Long-wavelength In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As large-area avalanche photodiodes and arrays

Large-area (500-/spl mu/m diameter) mesa-structure In/sub 0.53/Ga/sub 0.47/As-In/sub 0.52/Al/sub 0.48/As avalanche photodiodes (APDs) are reported. The dark current density was /spl sim/2.5/spl times/10/sup -2/ nA//spl mu/m/sup 2/ at 90% of breakdown; low surface leakage current density (/spl sim/4.2 pA//spl mu/m) was achieved with wet chemical etching and SiO/sub 2/ passivation. An 18 /spl times/ 18 APD array with uniform distributions of breakdown voltage, dark current, and multiplication gain has also been demonstrated. The APDs in the array achieved 3-dB bandwidth of /spl sim/8 GHz at low gain and a gain-bandwidth product of /spl sim/120 GHz.     

Evanescently coupled photodiodes integrating a double-stage taper for 40-Gb/s applications-compared performance with side-illuminated photodiodes

The design, fabrication, and performance of double-stage taper photodiodes (DSTPs) are reported. The objective of this work is to develop devices compatible with 40-Gb/s applications. Such devices require high efficiency, ultrawide band, high optical power handling capability, and compatibility with low-cost module fabrication. The integration of mode size converters improves both the coupling efficiency and the responsivity with a large fiber mode diameter. Responsivity of 0.6 A/W and 0.45 A/W are achieved with a 6-/spl mu/m fiber mode diameter and cleaved fiber, respectively, providing relaxed alignment tolerances (/spl plusmn/1.6 /spl mu/m and /spl plusmn/2 /spl mu/m, respectively), compatible with cost-effective packaging techniques. DSTPs also offer a wide bandwidth greater than 40 GHz and transverse-electric/transverse-magnetic polarization dependence lower than 0.2 dB. Furthermore, a DSTP saturation current as high as 11 mA results in optical power handling greater than +10 dBm and a high output voltage of 0.8 V. These capabilities allow the photodiode to drive the decision circuit without the need of a broad-band electrical amplifier. The DSTP devices presented here demonstrate higher responsivities with large fiber mode diameter and better optical power handling capabilities and are compared with classical side-illuminated photodiodes.     

Design and fabrication of a p-i-n photodiode with high responsivity and large alignment tolerances for 40-gb/s applications

This letter demonstrates an evanescently coupled p-i-n photodiode combined with a multimode diluted waveguide using a simple all 2-in InP processing that includes on-wafer mirrors etching and antireflection coating. A high responsivity of 0.81 A/W at 1.55 /spl mu/m with less than 0.4-dB polarization dependence and a large -1-dB vertical alignment tolerance of 2.70 /spl mu/m were achieved simultaneously with a bandwidth of 47 GHz. Stable operation for over 1000 h was obtained under bias stress and temperature at 200/spl deg/C.     

Edge-coupled InGaAs PIN photodiode with a light funnel waveguide

We have successfully fabricated InGaAs edge-coupled photodiodes (EC-PDs) with a light funnel integrated (LIFI) in front of the coupling aperture, called LIFI EC-PD, based on the self-terminated oxide polish (STOP), the crystallographic slope etching of InP, and the self-aligned diffusion (SAD) techniques. The LIFI EC-PD presents not only a lower dark current density (/spl sim/ 4.4 mA/cm/sup 2/) but also a higher responsivity (/spl sim/ 0.4 A/W) than that of the mesa EC-PD (27 mA/cm/sup 2/ and 0.26 A/W, respectively). Furthermore, the thick oxide film serves as the funnel in front of active-region aperture to enhance the coupling efficiency and to lower the bonding pad capacitance down to 50 fF. The lowered bonding pad capacitance can be beneficial in designing a device with a higher transit-time-limited frequency response of beyond 30 GHz. The LIFI EC-PD with a 1-/spl mu/m thick absorption layer exhibits a 3-dB bandwidth of 20 GHz and a responsivity of /spl sim/ 0.4 A/W.     

37-GHz bandwidth monolithically integrated InP HBT/evanescently coupled photodiode

In this letter, we demonstrate a monolithically integrated optoelectronic integrated circuit (OEIC) for 1.55-/spl mu/m wavelength application. The presented OEIC consists of an evanescently coupled photodiode (ECPD) and a single-stage common-base InP-InGaAs heterojunction bipolar transistor (HBT) amplifier. The guide structure was grown first by metal-organic chemical vapor deposition and pin/HBT was then regrown by molecular beam epitaxy. The ECPD exhibits a responsivity of 0.3 A/W and a -3-dB electrical bandwidth of 30 GHz. The photoreceiver demonstrates a -3-dB electrical bandwidth of 37 GHz with a transimpedance gain of 32 dB/spl middot//spl Omega/. This is, to our knowledge, the first ECPD/HBT ever reported for a monolithically integrated OEIC.     

Improvement of dark current using InP/InGaAsP transition layer in large-area InGaAs MSM photodetectors

A large-area InGaAs metal-semiconductor-metal (MSM) photodetector with 1/spl times/1 mm/sup 2/ photoactive area for free-space optical communication applications has been designed, fabricated, and characterized. Interdigitated electrodes of 2-/spl mu/m widths and 15-/spl mu/m spacings are designed to maximize the responsivity, and enable MSM photodetectors to reach a maximum responsivity at 1.53-/spl mu/m wavelength. By employing a two-step InP/InGaAsP transition layer, the dark current density of 45 fA//spl mu/m/sup 2/ was achieved at 10-V bias and at room temperature. Dark current-bias voltage curves were measured as a function of temperature from 40 to 270 K to estimate the activation energy. A 3-dB bandwidth of 210 MHz was obtained at a 10-V bias, and the measured result was compared with the designed bandwidth.     

A high-speed ITO-InAlAs-InGaAs Schottky-barrier photodetector

A high-speed and high-sensitivity vertical indium-tin-oxide-InAlAs-InGaAs Schottky barrier photodetector has been designed, fabricated, and characterized. The devices had dark current densities as low as 8.87/spl times/10/sup -5/ A/cm/sup -2/ at an applied bias of 5 V. The responsivity for all the devices tested ranged from 0.55-0.60 A/W at a wavelength of 1.31 /spl mu/m, and 0.563-0.583 A/W at 1.55 /spl mu/m. The 15-/spl mu/m diameter devices exhibited a -3 dB bandwidth of 19 and 25 GHz at a wavelength of 1.55 /spl mu/m and an applied bias of 5 and 10 V, respectively. These are the best values of responsivity and bandwidth for a vertical InGaAs-based Schottky-barrier photodetector reported to date.     

Multiquantum-well lasers with tapered active stripe for direct coupling to single-mode fiber

We demonstrate narrow beam divergence in 1.3-/spl mu/m wavelength multiquantum-well (MQW) lasers with an active stripe horizontally tapered over the whole cavity, for direct coupling to single mode-fibers. The lasers have reduced output beam divergence in a simple structure which does not contain an additional spot-size transformer. The fabricated laser shows narrow beam divergence of /spl sim/12/spl deg/, while a low-threshold current of 6.9 mA and a high efficiency of 0.62 mW/mA are realized. Furthermore, a direct-coupling efficiency to a single-mode fiber is -4.0-dB and -3-dB alignment tolerance is /spl plusmn/2.5 /spl mu/m.     

High-efficiency 1.3 /spl mu/m InAsP-GaInP MQW electroabsorption waveguide modulators for microwave fiber-optic links

High-efficiency electroabsorption waveguide modulators have been designed and fabricated using strain-compensated InAsP-GaInP multiple quantum wells at 1.32-/spl mu/m wavelength. A typical 200-/spl mu/m-long modulator exhibits a fiber-to-fiber optical insertion loss of 9 dB and an optical saturation intensity larger than 10 mW. The 3-dB electrical bandwidth is in excess of 20 GHz with a 50-/spl Omega/ load termination. When used in an analog microwave fiber-optic link without amplification, a RF link efficiency as high as -38 dB is achieved at 10 mW input optical carrier power. These analog link characteristics are the first reported using MQW electroabsorption waveguide modulators at 1.32 /spl mu/m.     

High-speed modulation of strain-compensated InGaAs-GaAsP-InGaP multiple-quantum-well lasers

Small signal direct modulation characteristics of InGaAs-GaAsP-InGaP multiple quantum well ridge waveguide lasers (4.5/spl times/220 /spl mu/m/sup 2/) are described. The compressive strain of four InGaAs quantum wells is compensated by the tensile strain of GaAsP barriers. The lasers have a threshold current of 8 mA and an internal differential quantum efficiency of 80%. A 3-dB bandwidth of 25 GHz is obtained at 54 mA. It is found that the strain-compensated lasers have a K factor as low as 0.15 ns, implying a maximum 3-dB bandwidth of 59 GHz.     

High-speed Modulation of InGaAs: Sb-GaAs-GaAsP quantum-well vertical-cavity surface-emitting lasers with 1.27-/spl mu/m emission wavelength

1.27-/spl mu/m InGaAs: Sb-GaAs-GaAsP vertical-cavity surface-emitting lasers (VCSELs) were grown by metal-organic chemical vapor deposition and exhibited excellent performance and temperature stability. The threshold current changes from 1.8 to 1.1 mA and the slope efficiency falls less than /spl sim/35% as the temperature raised from room temperature to 70/spl deg/C. With a bias current of only 5 mA, the 3-dB modulation frequency response was measured to be 8.36 GHz, which is appropriate for 10-Gb/s operation. The maximal bandwidth is measured to be 10.7 GHz with modulation current efficiency factor (MCEF) of /spl sim/5.25 GHz/(mA)/sup 1/2/. These VCSELs also demonstrate high-speed modulation up to 10 Gb/s from 25/spl deg/C to 70/spl deg/C.     

Interdigitated Ge p-i-n photodetectors fabricated on a Si substrate using graded SiGe buffer layers

We report an interdigitated p-i-n photodetector fabricated on a 1-/spl mu/m-thick Ge epitaxial layer grown on a Si substrate using a 10-/spl mu/m-thick graded SiGe buffer layer. A growth rate of 45 /spl Aring//s/spl sim/60 /spl Aring//s was achieved using low-energy plasma enhanced chemical vapor deposition. The Ge epitaxial layer had a threading dislocation density of 10/sup 5/ cm/sup -2/ and a rms surface roughness of 3.28 nm. The 3-dB bandwidth and the external quantum efficiency were measured on a photodetector having 1-/spl mu/m finger width and 2-/spl mu/m spacing with a 25/spl times/28 /spl mu/m/sup 2/ active area. At a wavelength of 1.3 /spl mu/m, the bandwidth was 2.2, 3.5, and 3.8 GHz at bias voltages of -1, -3, and -5 V, respectively. The dark current was 3.2 and 5.0 /spl mu/A at -3 and -5 V, respectively. This photodetector exhibited an external quantum efficiency of 49% at a wavelength of 1.3 /spl mu/m.     

High-sensitivity BiCMOS OEIC for optical storage systems

A new BiCMOS optoelectronic integrated circuit (OEIC) for applications in advanced optical storage systems is presented. It is optimized with respect to high sensitivity and high speed. The photodiode and the amplifier are monolithically integrated on the same substrate in a commercial 0.8-/spl mu/m BiCMOS process. Analytical expressions for the compensation capacitors and for the bandwidth of the OEIC are derived. Neglecting antireflection coating, no process modifications are necessary to produce the integrated photodiodes. A new offset compensation scheme is implemented in the amplifiers to allow for a small chip area and low power consumption. The OEIC shows a sensitivity of 43.3 mV//spl mu/W in combination with a -3-dB bandwidth of 60.2 MHz.     

40-Gbit/s OEIC on GaAs substrate through metamorphic buffer technology

An optoelectronic integrated circuit operating in the 1.55-/spl mu/m wavelength range was realized on GaAs substrate through metamorphic technology. High indium content layers, metamorphically grown on a GaAs substrate, were used to fabricate the optoelectronic integrated circuits (OEICs) with -3 dB bandwidth of 40 GHz and 210 V/W of calculated responsivity. The analog OEIC photoreceiver consists of a 12-/spl mu/m, top-illuminated p-i-n photodiode, and a traveling wave amplifier (TWA). This receiver shows 6 GHz wider bandwidth than a hybrid photoreceiver, which was built using comparable, but stand-alone metamorphic p-i-n diode and TWA. With the addition of a buffer amplifier, the OEIC shows 7 dB more gain than the hybrid counterpart. To our knowledge, this is the first 40 Gbit/s OEIC achieved on a GaAs substrate operating at 1.55 /spl mu/m.     

Millimeter-wave high-power 0.25-/spl mu/m gate-length AlGaN/GaN HEMTs on SiC substrates

Reports on the CW power performance at 20 and 30 GHz of 0.25 /spl mu/m /spl times/ 100 /spl mu/m AlGaN/GaN high electron mobility transistors (HEMTs) grown by MOCVD on semi-insulating SiC substrates. The devices exhibited current density of 1300 mA/mm, peak dc extrinsic transconductance of 275 mS/mm, unity current gain cutoff (f/sub T/) of 65 GHz, and maximum frequency of oscillation (f/sub max/) of 110 GHz. Saturated output power at 20 GHz was 6.4 W/mm with 16% power added efficiency (PAE), and output power at 1-dB compression at 30 GHz was 4.0 W/mm with 20% PAE. This is the highest power reported for 0.25-/spl mu/m gate-length devices at 20 GHz, and the 30 GHz results represent the highest frequency power data published to date on GaN-based devices.     

40-GHz coplanar waveguide bandpass filters on silicon substrate

We report a very simple process to fabricate high performance filter on Si at 40 GHz using proton implantation. The filter has only -3.4-dB loss at peak transmission of 40 GHz with a broad 9-GHz bandwidth. In sharp contrast, the filter on 1.5-/spl mu/m SiO/sub 2/ isolated Si has much worse transmission and reflection loss. This is the first demonstration of high performance filter at the millimeter-wave regime on Si with process compatible with current VLSI technology.     

A comparison of front- and backside-illuminated high-saturation power partially depleted absorber photodetectors

A systematic study of high-saturation-current p-i-n In/sub 0.53/Ga/sub 0.47/As photodiodes with a partially depleted absorber (PDA) has been made under front (p-side) and back (n-side) illumination. The photodiode structure consists of an In/sub 0.53/Ga/sub 0.47/As absorption region (450-nm p-InGaAs, 250-nm unintentionally doped InGaAs, and 60-nm n-InGaAs) sandwiched between p- and n-InP layers. For front illumination of a 34-/spl mu/m-diameter photodiode at 2-V bias the saturation currents were 23 and 24 mA at 10 and 1 GHz, respectively. Under similar conditions, backside-illumination resulted in saturation currents of 76 mA (10 GHz) and >160 mA (1 GHz). Backside illumination of a 100-/spl mu/m-diameter photodiode achieved a saturation current >400 mA. For the case of front illumination the device lateral resistance dominates whereas for backside illumination the response is determined primarily by the space charge effect.     

Small-signal modulation characteristics of p-doped 1.1- and 1.3-/spl mu/m quantum-dot lasers

We have investigated the small-signal modulation characteristics of 1.1and 1.3-/spl mu/m p-doped quantum-dot lasers in order to evaluate the potential of acceptor doping. The maximum measured 3-dB bandwidth of the 1.1- and 1.3-/spl mu/m lasers are 11 and 8 GHz, respectively, which are only marginally higher than those in the corresponding undoped devices.     

Lossless InAsP-InGaP modulator at 1.3 /spl mu/m for optical conversion of radio signals up to 40 GHz

We report on an InAsP-InGaP electroabsorption modulator at 1.3 /spl mu/m integrated with a semiconductor amplifier. The fiber-to-fiber insertion gain reaches +10 dB. The 50-/spl mu/m-long modulator section exhibits a bandwidth of 36 GHz and a -17-dB extinction ratio with 3-V drive voltage. The integrated amplifier produced an RF-link efficiency of -26 dB at 20 GHz without any external amplification.     

Integratable High Linearity Compact Waveguide Coupled Tapered InGaAsP Photodetectors

We investigate high linear response tapered photodiodes composed of bulk and multiquantum-well absorption layers based on the integratable asymmetric twin waveguide architecture. The tapered shape reduces space-charge induced nonlinearities, enhancing the saturation current densities at high input optical powers. The 1-dB compression current density for an InGaAsP bulk active layer photodiode (BPD) is in excess of (2.8 plusmn 0.3) kA/cm², compared to quantum-well photo- diodes (QWPD) in the same materials system that saturate at (1.2 plusmn 0.1) kA/cm². We find that the limited density of states of QWPDs leads to the early onset of current saturation. The BPD has a polarization sensitivity of S_Pol = (1.0 plusmn 0.5) dB and responsivity R = (0.3plusmn0.03) A/W at a wavelength of 1.55 mum, whereas the QWPD has S_Pol = (7plusmn1) dB and R = (0.55plusmn0.05) A/W, while both have a bandwidth of (11 plusmn 1) GHz.