Arvind I. D'Souza

Development of low dark current SiGe-detector arrays for visible-NIR imaging sensor

SiGe based Focal Plane Arrays offer a low cost alternative for developing visible- NIR focal plane arrays that will cover the spectral band from 0.4 to 1.6 microns. The attractive features of SiGe based IRFPAs will take advantage of Silicon based technology, that promises small feature size, low dark current and compatibility with the low power silicon CMOS circuits for signal processing. This paper discusses performance comparison for the SiGe based VIS-NIR Sensor with performance characteristics of InGaAs, InSb, and HgCdTe based IRFPAs. Various approaches including device designs are discussed for reducing the dark current in SiGe detector arrays; these include Superlattice, Quantum dot and Buried junction designs that have the potential of reducing the dark current by several orders of magnitude. The paper also discusses approaches to reduce the leakage current for small detector size and fabrication techniques. In addition several innovative approaches that have the potential of increasing the spectral response to 1.8 microns and beyond.

Fabrication of high-operating temperature (HOT), visible to MWIR, nCBn photon-trap detector arrays

We describe our recent efforts in developing visible to mid-wave (0.5 µm to 5.0 µm) broadband photon-trap InAsSb-based infrared detectors grown on GaAs substrates operating at high temperature (150-200K) with low dark current and high quantum efficiency. Utilizing an InAsSb absorber on GaAs substrates instead of an HgCdTe absorber will enable low-cost fabrication of large-format, high operating temperature focal plane arrays. We have utilized a novel detector design based-on pyramidal photon trapping InAsSb structures in conjunction with compound barrier-based device architecture to suppress both G-R dark current, as well as diffusion current through absorber volume reduction. Our optical simulation show that our engineered pyramid structures minimize the surface reflection compared to conventional diode structures acting as a broadband anti-reflective coating (AR). In addition, it exhibits > 70-80% absorption over the entire 0.5 µm to 5.0 µm spectral range while providing up to 3× reduction in absorber volume. Lattice-mismatched InAs0.82Sb0.18 with 5.25 µm cutoff at 200K was grown on GaAs substrates. 128×128/60μm and 1024×1024/18μm detector arrays that consist of bulk absorber as well as photon-trap pyramid structures were fabricated to compare the detector performance. The measured dark current density for the diodes with the pyramidal absorber was 3× lower that for the conventional diode with the bulk absorber, which is consistent with the volume reduction due to the creation of the pyramidal absorber topology. We have achieved high D* (< 1.0 x 1010 cm √Hz/W) and maintain very high (< 80 %) internal quantum efficiency over the entire band 0.5 to 5 µm spectral band at 200K.

HgCdTe multispectral infrared FPAs for remote sensing applications

Infrared (IR) remote sensing imaging applications require high-performance Focal Plane Arrays (FPAs) operating in all ranges of the IR spectrum. Short wavelength (SWIR; 1 to 3 micrometer), middle wavelength (MWIR; 3 to 5 micrometer), mid- long wavelength (MLWIR; 6 to 8 micrometer), long wavelength (LWIR; 8 to 14 micrometer), and very long wavelength (VLWIR; greater than 14 micrometer). These diverse spectral bands require high performance detectors and Read Out Integrated Circuits (ROICs) to perform the multi-spectral mission requirements. Significant progress in the design and fabrication of HgCdTe detector arrays and Read Out Integrated Circuits (ROICs) over the past few years has led to the demonstration of high resolution, low noise and large format reliable FPAs. Hybrid FPAs have been measured and their performance parameters are presented. Focal Plane Array D* performance values have been obtained for a multitude of spectral ranges and configurations that include; (1) (lambda) c equals 1.8 micrometer, 12 X 256 arrays operating at 295 K with median D* approximately 1.4 X 1012 cm Hz1/2/W, (2) (lambda) c equals 2.4 micrometer, 12 X 256 arrays operating at 250 K with median D* equals 1.6 X 1012 cm Hz1/2/W, detectors used are grown by MBE on lattice matched CdZnTe, (3) PACE-1 detectors with (lambda) c equals 2.5 micrometer, 1024 X 1024 arrays operating at 115 K with peak D* of 2.3 X 1013 cm Hz1/2/W at a background flux (phi) b equals 1.2 X 1011 ph/cm2- s, (4) MBE HgCdTe on Silicon MWIR detectors have been fabricated and the detector RoA performance for (lambda) co approximately 5.0 micrometer is in the 106 to 107 ohm-cm2 range at 78 K. (5) MBE HgCdTe on CdZnTe detectors, ((lambda) c equals 15.8 micrometer at 65 K), 128 X 128 array operating at 40 K with peak D* of 2.76 X 1011 cm Hz1/2/W at a background flux (phi) b equals 8.0 X 1015 ph/cm2-s. High performance 640 X 480 arrays imaging in the MWIR band have been fabricated on CdZnTe and PACE-1 substrates. The performance of these and additional FPAs will be presented.

HgCdTe and silicon detectors and FPAs for remote sensing applications

Photon detectors and focal plane arrays (FPAs) are fabricated from HgCdTe and silicon in many varieties. With appropriate choices for bandgap in HgCdTe, detector architecture, dopants, and operating temperature, HgCdTe and silicon can cover the spectral range from ultraviolet to the very-long-wavelength infrared (VLWIR), exhibit high internal gain to allow photon counting over this broad spectral range, and can be made in large array formats for imaging. DRS makes HgCdTe and silicon detectors and FPAs with unique architectures for a variety of applications. Detector characteristics of High Density Vertically Integrated Photodiode (HDVIP) HdCdTe detectors as well as Focal Plane Arrays (FPAs) are presented in this paper. MWIR[λc(78 K) = 5 μm] HDVIP detectors RoA performance was measured to within a factor or two or three of theoretical. In addition, 256 x 256 detector arrays were fabricated. Initial measurements had seven out of ten FPAs having operabilities greater than 99.45% with the best 256 x 256 array having only two inoperable pixels. LWIR [λc(78K)~10 μm] 640 X 480 arrays and a variety of single color linear arrays have also been fabricated. In addition, two-color arrays have been fabricated. DRS has explored HgCdTe avalanche photo diodes (APDs) in the λc = 2.2 μm to 5 μm range. The λc = 5 μm APDs have greater than 200 DC gain values at 8 Volts bias. Large-format to 10242 Arsenic-doped (Si:As, λc ~ 28 μm), Blocked-Impurity-Band (BIB) detectors have been developed for a variety of pixel formats and have been optimized for low, moderate, and high infrared backgrounds. Antimony-doped silicon (Si:Sb) BIB arrays having response to wavelengths > 40 μm have also been demonstrated. Avalanche processes in Si:As at low temperatures (~ 8 K) have led to two unique solid-state photon-counting detectors adapted to infrared and visible wavelengths. The infrared device is the solid-state photomultiplier (SSPM). A related device optimized for the visible spectral region is the visible-light photon counter (VLPC). The VLPC is a nearly ideal device for detection of small bunches of photons with excellent time resolution. Finally, DRS makes imaging arrays of pin-diodes utilizing the intrinsic silicon photoresponse to provide high performance over the 0.4-1.0 μm spectral range operating near room temperature. pin-diode arrays are particularly attractive as an alternative to charge-coupled devices (CCDs) for space applications where radiation hardening is needed. In addition, wire grid micropolarizers have been demonstrated and two color doped silicon detectors using diffractive microlenses are being developed. Precision alignment of sensor chips with respect to a base mounting plate has been demonstrated to be within 2 μm. A similar technique is also utilized to align single large detectors for sounder applications in focal plane arrays (FPAs). FPAs for space applications with the associated cold and warm electronics and packaging/cables have been fabricated.

Au- and Cu-doped HgCdTe HDVIP detectors

Detector characteristics of Au- and Cu-doped High Density Vertically Integrated Photodiode (HDVIP) detectors are presented in this paper. Individual photodiodes in test bars were examined by measuring I-V curves under dark and illuminated conditions at high bias values. Noise as a function of frequency has been measured on Au- and Cu-doped MWIR [λc(78 K) = 5 μ] HDVIP HgCdTe diodes at several temperatures under dark and illuminated conditions. No excess currents are observed above the photocurrents for reverse bias values out to 500 mV. Both Au- and Cu-doped detectors measured at 85 K, exhibit gain values between 40 and 50 at 8 V reverse bias. Gain values fell in this same range even when the flux incident on each type of detector was varied. The excess noise factor for the Cu-doped detectors ranged from 1.35 to 1.69 depending on the incident flux. Variation is probably due to measurement error. The noise at 8 V reverse bias is white for the Cu-doped detectors. The Au-doped detectors exhibited 1/f noise at 8 V reverse bias. At higher frequencies where the noise spectrum was quasi-white, the excess noise factor for the Au-doped detector was in the 1.0 to 1.5 range.

Cross-track infrared sounder FPA performance

The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Cross-track Infrared Sounder (CrIS) is an interferometric sensor that measures earth radiances at high spectral resolution, using the data to provide pressure, temperature and moisture profiles of the atmosphere. The pressure, temperature and moisture sounding data are used in weather prediction models that track storms, predict levels of precipitation etc. The CrIS instrument contains SWIR ((λc approximately 5 μm at 98K), MWIR (λc approximately 9 μm at 98K) LWIR (λc approximately 16 μm at 81K) Focal Plane Array (FPA) modules. A critical CrIS design selection was the use of photovoltaic (PV) detectors in all three spectral bands. PV detectors have the important benefits of high sensitivity and linearity. Each FPA modules consists of nine large (1000 μm diameter) photovoltaic detectors with accompanying cold preamplifiers. This paper describes the performance for all the modules forming the CrIS Detector Preamplifier Module (DPM). Molecular Beam Epitaxy (MBE) is used to grow the appropriate bandgap n-type Hg1-xCdxTe on lattice matched CdZnTe. SWIR, MWIR and LWIR 1000 μm diameter detectors have been manufactured using the Lateral Collection Diode (LCD) architecture. Custom pre-amplifiers have been designed to interface with the large SWIR, MWIR and LWIR detectors. The operating temperature is above 78K, permitting the use of passive radiators in spacecraft to cool the detectors. Recently fabricated 1000 micrometers diameter photovoltaic detectors have the measured performance parameters listed in the Table below. Expected D* performance from the detector/pre-amplifier models are also listed in the table. The D* values are calculated at the CrIS program peak wavelength specified for each spectral band.

LWIR, photovoltaic, Hg1-xCdxTe, and FPA performance for remote sensing applications

Focal plane arrays (FPAs), used for remote sensing applications, are required to operate at high temperatures and are subject to high terrestrial background fluxes. Typical remote sensing applications like cloud/weather imagery, sea- surface temperature measurements, ocean color characterization, and land-surface vegetation indices also require FPAs that operate from the visible through the LWIR portion of the spectrum. This combination of harsh requirements have driven the design of a unique LWIR FPA, that operates at 80 K under 300 K background conditions, with an operating spectral range from 11.5 micrometers to 12.5 micrometers , and a spectral cutoff of 13.5 micrometers . The FPA consists of 2 side by side arrays of 1 X 60 HgCdTe, (grown by molecular beam epitaxy) photovoltaic, detector arrays bump bonded to a custom CMOS Si readout. The 2 arrays are completely independent, and can be operated as such. The readout unit cell uses two, current-mode, analog building blocks; a Current Conveyor (CC1) and a dynamic current mirror. The CC1 has input impedance below 300 Ohms and an injection efficiency that is independent of the detector characteristics. This combination extracts high performance and excellent sensitivity from detectors whose average RoA values are approximately 1.7 Ohm-cm2 at T equals 80 K. The dynamic current mirror is used to subtract high background photocurrent while preserving excellent dynamic range. In addition to the performance enhancing readout, the detectors are manufactured with integral microlenses and operated in reverse bias to take advantage of their increased dynamic impedance. The dark currents associated with reverse bias operation are subtracted along with the background photocurrents by the dynamic current mirror. The expected and measured LWIR FPA performance will be presented. Measurements were performed on an LWIR FPA. Expected and measured FPA results are shown in the table below. The expected data are calculated from FPA models and compared to the measured values.

Room temperature SWIR sensing from colloidal quantum dot photodiode arrays

While InGaAs-based focal plane arrays (FPAs) provide excellent detectivity and low noise for SWIR imaging applications, wider scale adoption of systems capable of working in this spectral range is limited by high costs, limited spectral response, and costly integration with Si ROIC devices. RTI has demonstrated a novel photodiode technology based on IR-absorbing solution-processed PbS colloidal quantum dots (CQD) that can overcome these limitations of InGaAs FPAs. The most significant advantage of the CQD technology is ease of fabrication. The devices can be fabricated directly onto the ROIC substrate at low temperatures compatible with CMOS, and arrays can be fabricated at wafer scale. Further, device performance is not expected to degrade significantly with reduced pixel size. We present results for upward-looking detectors fabricated on Si substrates with sensitivity from the UV to ~1.7 µm. We further show devices fabricated with larger size CQDs that exhibit spectral sensitivity that extends from UV to 2 µm.

High-performance SWIR sensing from colloidal quantum dot photodiode arrays

RTI has demonstrated a novel photodiode technology based on IR-absorbing solution-processed PbS colloidal quantum dots (CQD) that can overcome the high cost, limited spectral response, and challenges in the reduction in pixel size associated with InGaAs focal plane arrays. The most significant advantage of the CQD technology is ease of fabrication. The devices can be fabricated directly onto the ROIC substrate at low temperatures compatible with CMOS, and arrays can be fabricated at wafer scale. Further, device performance is not expected to degrade significantly with reduced pixel size. We present results for upward-looking detectors fabricated on Si substrates with sensitivity from the UV to ~1.7 μm, compare these results to InGaAs detectors, and present measurements of the CQD detectors temperature dependent dark current.

Front Matter: Volume 8512

This PDF file contains the front matter associated with SPIE Proceedings Volume 8512, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.