We have fabricated GaN-based light-emitting diodes using transparent indium tin oxide (ITO) p contacts. ITO-contacted devices required an additional 2 V to drive 10 mA, as compared to similar devices with metal contacts. However, ITO has lower optical absorption at 420 nm (α=664 cm−1) than commonly used thin metal films (α=3×105 cm−1). Uniform luminescence was observed in ITO-contacted devices, indicating effective hole injection and current spreading.

Indium tin oxide contacts to gallium nitride optoelectronic devices

Technologies now exist for implementing dense surface-normal optical interconnections for silicon CMOS VLSI using hybrid integration techniques. The critical factors in determining the performance of the resulting photonic chip are the yield on the transceiver device arrays, the sensitivity and power dissipation of the receiver and transmitter circuits, and the total optical power budget available. The use of GaAs-AlGaAs multiple-quantum-well p-i-n diodes for on-chip detection and modulation is one effective means of implementing the optoelectronic transceivers. We discuss a potential roadmap for the scaling of this hybrid optoelectronic VLSI technology as CMOS linewidths shrink and the characteristics of the hybrid optoelectronic transceiver technology improve. An important general conclusion is that, unlike electrical interconnects, such dense optical interconnections directly to an electronic circuit will likely be able to scale in capacity to match the improved performance of future CMOS technology.

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Scaling optoelectronic-VLSI circuits into the 21st century: a technology roadmap

A fully embedded board-level guided-wave optical interconnection is presented to solve the packaging compatibility problem. All elements involved in providing high-speed optical communications within one board are demonstrated. Experimental results on a 12-channel linear array of thin-film polyimide waveguides, vertical-cavity surface-emitting lasers (VCSELs) (42 /spl mu/m), and silicon MSM photodetectors (10 /spl mu/m) suitable for a fully embedded implementation are provided. Two types of waveguide couplers, titled gratings and 45/spl deg/ total internal reflection mirrors, are fabricated within the polyimide waveguides. Thirty-five to near 100% coupling efficiencies are experimentally confirmed. By doing so, all the real estate of the PC board surface are occupied by electronics, and therefore one only observes the performance enhancement due to the employment of optical interconnection but does not worry about the interface problem between electronic and optoelectronic components unlike conventional approaches. A high speed 1-48 optical clock signal distribution network for Cray T-90 super computer is demonstrated. A waveguide propagation loss of 0.21 dB/cm at 850 nm was experimentally confirmed for the 1-48 clock signal distribution and for point-to-point interconnects. The feasibility of using polyimide as the interlayer dielectric material to form hybrid three-dimensional interconnects is also demonstrated. Finally, a waveguide bus architecture is presented, which provides a realistic bidirectional broadcasting transmission of optical signals. Such a structure is equivalent to such IEEE standard bus protocols as VME bus and FutureBus.

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Fully embedded board-level guided-wave optoelectronic interconnects

A discussion of CMOS readout technologies for infrared (IR) imaging systems is presented. First, the description of various types of IR detector materials and structures is given. The advances of detector fabrication technology and microelectronics process technology have led to the development of large format array of IR imaging detectors. For such large IR FPAs which is the critical component of the advanced infrared imaging system, general requirement and specifications are described. To support a good interface between the FPA and downstream signal processing stage, both conventional and CMOS readout techniques are presented and discussed. Finally, future development directions including the smart focal plane concept are also introduced.

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Focal-plane-arrays and CMOS readout techniques of infrared imaging systems

In this work indium tin oxide (ITO) films were prepared using electron beam evaporation to form Schottky contacts on n-type GaN films. The thermal stability of ITO on n-type GaN was also investigated by annealing the samples at various temperatures. In addition, current–voltage (I–V) measurements were taken to deduce the Schottky barrier heights. Owing to the large series resistance, the Norde method was used to plot the F(V)–V curves and the effective Schottky barrier heights were determined as well. The effective Schottky barrier heights were 0.68, 0.88, 0.94, and 0.95 eV for nonannealed, 400, 500, and 600 °C annealed samples, respectively. Results presented herein indicate that an increase of the barrier heights may be attributed to the formation of an interfacial layer at the ITO/GaN interface after annealing.

Effects of thermal annealing on the indium tin oxide Schottky contacts of n-GaN

The design, fabrication, and characterization of a 0.85- mu m sensitive photoreceiver is described. The monolithically integrated optoelectronic receiver is based on pseudomorphic InGaAs on GaAs modulation-doped field-effect transistors (MODFETs) and metal-semiconductor-metal (MSM) photodetectors. High-performance quarter-micrometer MODFETs with f/sub t/'s of approximately 70 GHz are utilized in a two-stage transimpedance amplifier. An asymmetric and a symmetric amplifier design are compared. The symmetric design is found to provide the desired zero DC offset voltage for a variety of supply voltages. Excess MSM-detector dark current and low-frequency internal gain are greatly reduced through the use of a silicon passivation layer and/or AlGaAs cap layer. Receiver transimpedances between 100 and 5000 Omega are obtained by varying the bias on an active feedback resistor. The parasitic capacitances of this common-gate feedback FET are studied. A transimpedance amplifier bandwidth as high as 14 GHz and an overall photoreceiver bandwidth of 11 GHz are measured. >

A MODFET-based optoelectronic integrated circuit receiver for optical interconnects

Metal-semiconductor-metal (MSM) photodiodes with interdigitated indium-tin-oxide (ITO) electrodes have been fabricated on undoped GaAs using reactive-ion-etching in a methane plasma. The responsivity of ITO MSMs is measured to be approximately 0.8 A/W, which is twice that of conventional Ti/Au MSMs under normal operational bias conditions at lambda =850 nm. This higher responsivity is attributed to the transparency of the ITO electrodes. ITO MSMs exhibited a linear optical response over a wider bias range than Ti/Au MSMs. Also, a higher breakdown voltage was measured for ITO MSMs. The bandwidths of ITO and Ti/Au MSMs fabricated on the same semiconductor layer with 1- mu m fingers and spaces were measured to be 6 and 9 GHz, respectively. The slower response of the ITO MSMs is due to the longer transit time of the carriers generated beneath the ITO electrodes. >

A comparative study of metal-semiconductor-metal photodetectors on GaAs with indium-tin-oxide and Ti/Au electrodes

The optical transmittance of indium-tin oxide (ITO) at a wavelength of 1.3 mu m has been improved by adding forming gas (H/sub 2//N/sub 2/) to the Ar sputtering gas. It is shown that the presence of H/sub 2/ in the plasma decreases the carrier concentration in ITO and increases the optical transmittance of a 320 nm-thick ITO film from 69.7% to 99.5%. The application of the high transmittance ITO to the fabrication of metal-semiconductor-metal (MSM) photodiodes on InAlAs/InGaAs heterostructures has resulted in an improvement of responsivity from 0.6 A/W to 0.76 A/W. This is double the responsivity of 0.39 A/W obtained for Ti/Au detectors. A 3-dB bandwidth of 6 GHz was obtained for the high transparency ITO device with 3 mu m fingers and gaps and with an area of 50 mu m*50 mu m. >

Application of indium-tin-oxide with improved transmittance at 1.3 mu m for MSM photodetectors

A process for enhancement/depletion (E/D)-mode GaAs/InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors (MODFETs) using citric acid: H2O2 solutions for selective wet gate recessing has been developed. Extrinsic DC transconductances gm as high as 450 and 600 mS/mm, and unity current-gain cutoff frequencies ft of 75 and 66 GHz at room temperature have been achieved for 0.3 μm gate-length depletion-mode MODFET (DFET) and enhancement-mode MODFET (EFET), respectively. Standard deviations for threshold voltage and transconductance of less than 16 mV and 25 mS/mm, respectively, have been achieved for both the DFETs and EFETs. Ring oscillators fabricated in direct-coupled FET logic technology have exhibited a propagation delay of 13 ps at room temperature.

Process for enhancement/depletion-mode GaAs/InGaAs/AlGaAs pseudomorphic MODFETs using selective wet gate recessing

A monolithically integrated optoelectronic receiver based on the pseudomorphic InGaAs on GaAs material system has been fabricated and tested for the first time Quarter-micrometer modulation-doped field-effect transistors (MODFETs) with f/sub t/'s of 66 GHz are used in the two-stage transimpedance amplifier design The circuit utilizes all active components including a metal-semiconductor-metal (MSM) photodetector and a common-gate FET active feedback resistor >

J.-W. Seo

Papers

A MODFET-based optoelectronic integrated circuit receiver for optical interconnects

A comparative study of metal-semiconductor-metal photodetectors on GaAs with indium-tin-oxide and Ti/Au electrodes

Application of indium-tin-oxide with improved transmittance at 1.3 mu m for MSM photodetectors

Process for enhancement/depletion-mode GaAs/InGaAs/AlGaAs pseudomorphic MODFETs using selective wet gate recessing

A high-performance AlGaAs/InGaAs/GaAs pseudomorphic MODFET-based monolithic optoelectronic receiver