Different type-II InGaAs/GaAsSb quantum well design structures on InP substrate for mid-infrared
emission has been modeled by six band k•p method. The dispersion relations, optical matrix element,
optical gain and spontaneous emission rate are calculated. The effects of the parameters of quantum wells
(thickness, composition) and properties of cladding layers were investigated. For injected carrier
concentration of 5×1012 cm-2, peak gain values around 2.6-2.7 μm wavelengths of the order of 1000 cm-1
can be achieved, which shows that type-II InGaAs/GaAsSb quantum wells are suitable for infrared laser
operation beyond 2μm at room temperature.
KEYWORDS: Avalanche photodiodes, Single photon, Data modeling, Negative feedback, Single photon detectors, Capacitance, Photodetectors, Sensors, Temperature metrology, Avalanche photodetectors
We report on progress in improving fundamental properties of InP-based single photon avalanche diodes (SPADs) and
recent trends for overcoming dominant performance limitations. Through experimental and modeling work focused on
the trade-off between dark count rate (DCR) and photon detection efficiency (PDE), we identify the key mechanisms
responsible for DCR over a range of operating temperatures and excess bias voltages. This work provides a detailed
description of temperature- and bias-dependent DCR thermal activation energy Ea(T,V), including the crossover from
low Ea for trap-assisted tunneling at temperatures below ~230 K to larger Ea for thermal generation at temperatures
approaching room temperature. By applying these findings to new device design and fabrication, the fundamental
tradeoff between PDE and DCR for InP/InGaAs SPADs designed for 1.55 μm photon detection has been managed so
that for PDE ~ 20%, devices routinely exhibit DCR values of a few kHz, while "hero" devices demonstrate that it is
possible to achieve sub-kHz DCR performance at temperatures readily accessible using thermoelectric coolers.
However, important limitations remain, particularly with respect to maximum count rates. Strategies adopted recently
to circumvent some of these present limitations include new circuit-based solutions involving high-speed very short-duration
gating as well as new monolithic chip-level concepts for obtaining improved performance through avalanche
self-quenching. We discuss these two approaches, and we describe recent results from devices with monolithically
integrated quench resistors that achieve rapid self-quenching, accompanied by evidence for a partial discharge of the
detector capacitance leading to charge flows as low as ~3 ×105 carriers associated with each avalanche event.
We report reduced afterpulsing for a high-performance InGaAs/InP single photon avalanche photodiode (SPAD) using a
gated-mode passive quenching with active reset (gated-PQAR) circuit. Photon detection efficiency (PDE) and dark count
probability (DCP) were measured at a gate repetition rate of 1 MHz. With a double-pulse measurement technique, the
afterpulsing probability was measured for various hold-off times. At 230K, 0.3% afterpulsing probability for a 10 ns
hold-off time was achieved with 13% PDE, 2×10-6 DCP and 0.4 ns effective gate width. For the same hold off time,
30% PDE and 1×10-5 DCP was achieved with 6% afterpulsing probability for an effective gate width of 0.7 ns.
The operation of InP-based single photon avalanche diodes (SPADs) in Geiger mode provides great utility for the
detection of single photons at near-infrared wavelengths between 1.0 and 1.6 μm. However, SPADs have performance
limitations with respect to photon counting rate and the absence of photon number resolution that, at the most
fundamental level, can be traced back to the positive feedback inherent in the impact ionization-driven avalanche
process. In this paper, we describe the inclusion of negative feedback with best-in-class InP-based single photon
avalanche diode (SPAD) structures to form negative feedback avalanche diodes (NFADs) in which many of the
present limitations of SPAD operation can be overcome. The use of thin film resistors as monolithic passive negative
feedback elements ensures rapid self-quenching with very low parasitic effects. We demonstrate a qualitative
difference in the performance of NFADs in the two regimes of small and large negative feedback. With small
feedback, we have studied the behavior of the persistent current prior to quenching, for which we have found
oscillatory behavior as well as an exponentially distributed duration. For large feedback, we find rapid quenching,
accompanied by evidence for a partial discharge of the detector capacitance, leading to charge flows as low as ~3 ×105
carriers associated with each avalanche event.
We present a new and innovative short-wave infrared (SWIR) hyperspectral imaging focal plane array (FPA)
concept for bulk and trace standoff explosives detection. The proposed technology combines conventional
uncooled InGaAs based SWIR imaging with the wavelength selectivity of a monolithically integrated solid-state
Fabry-Perot interferometer. Each pixel of the array consists of a group of sub-pixels in which each sub-pixel is
tuned to absorb a separate wavelength. The relative responses from the sub-pixels (i.e. wavelengths) are
compared to the spectral characteristics of explosives in the SWIR to detect and locate them within an imaged
scene among various background materials.
The novel technology will be compact, and consume low power such that it can be used as a handheld device or
mounted for persistent surveillance of crowded areas and checkpoints. The technology does not use any
scanning nor tuning apparatuses such as MEMS devices, and is therefore fast, compact, lightweight and not
susceptible to vibration. The technology is therefore ideal for man portable applications and unmanned vehicle
platforms. An eyesafe (covert) illuminator may be used to provide illumination in situations when ambient light
conditions are not sufficient. We will present a detailed design of the novel focal plane array and a theoretical
standoff distance and false rates study.
Goodrich, SUI has developed a 15 μm pitch, 1280 x 1024 pixel InGaAs focal plane array (FPA) with low noise, and
visible to near infrared (0.4 μm to 1.7 μm) wavelength response for day and night vision applications. The readout
integrated circuit (ROIC), which uses a capacitive transimpedance amplifier (CTIA) pixel, is designed to achieve a noise
level of less than 50 electrons, due to its small integration capacitor. The ROIC can be read out at 120 frames per second,
and has a dynamic range of 3000:1 using rolling, non-snapshot integration. The ROIC was fabricated in a standard
CMOS foundry process, and was bump-bonded to Vis-InGaAsTM detector arrays. SUI has successfully hybridized 15 μm
pitch 1280 x 1024 pixel FPAs, and produced imagery.
Under the DARPA Photon Counting Arrays (PCAR) program we have investigated technologies to reduce the overall noise level in InGaAs based imagers for identifying a man at 100m under low-light level imaging conditions. We report the results of our experiments comprising of 15 InGaAs wafers that were utilized to investigate lowering dark current in photodiode arrays. As a result of these experiments, we have achieved an ultra low dark current of 2nA/cm2 through technological advances in InGaAs detector design, epitaxial growth, and processing at a temperature of +12.3°C. The InGaAs photodiode array was hybridized to a low noise readout integrated circuit, also developed under this program. The focal plane array (FPA) achieves very high sensitivity in the shortwave infrared bands in addition to the visible response added via substrate removal process post hybridization. Based on our current room-temperature stabilized SWIR camera platform, these imagers enable a full day-night imaging capability and are responsive to currently fielded covert laser designators, illuminators, and rangefinders. In addition, improved haze penetration in the SWIR compared to the visible provides enhanced clarity in the imagery of a scene. In this paper we show the results of our dark current studies as well as FPA characterization of the camera built under this program.
The DARPA PCAR program is sponsoring the development of low noise, near infrared (1.5 &mgr;m wavelength) focal
plane arrays (FPAs) for night vision applications. The first phase of this work has produced a collection of 640 x 512
pixel, 20 &mgr;m pitch FPAs with low noise. The approach was to design four different read out integrated circuits
(ROICs), all compatible with the same bump-bonded InGaAs photodiode detector array. Two of the designs have
capacitive transimpedance amplifier (CTIA) pixels, each with a somewhat different amplifier design and with two
different sizes of small integration capacitors. The third design is a source follower per detector (SFD) pixel,
integrating on the detector capacitance. The fourth design also integrates on the detector capacitance, but uses a
moderate gain, in-pixel amplifier to boost the signal level, and also has a differential pixel output. All four designs
require off-chip correlated sampling to achieve the desired noise level. The correlated sampling is performed digitally
in the data acquisition software. Each design is capable of 30 frames per second read out rate, and has a dynamic range
of 1000:1 using a rolling, non-snapshot integration. The designs were fabricated in a standard CMOS foundry process,
and were bump-bonded to InGaAs detector arrays. All four designs are working without any significant design errors,
and are producing low noise imaging, with less than 50 electrons rms noise per pixel after correlated double sampling.
Military imaging is the largest application sector for shortwave infrared (SWIR) detectors, followed by spectroscopy (for the sorting of products and materials), and thermal sensing. Each application places different demands on the detectors, and fulfilling these requirements has driven the production of higher-quality, lower-cost imagers. The Visible SWIR Camera images digital pictures under day and starlight-only conditions, enabling the transmission of those images between soldiers on the battlefield. Additional functions are a windowing capability for comm link reception, and range-gating ability (viewing a specific depth of field at a specified range.) The combination of gated and video imaging is achieved through a high bandwidth pixel with a capacitive transimpedance amplifier (CTIA) design. Two different sensitivities in the CTIA pixel are achieved by switching between two feedback capacitor sizes, allowing for different illumination conditions. Anti-blooming is provided in the all solidstate gated camera, to prevent charge spreading from oversaturated pixels. All pixels are gated simultaneously for "snapshot" exposure. The low dark current and high bandwidth of the InGaAs photodetectors enables both high sensitivity imaging at long exposure times and high bandwidth at short exposure times. The spectral response of InGaAs extends from 0.9 μm to 1.7 μm, The Visible SWIR Camera is very reliable, in addition to being small and lightweight.
Extended wavelength InGaAs material is ideal for laser beam profiling applications from 1 micron to 2 microns wavelength. We report on a focal plane array and camera designed specifically for this application. The format of the camera is 320 x 256 pixels on a 25 micron pitch, and the operation is snapshot exposure with a 16 ms exposure time. The camera may be triggered for synchronization with laser pulses and has a 60 Hz maximum readout rate. Two challenges are encountered with extended wavelength InGaAs material compared to lattice matched material. The first is lower quantum efficiency at the shorter wavelengths due to transitional buffer layers that absorb at the shorter wavelengths. The second is the larger dark current caused by lattice mismatch between the InP substrate and the absorption layers. Neither challenge is a problem for laser beam profiling, since a large energy or power is available from the source. To accommodate the dark current, a gate modulated (GMOD) readout circuit is used, where the continuously variable capacity is increased to several million electrons. Both CW and pulsed illumination linearity are good, allowing accurate profiling. The temperature of the focal plane array is held near room temperature with a thermoelectric cooler for stability. To provide a corrected image, nonuniformity corrections for offset and gain are stored in the camera.
Hyperspectral imaging has been receiving much attention for its potential for high-resolution imaging and target recognition, chemical analysis and spectroscopy. In target recognition, identifying targets in cluttered and partially obscured environments requires the analysis of spectral content of the scenery. Spectroscopy type of applications can benefit from the real-time data collection of spatial and spectral content in a single image capture. We report on the design, simulation and fabrication of integrating MEMs tunable Fabry-Perot etalon filters with 2 dimensional InGaAs focal plane arrays for simultaneous spectral and spatial imaging. By tuning the transmission wavelength of the MEMs based filter, the spectral information is provided at each pixel of the photodiode array. The MEMs device is based on two InP/air-gap DBR reflectors, and a single wavelength air cavity that separates them. The selective etching of InGaAs forms the air gaps that suspend the quarter wavelength InP reflector layers. The top mirror reflectivity as well as the cavity air-gap is tuned by deflecting the suspended InP layer through a reverse biased p-i-n junction. Due to the high refractive index contrast of InP and air, the spectral width of the DBR reflectors is wide enough to block transmitted light from 1000nm to 1700nm, allowing the InGaAs absorber layer to detect only the MEMs filtered spectral content. A theoretical study on wide tuning range designs and the expected FWHM will be presented.
Resonant cavity enhanced (RCE) photodiodes are promising candidates for applications in optical communications and interconnects where ultrafast high-efficiency detection is very desirable. In RCE structures, the electrical function of the photodiode is largely unchanged, but optically it is subject to the effects of the cavity, mainly wavelength selectivity and a large enhancement of the resonant optical field. The increased optical field allows photodetectors to be made thinner and therefor faster in the transit-time limited operation, while simultaneously maintaining a high quantum efficiency at the resonant wavelengths. The combination of RCE detection scheme with Schottky photodiodes allows for fabrication of high-performance photodetectors with relatively simple material structure and fabrication process. In RCE Schottky photodiodes, a semi-transparent metalization can be used simultaneously as the electrical contact and the top reflector for the resonant cavity. Device performance is optimized by varying the thickness of the Schottky metalization and utilizing a dielectric matching layer. We present theoretical and experimental results on spectral and high-speed properties. We have demonstrated RCE Schottky photodiodes in (Al, In)GaAs/GaAs material system with temporal response of 10 ps full-width-at-half-maximum. These results were measurement setup limited and a conservative estimation of the bandwidth corresponds to more than 100 GHz. The photodiodes were designed and fabricated for 900 nm and 840 nm resonant wavelengths. The best measured quantum efficiency is around 50% which is slightly less than the theoretical prediction for these devices.
We describe a new method of sensing the linear polarization of light in a single mesa device structure by vertically integrating two photodetectors. The monolithic architecture eliminates the need for several discrete components, such as polarization filters and beam splitters, thus reducing critical alignment requirements and cost for various optical systems. Applications include the simplification of reading heads in magneto-optical (MO) data storage devices and constructing imaging arrays for polarization vision. In imaging, polarization sensing can extract additional information from a scene otherwise not noticeable to the human eye, facilitating remote sensing, material classification, and biological imaging. The operation principle of our vertical cavity polarization detector (VCPD) is based on a resonant cavity enhanced (RCE) photodetector, being vertically integrated with a conventional photodetector. The RCE detector is constructed by integrating a thin absorption region into an asymmetric Fabry-Perot cavity. The top reflector is formed by the semiconductor air interface, while the bottom mirror is a distributed Bragg reflector (DBR). For off-normal incidence of light, the reflectivity of the semiconductor-air interface and DBR are significantly different for TE (s) and TM (p) polarizations. Thus the RCE detector provides resonance enhancement for TE, capturing the TE polarized light in the top detector. For TM polarized light, both reflectivities are small, therefore, light is transmitted to and absorbed in the bottom detector. A large contrast in TE/TM response of the top and bottom detectors is achieved and the linear polarization can be computed from their relative responses. Experimental results displaying good agreement with simulation results have been recently achieved and are presented.
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