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.
Detectivity of mid-wave infrared (MWIR) detectors based on InAs/GaSb type II strained layer superlattices (T2SLs) can be significantly enhanced at select wavelengths by integrating the detector with a back-side illuminated plasmonic coupler. The application of a simple metal-T2SL structure directly on the GaSb substrate can result in radiation losses into the substrate due to the low refractive index of T2SL layer. However, insertion of a higher refractive index material, such as germanium (Ge), into the metal-SLS structure can confine the surface plasmon waveguide (SPW) modes to the surface. In this work, metal (Au)-Ge-T2SL structures are designed with an approximately 100 nm thick Ge layer. The T2SL layer utilized a p-i-n detector design with 8 monolayers (MLs) InAs/8 MLs GaSb. A plasmonic coupler was then realized inside the 300 μm circular apertures of these single element detectors by the formation of a corrugated metal (Au) surface. The T2SL single element detector integrated with an optimized plasmonic coupler design increased the quantum efficiency (QE) by a factor of three at an operating temperature of 77 K and 3 to 5 μm illumination wavelength, compared to a reference detector structure, and each structure exhibited the same level of dark current.
This paper investigates arrays of HgCdTe photon trapping detectors. Performance of volume reduced single mesas is
compared to volume reduced photon trap detectors. Good agreement with model trends is observed. Photon trap
detectors exhibit improved performance compared to single mesas, with measured noise equivalent temperature
difference (NEDT) of 40 mK and 100 mK at temperatures of 180 K and 200 K, with good operability. Performance as a
function of temperature has also been investigated.
We report on dual-band (mid-/long-wave infrared) InAs/GaSb strained layer superlattice detector
with nBn and pBp architectures. Two band response was registered with 50% cut-off wavelengths of 5μm
(both nBn and pBp detectors) and 9μm (nBn)/10μm (pBp). The maximum peak responsivity of MWIR
absorber equal to 1.6 A/W (at λ = 5 μm and V<sub>b</sub> = +1 V) and LWIR absorber equal to 1.2 A/W (at λ = 10 μm
and V<sub>b</sub> = -1 V) for nBn detector, with the corresponding values of D* were 1.2 x 10<sup>11</sup> Jones and 1.2 x 10<sup>10</sup>Jones for MWIR and LWIR absorbers, respectively (77 K). The maximum values of quantum efficiency were
estimated to 36% (MWIR) and 15% (LWIR) at V<sub>b</sub> = +1V and V<sub>b</sub> = -1V. For pBp detector, the responsivity
equal to 1.6 A/W (at λ = 5 μm and V<sub>b</sub> = +0.4 V) and 1.8 A/W (at λ = 9 μm and V<sub>b</sub> = -0.7 V) for MWIR and
LWIR absorbers was achieved with corresponding values of specific detectivity 5 x 10<sup>11</sup> Jones and 2.6 x 10<sup>10</sup>Jones, respectively. The maximum values of quantum efficiency were estimated to 41% (MWIR) and 25%
(LWIR) at V<sub>b</sub> = +0.4V and V<sub>b</sub> = -0.7V. Moreover, the diffusion-limited behavior of dark current at higher
temperatures was observed for MWIR absorber for pBp detector. The overall performance of the dual-band
InAs/GaSb SLS detectors with investigated designs showed comparable (nBn design) and superior (pBp
design) performance to the QWIP detectors both in MWIR and LWIR bands and comparable performance to
MCT detectors in MWIR band (nBn and pBp detector designs).
We report on the investigation of lateral diffusion of minority carriers in InAsSb based photodetectors with
the nBn design. Diffusion lengths (DL) were extracted from temperature dependent I-V measurements. The
behavior of DL as a function of applied bias, temperature, and composition of the barrier layer was
investigated. The obtained results suggest that lateral diffusion of minority carriers is not the limiting factor
for InAsSb based nBn MWIR detector performance at high temperatures (> 200K). The detector with an As
mole fraction of 10% in the barrier layer has demonstrated values of DL as low as 7 μm (V<sub>b</sub> = 0.05V) at 240K.
Our group is investigating nBn detectors based on bulk InAs(1-x)Sb(x) absorber (n) and contacts (n) with an AlAs(1-x)Sb(x)
barrier (B). The wide-band-gap barrier material exhibits a large conduction band offset and small valence band offset
with respect to the narrow-band-gap absorber material. An important matter to explore in this design is the barrier
parameters (material, composition and doping concentration) and how they effect the operation of the device. This paper
investigates AlAs(1-x)Sb(x) barriers with different compositions and doping levels and their effect on detector
characteristics, in particular, dark current density, responsivity and specific detectivity.
Oceanit Laboratories Inc. is collaborating with Raytheon Vision Systems (RVS) to develop a novel HgCdTe-based
position sensitive detector (PSD) that can ultimately be implemented in target detection and tracking or target
interception applications in the infrared spectral region.
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 <sub>0.15S</sub>b<sub>0.75</sub> (structure A) and AlAs<sub>0.10S</sub>b<sub>0.9</sub> (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 1.4x10<sup>12</sup> and 1.01x10<sup>12</sup> cmHz<sup>1/2</sup>/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,
Raytheon Visions Systems (RVS) is furthering its capability to deliver state-of-the-art high performance large
format HgCdTe focal plane arrays (FPAs) for dual-band long-wavelength infrared (LWIR) detection. Missile
seekers are designed to acquire targets of interest at long ranges and discriminate targets from clutter. The use of
dual-band long wavelength infrared detector technology provides the ability for these seekers to combine these
operations into the same package with enhanced performance. Increasing the format size of dual-band longwavelength
FPAs and tailoring the detector design for specific long-wavelength bands enables seekers to be
designed for increased field-of-view, longer target acquisition ranges, and improved accuracy. This paper will
review in further detail the aspects of detector design, MBE wafer growth, wafer fabrication, and detector
characterization that are contributing to development and demonstration of high performance large format dual-band
LWIR FPAs at RVS.
Raytheon Vision Systems (RVS) is developing two-color and large format single color FPAs fabricated from molecular beam epitaxy (MBE) grown HgCdTe triple layer heterojunction (TLHJ) wafers on CdZnTe substrates and double layer heterojunction (DLHJ) wafers on Si substrates, respectively. MBE material growth development has resulted in scaling TLHJ growth on CdZnTe substrates from 10cm<sup>2</sup> to 50cm<sup>2</sup>, long-wavelength infrared (LWIR) DLHJ growth on 4-inch Si substrates and the first demonstration of mid-wavelength infrared (MWIR) DLHJ growth on 6-inch Si substrates with low defect density (<1000cm<sup>-2</sup>) and excellent uniformity (composition<0.1%, cut-off wavelength Δcenter-edge<0.1μm). Advanced FPA fabrication techniques such as inductively coupled plasma (ICP) etching are being used to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance. Recent two-color detectors with MWIR and LWIR cut-off wavelengths of 5.5μm and 10.5μm, respectively, exhibit significant improvement in 78K LW performance with >70% quantum efficiency, diffusion limited reverse bias dark currents below 300pA and RA products (zero field-of-view, +150mV bias) in excess of 1×103 Ωcm<sup>2</sup>. Two-color 20μm unit-cell 1280×720 MWIR/LWIR FPAs with pixel response operability approaching 99% have been produced and high quality simultaneous imaging of the spectral bands has been achieved by mating the FPA to a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). Large format mega pixel 20μm unit-cell 2048×2048 and 25μm unit-cell 2560×512 FPAs have been demonstrated using DLHJ HgCdTe growth on Si substrates in the short wavelength infrared (SWIR) and MWIR spectral range. Recent imaging of 30μm unit-cell 256×256 LWIR FPAs with 10.0-10.7μm 78K cut-off wavelength and pixel response operability as high as 99.7% show the potential for extending HgCdTe/Si technology to LWIR wavelengths.
Raytheon Vision Systems (RVS) is developing two-color, large-format infrared FPAs to support the US Army's Third Generation FLIR systems. RVS has produced 640 x 480 two-color FPAs with a 20 micron pixel pitch. Work is also underway to demonstrate a 1280 x 720 two-color FPA in 2005. The FPA architecture has been designed to achieve nearly simultaneous temporal detection of the spectral bands while being producible for pixel dimensions as small as 20 microns. Raytheon's approach employs a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). This ROIC is coupled to bias-selectable two-color detector array with a single contact per pixel. The two-color detector arrays are fabricated from MBE-grown HgCdTe triple layer heterojunction (TLHJ) wafers. The single indium bump design is producible for 20 μm unit cells and exploits mature fabrication processes that are in production at RVS for Second Generation FPAs. This combination allows for the high temporal and spatial color registration while providing a low-cost, highly producible and robust manufacturing process. High-quality MWIR/LWIR (M/L) 640 x 480 TDMI FPAs with have been produced and imaged from multiple fabrication lots. These FPAs have LWIR cutoffs ranging to 11 micron at 78K. These 20 micron pixel FPAs have demonstrated excellent sensitivity and pixel operabilities exceeding 99%. NETDs less than 25 mK at f/5 have been demonstrated for both bands operating simultaneously.
HgCdTe offers significant advantages over other semiconductors which has made it the most widely utilized variable-gap material in infrared focal plane array (FPA) technology. However, one of the main limitations of the HgCdTe materials system has been the size of lattice-matched bulk CdZnTe substrates, used for epitaxially-grown HgCdTe, which are 30 cm<sup>2</sup> in size for production and have historically been difficult and expensive to scale in size. This limitation does not adequately support the increasing demand for larger FPA formats which now require sizes up to and beyond 2048 x 2048 and only a single die can be printed per wafer. Heteroepitaxial Si-based substrates offer a cost-effective technology that can be more readily scaled to large wafer sizes. Most of the effort in the IR community in the last 10 years has focused on growing HgCdTe directly on (112)Si substrates by MBE. At Raytheon we have scaled the MBE (112)HgCdTe/Si process originally developed at HRL for 3-in wafers, first to 4-in wafers and more recently to 6 in wafers. We have demonstrated a wide range of MWIR FPA formats up to 2560 x 512 in size and have found that their performance is comparable to arrays grown on bulk CdZnTe substrates by either MBE or LPE techniques. More recent work is focused on extending HgCdTe/Si technology to LWIR wavelengths. The goal of this paper is to review the current status of HgCdTe/Si technology both at Raytheon and the published work available from other organizations.
Raytheon Vision Systems (RVS) in collaboration with HRL Laboratories is contributing to the maturation and manufacturing readiness of third-generation two-color HgCdTe infrared staring focal plane arrays (FPAs). This paper will highlight data from the routine growth and fabrication of 256x256 30μm unit-cell staring FPAs that provide dual-color detection in the mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) spectral regions. FPAs configured for MWIR/MWIR, MWIR/LWIR and LWIR/LWIR detection are used for target identification, signature recognition and clutter rejection in a wide variety of space and ground-based applications. Optimized triple-layer-heterojunction (TLHJ) device designs and molecular beam epitaxy (MBE) growth using in-situ controls has contributed to individual bands in all two-color FPA configurations exhibiting high operability (>99%) and both performance and FPA functionality comparable to state-of-the-art single-color technology. The measured spectral cross talk from out-of-band radiation for either band is also typically less than 10%. An FPA architecture based on a single mesa, single indium bump, and sequential mode operation leverages current single-color processes in production while also providing compatibility with existing second-generation technologies.