Search

We study the effect of the addition of p- or n-type doping gases, trimethylboron (TMB) or phosphine, respectively, on cluster formation in the SiH4/H2 plasma used for the deposition of hydrogenated polymorphous silicon thin films. The formation of clusters is monitored using time-resolved measurements of the second harmonic of the radio frequency current J2. We show that the addition of PH3 does not change the cluster formation, while the addition of a small amount of trimethylboron strongly affects its charging behavior, preventing to some extent its agglomeration. The most ordered pm-Si:H thin films are obtained under conditions from which not only clusters but also larger silicon agglomerates are formed in the plasma and contribute to the deposition. The inhibiting role of TMB on the agglomeration and powder formation is evidenced by the smoothness of p-type films at high rates, as deduced from the sample surface topography obtained by atomic force microscopy measurements. [ABSTRACT FROM AUTHOR]

The electric field gradient (EFG) at the gold nucleus is calculated using a finite field approach, to make the extraction of the nuclear quadrupole moment Q from experimental nuclear quadrupole coupling constants possible. The four-component Dirac-Coulomb Hamiltonian serves as the framework, 51 of the 79 electrons are correlated by the relativistic Fock-space coupled cluster method with single and double excitations, and the contribution of the Gaunt term, the main part of the Breit interaction, is evaluated. Large basis sets (up to 26s22p18d12f8g5h uncontracted Gaussians) are employed. Energy splittings of the 2D5/2 and 2D3/2 levels, rather than level shifts, are used to extract the EFG constants, as the former remain linear with Q up to 10-5 a.u., whereas the latter display significant nonlinearity even at Q=10-8 a.u. Larger Q values lead to larger energy changes and better precision. Excellent agreement (0.1%) is obtained between Q values derived from 2D5/2 and 2D3/2 data. Systematic errors connected with neglecting triple and higher excitations, truncating the basis and orbital active space, and approximating the Gaunt contribution are evaluated. The final value of Q(197Au) is 521(7) mb. It is lower than the muonic 547(16) mb and agrees within error bounds with the recent value of 510(15) mb obtained from molecular calculations. [ABSTRACT FROM AUTHOR]

High-resolution scanning electron microscopy and cathodoluminescence spectroscopy measurements were performed to study the effect of postgrowth annealing on properties of ZnO films grown on GaAs substrates by rf sputtering. The films annealed at 550 °C show a well-oriented columnar structure and strong exciton emission at room temperature. Outdiffusion of gallium and arsenic from substrate into a ZnO film has been found to result in a different secondary electron dopant contrast, measured by the through-the lens secondary electron detector. Extended structural defects such as subgrain boundaries in ZnO assist Ga outdiffusion from the GaAs substrate and show a reduced secondary electron (SE) emission after annealing, while As doped ZnO adjacent to the ZnO/GaAs interface demonstrates an enhanced SE emission and the enhanced luminescence associated with donor-acceptor pairs and exciton bound to acceptors. [ABSTRACT FROM AUTHOR]

Studies the lower pressure limit of diamond growth using inductively coupled radio frequency plasma. Experimental details; Results and discussion; Conclusions.

Provides information on a study that investigated the radio frequency power dependence of structural, optical, electrical and optoelectronic properties of hydrogenated amorphous silicon carbon alloy films prepared by magnetron sputtering of silicon in methane-argon gas mixtures. Methodology of the study; Results and discussion on the study; Conclusions.

Microwave/radio-frequency-infrared multiple resonance has been used with an electric-resonance optothermal spectrometer to characterize a weak 21.6 MHz perturbation in the infrared spectrum of the ν14 C–O stretching vibration of 2-fluoroethanol. The infrared spectrum of 2-fluoroethanol was recorded at a resolution of ∼2 MHz using a tunable microwave-sideband CO2 laser. The spectrum is fit by an asymmetric-rotor Hamiltonian to a precision of 0.6 MHz, except for the transitions to the 413 upper state which are split into doublets by an interaction between the 413 level and a rotational level of a nearby background, or dark, vibrational state. Microwave/radio-frequency-infrared double and triple resonance reveals that the 413 level of the C–O stretching vibration is interacting with the 431 level of the dark state. The rotational constants determined for the dark state allow us to assign the perturbing state to the ν18+4ν21 combination vibration of the lowest energy conformer, where ν18 is the CCO bending vibration and ν21 is the C–C torsional vibration. From the weak ΔKa=2 matrix element between ν14 and ν18+4ν21 it is possible to derive a J=0 anharmonic interaction between these states of ∼3.5 GHz. [ABSTRACT FROM AUTHOR]

The structure of NH3·H2S has been determined from microwave and radiofrequency spectroscopy of this complex and its deuterated isotopomers, using molecular beam electric resonance techniques. Rotational constants, electric dipole moments and nitrogen quadrupole coupling constants were obtained from the spectra. The molecule was found to have a linear, hydrogen bonded structure with the ammonia as the proton acceptor. The N···S distance is 3.639 Å, the root-mean-square angular deviation of the NH3 axis from the N–S axis is 24.6° and the H2S C2 axis is 40.5° from the N–S axis. Although the molecule is an asymmetric rotor, first-order Stark effects were observed for K=1 rotational levels. These Stark effects are caused by torsional oscillations which are essentially ammonia monomer free internal rotation. Similar effects were observed for NH3·H2O and are reported here. [ABSTRACT FROM AUTHOR]

Radio frequency spectroscopy has been carried out on molecular beams of chloroacetylene to yield Stark and chlorine hyperfine properties for the ground and excited C–H stretching vibrational states. For HCC35Cl v=0, μ=0.440 079(1) D, eQq=-79 733.59(4) kHz, and CCl=1.412(4) kHz. For HCC35Cl v=1, μ=0.462 792(1) D, eQq=-79 744.59(5) kHz, and CCl=1.398(3) kHz. For HCC37Cl v=0, μ=0.439 954(1) D, eQq=-62 839.68(9) kHz, and CCl=1.142(6) kHz. For HCC37Cl v=1, μ=0.462 778(1) D, eQq=-62 848.64(10) kHz, and CCl=1.127(8) kHz. The above results are for J=1 levels, while J=1 and J=2 Stark data for the ground state were combined to give an electric polarizability anisotropy, α||–α⊥=3.87(2) Å3. [ABSTRACT FROM AUTHOR]

A capacitively coupled interdigital probe technique is described whereby the electrical characteristics of thin films at arbitrary frequencies in the range from about 100 kHz to beyond 1 GHz may be studied. Patterning of the film, deposition of electrodes, and bonding of electrical leads are unnecessary, so that all measurements are nondestructive and noninvasive. Preliminary measurements of the superconducting transition in thin films of the high-temperature superconductor YBa[sub 2]Cu[sub 3]O[sub 7-δ] in magnetic fields up to 1.5 T are presented and discussed. They show a large field-dependent peak, attributable to tunneling effects, just below the onset temperature. Anomalies are also seen at higher temperatures, providing evidence of a structural phase transition. [ABSTRACT FROM AUTHOR]

We propose an original analog method to perform instantaneous and quantitative spectralanalysis of microwave signals. An ensemble of nitrogen-vacancy (NV) centers held in a diamond plate is pumped by a 532nm laser. Its photoluminescence is imaged through an optical microscope and monitored by a digital camera. An incoming microwave signal is converted into a microwave field in the area of the NV centers by a loop shaped antenna. The resonances induced by the magnetic component of that field are detected through a decrease of the NV centers photoluminescence. A magnetic field gradient induces a Zeeman shift of the resonances and transforms the frequency information into spatial information, which allows for the simultaneous analysis of the microwave signal in the entire frequency bandwidth of the device. The time dependent spectralanalysis of an amplitude modulated microwave signal is demonstrated over a bandwidth of 600MHz, associated to a frequency resolution of 7MHz, and a refresh rate of 4ms. With such integration time, a field of a few hundreds of µW can be detected. Since the optical properties of NV centers can be maintained even in high magnetic field, we estimate that an optimized device could allow frequency analysis in a range of 30GHz, only limited by the amplitude of the magnetic field gradient. In addition, an increase of the NV centers quantity could lead both to an increase of the microwave sensitivity and to a decrease of the minimum refresh rate down to a few µs. [ABSTRACT FROM AUTHOR]

Pulsed field emission from cold carbon-nanotube cathodes placed in a radiofrequency resonant cavity was directly measured. The cathodes were located on the backplate of a conventional 1+1/2 -cell resonant cavity operating at 1.3-GHz and resulted in the production of bunch train with maximum average current close to 0.7 Ampère. The measured Fowler-Nordheim characteristic, transverse emittance, and pulse duration are presented and, when possible, compared to numerical simulations. The implications of our results to the promise of high-average-current electron sources are briefly discussed. [ABSTRACT FROM AUTHOR]

An exploratory study was carried out in the long-pulse tokamak Tore Supra, to determine if electric fields in the plasma around high-power, RF wave launchers could be measured with non-intrusive, passive, optical emission spectroscopy. The focus was in particular on the use of the external electric field Stark effect. The feasibility was found to be strongly dependent on the spatial extent of the electric fields and overlap between regions of strong (> ~ 1 kV/cm) electric fields and regions of plasma particle recycling and plasma-induced, spectral line emission. Most amenable to the measurement was the RF electric field in edge plasma, in front of a lower hybrid heating and current drive launcher. Electric field strengths and direction, derived from fitting the acquired spectra to a model including time-dependent Stark effect and the tokamak-range magnetic field Zeeman-effect, were found to be in good agreement with full-wave modeling of the observed launcher. [ABSTRACT FROM AUTHOR]

Nanosecond optical imaging spectroscopy is employed to investigate the spatio-temporal dynamics of the plasma plume expanding from a 4.2 mm-diameter, 20 mm-long cylindrical capacitively coupled electrothermal radiofrequency (rf) driven thruster using 10 W of power at 12.50 MHz and an argon pressure of 1.5 Torr. On-axis, the plume exhibits four distinct peaks of optical emission intensity within the rf period. The plume has a spherical shape with a transient radial extension (during half of the rf cycle) at the thruster exit plane due to an rf current to ground when the grounded electrode acts as an anode. [ABSTRACT FROM AUTHOR]

Nuclear magnetic resonance (NMR) Stark responses can occur in quadrupolar nuclei for an electric field oscillating at twice the usual NMR frequency (2ω0). Calibration of responses to an applied E field is needed to establish nuclear spins as probes of native E fields within material and molecular systems. We present an improved approach and apparatus for accurate measurement of quadrupolar Stark effects. Updated values of C14 (the response parameter in cubic crystals) were obtained for both 69Ga and 75As in GaAs. Keys to improvement include a modified implementation of voltage dividers to assess the 2ω0 amplitude, |E|, and the stabilization of divider response by reduction of stray couplings in 2ω0 circuitry. Finally, accuracy was enhanced by filtering sets of |E| through a linear response function that we established for the radiofrequency amplifier. Our approach is verified by two types of spectral results. Steady-state 2ω0 excitation to presaturate NMR spectra yielded C14 = (2.59 ± 0.06) × 1012 m-1 for 69Ga at room-temperature and 14.1 T. For 75As, we obtained (3.1 ± 0.1) × 1012 m-1. Both values reconcile with earlier results from 77 K and below 1 T, whereas current experiments are at room temperature and 14.1 T. Finally, we present results where few-microsecond pulses of the 2ω0 field induced small (tens of Hz) changes in high-resolution NMR line shapes. There too, spectra collected vs |E| agree with the model for response, further establishing the validity of our protocols to specify |E|. [ABSTRACT FROM AUTHOR]

A little more than a decade ago our team extended the field of defect spectroscopy in cryocrystals to solid 4He matrices, in both their body-centered cubic (bcc) and hexagonal close-packed (hcp) configurations. In this review paper we survey our pioneering activities in the field and compare our results to those obtained in the related fields of doped superfluid helium and doped helium nanodroplets, domains developed in parallel to our own efforts. We present experimental details of the sample preparation and the different spectroscopic techniques. Experimental results of purely optical spectroscopic studies in atoms, exciplexes, and dimers and their interpretation in terms of the so-called bubble model will be discussed. A large part of the paper is devoted to optically detected magnetic resonance (ODMR) processes in alkali atoms. The quantum nature of the helium matrix and the highly isotropic shape of the local trapping sites in the bcc phase make solid helium crystals ideal matrices for high-resolution spin physics experiments. We have investigated the matrix effects on both Zeeman and hyperfine magnetic resonance transitions and used ODMR to measure the forbidden electric tensor polarizability in the ground state of cesium. Several unexpected changes of the optical and spin properties during the bcc-hcp phase transition can be explained in terms of small bubble deformations. [ABSTRACT FROM AUTHOR]

A simple and widely applicable microwave spectroscopy system for low temperature matrices has been developed. We applied the technique of opto-thermal detection with a superconducting thin-film sensor (STS) for the detection of very weak absorption signals of molecules trapped in quantum solids such as solid para-hydrogen (p-H2). For testing of the system, the microwave transition of an ortho-hydrogen (o-H2) dimer in solid p-H2 was observed. The S/N ratio of the observed spectrum of the G1 transition at 1.4 K was approximately 8. The ultrahigh sensitivity of STS makes it possible not only to obtain sufficient sensitivity but also to make the apparatus very simple especially around the sample cell. [ABSTRACT FROM AUTHOR]

The cavity perturbation technique is an extremely powerful method for measuring the electrodynamic response of a material in the millimeter- and submillimeter spectral range (10 GHz–1 THz), particularly in the case of high-field/frequency magnetic resonance spectroscopy. However, the application of such techniques within the limited space of a high-field magnet presents significant technical challenges. We describe a 7.62 mm×7.62 mm (diameter×length) rotating cylindrical cavity which overcomes these problems. The cylinder is mounted transverse to the bore of the magnet, coupling is achieved through the sidewalls of the cavity, and the end plate is then rotated (by means of an external drive) instead of the body of the cavity itself. Therefore, rotation does not affect the cylindrical geometry, or the mechanical connections to the incoming waveguides. The TE011 mode frequency of the cavity is 51.863 GHz, with the possibility to work on higher-order modes to frequencies of order 350 GHz. Neither the quality factor (∼22 000 for the fundamental mode) or the coupling to the cavity are significantly affected for full 360° of rotation. The rotation mechanism provides excellent angle resolution (<0.1°), and is compact enough to enable measurements in the high-field (up to 45 T) magnets at the National High Magnetic Field Laboratory. Two-axis rotation capabilities are also possible in conjunction with split-pair magnet configurations. We present examples of angle-dependent measurements which illustrate the unique capabilities of this rotating cavity, including: high-field angle-dependent measurements of an unusual form of cyclotron resonance in anisotropic organic conductors; and angle-dependent high-frequency single-crystal electron paramagnetic resonance measurements in single-molecule magnets. [ABSTRACT FROM AUTHOR]

A commercially available high-speed, digital signal-averager is integrated into an existing time-domain radio frequency (rf) electron paramagnetic resonance (ERR) spectrometer/imager. Sensitivity enhancement by the integrated system is estimated by coherent averaging of free induction decay signals, obtained from narrow-line ErR spin probes, and its performance is compared with that of an existing custom-built averager. For the existing custom-built "Analytek" averager, the minimum realizable trigger rate was 50 kHz, whereas for the commercial EG&G 9826 system, due the spectrometer constraints, we set the retrigger rate to 133 kHz. Very short summing and down loading times of the latter enable good temporal resolution in phantom as well as in vivo rf Fourier transform ErR images, obtained by the single point imaging (SPI) modality. For two-dimensional and three-dimensional imaging using the SPI mode, a saving of time by a factor of > 2 could be achieved with the EG&G system compared with the Analytek. [ABSTRACT FROM AUTHOR]

An analysis is presented of the effect on power transfer and noise figure of attenuation in the radiofrequency coaxial cable (coax) between a NMR sample coil and the associated spectrometer. A matching network between the sample coil and the coax, which matches the coil to the coax characteristic impedance, is shown to decrease the noise temperature of the Coax by a factor of the Q of the coil. In addition, the usual two-capacitor matching network, which provides a perfect match only if both capacitors are variable, is shown to yield an acceptably small mismatch over a wide range of frequencies even if one capacitor is fixed. [ABSTRACT FROM AUTHOR]

We report the development of a novel radio frequency electron-spin-resonance spectrometer designed to provide measurements with high molar sensitivity and resolution in vivo. Radio frequency (250 MHz) is chosen to obtain good penetration in animal tissue and large aqueous samples with modest sacrifice of sensitivity. The spectrometer has a lumped component resonator and operates in continuous-wave mode. The spectrometer is capable of two-dimensional imaging, and with a modest addition should be capable of three-dimensional imaging. We demonstrate 3-mm spatial resolution for DPPH samples. For 10-ml samples of aqueous nitroxide, we demonstrate sensitivity (normalized to spectral width of 1 G) to 3×10-8-M concentrations and spectral resolution of 0.1 G. Spectra from nitroxide spin label injected into a live mouse are shown. [ABSTRACT FROM AUTHOR]

Reports on the synthesis of titanium silicide by continuous wave CO[sub2]-laser irradiation in air of a two-layer structure of polycrystalline silicon (Si) and titanium (Ti) deposited by radio-frequency sputtering on Si base. Method used to analyze the surface morphology; Data on the Rutherford backscattering spectrometry of arsenic-deposited Si/Ti/Si structure.

We present the coupling of atomic force microscopy (AFM) and nuclear magnetic resonance (NMR) technologies to enable topographical, mechanical, and chemical profiling of biological samples. Here, we fabricate and perform proof-of-concept testing of radiofrequency planar microcoils on commercial AFM cantilevers. The sensitive region of the coil was estimated to cover an approximate volume of 19.4 × 103 μm3 (19.4 pl). Functionality of the spectroscopic module of the prototype device is illustrated through the detection of 1Η resonance in deionized water. The acquired spectra depict combined NMR capability with AFM that may ultimately enable biophysical and biochemical studies at the single cell level. [ABSTRACT FROM AUTHOR]

In this letter, we propose a high speed and ultralow driving voltage traveling wave electro-optic (EO) modulator based on a dual horizontal slot waveguide. The proposed design harnesses the unique properties of ferroelectric materials such as LiNBO3 and exotic, organic EO polymers to tightly confine both the optical and rf modes in a nanoscale slot maximizing the nonlinear interaction with the electric field. Larger electrode separation allows for significant reduction in rf propagation loss. Simulations of the half-wavelength voltage-length product and electro-optic response of the proposed device reveal ultrabroadband operation, up to 250 GHz, and subvolt driving voltage for a 1 cm long modulator. [ABSTRACT FROM AUTHOR]

We compare low frequency noise in magnetic tunnel junctions with MgO barriers prepared by electron-beam evaporation with those prepared by radiofrequency sputtering, both showing a high tunneling magnetoresistance. The normalized noise parameter in the parallel state of junctions with evaporated barriers is at least one order of magnitude lower than that in junctions with sputtered barriers, and exhibits a weaker bias dependence. The lowest normalized noise is in the 10-11 μm2 range. A lower density of oxygen vacancies acting as charge trap states in the evaporated MgO is responsible for the lower noise. [ABSTRACT FROM AUTHOR]

In this letter, we propose a high speed and ultralow driving voltage traveling wave electro-optic (EO) modulator based on a dual horizontal slot waveguide. The proposed design harnesses the unique properties of ferroelectric materials such as LiNBO3 and exotic, organic EO polymers to tightly confine both the optical and rf modes in a nanoscale slot maximizing the nonlinear interaction with the electric field. Larger electrode separation allows for significant reduction in rf propagation loss. Simulations of the half-wavelength voltage-length product and electro-optic response of the proposed device reveal ultrabroadband operation, up to 250 GHz, and subvolt driving voltage for a 1 cm long modulator. [ABSTRACT FROM AUTHOR]

An alteration in the structure of the A6 relativistic magnetron is proposed, which introduces an extra degree of freedom to its design and enhances many of its quality factors. This modification involves the partial filling of the cavities of the device with a low-loss dielectric material. The operation of a dielectric-filled A6 is simulated; the results indicate single-mode operation at the desired π mode and a substantially cleaner rf spectrum. [ABSTRACT FROM AUTHOR]

The authors propose and demonstrate a miniaturized quadraxial probe that employs a differential feed technique for use in near-field rf/microwave transmission microscopy. Their quadraxial probe’s electric field measurements show higher electric field localization than a conventional coaxial (monopole) probe. The improved spatial resolution and more sensitive phase measurement of the quadraxial probe versus coaxial probe are further validated by a metal line scan experiment. [ABSTRACT FROM AUTHOR]

It is widely accepted that electrode insulation is unnecessary for generating radio-frequency (rf) atmospheric pressure glow discharges (APGDs). It is also known that rf APGDs with large discharge current are susceptible to the glow-to-arc transition. In this letter, a computational study is presented to demonstrate that dielectric barriers provide an effective control over unlimited current growth and allow rf APGDs to be operated at very high current densities with little danger of the glow-to-arc transition. Characteristics of electrode sheaths are used to show that the stability control is achieved by forcing the plasma-containing electrode unit to acquire positive differential conductivity. [ABSTRACT FROM AUTHOR]

A complete digital radio-frequency source suitable for broadband nuclear magnetic resonance applications is described. The use of a complete direct digital synthesis chip makes the design not only as a frequency- and phase-continuous tunable radio-frequency source, but also as an arbitrary wave form generator. This rf source is capable of generating a signal with frequency from dc up to 135 MHz directly. The whole design is mounted on a printed circuit board that can be inserted into one of the industry standard architecture slots in the personal computer and is controlled by the software. Details about the implementation and characteristics of the whole design are discussed in this article. [ABSTRACT FROM AUTHOR]

We present a technique for simultaneous determination of the uncorrelated and correlated timing and amplitude jitter of a pulse train from its radio-frequency spectrum. First, we present a robust analysis that separately identifies both correlated and uncorrelated jitter. Second, we show that all four noise components can be uniquely determined by combining the traditional integrating calculation with a nonintegrating computation. We present simulation results that confirm the accuracy of this technique. [ABSTRACT FROM AUTHOR]

We have discovered rotary resonances between rf field strength, ω[sub 1], and magic-angle spinning (MAS) frequency, ω[sub R], which dramatically enhance the sensitivity of triple quantum preparation and mixing in the multiple-quantum MAS experiment, particularly for quadrupolar nuclei having low gyromagnetic ratios or experiencing strong quadrupole couplings. Triple quantum excitation efficiency minima occur when 2ω[sub 1]=nω[sub R], where n is an integer, with significant maxima occurring between these minima. For triple quantum mixing we observe maxima when ω[sub 1]=nω[sub R]. In both preparation and mixing the pulse lengths required to reach maxima exceed one rotor period. We have combined these rotary resonance conditions into a new experiment called FASTER MQ-MAS, and have experimentally demonstrated a factor of 3 enhancement in sensitivity in comparison to conventional MQ-MAS. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have succeeded in preparing In[sub x]Ga[sub 1-x]As (0.2≤x≤0.8) nanocrystals with 3–5 nm in size embedded in SiO[sub 2] thin film by using the radio frequency magnetron cosputtering technique. The analyses of x-ray diffraction and Raman spectra strongly suggest the existence of In[sub x]Ga[sub 1-x]As nanocrystals in the matrices. It has been found that the optical absorption edge, lattice constant and Raman shift can be modulated by composition of In[sub x]Ga[sub 1-x]As by varying the effectively sputtered area ratio of InAs to GaAs on the target. The blueshift of the optical absorption edge is explained by the effective mass approximation method. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]