Cross-polarization measurements at Martlesham Heath (UK) using OTS

Slant-path propagation measurements using transmissions to and from the Ots satellite are being carried out at Martlesham Heath (Uk)at an elevation angle of 29.9°. The measurements allow the determination of the attenuation and cross-polarization for circularly-polarized signals at 14.5 and 11.8 GHz and attenuation and cross-polarization for a linearly-polarized signal at 11.6 GHz. This paper reports the results of cross-polar measurements made during the summer of 1978 together with examples of particularly severe cross-polar effects from 1979. The measurements are compared with both a Ccir curve relating cross-polar discrimination to fade depth for rain and theoretical curves supplied by Essex University. The results clearly demonstrate the superior cross-polar performance of linear polarization compared to circular polarization.     

Absolute measurement at 1 THz of the optical coupling of a F.I.R. conical antenna with a Josephson detector

We present the first absolute measurement of the optical coupling of a FIR antenna with a Josephson detector. We have studied a wide band FIR coupling system made of a conical antenna (discone) and a focussing paraboloid mirror. The calculated efficiency is (6.8±0.3)10^?2 for the optimum choice of parameters at 1 THz. The best measurements using a HCN laser at 891 GHz are in agreement while the average achieved coupling is (2.3±0.4)10^?2 reflecting the experimental difficulties.     

Terahertz Spectra of L-Ascorbic Acid and Thiamine Hydrochloride Studied by Terahertz Spectroscopy and Density Functional Theory

We have investigated the terahertz spectra of L-ascorbic acid and thiamine hydrochloride measured by terahertz time-domain spectroscopy (THz-TDS) and Fourier transform infrared spectroscopy (FTIR). The measured absorption spectra were demonstrated to be in good agreement with the results simulated by Density Functional Theory (DFT) using hybrid functional B3LYP with basis set of 6-31G (d), except with slight frequency shift and few peaks missing. We presented the comparison of measured spectra by the FTIR spectroscopy employing low temperature silicon bolometer as detector and the TDS system. The measured spectra of the L-ascorbic acid showed shoulder bands at 0.25, 1.1, 1.5, 1.82, 2.03, 2.30, 2.44, 2.67, 2.97, 3.12, and 3.40 THz, respectively. The spectra of the thiamine hydrochloride show shoulder bands at 0.48, 1.11, 1.57, 1.75, 1.92, 2.08, 2.31, 2.53, 2.69, 2.85, 3.12, 3.22, and 3.31 THz. Most absorption peaks of the two samples agree with the results simulated by Density Function Theory (DFT) method of Gaussian 09 software. In our work, more spectral peaks based on experimental and theoretical results were found in comparison to that of other groups, since we employed higher sensitive FTIR measurement system and considered the effect of number of molecule unit in simulation. The study suggests that the effect of intermolecular vibration is stronger than intramolecular interaction on the absorption bands in THz region.     

Simultaneous prediction of density and moisture content of wood by terahertz time domain spectroscopy

In this study, demonstration of simultaneous prediction of solid wood density and moisture content, both of which are critical in manufacturing operations, of 4 species (Aspen, Birch, Hemlock and Maple) was accomplished using terahertz time-domain spectroscopy (THz-TDS). THz measurements of wood at various moisture contents were taken for two orientations of the THz field (parallel and perpendicular) with respect to the visible grain. The real and imaginary parts of the dielectric function averaged over the frequency range of 0.1 to 0.2 THz had strong correlation with density and moisture content of the wood. We extend a model that has been applied previously to oven-dry wood to include the effects of moisture below the fiber saturation point by combining two effective medium models, which allows the dielectric function of water, air and oven-dry cell wall material to be modeled to give an effective dielectric function for the wood. A strong correlation between measured and predicted values for density and moisture content were observed.     

The pure rotational spectrum of water vapor—A millimeter,submillimeter, and far infrared analysis

Line positions, strengths, and absolute energies are calculated for the pure rotational spectrum of water in the region between 0 cm^?1 and 877 cm^?1. These calculations are done in the context of a reduced centrifugal distortion Hamiltonian, and are based on microwave measurements of this spectrum to 1 THz, and Far Infrared measurements between 32 cm^?1 and 715 cm^?1.     

Sixtieth order harmonic mixing to 4.25 THz using a Schottky diode

Harmonic mixing of mm-waves with radiation of 3.7 THz with different Schottky diodes is reported. The highest mixing order is 60 producing a beat between 72 GHz and 4.25 THz radiation. Such high frequency mixing has so far only been possible using cryogenic Josephson mixers. The present result permits substantial simplification of measurements of optical frequencies.     

A model for the complex permittivity of water at frequencies below 1 THz

Experimental permittivity data of liquid water, compiled from the open literature, were selectively applied to support a modeling strategy. Frequencies up to 1 THz and atmospheric temperatures are covered with an expression made up by two relaxation (Debye) terms. The double-Debye model reduces to one term when the high frequency limit is set at 100 GHz, and the model can be extended to 30 THz by adding two resonance (Lorentzian) terms. The scheme was carried out by employing nonlinear least-squares fitting routines to data we considered reliable.     

Structural,electrical and optical characterization of singlecrystal ErAs layers grown on GaAs by MBE

A detailed study is presented of the structural, electrical, and optical properties of ErAs films grown on GaAs by molecular beam epitaxy (MBE). ErAs layers 1500Å thick were grown successfully over a relatively wide range of substrate temperatures (420-580° C), although overgrowth of GaAs on ErAs was found to be difficult. In-situ reflection highenergy electron diffraction (RHEED), x-ray diffraction, and Rutherford backscattering (RBS) measurements all indicate single crystal growth. Analysis of X-ray rocking curves reveals that, over the range of substrate temperatures studied, strain due to the lattice mismatch between ErAs and GaAs is completely inelastically relieved in the 1500Å thick ErAs layers. Variable-temperature Hall measurements reveal metallic behaviour in all samples, with no pronounced dependence on substrate temperature. Spectrally narrow (0.6 meV) intra 4f-shell transitions of Er³⁺ (4f¹¹), at 1.54 μm, have been observed in ErAs epitaxial layers both in absorption (by Fourier transform infra-red spectroscopy, FTIR) and in emission (by cathodoluminescence). The crystal-field splittings observed in the FTIR spectra are consistent with the cubic(O _h)symmetry expected for the Er lattice site in unstrained ErAs, in good agreement with the x-ray analyses.     

Hydrogen content and annealing of memory quality silicon-oxynitride films

Optical measurements have been made to investigate the effects of post-deposition heating to 950° in vacuum on 750°-CVD silicon-nitride films which contain approximately 5 atomic percent oxygen. The refractive index and thick-ness, determined ellipsometrically at 5461 Å for 600 A films, were unchanged by annealing within the experimental uncertainties of the measurements. Annealing, however, caused a shift of the absorption edge toward higher energy, a decrease in the etch rate, a cracking of 4500 Å films on thick-Si substrates, and an increase in the peak absorption for the SiN band. These effects are interpreted as atomic reordering and densification of the films. The absence of any significant change in refractive index or in the posi-tion of the SiN band upon annealing implies that the oxygen content is unaffected by annealing. Multiple internal reflection measurements show a large hydrogen concentration (~ 7 atomic percent) initially bound to N or to Si in the films. The most striking effect of annealing is on the SiH centers which are annealed out by one hour at 900°C, and a 20 percent reduction is observed for the NH centers. Preliminary electrical measurements indicate an increase in conductance for films annealed at 800 and 850°C; and for the same applied voltage, dielectric failure is observed after annealing at 950°C. Exposure of dangling silicon and nitrogen bonds which were initially-decorated with hydrogen is suggested to explain the higher conductance after annealing.     

A comparison of MOS gate structures formed by depositing polycrystalline silicon on thin oxides using conventional low pressure chemical vapor deposition and rapid thermal chemical vapor deposition

MOS structures with 80Å-200Å thick gate oxides were fabricated using polycrystalline silicon gate electrodes deposited by rapid thermal chemical vapor deposition (RTCVD) and by conventional chemical vapor deposition (LPCVD). Polycrystalline silicon doping was achieved by BF₂ or As implantation followed by rapid thermal annealing (RTA). The Q-C method was employed to study the electrical properties of the capacitors through high and low frequencyC- V profiles. The electrical properties of the MOS structures show that the devices fabricated using RTCVD polycrystalline Si are comparable in quality to those with LPCVD polycrystalline silicon gate electrodes. However, the results also indicate that boron diffusion through the thin oxide is a problem regardless of the deposition technique used for polycrystalline silicon. Boron penetration is accompanied by a shift in threshold voltage, inversion layer capacitance and a high density of midgap interface traps. Dopant diffusion in polycrystalline silicon and through the thin gate oxides was also studied by secondary ion mass spectroscopy (SIMS). The findings of the SIMS analysis correlate well with the electrical measurements. The results indicate that significant boron diffusion can occur through an 80A oxide if an RTA temperature higher than 1000° C is used. On the other hand, both SIMS and electrical measurements suggest that As penetration into the substrate is negligible even at temperatures as high as 1100° C.     

Implementation of AES Key Schedule Using Look-Ahead Technique

The commencement of decryption process of Advanced Encryption Standard (AES) algorithm is dependent on availability of the last round key. In this paper, we propose a look-ahead technique for increasing the speed of implementation of AES key schedule using which the last round key can be made available fast. The other round keys can also be computed in a parallel path using the proposed technique. Applications such as key search engines need to be agile to key changes for decrypting given encrypted messages using all the keys in the available key space so that fast decryption is possible. The FPGA implementation results using Xilinx XC5VLX85 are also provided.     

Luminescent properties of AlGaAs grown by transient-mode liquid epitaxy

Transient-mode liquid epitaxy (TMLE), a method of non-equilibrium LPE growth, has been used to prepare thin n-type single-crystal layers of AlGaAs on GaAs substrates for the fabrication of infrared-emitting LED’s. It is well known that the concentration of Al in an epitaxial layer grown from the liquid phase is primarily dependent on solution composition, growth temperature and growth rate. During conventional LPE, the growth rate is usually made low enough that the equilibrium distribution coefficient can be assumed to prevail and the Al concentration for direct bandgap compositions will decrease as a layer grows. This, however, is not the case for material produced by TMLE. Since the growth rate during TMLE continually drops from an initially high value towards zero, the amount of Al incorporated in the epitaxial layer, and hence the bandgap energy, is not reduced as the layer is generated. The luminescence created at a diffused p-n junction is emitted through the p-side with very little absorption. Photoluminescence measurements using an argon-ion laser source (4579 Å) were made to determine the bandgap energies of AlGaAs epitaxial layers grown by TMLE for various lengths of time. Diodes fabricated from from this material had peak electroluminescence between 7900 and 8250 Å With half-intensity bandwidths of 200 A. In addition to a discussion of diode preparation, the spectral characteristics of diodes are compared with results obtained from photoluminescence measurements.     

Millimeter-wave absorption measurements in low-loss dielectrics using an untuned cavity resonator

Millimeter wave absorption measurements are usually complicated by scattering from the sample and by standing waves resulting from internal reflections. These problems can be largely overcome by using an untuned cavity resonator. The dielectric properties of the sample are determined from a measurement of the change in cavity Q produced by introduction of the sample into the cavity. For lamellar samples, Maxwell's equations can be used to find an explicit relationship between the change in cavity Q and the samples complex refractive index. Several measuring techniques based on the use of lamellar samples are presented, and their characteristics are illustrated both analytically and by laboratory measurements at 70 GHz on PTFE, polyethylene and plexiglas.     

High resolution molecular spectroscopy in the submillimeter region

Fourier Transform laboratory measurements have been carried out, for the first time in the 8–85 cm^?1 spectral region, with an unapodized resolution of 3.3. 10^?3 cm^?1 and a frequency accuracy of 2. 10^?4 cm^?1. Samples from spectra of several molecules namely: CO, O₃, H₂O₂, NO, NO₂, HNO₃, SO₂, H₂S, HOCL, NOCL, HNCO, ND₃ and AsH₃ are presented to show both the quality of the measurements and the type of information supplied by high resolution spectroscopy in the submillimeter region.     

Far-infrared observations of the galactic Cygnus region and Mars with a 60 cm diameter balloon-borne telescope

We have introduced secondary-mirror chopping in our balloon-borne far-infrared 60 cm Ø telescope equipped with He-cooled filters and a composite silicon bolometer with an NEP of 1.3 10^?14 W/Hz^1/2 for imaging, radiometry and spectroscopy of the Cygnus region at the wavelengths 80 μm, 130 μm and 310 μm. The modified system was flown on September 26/27, 1990 with a 380'000m³ hydrogen balloon at the CNES station at Aire/Adour, France. Observations were performed at a platform height of 39 km during 5 hrs. We made successful measurements on the complexes DR21 and S106 of the Cygnus region as well as on Mars at the wavelengths mentioned.     

Characterization of oxidized a-SiHx surfaces used in mis structures

Atomically clean and oxidized surfaces of a-SiH_x films, prepared by plasma decomposition of SiH₄ in a capacitative reactor, were studied using Auger and UV photoemission spectroscopy. The oxides were characterized in terms of the fine structure of the Si_LVV Auger transition. We also studied the electrical properties of MIS solar cell structures in which the a-SiH_x film was given the same treatment as the samples for surface studies and correlated the results of the two kinds of measurements. For thin (< 20 Å) oxides the photodiode open circuit voltage increases linearly with oxide thickness. This increase is found to be due to an increase in the barrier height and the built-in potential upon oxidation.     

GaAs Schottky barrier mixer diodes for the frequency range 1–10 THz

Recent technological advances have made possible the development of heterodyne receivers with high sensitivity and high spectral resolution for frequencies up to 3,000 GHz (3 THz). These receivers rely on GaAs Schottky barrier mixer diodes to translate the high-frequency signal to a lower frequency where amplification and signal processing are possible. In the frequency range from 1–10 THz several new effects will limit diode performance. These effects are discussed and guidelines for diode design are presented.     

Quasi-optical mixer for efficient sum and difference frequency sidebands generation at 1 THz mark

Construction details of recently developed mixer for generation of narrow band and continuously tunable submillimeter radiation are reviewed briefly. The measurements indicate the usability of such mixer design for the high resolution spectroscopical work at frequencies close to and above the 1 THz mark.     

A Unified Derivation of the Terahertz Spectra Generated by Photoconductors and Diodes

By a convolution approach for the spectra generated by photoconductive and photomixing THz emitters, the intrinsic response of the material can be separated from the spectrum of the optical pulse and that of any attached antenna. The spectrum turns out to be simply a product of the spectra of the individual components and can be calculated analytically. This result has impact on materials design and derivation of the optimum excitation conditions for photoconductors and photodiodes. Spectra of three technologically important devices, namely low-lifetime materials, semi-insulating materials and devices supporting ballistic transport, are calculated. The derived pulse shape is compared to experimental data.     

Calibration of the fir polarimeter on TFTR tokamak

The results of the first Faraday rotation measurement on the TFTR tokamak are presented. Data are reported on ohmic-as well as neutral-beam-heated plasmas including solid pellet injections. The procedure and the results of the calibration, are described. The effects of various errors in the measurements as well as the problem of cross coupling laser beams are studied.