon monocrystalline substrate by liquid phase epitaxy

Abstract We investigated the effects of growth temperature on the surface morphology of silicon layers grown by liquid phase epitaxy method on a (1 1 1) oriented silicon substrate in a relatively low-temperature range from 600°C to 900°C using Au–Bi alloy solvent. Layers are formed in the following growth sequence: (1) commencing in the form of initial nuclei as starting points of growth, (2) these structures grow along lateral and vertical directions as island structures, and (3) coalesce each other. Moreover, it was made clear that flat layers tend to cover the substrate completely at high growth temperature, in order to clarify the main factor that influences the surface morphology, an empirical model of the growth process was constructed. As a result, the temperature dependence of the surface morphology mainly depends on the temperature dependence of the preferential growth direction of Si on Si (1 1 1).

Molecular dynamics study on stopping powers of channeled He and Li ions in Si

A molecular dynamics simulation method for calculating the electronic stopping power for MeV energy He and Li ions in crystalline silicon is described. Two methods for calculating electronic stopping in channelling directions are studied. In the first method velocity dependent phase-shifts are calculated using a spherical average of a doping atom potential moving in an electron gas. In the second method an interpolation scheme between the density functional theory-calculation for low energies and a Bethe-like formula for high energies is used. The methods have been used to calculate stopping powers for He ions impinging in channeling directions of crystalline Si up to 3 MeV. The interpolation model has also been used to calculate channeling stopping power for Li ions up to 9 MeV. The simulation results are compared with recent experiments.

Charge-density analysis of spodumene (LiAlSi2O6), from ab initio Hartree?Fock calculations

An ab initio Hartree–Fock study of spodumene structure has been performed, and the wave function was used for a topological analysis of the electron density. The three non-equivalent oxygen atoms (O1, O2 and O3) of spodumene differed mainly in their valence shell charge concentration (VSCC). In particular, O1 shows a maximum of charge concentration along the Si–O bond path, and two other maxima, which can be regarded as lone pairs, point in the opposite direction with respect to the silicon position. O2 shows a torus of local charge concentration, whose axis is parallel to the Al–Si direction, with a bulge on the external side of the Si–O2–Al angle; even if no maximum exists in the Si direction, the valence shell of O2 appears to be strongly polarized toward the silicon; a slight polarization also exists along the Al–O bond path. A similar situation is displayed by O3, whose charge concentration has a torus-like shape, with the axis parallel to the Si–Si direction, and a bulge on the external side of the Si–O3–Si angle; as in the case of O2, a significant polarization of the valence shell of O3 is observed, towards the two Si directions. By recasting the calculated electron distribution in terms of the valence bond theory, a correlation has been found between structural details (bond lengths) and the hybridization state of the oxygen atoms.

Self-assembled NiSi quantum-dot arrays on epitaxial Si0.7Ge0.3 on (001)Si

Self-assembled NiSi quantum-dot arrays have been grown on relaxed epitaxial Si0.7Ge0.3 on(001)Si. The formation of the one-dimensional ordered structure is attributed to the nucleation of NiSi nanodots on the surface undulations induced by step bunching on the surface of SiGe film owing to the miscut of the wafers from normal to the (001)Si direction. The two-dimensional pseudohexagonal structure was achieved under the influence of repulsive stress between nanodots. Since the periodicity of surface bunching can be tuned with appropriate vicinality and misfit, the undulated templates promise to facilitate the growth of ordered silicide quantum dots with selected periodicity and size.

Electron loss from fast partially stripped C and O ions incident on crystal targets

We have measured energy spectra of electrons emitted in a backward direction of Si and Ge crystals while bombarded by 2.5 and 3.5 MeV/u C 4+, C 6+, O 5+ and O 8+. Under 〈1 1 0〉 channeling incidence conditions, the loss electron yield from the partially stripped ions is appreciably reduced (by a factor of 0.5–0.6 for C 4+) relative to the non-channeling case. This reduction can be directly related to the reduced charge states of the channeled ions in the crystals. Furthermore, the evolution of the pre-equilibrium charge states of the incident ions has been deduced from a comparison of the effective nuclear charges for the loss electron yield, the low-energy electron yield, and the binary-encounter electron yield reported previously.

Growth of zinc oxide nanowires by thermal evaporation on vicinal Si(1 0 0) substrate

The growth behavior of ZnO nanowires on an anisotropic crystalline substrate has been investigated in this work. On both the vicinal Si(1 0 0) substrate with a 6° miscut angle in the 0 1 ̄ 1 ̄ Si direction and the on-axis Si(1 0 0) substrate, ZnO nanowires have been directly grown by thermal evaporation using the ball-milled ZnO powders under controlled conditions without the presence of catalyst. In this work, we confirmed that the vicinal surface of the substrate leads to selective growth during the initial growth of ZnO nanowires. For the vicinal substrate that consists of terraces separated by steps, ZnO nanowires are preferentially grown near step-edge lines aligned along the 0 1 1 ̄ Si azimuth. For the on-axis substrate, on the other hand, ZnO nanowires are randomly grown on the whole surface of the substrate. In addition, the average diameter of the nanowires grown on the vicinal substrate is slightly thinner than that of the nanowires grown on the on-axis substrate. The vapor–solid model is also proposed to explain the growth behavior of ZnO nanowires in our synthesis process.

A Model of Skeletal Remodeling Should (but Does Not Currently) Include Mechanical Feedback

A Model of Skeletal Remodeling Should (but Does Not Currently) Include Mechanical Feedback

Epitaxial PbZr.52Ti.48O3 films on SrTiO3/(001)Si substrates deposited by sol–gel method

We report on the sol–gel deposition and characterization of high-quality, epitaxial films of PbZr.52Ti.48O3 (PZT) on (001)Si substrates, with a thickness range of 400 Å to 1 μm. The epitaxial growth of PZT on (001)Si is achieved using a thin template layer of SrTiO3, grown by molecular-beam epitaxy. The sol–gel PZT films have a typical surface roughness of 5 Å and exhibit well defined reflective high-energy electron diffraction patterns characteristic of smooth, epitaxial films. Using high-resolution transmission electron microscopy and double-crystal x-ray diffraction, we find that the PZT films are oriented with the c axis normal to the (001)Si plane and with the a axis lying along 〈110〉Si direction. Finally, we measure the electromechanical coupling coefficients and the surface acoustic wave velocities for our films as a function of thickness and compare our experimental data to previously published theoretical values for this system.

A quality control program for MR-guided focused ultrasound ablation therapy.

In this study, we propose a quality control program for MR‐guided focused ultrasound (FUS) ablation treatment to assess FUS beam positioning accuracy, FUS power delivery accuracy, MR imaging quality, and FUS ablation system safety. A total of 353 sonication points in Lucite cards were measured, the average placement errors were –0.06 mm in the SI direction and –0.04 mm in the LR direction. Temperature elevation was calculated from MR phase difference images and the measured water proton chemical shift (WPCS) temperature coefficient. WPCS temperature calibration for phantoms yielded a temperature coefficient of 0.011 ppm/°C. Sixteen experiments were conducted using six different phantoms to test the reliability of FUS power delivery. SNR and RF power calculated from phantom images were analyzed and stored at the MR console. A computer program was developed to integrate the system power delivery and the MR image quality control into one automated process. In the clinical trial at our institution, we expect this quality control program to be carried out before each patient treatment. If measured quality control values exceeds or below the preset values, a system service and retest should be conducted before the treatment.PACS number(s): 87.61.–c, 87.54.–n

Orientational order of hut clusters grown coherently on the substrate

Within the Tersoff approximation we obtain the analytic expressions for the elastic hut-hut and pyramid-pyramid interaction energies. Numerous experimental results show that the orientation of a Ge hut ridge is either parallel or perpendicular to the [010] direction of Si, and we treat these two possible hut orientations as spin variables in the two-dimensional (2D) Ising model. To study the thermal-equilibrium configuration of hut orientations, we map the Hamiltonians of the square-lattice and rectangular-lattice hut systems (SLHS and RLHS) onto that of the isotropic and anisotropic 2D Ising models with antiferromagnetic couplings, respectively, where the latter is accompanied by a magnetic field. We predict that a hut-orientation order-disorder phase transition takes place in the SLHS, and obtain the dependences of the transition temperature on the hut size, the aspect ratio of the hut base, and the hut-lattice spacing. For the case of the RLHS, the relations of critical temperatures to the hut-lattice constant, hut base aspect ratio, and size are also found by performing intensive Monte Carlo simulations.

Random and channeling stopping powers of He and Li ions in Si

In this work we have measured the electronic stopping powers of ${}^{4}\mathrm{He}$ and ${}^{7}\mathrm{Li}$ ions for channeling and random directions in Si as a function of the incident ion energy. The channeling $(〈100〉$ for Li, $〈110〉$ and $〈111〉$ for Li and He) and the random (Li only) measurements cover a wide energy range between 200 keV and 9 MeV. The Rutherford backscattering technique together with different multilayer targets has been employed in the present experiments. The results are compared to calculations carried out in the framework of the unitary convolution approximation, which takes into account the impact-parameter-dependent energy loss for each projectile charge state, with further refinements including screening effects in close collisions between the electrons of the target and the screened projectile.

Energy loss measurements of H 2 and H 3 molecular beams along random and [formula omitted] directions of Si

The energy loss of H 2 and H 3 molecules along the 〈1 1 0〉 and non-aligned directions of Si was measured through the Rutherford backscattering technique in combination with a SIMOX target. The experiments were performed at 300, 500 and 700 keV/atom. The results show the following features: First, the random and channeling molecular stopping powers are larger than the corresponding atomic one. Second, at a given energy per atom, the H 3 energy loss is larger than the one of H 2. Third, within the experimental errors, the channeling and random molecular stopping powers are equal. Finally, there are strong indications that the contribution of the screened Coulomb explosion to the energy straggling of molecular beam increases with the molecular velocity.

Microstructural differences of the two possible orientations of GaAs on vicinal (0 0 1) Si substrates

Two alternative orientations of a GaAs layer in respect to a vicinal (0 0 1) substrate, misoriented toward [1 1 0], are possible for most of the miscut angles; type A orientation having the GaAs [1 1 0] parallel to the [1 1 0] Si misorientation direction and type B with the GaAs [1 1 ̄ 0] parallel to the [1 1 0] Si direction. The dependence of the GaAs surface roughness and of the tilting between the GaAs and Si (0 0 1) planes on the GaAs/Si orientation and the miscut angle of the vicinal substrate, in the range of 0–9°, has been investigated. The GaAs/Si surface roughness was characteristic for preferential growth along [1 1 ̄ 0]. Thin GaAs films of approximately 2 μm thickness exhibited an almost constant rms roughness of 6.5–7.0 nm for the type A orientation, while the rms roughness varied in the range of 4.2–60 nm for the type B orientation, depending on the value of the miscut angle. The tilting angle between the GaAs and Si (0 0 1) planes exhibited different signs for the two types of orientation and much higher values for the type B orientation. A rather constant negative tilting was observed for type A samples, in the range of miscut angles of 1.5–7.5°. The results suggest that the GaAs/Si lattice tilting may be a sensitive index for the domain purity in the GaAs/Si films.

Extracting tongue muscle contraction patterns from tagged cine MRI

This study presents mechanically modeled 3-D volumetric strains for the tongue during speech for the syllable ‘‘sha.’’ Multiplanar tagged cine MRI (tMRI) provided input data for a B-spline, geometry-independent, cardiac tag tracking method, devised for the heart, which has been adapted for the tongue. Three sets of tMRI images with orthogonal tag planes were collected in 10 axial and 5 sagittal slices for 24 consecutive time phases. The sagittal slices were recorded twice, once each with a series of horizontal and vertical tag planes. The axial slices were recorded once with lengthwise (anterior-to-posterior) tag planes. These tag planes reflect deformations in the SI, AP, and RL directions, respectively. Within the tongue we tracked 3-D motion, calculated 3-D strains in each image plane, and reconstructed 3-D deformation for the entire volume. From the model muscle contraction patterns are inferred for genioglossus anterior, verticalis, and transverse. To infer muscle contraction we determined the lines of action for each muscle, and tracked their linear strains in the appropriate planes for all 24 time phases. Results showed that muscle compressions are consistent with expected muscle contractions, and we were able to distinguish between transverse and verticalis activity. [Work supported by NIH/NIDCD Grant No. DC01758.]

Anomalous elastic response of silicon to uniaxial shock compression on nanosecond time scales.

We have used x-ray diffraction with subnanosecond temporal resolution to measure the lattice parameters of orthogonal planes in shock compressed single crystals of silicon (Si) and copper (Cu). Despite uniaxial compression along the (400) direction of Si reducing the lattice spacing by nearly 11%, no observable changes occur in planes with normals orthogonal to the shock propagation direction. In contrast, shocked Cu shows prompt hydrostaticlike compression. These results are consistent with simple estimates of plastic strain rates based on dislocation velocity data.

Self-assembled growth of epitaxial erbium disilicide nanowires on silicon (001)

By choosing a material that has an appropriate asymmetric lattice mismatch to the host substrate, in this case ErSi2 on Si(001), it is possible to grow one-dimensional epitaxial crystals. ErSi2 nanowires are less than one nanometer high, a few nanometers wide, close to a micron long, crystallographically aligned to 〈110〉Si directions, straight, and atomically regular.

Molecular H 2 and H 3 energy loss measurements along the Si 〈111〉 direction

In the present contribution we present results on molecular H 2 and H 3 energy loss measurements in the 〈1 1 1〉 Si direction by using Rutherford backscattering/channeling spectrometry and a SIMOX target. With this aim, we have used atomic and molecular H 2 and H 3 beams with energies of 250, 400 and 800 keV/atom. From the present results it was determined that the H 2 molecular stopping is larger than the atomic one. Moreover, our measurements show that the H 3 stopping is of the order of the H 2 one. In addition we have determined that the coherent distances of the H 2 clusters vary between 20 and 33 nm.

TiN growth on Si by hybrid radical beam PLD

High-quality (good crystallinity and stoichiometry) titanium nitride (TiN) thin films were grown on Si(100) substrates by pulsed laser deposition (PLD) using a high-purity titanium target (99.99%) and nitrogen radical beam. The crystallinity, chemical composition, and depth profiles of the grown films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS), respectively. The XRD pattern indicated that the preferred growth of TiN(200) with an orientation parallel to the Si(100) direction was obtained and the nitrogen radical drastically improved crystallinity compared with that grown in ambient nitrogen gas. RBS spectra indicated that the combination of PLD and the nitrogen radical beam suppressed silicidation at the interface between the Si substrate and TiN thin film during growth. The XPS analysis revealed that this method achieved the synthesis of stoichiometric TiN films.

Ab Initio Studies of Disilazanes: Structures and Vibrational Properties of Hexachloro-, Hexamethyl-, and Tetrachlorodisilazane

The structures of hexachlorodisilazane, NH(SiCl3)2, and hexamethyldisilazane, NH(SiMe3)2, have been calculated at HF/6-31G* and MP2/6-31G* levels. Both contain planar HNSi2 skeletons and show the expected staggering of the substituents as seen along the Si‚‚‚Si direction, as previously found in the electron diffraction structure of the hexamethyl compound. Unlike the latter, however, the ab initio structures both belong to the C2 point group. Force fields for these two molecules and for tetrachlorodisilazane, NH(SiHCl 2)2, were calculated at the HF level and scaled to produce vibration frequencies for comparison with previously obtained spectra. For NH(SiCl3)2, all but five of the fundamentals are satisfactorily assigned. Seven scale factors could be refined, other factors being transferred from model compounds. For the hexamethyl compound, the interpretation was less detailed. In all three molecules the out-of-plane NH bending motion is found to be associated with fundamentals around or below 400 cm -1 , far lower than previous assignments. The force constant for this motion is greatly reduced as chlorine substituents are replaced by methyl groups, which shows the sensitivity of the stability of the planar system to the inductive effect of substituents. For tetrachlorodisilazane, NH(SiHCl2)2, earlier spectra were reanalyzed and evidence was found for the presence of two conformers, as suggested by previous ab initio investigations. The vibrational properties of the SiH bonds were explored by extending the ab initio calculations to a series of nonequilibrium structures, with the object of mapping variations in the SiH bond with change in torsional angle. This bond is strongest in the skeletal plane and weakest at right angles to the latter, which is in accordance with an n(N)-U*(Si-H) orbital interaction. However the orientation of the other silyl group also influences SiH bond strength. Long-range interactions are found between the stretching motions of both SiH and SiCl bonds. The former appear to arise from dipole-dipole interactions between the SiH bonds, but the source of the latter remains obscure. The form of the observed OSiH spectrum suggests that signal averaging conceals the splittings expected from both interactions and bond strength variation. Electrical properties of the SiH and SiCl bonds are examined.

Electronic stopping power of [formula omitted] axial channeled [formula omitted]Li ions in Si crystals

We report measurements on the electronic stopping power of 7 Li ions along the 〈1 0 0〉 Si direction in the energy region between 1 and 8 MeV. The measurements were carried out using the standard RBS/Channeling technique with SIMOX targets. The results show that the stopping power decreases for increasing energies, from 48 eV/Å at 1 MeV down to 19 eV/Å at 8 MeV. The general features obtained for the stopping power of 7 Li resemble those obtained for He ions measured previously.