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.     

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.     

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.     

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.     

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.     

Light intensity profiles of surface-emitting InGaAsP LED's: Impact on coupling to optical fibers

Light intensity profiles of 1.3-µm surface-emitting InGa AsP/InP LED's were measured using an infrared vidicon imaging technique. Typical light spot profiles were shown to have uniform intensity only over a small spot, with significant Gaussian tails around the periphery. Profiles were measured versus current. Narrowing of the light spot via current crowding was observed. A previous model for current crowding in stripe-geometry lasers was modified to calculate intensity profiles of circular-contact LED's. The model qualitatively predicts the observed changes in the profile shape with varying current and epitaxial-layer parameters. Measured efficiencies η of coupling LED power to optical fibers are usually lower than calculated for an ideal Lambertian source of uniform intensity. The observed profile is shown here to be responsible for the reduced η. For example, η of a uniform 25-µm spot is calculated to be ∼30-percent higher than that of a spot with typical Gaussian spreading and a width of 25 µm at half-maximum intensity, both butt-coupled to a 50-µm core, graded-index fiber. Lensing schemes to improve η were designed assuming uniform light spots, A ray-tracing calculation shows here that spherical lenses improve η equally well in the presence of Gaussian tails around the light spot.     

Efficient small-area GaAs-Ga1-xAlxAs heterostructure electroluminescent diodes coupled to optical fibers

A GaAs-Ga1-xAlxAs heterostructure prepared by liquid-phase epitaxy has been employed in the fabrication of efficient small-area (50-µm diameter) electroluminescent diodes, and the light output has been coupled into optical fibers. The light output through a short fiber was about 1 mW at a wavelength of 0.9 µm for a bias current of 200 mA dc. By comparison, the output from the most efficient diffused GaAs diodes of similar geometry was about 0.4 mW at this current.     

High-speed optical pulse transmission at 1.29-µm wavelength using low-loss single-mode fibers

Optical-fiber transmission experiments in the 1.3-μm wavelength region are reported. GaInAsP/InP double-heterostructure semiconductor laser emitting at 1.293 μm is modulated directly in nonreturn-to-zero (NRZ) codes at digit rates tanging from 100 Mbit/s to 1.2 Gbit/s. Its output is transmitted through low-loss GeO2-doped single-mode silica fibers in 11-km lengths. Transmitted optical signals are detected by a high-speed Ge avalanche photodiode. Overall loss of the 11-km optical fibers, including 11 splices, is 15.5 dB at 1.3 μm. Average received optical power levels necessary for 10^-9error rate are -39.9 dBm at 100 Mbit/s and -29.1 dBm at 1.2 Gbit/s. In the present system configuration, the repeater spacing is limited by loss rather than dispersion. It seems feasible that a more than 30 km repeater spacing at 100 Mbit/s and a more than 20 km even at 1.2 Gbit/s can be realized with low-loss silica fiber cables, whose loss is less than 1 dB/km. Distinctive features and problems associated with this experimental system and constituent devices are discussed.     

Long-wavelength optical fiber communication

Recent research on long-wavelength lightwave communication utilizing the wavelength region between 1.3 and 1.6 µm is reviewed with an eye toward future system development. The attraction of the long-wavelength region is the availability of the ultimately low-loss and wide-band features of the silica fiber, where minimum loss is 0.27 dB/km at a wavelength of 1.3 µm and 0.16 dB/km at 1.55 µm. The single-mode fiber has found its first significant applications in long-wavelength systems. The specific characteristics of lightwave components are discussed with focus on physical fundamentals. The practical performance of fibers and lightwave devices is surveyed. The dynamic properties of long-wavelength laser diodes are discussed in relation to fiber characteristics. The noise characteristics of long-wavelength detectors are considered for the purpose of specifying the repeater spacing. Some system studies are reviewed, for example, 1.3-µm-wavelength lightwave systems, which have demonstrated bandwidth-distance products of about 40 GHz ċ km. Various approaches to extend the capacity of long-wavelength lightwave transmission are given. In the future, the 1.5-µm wavelength system could operate at the lowest loss wavelength region extending from 1.5 to 1.65 µm. Much higher performance, for example, bandwidth-distance products of 185 GHz ċ km, achieved by further continuation of research and development on lightwave sources as well as fibers. Because of the author's familiarity with work in Japan, that work is emphasized and most frequently cited.     

Characterization of InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

High-performance pseudomorphic InyGa1-yAs/Al0.15- Ga0.85As (0.05 le y le 0.2) MODFET's grown by MBE have been characterized at dc (300 and 77 K) and RF frequencies. Transconductances as high as 310 and 380 mS/mm and drain currents as high as 290 and 310 mA/mm were obtained at 300 and 77 K, respectively, for 1-µm gate lengths and 3-µm source-drain spacing devices. Lack of persistent trapping effects,I-Vcollapse, and threshold voltage shifts observed with these devices are attributed to the use of low mole fraction AlxGa1-xAs while still maintaining 2DEG concentrations of about 1.3 × 10¹²cm^-2. Detailed microwave S-parameter measurements indicate a current gain cut-off frequency Of 24.5 GHz Wheny = 0.20, which is as much as 100 percent better than similar GaAs/AlGaAs MODFET structures, and a maximum frequency of oscillation of 40 GHz. These superior results are in part due to the higher electron velocity of InGaAs as compared with GaAs. Velocity field measurement performed up to 3 kV/cm using the magnetoresistance method indicates an electron saturation velocity of greater than 1.7 × 10⁷cm/s at 77 K fory = 0.15, which is 20 percent higher than GaAs/AlGaAs MODFET's of similar structure.     

A study of high-speed normally off and normally on Al0.5Ga0.5As heterojunction gate GaAs FET's (HJFET)

The dc, small-signal microwave, and large-signal switching performance of normally off and normally on Al0.5Ga0.5As gate heterojunction GaAs field-effect transistors (HJFET) with submicrometer gate lengths are reported. The structure of both types of devices comprises an n-type 10¹⁷-cm^-3Sn-doped active layer on a Cr-doped GaAs substrate, a p-type 10¹⁸-cm^-3Ge-doped Al0.5Ga0.5As gate layer and a p⁺-type 5 × 10¹⁸-cm^-3Ge-doped GaAs "contact and cap" layer on the top of the gate. The gate structure is obtained by selectively etching the p⁺-type GaAs and Al0.5Ga0.5As. Undercutting of the Al0.5Ga0.5As layer results in submicrometer gate lengths, and the resulting p⁺-GaAs overhang is used to self-align the source and the drain with respect to the gate. Normally off GaAs FET's with 0.5- to 0.7-µm long heterojunction gates exhibit maximum available power gains (MAG) of about 9 dB at 2 GHz. Large-signal pulse measurements indicate an intrinsic propagation delay of 40 ps with an arbitrarily chosen 100-Ω drain load resistance in a 50-Ω microstrip circuit. Normally on FET's with submicrometer gate lengths (∼0.6 µm) having a total gate periphery of 300 µm and a corresponding dc transconductance of 20-30 mmhos exhibit a MAG of 9.5 dB at 8 GHz. The internal propagation delay time measured under the same conditions as above is about 20 ps.     

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.     

Monolithic integration of GaAs/AlGaAs double-heterostructure LED's and Si MOSFET's

Fully monolithic integration of interconnected GaAs/Al-GaAs double-heterostructure LED's and Si MOSFET's is demonstrated for the first time. The Si MOSFET's, with a gate length of 5 µm and gate width of 1.6 mm, have almost the same characteristics as those of control devices fabricated on a separate Si wafer. The LED output collected by a microscope lens with a numerical aperture of 0.65 is about 6.5 µW at 100- mA dc current. LED modulation rates up to 27 Mbit/s have been achieved by applying a stream of voltage pulses to the MOSFET gate. The modulation rate is limited by the speed of the MOSFET.     

Ga0.4In0.6As/Al0.55In0.45As pseudomorphic modulation-doped field-effect transistors

High-performance pseudomorphic Ga0.4In0.6As/ Al0.55In0.45As modulation-doped field-effect transistors (MODFET's) grown by MBE on InP have been fabricated and characterized. DC transconductances as high as 271, 227, and 197 mS/mm were obtained at 300K for 1.6-µm and 2.9-µm gate-length enhancement-mode and 2-µm depletion-mode devices, respectively. An average electron velocity as high as 2.36 × 10⁷cm/s has been inferred for the 1.6-µm devices, which is higher than previously reported values for 1-µm gate-length Ga0.47In0.53As/Al0.48In0.52As MODFET's. The higher bandgap Al0.55In0.45As pseudomorphic barrier also offers the advantages of a larger conduction-band discontinuity and a higher Schottky barrier height.     

0.4-µm gate-length devices fabricated by contrast-enhanced lithography

This letter describes the fabrication of submicrometer polysilicon-gate MOS devices by an advanced optical process called contrast enhancement. Functional devices having gate lengths as small as 0.4 µm were fabricated with this process. Contrast-enhanced lithography (CEL) allows usable photoresist patterns to be fabricated at smaller dimensions than is possible with conventional resist. The simultaneous replication of mask dimensions for isolated lines at 0.35 µm and above was achieved in this work using a single exposure on an Optimetrix 10:1 DSW system. Contrast enhancement has been applied to the fabrication of n-channel MOS devices having gate lengths from 0.4 to 1.5 µm in steps of 0.1 µm. Long-channel devices were also fabricated. The transconductance of the 0.4-µm devices is 40 mS/mm at Vds= 5 V. Threshold voltages (Vds= 0) are nearly independent of gate length, ranging from 1.21 to 1.31 V over the 7.5- to 0.4-µm range in gate length. The effective mobility for long-channel devices is 430 cm²/V.s.     

Low-power high-speed InP MISFET direct-coupled FET logic

High-dynamic-range n-channel InP MISFET direct-coupled FET logic ring oscillator and inverter integrated circuits with minimum observed propagation delay per staget_{pd} = 62ps with associated power delay product of 41 fJ and minimum observed power delay productPt_{pd} = 22fJ with associated delay of 84 ps have been fabricated on Fe-doped semi-insulating substrate material using ion implantation for contact and load channel regions and pyrolytic SiO2as the gate insulator. Accumulation-type enhancement-mode MISFET structures with source-drain separations of 1.5 µm and gate metallization lengths of 3.0 µm were employed as driver devices while both MESFET's and 1.5-µm-length ungated "velocity saturation" structures were used as loads. WithV_{DD} = 4.5V representative inverter structures exhibited logic swings of 3.58 V, noise margins of 1.00 and 0.92 V, and dc gain in the linear region of 2.2.     

Electrical properties of tellurium thin films

Semiconducting properties of evaporated tellurium thin films, in the thickness range of 100 to 400 Å, are studied and correlated with observed structural properties. It is found that less-than-monolayer gold films can act as nucleation sites and stimulate the growth of large crystallites in deposited Te films. The Au-nucleated Te films are preferentially oriented with the c axis in the substrate plane and have crystallite diameters ranging from 2 to 5 µm. Hall mobilities as high as 250 cm²/V ċ s are observed in 400-Å Au-nucleated films with 5-µm crystallites. These large-grain films exhibit a temperature dependence for mobility of the form µ ∼ T^3/2between 85°K and 250°K, while the carrier concentrations in the films do not change appreciably with temperature. Transconductances greater than 1000 µmhos are achieved for Au-nucleated Te thin-film transistors with 3-mil channels (operating with a saturated drain current of 1 mA). Several devices exhibit field-effect mobilities greater than 100 cm²/V ċ s, a value consistent with the observed Hall mobilities for similar films. Transconductance measurements indicate that Te thin-film transistor (TFT) instabilities result primarily from hole trapping at the Te-insulator interface. It is possible to alter the threshold voltage of Te TFTs by applying a gate bias at room temperature. Improved stability (changes in V0less than 50 mV in 1 h) is observed at 77°K. From the observed changes in threshold, a lower limit of the trapping-state density at the surface is inferred to be 5×10¹²traps/cm². The surface-state density at the Te-SiO interface is estimated to be less than 6×10¹²surface states/cm²ċ eV as determined from capacitance and conductance measurements.     

An Hg0.3Cd0.7Te avalanche photodiode for optical-fiber transmission systems at λ = 1.3 µm

The purpose of this paper is the characterization of Hg0.3Cd0.7Te avalanche photodiodes at γ = 1.3 µm. These devices are manufactured by tile Société Anonyme des Télécommunications. The multiplication noise for these APD's is measured. The value of the ratiok= β/α is deduced from noise measurements, β and α being, respectively, the hole and electron ionization coefficients. It is shown that these HgCdTe APD's are promising candidates for detectors of 1.3-µm optical communication.     

Performance characteristics and extended lifetime data for InGaAsP/InP LED's

The performance and lifetest results of InGaAsP/InP LED's emitting at 1.27 µm are described. At a current density of 10 kA/cm², 40 µW of optical power is coupled into a 63 µm core, 0.21 NA optical fiber, and the projected lifetime at room temperature is approximately estimated to be 8 × 10¹⁰hours.