摘要:

Structural modifications induced by pulsed laser irradiations in the surface layers of glassy carbon have been monitored by reflection high energy electron diffraction, Raman spectroscopy, and electron energy loss spectroscopy. The glassy carbon samples were irradiated by 30 superimposed laser pulses (&lgr;´=6.943 nm). The energy density (100–500 mJ/cm2per pulse) delivered to the material and the repetition rate of the laser (0.05 Hz) have been chosen so that the temperature increase of the irradiated surface layers was below the melting point of the glassy carbon. The combined use of the analysis techniques indicated that the beginning of the solid state processes, leading to microstructural modifications of the surface layers, occurs at energy density of 300 mJ/cm2. An increase of the average crystalline size of graphitic clusters occurs upon radiation performed at fluences of 300 and 400 mJ/cm2, whereas at higher energy density the material undergoes complete amorphization. The analysis of chemical state and microstructure of the irradiated samples clearly demonstrates that graphitization or, conversely amorphization of glassy carbon surface layers can be achieved by a proper choice of the laser irradiation conditions. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359558

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The growth of GaAs on (100)ScxEr1−xAs/GaAs (x=0 and 0.3) by molecular‐beam epitaxy has been studied using transmission electron microscopy. The initial stages of the three‐dimensional growth of GaAs on lattice‐matched Sc0.3Er0.7As have been investigated at different growth temperatures. Besides (100) epitactic GaAs, there are also areas of {111}‐ and {122}‐oriented GaAs observed on ScxEr1−xAs. The latter has a simple twin relationship with the neighboring (100) GaAs. Areas of {111}‐oriented GaAs have been observed only at growth temperatures above 400 °C and with moderate growth rates. The GaAs islands grown at 480 °C are faceted on {110} and {111} low‐index planes. These GaAs islands are elongated along ⟨011⟩ directions. The anisotropy of the island shape is greater at higher growth temperatures. The growth of GaAs on ScxEr1−xAs as islands is considered to be due to chemical rather than strain effects. The morphology of GaAs layers grown on ScxEr1−xAs is shown to have a strong dependence on the growth conditions. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359454

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Lattice‐matched heterostructures of GaAs/Sc0.3Er0.7As/GaAs have been grown on GaAs substrates with various orientations by molecular beam epitaxy and characterized by transmission electron microscopy. The Sc0.3Er0.7As layer usually has good epitaxy on GaAs; the purpose of this study is to investigate the overgrowth of GaAs on Sc0.3Er0.7As. In comparison with the samples grown on nominal (100) substrates, the epitactic growth of GaAs on Sc0.3Er0.7As is slightly improved on a vicinal substrate oriented (100) 6° off towards (111)A. {311} and {211} are shown to be preferred orientations for the epitactic growth of GaAs/Sc0.3Er0.7As/GaAs heterostructures. The epilayers grown on {311} and {211} substrates are epitactically well aligned, and the density of planar defects (stacking faults and microtwins) in the overgrown GaAs layer is significantly reduced. It is suggested that the mixed {111} and {100} character surfaces assist the nucleation of GaAs on Sc0.3Er0.7As. Stacking faults and microtwins are the major defects for the epilayers grown on (110) substrates. For samples grown on (111)B‐oriented substrates, twinning at the interfaces occurs frequently even when selftwinning inside each material is eliminated. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359455

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We show that annealing of the buried oxide layer used for device isolation generates point defects in the SiO2film and that this defect generation is independent of temperature above 1000 °C. Electron paramagnetic resonance data obtained on thermally grown buried oxides and those fabricated by ion implantation indicate that the defect is intrinsic to the structure of SiO2and is associated with an oxygen deficient environment. The similarity in the generation of the defects studied here and the formation of SiO observed in earlier studies of low pressure high temperature oxidation suggests that the formation of the buried oxide defect is related to the reduction of SiO2and the release of SiO. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359456

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The undercooling and nucleation of Al4Mn alloy have been investigated by containerless solidification in a 1.2 m long drop tube, where molten droplets of the alloy solidified in the vacuum. Droplets in various sizes were collected at the bottom of the tube. It is found by x‐ray and electron diffractions that besides the orthorhombic Al6Mn and &bgr;‐Mn phases, two approximants of the decagonal quasicrystal, one approximant of the icosahedral quasicrystal and decagonal domains can be observed in the as‐solidified droplets. However, due to the different cooling rate or undercooling in the samples of different sizes, the phases mentioned above will appear in different amounts. The relationship between the phase composition and the sample size is discussed on the basis of the classical nucleation and growth theories. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359457

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Using x‐ray diffraction and ellipsometry we have studied the incorporation process of SnTe in GaAs for n‐type doping. Combining these two techniques allows us to decide whether SnTe is incorporated pairwise, as has been proposed in the literature. We found SnTe doping to change theE1andE1+&Dgr;1critical point parameters in a way similar to that previously reported for n‐type Si‐doped GaAs. X‐ray diffraction and Hall measurements show that the free carrier concentration is more than 1/2 of the [Sn]+[Te] concentration. We thus conclude that a large proportion of SnTe is incorporated as independent Sn and Te dopant atoms. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359458

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The spectral line shapes and the absorption frequencies of the oxygen related infrared active vibrational modes in amorphous SiO2are studied both experimentally and by simulation. Experimental data were obtained on oxides grown thermally in the temperature range from 800 to 1050 °C and on oxides deposited at 300 °C by plasma enhanced chemical deposition. The transverse optic (TO) mode centered around 1090 cm−1is found to have a line shape and peak frequency which varies significantly with film thickness while the longitudinal optic (LO) mode at 1256 cm−1is invariant. Data on both modes and on refractive index is used to demonstrate consistently that the 800 °C grown oxide is 1.6%–2.0% denser than that grown at 1050 °C. For thin oxides (<10 nm) there is evidence from both TO and LO modes that interfacial oxide is denser than the ‘‘bulk.’’ The data on deposited oxides suggest that caution must be exercised in extending the analysis to their case. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359459

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We study the elastic properties of rigid fiber‐reinforced composites with perfect bonding between fibers and matrix, and also with sliding boundary conditions. In the dilute region, there exists an exact analytical solution. Around the rigidity threshold we find the elastic moduli and Poisson’s ratio by decomposing the deformation into a compression mode and a rotation mode. For perfect bonding, both modes are important, whereas only the compression mode is operative for sliding boundary conditions. We employ the digital‐image‐based method and a finite element analysis to perform computer simulations which confirm our analytical predictions. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359460

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We derive rigorous three‐point upper and lower bounds on the effective bulk and shear moduli of a two‐phase material composed of equisized spheres randomly distributed throughout a matrix. Our approach is analogous to previously derived three‐point cluster bounds on the effective conductivity of suspensions of spheres. Our bounds on the effective elastic moduli are then compared to other known three‐point bounds for statistically homogeneous and isotropic random materials. For the case of totally impenetrable spheres, the bulk modulus bounds are shown to be equivalent to the Beran–Molyneux bounds, and the shear modulus bounds are compared to the McCoy and Milton–Phan‐Thien bounds. For the case of fully penetrable spheres, our bounds are shown to be simple analytical expressions, in contrast to the numerical quadratures required to evaluate the other three‐point bounds. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359461

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The values of the fourth order elastic moduli (FOEM) for the diamond structure elements, silicon and germanium, have been calculated. The derivation utilizes the Keating model, assuming that both the third and fourth order terms in the interaction potential are composed of a bond‐stretch, an angle‐bend, and a mixed bond‐stretch angle‐bend term. The three force constants in the fourth order interaction potential are evaluated from the measurements of the sound velocity under pressure, hence, numerical values for all 11 FOEM may be obtained. The set of FOEM thus calculated shows markedly different characteristics than the analogous quantities for other materials, e.g., the alkali halides. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.359462

出版商:AIP    

年代:1995

数据来源: AIP