摘要:

Experiments of water infiltration in a vertical plane through layered soils were carried out in the laboratory. We have followed the fingering dynamics by using image techniques. The fractal theory has been applied to describe the fingering phenomena. We also have simulated a modified site gradient invasion percolation model where most of the physical ingredients relevant to the problem have been incorporated. In contrast to what occurs with the standard gradient invasion percolation model, we have found the formation ofmanyfingers. Taking into account the bond number, the number of fingers, and the wetted area of the experiments, we have fitted the parameters of our model. This opens the possibility of a theoretical prediction of theporesizedistributionof the soil. The results are then compared with the experimental data. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360044

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Perchloroethylene (PCE) is one of a class of industrial solvents for which alternate disposal strategies are being investigated. Among these strategies is plasma remediation. In this paper we report on a computational study of the plasma remediation of PCE using repetitively pulsed dielectric barrier discharges. We discuss the dominant reaction pathways in the remediation of PCE and show that remediation is significantly improved in humid gas mixtures. Although plasma remediation does not totally oxidize PCE to the desired end‐products (CO2and HCl), the dominant end‐products are more easily treated than PCE and can be disposed of using conventional methods. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360045

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A comparison between laser welding under vacuum and atmospheric pressure conditions was performed. By measuring plasma size, electron density, and electron temperature, plasma suppression at low pressure (below 20 Torr) is observed, associated with an electron density and temperature decrease with reduction of pressure. These microscopic parameters were then correlated to the ambient pressure according to Saha’s law. The electron density in the plasma plume is controlled by the ambient pressure. The influence of the plasma suppression on the welded seam was also studied. New information concerning the energy deposition process inside the keyhole was obtained: Under reduced pressure conditions, the laser light is mainly absorbed by Fresnel reflection and a constant energy deposition per unit length occurs. At atmospheric pressure the laser is absorbed by a combination of Fresnel effect and inverse bremsstrahlung plasma process. The power deposition can be approximated by an inverse exponential law. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360046

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We construct a mathematical model of interface fluctuations in Czochralski crystal growth based on the thermal balance across the growing interface. The model describes time‐dependent growth rate in relation to fluctuations in crystal pull rate and melt flow viewed as external perturbations. Complexities in the flow of a silicon melt are characterized in terms of time series forecast about melt thermal fluctuations actually observed beneath a growing crystal. The melt exhibits self‐affine random motion with spatial and temporal structure specific to crucible rotation rate. The influence of the complex flow to crystal growth is discussed on the basis of the model proposed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360047

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A deep level transient spectroscopy study of native and radiation induced defects in metal organic chemical vapor depositionnonp6H‐SiC diodes has been conducted. Three majority carrier levels were found, atEv+0.50, +0.55, and +0.69 eV, and three minority carrier deep levels were found, atEc−0.38, −0.48, and −0.58 eV. These six levels were initially observed in the unirradiated materials. Their concentration increased 2 to 13‐fold after irradiation to a fluence of 2×1011cm−2of 5.5 MeV alpha particles. In addition the carrier removal was monitored during irradiation, and a carrier removal rate of 7.8×104cm−1for 5.5 MeV alpha particles was measured. When compared with a similar study of alpha particle irradiation of InP, the results suggest that SiC has radiation resistance comparable to that of InP, another highly radiation resistant semiconductor. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360048

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Concentration dependent diffusion of Zn during metalorganic vapor phase epitaxy from a Zn‐doped InP layer into the adjacent undoped InP buffer layer were studied systematically using secondary ion mass spectroscopy and carrier concentration profiling. Under the condition that the growth rate of the Zn‐doped film is faster than the interdiffusion of Zn into the underlying undoped buffer layer, the diffusion problem can be treated as a one‐dimensional diffusion couple between two semi‐infinite media. Furthermore, Zn diffusion during the optimized growth condition for InP completely eliminates the thermal decomposition problem encountered in the sealed ampoule and open tube diffusions and also maintains all the intrinsic point defects at their thermodynamic equilibrium concentrations. With an optimal growth temperature at 625 °C and a maximum Zn flow below the incorporation limit for substitutional Zn to ensure that the dominant Zn are incorporated substitutionally, the diffusion profiles of Zn across the interface in this simple and clean system are simulated using a concentration dependent diffusivity. A third power concentration dependence of the effective diffusion coefficient has been confirmed, which applies to both Frank–Turnbull and kickout interstitial‐substitutional equilibrium mechanisms using an interstitial‐substitutional diffusion model. This indicates a +2 charge state of the fast diffusing Zn interstitials. The extrapolated curve into high‐concentration diffusion source regime used by sealed ampoule diffusion experiments generally agrees with the published results although the dominant Zn atoms found in the high‐concentration diffusion source regime form complexes with phosphorous vacancies in a neutral state. The enhanced diffusion due to excess interstitials is discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360049

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The effect of ion‐implantation‐induced damage on the resistivity of n‐type GaN has been investigated. H, He, and N ions were studied. The resistivity as a function of temperature, implant concentration, and post‐implant annealing temperature has been examined. Helium implantation produced material with an as‐implanted resistivity of 1010&OHgr;‐cm. He‐implanted material remained highly resistive after an 800 °C furnace anneal. The damage associated with H implantation had a significant anneal stage at 250 °C and the details of the as‐implanted resistivity were sample dependent. N implants had to be annealed at 400 °C to optimize the resulting resistivity but were then thermally stable to over 800 °C. The 300 °C resistivity of thermally stabilized He‐ and N‐ implanted layers was 104&OHgr;‐cm, whereas for H‐implanted layers the 300 °C resistivity was less than 10 &OHgr;‐cm. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360712

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The gettering of copper by keV implantation of germanium into silicon is investigated. Germanium is implanted at a fixed energy with varying doses into the front side of silicon samples. Copper is thermally evaporated on the backside of the samples and then annealed at 900 °C for 1 h and 10 h, respectively, to allow in‐diffusion of the transition metal. Rutherford backscattering spectroscopy, secondary‐ion‐mass spectroscopy, and cross‐section transmission electron microscopy are used to demonstrate that gettering of copper is achieved through stacking faults created by heavy dose germanium implantation and solid‐phase epitaxy. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360050

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The crystallization of amorphous carbon films, under inert atmospheres, occurs at annealing temperatures above 800 °C. In this work we have found that when the annealing of carbon films is performed under atmospheric conditions, crystallization occurs at temperatures as low as 200 °C. The catalytic effect of oxygen in the crystallization process is understood in terms of the generation of a porous structure in the carbon film due to the vaporization of carbon oxides. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360051

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A study of the temporal response of a biased planar nematic liquid crystal to short (∼10 &mgr;s) voltage pulses is presented. The resulting optical phase shift varies quadratically with time during the first 2 &mgr;s after switching the voltage on and then linearly. A theoretical model is developped starting from the Leslie‐Ericksen theory which describes the investigated phase modulation in the microsecond time scale. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.360052

出版商:AIP    

年代:1995

数据来源: AIP