The development of optical fibers with extremely low loss and near zero pulse dispersion in the 1.30-1.55 pm spectral range has generated considerable interest in emitters and detectors for use in optical fiber communication systems utilizing these wavelengths. The InGaAsP quaternary alloy, lattice matched to InP, is one of at least three different semi-conductor alloys being evaluated for detector applications in these systems. In this paper we will review some of the previous results obtained in InGaAsP/InP photodetectors, and discuss the possible mechanisms responsible for the large dark current observed in some of these devices. The material properties and device structures which minimize the dark current are described, and the possibilities of achieving efficient avalanche photodiodes using these materials are evaluated.
The growth and fabrication of GaAlAsSb/GaSb avalanche photodiodes is discussed. The leakage current, gain, and uniformity of heterojunction, ion-implanted and Schottky-barrier structures are examined. In addition, the effect of the valence band spin orbit splitting upon avalanche noise is briefly discussed. It is shown that the prevalent problem of these devices is a high surface leakage current.
High performance inverted-mesa GaInAsP/InP avalanche photodiodes responding out to 1.25 pm haye been fabricated. Uniform avalanche gains, M, of 700, dark current densities of 3 x 10-6 A/cm2 at M = 10, and an excess noise factor of -3 at M = 10 have been achieved by placing the p-n junction in the InP and using a new passivation technique. Pulse-response risetimes of less than 160 psec, limited by the risetime of the mode-locked Nd:YAG laser pulse, were measured with an avalanche gain of 40. Avalanche photodiodes (APDs) of the III-V quaternary alloy GaxIn1-xAsyP1-y on InP substrates have been under development in several laboratories for use in the 1.0-1.6 pm spectral region of interest for fiber optics applications. Typically, these devices, which have had the p-n junction located in the GaInAsP layer, have exhibited large values of dark current at biases sufficient to achieve gain, a condition that severely degrades the signal-to-noise performance and limits the gain to low values. Recently, however, Nishida et al. 1,2 have demonstrated in diffused structures that reductions in leakage current and increases in gain can be obtained by placing the p-n junction in the InP, so the high-field region is in the InP while the photogeneration region is in the GaInAsP. The noise performance of these devices was not discussed, but the low temperature (-190°C) noise characteristics of more conventional GaInAsP APDs were very recently published.3
In .53Ga 47As/InP photodiodes and hybrid InP-InGaAsP APDs designed for long wavelength optical fiber communication are discussed. The photodiodes have rise and fall times <0.5 nsec, dark currents in the nanoamp range, subpicofarad capacitance, and high quantum efficiency. The APDs have a long wavelength cutoff of -1.25 μm, have negligible dark current (ID <6 nA at a gain of M = 30), and have an excess noise factor F given by F/M = 0.42 ± 0.10 = const for 5 ≤M≤ 5 35. The detectors are packaged in sealed micro-window packages that allow nearly 100% coupling efficiency to optical fibers with core diameters up to 100 pm (NA 0.30).
Hgi-xCdxTe epitaxial layers have been successfully grown in various compositions, for 1-3 μm applications. n+/p junctions are formed either by a standard B-implantation into as-grown p-type layers or by doubly grown p- and n-layers. The SWIR HgCdTe photodiodes exhibit quantum efficiencies of 55-65% without AR coating. For the diodes with 1.39 μm cut-off at room temperature, the zero bias detector resistance-area (RoA) product is 4 x 10 4 Ω-cm2, and the dark current density is ~ 1 x 10 -4 A/cm2 at half-breakdown voltage. The same values of ~ 104 Ω-cm2 RoA products have also been measured for 2.4 μm cut-off photodiodes at 195K. The energy gap and temperature dependence of RoA product is in excellent agreement with the bulk limited generation-recombination model. The breakdown voltages of SWIR diodes vary from 12 volts to greater than 130 volts, depending on the Cd composition (x) and base carrier concentrations.
The transport properties of In0.53Ga0.47As indicate that this ternary material is a strong candidate for future FET's. Early results reported for FET's prepared from this material are reviewed and compared. The advantage of the JFET for fabrication of PIN-FET integrated optical receiver circuits is pointed out and updated results are presented for these devices. Transconductance values are presently 50 mS/mm, while work aimed at further reduction of gate dimensions is expected to yield even higher values.
The sensitivities of three different detector/pre-amplifier configurations suitable as 1.3 pm optical fiber receivers -- a heterojunction bipolar phototransistor, a p-i-n photo-diode with a FET preamplifier, and an avalanche photodiode with a FET preamplifier -- are calculated and compared using recently published performance data. Use of a state-of-the-art low capacitance, high transconductance FET preamplifier promises to increase the sensitivity of a pin-FET receiver to the point that it is only 6-7 db less than that of a low noise, low leakage APD detector with the same FET preamplifier. Heterojunction phototran: sistors, however, have the simplest structure of any of the three receivers, are monolithic, and require the least amount of control circuitry, while having sensitivities comparable to those of the advanced pin-FET receivers.
The theory of operation of the heterojunction phototransistor is reviewed and the limitations on gain and speed-of-response are examined in the context of fiber-optic systems requirements. The response of the base potential is shown to depend on the input optical power yielding a power dependent gain-bandwidth product, fT. Model calculations assuming an optimized device structure with peak response in the 1. μm - 1.55 μm spectral region are used to demonstrate the consequences of this dependence. The results indicate that the HPT can have sufficient gain and speed-of-response if a dc bias current is used. A comparison of the S/N of the HPT and an APD is presented to clarify system applicability.
High performance GaAlAs/GaAs heterostructure avalanche photodiodes (APD) have been fabricated. The spectral response of these devices are from 0.48 μm to 0.89 μm. The quantum efficiency at unity gain is as high as 95%. Microwave gain (273 MHz) of 42 dB has been observed in these devices. Dark current is extremely low ~ l0 -12 A at half the breakdown voltage. Low noise characteristics of the Read structure APD cannot be verified at this moment because of the poorer performance than the traditional LPE heterostructure APD. With careful design, III-V heterostructure photodiode can have response time as low as 5 ps.
The operational principles and performance of InP photoconductive switches are reviewed. The results suggest that these devices may be better suited for high-speed photodetection and signal processing applications than comparable Si or GaAs devices. The switches have a response time of -50 psec, an off-state impedance of 10 fF and 100 MΩ and an on-state impedance of 45 Ω for 40 pJ of incident energy. In a variety of experiments with cw mode-locked lasers, the switches have been used as fast optical pulse detectors, high-speed pulse-train generators and as wide-band analog samplers. In the latter application, a 70-MHz sine wave has been sampled at 275 MS/sec with 98% accuracy. Extension of this sampling application to analog-to-digital-conversion demultiplexing operation is proposed.
The fundamental principles of basic semiconductor optical waveguide components are reviewed. Fabrication of these components require similar material and processing technologies as those used for advanced injection lasers. Compatibility of material and processing permits integration of optical and electronic components to form circuits of potentially high complexity. The monolithic integration of optical lasers and amplifiers with other waveguide components requires a high degree of control of composition and thick-ness of layer growth and advanced processing technologies which are reviewed briefly. Present material and process technologies make monolithic integration particularly attractive for controlling and improving the performance of, injection lasers, for permitting novel high speed modulation schemes, and for a number of switching applications. The integrated optic circuit design must allow for relatively large fabrication tolerances for the individual components without causing significant performance degradation or potential reliability problems. The feasibility of integration is demonstrated with a few examples such as optical switches, optical isolators and electrooptically tunable injection lasers.
Recent progress in the development of integrated optical circuits using the III-V compound semiconductors is reviewed. The application of double heterostructure configurations is emphasized, not only for optical sources, but also for detectors, with reference to both the A.GaAs/GaAs system and the InGaAsP quaternary. Devices utilizing periodic corrugations are described briefly, whereas alternate attempts to fabricate optical circuits by etching or sputtering techniques are discussed in more detail. Recent advances in processing techniques suitable for optical integration, such as reactive-ion etching, and the use of lasers or electron beams for device processing, are described.
In this paper, we discuss the operation of a non-planar GaAs/GaA1As laser grown by metalorganic chemical vapor deposition. The non-planar structure is achieved by using a self-aligned masking technique. The masking causes lateral spatial variations to develop during the growth of the active and/or cladding regions. These lasers inherently have both lateral current confinement and lateral real refractive index waveguidance. Threshold currents of ≈40 mamps and differential quantum efficiencies of 32% are measured very reproducibly over a wafer.
The optical properties of GaAs make it a very useful material for the fabrication of optical emitters and detectors. GaAs also possesses electronic properties which allow the fabrication of high speed electronic devices which are superior to conventional silicon devices. Monolithic optoelectronic circuits are formed by the integration of optical and electronic devices on a single GaAs substrate. Integration of many devices is most easily accomplished on a semi-insulating (SI) sub-strate. Several laser structures have been fabricated on SI GaAs substrates. Some of these lasers have been integrated with Gunn diodes and with metal semiconductor field effect transistors (MESFETs). An integrated optical repeater has been demonstrated in which MESFETs are used for optical detection and electronic amplification, and a laser is used to regenerate the optical signal. Monolithic optoelectronic circuits have also been constructed on conducting substrates. A heterojunction bipolar transistor driver has been integrated with a laser on an n-type GaAs substrate.
Recent improvements in fiber optic semiconductor components (i.e. lasers and detectors) will lead to the rapid deployment of fiber optic systems in the next few years. This does not, however, signal the end of research in semiconductor optoelectronics, but rather the opening of new and ultimately more widespread application of optoelectronic devices. New and powerful signal processing and data transmission functions will become possible by the marriage of optical and electronic functions on a single monolithic wafer.
Various configurations of integrated photodetectors and optical guided wave structures will be discussed. Channel waveguide arrays integrated with charge-coupled devices (CCDs) formed on silicon have been used to perform transversal filtering, log detection, high quantum efficiency detection of X = .82μm, and high resolution imaging without requiring small detector array spacings. The latter is accomplished through use of a fan-out channel waveguide array. We shall also describe how computer simulation is utilized to predict crosstalk in linear imaging arrays. The simulation is based on analysis which includes the effects of focusing of the input light, the use of Kettler's equations to describe propagation in a lossy medium, and a solution of the generalized diffusion equation for excited carriers. Crosstalk for a conventional linear image array and one employing a fan-out channel waveguide array is evaluated and compared.
An advanced high speed imager preprocessor silicon micro electronic circuit for use with integrated and bulk real time A-0 spectrum analyzers is being developed. The circuits will be capable of analyzing and sorting by amplitude, frequency, and time of arrival, wide dynamic range optical signals presented at its focal plane.