Lourdes Enríquez

Atomistic analysis of defect evolution and transient enhanced diffusion in silicon

Kinetic Monte Carlo simulations are used to analyze the ripening and dissolution of small Si interstitial clusters and {113} defects, and its influence on transient enhanced diffusion of dopants in silicon. The evolution of Si interstitial defects is studied in terms of the probabilities of emitted Si interstitials being recaptured by other defects or in turn being annihilated at the surface. These two probabilities are related to the average distance among defects and their distance to the surface, respectively. During the initial stages of the defect ripening, when the defect concentration is high enough and the distance among them is small, Si interstitials are mostly exchanged among defects with a minimal loss of them to the surface. Only when defects grow to large sizes and their concentration decreases, the loss of Si interstitials through diffusion to the surface prevails, causing their dissolution. The presence of large and stable defects near the surface is also possible when the implant energy i...

Parallel Continuous-Time

A parallel multibit continuous-time (CT) DeltaSigma analog-to-digital converter for an orthogonal-frequency-division-multiplexing (OFDM) ultrawideband receiver intended to operate according to the IEEE 802.15.3a or the ECMA 368 (ISO/IEC 26907) standards has been designed. The overall CT DeltaSigma converter consists of two modulators covering two unequal subbands (low-pass (LP) and bandpass (BP) subbands) that are arranged to operate in parallel and whose respective noise transfer functions (NTFs) are designed to match its corresponding frequency band. The composite NTF for the overall converter is defined as the minimum gain value out of these two individual NTFs. The LP and BP subbands were designed by using third- and fourth-order modulators, respectively, based on a 3-bit quantizer and operating at a clock frequency of 1056 MHz. NTF zero locations were optimized according to the criterion that all the in-band composite NTF gain maxima have approximately the same value. Combining OFDM signal characteristics and converter parameters, the effect of the quantization noise on the overall converter performance has been analytically derived. A simulation program has been realized to verify the performance of the converter.

\Delta\Sigma

This paper presents a digital correction technique for wide-band multibit error-feedback (EF) digital-to-analog converters (DACs). The integral nonlinearity (INL) error of the multibit DAC is estimated (on line or off line) by a calibration analog-to-digital converter (CADC) and stored in a random-access memory table. The INL values are then used to compensate for the multibit DACs distortion by a simple digital addition. The accuracy requirements for the error estimates are derived. These requirements can be significantly relaxed when the correction is combined with data-weighted averaging (DWA). Simulation and discrete-component measurement results are presented for a fourth-order 5-bit EF DAC. The results show a 14-bit DAC operating at an oversampling ratio of 8, which is suitable for digital subscriber line applications. The correction uses simple digital circuitry and a 3-bit CADC enhanced by DWA.

ADC for OFDM UWB Receivers

In this work the design of a continuous-time @D@S modulator for Gigabit Ethernet applications is presented. The input bandwidth and oversampling ratio are, respectively, 62.5MHz and 8, resulting in a clock frequency of 1GHz. It was designed and implemented in a standard 90nm CMOS technology. The active area of the modulator measures 0.0207mm^2. It consists of a loop filter based on RC-opamp integrators and a 3-bit quantizer which includes a data weighted averaging scrambler. A digital tuning scheme to deal with process variations has also been included. System level simulations including several non-ideal effects have been carried out in order to determine in detail the performance of the converter. Experimental results show a resolution of 7.1 effective bits, and a power consumption of 10.8mW from a nominal power supply of 1V.

Nonlinearity correction for multibit /spl Delta//spl Sigma/ DACs

In this work we have applied the admittance spectroscopy technique to characterize the DX centers in AlxGa1−xAs alloys doped with silicon. Our experimental results reveal the existence of two DX centers related to silicon in AlxGa1−xAs alloys, named DX‐I and DX‐II centers, with thermal activation energies of 0.370 and 0.415 eV, respectively. These values are lower than those obtained by other authors using capacitance techniques. To explain this disagreement it should be noticed that capacitance techniques can be affected by the nonexponential behavior of the thermal emission transients of the DX centers in AlxGa1−xAs alloys.

A 1-GHz, multibit, continuous-time, delta-sigma ADC for Gigabit Ethernet

Deep‐level transient spectroscopy (DLTS) and constant‐capacitance DLTS (CC‐DLTS) techniques have been used to investigate selenium‐related DX centers in AlGaAs alloys. The value of the thermal activation energy obtained by both techniques was the same (0.21 eV); however, experimental curves show some important differences. While CC‐DLTS curves show only one peak, which reveals that there exists only one DX center in Se‐doped AlGaAs, in DLTS curves it is possible to resolve up to two peaks lying at a lower temperature than the one observed by CC‐DLTS. This disagreement may be due to the fact that DLTS measurements are strongly affected by refilling effects which occur in the edge zone of the space‐charge region during capacitance transients performed at constant voltage. These effects accelerate the capacitance transients and can lead to too high thermal‐emission rates. In contrast, these effects do not affect CC‐DLTS curves, because in constant‐capacitance voltage transients the edge of the space‐charge r...

Characterization of the DX centers in AlGaAs:Si by admittance spectroscopy

A detailed analysis of the impact of a hysteretic quantizer on a multibit, Sigma-Delta modulator has been carried out in this paper. Both discrete-time and continuous-time modulators have been considered. A qualitative modeling of the hysteretic quantizer based on a hysteretic block followed by an ideal quantizer was proposed. Due to the hysteresis effect, the quantizer output signal is delayed and distorted with respect to the quantizer input signal, where the delay causes a phase-shift independent on the signal frequency. Yet, the effect of the hysteresis depends on the input signal amplitude. This model was validated by using system-level simulations for a second order, 3-bit, discrete-time Sigma-Delta modulator. A linear model for hysteresis was derived by assuming a narrow hysteresis cycle. The quantizer input signal plays a fundamental role in the discussion. In order to include this signal into the linear analysis some approximations are proposed. The quantizer output signal is decomposed by the use of the Fourier series analysis only into the in-phase and quadrature components (with respect to the input signal) whose Fourier series coefficients can be analytically calculated. A quantitative analysis for both a second order, 3-bit, DT and CT Sigma-Delta modulators including a hysteretic quantizer was carried out. For the CT modulator, finite GBW in amplifiers, excess loop delay, and a hysteretic quantizer were considered separately and combined. A good agreement with both system-level simulations and experimental results is found, despite the approximations considered for the quantizer input signal.

Influence of refilling effects on deep-level transient spectroscopy measurements in Se-doped AlxGa1-xAs

In this work, an approach for the simulation of the effect of noise sources in the performance of continuous‐time ΔΣ modulators is presented. Electrical noise including thermal noise, 1/f noise and clock jitter are included in a simulation program and their impact on the system performance is analyzed.

Detailed analysis of the effect of a hysteretic quantizer in a multibit, Sigma-Delta modulator

In this article a current-mode, second-order, CT Sigma-Delta modulator operating at a sampling frequency of 25 MHz and with an oversampling ratio of 64 has been designed. The modulator consists of current-mode, differential integrators based on a single-stage folded cascode topology and switched capacitors DACs cells generating exponential feedback waveforms. The modulator was analysed in detail from both a system- and a circuit-level point of view. A ‘short’ exponential DAC pulse, namely, an exponential pulse whose time constant is lower than the pulse width, was used to improve jitter rejection. Modulator feedback coefficients were analytically derived for an exponential feedback waveform. Jitter considerations paying special attention to its impact on CT Sigma-Delta modulators were presented, and two different contributions distinguished: independent jitter and accumulated jitter. Finally, functional- and transistor-level simulations have been accomplished to obtain significant features of the modulator performance.

Noise Simulation of Continuous‐Time ΣΔ Modulators

A novel DFT-based Reaction Kinetics (DFT-RK) simulation approach, employed in combination with real-time data from reaction monitoring instrumentation (like UV-vis, FTIR, Raman, and 2D NMR benchtop spectrometers), is shown to provide a detailed methodology for the analysis and design of complex synthetic chemistry schemes. As an example, it is applied to the opening of epoxides by titanocene in THF, a catalytic system with abundant experimental data available. Through a DFT-RK analysis of real-time IR data, we have developed a comprehensive mechanistic model that opens new perspectives to understand previous experiments. Although derived specifically from the opening of epoxides, the prediction capabilities of the model, built on elementary reactions, together with its practical side (reaction kinetics simulations of real experimental conditions) make it a useful simulation tool for the design of new experiments, as well as for the conception and development of improved versions of the reagents. From the perspective of the methodology employed, because both the computational (DFT-RK) and the experimental (spectroscopic data) components can follow the time evolution of several species simultaneously, it is expected to provide a helpful tool for the study of complex systems in synthetic chemistry.