Layer-by-layer oxidation of Si(001) surfaces has been studied by scanning reflection electron microscopy (SREM). The oxidation kinetics of the top and second layers were independently investigated from the change in oxygen Auger peak intensity calibrated from the SREM observation. A barrierless oxidation of the first subsurface layer, as well as oxygen chemisorption onto the top layer, occurs at room temperature. The energy barrier of the second-layer oxidation was found to be 0.3 eV. The initial oxidation kinetics are discussed based on first-principles calculations.[S0031-9007(97)04959-4] PACS numbers: 81.65. Mq, Oxidation of Si surfaces is important for technological application of electronic devices [1][2][3]. Although many kinds of surface analyses have been used to study oxygen adsorption kinetics onto Si surfaces [4][5][6][7][8][9], the oxidation kinetics of subsurface layers, which determine oxide film growth, have not been studied in detail. This is because of the difficulty of experimentally and independently analyzing the oxidation processes of specific subsurface layers. In this Letter we used scanning reflection electron microscopy (SREM [10]) combined with Auger electron and x-ray photoelectron spectroscopy (AES and XPS) to investigate the initial oxidation of Si(001) surfaces. Our combined analysis has a great advantage for observing layer-by-layer oxidation of subsurface layers, as well as the step and terrace configurations buried with oxide layers. We report, for the first time, reaction barriers of the uppermost and second layer oxidations, and discuss our experimental results based on first-principles calculations.Our experiments were carried out using an ultrahighvacuum surface analysis system that performs SREM, AES, and XPS [11]. This system is equipped with a thermal field emission electron gun, a precision energy analyzer (a spherical capacitor analyzer), and a conventional x-ray source (Mg Ka excitation). A 30-keV electron beam with a 2-nm diameter was used for the SREM with a low incident angle of about 2 ± to the surface. The AES measurement could be performed simultaneously by using the electron gun for SREM at an incident angle and detection angle of about 2 ± and 73 ± to the sample surface, respectively. The XPS was performed with a 60 ± takeoff angle with respect to the normal to the surface. A Si(001)-͑2 3 1͒ surface was prepared by flash heating with a direct current. Oxidation of the surfaces was carried out by introducing molecular oxygen into the analysis chamber. Since the electron gun and the energy analyzer were independently evacuated, the AES measurement could be performed under oxygen pressure on the order of 10 26 Torr.As we previously reported, since SREM images of SiO 2 ͞Si systems are obtained by recording the intensity change in reflection spots from a crystal Si substrate covered with an amorphous oxide layer, the interfacial structure can be observed without the need to remove the SiO 2 overlayer [12][13][14]. Figures 1(a)-1(d) show SREM images of Si(001) surfaces...

We report a 19-year-old woman who was diagnosed as polycystic ovary. Hypothyroidism with a markedly elevated TSHlevel and an enlarged pituitary gland on MRI were noted. The 123I uptake was decreased to 6.5%. After treatment with thyroid hormone, regression of the enlarged pituitary and the ovarian cysts was observed. In the present case, hypothyroidism was considered to have caused a reversible enlargement of the pituitary gland and concomitant polycystic ovary. Weconcluded that the polycystic ovary might have resulted from the effects of an excessive amount of TSHon immature ovaries. (Internal Medicine 40: 751-755, 2001)

To discuss the X-ray dynamical diffraction when the imaginary part of the X-ray polarizability is larger than the real part, the dispersion surface is studied as a function of the ratio between the real and the imaginary parts of the polarizability. The dispersion surface in the Laue case when the real part is zero has a similar form to that in the Bragg case when the imaginary part is zero. The relations between the dispersion surface and the diffracted intensity are studied in some special cases. The abnormal absorption and the abnormal transmission effect are related to the features of the dispersion surface. AbstractIn 1912, von Laue first described X-ray diffraction by approximating as plane waves the spherical waves radiated by atoms in a crystal. Darwin recognized that © 1995 International Union of Crystallography Printed in Great Britain -all rights reserved this approximation is valid only in the limit of very small crystals, and published in 1914 the more general spherical-wave theory based on the reflectivity of individual atomic planes. The Darwin theory is extended here to surface Bragg diffraction from a single-crystalline Acta Crystallographica Section A

We investigated the structures of the two-dimensional Au islands grown on Ir͑111͒ and Pt͑111͒ using scanning tunneling microscopy and Monte Carlo simulations. We found that the Au islands have dendritic, triangularly compact, and irregularly compact shapes depending on the underlying substrate. We also found a strong layer dependence of the island density. Monte Carlo simulation results demonstrate that the fractal dimensions of these dendritic islands are determined by competition between terrace diffusion and edge diffusion of adatoms. Furthermore, the fractal dimension changes continuously from about 1.7 to 2 as a function of the edge diffusion barrier, which is discussed on the basis of the generalized diffusion limited aggregation model.