High speed, efficient photodetectors are difficult to fabricate in standard silicon fabrication processes due to the long absorption length of silicon. However, high performance servers will soon require dense optical interconnects with low cost and high reliability, and this trend favors monolithic silicon receivers over hybrid counterparts. Recently, lateral PIN photodiode structures have been demonstrated in silicon CMOS technology with little or no process modifications. Optical receivers based on these detectors have achieved record performance in terms of speed and sensitivity. This paper will discuss the advantages, issues and recent advances in silicon-based photodetectors and optical receivers. This includes the fastest photodetector ever implemented in a standard bulk CMOS process, a 13.9 Gb/s lateral trench detector implemented in a modified EDRAM process, and a >15 GHz pure germanium photodiode grown directly on a silicon substrate.

We present the results of our studies of the quantum efficiency of Si photodiode close to the bandgap and some factors influencing Si photodiode sensitivity. To calculate the carrier collection efficiency function P(x), we used a corrected, explicit expression that takes into account the rear surface reflection of the light [1]. The method of calculation involves solution of the integral equation and allows determination of the carrier collection efficiency function with a high precision. We show that the exact integral equation for P(x) could be often, but not always replaced with the approximations similar to those proposed previously in [2,3]. However, employing the method proposed in this work allows quantifying such factors as e.g. the reflectance of the rear surface of the die and optical scattering within the diffusion layer, which is vital in designing highly efficient Si photodiodes in the near infrared spectral range. 1. C. Hicks, M. Kalatsky, R.A. Metzler, and A.O. Goushcha. Applied Optics, Vol. 42, No. 22. In press 2. L. Werner, J. Fischer, U. Johannsen and J. Hartmann, Metrologia 37, 279-284 (2000). 3. T.R. Gentile, J.M. Houston, and C.L. Cromer, Appl. Opt. 35, 4392-4403 (1996).

Arrays of embedded bipolar junction transistor (BJT) photo detectors (PD) and a parallel mixed-signal processing system were fabricated as a silicon complementary metal oxide semiconductor (Si-CMOS) circuit for the integration optical sensors on the surface of the chip. The circuit was fabricated with AMI 1.5um n-well CMOS process and the embedded PNP BJT PD has a pixel size of 8um by 8um. BJT PD was chosen to take advantage of its higher gain amplification of photo current than that of PiN type detectors since the target application is a low-speed and high-sensitivity sensor. The photo current generated by BJT PD is manipulated by mixed-signal processing system, which consists of parallel first order low-pass delta-sigma oversampling analog-to-digital converters (ADC). There are 8 parallel ADCs on the chip and a group of 8 BJT PDs are selected with CMOS switches. An array of PD is composed of three or six groups of PDs depending on the number of rows.

A new class of highly sensitive silicon photodetectors is based on the internal discrete amplification mechanism developed by Amplification Technologies, Inc. The key parameters of the novel photodetectors are high speed and ultra low excess noise at high levels of gain. The photodetectors work both * in the photon counting mode and * for analog proportional detection of few-photon light pulses. Performance parameters of these solid-state devices are comparable to those of vacuum PMTs, and even exceed them for some applications. The main parameters of the photodetectors in photon counting mode are the following: * short rise-fall time of one-electron pulse - less than 400 ps * high count speed - up to 500 MHz (in time gating mode) * timing resolution - 200 ps * fine one-electron pulse height distribution due to low (less than 1.05) excess noise factor

AlGaAsSb/AlGaSb heterostructures offer the ability to realize high-performance devices for 1550 nm high-speed optical interconnect applications. In this context, we present the design, fabrication, integration and characterization of 10 GHz p-i-n photodetectors in this material system. This effort has involved an investigation into inductively coupled plasma (ICP) etching of these materials and the development of a novel process for their conductive polymer based flip chip die attach.

As optoelectronic devices increase in speed, the measurement system used to characterize these devices must have sufficient bandwidth and minimum parasitic loading during test to accurately determine the intrinsic performance of the device under test. Conventional electrical measurement systems have an intrinsic bandwidth due to the available components for test and have parasitic loading due to direct electrical contact to the device under the test. Electro-optic sampling is an excellent measurement technique for characterizing ultra-fast devices because it has high bandwidth, is non-contact, is non-destructive, and relatively non-invasive. In this paper, an optical fiber-based electro-optic sampling system is designed and used for characterizing high speed InGaAs thin film MSM photodetectors. A fiber laser which is operating at 1556 nm wavelength was used for the sampling and excitation beam. Optical fibers were used to connect each component in the system for flexibility. InGaAs thin film MSM photodetectors were fabricated and characterized. InGaAs thin film MSM photodetectors were bonded onto a coplanar strip line deposited on a benzocyclobutene (BCB)-coated glass substrate for characterization. These thin film photodetectors show high speed operation combined with high responsivity and large detection area compared to P-I-N photodetectors operating at similar speeds

As an alternative approach to current electrical interconnection technology, optical interconnections at high speeds offer several potential advantages including small footprint, simple system design (in comparison to transmission lines), and immunity to electromagnetic interference. There are a number of approaches to integrating optical signal paths in electrical interconnection substrates such as backplanes, boards, and modules. One approach utilizes the heterogeneous integration of thin film optoelectronic (OE) devices embedded in waveguides. Optical signals can be coupled in from external fibers or from thin film lasers integrated onto the substrate, propagated, distributed, and processed in a planar waveguide format, and then coupled from the waveguide to an embedded thin film photodetector by evanescent field or direct coupling. This approach achieves alignment through assembly and successive masking layers and thus minimizes alignment issues. In addition, the integrated optical signal distribution system can be integrated onto the electrical interconnection substrate after the substrate has been fabricated using post processing, thus, the board facility is not impacted through the integration of the optical links. In this paper, a discussion of the fabrication processes as well as coupling efficiency and speed measurement results for thin film InGaAs PDs embedded in polymer waveguides integrated onto Si substrates is included. These results are compared to theoretical estimates of the coupling efficiency, which was estimated using the finite difference beam propagation method.

We have fabricated and characterized the first resonant cavity enhanced (RCE) germanium photodetectors on double silicon-on-insulator substrates (Ge/DSOI) for operation around the 1550 nm communication wavelength. The Ge layer is grown through a novel two-step UHV/CVD process, while the underlying double-SOI substrate is formed through an ion-cut process. Absorption measurements of an undoped Ge-on-Si (Ge/Si) structure reveal a red-shift of the Ge absorption edge in the NIR, due primarily to a strain-induced bandgap narrowing within the Ge film. By using the strained-Ge absorption coefficients extracted from the absorption measurements, in conjunction with the known properties of the DSOI substrate, we were able to design strained-Ge/DSOI photodetectors optimized for 1550 nm operation. We predict a quantum efficiency of 76% at 1550 nm for a Ge layer thickness of only 860 nm as a result of both strain-induced and resonant cavity enhancement, compared to 2.3% for the same unstrained Ge thickness in a single-pass configuration. We also estimate a transit-time limited bandwidth of 28 GHz. Although the fabricated Ge/DSOI photodetectors were not optimized for 1550 nm operation, we were able to demonstrate an over four-fold improvement in the quantum efficiency, compared to its single-pass counterpart.

The single-photon detection efficiency of various commercial InGaAs/InP avalanche photodiodes (APDs) operated in the Geiger mode has been reported previously. These studies showed substantial photon detection efficiency variation between individual devices, but did not indicate what device parameters might be responsible for this variation. We present data on the external single-photon detection efficiency of APDs operated as near-infrared single photon counters, and show how detection efficiency is related to both device design and operating conditions. We have fabricated APDs with near-infrared single-photon detection efficiency exceeding 50% at 10% excess bias, demonstrating that InGaAs/InP APDs of the proper design are well suited to many practical applications of photon counting in the 1.0 to 1.7 micron wavelength band.

We report on the fabrication of quantum grid infrared photodetector (QGIP) arrays and demonstrate their feasibility for use as multi-channel long wavelength infrared spectrometers. The quantum well infrared photodetector (QWIP) material structure was designed to exhibit broadband absorption in the wavelength range of 7 μm to 16 μm. By fabricating QGIP devices with this QWIP material, scattering of light at an individual wavelength of interest within the material absorption range can create narrow band detection in each device. Arrays of QGIP devices with varying geometry, each tailored to respond to a discrete wavelength were fabricated. Details of the epi-growth, processing steps taken to fabricate required device features for narrow band absorption of the QGIP devices, and characterization methods will be discussed.

Photodetectors (PDs) are an important active device in optoelectronic integrated circuits (OEICs), and, for shorter haul interconnections where circuit (e.g. transimpedance amplifier (TIA)) noise may be the dominant noise in receivers, metal-semiconductor-metal photodiodes (MSM PDs) are attractive due to their low capacitance per unit area compared to PIN photodetectors and the ease of monolithic integration with field effect transistors (FETs). Inverted-MSM PDs (I-MSM PDs), which are thin film MSM PDs with the fingers on the bottom of the device, have demonstrated higher responsivities compared to conventional MSM PDs while maintaining small capacitance per unit area, low dark current (~nA), and high speed. However, the modeling of MSM PDs and I-MSM PDs for insertion into circuit simulators for integrated PD/TIA modeling has not been reported. In this paper, an accurate high-frequency equivalent circuit-level model of thin film I-MSM PDs is obtained using an on-wafer measurement-based modeling technique. This circuit-level model of MSM PDs can be used for capacitance sensitive preamplifier design for co-optimization with widely used simulators (ADS and HSPICE). The obtained circuit-level model shows good agreement with measured s-parameters.

The trapping mechanisms at the origin of the persistent photocurrent effects in GaN-based devices have been studied on different time scales by characterizing a low barrier metal-semiconductor-metal GaN-based photodetector in the temperature range between room temperature and 500 K. The active material of the metal-semiconductor-metal device consists of a thin film of GaN grown by metal organic chemical vapour deposition. The Arrhenius plots obtained by the analysis of the decay times of the photocurrent as a function of the temperature on time scales from millisecond up to hours allowed us to calculate the activation energies of the mechanisms responsible for the persistent photocurrent. The activation energies derived from the decay times on the time scale of hours have been attributed to gallium vacancies (VGa), gallium antisites (GaN) and carbon impurities, whereas GaN excitonic resonances resulted to be responsible for the persistent photocurrent on the millisecond time scale. Finally, the influence of the decay times has been correlated with the photocurrent gain of the device, which resulted to be as high as 4.1×10⁵ at RT and 0.85×105 at 450 K.

Lincoln Laboratory has developed 32 x 32-pixel ladar focal planes comprising silicon geiger-mode avalanche photodiodes and high-speed all-digital CMOS timing circuitry in each pixel. In Geiger mode operation, the APD can detect as little as a single photon, producing a digital CMOS-compatible voltage pulse. This pulse is used to stop a high-speed counter in the pixel circuit, thus digitizing the time of arrival of the optical pulse. This "photon-to-digital conversion" simultaneously achieves single-photon sensitivity and 0.5-ns timing. We discuss the development of these focal planes and present imagery from ladar systems that use them.

The paper discusses opto-electrical properties of a 32x16 (and 16x16) element pin photodiode array built on a 30-μm thick single silicon die. The element size is ca. 1 mm square or smaller and the gaps between adjacent elements are as small as 100 μm. The arrays can be tiled facilitating the building of large scale photodetector matrices. The arrays have superior optical and electrical characteristics and are designed to work at zero Volts bias. The internal quantum efficiency is close to 100% within the spectral range 500 - 800 nm and could be tuned to a maximum value outside that spectral interval. The cross talk is smaller than 0.5% within the spectral range 400 to 1000 nm. The arrays are characterized with very low leakage currents, high shunt resistance, and low capacitance. Some other parameters of the array like the frequency bandwidth and capacitance are also discussed.

We provide the theory of operation of a recently proposed sensor array. The sensor is designed to detect changes in incident optical intensity that result when a non-uniform intensity distribution on the sensor is displaces laterally. This property allows the device to be used for optical vibration detection. The sensor is based on four-point measurements of photoconductance, which makes the device scalable and easy to fabricate. Some preliminary experimental results obtained with a prototype are given.

Efforts to exploit reduced dimensionality systems in semiconductor devices are presently driven by the continuing need to improve speed performance, transport efficiency, device density, and power management. In this work, we investigate the performance of novel GaAs/AlGaAs and InGaAs/InAlAs heterostructures for high-speed photodetector devices. First, a modulation-doped AlGaAs/GaAs device, suitable for monolithic integration with planar HEMT and FET devices, produces a built-in electric field that aids in the high-speed collection of photogenerated carriers. Surface Schottky electrodes on this structure form a planar interdigitated metal-semiconductor-metal (MSM) device for use at 850-nm wavelength. A second structure, an InGaAs/InAlAs quantum-well MSM photodetector for use at 1550-nm wavelength, utilizes recessed electrodes to contact directly the two-dimensional (2D) transport channel. Unfortunately, rather low Schottky barrier heights on undoped InGaAs lead to excessive dark currents when metal contacts are deposited directly on this material. To remedy this situation, we propose to form barrier-enhancement regions between the optically active 2D-quantum well and the lateral 3D-metal contacts by means of ion-implantation-induced quantum-well intermixing. Results indicate a reduction in dark current of nearly three orders of magnitude. Additionally, the high-speed performance appears not to be adversely affected under normal operating conditions by the potentially deleterious effects of carrier emission and accumulation at these heterojunction interfaces. The Fourier transform of a simulated transient current response to a light impulse indicates an electrical 3-dB bandwidth in excess of 50 GHz in a device with a recessed electrode gap of 1 μm.

In this paper we introduce the concept and technique of optical beam induced current (OBIC) generation at radio frequencies. The method is combined with lateral raster scanning of a tightly focused spot so as to generate a mapping of high spatial resolution. We demonstrate experimentally that if a mode-locked laser is used to excite the sample then the frequency transfer function of the optically active device is readily obtained with at least 1 µm spatial resolution, in real time. In addition, with the help of an appropriate electronic arrangement, we demonstrate how to obtain pseudocolored OBIC images of the sample.

The III-V nitrides (GaN and AlGaN) wide band gap semiconductors have been recognized recently as a very important technological material system for fabricating optoelectronic devices operating in the blue/ultraviolet (UV) spectral region and electronic devices capable of operating under high-power and high-temperature conditions. These materials are remarkably tolerant to aggressive environments, due to its thermal stability and radiation hardness and are excellent photodetector materials to cover the 240-360 nm range. A key advantage of III-nitrides detectors over competing devices based on semiconductors with smaller bandgaps is the long wavelength response cut-off, which is directly related to the bandgap of the material in the active region and thus does not require external filters. Metal-semiconductor-metal (MSM) photodiodes are of interest for many applications because of their relatively simple fabrication process, low dark currents, low noise, and fast response time. In this work, AlGaN-based MSM photodetectors with nickel (Ni) Schottky contacts were fabricated and characterized. A comparative study of the photodiodes characteristics were carried out. The thermal stability of the contacts at various annealing temperatures (300°C-700°C) was investigated. Cryogenic cooling after heat treatment was also performed to determine the effects of this treatment on the electrical characteristics of the devices. Electrical characterization was performed by current-voltage (I-V) measurement to investigate the Schottky contact properties of the photodetectors.