摘要:

Extended x‐ray‐absorption fine‐structure measurements have been performed at the AsKedge of GaAs/InP and InAs/InP strained single heterostructures grown by molecular‐beam epitaxy. The results show a substantial conservation of the bond length in the strained epitaxial layers with respect to the corresponding bulk materials. The measures point out the formation of a thick InAsP (or InGaAsP) layer at the GaAs/InP interface. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359355

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A new model has been developed for hydrogen retention and trapping in and release from graphite. Two different regions in the graphite with different hydrogen transport and trapping behaviors are distinguished, the bulk region within, and the surface region on graphite crystallites. The model incorporates new experimental results related to atom diffusion and recombination on inner surfaces. Recombination is explained from a fundamental viewpoint by linking it to diffusion using a classical expression. The model is applied to a number of reemission and thermal desorption experiments, in particular, the reemission of hydrogen atoms during irradiation with energetic hydrogen ions and the formation of HD during irradiation with H+and D+or during thermal desorption of graphite that was preimplanted with H+and D+ions with different energies. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359356

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The nature of keV ion damage buildup and amorphization in AlxGa1−xAs at liquid‐nitrogen temperature is investigated for various Al compositions using Rutherford backscattering channeling, transmission electron microscopy, andinsitutime‐resolved‐reflectivity techniques. Two distinct damage buildup processes are observed in AlxGa1−xAs depending on Al content. At low Al content, the behavior is similar to GaAs whereby collisional disorder is ‘‘frozen in’’ and amorphization proceeds with increasing dose via the overlap of damage cascades and small amorphous zones created by individual ion tracks. However, some dynamic annealing occurs during implantation in AlGaAs and this effect is accentuated with increasing Al content. For high Al content, crystallinity is retained at moderate ion damage with disorder building up in the form of stacking faults, planar, and other extended defects. In the latter case, amorphization is nucleation limited and proceeds abruptly when the level of crystalline disorder exceeds a critical level. The amorphization threshold dose increases with increasing Al composition by over two orders of magnitude from GaAs to AlAs. Dynamic annealing and damage creation processes during implantation compete very strongly in AlxGa1−xAs even at liquid‐nitrogen temperatures. This behavior is discussed in terms of both the availability of very fast mobile defects and bonding configurational changes related to the Al sublattice in AlxGa1−xAs of high Al content. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359358

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A high‐resolution transmission electron microscopy study of the solid phase crystallization of amorphous silicon thin films deposited on SiO2at 520 °C by low pressure chemical vapor deposition and annealed at 550 °C in a dry N2ambient was carried out so that the grain growth mechanism, various types of defects, and the origins of defect formation could be understood on an atomic level. Silicon crystallites formed at the initial stage of the crystallization had a circular shape and grains had a branched elliptical or a dendritic shape. Many twins, of which {111} coherent boundaries were parallel to the long axis of a grain, were observed in the interior of all the elongated grains. In addition to twins, the following defects were observed in the grain: intrinsic stacking faults, extrinsic stacking faults, perfect dislocations, extended screw dislocations, and Shockley partial dislocations. These defects were formed by the following reasons: errors in the stacking sequence at the amorphous/crystalline interface; jumps of a twin plane; the intersecting of two crystal growth fronts slightly misoriented; and the intersecting of two twin planes at the amorphous/crystalline interface. Among those defects, twins and stacking faults provided a preferable nucleation site for an atomic step of a {111} plane. As a result, it was concluded that grain growth in the ⟨112⟩ direction along the {111} plane parallel to the long axis of a grain was accelerated by twins and stacking faults. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359359

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The combined effect of the misfit strain and the strain caused by a neighboring defect on the activation energy of nucleation of dislocation loops is calculated. Defects of different sizes and shapes and located at different distances from the loop are considered. At very low mismatches (<0.5%) and with very small defects, the activation energy is not sufficiently reduced and large layer thicknesses are required for nucleation. At mismatches of 1% or more, and with defect sizes of 1.5 nm or larger, heterogeneous nucleation at growth temperatures becomes possible. These defects are more efficient in reducing the energy when they are at the center of the loop. Though impurities located within the core of the dislocations can reduce the core parameter substantially and therefore reduce the activation energy, in practice this is unlikely to occur. Very large defects such as SiO2and SiC precipitates reduce the activation energy of nucleation over large distances thereby inducing the nucleation of several loops which results in very rapid relaxation of strain. In highly mismatched layers (4%–8%) homogeneous nucleation occurs at about 400–500 °C. Why the periodic arrangement of misfit dislocations is observed only in the highly mismatched layers is explained. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359375

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Some diffusion problems in the impression and diffusional creep of anisotropic materials are analyzed. It is found that at the limit of low stress, both the diffusional creep rate and impression velocity are proportional to the applied stress. For a parallelepiped crystal under simple tension or compression in theZdirection, the diffusional creep rate depends on all three principal diffusivities,DX,DY, andDZ. Two limiting cases depend on the quantity &sqrt;DXDY/DZ. When this quantity is large the creep rate is proportional to &sqrt;DZbut when it is small the creep rate is independent ofDZ. For a cylindrical crystal under tension or compression in the axialZdirection, the creep rate depends onDr/DZ. When the ratio is large, the creep rate is the same as the isotropic case except that the effective diffusivity is &sqrt;DrDZ. When the ratio is small, the creep rate is proportional toDrand independent ofDZ. For the impression creep of a half‐space the punch velocity is proportional to the geometric mean of the principal diffusivities parallel and perpendicular to the loading direction, and inversely proportional to the punch dimension. For impression creep of a thin film deposited on an impermeable substrate under the same punching stress, the impression velocity is dependent only on the diffusivity parallel to the thin film, and inversely proportional to the square of the punch dimension. Without the substrate, the impression velocity is dependent only on the diffusivity perpendicular to the thin film, inversely proportional to the thickness of the film, and independent of the punch dimension. ©1994 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359524

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Proton implantation into neutron transmutation doped (NTD) silicon generates three hydrogen‐related effective mass‐like donors with binding energies between 35 and 44 meV. The donors are formed by the complexing of hydrogen with damage induced by the irradiation. Our results indicate residual defects even in well annealed NTD silicon. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359376

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Here, we present scanning tunneling microscopy images of antiferroelectric liquid‐crystalline (AFELC) molecules observed at different elevated temperatures corresponding to different bulk mesophases of the material. These images show evidence for a commensurate changing of the molecular arrangement on the graphite substrate as a function of temperature. Furthermore, a nontrivial angular relationship between the graphite substrate and the AFELC domains was obtained, indicating that there may be a stronger, polar (electrostatic) interaction between the AFELC molecules than between the adsorbate and the substrate. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359378

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The stressed crystal that has been obtained during the crystallization process is considered with particular attention given to the comparative numerical analysis of composition and temperature gradients as the causes of the stressed state. The numerical calculations are referred to the directional solidification method. It is shown that the contribution of diffusion induced stresses to the total stressed state of a crystal in the growth process may be comparable to the contribution from thermostresses. In this article, as a consequence of the main thermodynamical laws the relations obtained by Larche´ and Cahn [Acta Metall.21, 1051 (1973)] are generalized to the case of a coupling dependence of elastic moduli on temperature and composition. The relations provide the important connection between the elastic modulus of compound materials measured using acoustic methods (at constant entropy and chemical potentials) and those that are used in practice (at constant temperature and composition). ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359379

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The ablation of free‐standing thin aluminum films by a nanosecond laser pulse was investigated by time‐resolved transmission electron microscopy and numerical simulation. It was established that thin film geometry is particularly suited to furnish information on the mechanism of evaporation and the surface tension of the melt. In the case of aluminum the surface tension &sgr; as function of temperature can be approximated by two linear sections with a coefficient −0.3×10−3N/K m from the melting point 933 K up to 3000 K and −0.02×10−3N/K m above 3000 K, respectively, with &sgr;(933 K)=0.9 N/m and &sgr;(8500 K)=0. At lower pulse energies the films disintegrated predominantly by thermocapillary flow. Higher pulse energies produced volume evaporation, and a nonmonotonous flow, explained by recoil from evaporating atoms and thermocapillarity. The familiar equations of energy and motion, which presuppose separate and coherent vapor and liquid phases, were not adequate to describe the ablation of the hottest zone. Surface evaporation seemed to be marginal at all laser pulse energies used. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359380

出版商:AIP    

年代:1995

数据来源: AIP