Toshiharu Irisawa

Growth of a polyhedral crystal from solution and its morphological stability

Abstract When a polyhedral crystal grows from solution in a stable way, the supersaturation is not uniform over its interface. The rate of stable growth of a cubic crystal is determined by taking account of three-dimensional diffusion field surrounding it and growth kinetics on the interface. It depends on both supersaturation σ∞ at infinity and crystal size L. Then, the shape stability of the growing polyhderal crystal is discussed, and the curve of stability limit, σ∗∗∞ versus L, is obtained by numerical calculations.

Periodic changes in the structure of a surface growing under MBE conditions

Abstract Periodic changes in the structure of a surface growing by the process of MBE are investigated theoretically by means of a Monte Carlo simulation. The conditions for oscillations of the RHEED intensity are obtained for singular and stepped surfaces.

Scaling Laws in Thermal Relaxation of Fractal Aggregates

We study shape relaxation of a two-dimensional fractal aggregate, during annealing after the rapid crystallization on a substrate. The edge length of the aggregate decreases with time in a power law with the exponent (d - 1 - D)/(β + 1), where d = 2 is the spatial dimension, D is the fractal dimension and β depends on the relaxation mechanism as to be 3, 2 and 1 for edge diffusion, surface diffusion and edge kinetics, respectively. With Monte Carlo simulation, we confirm the predicted exponents for the diffusion-limited aggregation with two different diffusion mechanisms.

Effects of impurities on lateral growth of crystals

Abstract The effect of impurities on the step velocity of a growing crystal and, hence, the growth rate of the crystal is formulated on the basis of step pinning theory for the growth from dilute nutrients. The step velocity is obtained by solving two simultaneous equations, i.e. one gives step velocity as a function of supersaturation and impurity density, and the other gives impurity density as a function of step velocity and impurity current. A graphical solution of the equation gives us clear explanation about pairing and instability of step trains. The minimum supersaturation needed for crystal growth is proved to be approximated by 1 3 – 1 2 power of the current of impinging impurity molecules. Monte Carlo simulation of solid on solid model also verified the step bending and the dependence of the minimum supersaturation vs. current of impinging impurity molecules.

Spin polarization vectors of field emitted electrons from Fe/W tips

The atomic and electronic structures at the apex of W tips were studied by means of field ion microscopy and field emission microscopy, before and after the thermal deposition of a 5 nm Fe film. Two geometries of W tip, a conventional hemi-spherical type and a chisel (flat needle) type, were prepared. The hemispherical and the chisel W tips had a 110 direction parallel and perpendicular to the tip axis, respectively. The coated Fe films were found to be most likely in a non-crystalline phase, and to have a lower work function leading to a drastic change in electron emission from the apexes. The spin-polarization vectors of field-emitted electrons from these Fe/W tips were investigated with a Mott detector with a rotatable mechanism of tips. A similar absolute value of the spin-polarization vector was obtained for each Fe/W, while the direction of the spin-polarization vector was dependent on the shape of the apex. The angle from the tip axis was θ=45° for the hemispherical apex and θ=66° for the chisel apex. A spin-polarized scanning tunneling microscopy setup with a rotation mechanism of such Fe/W tips made it possible to detect both the in-plane and the out-of-plane spin component of a sample magnetization.

Minimum Size of Oxygen Precipitates in Czochralski Silicon Wafers Detected by Improved Light Scattering Tomography

The minimum size of oxygen precipitates within Czochralski-grown (CZ) Si wafers detectable by light scattering tomography (LST) is completely dependent upon the correctness of the intensity measurement of scattered light, because the intensity is exactly proportional to the size of the precipitates. To assure precise intensity measurement, an adequately adjusted intensity and a well-shaped focusing of the laser beam were realized by instrument enhancement for illumination, and analytical software for brightness correction of and noise elimination from the light scattering tomograms were installed in a computer. With this well-developed system, the minimum diameters of oxygen precipitates in CZ Si wafers detected by improved LST was quantitatively determined using 1.06 µm radiation from a Nd:YAG laser to be 10 nm, compared with oxygen precipitates which were directly measured by transmission electron microscopy (TEM).

Structural feature of surface in MBE growth Monte Carlo simulation

The Monte Carlo study of the surface structure under MBE growth conditions is carried out. Characteristic surface diffusion field is obtained with respect to each growth mode. Two-dimensional nucleation on the terrace is investigated in atomic level using the time evolution of the density of clusters. Concerning the anisotropy of surface structure, the main reason for this is not the surface diffusion anisotropy but the anisotropic bond energy. The stoichiometric A-B crystal with the layer-by-layer mode is obtained in a range Φ AA = Φ BB = Φ AB /r, r ≥ 1, J A = J B , where Φ AA , Φ BB and Φ AB are the bond energies of A-A, B-B and A-B bonds, J A and J B are the flux of A and B atoms, respectively. It is found that the orderliness S of A-B crystal is a function of (2Φ AB /kT-Δμ/kT), where Δμ/kT is the parameter representing the intensity of the flux.

Electron-bombarded 〈110〉-oriented tungsten tips for stable tunneling electron emission

A clean tungsten (W) tip apex with a robust atomic plane is required for producing a stable tunneling electron emission under strong electric fields. Because a tip apex fabricated from a wire by aqueous chemical etching is covered by impurity layers, heating treatment in ultra-high vacuum is experimentally known to be necessary. However, strong heating frequently melts the tip apex and causes unstable electron emissions. We investigated quantitatively the tip apex and found a useful method to prepare a tip with stable tunneling electron emissions by controlling electron-bombardment heating power. Careful characterizations of the tip structures were performed with combinations of using field emission I-V curves, scanning electron microscopy, X-ray diffraction (transmitted Debye-Scherrer and Laue) with micro-parabola capillary, field ion microscopy, and field emission microscopy. Tips were chemically etched from (1) polycrystalline W wires (grain size ∼1000 nm) and (2) long-time heated W wires (grain size larger than 1 mm). Heating by 10-40 W (10 s) was found to be good enough to remove oxide layers and produced stable electron emission; however, around 60 W (10 s) heating was threshold power to increase the tip radius, typically +10 ± 5 nm (onset of melting). Further, the grain size of ∼1000 nm was necessary to obtain a conical shape tip apex.

Periodic changes in the structure of a surface growing under MBE conditions and RHEED oscillation

The periodic change of the structure and the flatness of a growing surface under MBE conditions are theoretically investigated. The growth of the (001) face of the simple cubic lattice is simulated by using Gilmer and Bennemas model for vapor growth. We discuss the properties of the RHEED oscillation by combining this simulation and a kinematical formula for RHEED intensity. In MBE growth, the lifetime τs of an adatom before re-evaporation is much larger than the lifetime τc of an adatom before capture by another adatom. If J is the incident beam flux and Ds is the surface diffusion coefficient of adatoms, τc = (JDs)−12. The results of the Monte Carlo simulation and the RHEED intensities calculated for the simulated growth are interpreted in terms of the lifetime τu and the mean diffusion length λc inτc. We obtain a diagram predicting the growth conditions under which the periodic change of a growing surface causing RHEED oscillation occurs.

Influence of surface diffusion on the structure of growing crystal surface

Abstract The structure of a growing crystal surface is investigated theoretically and by means of computer simulations. It is found that surface diffusion plays an important role in flattening the growing surface. The territory of a nucleus, i.e. one of the characteristic scales in multinucleation growth, is larger on the surface where the diffusion of adsorbed units on the surface is easier. It is also evaluated by computer simulations calculating the correlation function of the local surface height. Good agreement is found between simulation results and theory if the back-force effect is taken into account.