摘要:

High‐angle neutron powder diffraction was used to investigate the grain size dependence of the Debye–Waller parameter (DWP) of nanocrystalline and coarse‐grained chromium samples. The DWP measured at 20 K depends linearly on inverse grain size and is consistent with a two‐state model in which defects with short‐ranged displacement fields are present in differing concentrations in two distinct microstructural regions within individual grains. One possible model microstructure consists of grain boundary and free surface regions with significantly larger concentrations of point defects than in grain interiors. Evidence is also seen for an enhancement of the temperature‐dependent component of the DWP of chromium with decreasing grain size, indicating different behavior than seen previously for nanocrystalline palladium. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359035

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Spatial fluctuations of lattice strain in an as‐grown Czochralski‐grown silicon wafer, in which a ring‐shaped region of densely distributed oxidation‐induced stacking faults appears after oxidation thermal treatment, are measured by double‐crystal reflection topography with synchrotron radiation. The measured lattice strain is isolated into local variations in lattice dilation and inclination angle from an average plane. The variation profile of the lattice spacing shows a small valley in the ring‐shaped region, while showing a peak just outside the ring‐shaped region. The relation between the lattice strain and anomalous oxygen precipitation is discussed. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359036

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Solid‐state reactions in mixtures of nanometer‐sized Cu and Zr as well as Ni and Zr crystallites—produced by inert‐gas condensation followed byinsitucompaction—have been investigated by x‐ray diffraction and thermal analysis. Nanocrystalline particle mixtures and multilayered samples showed qualitatively the same behavior: A solid‐state amorphization reaction occurs in NixZr1−xsamples but not in CuxZr1−x. The results are discussed in the framework of a heterogeneous nucleation and growth process for the amorphous phase. Comparison with results from ball‐milling experiments and mechanically prepared multilayers reveals the significance of additional external driving forces. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359037

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Molecular beam epitaxy of PbTe on BaF2(111) is studied using reflection high‐energy electron diffraction (RHEED). The influence of growth parameters (substrate temperature and growth rate) on surface kinetics and the steady‐state growth surface morphology is investigated employing dynamical RHEED measurements (RHEED oscillations). For a well adjusted stoichiometric PbTe beam flux composition, two‐dimensional layer‐by‐layer growth can be achieved from substrate temperatures as high as 410 °C down to temperatures below 95 °C, with a maximum number of 230 RHEED oscillations observed at substrate temperatures in the 160 °C range. At temperatures above 400 °C, the growth kinetics start to be modified by PbTe reevaporation from the layer surface. The dependence of the RHEED oscillations on substrate temperature and growth rate indicates the importance of adatom surface diffusion for the surface morphology developed under steady‐state growth conditions, and for all growth conditions, a close correlation between steady‐state growth surface step density and damping of RHEED oscillations is observed. Furthermore, it is shown that even very small changes in the beam flux composition have a dramatic influence on the RHEED intensity oscillations as well as the surface processes involved in the growth. With only a small additional Te2flux used for the growth, an abrupt growth mode transition from layer‐by‐layer to step flow growth is induced. This is the first evidence that PbTe molecules impinging on the layer surface do not dissociate upon adsorption, but remain in a molecular state until incorporated in the crystal lattice. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359038

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The defects due to oxygen precipitation in Czochralski grown silicon single crystals annealed for 216 h at 750 °C and with oxygen concentration varying between 18 and 12×1017atoms per cm3(according to DIN 50438) were studied by means of small angle neutron scattering and &ggr;‐ray diffractometry probing the same volume elements in the sample. The size and the shape of the SiO2precipitates were determined by means of small angle neutron scattering. In the center of the disk‐shaped sample of 10 cm diameter one finds spherical precipitates with a radius of ≊20 A˚, at the border the precipitates are of plate‐like shape, ≊146×146×41 A˚ in dimension. Thek‐space distribution of the diffuse scattering caused by the strain field of the SiO2precipitates has been determined by means of a double‐crystal diffractometer and 316 keV &ggr; radiation. Modeling with Huang and Stokes–Wilson theory suggests that the relatively small SiO2precipitates are loosely bound in larger clusters with a radius of ≊2000 A˚ in which the crystal matrix is strongly distorted. With the &ggr;‐ray diffractometer operated in the single‐crystal mode the thickness dependence of the integrated reflecting power was measured from which the static Debye–Waller factor for the 2 2 0 reflection is determined using statistical dynamical theory. The results confirm qualitatively the cluster model. In the center of the sample the size of the precipitates could be calculated by additional measurement of the static Debye–Waller factor of the 4 4 0 reflection. The result is in excellent agreement with the small angle neutron scattering data. Combining all the experimental data taken in the oxygen rich center of the annealed silicon crystal the average distance between the centers of the SiO2precipitates in the clusters is ≊80 A˚ and the distance between the large clusters is ≊40 000 A˚. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359039

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A scanning tunneling microscope (STM) in ultrahigh vacuum has been used to investigate the growth, morphology, and surface atomic structure of ultrathin titanium silicide films on Si(100) substrates. Microstructural considerations have been used to identify various stages of the silicide growth. Methods for STM crystallography have been developed and used to identify possible epitaxial silicide/silicon relationships based on morphological considerations. Atomic resolution images of a titanium silicide crystallite have identified a 2×2 silicon termination of a C54‐TiSi2(111) surface. It is shown that unambiguous identification of epitaxial relationships requires images of the atomic structure of the silicide crystallite surfaces in addition to morphological information. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359040

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

In this paper we report on the time behavior of the apparent pitch of a cholesteric mixture undergoing a step‐like dc electric field applied parallel to the helical axis. We show that the relative variation of the apparent pitch &Dgr;&lgr;(t)/&lgr;0decreases from the initial value to negative ones (&lgr;<&lgr;0) by two different characteristic times and tends finally to a saturation value with &Dgr;&lgr;/&lgr;0<0 after a long time. An attempt to explain this phenomenon in a (semi‐)quantitative way has been made by considering relaxation mechanisms on different spatial scales. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359548

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Single‐crystal Si has been implanted with O+ions at 2 MeV energy, to doses from 0.1 to 2×1018O+ cm−2and at substrate temperatures between 90 K and 700 °C. Infrared reflection spectra in the transparent region 1500–7000 cm−1were measured and interference fringes were observed. A detailed theoretical analysis of the interference fringes yields refractive index profiles which provide accurate measurements of the range and straggle of the implanted ions, the Si overlayer crystallinity, and the oxygen concentration depth profile. The implantation temperature influences the crystallinity of the Si overlayer but not the oxygen distribution. The measured range parameters are in agreement with theory. The 2 MeV results are compared to results from 200 and 90 keV implants, which show that the method is applicable for a wide range of separation by implantation of oxygen structures. Rutherford backscattering spectroscopy and ion channeling results are in good agreement with the infrared results. It is concluded that infrared spectroscopy is the most convenient and inexpensive nondestructive technique to investigate structures formed by MeV and lower‐energy O+implantation. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359041

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Ge‐22 at. %Au/Au bilayer films annealed at various temperatures have been observed by transmission electron microscopy. The as‐deposited bilayer film is composed of crystalline Ge,Au and metastable Ge0.4Au0.6phases. The Au grain size is nonuniform, and no amorphous Ge is observed. During annealing at 60–350 °C, patterns with several wide branches are generated that are different from the dense branching fractals induced by crystallization of amorphous Ge ina‐Ge/Au films. The fractal dimension of the pattern (measured by the box‐counting method) increases with the annealing temperature. The agglomeration of small Au grains to coarse Au grains in the Ge‐Au/Au bilayer film may be the mechanism for the fractal structure formation. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359042

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

TiO2thin films have been prepared and their structure analyzed by XRD at grazing angles and x‐ray absorption spectroscopies (EXAFS/XANES). The deposition method is an ion beam induced chemical vapor deposition procedure (IBICVD) consisting of the bombardment of a substrate with accelerated O+2ions while the vapor of a volatile metallic precursor is directed on its surface. The structure of the films was dependent on the type of substrate (glass, fused quartz, and sapphire) and on the conditions used for their preparation (temperature of substrate and beam energy of 1 or 10 keV). Thus, while on glass and silica/sapphire, an amorphous structure is formed when the substrate is at 300 K during preparation, a rutile‐rich structure is obtained if the substrate is at 573 K during deposition or, even to a larger degree, when using ions of 10 keV. The amorphous structure of the films grown on glass yield 100% anatase after calcining at 773 K. On sapphire, the crystallization was less perfect and besides anatase, rutile and amorphous phases were detected after calcination at that temperature. The films prepared at 573 K or with O+2ions of 10 keV yield after calcining a well ordered structure of rutile. A detailed analysis of the EXAFS and XANES spectra has permitted a better characterization of the amorphous and crystalline phases in the films and provided a better understanding of the crystallization processes during calcination. The contribution of the different factors controlling the crystallization of the films is discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359043

出版商:AIP    

年代:1995

数据来源: AIP