Interface studies of ALD-grown metal oxide insulators on Ge and III–V semiconductors (Invited Paper)

Atomic layer deposited (ALD) HfO₂/GeO_xN_y/Ge(1 0 0) and Al₂O₃/In_0.53Ga_0.47As(1 0 0) ? 4 × 2 gate stacks were analyzed both by MOS capacitor electrical characterization and by advanced physical characterization to correlate the presence of electrically-active defects with chemical bonding across the insulator/channel interface. By controlled in situ plasma nitridation of Ge and post-ALD annealing, the capacitance-derived equivalent oxide thickness was reduced to 1.3 nm for 5 nm HfO₂ layers, and mid-gap density of interface states, D_it = 3 × 10¹¹ cm^?2 eV^?1, was obtained. In contrast to the Ge case, where an engineered interface layer greatly improves electrical characteristics, we show that ALD-Al₂O₃ deposited on the In_0.53Ga_0.47As (1 0 0) ? 4 × 2 surface after in situ thermal desorption in the ALD chamber of a protective As cap results in an atomically-abrupt and unpinned interface. By avoiding subcutaneous oxidation of the InGaAs channel during Al₂O₃ deposition, a relatively passive gate oxide/III–V interface is formed.     

QoS Aware Multi-Channel Scheduling for IEEE 802.15.3 Networks

From a multimedia applications perspective, there is an ever increasing demand for wireless devices with higher bandwidth to support high data rate flows. One possible solution to support the demand for higher bandwidth is to utilize the full spectrum by simultaneously using multiple channels for transmission. Recent approval by the Federal Communications Commission (FCC) has led to considerable interest in exploiting Ultra Wideband (UWB) access on an unlicensed basis in the 3.1--10.6 GHz band. Currently, the IEEE TG802.15.3a standards group is in the process of developing an alternative high-speed link layer design conformable with the IEEE 802.15.3 Wireless Personal Area Network (WPAN) multiple access (MAC) protocol. One of the proposals, based on the concept of Orthogonal Frequency Division Multiplexing (OFDM), divides the spectrum into multiple bands and achieves channelization through the use of different time-frequency codes. These multiple channels can help satisfy the increasing demand for higher bandwidth in order to support high data rate multimedia applications. In this paper, we present a QoS-aware, multi-channel scheduling algorithm that simultaneously utilizes the various channels available in the UWB network. Aniruddha Rangnekar is a doctoral student in the Department of Computer Science and Electrical Engineering at the University of Maryland, Baltimore County. He received the B.E. degree in Computer Engineering from the University of Pune, India in 1998 and a M.S. in Computer Science from the University of Maryland, Baltimore County in 2001. From January 2002 to date, he has been involved in graduate research in University of Maryland, Baltimore County. During the summer of 2004, he worked as the MAC development engineer at Staccato Communications, San Diego, CA. His current interests are in the areas of wireless ad hoc networks, multicast routing protocols, ultra wideband communications and MAC protocol development. He is a member of the MACSim group of the Multiband OFDM alliance (MBOA). Krishna M. Sivalingam is an Associate Professor in the Dept. of CSEE at University of Maryland, Baltimore County. Previously, he was with the School of EECS at Washington State University, Pullman from 1997 until 2002; and with the University of North Carolina Greensboro from 1994 until 1997. He has also conducted research at Lucent Technologies' Bell Labs in Murray Hill, NJ, and at AT&T Labs in Whippany, NJ. He received his Ph.D. and M.S. degrees in Computer Science from State University of New York at Buffalo in 1994 and 1990 respectively; and his B.E. degree in Computer Science and Engineering in 1988 from Anna University, Chennai (Madras), India. While at SUNY Buffalo, he was a Presidential Fellow from 1988 to 1991. His research interests include wireless networks, optical wavelength division multiplexed networks, and performance evaluation. He holds three patents in wireless networks and has published several research articles including more than thirty journal publications. He has published an edited book on Wireless Sensor Networks in 2004 and edited books on optical WDM networks in 2000 and 2004. He served as a Guest Co-Editor for special issues of the ACM MONET journal on “Wireless Sensor Networks” in 2003 and 2004; and an issue of the IEEE Journal on Selected Areas in Communications on optical WDM networks (2000). He is co-recipient of the Best Paper Award at the IEEE International Conference on Networks 2000 held in Singapore. His work has been supported by several sources including AFOSR, NSF, Cisco, Intel and Laboratory for Telecommunication Sciences. He is a member of the Editorial Board for ACM Wireless Networks Journal, IEEE Transactions on Mobile Computing, Ad Hoc and Sensor Wireless Networks Journal, and KICS Journal of Computer Networks. He serves as Steering Committee Co-Chair for the International Conference on Broadband Networks (BroadNets) that was created in 2004. He is currently serving as General Co-Vice-Chair for the Second Annual International Mobiquitous conference to be held in San Diego in 2005 and as General Co-Chair for the First International Conference on Security and Privacy for Emerging Areas in Communication Networks to be held in Athens, Greece in Sep. 2005. He served as Technical Program Co-Chair for the First IEEE Conference on Sensor and Ad Hoc Communications and Networks (SECON) held at Santa Clara, CA in 2004; as General Co-Chair for SPIE Opticomm 2003 (Dallas, TX) and for ACM Intl. Workshop on Wireless Sensor Networks and Applications (WSNA) 2003 held in conjunction with ACM MobiCom 2003 at San Diego, CA; as Technical Program Co-Chair of SPIE/IEEE/ACM OptiComm conference at Boston, MA in July 2002; and as Workshop Co-Chair for WSNA 2002 held in conjunction with ACM MobiCom 2002 at Atlanta, GA in Sep 2002. He is a Senior Member of IEEE and a member of ACM.     

Spherically symmetric static solutions of the Einstein-Maxwell equations

We report a new formalism to obtain solutions of Einstein-Maxwell’s equations for static spheres assuming the matter content to be a charged perfect fluid of null-conductivity. Defining three new variablesu=4πεr ²,ν=4πpr ^{2 2} andw=(4π/3)(ρ+ε)r ² whereε, ρ andε denote respectively energy densities of the electric, matter and free gravitational fields whereasp is the fluid pressure, Einstein’s field equations are rewritten in an elegant form. The solutions given by Bonnor [1], Nduka [2], Cooperstock and De la Cruz [3], Mehra [4], Tikekar [5,6], Xingxiang [7], Patino and Rago [8] are all shown to possess simple relations betweenu, v, andw whereas Pant and Sah’s [9] solution for which all the three functions,u, v, andw are constants is a trivial case of the present formalism, We have presented six new solutions with ε = 2ρ. For the first three solutionsw andu are constants withv as a variable whereas the remaining three solutions satisfy the equation of state for isothermal gas;v =kw =-ku where (i)k is an arbitrary constant but not equal to 1 or 1/3 (ii)k = 1 and (iii)k = 1/3. We also obtained a generalization of Cooperstock and De la Cruz’s [3] solution which is regular for 2ρ > ε but singular for 2ρ ≤ ε.     

Electrical Characterization of Different Passivation Treatments for Long-Wave Infrared InAs/GaSb Strained Layer Superlattice Photodiodes

Silicon dioxide (SiO₂), silicon nitride (Si _x N _y ), and zinc sulfide (ZnS) with ammonium sulfide [(NH₄)₂S] as a prepassivation surface treatment were compared as passivants for InAs/GaSb strained layer superlattice detectors with a 0% cutoff wavelength of ∼10 μm. SiO₂ did not show significant improvement and the zero-bias resistance-area product (R ₀ A) was 0.72 Ω-cm² at 77 K. Si _x N _y passivation showed a nominal improvement with an R ₀ A value of 4.1 Ω-cm² at 77 K. ZnS with (NH₄)₂S treatment outperformed others significantly, improving the R ₀ A value to 492 Ω-cm² at 77 K. Variable-area diode measurements indicated a bulk-limited R ₀ A value of 722 Ω-cm². ZnS-passivated diodes exhibited maximum surface resistivity with a value of 2500 Ω-cm.     

Secure image transmission through crypto‐OFDM system using Rubik's cube algorithm over an AWGN channel

Confidentiality of information must be maintained when it is required to be transmitted over a communication channel or when it is stored in a computer for further information access. Establishing a highly reliable and secure means of wireless communication for the transfer of digital data (text, images, audio, and video) from source to destination is becoming a prime requirement in present‐day wireless communications. Security can be achieved at the network level as well as at the data level. Data‐level security is made through cryptographic techniques. This paper emphasizes data‐level security aspects in a wireless communication system for the secure transmission of images over an AWGN channel condition. Here, a crypto orthogonal frequency division multiplexing system (crypto‐OFDM system) is designed using Rubik's cube encryption algorithm scheme for secured transmission of images under the MATLAB environment. The quality of the image transmission is analyzed using peak signal to noise ratio (PSNR) and BER at different SNR conditions. The quality of the encryption algorithm was also tested with statistical metrics, which are the histogram, NPCR, UACI, entropy, correlation coefficient analysis, etc. The numerical results reveal that a DCT‐based crypto‐OFDM system with Rubik's cube algorithm show a better performance over earlier cryptosystems and also perform superior to the original/basic OFDM system. The statistical analysis tests prove that Rubik's cube algorithm is one of the most robust algorithms for security point of view and is easy to implement, as Rubik's cube algorithm makes use of natural characteristics of pixel values, which form the basis for designing the secret key .     

Intersubband gain and stimulated emission in long-wavelength(λ=13 μm) intersubband In(Ga)As-GaAs quantum-dotelectroluminescent devices

The dynamics of injected carriers and the conditions for intersubband gain and population inversion in In(Ga)As-GaAs self-organized quantum dots have been studied. Direct femtosecond pump-probe spectroscopy as a function of temperature and excitation density confirms earlier results and shows a long (>100 ps) electron relaxation time between the excited states and ground state in the dots. Intersubband gains as high as 170 cm^-1 are calculated in the dots. Far-infrared spontaneous emission centered around 13 μm is observed in edge-emitting light-emitting diodes. Stimulated emission, with a distinct threshold around 1.1 kA/cm² in the light-current characteristics, is observed in plasmon-enhanced waveguide devices. The intersubband threshold occurs after a threshold is observed for interband lasing (~1 μm) in the same device     

Interference and Complexity Reduction in Multi-stage Multi-user Detection in DS-CDMA

Multi-stage parallel interference cancellation (PIC) technique gives good performance compared to successive interference cancellation (SIC) method, but biased decision statistic and complexity problems are raised due to imperfect estimation of multiple access interference (MAI) as number of stages increases. partial parallel interference cancellation (PPIC) technique is proposed to cancel the interference partially stage by stage to overcome biased problem. The complexity reduction for PIC detection is based on the convergence nature of interference cancellation which is called the difference PIC (D-PIC) detection technique. In this paper we combine (PPIC and DPIC) these two techniques and propose a multi-stage multi-user (PD-PIC) detector for performance improvement and complexity reduction compared to conventional PIC detector. The performance is degraded as the number of users increases in each technique.     

Robust Chaotic Communication Based on Indirect Coupling Synchronization

This paper proposes a chaotic communication approach using indirect coupled synchronization scheme with high power encrypted signals. The proposed scheme is carefully designed so that the encrypted signal does not deteriorate the synchronization unlike in traditional communication methods. The synchronization problem is solved using observer-based controller. The advantages of this approach are the general and systematic feedback observer design methodology suitable for convergence rate of synchronization; flexibility in selection of chaotic signals for cryptosystem secure key generator; and improvement of the frequency-domain characteristics of the transmitted message. Computer simulations show that the synchronization between the transmitter and the receiver is more robust for different amplitude values of the information signal, even in the presence of external disturbances.     

Wafer‐Level Self‐Organized Copolymer Templates for Nanolithography with Sub‐50 nm Feature and Spatial Resolutions

Robust lithographic templates, with sub‐50 nm feature and spatial resolutions, that exhibit high patterning integrity across a full‐wafer are demonstrated using self‐organized copolymer reverse micelles on 100 mm Si wafers. A variation of less than 5% in the feature size and periodicity of polymeric templates across the entire wafer is achieved simply by controlling the spin‐coating process. Lithographic pattern transfer using these templates yields Si nanopillar arrays spanning the entire wafer surface and exhibiting high uniformity inherited from the original templates. The variation in geometric characteristics of the pillar arrays across the full‐wafer surface is validated to be less than 5% using reflectance spectroscopy. The physical basis of the change in reflectance with respect to sub‐10 nm variations in geometric parameters of pillar arrays is shown by theoretical modelling and simulations. Successful fabrication of highly durable TiO₂ masks for nanolithography with sub‐50 nm feature width and spatial resolutions is achieved through highly controlled vapour phase processing of reverse micelle templates. This allows lithographic pattern‐transfer of organic templates with a feature thickness and separation of less than 10 nm, which is otherwise not possible through other approaches reported in literature.     

Study of Surface Treatments on InAs/GaSb Superlattice LWIR Detectors

We report on the comparison of mesa sidewall profiles of InAs/GaSb strained-layer superlattice (SLS) detector structures (λ _{50% cutoff} ≈ 14 μm at V _bias = 0 V and T = 30 K) obtained after (a) a conventional BCl₃-based inductively coupled plasma etch, (b) a chemical etch (H₂O₂:HCl:H₂O, 1:1:4), and (c) a combination of both etches. We found that the smoothest sidewall profile with reasonable undercut (~5 μm) was obtained after chemical etch only. The chemical etch was optimized primarily using an n-type GaSb substrate. During this process, numerous chemical etchants were examined. GaSb n-type substrates were chosen for this study in preference over InAs substrates due to their high chemical reactivity and the complicated composition of the native oxide. In addition, SLS detectors are usually grown on GaSb substrates and, after hybridization of the focal-plane array to the readout integrated circuit, the GaSb substrate is etched away using a combination of wet and dry etching techniques. We found that H₂O₂:HCl:H₂O (1:1:4) etching solution provided the smoothest etched surface of GaSb, with a root-mean-square roughness of 1.59 nm.