InGaAsP/InP 1.3-µm wavelength surface-emitting LED's for high-speed short-haul optical communication systems

Performance and reliability for InGaAsP/InP 1.3-µm wavelength high-speed surface-emitting DH light emitting diodes (LED's) have been investigated. High-speed and high-radiance performances were obtained by the optimal design of both structural parameters and LED driving circuit. Rise and fall times were both 350 ps and peak optical power coupled to a 50-µm core 0.20 NA graded-index fiber at the 100-mA pulse current was - 15.8 dBm with 6-dB optical ON/OFF ratio. A 2-Gbit/s non-return-to-zero (NRZ) pulse transmission over a 500-m span was carried out, Feasibility of using surface-emitting LED's in a high-speed optical communication system has been confirmed. Accelerated aging tests on high-speed LED's were carried out. The half-power lifetimes have been estimated to be more than 1 × 10⁸h at 50°C ambient temperature.     

A study of far-field patterns from high performance 1.3-µm InGaAsP-InP edge-emitting LED's

Liquid-phase epitaxy InGaAsP-InP 1.3-µm edge-emitting LED's are fabricated with a very simple Schottky-delineated stripe structure. With a stripe 50 µm wide and 200-250 µm long, typical characteristics of these devices include 170 µW of optical power coupled into a 60-µm core 0.2-NA graded index fiber, 600-Å spectral halfwidths, 2-5-ns risetimes. Unlike GaAs-GaAlAs edge-emitting LED's, best results of coupling efficiency are obtained with InGaAsP-InP LED's whose active layer thickness is in the range 0.12-0.15 µm, due to asymmetry in the far-field patterns. Our study of these far-field patterns shows that this asymmetry is governed by the quality of the active layer material located near the InGaAsP-nInP hetero-interface.     

GaInAsP/InP fast, high-radiance, 1.05-1.3-µm wavelength LED's with efficient lens coupling to small numerical aperture Silica optical fibers

Power levels up to 100 µW have been launched from GaInAsP LED's with 14-µm-diameter emitting regions into low-loss small numerical aperture (NA) silica fibers at a dc drive level of only 25 mA. A maximum launch power of 206 µW at 100-mA dc was obtained from slightly larger devices. The high coupling efficiency was achieved using truncated spheres of Ti2O3:SiO2glass as microlenses. Gains over the butt coupled case exceeded a factor of twelve for the small-area devices. The high operating current densities (2-20 kA/cm²) for the small-area devices resulted in modulation bandwidths extending to beyond 300 MHz (-3 dB optical). The surface-emitting LED's showed an enhanced performance over edge-emitting LED's fabricated from similar material. Linewidths of the devices, which were prepared by liquid-phase epitaxy with step followed by ramp cooling, were approximately 3 kT. Even with the relatively broad linewidth, material dispersion limits in silica fibers exceeding 1 GHz . km around 1.3 µm are predicted. These devices are suitable for long-haul, wide-bandwidth fiber links operating in the 1.3-µm window.     

Junction-current-confinement planar light-emitting diodes and optical coupling into large-core diameter fibers using lenses

High-radiance AlGaAs-GaAs double-heterostructure light-emitting diodes utilizing junction current confinement are described. Diode resistance and junction ideality factor are investigated as a function of emission diameters from 10 to 75 µm. Near-field intensity profiles indicate tight current confinement over the full range of emission diameters. Rise-time measurements are consistent with a simple carrier lifetime model for >25-µm emission diameters. An effective radiative-recombination constant, B = 1.5(±0.5) × 10-¹⁰cm³/s is deduced from the rise-time data and model. Peak wavelength and spectral width data are discussed in terms of junction current density and temperature. With decreasing emission diameter, the optical coupling efficiencies into 100- and 200-µm core diam high-numerical-aperture fibers increased from 10 to 25 percent and 25 to 50 percent, respectivley, using spherical glass lenses.     

High speed response InGaAsP/InP DH LED's in the 1 µm wavelength region

The frequency response of a fiber-coupled LED with a small diameter contact has been studied. It has been shown that the current spreading occurring at the contact edge causes an appreciable response speed reduction. However, this was found to be eliminated by a monolithic lens formed on the LED surface, which can make only the light generated at the center of the contact coupled into a fiber. This technique was successfully applied to fabricate high speed InGaAsP/ InP DH LED's at wavelengths of 1.15 µm, 1.3 µm, and 1.5 µm. The 1.3 µm LED's exhibited the -1.5 dB cutoff frequency of 80 MHz, typically at 100 mA current without reduction of the coupled power.     

Two-layer resist structure for electron-beam fabrication of a submicrometer gate length GaAs device

A two-layer resist structure using EBR-9 and PMMA for fabricating a fine metal line with a mushroom-like cross-sectional profile is reported. The structure provides T-shaped resist cavities with undercut profiles using electron-beam exposure. With the optimum developing condition, the bottom opening is as small as 0.1 µm, and the top opening is wide enough not to require an additional exposure in order to obtain a mushroom-like metal lift-off pattern. A Monte Carlo calculation is carried out in order to analyze the profile of the two-layer resist structure, and it is shown that an undercut T-shaped resist profile with a 0.1-µm bottom opening can be obtained using a high-sensitivity resist on a low-sensitivity resist structure. A 0.15-µn mushroom-like lift-off metal profile has been fabricated on a 0.1-µm recessed GaAs substrate by the use of this resist structure.     

Silicon-on-insulator bipolar transistors

Thin-film lateral n-p-n bipolar transistors (BJT) have been fabricated in moving melt zone recrystallized silicon on a 0.5-µm silicon dioxide substrate thermally grown on bulk silicon. Current-voltage characteristics of devices with different base widths (5 and 10 µm) have been analyzed. The use of a metal gate over oxide covering the base region has allowed the devices to be operated as n-channel MOSFET's as well thus surface effects on device characteristics have been investigated under varying gate-bias voltages. Maximum dc current gain values of 2.5 were achieved with a 5-µm base width and values around 0.5 with a 10-µm base width. Higher gain values were impeded by onset of high-level injection which occurred at low currents because of light base doping of these devices.     

Multimode-fiber technology for digital transmission

Multimode-fiber systems are presently being installed to meet some of the burgeoning demands for digital transmission in the telecommunications industry. These first-generation systems operate near 0.85-µm wavelength with laser transmitters and avalanche-photodiode receivers. Second-generation multimode systems may use simpler and more reliable LED's and p-i-n photodiodes operating near 1.3 µm, where fibers exhibit much lower loss and dispersion. This paper summarizes the state of the art of multimode-fiber digital transmission with special emphasis on emerging technologies for operation in the 1.1-1.7- µm wavelength region. Graded-index multimode fibers, lasers, LED's, photodetectors, receiver sensitivities, and noise penalties are considered. Finally, some of the requirements and challenges in applying these technologies are discussed.     

Comparison of surface- and edge-emitting LED's for use in fiber-optical communications

The performance of state-of-the-art double-heterojunction (DH) surface and edge emitters are compared with respect to their use in high-data-rate fiber-optical communication systems. Thick-window (20-25-µm) surface emitters with 2-2.5-µm thick active layers and emitting up to 15-mW optical power at 300 mA have been fabricated. For edge emitters, we use very-high-radiance-type devices with ≃ 500-Å thick active layers. For these two types of LED's we examine differences in structure and light coupling efficiency to fibers of various numerical apertures (NA). For typically good devices we compare the diodes' output power capabilities, the powers coupled into step- and graded-index fibers of various NA, and their respective frequency response. For the same drive current level, we find that edge emitters couple more power than surface emitters into fibers with NAlsim 0.3. The edge emitters also have ≈ 5 times larger bandwidths. We estimate that an edge emitter can couple 5-6 times more power into low numerical aperture (NAlsim 0.2) fibers than a surface emitter of the same bandwidth. We conclude that edge emitters are preferred to surface emitters for optical data rates above 20 Mbits/s.     

A milky tinoxide on glass (MTG) substrate thin undoped layer p-i-n amorphous silicon solar cell with improved stability and relatively high efficiency

It has been clarified that p-i-n type a-Si:H solar cells fabricated on large-grain SnO2on glass (milky tin oxide on glass (MTG)) substrates with thin (∼ 0.2-µm) undoped (i-) layers have comparable high conversion efficiencies (η > 7 percent) and higher stability against the prolonged illumination of intense light when compared with thicker (∼ 0.5m-µm) i-layer cells with conventional transparent conductive oxide. The observed high stability was explained qualitatively by the dependence of the recombination rate of photogenerated (PG) carriers in undoped a-Si:H on the i-layer thickness.     

High-speed high-power 1.3-µm InGaAsP/InP surface-emitting LED's for short-haul wide-bandwidth optical-fiber communications

High-speed high-power double-heterostructure 1.3-µm InGaAsP/InP LED's have been developed for use in short-haul wide-bandwidth fiber-optics systems. At 150-mA dc, devices typically launch - 11.7 dBm into a 62-µm core graded index (GI) fiber. Optical bandwidth is typically 690 MHz with a 50-mA prebias. Modulation capability was demonstrated at 1-Gbit/s NRZ with - 15.7 dBm of peak power launched into the fiber. Rise and fall times of 340 and 780 ps, respectively, were achieved. Reliability data indicates a median life of ∼ 10⁸h for anticipated operating conditions.     

Recent advances in the performance and reliability of InGaAsP LED's for lightwave communication systems

High radiance LED's are being exploited in lightwave applications where their low cost, wide temperature range of operation, and outstanding reliability outweigh the power and modulation advantages of injection laser diodes. Development activity and commercial offerings of LED's have been rapidly moving toward InGaAsP LED's to take advantage of the low attenuation and chromatic dispersion minimum at the 1.3-µm emission wavelength. This paper reviews recent advances in the performance, reliability, and system application of InGaAsP LED's.     

A dc and microwave comparison of GaAs MESFET's on GaAs and Si substrates

In order to assess GaAs on Si technology, we have made a performance comparison of GaAs MESFET's grown and fabricated on Si and GaAs substrates under identical conditions and report the first microwave results. The GaAs MESFET's on Si with 1.2-µm gate length (290-µm width) exhibited transconductances (gm) of 180 mS/mm with good saturation and pinchoff whereas their counterparts on GaAs substrates exhibited gmof 170 mS/mm. A current gain cut-off frequency of 13.5 GHz was obtained, which compares with 12.9 GHz observed in similar-geometry GaAs MESFET's on GaAs substrates. The other circuit parameters determined from S-parameter measurements up to 18 GHz showed that whether the substrate is Si or GaAs does not seem to make a difference. Additionally, the microwave performance of these devices was about the same as that obtained in devices with identical geometry fabricated at Tektronix on GaAs substrates. The side-gating effect has also been measured in both types of devices with less than 10-percent decrease in drain current when 5 V is applied to a pad situated 5 µm away from the source. The magnitude of the sidegating effect was identical to within experimental determination for all side-gate biases in the studied range of 0 to -5 V. The light sensitivity of this effect was also very small with a change in drain current of less that 1 percent between dark and light conditions for a side gate bias of -5 V and a spacing of 5 µm. Carrier saturation velocity depth profiles showed that for both MESFET's on GaAs and Si substrates, the velocity was constant at 1.5 × 10⁷cm/s to within 100-150 Å of the active layer-buffer layer interface.     

A fast-transient high-PSR output-capacitor-free low-dropout regulator for low-power embedded systems

A low-power fast-transient output-capacitor-free low-dropout regulator (LDO) with high power-supply rejection (PSR) is presented in this paper. The proposed LDO utilizes a non-symmetrical class-AB amplifier as the input stage to improve the transient performances. Meanwhile, PSR enhancement circuit, which only consumes 0.2-µA quiescent current at light load, is utilized to form a feedforward cancellation path for improving PSR over wide frequency range. The LDO has been designed and simulated in a mixed signal 0.13-µm CMOS process. From the post simulation results, the LDO is capable of delivering 100-mA output current at 0.2-V dropout voltage, with 3.8-µA quiescent current at light load. The undershoot, the overshoot and the 1 % settling time of the proposed LDO with load current switching from 50 µA to 100 mA in 300 ns are about 100 mV, 100 mV and 1 µs, respectively. With the help of proposed PSR enhancement technique, the LDO achieves a PSR of ?69 dB at 100 kHz frequency for a 100-mA load current.     

Flattening of dynamic dielectric phase grating and single-mode lasing under the conditions of transverse oscillations of luminous flux

Emission spectra and the intensity patterns of InAsSbP/InAsSb/InAsSbP-based diode lasers with different cavity lengths and a spectral range of 3–4 μm were studied. It is ascertained experimentally that a 200-to 300-μm-long and 18-μm-wide laser cavity ensures single-mode lasing, during which the wavelengthdecreases with current by about 100 ? and the top of the intensity pattern becomes bimodal, thus indicating that there are transverse spatial oscillations of laser flux in the cavity. In a 300-to 500-μm-long cavity, several tens of modes are generated mainly on the long-wavelength side of the mode that prevailed initially at the lasing threshold and spatial oscillations of laser flux are not observed. Single-mode lasing is attained due to the fact that the transverse oscillations of laser flux flatten the dynamic dielectric phase grating, which is usually produced by the interaction between lasing modes, and prevent an increase in amplification on the long-wavelength side of the dominant mode. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 11, 2002, pp. 1388–1392. Original Russian Text Copyright ? 2002 by Astakhova, Danilova, Imenkov, Kolchanova, Yakovlev.     

On the reliability of 1.3-µm InGaAsP/InP edge-emitting LED́s for optical-fiber communication

This study of the reliability of 1.3-μm double heterojunction edge-emitting LED's indicates that edge-emitting LED's mounted with Au0.8Sn0.2solder have an activation energy of 0.9 eV for degradation and extrapolated lifetimes of2 times 10^{8}h at room temperature. A study of 1.3-μm LED's grown by LPE and VPE show them to be comparable in operating life. The temperature dependence of the light output (P) of the edge-emitting LED's is given byP alpha exp(-Delta T/75K). The study also showed that lattice mismatch up to 0.31 percent at the InGaAsP/InP heterojunction does not effect reliability.     

Optimization of the composition of a new MRS-type negative resist

Sensitizer concentration is optimized for a new negative photoresist, MRL (Micro Resist for Longer wavelengths) with the assistance of computer simulation. The resist, which has photosensitivity in the ordinary UV region, resembles a deep UV resist MRS in terms of light absorption characteristic. It is found that a photosensitizer concentration of 20 wt% (based on the resin) is suitable for a reduction projection exposure system that utilizes UV light at 365 nm. A steep profile resist image of 0.7-µm lines and 0.7-µm spaces in a 1.0-µm thick resist layer is obtained using the MRL of optimized composition and the exposure system.     

InGaPAs-InP double-heterojunction high-radiance LED's

InGaPAs-InP double-heterojunction (DH) high-radiance LED's (λ ∼ 1.05-1.3 µm) have been fabricated by liquid-phase epitaxy (LPE) at constant temperature. The crystal growth procedure is described and the influence of InP substrate crystalline perfection is discussed. LED's with a high-radiance geometry suitable for coupling to an optical fiber have been fabricated. The four-layer double-heterostructure LED's have low forward-biased resistances. Typical external quantum efficiencies of ∼1.5 percent and narrow emission linewidths (∼56 nm, typical), have been measured for LED's (λ ∼ 1.08 µm) with an InGaPAs active layer thickness of 1.6 µm and an active layer carrier concentration ofN_{A} - N_{D} approx 2.8 \× 10^{16}cm^-3. The dependence of LED emission linewidth upon active layer doping is reported. Transient measurements show that the LED rise time is dependent upon current density for high-injection conditions. Preliminary lifetest results demonstrate only slight LED degradation after operation at 50 and 70°C for times up to ∼3500 h.     

High-accuracy physical modeling of submicrometer MOSFET's

When short-channel MOSFET transistor models are compared to experimental data, the uncertainty in some of the physical input variables often requires that some of the input variables be adjusted to fit the data. This uncertainty is increased by a lack of knowledge of process sensitivity information on critical parameters. These uncertainties have been eliminated using a two-dimensional finite-element model of a MOSFET with no free parameters. The model is compared to four self-aligned silicon-gate n-channel MOSFET's with channel lengths of 0.80, 1.83, 2.19, and 8.17 µm. The 0.80, 1.83, and 8.17-µm devices have phosphorus sources and drains. The 2.19-µm device has an arsenic source and drain. These devices span the range of channel lengths from a short-channel device, totally dominated by velocity saturation and source-drain profile shape, to a long-channel device, well characterized by a long-channel model. Using the data obtained from the measurements described in this work, it is possible to model the drain current for all of the transistors studied without adjustable parameters. Transistors with 0.80-µm channel length differ in model input from those with 8.17-µm channel length only in the length of the polysilicon gate. If sufficiently accurate parameters are available, these methods allow the characteristics of submicrometer transistors to be predicted with ±5-percent accuracy. These simulations show that the observed short-channel effects can be accounted for by existing mobility data and a simple empirical model of these data. Triode and saturation effects are dominated by two-dimensional drain field penetration of the channel region. Subthreshold effects are caused by distortion of fields in the entire channel region by the drain field.