摘要:

On a very flat GaAs surface we grow a 1/6 monolayer amount of GaAs by metalorganic vapor‐phase epitaxy and observe two‐dimensional (2D) nuclei by high‐vacuum scanning tunneling microscopy. From the 2D nucleus densities we calculate the surface diffusion coefficient of 2×10−6cm2/s at 530 °C. During growth, the bunched step (multistep) separation saturates and is independent of the substrate misorientation angle. These results can be explained by a mechanism that takes into account both 2D nuclei formation on a wider terrace and their coalescence on ascending steps. A step‐bunching simulation based on our model reveals that the saturated multistep separation is proportional to the 2D nucleus separation, i.e., the inverse of the square root of the density. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360053

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

X‐ray diffraction measurements for stoichiometric TaC single crystals encapsulated in carbon nanotubes in the temperature range from 7 to 273 K are reported. Mean‐square displacements of Ta and C atoms, ⟨u2⟩, in the encapsulated crystals are derived from the integrated intensities of the (hkl) Bragg reflections by assuming ⟨uTa2⟩=⟨uC2⟩. The experimental value of &sqrt;⟨u2⟩ at 273 K is in agreement with the rippled displacements of the bulk TaC (100) surface atoms, which have been explored in a low energy electron diffraction measurement. The effective Debye temperature &THgr;′ derived from its temperature dependence is 340±10 K, which is about 70% of that for bulk TaC. The predicted softening of transverse acoustic surface phonon mode (the Rayleigh mode) along theM¯ symmetry axis in the surface Brillouin zone, by Ishida and Terakura, is in good accord with the decrease of &THgr;′. Linear thermal expansivities, &Dgr;a/a273, for the encapsulated crystals and for bulky crystalline particles show a marked difference below 150 K, wherea273is the lattice constant of TaC at 273 K. The predicted surface phonon mode (the Rayleigh mode), with a small anomaly, is adequate for explaining the starting temperature of the observed difference. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360754

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Raman spectroscopy has been used to investigate the effects of dynamic and postimplantation annealing on glassy carbon implanted with 50 keV C ions to a dose of 5×1016ions/cm2. The postimplantation annealing of damage in the ion‐beam modified material was found to occur in two stages as a function of postimplantation annealing temperatureTa. These occur for 500<Ta<800 K andTa≳1300 K and correspond to the thermal energy required to activate C interstitials and vacancies, respectively. Once mobile these defects diffuse through the implanted layer, reducing bond angle disorder which leads to an increase in graphitic order as interstitial‐vacancy recombination occurs. The effects of the ion‐beam irradiation on the final structure of glassy carbon were found to be a strong function of the temperature of the sample during the irradiation,Ti. This dependence is interpreted in terms of dynamic annealing and radiation‐enhanced diffusion. Three temperature regimes were found to be important. ForTi<300 K defect motion during irradiation is suppressed. For 300<Ti<600 K, the mobility of C interstitials during irradiation results in dynamic annealing which prevents amorphization, with the result that the ion irradiation creates a highly disordered, but essentially graphitically bonded carbon. ForTi≳600 K, vacancy mobilities are sufficiently high such that most ion‐beam‐induced defects are dynamically annealed and, forTi≳800 K the unimplanted glassy carbon microstructure is retained following the ion‐beam irradiation. Finally, activation energies for interstitial and vacancy mobilities were determined and found to compare favorably with those found in other forms of carbon. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360054

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The crystallization of vacuum evaporated selenium on glass, indium tin oxide, and polycarbonate substrates held at 70 °C was investigated using a near infrared microscope and scanning electron microscopy. The crystallization, observed merely at the Se/substrate interface, was found greatly enhanced and the structure of the crystallites modified if the substrates are soaked in water prior to the selenium deposition. It is suggested that residual water molecules entrapped in the substrate act as additional nucleation sites and induce the crystallization of selenium. The morphology of the individual crystallite depends only on the type of substrate; it is not affected by water‐soaking or by post‐annealing of the Se films in air. For all surface compositions and treatments, no chemical bond was formed between the substrate and the selenium film as revealed by x‐ray photoelectron spectroscopy. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360055

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Predominant parameters in the shock‐induced transition from graphite materials to diamond were examined in the present study by using quenching and powder methods under pressures of 50–60 GPa and 80–90 GPa, respectively, in the temperature range from 750 to 3500 K. Effects of the material parameters of the starting graphite—i.e., crystallite size and crystallinity—were distinguished from effects of the experimental parameters by standardizing the shock conditions for the materials examined. In addition, only a few graphite materials possessing wholly identical parameters throughout were selected as starting materials. Detailed characterization by transmission electron microscopy and electron energy‐loss spectroscopy revealed that the transition ratios of diamond and the degrees of graphitization varied with those parameters. The various changes observed were plotted in terms of pressure, temperature, and material‐parameter axes to create a tentative pressure‐temperature‐material diagram representing the behavior of the graphite materials under shock compression. The material parameters of the initial graphite structure primarily affected the diamond transition: The lower the crystallinity and crystallite size, the more easily the diamond transition occurred in the case of a reconstructive mechanism. Smaller crystallite size and lower crystallinity elevated the initial energy states of the graphite materials because of surface energy and strain energy, making it relatively easier to transcend the activation‐energy barrier to diamond transition. Temperature was fairly effective and pressure ineffective in regard to the diamond transition, a result consistent with the belief that the transition is a diffusion‐controlled process. Moreover, differentiation of the transition pathways, the diamond transition, and graphitization fit a concept of alternative metastable behavior; graphitization was more favored kinetically than diamond transition under the shock conditions examined. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.360056

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The possibility for the synthesis of CN compounds by high‐dose N implantation of glassy carbon is investigated. The changes in volume and surface morphology and the retained N concentration as a function of ion dose are reported. For both N and C (used as a control), implantation initially induces compaction in the surface region that saturates at a density above 2.6 g/cm3. After formation of this dense surface layer, additional implantation causes the material to expand to accommodate the implanted ions and vacancies formed during the implantation process. For N the swelling is initially linear in fluence up to 5×1017/cm2with a volume increase of 16 A˚3per added N atom; this is twice the volume per atom in the compacted substrate. Above a dose of 5×1017N/cm2the swelling behavior is more complex. A phase change is observed to occur at a dose between 5×1017and 1×1018/cm2, which is concomitant with at least some of the implanted N being driven to the surface. In addition, micron‐sized surface features are seen by scanning electron microscopy, and a significant surface roughening occurs. Ion backscattering spectra have been used to characterize the implanted N profile. The maximum retained concentration was found to be 30 at. %. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360057

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The effects of He ion irradiation have been investigated in high electron mobility transistors. Heterojunctions have been also used to monitor the two‐dimensional electron‐gas degradation. Devices with different layer structures have been employed for the better understanding of failure mechanism sources. Finally, introduction of a charge control model allowed the determination of buffer layer degradation. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360058

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

To examine the possibility of generating complexes of iron and phosphorus atoms in silicon, neutral interstitial iron in phosphorus doped silicon crystals with various concentrations were measured with the electron spin resonance (ESR) method after cooling from high temperature at various cooling rates. This concentration was found to depend on the cooling rate when the concentration of phosphorus was high: The concentration of interstitial iron was low only when the concentration of phosphorus was high and the cooling rate was slow. This result shows that some iron atoms form complexes with phosphorus atoms during cooling and become ESR‐inactive. Annealing behaviors of these complexes monitored as to the concentration of neutral interstitial iron are described as a combination of two processes, namely, the release of iron atoms from iron‐phosphorus complexes and the diffusion of iron to quenched‐in defects which are concluded to exist based on the dependence of annealing behavior on the cooling rate. From the analysis of annealing behaviors, the binding energy between iron and phosphorus atoms and the activation energy of diffusion of iron were determined to be 0.90 and 0.80 eV, respectively. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360059

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The fracture behavior of synthetic diamond has been investigated using indentation methods and by the tensile testing of pre‐notched fracture‐mechanics type samples. Specifically, the fracture toughness of free‐standing diamond plates, grown by chemically‐vapor deposited (CVD) methods, was measured using Vickers indentations and by the use of disk‐shaped compact‐tension specimens; the latter method provides an evaluation of the through‐thickness fracture properties, whereas the indentation method was performed on the nucleation surface of the sample. Measured fracture toughness (Kc) values were found to be approximately 5–6 MPa&sqrt;m by both methods, indicating that the fracture resistance of CVD diamond does not vary appreciably with grain size (within the certainty of the testing procedures). Complications, however, arose with the fracture‐mechanics testing regarding crack initiation from a relatively blunt notch; further work is needed to develop pre‐cracking methods to permit more reliable fracture toughness testing of diamond. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360060

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The shock initiation of the insensitive high‐explosive LX‐17, which contains 92.5% triaminotrinitrobenzene and 7.5% Kel‐F binder, was studied under simulated accident conditions in which two shock waves interact producing locally high pressures and temperatures. Two experimental geometries were studied using embedded manganin pressure gauges to measure the increases in pressure due to exothermic reaction at various locations as functions of time. These pressure histories were compared to ignition and growth reactive flow model calculations to determine whether a second shock compression of reacting LX‐17 caused unusually rapid reaction rates and thus more extreme hazards. One experiment used a tandem flyer plate of aluminum and steel separated by a gap to shock the LX‐17 charge, allow it to rarify, and then reshock the damaged charge to even higher pressures. These experiments revealed no significant enhancement of the LX‐17 reaction rates under this shock, rarefaction, and reshock loading. The second experiment used a grooved flyer plate to produce a subcritical shock wave in LX‐17, which then diverged and collided, producing a Mach stem interaction at the charge axis. The threshold conditions under which the Mach stem grew to detonation were measured. The standard LX‐17 ignition and growth model yielded excellent agreement with the pressure gauge records in the Mach stem interaction region. The formation of Mach stem interactions by nearly simultaneous multiple high‐velocity impacts was identified as a serious shock initiation hazard for heterogeneous solid explosives.

ISSN:0021-8979    

DOI:10.1063/1.360061

出版商:AIP    

年代:1995

数据来源: AIP