Metalorganic Chemical Vapor Deposition of CdTe(133) Epilayers on Si(211) Substrates

Single-crystalline CdTe(133) films have been grown by metalorganic chemical vapor deposition on Si(211) substrates. We studied the effect of various growth parameters on the surface morphology and structural quality of CdTe films. Proper oxide removal from the Si substrate is considered to be the principal factor that influences both the morphology and epitaxial quality of the CdTe films. In order to obtain single-crystalline CdTe(133) films, a two-stage growth method was used, i.e., a low-temperature buffer layer step and a high- temperature growth step. Even when the low-temperature buffer layer shows polycrystalline structure, the overgrown layer shows single-crystalline structure. During the subsequent high-temperature growth, two-dimensional crystallites grow faster than other, randomly distributed crystallites in the buffer layer. This is because the capturing of adatoms by steps occurs more easily due to increased adatom mobility. From the viewpoint of crystallographic orientation, it is assumed that the surface structure of Si(211) consists of (111) terrace and (100) step planes with an interplanar angle of 54.8°. This surface structure may provide many preferable nucleation sites for adatoms compared with nominally flat Si(100) or (111) surfaces. The surface morphology of the resulting films shows macroscopic rectangular-shaped terrace—step structures that are considered to be a (111) terrace with two {112} step planes directed toward 〈110〉.     

Amorphous Ta-nanocrystalline RuOx diffusion barrier for lower electrode of high density memory devices

The effects of the amount of RuO₂ added in the Ta film on the electrical properties of a Ta-RuO₂ diffusion barrier were investigated using n⁺⁺-poly-Si substrate at a temperature range of 650–800°C. For the Ta layer prepared without RuO₂ addition, Ta₂O₅ phase formed after annealing at 650°C by reaction between Ta and external oxygen, leading to a higher total resistance and a non-linear I-V curve. Meanwhile, in the case of the Ta film being deposited with RuO₂ incorporation, not only a lower total resistance and ohmic characteristics exhibited, but also the bottom electrode structure was retained up to 800°C, attributing to the formation of a conductive RuO₂ crystalline phase in the barrier film by reaction with the indiffused oxygen because of a Ta amorphous structure formed by chemially strong Ta-O or Ta-Ru-O bonds and a large amount of conductive RuO₂ added. Since a kinetic barrier for nucleation in formation of the crystalline Ta₂O₅ phase from an amorphous Ta(O) phase is much higher than that of crystalline RuO₂ phase from nanocrystalline RuO_x phase, the formation of the RuO₂ phase by reaction between the indiffused oxygen and the RuO_x nanocrystallites is kinetically more favorable than that of Ta₂O₅ phase.     

Performance of 2-D RAKE receiver in a correlatedfrequency-selective Nakagami-fading

The bit error rate (BER) performance and the characteristics of a two-dimensional (2-D) RAKE receiver operating in a correlated frequency-selective Nakagami-fading environment are analyzed. Correlated fading between array elements whose fading statistics are identical across the same RAKE branch, as well as an arbitrary number of RAKE-branches with arbitrary finding statistics, are assumed. We derived an approximated signal-to-noise ratio (SNR) statistics for one RAKE branch with correlated multiple antennas, which is extended to that for multiple RAKE branches with arbitrary fading statistics, i.e., a 2-D RAKE receiver. The receiver's performance and characteristics are analyzed using the cumulative distribution function of the SNR at the 2-D RAKE receiver output and the BER under various conditions, Numerical results show that the improvement In performance of the 2-D RAKE receiver is brought about by the average SNR and diversity gains, which are identified by two parameters specifying the gamma distribution of SNR     

Experimental demonstration of enhancing pump absorption rate in cladding-pumped ytterbium-doped fiber lasers using pump-coupling long-period fiber gratings

A long-period fiber grating (LPFG) was inscribed in a single-mode fiber and was spliced with a ytterbium (Yb)-doped double-clad fiber in order to couple pump radiation of the inner cladding into the core in a cladding-pumped fiber laser. The use of an LPFG permits a partial core-pumping scheme in a cladding-pumping fiber laser because a portion of the pump radiation can be coupled to the core by LPFG. The enhancement of the pump absorption of a Yb-doped cladding-pumped fiber laser as the result of pump coupling by LPFG was 35%, and the maximal output power increased by up to 55% when a 20-W pump source is used.     

High average power, high repetition rate, picosecond pulsed fiber master oscillator power amplifier source seeded by a gain-switched laser diode at 1060 nm

We demonstrate a pulsed ytterbium-doped fiber master-oscillator power amplifier source at 1060 nm producing over 300 W of average power in 20-ps pulses at 1-GHz repetition rate. The pulses generated by a gain-switched diode were compressed by a chirped fiber Bragg grating and amplified without any distortion with excellent spectral quality. This fiber master oscillator power amplifier system offers versatility and potential for further power scaling.     

Near‐Optimum Blind Decision Feedback Equalization for ATSC Digital Television Receivers

This paper presents a near‐optimum blind decision feedback equalizer (DFE) for the receivers of Advanced Television Systems Committee (ATSC) digital television. By adopting a modified trellis decoder (MTD) with a trace‐ back depth of 1 for the decision device in the DFE, we obtain a hardware‐efficient, blind DFE approaching the performance of an optimum DFE which has no error propagation. In the MTD, the absolute distance is used rather than the squared Euclidean distance for the computation of the branch metrics. This results in a reduction of the computational complexity over the original trellis decoding scheme. Compared to the conventional slicer, the MTD shows an outstanding performance improvement in decision error probability and is comparable to the original trellis decoder using the Euclidean distance. Reducing error propagation by use of the MTD in the DFE leads to the improvement of convergence performance in terms of convergence speed and residual error. Simulation results show that the proposed blind DFE performs much better than the blind DFE with the slicer, and the difference is prominent at the trellis decoder following the blind DFE.     

Computation sharing programmable FIR filter for low-power and high-performance applications

This paper presents a programmable digital finite-impulse response (FIR) filter for high-performance and low-power applications. The architecture is based on a computation sharing multiplier (CSHM) which specifically targets computation re-use in vector-scalar products and can be effectively used in the low-complexity programmable FIR filter design. Efficient circuit-level techniques, namely a new carry-select adder and conditional capture flip-flop (CCFF), are also used to further improve power and performance. A 10-tap programmable FIR filter was implemented and fabricated in CMOS 0.25-/spl mu/m technology based on the proposed architectural and circuit-level techniques. The chip's core contains approximately 130 K transistors and occupies 9.93 mm/sup 2/ area.     

Environmental effects on the electrical behavior of pentacene thin-film transistors with a poly(methyl methacrylate) gate insulator

The electrical properties of top-contact pentacene thin-film transistors (TFTs) with a poly(methyl methacrylate) (PMMA) gate dielectric were analyzed in air and vacuum environments. Compared to the vacuum case, the pentacene TFT in air exhibited lower drain currents and more pronounced shifts in the threshold voltage upon reversal of the gate voltage sweep direction, together with a decrease in the field-effect mobility. These characteristic variations were explained in terms of two distinctive actions of polar H₂O molecules in pentacene TFT. H₂O molecules were suggested to diffuse under the source and drain contacts and interrupt the charge injection into the pentacene film, whereas those that permeate at the pentacene/PMMA interface retard hole depletion in and around the TFT channel. The diffusion process was much slower than the permeation process. The degraded TFT characteristics in air could be recovered mostly by storing the device under vacuum, which suggests that the air instability of TFTs is due mainly to the physical adsorption of H₂O molecules within the pentacene film.     

Consideration of the Actual Current-Spreading Length of GaN-Based Light-Emitting Diodes for High-Efficiency Design

Based on the proposed experimental method, the current spreading length of GaN-based light-emitting diodes (LEDs) was measured and analyzed for practical device design. In this study, Thompson's and Guo's models, which are categorized according to vertical series resistance (in particular, p-type contact resistance), were used to extract device parameters. It was shown that the measured current spreading length strongly depends on the injected current density. For LEDs fabricated with low-resistance p-type contacts, this behavior could be explained in terms of the accelerated current crowding with higher current densities occurring as a result of the reduced voltage drop across the junction, which is in good agreement with Thompson's relation. However, for LEDs fabricated with high-resistance p-contacts, unlike Guo's prediction, the measured current spreading length also showed a strong dependence on the injected current density. This was attributed to thermal heating at the p-contact, resulting in the reduction of the voltage drop across the p-contact and so junction voltage, which is also in agreement with Thompson's model. Based on the measured parameters and the design rule, efficient p-type reflectors, namely, hybrid reflectors were designed. Compared with conventional ones, LEDs fabricated with the hybrid reflectors exhibited better output power at a reasonable forward voltage, indicating that the proposed method is effective in understanding the actual current spreading and hence the practical design of high-efficiency LEDs.     

Transmission of Traffic Information Using a Terrestrial Digital Multimedia Broadcasting System

This letter introduces an efficient transmission of traffic information through a terrestrial digital multimedia broadcasting system, which is a multimedia and mobility empowered option of the European digital audio broadcasting system. By adapting Korean characteristic traffic information into the transport protocol expert group messages in the traffic information delivery, a highly efficient traffic information system was implemented and tested in Korea.