Inhomogeneity of the deep center el2 in GaAs observed by direct infra-red imaging

The distribution of the dominant deep trap EL2 in 7.5cm diameter crystals of semi-insulating GaAs is studied by whole slice infrared imaging. Very significant fluctuations in the neutral EL2 concentration ([EL2]ℴ) are observed, corresponding at most to variations in [EL2]ℴ of up to 80%. The different sorts of fine structure, namely cell structure and bands of high infrared absorption ("sheets" and “streamers”) lying in (110) planes running down the <001> growth directions, are described.     

Effect of tin impurity on the photoconductivity kinetics of thin amorphous layers of arsenic selenide

In thermally sputtered As₂Se₃ and As₂Se₃ + 0.1 at. % Sn films the tin impurity strongly influences the photoconductivity kinetics under stepped optical excitation. The tin quenches the “spike” on the section of increasing photocurrent, eliminates the dependence of the form of the decrease on the excitation intensity, and leads to a temperature-dependent delay in recombination onset. The effect of the impurity is attributed to an increase in trapping in deep localized states produced by the introduction of tin. Fiz. Tekh. Poluprovodn. 31, 836–840 (July 1997)     

ATM cell encryption and key update synchronization

This paper presents a data compaction/randomization based approach as a mode of block encryption for ATM (Asynchronous Transfer Mode) cells. The presented approach converts a plaintext into pseudo‐random plaintext before ciphering to conceal patterns in the plaintext. The underlying idea behind this scheme is the Shannon's principles of “confusion” and “diffusion” which involve breaking dependencies and introducing as much randomness as possible into the ciphertext. In this scheme, confusion and diffusion are introduced into the system by first compressing the ATM cell payload and then spreading a continuously changing random data over the entire content of the cell. As a mode of operation for block ciphering, this scheme offers the following attractive features:(i) plaintext patterns are pseudo‐randomized and chained with ciphertext (thereby, preventing against “dictionary”, “known plaintext”, and “statistical analysis” attacks), (ii) it is self‐synchronizing, (iii) cell loss has no additional negative effect, (iv) no IV (Initialization Vector) storage is required, (v) it is encryption‐algorithm independent, (vi) there is no cell‐to‐cell dependency (no feedback from previous cells), and (vii) it is highly scalable (i.e., cells from the same stream can be ciphered and deciphered in parallel). This paper also presents a secure mechanism for in‐band synchronization of encryption/decryption key updates using a “marker‐cell” that is carried within the data channel. An important aspect of both the above mechanisms is that they do not require any changes to the ATM cell header or ATM infrastructure. This revised version was published online in June 2006 with corrections to the Cover Date.     

Optical and electrical effects of charges on defects in non-metals

Electrically charged defects, which are unavoidable in solids, create very strong electric fields that affect many properties of the host. In an insulator with ∿ 10 ppm charged impurities, for example, the average field strength is F ∿ 50 kV/cm. The polar lattice vibrations, which are defects of a different sort, create fields in ionic solids that are typically lO⁶V/cm at room temperature. Perhaps the most fundamental effect of these charges is the alteration of the density of electronic states. Near energy band edges this produces “band tails” that have important consequences in most electron transport properties quite apart from ordinary impurity scattering. Furthermore, the matrix elements for optical transitions among states near band edges are also affected with several significant consequences in the optical properties. Aside from these fundamental physical consequences of charged defects, a number of important practical effects occur, including the domination of semiconductor laser properties, limitation of light transmission in quartz fibers and behavior of amorphous semi-conductors.     

Two-electron photo-ionization from deep states in semiconductors

Photo-ionization (P-I) from deep states in semiconductors can be used to explore the band structure of the crystal as well as to give detailed information about the symmetry and other properties of the bound states themselves. When 2 electrons are bound to a defect the P-I spectrum at high photon energy reflects the initial and final state properties of both electrons. Only at low energies, where one electron is transferred into its 1-electron ground state does the P-I process of a 2-electron state resemble that of a 1-electron state. This fact offers a resolution of the two different interpretations of P-I from the 0^- state in GaP as reported by Henry et al. from photo-capacitance studies and by Grimmeiss et al. from photo-conductivity. In the latter the 4 lowest enercry thresholds (E_i = 0.65 to 1.19 eV) correspond to “1-electron” P-I with the 0 center left in its neutral ground state. Only above about 1.4 eV do the “2-electron” processes appear. In photo-capacitance the “2-electron” processes dominate (the “1-electron” thresholds being too weak or too diffuse to be identified), and the spectrum is strongly modified by relaxation of the lattice around the 0⁺ ion.     

Observation of differences in the quenching of the Photocurrent in GaAs containing EL2 and EL0

The quenching of the photocurrent and photo-Hall effect of several undoped semi-insulating gallium arsenide samples has been measured and compared with the deep-level photoluminescence spectra from neighboring samples. Samples that show either EL2 (0.68 eV) or ELO (0.63 eV) photoluminescence have distinctly different photocurrent quenching behaviors. EL2 samples show a photocurrent decrease of several orders of magnitude, and a change fromn-type to p-type conduction during quenching at 80 K with 1.1 eV light. ELO samples show a reduction in photocurrent of less than an order of magnitude with no change in the carrier type at this temperature. Photo-Hall effect experiments at 80 K indicate that the conduction isn-type for the ELO samples, but changes fromn- to p-type during the quench for the EL2 samples. The temperature dependence of the quenching has also been studied. EL2 samples show little variation in the range 10-80 K, while ELO samples show significant quenching similar to EL2 after the temperature is reduced below 70 K. These results indicate that defects other than EL2 can significantly affect photocurrent quenching experiments.     

Relationship between near bandedge absorption and photoluminescence efficiency in semi-insulating GaAs

We demonstrate that near bandedge photoluminescence efficiency in SI bulk GaAs can be increased by low temperature photo-quenching of native point defects in the material. These defects cause infrared absorption at photon energies just below the bandgap energy in cooled samples and their concentrations anti-correlate with those of EL2 in unannealed crystals. This absorption has been previously termed “Reverse Contrast.” The increase in PL efficiency is almost an exponential function of the photoquenching time and proportional to the Reverse Contrast absorption coefficient.     

Characteristics of radio frequency paths of on-board aviation navigation-landing equipment

Methodologies and results of researching high-frequency paths in on-board navigationlanding equipment of meter frequency band are presented in this article. Recommendations on their design using international recommendations “ARINC” and “Rules of planes’ flight eligibility RPFE-3” are given.     

Experimental study on a novel polarization state generator

Duetothe spoilageinduced bythe polarization-relatedeffects (PRE) in high-speed(≥10 Gb/s/channel) opti-cal fiber communication systems ,it is necessary to in-vestigate the effect of PRE on optical components andentire transmission systems .In general , PR…     

Extending a DWDM Optical Network Test System to 12 Gbps x4 Channels

A “Development Platform” for prototyping new multi-GHz ATE has recently been introduced (Keezer et al. 2009). The first application was a multi-channel test system for characterizing an optical network switch operating at 2.5 Gbps per channel (Keezer et al. 2009, 2010). Nine transmitter channels (TX) and nine receivers (RX) were used to test the Dense Wavelength Division Multiplexing (DWDM) switching network. This present paper incorporates elements from the prototype designs into full-feature test modules targeting applications between 2.5 Gbps and 24.0 Gbps per channel. Specifically, the optical test system is extended for burst-mode 12 Gbps DWDM packets (more than 4-times the rate of the original system). Using 8 TX channels, an aggregate data rate of 96 Gbps is achieved. Alternatively, some modules can be configured to double the channel-count (up to 18) while operating at the lower 2.5 Gbps rate (45 Gbps aggregate rate). Lower rates permit use of lower-cost optical components. Two new modules are described with testability features such as: (1) support for “loopback” testing of DUT output-to-inputs, (2) DC electrical tests, (3) 2-to-1 multiplexing up to 24 Gbps, (4) ATE self-test/calibration loopback paths. Recently multiple Development Platforms have been constructed that can operate either independently or synchronized using very low-jitter (~1 ps RMS) clock distribution paths.     

Solid- and gas-source “hybrid” Si molecular beam epitaxy for a Si1−xGex/Si single quantum well electroluminescent device

A p-i-n diode for a Si_1−xGe_x/Si single quantum well (SQW) electroluminescent (EL) device was successfully fabricated by solid-source (SS) and gas-source (GS) “hybrid” Si molecular beam epitaxy (MBE). First, the undoped SQW layer was grown on a p-type Si(100) substrate by GSMBE using disilane (Si₂H₆) and germane (GeH₄). Then the n-type Si contact layer was regrown by SSMBE after transferring the sample through the air. A (2 × 1) reconstruction was observed on a GSMBE-prepared Si surface even after the sample was exposed to air for 15 h. The excellent quality of the EL p-i-n device was shown by the sharpest emission lines, ≈5.5 meV, ever reported in the EL spectra of an SiGe system. Linear polarization along the SQW plane was also observed for no-phonon replica of EL.     

Preparation and characteristics of white EL display panels

Two methods for preparing white emitting EL panels are well known in principle: (1) blending a blue and an orange emitting EL phosphor and (2) partially cascading of green EL emission with a red photoluminescent material, typically an organic fluorescent dye. Both methods have their advantages and shortcomings. Panels containing phosphor blends are readily viewed even in moderately bright ambient light but the “ white” color is reasonably stable only over limited times because the blue phosphor comnonent deteriorates faster than the orange. Cascading panels show very long lives, depending on the green EL phosphor involved, without any visible color change but these panels can be used only under restricted ambient light conditions. Brightnesses and efficiencies of both kinds of panels are about comparable     

Electrical and luminescent properties and the spectra of deep centers in GaMnN/InGaN light-emitting diodes

Electrical and electroluminescent properties were studied for GaN/InGaN light-emitting diodes (LEDs) with the n-GaN layer up and with the top portion of the n layer made of undoped GaMnN to allow polarization modulation by applying an external magnetic field (so-called “spin-LEDs”). The contact annealing temperature was kept to 750°C, which is the thermal stability limit for retaining room-temperature magnetic ordering in the GaMnN layer. Measurable electroluminescence (EL) was obtained in these structures at threshold voltages of ∼15 V, with a lower EL signal compared to control LEDs without Mn. This is related to the existence of two parasitic junctions between the metal and the lower contact p-type layer and between the GaMnN and the n-GaN in the top contact layer.     

Influence of substoichiometry,hydrogen content and crystallinity on the optical and electrical properties of HxWOy thin films

We have prepared H_xWO_y amorphous thin films both by evaporation of tungsten trioxide powder and by cathodic sputtering of a tungsten target in an argon/oxygen/hydrogen reactive gas mixture. The evaporated layers have the composition H_xWO_2.7 (0.2 < × < 0.5). Their oxygen content seems rather insensitive to the evaporation parameters. We do not observe any correlation between x and these parameters. Evaporated virgin layers are nearly transparent. Annealing,under vacuum leaves y unchanged, under oxygen increases y to 3. Annealing of the virgin layer under vacuum induces the growth of the 1.38 eV absorption band (giving blue coloration) and a decrease of the activation energy for conduction. Annealing the blue layers in oxygen destroys the 1.38 eV band and increases the activation energy for conduction. In both cases annealing at high temperature induces a microcrystalline phase with an absorption band centered about 0.72 eV (giving also a blue coloration) and a jump in electronic conductivity. As in the case of the 1.38 eV band, an increase of the intensity of the 0.72 eV band induces a decrease of the activation energy for conduction. The two bands are interpreted as polaronic like. They can be induced in transparent layers without any change in global composition by excitation of the hydrogen atoms from a “ passive” state to an “ active” state. In addition to the hydrogen content, the existence of the 1.38 eV band requires some substoichiometry. The study of the optical and electrical properties of amorphous H_xWO_y sputtered layers, supports our previous conclusions about the composition range (C) for the coloration capability of transparent thin films. In addition there is a composition range (B) where the virgin layers are blue, and a composition range (M) where they have a metallic like behavior. On the other side of (C), there is a range (C’) where uv illumination only induces a decrease in the activation energy for conduction, then a range (T) where the layers are completely nonresponsive. One can pass from (T) to (B) through (C') and (C) either at constant hydrogen content by increasing the departure from stochiometry, or at constant substoichiometry by increasing the hydrogen content. A part of this work was presented at EMC Cornell, New York July 1, 1977.     

Mobility of modulation doped AlGaAs/low-temperature MBE-grown GaAs heterostructures

A study of the mobility of a novel modulation doped heterostructure in which the channel region is made of low-temperature molecular beam epitaxially grown GaAs (LT-GaAs) and all other layers are grown at normal temperatures is presented for the first time. The resistivity of the as-grown samples(in- situ annealed) is very high, as is that of single layers of bulk LT-GaAs. However, in the presence of light, the resistivity of the LT-GaAs modulation-doped field effect transistor (MODFET) is significantly lower, facilitating reliable Hall measurements. We speculate that the observed decrease in resistivity of the LT-GaAs MODFET is due to the formation of a two-dimensional electron gas (2DEG) at the heterointerface under illumination. A number of samples grown under different growth conditions were investigated. Mobilities for these samples were found to be in the range of 250 to 750 cm²Vs at 300K and ∼3000 to 5500 cm²Vs at 77K. A first-order computer simulation was implemented to calculate the mobility of the 2DEG using the relaxation-time approximation to solve the Boltzmann equation, taking into account different scattering mechanisms. Scattering by the arsenic clusters and by ionized impurities in the LT-GaAs MODFET channel are found to be the two dominant mechanisms limiting the mobility of the LT-GaAs MODFET samples. Experimental values are in good agreement with theoretical results.     

Processing-texture relationships in dual-unbalanced magnetron deposition of TiN films

There has been sustained interest in using TiN and other sputter deposited thin film materials in electronics applications, such as barrier coatings. However, it is difficult to produce “pin-hole free” coatings using conventional magnetron sputtering, since the high bias potentials required to produce dense films often result in substrate damage. “Unbalanced” magnetron sputtering may offer a low energy alternative since the ion-to-deposited-atom ratio can be greatly increased, permitting the ion-bombardment energy to be reduced to <200 eV, without sacrificing film density, hardness, or adhesion. As has been demonstrated previously, ion energy can have a profound effect on film texture, but what affect the “substitution” of ion flux for ion energy will have on film texture has not been determined. In this work, TiN films were deposited onto M2 steel via “unbalanced” magnetron sputtering in an attempt to correlate changes in film texture and film stress, with ion energy and flux.     

Input-Matching and Offset-Cancelling Networks for Limiting Amplifiers in Optical Communication Systems

The diffusion of optical communication systems in the access network and for short-haul datacom applications requires the use of low-cost plastic packages: the functional block most affected is the limiting amplifier, that is often the first stage of the Clock and Data Recovery (CDR) IC. In this paper we illustrate the design issues of the input-matching and offset-cancelling network for a differential limiting amplifier for optical communication systems, with particular emphasis on the effect of bond wires. We discuss the limitations of passive feedback networks when used both for offset suppression and for input matching, and propose a topology that overcomes such limitations by using an active feedback loop. A 50 Ω-loaded differential pair is used to achieve input matching and high offset suppression, and its buffering action desensitizes the input matching from the effect of the bond wires connecting off-chip filtering capacitors. Very good performance even with low cost plastic packages can be achieved by solving the trade-off between power consumption, offset suppression and the value of the low-pass filtering capacitors. Design examples of CDR IC's for 2.5 Gb/s optical systems are presented to compare the proposed topology with solutions based on passive feedback networks. Marco Balsi received the laurea (M.Sc.) degree in 1991 and the dottorato di ricerca (Ph.D.) in 1995 in Electronic Engineering from University of Roma “La Sapienza”, Roma, Italy. Since 1996 he is with “La Sapienza” University as ricercatore (assistant professor). He is engaged in research in nonlinear and soft-computing-based signal processing (especially for biomedical imaging), artificial vision, mechatronics, and anti-personnel mine detection. He has published about 60 paper in international journals and refereed conferences. Francesco Centurelli received the laurea degree (cum laude) and the Ph.D. degree in electronic engineering from the University of Roma “La Sapienza”, Roma, Italy, in 1995 and 2000, respectively. He is currently doing postdoctoral work with the Electronic Engineering Department of the University of Roma “La Sapienza.” His research interests include system-level analysis and design of clock recovery circuits and high-speed analog integrated circuits, with particular emphasis on gigabit-rate optical communication systems. Andrea Pallotta received the M.S. degree in electronic engineering from the University of Ancona, Ancona, Italy. From 1991 to 1999, he was with Italtel, Italy, where he joined several European Community research projects in the field of SDH and WDM fiber-optic transmission systems for both transport and access networks. From 1999 to 2000, he was with Siemens Information and Communication Networks, where he was responsible for the SDH Cross-Connect advanced development group. In September 2000, he joined the STMicroelectronics Company, where he is currently responsible for the electrooptical interface design group. His research interests include GaAs and silicon high-speed ICs, active optical devices, high-speed TX and RX optical modules, and fiber-optic transmission system engineering. Alessandro Trifiletti was born in Roma, Italy, in 1959. In 1991, he joined the Electronic Engineering Department of the University of Roma “La Sapienza” as a Research Assistant and is currently an Assistant Professor. His research interests include high-speed circuit design techniques and III-V device modeling.     

Photo CVD system for silicon nitride film

An ultraviolet light excitation photo CVD system for silicon nitride film deposition, in which the use of mercury photo-sensitizer and the undesirable wall deposition onto the optical window inside are eliminated, has been developed. The elimination of the use of mercury sensitizer is achieved by employing direct photolysis of SiH₄/NH₃ gas mixture, using 185 nm light emitted from a low pressure mercury lamp. The wall deposition prevention is achieved by inserting an optically transparent “separator plate” with a number of through-holes on its plane area underneath the optical window and by draining inert gas into the reaction chamber through this “separator plate.” With this system, silicon nitride films have been deposited without marked degradation of deposition rate, keeping a reasonable deposition rate of about 40 A/minute. The inert gas used for wall deposition prevention has no influence on the properties of deposited films.     

Characterization of subthreshold MOS mismatch in transistors for VLSI systems

MOS transistor mismatch is revisited in the context of subthreshold operation and VLSI systems. We report experimental measurements from large transistor arrays with device sizes typical for digital and analog VLSI systems (areas between 9 and 400μm²). These are fabricated at different production qualified facilities in 40-nm gate oxide,n-well andp-well, mask lithography processes. Within the small area of our test-strips (3 mm²), transistor mismatch can be classified into four categories: random variations, “edge,” “striation,” and “gradient” effects. The edge effect manifests itself as a dependence of the transistor current on its position with reference to the surrounding structures. Contrary to what was previously believed, edge effects extend beyond the outer most devices in the array. The striation effect exhibits itself as a position-dependent variation in transistor current following a sinusoidal oscillation in space of slowly varying frequency. The gradient effect is also a position-dependent spatial variation but of much lower frequency. When systematic effects are removed from the data, the random variations follow an inverse linear dependence on the square root of transistor area.     

A New Nonlinear Optically Active Donor–Acceptor-Type Conjugated Polymer: Synthesis and Electrochemical and Optical Characterization

A new donor–acceptor-type poly[3-{5-[3,4-didodecyloxy-5-(1,3,4-oxadiazol- 2-yl)thiophen-2-yl]-1,3,4-oxadiazol-2-yl}-9-dodecyl-9H-carbazole] (P) has been synthesized through multistep reactions. The new polymer P exhibited good thermal stability and film-forming behavior. The electrochemical band gap is estimated to be 2.15 eV. The polymer emits intense green fluorescence in the solid state. Third-order nonlinear optical (NLO) studies showed that the strong absorptive nonlinearity observed for the polymer is of the optical limiting type, which is due to an “effective” three-photon absorption (3PA) process. This 3PA process can have potential applications in photonic devices. The studies revealed that the new polymer P is a promising material for development of efficient optoelectronic devices.