Volume 77, Issue 7, 01 April 1995
Index of content:
77(1995); http://dx.doi.org/10.1063/1.358700View Description Hide Description
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 isotopesuperlattices consisting of alternating layers of n atomic planes of 70Ge and 74Ge reveal a host of zone‐center phonons due to Brillouin‐zone folding. At n≳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 pure 12C diamond crystals exhibit thermal conductivities as high as 410 W cm−1 K−1 at 104 K, leading to projected values of over 2000 W cm−1 K−1 near 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 Geisotopesuperlattices with 0.1–1 μm 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.
77(1995); http://dx.doi.org/10.1063/1.358701View Description Hide Description
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.
77(1995); http://dx.doi.org/10.1063/1.358702View Description Hide Description
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.
77(1995); http://dx.doi.org/10.1063/1.358703View Description Hide Description
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 Ω‐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 microwavetechnology,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’sequations 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.
Optical second‐harmonic generation as a probe of the temperature dependence of the distribution of sites in a poly(methyl methacrylate) polymer doped with disperse red 1 azo dye77(1995); http://dx.doi.org/10.1063/1.358704View Description Hide Description
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 α and β 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 β 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.
Enhancement of the nonreciprocal magneto‐optic effect of TM modes using iron garnet double layers with opposite Faraday rotation77(1995); http://dx.doi.org/10.1063/1.359516View Description Hide Description
Garnetfilms of composition Lu3−x Bi x Fe5−y Ga y O12 are grown by liquid‐phaseepitaxy on ‐oriented substrates of gadolinium gallium garnet.Faraday rotation and saturation magnetization are measured as a function of substitution levels, which range up to x=1.4 and y=1.8, respectively. Nonreciprocal propagation of the TM0 is studied at a wavelength of 1.3 μm. 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.
77(1995); http://dx.doi.org/10.1063/1.358705View Description Hide Description
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 3 P) and Ne*(3s 3 P 2) atoms in the discharge using He/Ar/F2 and Ne/Ar/F2 gas mixtures were studied. The peak of Ne* and He* atom densities were 1.7×1012 cm−3 and 2.1×1012 cm−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/F2 gas 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.
77(1995); http://dx.doi.org/10.1063/1.358706View Description Hide Description
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.
77(1995); http://dx.doi.org/10.1063/1.358707View Description Hide Description
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 medium U p ∼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 ‖E M1‖ 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 ‖E M2‖. The model allows the interpretation of scaling laws observed experimentally: an inverse relationship of the field strength with the dielectric constant ε; a dependence of the field strength on the length l of the gap (size effect). It is demonstrated that, provided the total amount of charges is bounded when l→∞, the field strength ‖E M2‖ is necessarily a decreasing function of l, 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.
77(1995); http://dx.doi.org/10.1063/1.358708View Description Hide Description
Time‐ and space‐resolved emission and laser‐induced fluorescencespectroscopicmeasurements 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−2 up to values of about 500 MW cm−2 typically 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.
77(1995); http://dx.doi.org/10.1063/1.358709View Description Hide Description
The sheath zone near a plane probe inmersed in an electronegative plasma is analyzed. A theoretical model for the sheath zone was used to derive a new expression for the Bohm criterion. The new expression holds at any negative probe potential. Solving the model equations allows one to obtain a wealth of information including the spatial distribution of various quantities (electric field, potential, and particle densities), the sheath thickness, and the floating potential.
77(1995); http://dx.doi.org/10.1063/1.358710View Description Hide Description
Nanocrystalline TiN thin films were synthesized by Xe+ ion‐beam‐enhanced deposition at room temperature. X‐ray diffraction was used to study different samples prepared by different energy Xe+ ions. The results indicated that different ion energies can introduce different phases. Two metastable phases (Ti2N) can be formed at higher Xe+ energies than 40 keV. The crystalline phases are dependent on the temperatures in the collision cascades, which are induced by Xe+ ion bombardment with different energies. The films were always composed of nanocrystallites. Therefore a lot of surface and interface structures are in the film. The grain sizes grow with the increase of the bombarding ion energy. The effect is caused by the increase of cascade size and local temperature. The lattice parameters are found to increase with the rise of defect concentration and to decrease with the increase of surface structure. These films often have preferential orientation. The relationship between preferential orientation and process parameters is discussed.
77(1995); http://dx.doi.org/10.1063/1.358711View Description Hide Description
Single crystals of lithium niobate (LiNbO3) with y‐cut orientation were irradiated at GANIL with 5.17 MeV/amu 155Gd ions at a fluence ranging from 1.5×1011 to 6.0×1011 ions cm−2. After irradiation the samples were implanted with europium ions of 70 keV energy at a fixed fluence of 5×1016 ions cm−2. Both irradiations and implantations were performed at room temperature. Thermal treatments in air, in the range 300–1100 °C, were applied to investigate the influence of the preirradiation damage on the thermal evolution of Eu implanted LiNbO3 crystals. Transmission electron microscopy micrographs exhibit the amorphous track formed by high‐energy irradiations.Rutherford backscattering spectrometry results show that these amorphous tracks enhance the europiumdiffusion in depth of the LiNbO3. A diffusion coefficient and an activation energy were estimated to be about 4.2×10−15 cm2 s−1 and 0.42 eV, respectively.
77(1995); http://dx.doi.org/10.1063/1.358712View Description Hide Description
Fibrous and crystal structures of a helical polymer, poly‐L‐lactic acid (PLLA), were analyzed by using x‐ray diffraction experiments. It was confirmed that the molecular residues were arranged on a nonintegral 10/3 helix as De Santis and Kovacs [Biopolymers6, 299 (1968)] reported. The atomic positions in a monomeric unit, which were proposed by Hoogsteen, Postema, Pennings, ten Brinke, and Zugenmaier [Macromolecules 23, 634 (1990)], were validated. However, the previous reports on the positions of the two helical chains were found to be in error. The correct positions were determined. The second helical chain shifts from the base center by 0.45, 0.25, and 0.61 Å along a, b, and c axes. Besides, the second chain rotates by 2.46° with respect to the first. Distribution function of the crystallites in various drawn fibers were determined as a function of spiral angle. Optical gyrations of PLLA and poly‐D‐lactic acid fibers were successfully measured by using high accuracy universal polarimeter, as functions of temperature and drawing ratio. By using x‐ray data of the change of the fibrous structure by drawing treatments, the gyration tensor components of PLLA could be calculated. It is of great interest that gyration tensor component g 33 along the helical axis is extremely large, ∼(3.85±0.69)×10−2, which corresponds to a rotatory power of (9.2±1.7)×103°/mm, about two orders of magnitude larger than those of ordinary crystals. This is the first experimental evidence that helical polymers will produce enormous optical activity in the solid state. Helical polymers will be important for the elucidation of gyro‐optical properties of solids and promising for new optical applications utilizing their large optical activity.
Effect of rapid thermal annealing on the strain relaxation in heavily boron doped silicon epitaxial layer77(1995); http://dx.doi.org/10.1063/1.358713View Description Hide Description
The lattice parameters of heavily borondopedsiliconhomoepitaxial layers before and after rapid thermal annealing at different temperatures (800–1100 °C) are characterized by x‐ray double crystal diffraction. The results illustrate that although the strain relaxation occurs after the rapid thermal annealing(RTA) treatment, the improvement of the crystalline quality of epitaxial layers are observed at RTA temperatures higher than 1000 °C. The proportional relationship between the lattice mismatch and the substitutional boron concentration with the lattice contract coefficient β=5.3 (in units 10−24 cm3) is valid up to the concentration of 3×1020 cm−3.
77(1995); http://dx.doi.org/10.1063/1.358714View Description Hide Description
High‐dose carbon‐ion‐implanted Si samples have been analyzed by infrared spectroscopy, Raman scattering, and x‐ray photoelectron spectroscopy(XPS) correlated with transmission electron microscopy. Samples were implanted at room temperature and 500 °C with doses between 1017 and 1018 C+/cm2. Some of the samples were implanted at room temperature with the surface covered by a capping oxide layer. Implanting at room temperature leads to the formation of a surface carbon‐rich amorphous layer, in addition to the buried implanted layer. The dependence of this layer on the capping oxide suggests this layer to be determined by carbon migration toward the surface, rather than surface contamination. Implanting at 500 °C, no carbon‐rich surface layer is observed and the SiC buried layer is formed by crystalline β‐SiC precipitates aligned with the Si matrix. The concentration of SiC in this region as measured by XPS is higher than for the room‐temperature implantation.
77(1995); http://dx.doi.org/10.1063/1.358715View Description Hide Description
The deep level transient spectroscopy technique has been used to study the EL2 defect in n‐type semiconducting GaAs subjected to 1 MeV fast neutrons at room temperature. After neutron irradiation, the EL3 defect which is usually detected between 180 and 210 K disapperared and the EL2 defect measured between 280 and 320 K was found to remain single exponential (E c −0.820 eV) despite the creation of a broad U band measured between 100 and 270 K. From this result, together with our earlier reports on the double exponential capacitance transient of the EL2 defect after 1 MeV electron irradiation [Lai, Nener, Faraone, Nassibian, and Hotchkis, J. Appl. Phys. 73, 640 (1993)] and the behavior of the electron irradiated EL2 defect upon isochronal annealing [Lai and Nener, J. Appl. Phys. 75, 2354 (1994)], we observe a difference in the behavior of the EL2 defect after neutron and electron irradiation. The results of the present study indicate that the EL2‐B level reported in an earlier work is not due to any interaction of the stable EL2 (or EL2‐A) level with either the U‐band or EL6 defect. The EL2 defect is likely to be a complex defect which can manifest itself as a number of different defect levels depending on the particular details of the irradiation used. The U band is likely to be a cluster defect caused by the large number of atoms displaced from lattice sites by the fast neutrons, and is not likely to be due to any interaction mechanism between the EL2 and EL6 defects.
77(1995); http://dx.doi.org/10.1063/1.358716View Description Hide Description
Distribution of light‐induced defect states in undoped amorphous silicon has been studied by using the modulated photoconductivityspectroscopy technique. The experimental results show that a large increase of the neutral defect states occurs, and the positively charged defect states grow particularly in the midgap energy range. The qualitative features of the measured energy distribution agree well with the theoretical prediction from the current defect formation model, although a quantitative comparison with respect to the magnitude of density‐of‐states reveals a discrepancy between theory and experiment.
Effect of energetic particle bombardment during sputter deposition on the properties of exchange‐biased spin‐valve multilayers77(1995); http://dx.doi.org/10.1063/1.359565View Description Hide Description
The effect of energetic particle bombardment during sputter deposition on the physical (magnetoresistance and interlayer coupling) and microstructural (roughness and interface thickness) properties of exchange‐biased spin‐valve multilayers is investigated. An increasing pressure leads, through enhanced stopping of energetic particles by collisions with the background gas, to a decrease of the interfacial intermixing by collisions during growth, and to a more rough, void‐rich structure. These microstructural changes lead to an increase of the transmissivity of the interfaces for conduction electrons, as well as to an increase of the ferromagnetic interlayer coupling with increasing pressure.
77(1995); http://dx.doi.org/10.1063/1.358649View Description Hide Description
Amorphization of 6H‐SiC with 200 keV Ge+ ions at room temperature and subsequent ion‐beam‐induced epitaxialcrystallization (IBIEC) with 300 keV Si+ ions at 480 °C have been studied by Rutherford backscattering spectrometry/channeling and transmission electron microscopyanalysis. Experimental results on amorphous layer thicknesses have been compared with trim calculations in association with the critical energy density model. Density changes during amorphization have been observed by step height measurements. Particular attention has been directed to the crystal quality and a possible polytype transformation during the IBIEC regrowth. The IBIEC process consists of two stages and results in a multilayer structure. In the initial phase an epitaxialgrowth of 6H‐SiC has been obtained. With increasing IBIEC dose the epitaxialgrowth changes to columnar growth and is stopped by polycrystallization of 3C polytype in the near‐surface region.