Correlation between Fe–Zn Interdiffusion Observed by Scanning Capacitance Microscopy and Device Characteristics of Electro-Absorption Modulators

Zn diffusion into the multi quantum well (MQW) absorption layer of semi-insulating InP-buried electroabsorption (EA) modulators is investigated by two methods: One is a novel experimental method in which a planer-structured test sample is used to evaluate the diffusion. The other is a simulation of the extinction characteristics of Zn-diffused MQW absorption layers at various the Zn concentrations. The Zn diffusion experiment shows that Zn diffusion is suppressed by the use of Ru-InP burying layers. Comparing device characteristics to the simulation results shows that Zn diffusion into the absorption layer is effectively suppressed in a Ru-doped InP-buried EA modulator so excellent extinction characteristics can be achieved. These results indicate that Ru-doped InP-buried EA modulators are superior to Fe-doped InP-buried ones.

In order to demonstrate its unique capabilities, a combined microscope comprised of a scanning capacitance microscope and a contact mode atomic force microscope was used to image a buried structure. The atomic force microscopy images surface topography, while the scanning capacitance microscope simultaneously images buried structure. By applying a modulation technique, the voltage derivative of the capacitance (dC/dV) was used as a scanning capacitance microscope signal. The sample used for the demonstration was a silicon wafer with metal lines 4 nm in thickness covered by a 40-nm-thick layer of silicon oxide. The scanning capacitance microscope image was dependent on the bias voltage applied to the probe and the sample. Since the scanning capacitance microscope signal above the metal lines is zero, the bias voltage dependency of the contrast in scanning capacitance microscope images corresponds directly to the scanning capacitance microscope signal versus V at locations where the metal line is not buried. However, the bias dependency of the contrast did not agree with the scanning capacitance microscope signal observed between metal lines. A possible mechanism for the discrepancy between these bias dependencies is discussed based upon charge deposition on the sample surface caused by tip contact and lateral movement of the charges in the adsorbed water on the sample surface.

Scanning capacitance microscopy (SCM) is a powerful tool for two-dimensional (2D) dopant/carrier profiling. Currently noise limits the accuracy of 2D dopant profiles obtained by SCM. In an effort to reduce noise, a systematic analysis of different SCM noise sources is provided. The main noise sources during SCM measurements are capacitance sensor noise and oxide–semiconductor surface induced noise. For adequate tip size, the dominant noise in SCM measurements is caused by variations in the quality of surface. On as-polished surfaces, nonstationary noise is observed. This noise is likely caused by the variations in the density of oxide traps. Tip induced charging of these traps and local variations or fluctuations in discharge time during SCM imaging cause the noise level and noise pattern to be different from image to image. Heat treatment under ultraviolet irradiation or in a hydrogen ambient is found to be an effective way to reduce or even eliminate this type of SCM noise. Stationary surface noise is mostly created by the variations in the oxide thickness. This type of noise correlates with topographic roughness and is very consistent during SCM measurements. By reducing the topographic roughness, the stationary surface noise may be reduced to the level of ∼10−2 of the depletion SCM signal for typical experimental conditions. It is shown that the capacitance sensor noise depends on the capacitance sensor tuning parameters and under proper conditions can be reduced to a negligible level for standard probe tips used in SCM measurements.

Scanning capacitance microscope (SCM) images, and the two-dimensional (2D) dopant profiles extracted from them, show poor reproducibility from laboratory to laboratory. Major factors contributing to SCM image variability include: poor sample surface and oxide quality, excess carrier generation from stray light, reduced sensor dynamic range from stray capacitance, and use of nonoptimal SCM operating voltages. This article discusses the sources of SCM image variability, how they affect the measured SCM images, and possible approaches for mitigating their effects. Recommended procedures for extracting quantitative 2D are discussed. Finally, a set of informal research materials is introduced consisting of a complementary metal-oxide-semiconductor transistor pair, an identical pair without metallization, and a pair of transistor-like structures with the conductivity type of the source/drains reversed. These structures are intended for use with the FASTC2D software to help improve laboratory-to-laboratory dopant profile reproducibility.

Scanning capacitance microscopy (SCM) has been applied to microscopic characterization of electrical properties of silicon-on-insulator (SOI) wafers. Two kinds of capacitance-voltage (C-V) methods have been proposed for separately characterizing the electrical properties of a gate oxide, an SOI layer, a buried oxide (BOX) layer. a Si substrate, and their interfaces: (i) a front-gate C-V method whereby the electrical properties of the gate oxide and front SOI (the gate oxide/SOI) interface can be characterized. arid (ii) a back-gate C-V method for the characterization of the electrical properties of the BOX layer. back SOI (the BOX/SOI) interface, and the BOX/Si substrate interface. Furthermore, SCM images of the sampled SOI wafer have been obtained for visualizing the microscopic spatial distribution of electrical properties of SOI wafers by using the proposed C-V methods. These SCM images revealed the fluctuation in tile oxide charges arid interface traps. SCM has been demonstrated to be an effective tool for microscopic electrical characterization of SOI wafers.

The input and output waveguides are integrated with a multiple-quantum-well (MQW) electro-absorption (EA) modulator to achieve ultra-high-speed modulation. This MQW- EA modulator with integrated waveguides has a larger modulation bandwidth due to a shorter modulation region, while maintaining a device length long enough for fabrication and packaging. An optimized device had a large modulation bandwidth of 50 GHz, a low driving voltage of less than 3 V, and a low fiber-to-fiber insertion loss of 8 dB, which are enough for 40-Gbit/s modulation. The advantage of packaging the modulator with integrated waveguides is also demonstrated: an assembled prototype module showed a large modulation bandwidth of more than 40 GHz. These results demonstrate that the MQW-EA modulator with integrated waveguides is advantageous in terms of ultra- high-speed operation as well as packaging.

We report scanning capacitance microscopy (SCM) images of a working p-channel metal–oxide–semiconductor field-effect transistor (P-MOSFET) during device operation. Independent bias voltages were applied to the source/gate/drain/well regions of the MOSFET during SCM imaging, and the effect of these voltages on the SCM images is discussed.

The accuracy with which two-dimensional carrier profiles can be extracted from scanning capacitance microscopy (SCM) images of doped structures in silicon depends on the model used to interpret the SCM differential capacitance data. This work validates models of the SCM by comparing the calculated SCM signal to the measured signal for a variety of sample parameters, measurement conditions, and capacitance sensors from different manufacturers. The magnitude of the capacitance sensor high-frequency voltage is measured and its effect on ΔC–V curves and extracted carrier profiles is quantified. Two nonidealities commonly observed in SCM signals, U-shaped C–V curves and double zero crossing in the SCM signal at the p–n junctions, are related to the measurement parameters and explained.

Chapter 7 - Electroabsorption Modulators

A Soliton transmission system is a promising candidate as a future ultra-long distance and large capacity optical fiber transmission system. Major limiting factor of soliton systems is the timing jitter due to Gordon-Haus effect[1]-[3]. Although the soliton control techniques to reduce the timing jitter both in frequency domain[4]-[8] and time domain[9] have been intensively studied, it is important to demonstrate experimentally the Gordon-Haus limit for the most basic soliton transmission system[10],[11], which has not clearly been shown yet because the maximum transmission length is sometimes affected by other experimental factors such as the initial chirp of the pulse source[6] and/or insufficient pulse separation[7]. Previously, we have proposed a simple technique to generate a nearly transform-limited optical short pulse with variable repetition rate and variable pulse width by using a sinusoidally driven InGaAsP electroabsorption (EA) modulator[12]. In this paper, we investigate and demonstrate an experimental maximum transmission length at 5Gbit/s for the most simple soliton transmission system by using optical short pulses generated by the EA modulator.

Performance improvement of 1.3 μm InAlGaAs MQW modulators grown by MOVPE using C-doped InAl(Ga)As cladding layers

This paper explores the use of secondary electron (SE) doping contrast in scanning electron microscopy (SEM) for visualizing and quantifying doping in silicon carbide (SiC), a vital wide bandgap semiconductor for power electronics. Traditional methods, such as scanning spreading resistance microscopy and scanning capacitance microscopy, face challenges with SiC due to its high hardness. We propose a theoretical model based on potential differences between n- and p-doped areas, differing from the conventional surface band bending theory. The experimental data align well with our model, revealing insights into SE yield variations due to doping concentrations. The study also examines surface patch fields above p–n junctions and the effect of sample bias on SE yield, differentiating the impacts of surface band-bending and external patch fields. Our findings show strong agreement between theory and experiment, explaining contrast characteristics in energy-filtered secondary electron images. This work enhances the understanding of doping contrast mechanisms in SEM, enabling the development of a quantitative dopant profiling method for SiC.

Scanning capacitance microscopy (SCM) using a dV/dC signal has been proposed for local mapping of interface traps in an insulator/semiconductor structure. In conventional SCM measurements using a dC/dV signal, it is difficult to accurately analyze the spatial distribution of interface traps from the obtained SCM images because the dC/dV signal is inversely proportional to the interface trap density. In the proposed technique, however, the spatial distribution of the interface traps can be characterized using the dV/dC signal, which is directly proportional to the interface trap density. The effectiveness of the proposed technique has been demonstrated by characterizing the spatial distributions of the interface traps in HfSiO/Si structures before and after H2 annealing. The dV/dC images obtained reveal that the spatial variation in the interface trap density is reduced by H2 annealing. Consequently, SCM measurements using a dV/dC signal have been verified to effectively characterize the spatial distribution of interface traps.

The fabrication process and characteristics, including 5-Gb/s transmission, of an integrated light source consisting of a lambda /4-shifted distributed feedback laser and an electroabsorption (EA) modulator are discussed. By introducing a semi-insulating (SI) InP on the butt-joint region, both the large electrical isolation resistance (>10 M Omega ) between the laser and the modulator and a high optical coupling efficiency (>80%) between them are achieved. A typical threshold current was 50-70 mA, and single-mode operation at 1.576 mu m was maintained up to 5.5-mW output power. The modulation voltage to swing between 90% transmission and 10% transmission was 6.2-12 V, depending on the modulator length in the range 1000-400 mu m. The 3-dB bandwidth is 7.7 GHz and a linewidth enhancement factor ( alpha ) of 0.9 is estimated from the sideband-to-carrier ratio of the spectra.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An ultra-compact electro-absorption (EA) modulator operating around 1.55-μm telecom wavelengths is proposed and theoretically investigated. The modulator is comprised of a stack of TiN/HfO2</ITO/Cu conformally deposited on a single-mode stripe waveguide to form a hybrid plasmonic waveguide (HPW). Since the thin ITO layer can behave as a semiconductor, the stack itself forms a MOS capacitor. A voltage is applied between the Cu and TiN layers to change the electron concentration of ITO (NITO), which in turn changes its permittivity as well as the propagation loss of HPW. For a HPW comprised of a Cu/3-nm-ITO/5-nm-HfO2/5-nm-TiN stack on a 400-nm × 340-nm-Si stripe waveguide, the propagation loss for the 1.55-μm TE (TM) mode increases from 1.6 (1.4) to 23.2 (23.9) dB/μm when the average NITO in the 3-nm ITO layer increases from 2 × 10(20) to 7 × 10(20) cm(-3), which is achieved by varying the voltage from -2 to 4 V if the initial NITO is 3.5 × 10(20) cm(-3). As a result, a 1-μm-long EA modulator inserted in the 400-nm × 340-nm-Si stripe waveguide exhibits insertion loss of 2.9 (3.2) dB and modulation depth of 19.9 (15.2) dB for the TE (TM) mode. The modulation speed is ~11 GHz, limited by the RC delay, and the energy consumption is ~0.4 pJ/bit. The stack can also be deposited on a low-index-contrast waveguide such as Si3N4. For example, a 4-μm-long EA modulator inserted in an 800-nm × 600-nm-Si3N4 stripe waveguide exhibits insertion loss of 6.3 (3.5) dB and modulation depth of 16.5 (15.8) dB for the TE (TM) mode. The influences of the ITO, TiN, HfO2 layers and the beneath dielectric core, as well as the processing tolerance, on the performance of the proposed EA modulator are systematically investigated.