摘要:

Scientific interest, technological promise, and increased availability of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies with isotopically controlled semiconductor crystals. This review of mostly recent activities begins with an introduction to some past classical experiments which have been performed on isotopically controlled semiconductors.A review of the natural isotopic composition of the relevant elements follows. Some materials aspects resulting in part from the high costs of enriched isotopes are discussed next. Raman spectroscopy studies with a number of isotopically pure and deliberately mixed Ge bulk crystals show that the Brillouin‐zone‐center optical phonons are not localized. Their lifetime is almost independent of isotopic disorder, leading to homogeneous Raman line broadening. Studies with short period isotope superlattices consisting of alternating layers ofnatomic planes of70Ge and74Ge reveal a host of zone‐center phonons due to Brillouin‐zone folding. Atn≳40 one observes two phonon lines at frequencies corresponding to the bulk values of the two isotopes. In natural diamond, isotope scattering of the low‐energy phonons, which are responsible for the thermal conductivity, is very strongly affected by small isotope disorder. Isotopically pure12C diamond crystals exhibit thermal conductivities as high as 410 W cm−1 K−1at 104 K, leading to projected values of over 2000 W cm−1 K−1near 80 K.The changes in phonon properties with isotopic composition also weakly affect the electronic band structures and the lattice constants. The latter isotope dependence is most relevant for future standards of length based on crystal lattice constants. Capture of thermal neutrons by isotope nuclei followed by nuclear decay produces new elements, resulting in a very large number of possibilities for isotope selective doping of semiconductors. This neutron transmutation of isotope nuclei, already used for homogeneous doping of floating zone Si with P, holds perhaps the biggest promises for isotopically controlled semiconductors and is discussed in some detail. Local vibrational modes of low‐mass impurities are sensitive to the mass of the impurity as well as the masses of the host atoms neighboring the impurity. High‐resolution infrared‐absorption studies of O in Ge crystals of different isotopic composition demonstrate the extreme simplification in such spectra which is achieved by isotope control. Interdiffusion of GaAs and Ge isotope superlattices with 0.1–1 &mgr;m thick layers have been studied with secondary‐ion‐mass spectroscopy. This kind of internal diffusion avoids the problems with surface effects and can produce accurate data without the need for radioactive tracers. The review closes with an outlook on the exciting future possibilities offered through isotope control of a wide range of semiconductor materials. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358700

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Systems for focusing a light ion beam onto an inertial confinement fusion (ICF) target are studied using a thin lens model to compute ion trajectories. Three focusing systems are analyzed. The systems are (1) an uncorrected single focusing lens; (2) a focusing lens with chromatic correction due to the self magnetic field of the beam; and (3) the combination of a focusing lens, a defocusing lens for divergence correction, and chromatic correction from the beam’s self field. Systems (2) and (3) both produce energy efficiencies of greater than 50% for transporting an ion beam with 6 mrad divergence over 4 m and focusing it onto a 1 cm radius target. The predictions of the thin lens model are compared to predictions made by codes using numerical integration of particle trajectories. The implications for ICF applications are discussed. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358701

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The propagation in sea water of a low‐frequency electromagnetic pulse generated by an electric dipole is investigated analytically. The dipole is excited by a rectangular current pulse with a finite, nonzero rise and decay time. In order to obtain an explicit formula for the field in the equatorial plane of the dipole source that is uniformly valid in distance and time, Fourier‐transform methods are applied. Certain limiting forms of the current pulse are studied separately. Simple analytic expressions of the field are obtained, compared to previous results, and examined thoroughly. The effect of the finite rise and decay time is discussed. It is noted that the present analysis may be used for studying pulse propagation in any highly conducting medium besides sea water. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.358702

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The uniaxial bianisotropic‐ferrite medium is a generalization of the well‐studied magnetically biased ferrite and uniaxial material. It can be manufactured either by immersing randomly oriented short helices and &OHgr;‐shaped particles in a magnetically biased ferrite, or by arranging short conductive helices in a magnetized ferrite in a certain manner. It has potential applications in microwave technology, antenna design, and antireflection shielding. In the present consideration, based on the concept of characteristic waves and the method of angular spectral expansion, field representations in uniaxial bianisotropic‐ferrite medium are developed. The analysis reveals the solutions of source‐free Maxwell’s equations for uniaxial bianisotropic‐ferrite medium can be represented in sum‐integral forms of the circular cylindrical vector wave functions. The addition theorem of vector wave functions for uniaxial bianisotropic‐ferrite medium can be straightforwardly derived from that of vector wave functions for isotropic medium. An application of the proposed theory in scattering is presented to show how to use these formulations in a practical way. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358703

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

We have applied electric‐field‐induced optical second‐harmonic generation to measure the distribution of chromophore sites in a dye‐doped polymer [disperse red 1 in poly (methyl methacrylate)] in terms of the standard &agr; and &bgr; parameter, that is, the decay rate and the breadth parameter. These parameters have been measured from 100 to 430 K and are modeled with a smooth function over that temperature range. Over a narrow region of temperature where second‐harmonic data from other groups exist, we find that our temperature dependence of &bgr; is in excellent agreement with these other experiments. In principle, our method can be applied to any glassy polymer to characterize the temperature dependence of the distribution of nonlinear‐optical dye sites. ©1995 American Institute of Physics. 

ISSN:0021-8979    

DOI:10.1063/1.358704

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Garnet films of composition Lu3−xBixFe5−yGayO12are grown by liquid‐phase epitaxy on [111]‐oriented substrates of gadolinium gallium garnet. Faraday rotation and saturation magnetization are measured as a function of substitution levels, which range up tox=1.4 andy=1.8, respectively. Nonreciprocal propagation of the TM0is studied at a wavelength of 1.3 &mgr;m. It is shown that the difference between forward and backward propagation constants can be optimized using double layers with opposite sign of the Faraday rotation. Agreement between experiments and calculations is excellent. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.359516

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Time‐dependent number densities of excited atoms in a discharge‐pumped ArF excimer laser were measured through laser absorption probing with a cw dye laser. The He*(2p 3P) and Ne*(3s 3P2) atoms in the discharge using He/Ar/F2and Ne/Ar/F2gas mixtures were studied. The peak of Ne* and He* atom densities were 1.7×1012cm−3and 2.1×1012cm−3, respectively. The number densities of He*, the laser output power, and the discharge voltage were also measured with varying He/Ne mixture ratio in He/Ne/Ar/F2gas mixtures. Effects of He and Ne buffer gases on the excitation mechanisms of atoms are discussed on the basis of those measured results, and compared with the case of XeCl excimer lasers. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358705

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A method for analyzing strong cylindrical blasts in cases where the source mass is not negligible is presented. Analytic expressions for the shock position as a function of time and shock speed and pressure as functions of position are derived. An approximation to classical Taylor–Sedov theory is extended to the near‐field regime where the source mass is not negligible. Results from example calculations are compared to Taylor–Sedov theory as well as thermodynamic free expansion. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358706

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

The trapping of charges in hard dielectric materials has been recently described [G. Blaise, IEEE Trans. Electr. Insul.EI‐28, 437 (1993)] on the basis of the polaron concept. A local increase of the internal energy of the mediumUp∼5–10 eV results from the polarization around a trapped charge. The relaxation of the lattice after a rapid detrapping of charges from their site releases the local excess of internal energy to the medium, producing breakdown when critical conditions are reached. This scenario is applied to the electric breakdown of a parallel‐plate capacitor in which the space charge results from the injection of electrons at the cathode. Breakdown at the cathode is produced by the destabilization of a static space charge when the field in dielectric gap exceeds the detrapping field of charges. This determines the minimum value ‖EM1‖ of the field strength. At the anode breakdown is due to the trapping‐detrapping of flowing charges. This corresponds to the maximum value of the field strength ‖EM2‖. The model allows the interpretation of scaling laws observed experimentally: an inverse relationship of the field strength with the dielectric constant &egr;; a dependence of the field strength on the lengthlof the gap (size effect). It is demonstrated that, provided the total amount of charges is bounded whenl→∞, the field strength ‖EM2‖ is necessarily a decreasing function ofl, tending toward a finite value. This interpretation of breakdown applies to bulk breakdown observed in small‐gap capacitors and surface flashover observed in large‐gap capacitors as well. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358707

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Time‐ and space‐resolved emission and laser‐induced fluorescence spectroscopic measurements were performed to investigate vaporization and plasma formation resulting from excimer laser irradiation of titanium targets in a low‐pressure nitrogen atmosphere. Measurement series have been done by varying the laser intensity from the vaporization threshold at 25 MW cm−2up to values of about 500 MW cm−2typically applied in pulsed laser deposition processing of titanium nitride films. Thus, the transition from thermal evaporation to the high‐density plasma formation process, leading to the production of reactive species and high‐energy ions, was evidenced. An interesting result for the comprehension of the reactive deposition process was the observation of a quantity of dissociated and ionized nitrogen, which is transported with the plasma front in the direction of the substrate. ©1995 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.358708

出版商:AIP    

年代:1995

数据来源: AIP