Our theoretical study of step bunching induced by the drift of adatoms is reviewed. The theory may apply to the bunching in Si(111) vicinal faces with direct current heating. In the occurrence of evaporation of adatoms (nonconserved system), a vicinal face is unstable when the velocity of the down-hill drift exceeds a critical value. After the instability, the terrace width L grows as L∼tβ with the value of β smaller than 1/2 in an early stage, and saturates if the drift velocity is small. The saturation of L is expected from the Benney eyuation, which governs the surface profile near the threshold of instability. With increasing drift velocity the bunch size grows via coalescence and the exponent β approaches 1/2. When the evaporation is neglected (conserved system), a vicinal face is always unstable with the drift. The terrace width grows by coalescence of bunches and f ti β≈1/2 independent of the form of the step repulsion.
A systematic study of specimen heating current effects on Si(111) and (001) vicinal surfaces is summarized briefly in this paper. On the (111) surfaces current direction dependent step bunching and its temperature dependence reported elsewhere on small miscut surfaces were found to be valid also on vicinal surfaces with miscut angles more than 10°. We newly found that below 1200°C step wandering occurs under the current at which step bunching does not take place. On the (001) vicinal surfaces, step bunching independent of the current direction and anisotropy in current effects were observed.
We present a highly reliable statistical mechanical approach to calculate step-related quantities on the vicinal surface, which is comprised of the imaginary path-weight random walk method and the numerical renormalization group method. We applied the method to the studies of Si(001), Si(111) 7×7 and Si(111) 1×1 surfaces. From the effective kink energy, we obtain anisotropic step tension, step stiffness, equilibrium island shape, and step interaction coefficient, which allows quantitative comparison with experiments.
A step plays an important role in the growth process by molecular beam epitaxy, as it serves an active site for incorporating adatoms on the surface. The shape of steps changes by incorporating an adatom, which in turn changes the behavior of adatoms around the step. The shape and the motion of steps are well correlated with adatom kinetics during the growth. Based on a Monte Carlo simulation, such a step dynamics is studied of the epitaxial growth on vicinal surfaces of Si(100) and GaAs(100). There are two types of monatomic height steps on Si(100) and two types of biatomic height steps on GaAs(100). Common and different features during the growth on Si and GaAs are pointed out by showing the morphologies as well as the surface step density variations.
Dynamic processes of the step bunching on a Si(001) vicinal surface during Au deposition were studied by using reflection electron microscopy. During the growth of the Si(001) 5×3.2-Au terraces, the step bunching occurs and the evolution of the terrace width is fitted in a form of ta with respect to time t for temperature between 780 and 900°C. The exponent α is 1/4 at 820°C, increases gradually with temperature and is 1/2 above 870°C. This suggests that step bunching kinetics changes with temperature. It was found that the kinetics is governed by surface diffusion in low temperature region and by evaporation in high temperature region. Dynamic processes during debunching were also analyzed in the same way. Time evolution of terrace width can be fitted by a function of A* (to-t)α'. It is unity below 820°C and decreases with increasing substrate temperature to reach to 1/2 above 870°C. This suggests that mass transport kinetics changes with temperature and the transport of Au is the rate determining process below 820 °C.
Using ultrahigh vacuum scanning electron microscopy, we investigated the transformation processes of holes and hillocks on vicinal Si(111) surfaces during annealing at around 1200°C. A hole and a hillock decay symmetrically with one another, their shapes are complementary and the decay rate is almost the same except during the final stage, where hillocks decay faster than holes. Flat (111) planes appear both at the bottom of the hole and at the top of the hillock due to layer-by-layer decay. As a result of transformation, regularly arranged atomic-step bunches are formed in both cases. The electric field used for sample heating was found to greatly affect the final shape of step bunches. As for the transformation of holes, step-flow sublimation becomes the dominant process for holes larger than the adatom diffusion length, while layer-by-layer filling-in of the bottom terrace is the dominant process for holes smaller than the adatom diffusion length.
Thermal relaxation of isolated single silicon two dimensional (2D) islands, 2D vacancy islands and pyramid type islands, produced on the Si(111)(7×7) surface by using a tip of a scanning tunneling microscope are observed by temperature variable scanning tunneling microscopy. Thermal decay rates of silicon islands are measured at various substrate temperatures of 650 K-850 K. Effects of the probe tip on the rates are measured and reduced rates are determined without the tip effects. The sizes of islands depend on time with a function form of (t0-t)α. For single bilayer islands, α was 1 without the tip effects and 2/3 with the tip effects. From these results, we conclude that the intrinsic decay process of islands is attachment-detachment limited but is diffusion limited with the tip effect. Activation energies for the intrinsic decay of single 2D islands and 2 D vacancy islands are 1.5 eV and 1.3 eV, respectively. Characteristic (5×5) islands with long life times are found during decay. The (5×5) structure is also observed on pyramid type island surfaces. We discuss the results in terms of two dimensional vapor phase processes.
A preliminary theory of the mechano-catalytic water-splitting proposed in 1998. The friction between the stirring rod and the glass surface generates an enough electriostatic field for the field-emission of positive holes from the semiconductors. The emitted holes react with water molecules to evolve oxygen molecules and protons, which go to the glass surface and combine with the electrons there to evolve hydrogen molecule. However, it leaves behind to clarify the role of the electrodes.
Infrared reflection absorption spectroscopy (IRRAS) has been adopted for the high sensitive and high resolution vibrational spectrum measurements on the metal surfaces. In recent years, IRRAS using buried metal layer substrates (BML-IRRAS) has been developed. This technique provides sufficient sensitivity for the submonolayer adsorbates on the semiconductor and insulator surfaces. The synchrotron radiation stimulated process, which has characteristics of high spatial resolution, low damage, and unique reaction selectivities, is expected to realize a practical nano-process technique in future. We are developing the new BML-IRRAS technique and trying to apply it to the monitoring of the SR stimulated surface reactions. The BML-IRRAS technique is useful in the in-situ observations of the surface reaction, especially of the beam-induced reactions. In this report, our BML-IRRAS technique and recent results of our experiments using this technique with thermal and SR stimulated reaction on the hydrogen terminated Si(100) surfaces are introduced.
Microstructural and microanalytical investigations were carried out for traditional Japanese clay ceramics called Sekishu-kawara. In this study, optical microscopy as well as scanning- and transmission-electron microscopy were employed. Specimens under investigation are (A) the original wet materials dried in air for several days, (B) the materials dried at 773 K, and (C) the same materials fired and sintered at 1493 K in a tunnel kiln. Not only some clear SEM images but also the TEM images, which were taken for the first time by us, for the clay ceramics are presented. TEM mi-crographs of specimen C show that mullite crystals, grown in the firing and sintering process, are in the shape of strip, 5 nm×50 nm×500 nm in largest, and are dispersed, in the amorphous glass. SEM micrographs of specimen C, on the other hand, show that the clay ceramics are porous and contain considerable amount of tunnel-shaped open spaces called continuous pores. The continuous pores are constructed by a complicated, three dimensional shape of amorphous glass, in which the mullite strips exist.