摘要:

The directional solidification of aqueous ammonium chloride solutions has been considered by theoretical and experimental approaches to investigate the influence of convection on solidification. By comparing the growth of crystal from experiments in which convective flows occur and that from theoretical analysis in which no convection is considered, the effect on solidification due to each individual convective flow, such as the double‐diffusive salt‐finger convection above the melt/mush interface, the buoyant flow in the mushy layer, the plume flow issued directly from the interior of the mush into the bulk melt, and the mushy‐layer mode circulation induced by the subcritical instability and so on, is identified. It is found, in general, that the salt‐finger convection and mushy‐layer mode circulation inhibit the growth of the dendritic mushy zone and the plume flow enhances the growth of eutectic solid layer while suppresses the mushy layer. The buoyant flow effect is virtually negligible.

ISSN:0021-8979    

DOI:10.1063/1.354979

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Continuous 1‐&mgr;m‐thick diamond films have been grown by chemical vapor deposition (CVD) at ∼900 °C on pure titanium and on a Ti alloy, Ti‐6Al‐4V. The diamond film exhibits good adhesion to the substrates in spite of the presence of ∼7 GPa of in‐plane residual stress which arises from the large differences in thermal expansion coefficients between diamond and titanium. The interface between the CVD diamond film and the substrate was exposed by deforming the substrate, thereby removing parts of the diamond film, under both ultrahigh vacuum and ambient conditions. After fracture, both the substrate and diamond film sides of the interface were characterized by a combination of x‐ray photoelectron spectroscopy (XPS), scanning Auger microscopy, secondary electron microscopy, and Raman microprobe spectroscopy. The substrate fracture surface is inhomogeneous, containing some areas of diamond and amorphous carbon. XPS analysis revealed that carbon and oxygen are present on the substrate fracture surface. Micron‐size areas of Ti were also found on the diamond fracture surface. Raman spectroscopy of the substrate fracture surfaces found evidence for the presence of amorphous, nonstoichiometric titanium oxides; no evidence of crystalline TiC or stoichiometric TiO2was seen. Analysis of the XPS core level structure of the Ti and C spectra confirmed the presence of titanium carbide; little evidence of metallic titanium was seen in the interfacial region. Differences in the structure of the substrate fracture surface between titanium and the Ti alloy were also seen. The interface at the diamond/Ti‐6Al‐6V alloy was more heavily oxidized than the diamond/titanium interface. Depth profiling studies also revealed a thicker oxygen‐containing surface layer on the alloy fracture surface. The presence of diamond and Ti compounds on both sides of the exposed interfaces indicates that the fracture did not occur discretely at the diamond/reaction layer interface. From these findings we propose a model of the failure region of the highly adherent diamond/titanium system.

ISSN:0021-8979    

DOI:10.1063/1.354980

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Bismuth titanate thin films were deposited by the excimer laser ablation technique directly onto bare silicon substrates, SiO2and Si3N4coated silicon. The impact of process parameters such as gas pressure, laser fluence, processing temperature, and the presence of an oxygen plasma were studied with regards to the ferroelectric‐semiconductor interface. The density of interfacial surface state (Nss) at the flatband voltage was found to be on the order of 1012–1014eV−1 cm−2. Hysteretic capacitance‐voltage data indicated charge injection from the substrate was the dominant mechanism, masking any polarization mode. Films deposited on SiO2coated silicon did, however, exhibit polarization type switching.

ISSN:0021-8979    

DOI:10.1063/1.354981

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Turbulent premixed oxygen‐acetylene flames have been used to synthesize polycrystalline diamond films on molybdenum substrates at temperatures ranging from 500 to 1300 °C and facetted single crystals on mm‐sized natural diamond substrates at temperatures of 1200–1300 °C. Turbulence was achieved by increasing the torch’s orifice diameter and/or the flow velocity; the presence of turbulence was confirmed by observations of changes in the flame shape, measurements of the flame’s noise spectrum, and calculations of the Reynolds number. The optical emission spectra of several diamond‐growing turbulent flames were also compared to the spectra of laminar flames. The variation in diamond quality with temperature and oxygen acetylene flow ratio was studied with one or more of the following techniques: Raman spectroscopy, scanning and transmission electron microscopy, infrared spectroscopy, and photoluminescence spectroscopy. Crystals grown on molybdenum at temperatures of 600–1100 °C were observed to be transparent, and under the proper conditions the quality of diamond grown epitaxially in a turbulent flame equals that of natural type‐IIa diamond. Although this enhanced quality has only been observed for fairly lean flames, the growth rate for type‐IIa quality diamonds can still exceed 35 &mgr;m/h.

ISSN:0021-8979    

DOI:10.1063/1.354982

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Concentration profiles and diffusion coefficients of deuterated polystyrene chains (27 000–104 000 amu) diffusing into hydrogenated polystyrene (h‐PS) matrices (90 000 and 590 000 amu) were measured by forward recoil of 2.0 MeV He ions. This experimental procedure allows us to detect the diffusion coefficient in the range 3×10−15–3×10−13cm2/s. The irradiation of theh‐PS layer with ion beam changes the interdiffusion properties. The diffusion coefficient, measured inh‐PS irradiated with 300 keV H+in the 5×1011–2×1014ions/cm2fluence range, does not change at low fluence, instead it is reduced for a threshold fluence whose value depends on the matrix molecular weight. High fluence irradiation ofh‐PS decreases the polymer solubility because crosslinks between the original chains, produced by ion irradiation, yield a three‐dimensional network (gel). By combining diffusion and solubility measurements, for both molecular weights, we observed that the diffusion process is slowed down when, in the irradiatedh‐PS is produced a gel fraction of 0.8, which corresponds to a polymer structure containing an average of 1.0 crosslink per chain. This result does not depend on the marker molecular weight.

ISSN:0021-8979    

DOI:10.1063/1.354983

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The dynamics of a fragmentation model is examined from the point of view of numerical simulation and rate equations. The model includes effects of temperature. The numbern(s,t) of fragments of sizesat timetis obtained and is found to obey the scaling formn(s,t)∼s−&tgr;tws&ggr;e−&rgr;tf(s/tz) wheref(x) is a crossover function satisfyingf(x)&bartil;1 forx≪1 andf(x)≪1 forx≫1. The dependence of the critical exponents &tgr;,w, &ggr;, andzon space dimensionalitydis studied fromd=1 to 5. The result of the dynamics on fractal and nonfractal objects as well as on square and triangular lattices is also examined.

ISSN:0021-8979    

DOI:10.1063/1.354984

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Spin‐coated solid films of the gold‐cluster compound Dodeca‐(triphenylphosphine), hexa(chloro)pentapentacontagold Au55(PPh3)12Cl6are irradiated with a focused 20‐keV Ga+focused‐ion beam. The writing speeds on the substrate were ranging from 50 up to 2000 &mgr;m/s. This treatment locally decreases the solubility of the metalorganic precursor layer in CH2Cl2. Removal of the nonirradiated part of the surface layer with this solvent, followed by thermal decomposition of the remaining metalorganic nanostructures, leads to conducting gold lines. The width (150–360 nm) and height (20–80 nm) of these metallic lines depend on the ion dose and the original film height of the gold compound. The possible interactions leading to the fixation are briefly discussed.

ISSN:0021-8979    

DOI:10.1063/1.354985

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Extremely low resistance nonalloyed Ti/Pt/Au contacts have been formed ton‐InGaAs,p‐InGaAs, andn‐InAs/InGaAs layers with doping concentrations ranging from 1 to 5×1019cm−3forn‐type and from 2 to 1×1020cm−3forp‐type material. The comparative studies reveal specific contact resistances as low as 1.7×10−8&OHgr; cm2for then‐InAs/InGaAs system, while the best values obtained forn‐InGaAs andp‐InGaAs are 4.3 and 4.8×10−8&OHgr; cm2, respectively. The electrical behavior of the contacts is discussed in relation to the effect of ion beam cleaning and post‐deposition annealing. The use of low energy (60 eV) Ar+bombardment provides atomically clean, contamination free surfaces. Very rapid thermal processing at 400 °C for 1 s was successfully employed in order to restore the original properties of the subsurface layer disordered during ion bombardment. This fabrication sequence ensures formation of intimate contacts without interfacial films and carrier compensation effects. The variation of the specific contact resistance with the reciprocal square root of the carrier concentration indicates that tunneling is the dominant mechanism of current flow through the metal‐semiconductor junction. The tunneling origin of the contact characteristics has also been elucidated from the temperature dependence of the contact resistance.

ISSN:0021-8979    

DOI:10.1063/1.354986

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Interface traps are created in metal‐oxide‐semiconductor field‐effect transistors when hot carrier stressing is done with the maximum substrate current biasing condition (Vd=6.0 V andVg=2.9 V). Unlike trapped oxide charge, the interface traps are not annealed by keeping the device at room temperature for 24 h. However, by applying reverse bias with high positive drain voltageVdand negative gate voltageVg, the hot carrier induced interface traps can be completely annealed out. This is confirmed by both transconductance and charge pumping measurements. There is a direct relationship between the substrate current and annealing of interface states by reverse stressing. The possible mechanism of interface state annealing is discussed.

ISSN:0021-8979    

DOI:10.1063/1.354987

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Photoconductance investigations with square light pulses have been made on semiconducting thin films of GaInAs doped with silicon. Two wavelengths are used: 870 and 1300 nm. One unexpected evolution of the photoconductance transients resulting from traps is observed when the wavelength is varied. A model based on the representation of each trap by a well located between two disymmetrical potential barriers is consistent with the experimental results.

ISSN:0021-8979    

DOI:10.1063/1.354988

出版商:AIP    

年代:1993

数据来源: AIP