In mid-wavelength infrared detectors built with the InAs/InAsSb superlattice, a study of the absorption behavior near the cutoff wavelength was conducted. Two results are, first, an accurate method for determining the bandgap energy from the spectral response of a fabricated device, and second, an analysis of the Urbach tail of the absorption showing that its characteristic energy is consistent with the dominant phonon of InAs. Additionally, a temperature-dependent measurement of the transport of photo-generated holes in nBn devices can be used to identify the effects of localization due to layer nonuniformities, and to measure the energy step at hole-blocking features.
We have studied infrared absorption near the bandgap energy in mid-wavelength (MWIR) III-V photon detectors built in the nBn configuration. The absorbing material is the InAs/InAsSb superlattice. We show that in a practical device near the infrared (IR) cutoff, the spectral response curve as a function of photon energy is proportional to the absorption coefficient, to a good approximation. Thus, in the near-gap range, the energy dependence of the device spectral response is a reliable proxy for the energy dependence of the absorption coefficient. We demonstrate this by means of an expansion of the Hovel equations in powers of the product of the absorption coefficient and hole diffusion length. One application of this result is that the point of maximum slope of the spectral response curve can be used to locate the true bandgap energy. This result also facilitates a study of absorption in the Urbach tail, which occurs at sub-gap energies. The temperature dependence of the Urbach steepness parameter was found to be consistent with the dominant phonon energy of InAs.
Random telegraph signal (RTS) noise is ubiquitous in electronic and electro-optical devices, having been observed in MOSFETs and photodiode arrays. For imaging arrays, in particular, RTS noise (blinking pixels or "blinkers") deteriorates system performance through poor nonuniformity correction (NUC) stability and degrades image quality with blinking pixel behavior that can distract human operators and confuse computer vision algorithms. To date, there exists no universally accepted identification method or description of RTS noise in photodetectors, nor a conventional analysis approach to determine its origin. Current approaches typically focus on spectral properties (RTS noise is characterized by a Lorentzian power spectrum), which can be expensive to compute through Fourier methods, and analysis is usually performed on only a small sample of pixels. Here, we propose a method to identify and characterize blinkers by training a hidden Markov model (HMM) to extract the principal parameters governing blinking behavior, including the underlying state space, the state transition probabilities, and the distribution of state output levels. We find evidence to support classifying blinking behavior with HMM parameters; the variation of the model parameters with extrinsic variables, such as the temperature and applied bias, give some indication of the underlying physical mechanisms. Specifically, we find the timescale of the blink current is longer than typical electron{phonon, electron{electron, and electron{photon interactions, which leads to the suggestion that the blinking mechanism may be related to trap occupation dynamics.
Valence band features affecting carrier transport in III-V superlattice nBn detectors.
David R. Rhiger and Edward P. Smith,
Raytheon Vision Systems.
We have investigated non-ideal features occurring in the valence band profile of nBn detectors which affect the transport of minority-carrier holes representing the IR signal. The objectives are to reduce dark currents and improve quantum efficiency. The nBn device consists of an n-type absorber several microns thick, plus a very thin electron barrier B and a thin n-type collector (top contact region). In this investigation, the absorber and collector were built with the InAs/InAsSb superlattice. For normal operation, holes generated by photons in the absorber must flow to the collector. Current is promoted by a combination of diffusion and electric field drift. However, in some cases the transport of holes is limited by (1) absorber-barrier valence band misalignment, (2) bandgap difference between collector and absorber, or (3) possible localization sites in the absorber due to compositional fluctuations. These characteristics, when combined with the known limitations of hole diffusion length, can adversely affect the quantum efficiency, and require the application of an operating bias that is larger than otherwise necessary. We have been able to identify and measure these valence band features by analyzing device characteristics as a function of temperature, bias voltage, photon flux, and wavelength dependence of the response. Examples will be presented.
This work was supported by Dr. Meimei Tidrow of NVESD, Contract Number W15P7T-06-D-E402, Task BD30, Agreement No. S08-092256, Purchase Order P000006939.
Improved LWIR sensors are needed for defense applications. We report an advance in sensor
technology based on diodes in type-II strained layer superlattice structures built in the
InAs/GaSb/AlSb materials system. A key feature of the devices is a pair of complementary barriers,
namely, an electron barrier and a hole barrier formed at different depths in the growth sequence. The
structure is known as CBIRD. This work is a collaborative effort between Raytheon Vision Systems
and Jet Propulsion Laboratory, with design and growth being performed at JPL, and processing and
testing at RVS. We have analyzed the current-voltage characteristics as functions of temperature and
junction area, and have measured the spectral response and quantum efficiency as functions of bias
voltage. From the temperature dependence of the dark current in a typical case, we infer that the
effective barrier height is 0.175 eV. This indicates that dark current is limited by the barriers rather
than diffusion or GR mechanisms occurring within the absorber region where the bandgap is 0.13 eV.
The barriers prove to be very effective in suppressing the dark current. In the case of a detector
having a cutoff wavelength of 9.24 μm, we find R0A > 105 ohm cm2 at 78 K, as compared with about
100 ohm cm2 for an InAs/GaSb homojunction of the same cutoff. For good photo response, the
device must be biased to typically -200 or -250 mV. In this condition we find the internal quantum
efficiency to be greater than 50%, while the RA remains above 104 ohm cm2. Thus, the device shows
both high RA and good quantum efficiency at the same operating bias. We have also measured the
capacitance of the CBIRD device as functions of bias and frequency to help characterize the behavior
of the barriers. A 256×256 focal plane array was fabricated with this structure which showed at 78K a
responsivity operability of more than 99%.
The development of InAsSb detectors based on the nBn design for the mid-wave infrared (MWIR) spectral region is
discussed. Comparisons of optical and electrical properties of InAsSb photodetectors with two different barrier material,
namely, AlAs 0.15Sb0.75 (structure A) and AlAs0.10Sb0.9 (structure B) are reported. The dark current density in the
AlAs0.15Sb0.85 is lower possibly due to the larger valence band offset. Clear room temperature spectral responses is
observed and a specific detectivity (D*) of 1.4x1012 and 1.01x1012 cmHz1/2/W at 0.2 V, and a responsivity of 0.87 and
1.66 A/W under 0.2 V biasing at 77 K and 3.5 μm, assuming unity gain, was obtained for structures A and B,
respectively.
This paper reviews the historical progress of HgCdTe material and device development at Raytheon Vision Systems
starting with the initial work in 1965 at what was then the Santa Barbara Research Center, a subsidiary of the Hughes
Aircraft Company and progressing up to the present time. Because of the long history, all the details cannot be presented
in a single paper; instead, we focus only on a few major accomplishments. In HgCdTe material preparation these
include: the early bulk single crystal growth methods; the advent of liquid phase epitaxial growth from Hg melts; and,
the most recent molecular beam epitaxial methods. For IR photodetector devices, we started with just single element
detectors operating either in photoconductive or photovoltaic mode, then progressed to multi-element linear arrays, then
to 2-D arrays on Si read-out circuits and, finally to the very large focal plane (>2k × 2k), dual-band, and APD arrays of
today. Some applications of these devices in IR systems will be presented. Technical issues will be discussed only to
the extent necessary to support the historical narrative. Some interesting anecdotes will be included.
We have demonstrated the use of bulk antimonide based materials and type-II antimonide
based superlattices in the development of large area mid wavelength infrared (MWIR) focal
plane arrays (FPAs) as well as smaller format long wavelength infrared (LWIR) arrays.
Barrier infrared photodetectors (BIRDs) and superlattice-based infrared photodetectors are
expected to outperform traditional III-V MWIR and LWIR imaging technologies and are
expected to offer significant advantages over II-VI material based FPAs. We have used
molecular beam epitaxy (MBE) technology to grow InAs/GaSb superlattice pin photodiode
and bulk InAsSb structures on GaSb substrates. The coupled quantum well superlattice
device offers additional control in wavelength tuning via quantum well sizes and ternary
composition. Furthermore, we have fabricated mid-wavelength 1024x1024 pixels superlattice
imaging FPAs, 640x512 MWIR arrays based on the BIRD concept, and 256x256 LWIR
arrays based on pin superlattice structures. These initial FPA have produced excellent
infrared imagery.
We report on the status of focal plane arrays (FPAs) based on GaSb/InAs type-II superlattice diodes grown by molecular beam epitaxy (MBE) and designed for infrared absorption in the 2-5μm and 8-10μm bands. Recent LWIR devices have produced differential
resistance-area product greater than 100 Ohmcm2 at 80K with a long wavelength cutoff of approximately 10μm. The measured quantum efficiency of these front-side illuminated devices is close to 25% in the 8-9 μm range. MWIR devices have produced detectivities as high as 8x1013 Jones with a differential
resistance-area product greater than 3x107 Ohmcm2 at 80K with a long wavelength cutoff of approximately 3.7μm. The measured quantum efficiency of these
front-side illuminated MWIR devices is close to 40% in the 2-3μm range at low temperature and increases to over 60% near room temperature. Initial results on SiO2 and epitaxial-regrowth based passivation techniques are also presented, as well as images from the first lot of 1kx1k MWIR arrays and our latest 256x256 LWIR arrays.
We report on the status of GaSb/InAs type-II superlattice diodes grown by molecular beam
epitaxy (MBE) and designed for infrared absorption in the 2-5μm and 8-12μm bands.
Recent LWIR devices have produced detectivities as high as 8x1010 Jones with a differential
resistance-area product greater than 6 Ohmcm2 at 80K with a long wavelength cutoff of
approximately 12μm. The measured quantum efficiency of these front-side illuminated
devices is close to 30% in the 10-11μm range. MWIR devices have produced detectivities as
high as 8x1013 Jones with a differential resistance-area product greater than 3x107 Ohmcm2 at
80K with a long wavelength cutoff of approximately 3.7μm. The measured quantum
efficiency of these front-side illuminated MWIR devices is close to 40% in the 2-3μm range
at low temperature and increases to over 60% near room temperature. Initial results on SiO2
and epitaxial-regrowth based passivation techniques are also presented, as well as images
from the first lot of LWIR arrays.
We report progress in the development of long wavelength infrared (LWIR) focal plane arrays (FPAs) built on type-II
strained layer InAs/GaSb superlattice materials. Work at Raytheon Vision Systems and Jet Propulsion Laboratory has
led to successful devices with cutoff wavelengths in the 10 to 12 μm range. Pixels have been formed by wet etching
and surface passivation by plasma-deposited silicon dioxide. We present test results on arrays hybridized with indium
bump bonding to silicon readout integrated circuits, as well as analyses of current-voltage characteristics of individual
diodes. In particular, we find that, at temperatures below about 70 K the leakage current is dominated by generation-recombination
effects near zero bias and by trap-assisted tunneling in reverse bias. Although other authors have
demonstrated imaging for SWIR and MWIR type-II superlattice devices, to our knowledge no one has done so prior to
2006 in the LWIR range. We have obtained both still and video imaging with 256×256 arrays with 30-μm pixels
operating at 78 K, having high operability and a cutoff wavelength of 10.5 μm.
We report on the status of GaSb/InAs type-II superlattice diodes grown by molecular beam epitaxy (MBE) and designed for infrared absorption in the 2-5μm and 8-12&mgr;m bands. Recent LWIR devices have produced detectivities as high as 8x1010 Jones with a differential resistance-area product greater than 6 Ohmcm2 at 80K with a long wavelength cutoff of approximately 12&mgr;m. The measured quantum efficiency of these front-side illuminated devices is close to 30% in the 10-11μm range. MWIR devices have produced detectivities as high as 8x1013 Jones with a differential resistance-area product greater than 3x107 Ohmcm2 at 80K with a long wavelength cutoff of approximately 3.7μm. The measured quantum efficiency of these front-side illuminated MWIR devices is close to 40% in the 2-3μm range at low temperature and increases to over 60% near room temperature. Initial results on SiO2 and epitaxial-regrowth based passivation techniques are also presented.
We report on progress in the development of a device fabrication process for type-II strained layer superlattice IR detectors, composed of InAs/GaSb or InAs/GaInSb. Steps of the process include etching the mesas, cleaning up the surface, and applying a surface passivation treatment. Certain etchants have been evaluated and calibrated. The surface has been studied with single wavelength ellipsometry and results have been compared with modeled ellipsometry values, revealing effects of surface residues and surface roughness. An initial investigation of ammonium sulfide treatment for surface passivation has been made. Initial measurements of the IR transmission of the GaSb substrate have also been made to determine how much thinning is needed for back side illuminated operation of the IR detectors.
KEYWORDS: Sensors, Electronics, Analog electronics, X-ray telescopes, Calibration, X-ray astronomy, Indium, Signal detection, Space telescopes, Hard x-rays
Large area, high spatial resolution CdZnTe pixel detectors are being developed for hard X-ray astronomy. We have designed and fabricated custom readout chips and bump-bond these to pixelated CdZnTe crystals using indium bump bonding technology. The resulting detectors have 16 x 16 pixels with 300 micron pitch, enabling low noise operation and permitting detailed imaging. These devices are ideally suited for the focal plane of future high-resolution hard x-ray focusing telescopes now being considered, such as the HXT on Constellation-X. An initial demonstration using the sparse read-out capabilities of these detectors is presented.
Further progress has been made in the development of the Modified Vertical Bridgman method for the growth of CdZnTe crystals for fabrication of x-ray and gamma-ray detectors to operate at room temperature. Specifically, the diameter of the grown ingots has been increased from 2 to 3 inches. High quality, large volume (up to 6 in3) twin-free single crystals have been produced. Detectors fabricated with this material show sharp energy resolution and good uniformity.
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