Spl Tau Research Articles

Relaxation time and thermal stability in magneto-optical recording media

The time dependence of the magnetization process in amorphous TbFeCo thin films and (TbFeCo/Pt) multilayers is studied. By assuming a Gaussian distribution of relaxation times for magnetic aftereffect, the non-Arrhenius behavior of the magnetization decay is found in almost all samples. From a fitting of experimental data one can determine the mean value of relaxation time /spl tau//sub 0/ and also its width /spl Delta//spl tau//sub 0/. The multilayers of TbFeCo/Pt show a high stability compared to single layers of TbFeCo with also a small activation volume.

A microwave frequency reference based on VCSEL-driven dark line resonances in Cs vapor

Dark line resonances, narrowed with a buffer gas to less than 100 Hz width, are observed in a Cs vapor cell using a directly modulated vertical-cavity surface-emitting laser (VCSEL). An external oscillator locked to one of these resonances exhibits a short-term stability of /spl sigma//sub y/(/spl tau/)=9.3/spl times/10/sup -12///spl radic//spl tau/, which drops to 1.6/spl times/10/sup -12/ at 100 s. A physics package for a frequency-reference based on this design could be compact, low-power, and simple to implement.

Cross-current instability generated by pick-up ions in the environment of a large spacecraft in low Earth orbit

Measurements from the space shuttle flights have revealed that a large spacecraft in a low Earth orbit is accompanied by an extensive gas cloud, which is primarily made up of water. The charge exchange between the water molecule and the ionospheric O/sup +/ ions produces a water ion beam traversing downstream of the spacecraft. The authors present results from a study on the linear and nonlinear behaviors of the waves generated by the interaction of the water ion beams with the ionospheric plasma. Since velocity distribution function is key to the understanding of the wave generation process, they have performed a test particle simulation to determine the nature of H/sub 2/O/sup +/ ions velocity distribution function. The simulations show that at, the time scales shorter than the ion-cyclotron period /spl tau//sub c/, the distribution function can be described by a beam. On the other hand, when the time scales are larger than /spl tau//sub c/, a ring distribution forms. A brief description of the linear instabilities driven by an ion beam streaming across a magnetic field in a plasma is presented. In order to study the nonlinear evolution of the waves, the author has performed 2.5-dimensional particle-in-cell (PIC) simulations which show that transient amplitudes of the waves can be up to a few Volts/m at the lower-hybrid frequency. The frequency spectrum of the waves from the simulation shows that the water ion beam produces a broadband electric field noise extending in frequency up to a few times the ion plasma frequency. The simulations also reveal an enhanced electric field noise level near the electron cyclotron harmonics.

Carrier transport related analysis of high-power AlGaN/GaN HEMT structures

Shubnikov-de Haas (SdH) oscillation and Hall measurement results were compared with HEMT DC and RF characteristics for two different MOCVD grown AlGaN-GaN HEMT structures on semiinsulating 4H-SiC substrates. A HEMT with a 40-nm, highly doped AlGaN cap layer exhibited an electron mobility of 1500 cm/sup 2//V/s and a sheet concentration of 9/spl times/10/sup 12/ cm at 300 K (7900 cm/sup 2//V/s and 8/spl times/10/sup 12/ cm/sup -2/ at 80 K), but showed a high threshold voltage and high DC output conductance. A 27-nm AlGaN cap with a thinner, lightly doped donor layer yielded similar Hall values, but lower threshold voltage and output conductance and demonstrated a high CW power density of 6.9 W/mm at 10 GHz. The 2DEG of this improved structure had a sheet concentration of n/sub SdH/=7.8/spl times/10/sup 12/ cm/sup -2/ and a high quantum scattering lifetime of /spl tau//sub q/=1.5/spl times/10/sup -13/ s at 4.2 K compared to n/sub SdH/=8.24/spl times/10/sup 12/ cm/sup -2/ and /spl tau//sub q/=1.72/spl times/10/sup -13/ s for the thick AlGaN cap layer structure, Despite the excellent characteristics of the films, the SdH oscillations still indicate a slight parallel conduction and a weak localization of electrons. These results indicate that good channel quality and high sheet carrier density are not the only HEMT attributes required for good transistor performance.

Possibility of Nd/sub 1.9/Ba/sub 1.1/Cu/sub 3/O/sub 7+δ/ and Pr/sub 1.14/Ba/sub 1.86/Cu/sub 3/O/sub 7-δ/ single crystals for insulator in high-speed superconducting circuits

Dielectric properties of both Nd/sub 1.9/Ba/sub 1.1/Cu/sub 3/O/sub 7+/spl delta// (NBCO-213) and Pr/sub 1.14/Ba/sub 1.86/Cu/sub 3/O/sub 7-/spl delta// (Pr-rich PBCO) single crystals have been examined at low temperature. These materials have good lattice matching to high-T/sub c/ superconductors (HTS), but they are conductive at room temperature. Below 80 K, they are insulators with low dielectric constants, /spl epsi//sub /spl tau// below 25, and low dielectric loss tan /spl delta/ below 0.1 at 100 kHz. The value of /spl epsi//sub /spl tau// is suitable for insulators in integrated circuits using strip line widths of 10 /spl mu/m order, providing short delay time, no excitation of surface wave, and low radiation loss. The value of tan /spl delta/ is comparable to loss of superconducting surface resistance above 100 GHz. These results indicate the applicability for the insulator layers in multilayer superconducting electronic devices such as Single Flux Quantum (SFQ) circuits operated at high speed.

GaN/AlGaN high electron mobility transistors with f of 110 GHz

The performance of 50 nm gate length GaN/AlGaN HEMTs is reported. The device layers were grown by MBE directly onto an SiC substrate. Devices exhibit a maximum drain current density of 1.2 A/mm, an extrinsic f/sub /spl tau// of 110 GHz and an f/sub max/ of over 140 GHz. The f/sub /spl tau// of 110 GHz is the highest reported to date for a GaN/AlGaN HEMT, a nearly 50% increase over the previous record.

Layered Al2O3-TiO2 composite dielectric resonators

Aluminium oxide displays a very low tan/spl delta/ at microwave frequencies. It also possesses a remarkably high thermal conductivity, ideal for heat dissipation in high power satellite filters. However, its temperature coefficient of the resonant frequency (/spl tau//sub f/) is approximately 60 ppm/K. It is shown that the application of a film of titanium oxide which has a T/sub f/ of opposite sign (45O ppm/K) produces a composite in which the /spl tau//sub f/ can be tuned to be zero over a wide temperature range. The tan/spl delta/ of the composite at zero T/sub f/ is 3.3/spl times/10/sup 5/ (Q=30000) at room temperature and at 10 GHz.

Nanosecond magnetization reversal in high coercivity thin films

We used a wide-field Kerr microscope to measure magnetization reversal in high coercivity thin film media that were subjected to nanosecond field pulses. Coplanar waveguides were used as a field source. Two different samples of CoCr/sub 10/Ta/sub 4/ were measured. Sample A had a coercivity of 83 kA/m and sample B had a coercivity of 167 kA/m. For sample A, we find that after a step change in H, the magnetization initially relaxes exponentially with a time constant of 5 ns, and then relaxes logarithmically. We interpret this result as indicating a transition from dynamic reversal to thermal relaxation. In higher fields, the exponential relaxation time decreases according to /spl tau/=S/sub w//(H-H/sub 0/), where S/sub w/=29.7 /spl mu/s/spl middot/A/spl middot/m/sup -1/ (373 ns/spl middot/Oe). For sample B, only logarithmic relaxation is observed, implying that the dynamic magnetization response time is subnanosecond. We observe correlated regions of reversed magnetization in our Kerr images of sample A with a typical correlation length of 1 /spl mu/m along the applied field direction. We propose a microscopic model of nucleation and growth of reversed regions by analogy to viscous domain wall motion.

New algorithm for multidimensional type-III DCT

New algorithms are proposed for the type-III multidimensional discrete cosine transform (MD-DCT-III). The polynomial transform is used to convert the type-III MD-DCT into a series of one-dimensional type-III discrete cosine transforms (1-D-DCT-III). The algorithms achieve considerable savings on the number of operations compared to the row column method. For computing an r-dimensional DCT-III, the number of multiplications required by the proposed algorithm is only 1//spl tau/ times that needed by the row-column method, and the number of additions is also reduced. Compared to other known fast algorithms for two-dimensional- and MD-DCTs, the proposed method uses about the same number of operations. However, advantages such as better computational structure and flexibility on the choice of dimensional sizes can be achieved.

A precise numerical prediction of effective dielectric constant for polymer-ceramic composite based on effective-medium theory

Nanostructure polymer-ceramic composite with high dielectric constant (/spl epsiv//sub /spl tau///spl sim/90) has been developed for embedded capacitor application. This polymer-ceramic system consists of lead magnesium niobate-lead titanate (PMN-PT) ceramic particle and modified high-dielectric constant low-viscosity epoxy resin. In order to obtain precise prediction of effective dielectric constant of this composite, an empirical prediction model based on self-consistent theory is proposed. The electrical polarization mechanism and interaction between epoxy resin and ceramic filler has been studied. This model can establish the relevant constitutional parameters of polymer-ceramic composite materials such as particle shape, composition, and connectivity that determine the dielectric properties of the composite. This model is simpler, uses fewer parameters and its prediction compares better with experiment (error <10%). The precision and simplicity of the model can be exploited for predictions of the properties and design of nanostructure ferroelectric polymer-ceramic composites. The effective-medium theory (EMT) has been proved a good tool to predict effective properties of nanocomposites.

Uncooled InAs-GaSb type-II infrared detectors grown on GaAs substrates for the 8-12-μm atmospheric window

We report on the growth and characterization of type-II infrared detectors with an InAs-GaSb superlattice active layer for the 8-12-/spl mu/m atmospheric window at 300 K. The material was grown by molecular beam epitaxy on semi-insulating GaAs substrates. Photoconductive detectors fabricated from the superlattices showed 80% cutoff at about 12 /spl mu/m at room temperature. The responsivity of the device is about 2 mA/W with a 1-V bias (E=5 V/cm) and the maximum measured detectivity of the device is 1.3/spl times/10/sup 8/ cm.Hz/sup 1/2//W at 11 /spl mu/m at room temperature. The detector shows very weak temperature sensitivity. Also, the extracted effective carrier lifetime, /spl tau/=26 ns, is an order of magnitude longer than the carrier lifetime in HgCdTe with similar bandgap and carrier concentration.

A new common-emitter hybrid-π small-signal equivalent circuit for bipolar transistors with significant neutral base recombination

The linear superposition approach to the modeling of small-signal parameters in the presence of substantial base recombination, which involves a virtual transistor without base recombination, is identified to cause incorrect emitter current modeling. All of the terminal current changes can be correctly modeled by using the measured forced-V/sub BE/ Early voltage in a new equivalent circuit, which properly accounts for NBR and Early effect in a physically consistent manner. As a result, practical situations of small collector-base resistance (/spl tau//sub /spl mu//) can be properly handled, /spl tau//sub /spl mu// is related to the ac current-drive and ac voltage-drive Early voltages, which facilitates parameter extraction and circuit modeling. Measurements on state-of-the-art UHV/CVD SiGe HBT's show that the conventional assumption that /spl tau//sub /spl mu// is far larger than the forced-V/sub BE/ output resistance /spl tau//sub 0/ does not apply to devices with significant NBR. In practice, /spl tau//sub /spl mu// can be comparable to (and smaller than) /spl tau//sub 0/ depending on the device processing, profiles and operating temperature. Temperature dependent data are presented, and circuit implications are discussed based on the new equivalent circuit.

Direct extraction and numerical simulation of the base and collector delay times in double heterojunction bipolar transistors

A new method is presented to evaluate the base and collector transit times, /spl tau//sub B/ and /spl tau//sub C/ in heterojunction bipolar transistors (BBT's) from the phase and magnitude of the common-base current gain, /spl alpha/(/spl omega/), which itself was directly extracted from measured S-parameter data. The method is applied to InGaP/GaAs single and double HBT's. A smaller cutoff frequency in the latter device is attributed to /spl tau//sub B/ and /spl tau//sub C/ due to two effects: trapping of electrons in the conduction band triangular barrier existing at the base-collector (B-C) heterojunction and smaller saturation velocity of electrons in InGaP as compared to GaAs. Finally, a new B-C design of InGaP/GaAs DNBT's is proposed to partially compensate the transit time effects. Numerical simulation of the cutoff frequency demonstrates the superiority of the proposed structure for high-frequency applications.

Radiation damage effects by 25 MeV protons and thermal annealing effects on thallium bromide nuclear radiation detectors

In this study, TlBr detectors were irradiated with 25 MeV protons accelerated by an AVF cyclotron. Isothermal annealing was performed to restore the performance of the detectors. In order to characterize the radiation damage and thermal annealing effects on the TlBr detectors, we measured current-voltage (I-V) characteristics, mobility-lifetime (/spl mu//spl tau/) products and spectrometric responses. The I-V and /spl mu//spl tau/ measurements suggest that electron traps have been induced by 25 MeV protons in the TlBr crystals. X- and /spl gamma/-ray energy spectra were measured for two different electronic conditions: the electric signals induced mainly by electron carriers traversing the crystal were used for one case and the signal induced by hole carriers were used in the other case. After irradiation of 25 MeV protons, the /sup 241/Am X- and /spl gamma/-ray spectra obtained in the former showed significantly degraded energy resolution. No degradation of energy resolution, however, was observed in the latter case. Noticeable improvements of the degraded detector performance have been observed after the thermal annealing.

Draft revision of IEEE STD 1139-1988 standard definitions of physical quantities for fundamental frequency and time metrology — random instabilities

This is a draft revision of IEEE Std 1139-1988 Standard Definitions of Physical Quantities for Fundamental Frequency and Time Metrology. This draft standard covers the fundamental metrology for describing random instabilities of importance to frequency and time metrology. Quantities covered include frequency, amplitude, and phase instabilities; spectral densities of frequency, amplitude, and phase fluctuations; variances of frequency and phase fluctuations; time prediction; and confidence limits when estimating the variance from a finite data set. The standard unit of measure for characterizing phase and frequency instabilities in the frequency domain is L(f), defined as one half of the double-sideband spectral density of phase fluctuations. In the time domain, the standard unit of measure of frequency and phase instabilities is the fully overlapped Allan deviation /spl sigma//sub y/(/spl tau/) or the fully overlapped modified Allan deviation Mod /sub /spl sigma//(/spl tau/).

A new bistable laser diode configuration for all optical switching

A new bistable laser diode configuration for all optical switching has been suggested and its equivalent circuit model is developed. For switching the device to the high state (set) a TE mode optical pulse is required, which is the oscillating mode of the laser, whereas a TM mode pulse is used to bring the output to a low state (reset). But the wavelength of the set and reset pulses are the same as that of the lasing wavelength of the device. The static and dynamic characteristics are studied by simulating the equivalent circuit model using the circuit simulation program PSPICE. It is found that as the carrier lifetime /spl tau//sub a/ of the absorption section increases, the width of the optical pulse required for reset increases nonlinearly. The rise and fall times are found to be 0.15 and 0.16 ns, respectively, for a /spl tau//sub a/, of 3 ns.

Miller and noise effects in a synchronizing flip-flop

The effects of Miller coupling and thermal noise on a synchronizing flip-flop are described. Data on the metastability characteristics of the flip-flop are gathered and analyzed. True metastability is distinguished from the deterministic region. A worst case mean-time-between-failure bound is established. A simple and accurate test method is presented. A simple jamb latch was used with driving circuits of two different strengths to determine the role of input strength on T/sub m/ and /spl tau/. The flip-flop was fabricated on a 0.25-/spl mu/m CMOS process.

First year results of the international comparison of UTC(CENAM)

The Centro Nacional de Metrologia (National Center of Metrology), CENAM, in Mexico, has been contributing to UTC since March 1996 with six HP5071A high performance frequency standards. The UTC(CENAM) (or UTC(CNM) as it is labeled in BIPM Circular T) is presently generated from a single HP5071A clock which is operated under controlled temperature and humidity conditions. In this report we present results of the first year (from March 96 to March 97) of international comparison of UTC(CENAM). We use the common-view time transfer technique and follow the BIPM schedule for North-West America to make that comparison through the BIPM. Analysing the BIPM Circular T CENAM's data, we find the fractional frequency instability of our Master Clock to be about 2/spl times/10/sup -15/ for an integration time (/spl tau/) of 81 days, and an intrinsic frequency offset of about 1,5/spl times/10/sup -13/.

Predicting the outage performance of an equalized-QPSK system over indoor wireless channels

The need to design wide-band wireless indoor systems with low outage probabilities (on the order of 1% to 2%) implies the need to study system performance at the 99th percentile as a function of several variables. We investigate the performance of a 15-MBaud quantenary phase-shift-keying (QPSK) system combined with decision-feedback equalization. The study attempts to identify, and quantify, trends in the system outage performance as a function of the equalizer span, the ensemble averaged RMS-delay-spread (/spl tau/~/sub RMS/) of the environment, and the ensemble averaged input signal-to-noise ratio (SNR). We introduce a novel figure of merit 99%-SNR-loss, which when plotted as a function of the normalized-equalizer-span shows a trend in the system's outage performance as a function of the desired parameters. These suggest a pattern that can help predict, to the first order, the performance of such a system without the need to resort time-consuming simulations. The results suggest a 0.79-dB improvement in the 99%-SNR-loss per normalized equalizer and a 2-dB improvement with a doubling of /spl tau/~/sub RMS/ with the same normalized equalizer span.

Amorphous silicon-based unipolar detector for color recognition

The device performance of a novel color sensitive diode (p-i-i-i-n-diode)-based on amorphous silicon is discussed. The detectors are developed with regard to the /spl mu//spl tau/-(carrier mobility/spl times/lifetime)-product and the bandgap of the different i-layers resulting in a voltage controlled spectral response. Since the linear independence of the spectral response curves and a linear response of the photocurrent on the incident light intensity is a prerequisite for the generation of a red-green-blue signal, the influence of light intensity on the color separation is examined by spectral response measurements under different monochromatic bias illumination. Additionally, numerical simulations are carried out to study the optoelectronic properties of these devices. Finally, a deconvolution method is presented which allows the transformation of the measured spectral response curves into a red-green-blue signal.