ALBERT

Notizen: Thin films of La1/2Na1/2TiO3 fabricated by pulsed laser deposition show interesting dielectric properties exhibiting quantum paraelectric-type temperature-insensitive dielectric constant below 50 K. The dielectric constant (εr∼180) is also very stable under electric bias up to 4×104 V/cm. These properties indicate that these films coupled with high temperature superconductors have a great potential for microwave applications. © 2000 American Institute of Physics.

Notizen: Multiple-energy box profile elevated-temperature (700 °C) phosphorus ion implantations were performed into 4H–SiC in the doping range of 1×1017–1×1020 cm−3. The implanted material was annealed at 1500, 1600, or 1650 °C with an AIN encapsulant to prevent degradation of the SiC surface. Within this temperature range the sheet resistance does not change significantly for a given dose. The percentage of electrical activation of the P donors initially decreased with increasing implant dose for P-implant concentration up to 3×1019 cm−3 and then increased again at higher doses. For 1×1020 cm−3 P implant, a carrier concentration of 4×1019 cm−3 was measured at room temperature. In the 1017 cm−3 P doping concentration range substitutional activation greater than 85% was measured. Despite performing the implants at 700 °C, a significant amount of as-implanted damage was observed in the Rutherford backscattering (RBS) spectrum, even for 1018 cm−3 range P implantations. The RBS yield after annealing is near the virgin level for P concentrations up to 1×1019 cm−3, but above this concentration the RBS yield is above the virgin level, indicating a significant amount of residual lattice damage in the crystal. © 2000 American Institute of Physics.

Notizen: A detailed investigation of electrostatic dust wave modes in unmagnetized dusty plasmas consisting of electrons, ions and dust grains has been carried out over a wide range of dust fugacity and wave frequency by using fluid as well as kinetic (Vlasov) theories. The dust fugacity parameter is defined by f≡4πnd0λD2R∼ND R/λD where nd0, λD and R are respectively the dust number density, the plasma Debye length and the grain size (radius), and ND=4πnd0λD3/3 is the dust plasma parameter. Dusty plasmas are considered to be tenuous, dilute or dense according as f(very-much-less-than)1, ∼1, or (very-much-greater-than)1. In particular, attention is focused on the "dust–acoustic waves" (DAWs) and the "dust–Coulomb waves" (DCWs) which exist in the tenuous (low fugacity) and the dense (high fugacity) regimes, respectively, when the wave frequency is much smaller than the grain charging frequency. Unlike the DAWs, which exist even with constant grain charge, the DCWs [N. N. Rao, Phys. Plasmas 6, 4414 (1999)] are the normal modes associated with grain charge fluctuations, and exist in dense dusty plasmas. In the long wavelength limit, the DCW phase speed scales as ∼CDA/f where CDA is the DAW phase speed. In the dilute (medium fugacity) regime, the two modes merge into a single mode, which may be called the "dust charge–density wave" (DCDW) since the latter involves contributions arising from both the DAW and the DCW. On the other hand, for frequencies much larger than the charging frequency, DAWs are shown to exist also in the dilute regime. The real frequency as well as the damping rate in each case are explicitly calculated from both the fluid as well the kinetic theories, and a comparison between the two has been carried out. In the allowed fugacity regimes (tenuous, dilute or dense), all the three waves are weakly damped and, hence, can propagate as normal modes. The present analysis of wave propagation in dusty plasmas over different fugacity regimes suggests the introduction of a new length scale defined by λR≡dWSdWS/3Rδ, where dWS is the Wigner–Seitz radius and δ is a parameter related to the charging frequencies. This length scale which governs the dispersive properties of the DCW modes is most useful in the dense regime, and plays a role which is very similar to that of the Debye length in the tenuous regime. The ratio of λR to λD is a measure of the dust fugacity, and is given through fδ=λD2/λR2. The very recent experimental observation [S. Nunomura et al., Phys. Rev. Lett. 83, 1970 (1999)] on a self-excited instability associated with grain charge fluctuations may be an indication of DCWs in the strong coupling regime. The possibility of the existence of a dust thermal wave (DTW) in the super-dense regime has been pointed out. A heuristic, but simple, derivation of DCWs based on grain dynamics but supplemented by physical inputs from the plasma response has also been presented. © 2000 American Institute of Physics.

Notizen: The existence of ultra low-frequency wave modes in dusty plasmas has been investigated over a wide range of dust fugacity [defined by f≡4πnd0λD2R, where nd0 is the dust number density, λD is the plasma Debye length, and R is the grain size (radius)] and the grain charging frequency (ω1) by numerically solving the dispersion relation obtained from the kinetic (Vlasov) theory. A detailed comparison between the numerical and the analytical results applicable for the tenuous (low fugacity, f(very-much-less-than)1), the dilute (medium fugacity, f∼1), and the dense (high fugacity, f(very-much-greater-than)1) regimes has been carried out. In the long wavelength limit and for frequencies ω(very-much-less-than)ω1, the dispersion curves obtained from the numerical solutions of the real as well as the complex (kinetic) dispersion relations agree, both qualitatively and quantitatively, with the analytical expressions derived from the fluid and the kinetic theories, and are thus identified with the ultra low-frequency electrostatic dust modes, namely, the dust-acoustic wave (DAW), the dust charge-density wave (DCDW) and the dust-Coulomb wave (DCW) discussed earlier [N. N. Rao, Phys. Plasmas 6, 4414 (1999); 7, 795 (2000)]. In particular, the analytical scaling between the phase speeds of the DCWs and the DAWs predicted from theoretical considerations, namely, (ω/k)DCW=(ω/k)DAW/fδ (where δ is the ratio of the charging frequencies) is in excellent agreement with the numerical results. A simple physical picture of the DCWs has been proposed by defining an effective pressure called "Coulomb pressure" as PC≡nd0qd02/R, where qd0 is the grain surface charge. Accordingly, the DCW dispersion relation is given, in the lowest order, by (ω/k)DCW=PC/ρdδ, where ρd≡nd0md is the dust mass density. Thus, the DCWs which are driven by the Coulomb pressure can be considered as the electrostatic analogue of the hydromagnetic (Alfvén or magnetoacoustic) waves which are driven by the magnetic field pressure. For the frequency regime ω(very-much-greater-than)ω1, the numerical results confirm the existence of only the DAWs, while the DCWs are absent as predicted by the fluid and the kinetic theories. The wave damping rates due to the charge fluctuations as well as the wave–particle interactions (Landau type) have been numerically computed and compared with the analytical results. For the tenuous as well as the dilute regimes, there is good agreement between the numerical and the analytical results at small wave numbers. However, at larger wave numbers and for the dense regime, the damping rates are underestimated by the theoretical expressions obtained under suitable approximations. Some comments on the sources for these differences have also been presented. © 2000 American Institute of Physics.

Notizen: The ternary intermetallic compound Gd3Fe28.2W0.8 with a Nd3Fe29−xTix-type structure was synthesized with W as a stabilizing element, and its crystal structure and magnetic properties were investigated by means of x-ray powder diffraction and magnetic measurements. The space group of Gd3Fe28.2W0.8 is A2/m with lattice parameters a=10.5812 Å, b=8.5338 Å, c=9.6986 Å, and β=96.86°. Rietveld refinement of the crystal structure reveals that W atoms occupy exclusively three crystallographic positions (4g, 4i3, 4i4) with only one neighboring Gd atom. The preferential occupation of W atoms can be understood in terms of the site volume and enthalpy of mixing. The compound exhibits a planar magnetocrystalline anisotropy perpendicular to the [102] direction. The Curie temperature Tc, saturation moment Ms, and anisotropy field HA are 502 K, 38.6 μB/f.u., and 7.5 T, respectively. © 2000 American Institute of Physics.

Notizen: Heteroepitaxial growth of InSb was performed on Si(111)–(7×7) and Si(111)–In(4×1) surface phases over a wide temperature range, by optimizing the growth rate and substrate temperature. When the heteroepitaxy was performed on the Si(111)–In(4×1) surface, the In(4×1) reconstruction modified the growth process depending on the growth temperature. At low temperatures, the In(4×1) reconstruction contributes mildly to the growth, and as the growth temperature increases, it starts degrading the quality of the films. For temperatures above 300 °C, the In(4×1) reconstruction virtually destroys the growth. In the present article, we illustrate this behavior using the growth of InSb on both Si(111)–(7×7) and Si(111)–In(4×1) surfaces at 210, 250, and 300 °C. Based on reflection high-energy electron diffraction observations, we discuss the initial stages of growth. A model for the interface formation is proposed based on our earlier results suggesting the temperature-dependent modification of In-induced surface phases on Si(001) and Si(111) surfaces during Sb deposition. © 2000 American Institute of Physics.

Notizen: An electron cyclotron resonance (ECR) plasma source (10 cm in diameter) has been developed for ion assisted sputter deposition of thin films. Variation of plasma parameters like ion density, electron temperature, plasma potential, and floating potential as a function of pressure and microwave power has been studied using Langmuir probe analysis. The ECR source gives an ion density of 1.01×1011/cm3 at a distance of 8 cm from the source exit at a pressure of 8×10−4 mbar and 400 W of microwave power. The uniformity of the plasma parameters at the substrate position was found to be ±2% over a diameter of 12 cm. Thin films of copper and silicon nitride have been deposited by rf sputtering in the presence of ECR plasma. The properties showed a significant change at an ECR power of 100 W. © 2000 American Institute of Physics.

Notizen: Low-cost instruments for measurement in medicine, biotechnology, and environmental monitoring are presented. Recent developments in optoelectronic technology enable practical compact designs. This article presents the available types of light emitters, detectors, and wavelength selection components that are used in low-cost instruments. The main spectroscopic techniques (absorption, reflectance, luminescence intensity, lifetime, and polarization, evanescent wave and surface plasmon resonance) that are used with these instruments are described. Numerous examples of devices for a broad variety of biomedical measurements are presented. © 2000 American Institute of Physics.

Notizen: Barium ferrite thin films with perpendicular c-axis orientation and small grain size (about 300 Å) were successfully fabricated with careful control of sputtering conditions. The c-axis orientation of barium ferrite thin films is most sensitive to the oxygen partial pressure during deposition. All samples with oxygen gas during deposition have a random c-axis texture, as indicated by existence of both weak (00l) peaks and (106) peaks. All the samples without oxygen gas during deposition show only strong (00l) peaks, which indicate excellent perpendicular c-axis orientation. Transmission electron microscopy results show that oxygen gas promotes the growth of in-plane and/or randomly oriented grains. The effect of the Pt interlayer on the barium-rich films was also studied. The Pt interlayer was found to be very effective in improving c-axis orientation of barium-rich films. A relative increase in perpendicular nucleation sites over in-plane and/or random nucleation sites contributes to the improvement in perpendicular c-axis orientation. © 2000 American Institute of Physics.

Notizen: Electron cyclotron resonance plasma based sputtering is characterized by low pressure operation with high ion density. The distribution of sputtered flux on the substrate at different pressures and target–substrate distances has been simulated using Monte Carlo methods and compared with experimental results. It has been shown that due to cylindrical geometry of the target, at low pressures, the variation as a function of distance is different from conventional sputtering. At high pressures, however, the uniformity of sputtered flux increases with the target–substrate distance. Using the simulated data the variation of the thickness with sputtering pressure and target–substrate distance has been studied. © 2000 American Institute of Physics.