GaSb-based infrared (IR) photodetector technology progression is toward larger-format focal plane arrays (FPAs). This requires a performance-based and cost-based manufacturing process on larger diameter substrates for improved throughput, volume, and yield. IQE has demonstrated molecular beam epitaxy (MBE) growth processes for barrier-design detectors (nBn) in multi-wafer configurations on 4-inch and 5-inch diameter GaSb substrates, and via a metamorphic process on 4-inch and 6-inch GaAs substrates. Recently we took the next step in this progression, growing nBn detectors on 6-inch Si substrates coated with CVD-grown Ge, opening the door for potential integration with Si-based electronic circuitry. Here, we compare the epiwafer characteristics (morphology, x-ray, PL) and diode performance (turn-on, QE, cutoff wavelength) of this M-nBn on Ge-Si with the same M-nBn on GaAs and the corresponding nBn structure grown on native GaSb substrate. Similar performance was obtained on all three types of substrates. We also present FPA data based on a 640×512 pixel, 15 μm pitch process without substrate removal, where QE ~ 80%, NE▵T < 20 mK, and operability <99% was demonstrated. The results represent an important technological path toward next-generation large-format IR detector array applications.
An infrared sensor technology that has made quick progress in recent years is the photodiode based on Type-II
InAs/(In)GaSb strained layer superlattices (SLS). We have developed Focal Plane Arrays (FPAs) with up to a million
pixels, quantum efficiency exceeding 50%, and cutoff wavelength ~ 10 microns. SLS offers the promise of the high
quantum efficiency and operating temperature of longwave infrared mercury cadmium telluride (MCT) at the price point
of midwave infrared indium antimonide (InSb). That promise is rapidly being fulfilled. This paper presents the current
state-of-the-art of this sensor technology at this critical stage of its evolution.
In the last few years infrared focal plane arrays based on Type-I GaAs/AlGaAs quantum well infrared photodetectors
(QWIPs) have been commercialized, providing excellent cost-effective imaging for security and surveillance and gas
imaging applications. A second cooled infrared sensor technology that has made significant advances in recent years is
photodiodes based on Type-II InAs/(In)GaSb strained layer superlattices (SLS). Imaging chips with upto a million
pixels, quantum efficiency exceeding 50%, and cutoff wavelength exceeding 10 microns have been recently
demonstrated. SLS offers the promise of the high quantum efficiency and operating temperature of longwave infrared
mercury cadmium telluride (MCT) at the price point of QWIP and midwave infrared indium antimonide (InSb). That
promise is rapidly being fulfilled. This paper presents the current state-of-the-art of both these sensor technologies at
this critical stage of their evolution.
The infrared photon flux emitted by an object depends not only on its temperature but also on a
proportionality factor referred to as its emissivity. Since the latter parameter is usually not known
quantitatively a priori, any temperature determination based on single-band radiometric
measurements suffers from an inherent uncertainty. Recording photon fluxes in two separate
spectral bands can in principle circumvent this limitation. The technique amounts to solving a
system of two equations in two unknowns, namely, temperature and emissivity. The temperature
derived in this manner can be considered absolute in the sense that it is independent of the
emissivity, as long as that emissivity is the same in both bands. QmagiQ has previously
developed a 320x256 midwave/longwave staring focal plane array which has been packaged into
a dual-band laboratory camera. The camera in question constitutes a natural tool to generate
simultaneous and independent emissivity maps and temperature maps of entire two-dimensional
scenes, rather than at a single point on an object of interest. We describe a series of measurements
we have performed on a variety of targets of different emissivities and temperatures. We examine
various factors that affect the accuracy of the technique. They include the influence of the
ambient radiation reflected off the target, which must be properly accounted for and subtracted
from the collected signal in order to lead to the true target temperature. We also quantify the
consequences of spectrally varying emissivities.
The Thermal Infrared Sensor (TIRS) is a QWIP based instrument intended to supplement the Operational Land Imager
(OLI) for the Landsat Data Continuity Mission (LDCM) . The TIRS instrument is a dual channel far infrared imager
with the two bands centered at 10.8μm and 12.0μm. The focal plane assembly (FPA) consists of three 640x512 GaAs
Quantum Well Infrared Photodetector (QWIP) arrays precisely mounted to a silicon carrier substrate that is mounted on
an invar baseplate. The two spectral bands are defined by bandpass filters mounted in close proximity to the detector
surfaces. The focal plane operating temperature is 43K. The QWIP arrays are hybridized to Indigo ISC9803 readout
integrated circuits (ROICs). Two varieties of QWIP detector arrays are being developed for this project, a corrugated
surface structure QWIP and a grating surface structure QWIP. This paper will describe the TIRS system noise
equivalent temperature difference sensitivity as it affects the QWIP focal plane performance requirements: spectral
response, dark current, conversion efficiency, read noise, temperature stability, pixel uniformity, optical crosstalk and
pixel yield. Additional mechanical constraints as well as qualification through Technology Readiness Level 6 (TRL 6)
will also be discussed.
We present the performance of longwave infrared focal plane arrays (FPAs) made from Type-II InAs/GaSb strained
layer superlattice (SLS) photodiodes. In 320x256 FPAs operating at 77K, we measure cutoff wavelength ~ 8.5 μm, dark
current density ~ 10-5 A/cm2, quantum efficiency > 5% (with 2 μm -thick absorber photodiode), and pixel operability ~
96%. Device physics and FPA performance are graphed. Current challenges are discussed.
The development of type-II InAs/(In,Ga)Sb superlattice (SL) detectors with nBn design for single-color and
dual-color operation in MWIR and LWIR spectral regions are discussed. First, a 320 x 256 focal plane array (FPA) with
cutoff wavelength of 4.2 μm at 77K with average value of dark current density equal to 1 x 10-7 A/cm2 at Vb=0.7V (77
K) is reported. FPA reveals NEDT values of 23.8 mK for 16.3 ms integration time and f/4 optics. At 77K, the peak
responsivity and detectivity of FPA are estimated, respectively, to be 1.5 A/W and 6.4 x 1011 Jones, at 4 μm. Next,
implementation of the nBn concept on design of SL LWIR detectors is presented. The fabrication of single element nBn
based long wave infrared (LWIR ) with λc ~ 8.0 μm at Vb = +0.9 V and T = 100K detectors are reported. The bias
dependent polarity can be exploited to obtain two color response (λc1 ~ 3.5 μm and λc2 ~ 8.0 μm) under different polarity
of applied bias. The design and fabrication of this two color detector is presented. The dual band response (λc1 ~ 4.5 μm
and λc2 ~ 8 μm) is achieved by changing the polarity of applied bias. The spectral response cutoff wavelength shifts
from MWIR to LWIR when the applied bias voltage varies within a very small bias range (~100 mV).
QmagiQ LLC, has recently completed building and testing high operability two-color Quantum Well Infrared Photodetector (QWIP) focal plane arrays (FPAs). The 320 x 256 format dual-band FPAs feature 40-micron pixels of spatially registered QWIP detectors based on III-V materials. The vertically stacked detectors in this specific midwave/longwave (MW/LW) design are tuned to absorb in the respective 4-5 and 8-9 micron spectral ranges. The ISC0006 Readout Integrated Circuit (ROIC) developed by FLIR Systems Inc. and used in these FPAs features direct injection (DI) input circuitry for high charge storage with each unit cell containing dual integration capacitors, allowing simultaneous scene sampling and readout for the two distinct wavelength bands. Initial FPAs feature pixel operabilities better than 99%. Focal plane array test results and sample images will be presented.