InGaAs-based focal plane arrays are an unrivaled uncooled SWIR technology. Prior analytical models of InGaAs have
been inaccurate at predicting the ultimate dark current limits for tight-pitch arrays. By going back to first-principles, we
have developed an improved analytic model. This model clarifies how tight pitch arrays suppress diffusion current and
why bulk generation-recombination is not a limiting factor in today's devices. We can thus explain our experimental
arrays with dark currents of 0.5nA/cm2 at 20C and <0.1nA/cm2 at 7C as well why we believe another order of
magnitude decrease in dark current is theoretically possible.
There are few choices when identifying detector materials for use in the SWIR wavelength band. We have exploited the
direct-bandgap InGaAs material system to achieve superior room temperature (293°K) dark current. We have
demonstrated sensitivity from 400nm through 2.6um with this material system and thus provide the opportunity to sense
not only the visible, but also the J-band (1.25um), H-band (1.65um) and K-band (2.2um) windows. This paper discusses
the advantages of our hybridized CMOS-InGaAs material system versus other potential SWIR material systems.
The monolithic planar InGaAs detector array enables 100% fill factor and thus, high external quantum efficiency. We
have achieved room-temperature pixel dark current of 2.8fA and shot noise of 110 electrons per pixel per second. Low
dark current at +300K allows uncooled packaging options, affording the system designer dramatic reductions in size,
weight (cameras <28grams), and power (<2.5W). Commercially available InGaAs pin arrays have shown diode lifetime
mean time between failures (MTBF) of 1011hours for planar InGaAs detectors1, far exceeding telecom-grade reliability
requirements. The use of a hybrid CMOS-InGaAs system allows best of breed materials to be used and permits efficient, cost-effective,
volume integration. Moreover, we will discuss how the InGaAsP material system is compatible with CMOS monolithic
integration. Taken together, these advantages, we believe, make InGaAs the obvious choice for all future SWIR
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.
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.
We report on the recent production release of our 320x240 pixel InGaAs/InP focal plane array and camera for visible and short-wavelength infrared light imaging. For this camera, we have fabricated a substrate-removed backside-illuminated InGaAs/InP photodiode array hybridized to a silicon read out integrated circuit (ROIC). Removing the InP substrate from the focal plane array allows visible wavelengths, which would otherwise be absorbed by the InP substrate due to its 920 nm wavelength cut-off, to reach the pixels’ active region. Quantum efficiency is approximately 15% at 500 nm, 70% at 850 nm, 85% at 1310 nm and 80% at 1550 nm. This focal plane array is useable for visible imaging as well as imaging eye-safe lasers and is of particular interest for day and low light level imaging as well as hyperspectral imaging.
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.