OPTOELECTRONIC INTEGRATED CIRCUIT8

R. r. teheny Bell Communications Research, Inc.

Red Bank, NJ 07701

ABSTRACT

The monolithic integration of optical and electronic devices on a single chip, OptoElectronic Integrated Circuits (OEICs) , represents an emerging device technology with potential to meet a broad range of future telecommunications and computing systems needs. As in the case of entirely electronic circuits, monolithic integration offers advantages of compactness, reliability, reduced cost in volume p\roduction, and the possibility for performance improvements from reduced parasitics. These advantages have been widely recognize for some time but the problems associated with integrated components having very different materials and structural requirements have presented formidable barriers to demonstration of components that compete in performance with hybrid integration. occurring however, with continuing advances in device design and fabrication technologies, as well as in materials growth and processing technologies, that are today providing a strong technology push for continued research on OEICs. At the same time, as semiconductor optoelectronic components are incorporated into an increasing variety of communications and computing systems, and demand for low cost, high volume components grows, we see a market pull for OEIC innovation to satisfy emerging systems needs.

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INTRODUCTION

The current trend in optoelectronic device research for communications systems is away from point-to-point links and toward more versatile systems designed to exploit the very broad band capability of optics. For telecommunications the focus of research has shifted from long-haul systems with relatively low optoelectronic component counts, cost shared among a large number of subscribers, to distribution in the local loop where the wide bandwidth of fiber offers the possibility for greatly enhanced services,

but where the cost of components can only be shared among only a few end users. computing systems the trend is towards distributed data networks, with the capability of rapid reconfiguration, linking remote locations to high speed processing and extended data bases. With the ever increasing requirement for high speed inter-connects even for relatively short spans, such as cabinet-to-cabinet or accross the back plane of complex systems, the bandwidth and low loss of optical technologies offers advantages over the physical size and complexity of copper interconnection. Even down to the level of interchip connection of VLSI circuits, the density and complexity of traditional wire bonding and circuit board connections is stimulating research interest for possible optical alternatives [l].

A typical optical system consists of a source, transmission medium (either fiber, optical waveguide or free space), and a receiver. For spans greater than a few tens of kilometers glass fiber is the medium of choice, and, for lowest loss and signal dispersion, operation in the wavelength bands at 1.3 and 1.5 pm is dictated. These wavelengths require use of components fabricated from InP based materials. For shorter spans where loss or pulse dispersion are not so critical, or free space transmission is an option, shorter wavelength GaAs based devices can be used. However, regardless of the optoelectronic material technology, the effective information bandwidth of the transmission medium far exceeds the capability of the practical tuning range of discrete sources and detectors, or the bandwidth of electronic transmitting and receiving circuits.

In fact the range of operating wavelengths available to optical systems is so large that it provides an effective means for routing signals using wavelength division multiplexing, providing wavelength selective elements are incorporated into the signal distribution system. Therefore, future optical systems are expected to incorporate transmitters and receivers with some degree of wavelength selectivity, as well as passive transmission systems with wavelength

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selectivity for routing signals. For these systems the integration of both passive and active components, such as illustrated in Fig 1. onto a single chip offers significant advantages in size and reliability over hybrid realization of the same functionality [ 2 ] .

What characterizes these applications for replacing traditional electronic interconnection technologies with optical is the requirement for low cost, reliable means for conversion between electronic and optical signal formats with minimum cost or loss of performance. In most cases research or demonstration systems aimed at demonstrating the capability of optical interconnections are realized using hybrid components that tend to be expensive, bulky and unreliable for mass distribution applications. Monolithic integration offers advantages for low cost through batch processing, and greater reliability through elimination of packaging related factors that can degrade reliability. Integration also minimizes the influence of parasitics that limit high speed performance and sensitivity in receivers.

NATEPIALS ISSUES

With all these long recognized advantages, what is limiting the rapid introduction of OEIC components? The answer lies in part in the complex material structures required to create high performance components. For example today's state-of-the-art lasers diodes, capable of efficient tuning over a range of wavelengths, consist of couples structures designed to control the flow of current through the active region, to control the optical mode of the emitted light for good output beam characteristics, and to provide wavelength selective elements for frequency control. High performance transistor designs, such as low noise HEMTs and heterojunction bipolar transistors, require tight control over material structure and doping. sensitive photodetection using pin diodes or APDs, and the realization of efficient passive waveguide devices for optical signal distribution, each have their unique material requirements. Integration of these components either requires some compromise in performance or a means of realizing the required material structure for each type of component on different regions of the substrate. requirement of OEIC technology that has impeded their rapid development, but continuous advancement of materials growth and processing technologies are rapidly eliminating many of these limitations. Figure 2 illustrates how a combination of detector (MSM), HEMT transistor and other circuit elements can be incorporated onto a single chip by growth in an etched well

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with etch removal of materials from areas where it is not required to achieve a planar surface [ 3 ] .

TLZMISIIITTERS

Advances in laser diode design over the past decade have resulted in techniques for incorporating optical waveguides and frequency selective elements with the laser active region to produce frequency stabilized and tunable laser devices. This technology has advanced to the point that researchers at AThT Bell Labs have demonstrated the integrated optical transmitter illustrated in Fig. 3, [ 4 ] . This research device incorporates multiple, wavelength tunable, lasers with output waveguide couplers and optical amplifiers to produce a source with an extended tuning range. This approach to integration that doesn't include any electronic components is referred to as PIC, Photonic Integrated Circuits. PIC fabrication relies on the controlled etching and regrowth of the material structures to achieve the individual components. These material growth and processing technologies have also been applied to the fabrication of integrated optical mixer incorporating; laser U) oscillator, waveguide couplers for balanced distribution of signal and LO, and dual photodetectors for coherent detection [ 5 , 6 ] . This accomplishment comes close to realizing most of the optical component functions illustrated in Fig. 1. While currently still representing a considerable fabrication challenge, these PICs will be essential components for WDM systems using densely packed source wavelengths.

Integration of high speed drive transistors with lasers has also been demonstrated. Here the goal is to more efficiently drive the laser at high speed by elimination of the inductance associated with bond wires. Integration with high performance electronic components has also been demonstrated to allow incorporation of high speed multiplexiers circuits with the laser as well [ 7 , 8 ] .

I(ECE1VERS

There has perhaps been greater effort devoted to integrating photodetectors with receiver electronics than any other type of OEIC. This is in part because the integration of a relatively simple structure like a pin diode with receiver electronics seems like a relatively straight forward process and in part because the Figure of Merit for an optical receiver (FOM-Input Transistor Transconductance, GJNet Input Capacitance Squared, $) can be improved significantly

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using integration to minimize parasitic capacitance at the transistor input node. In fact the integration of a pin diode is not a simple problem. To avoid the difficulties associated with fabrication of pin diodes with electronic components, an alternative design, based on the use of reversed biased back-to-back Shottky diodes to form a Metal-Semiconductor-Metal detector, has evolved as the preferred detector for OEIC receiver applications. Since the detector metalization can be done at the same fabrication step as the FET gate metalization the use of MSM detectors greatly simplifies processing, but these detectors have an additional advantages in their design permits simple planar contacts and which exhibit nearly an order of magnitude less capacitance per unit detector area than the traditional pin design [9].

First realized in GaAs circuits MSM detectors have become the detector of choice for OEICs and have been used in the very complex reliever circuits including the OEIC reported by researchers at IBM that integrates detectors, amplifiers and timing recovery onto a single MSI level IC [lo]. While in principal capable of greater sensitivity than hybrid receivers, OEICs for use at 1.3 or 1.5 jbm have lagged behind GaAs based circuits due to difficulties in realizing high performance transistors in the InP based material system. This situation has recently improved [3, 11, 121 and currently OEIC receivers with, sensitivities approaching that of commercial hybrid receivers are being reported, see Fig 4 [13]. OEIC receivers should in principal out perform hybrid circuits, but the advantage that a hybrid circuit designer has in selecting the best components for each circuit element still provides a competitive advantage for Were" experiments.

FUTURE PROSPECTS

Until recently OEIC research has been focused primarily on demonstrating what can be accomplished within the existing materials and materials processing technologies. In the past two years significant advances in both the scale and performance of OEICs have been realized. The current trend towards increasing complexity in optoelectronic component designs can be expected to continue driving advances in materials and processing technologies and this will maintain the momentum for improvements in OEIC components. For the future we expect OEIC designs to be increasingly targeted to meet specific systems applications where they can begin to significantly impact system performance, particularly in those applications requiring arrays of devices.

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References

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[ll] W. S. Lee, D. A. H. Spear, M. J.

(1990)

Integrated Coherent Optical Receiver

Frequency Controlled Laser

Figure 1

High Performance InAlAsllnGaAs MSM - HEMT ONC Receiver Technology

MSM S C H O W RESISTOR D€IECTOR HEMT DIODE

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[ U ] H. Yano, K. Aga, H. Kamei, G. Sasaki, and H. Hayashi, lgMonolithic pin-HEMT Receiver with Internal Equalizer for Long-wavelength Fiber Optic Communicationsgg; Electr. Lettrs., B, 305 (1990)

[13] 0. Wada, Extended Abstracta : 5th European Conf. on Integrated Optics, ECI0'89 (Paris) SPIE U, 1141 (1989), also cited in Ref. 1

Three Tunable DBR Lasers Photonic lntearated Circuit

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