Mechanisms for the formation of low temperature,non-alloyed Au-Ge ohmic contacts to n-GaAs

Low temperature, non-alloyed Au-Ge contact formation ton-GaAs is a multi-step pro-cess. During the first 5 min of annealing at 320° C the Au and Ge segregate into regions a few microns in size and extend over the entire thickness of the metal layer and sig-nificant in-diffusion of the Au and Ge and out-diffusion of the Ga and As occurs. This intermixing reduces the barrier height from 0.75 to 0.40 eV. The contact does not show ohmic behavior until it has been annealed for 3 hr. During this time Ge continues to in-diffuse but at a slower rate than it did initially. The rate of Ge in-diffusion is en-hanced by the presence of Au since samples containing less Au require longer anneals to show ohmic behavior and have higher specific contact resistances. The presence of excess As, which is prevented from evaporating by a Si₃N₄ cap has the opposite effect since capped layers have higher specific contact resistances. Au-Ge phases appear after approximately 3 hr of annealing, therefore, Au-Ge phases cannot be responsible for the reduction in barrier height. The interface morphology is smooth, differing from that of pure Au and alloyed contacts that often contain spiking of the metals into the semi-conductor. The orientation relationship for the Au grains differs from that of pure Au. Work performedat U.S. Army ETDL, Fort Monmouth, NJ 07703. Work performed at U.S.Army ETDL, Fort Monmouth, NJ 07703.     

Multilevel metal interconnection utilizing CVD tungsten and liftoff processing

Interconnections between semiconductor devices in integrated circuits continue to present difficult problems in the tradeoffs between RC time constants, production worthiness, reliability, structural complexity, and compactibility for any single technology. A process and structure has been demonstrated for integrated circuit interconnections which uses a conformai tungsten layer deposited by chemical vapor deposition to provide step coverage into via holes of variable height. The film is then patterned with a via interconnect pattern designed for liftoff processing, layers of chromium copper and chromium are then depositedinsitu on the wafers by way of evaporation. The undesired material is lifted off in a solvent process and the resulting metal pattern is used as the mask for the reactive ion etching of CVD tungsten. This combination of materials and process allows for high conductivity reliable interconnections with negligable step coverage problems. Processing and test information will be presented in the paper.     

Creating an intelligent optical network worldwide interoperability demonstration

The Optical Internetworking Forum (OIF) conducted a world interoperability demonstration in conjunction with SUPERCOMM held in Chicago June 22-24, 2004. The demonstration was executed on a global stage with seven carriers across three continents internetworking through an intelligent control plane in a multivendor environment of 15 vendor participants. OIF's demonstration successfully included dynamic end-to-end connection management between client devices and transport network elements in a multidomain, multinode environment. The global connectivity included network elements (NEs) that incorporated the OIF optical user-to-network interface (UNI) and external network-to-network interfaces (E-NNI) among various vendors. Furthermore, carriers were able to test the new Ethernet over synchronous optical network/synchronous digital hierarchy (SONET/SDH) capabilities utilizing the generic framing procedure (GFP-F), virtual concatenation (VCAT), and the link capacity adjustment scheme (LCAS), based on International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) recommendations. This article described the technical approach and execution of this demonstration.     

T-Ray Sensing and Imaging

T-ray wavelengths are long enough to pass through dry, nonpolar objects opaque at visible wavelengths, but short enough to be manipulated by optical components to form an image. Sensing in this band potentially provides advantages in a number of areas of interest to security and defense such as screening of personnel for hidden objects and the retection of chemical and biological agents. Several private companies are developing smaller, reliable cheaper systems allowing for commercialization and this motivates us to review a number of promising applications within this paper. While there are a number of challenges to be overcome there is little doubt that T-ray technology will play a significant role in the near future for advancement of security, public health, and defense.     

Tractography gone wild: probabilistic fibre tracking using the wild bootstrap with diffusion tensor MRI

Diffusion tensor magnetic resonance imaging (DT-MRI) permits the noninvasive assessment of tissue microstructure and, with fibre-tracking algorithms, allows for the 3-D trajectories of white matter fasciculi to be reconstructed noninvasively. Probabilistic algorithms allow one to assign a "confidence" to a given reconstructed pathway--but often rely on a priori assumptions about sources of uncertainty in the data. Bootstrap methods have been proposed as a way of circumventing this problem, deriving the uncertainty from the data themselves--but acquisition times for data amenable to precise and robust bootstrapping are clinically prohibitive. By combining the wild bootstrap, recently introduced to the DT-MRI literature, with tractography, we show how confidence can be assigned to reconstructed trajectories using data collected in a fraction of the time required for regular bootstrapping. We compare in vivo wild bootstrap tracking results with regular tracking results and show that results are comparable. This approach therefore allows users who have collected data sets for use with deterministic tracking algorithms, rather than those specifically designed for bootstrapping, to be able to apply bootstrap analyses and retrospectively assign confidence to their reconstructed trajectories with minimum additional effort.     

Prophet inequalities for cost of observation stopping problems

Comparisons are made between the expected gain of a prophet (an observer with complete foresight) and the maximal expected gain of a gambler (using only non-anticipating stopping times) observing a sequence of independent, uniformly bounded random variables where a non-negative fixed cost is charged for each observation. Sharp universal bounds are obtained under various restrictions on the cost and the length of the sequence. For example, it is shown for X₁, X₂, … independent, [0, 1]-valued random variables that for all c ≥ 0 and all n ≥ 1 that E(max_{1 ≤ j ≤ n}(X_j − jc)) − sup_{t Tn} E(X_t − tc) ≤ 1/e, where T_n is the collection of all stopping times t which are less than or equal to n almost surely.     

The 802.11n MIMO-OFDM Standard for Wireless LAN and Beyond

An overview is given of the new IEEE 802.11n standard. This is the first wireless LAN standard based on MIMO-OFDM, a technique pioneered by Airgo Networks to give a significant performance increase in both range and rate relative to conventional wireless LAN. Performance results show that net user throughputs over 100 Mbps are achievable, which is about four times larger than the maximum achievable throughput using IEEE 802.11a/g. For the same throughput, MIMO-OFDM achieves a range that is about 3 times larger than non-MIMO systems. This significant improvement in range-rate performance makes MIMO-OFDM the ideal solution not only for wireless LAN, but also for home entertainment networks and 4G networks. Richard van Nee received the M.Sc. degree in Electrical Engineering from Twente University in Enschede, the Netherlands, in 1990. In May 1995, he received the PhD degree from Delft University of Technology. From 1995 to 2000, he worked for Lucent Technologies Bell Labs on wireless LAN transmission techniques. He was one of the original proposers of the CCK and OFDM modulation techniques which were adopted by the IEEE 802.11b and IEEE802.11a wireless LAN standards. In 2001, he cofounded Airgo Networks that developed the first MIMO-OFDM modem for wireless LAN and which techniques form the basis of the IEEE 802.11n standard. Together with Ramjee Prasad, he wrote a book on OFDM, entitled ‘OFDM for Mobile Multimedia Communications.’ In 2002 he received the Dutch Veder award for his contributions to standardization of wireless communications.     

Singly generated planar algebras of small dimension, Part II

We classified in Bisch and Jones (Duke Math. J. 101 (2000) 41) all spherical C∗-planar algebras generated by a non-trivial 2-box subject to the condition that the dimension of N′∩M₂ is ?12. We showed that they are given by the Fuss-Catalan systems discovered in Bisch and Jones (Invent. Math. 128 (1997) 89) and one exceptional planar algebra. In the present paper, we extend these results and show that there is only one spherical C∗-planar algebra generated by a single non-trivial 2-box if the dimension of N′∩M₂ is 13. It is given by the standard invariant of the crossed product subfactor , where D₅ denotes the dihedral group with 10 elements.     

Pilotless Frame Synchronization via LDPC Code Constraint Feedback

In traditional receiver architectures, frame synchronization is performed using pilot symbols and a correlation rule. In this paper we show that outputs from the constraint node side of a bipartite decoding graph can be used to achieve frame synchronization in a pilotless low-density parity-check (LDPC) coded transmission, thereby avoiding the bandwidth cost inherent in use of pilot symbols. The complexity of the frame synchronizer is kept relatively low due to its XOR-based approach.     

Modeling synchronization and suppression of spontaneous otoacoustic emissions using Van der Pol oscillators: effects of aspirin administration

Many of the aspects of the interaction of spontaneous otoacoustic emissions with external tones (suppression and synchronization) can be qualitatively simulated by the behavior of a single driven Van der Pol oscillator. Analytical and numerical investigations of a model of spontaneous otoacoustic emissions based on such an oscillator (with appropriate parametric changes in the nonlinear and negative damping components) lead to predictions of the nature of the changes in suppression and synchronization (frequency-locking) tuning curves when the levels of spontaneous otoacoustic emissions are modified. Observations of the suppression and synchronization of spontaneous otoacoustic emissions by external tones of different frequencies and levels were obtained while the levels of spontaneous emissions were altered by aspirin administration. Modeling an emission as a single Van der Pol oscillator qualitatively accounts for: (1) the reduction of the level of an external tone required to suppress the emission by a decibel amount equivalent to the level reduction induced by aspirin administration; (2) the broadening of the frequency-locking tuning curve of an emission whose level is reduced; and (3) the pulling of the emission frequency by an external tone. It does not account for: (1) the observed asymmetry in the slopes of the external-tone suppression curves (more gradual for frequencies of the suppressor tone higher, rather than lower, than that of the emission); and (2) the frequency pushing of the emission by an external tone.