HgCdTe-based FPAs that can be used in high neutron radiation environments were designed and fabricated by EPIR, and tests using Fermi National Accelerator Laboratory’s neutron beam confirmed that these FPAs can maintain imaging functionality while exposed to fluxes up to low-1E13 neutron per squared centimeter accumulated neutron exposure. Monte Carlo N-Particle (MCNP) simulations were used to find that the energy deposited into the HgCdTe FPA can come from not only directly impinging neutrons but also scattered neutrons and subsequently generated protons, electrons and photons, confirming that our neutron-hardened designs are also hardened against other high energy particles. To mitigate radiation damage, we redesigned the optical system of the camera using modeling and simulation by utilizing MCNP code during our camera design. By properly choosing mirror substrate material and coating as well as the corresponding optical system and the camera design, we can filter out harmful radiation flux while still collecting the MWIR signal with high efficiency, thereby significantly reducing camera and image system performance degeneration under high-energy high-flux neutron beams.
KEYWORDS: Dark current, Sunlight, Signal to noise ratio, Quantum efficiency, Continuous wave operation, Active imaging, Sensors, Quantum reading, Performance modeling, Imaging systems
In the last half-decade, the extended shortwave infrared (eSWIR) atmospheric band has become a focus of investigation for its potential to provide better object discrimination at range than the visible, as well as the near, shortwave, midwave, and longwave infrared bands, particularly in degraded visual environments such as smoke, dust, and smog. However, any detection band is only as useful as the best available detector, and thus an investigation into the design of detectors for use in the eSWIR band is necessary before standards are established and applications put into practice. This study examines the relationship between detector parameters and targeting performance in the eSWIR band for both passive and active detection. The effects of pixel pitch, dark current, read noise, frame rate, quantum efficiency, and well depth are examined and ranked in importance to an eSWIR system’s performance.
Pixel pitch size reduction was not the focus in early infrared (IR) detector development for a long time with pixel pitch remained at 24 μm or above. Pitch size reduction today is the key enabler for cost-efficient manufacturing of large format arrays and allows compact IR-systems with high spatial resolution. When mastered the smaller pixel pitch geometries will provide consistent range performance in a smaller package, minimized aliasing and false alarm rates, ability to use faster F/# optics and shorter focal length for long range identification and optimized size, weight and power (SWaP) characteristics. Advanced integration technologies (including three-dimensional integration) are necessary to realize small pitch arrays.
EPIR, Inc. has developed thermomechanical stress aware approach for advanced integration of IR focal plane arrays (IRFPAs) – MoDiBI. As intended, MoDiBI allows for favorably addressing the reliability concerns associated with the conventional integration approaches. The current work focuses on extending MoDiBI to small pixel pitch, large format IRFPA integration. Strategies for optimizing the thermal stress induced in the hybridized assembly during thermal cycling, thereby helping in reducing the fatal failures experienced by IRFPAs will be discussed. Applicability of MoDiBI to 1280×720, 8µm pitch IRFPAs will be presented.
Several MWIR nBn HgCdTe devices grown on silicon were studied. While several parameters are varied in the study, the devices can most usefully be put into 2 groups: those with a type 3 HgTe/CdTe superlattice barrier and those simply with a wider bandgap alloy barrier. Other groups have shown the potential advantage of a super-lattice barrier. Many devices were grown and fabricated, and were run through several optical and electrical tests to evaluate their properties. Utilizing the finite volume method based semiconductor device modeling software Devsim, these devices were simulated to extract further material parameters.
Novel integration method that addresses thermo-mechanical reliability of the IRFPA hybrid assembly in advanced three-dimensional integration scheme requires optimization by engineering materials used for vertical integration and geometry engineering of the assemblies to be integrated. We present such optimization scheme and applicability of this method to vertical integration of HgCdTe and Type-II Superlattice (T2SL) based IRFPA.
HgCdTe is one of the most important materials for fabrication of infrared detectors and focal plane arrays (FPAs) deployed in environments where high energy particle, such as protons and neutrons, are present. We designed and fabricated HgCdTe-based FPAs that can be used in high neutron radiation environment and we measured their characteristics. The influence of the radiation on the infrared FPAs and cameras are presented. HgCdTe material and devices are capable of maintaining high performances under high energy neutron irradiation environment. For the MWIR FPA directly facing 2.59×108 n/cm2·s neutron flux beam (with the highest energy 66 MeV) for 1 hour, the noise equivalent differential temperature (NEDT) increased ~ 8 times after irradiation. However NEDT decreased to 33 mK (compared to the original value of 21 mK) after one warming-up (to room temperature) and cooling-down cycle. The NEDT for the MWIR FPAs mounted parallel to the beam did not degrade (16 mK and 28 mK before irradiation, changed to 18 mK and 26 mK after irradiation, respectively).
Higher operating temperatures can be provided by a system based on HgCdTe infrared detectors utilizing optimized material parameters (i.e. doping and defects) and non-equilibrium device architectures that suppress detector diffusion current. A PIN (P+/Intrinsic/N+) hetero-junction photodiode can eliminate Auger generated diffusion current, resulting in a diode dominated by Shockley-Read-Hall (SRH) depletion current. The limiting dark current in the PIN diode will be determined by the SRH lifetime of the HgCdTe material. We are optimizing processes for material growth and post-annealing, to improve in SRH lifetimes and hence depletion dark current and performance at higher operating temperatures. We report on the growth of high-quality n-type long wavelength infrared (LWIR) HgCdTe (cutoff wavelength 10 μm at 77 K) layers grown on CdTe/Si and CdZnTe substrates by molecular beam epitaxy (MBE). A low indium concentration in the absorber layer of ~1x10-14 is confirmed by secondary ion mass spectrometry (SIMS). In order to reduce potential SRH centers and enable low doping levels, we are applying gettering processes to reduce impurity levels below what is achievable by best practices in MBE growth. Concentration profiles of impurities such as Na and K are seen to getter to top surface and interface with the substrate, and are seen to be dramatically reduced in the absorber layer as annealing temperature is increased. We are also studying the effect of anneals on the sharpness of heterojunctions created by MBE growth. The potential benefits of heterojunction devices in suppressing dark current, particularly structures with lower thickness, can only be realized if interdiffusion can be controlled. We have successfully fit SIMS Cd profiles to a model that incorporates the temperature and composition dependence of the interdiffusion coefficient. These results, along with dependence of As diffusion coefficients vs. temperature and Hg pressure lead us to propose and test alternative annealing profiles that better preserve the heterojunctions while maintaining an appropriate amount of As diffusion for junction formation.
We report high-quality n-type extended short wavelength infrared (eSWIR) HgCdTe (cutoff wavelength ~2.59 μm at 77 K) layers grown on three-inch diameter CdTe/Si substrates by molecular beam epitaxy (MBE). This material is used to fabricate test diodes and arrays with a planar device architecture using arsenic implantation to achieve p-type doping. We use different variations of a test structure with a guarded design to compensate for the lateral leakage current of traditional test diodes. These test diodes with guarded arrays characterize the electrical performance of the active 640 × 512 format, 15 μm pitch detector array.
High performance infrared sensors are vulnerable to slight changes in defect densities and locations. For example in a space application where such sensors are exposed to proton irradiation capable of generating point defects the sensors are known to suffer performance degradation. The degradation can generally be observed in terms of dark current density and responsivity degradations. Here we report results of MWIR HgCdTe/CdZnTe single element diodes dark current densities before and after exposure to 63MeV protons at room temperature to a total ionizing dose of 100 kRad(Si). We find the irradiated diodes as a group show some signs of proton-induced damage in dark current.
Focal plane array (FPA) technology is mature and is widely used for imaging applications. However, FPAs have
broadband responses which limit their ability to provide high performance in hyperspectral applications such as
detection of buried explosives, and identifying the presence of explosive chemicals and their concentrations. EPIR is
currently developing Micro-Opto-Electro-Mechanical System (MOEMS) Fabry-Perot interferometer filter (FPF) devices
for FPAs. In this paper, we present our approach to MOEMS FPF design and fabrication that will meet the size
requirements for large format FPA hyperspectral imaging. We also report the performance of our FPF resonance cavity,
capable of up to 3 μm change gap in tens of nanometer increments.
Exposure to proton radiation degrades the performance of wavelength infrared (MWIR) and long wavelength infrared (LWIR) HgCdTe photodetectors to varying degrees depending on the dose and energy of the incident particles. We report an experimental characterization of test devices of multiple sizes and configurations designed to investigate the effect proton radiation has on detector performance. Photodetector devices, from test element devices to fully functional focal plane arrays, are processed into MWIR and LWIR HgCdTe material grown by molecular beam epitaxy (MBE), in both single and two-color architectures, on CdZnTe and CdTe-buffered Si substrates. The devices receive doses of 30 krad(Si) and 100 krad(Si) from an incident beam of 63 MeV protons. The lower dose induces negligible degradation. At the higher dose, MWIR detectors begin to show reduced activation energy for higher temperatures, while LWIR detectors are more strongly affected with the activation energy being halved following proton irradiation.
We report the development of high performance low cost SWIR infrared detectors from MBEgrown HgCdTe on 3-inch CdTe-buffered silicon substrates. The experimental findings demonstrate that despite the large lattice mismatch between HgCdTe and Si substrate, the materials and detector performances are sufficiently better than those reported for III-V mixed crystals. High minority carrier lifetime of the order 3 μs at room temperature was measured on the as grown material. Photodetectors fabricated from this material produced low dark current densities on the order of 10-6 A/cm2 and 10-3 A/cm2 at 200K and 300K. Quantum efficiency exceeding 70% at 2.0 μm, without antireflective coating, was measured on single element detectors. Further, 320 X 256, 30 μm pitch FPA’s have been fabricated with this HgCdTe on Si material and dark current operability of ~ 99.5% (mean dark current of 30 pA/Pixel) at 200K has been demonstrated.
Imaging spectrometry can be utilized in the midwave infrared (MWIR) and long wave infrared
(LWIR) bands to detect, identify and map complex chemical agents based on their rotational and
vibrational emission spectra. Hyperspectral datasets are typically obtained using grating or
Fourier transform spectrometers to separate the incoming light into spectral bands. At present,
these spectrometers are large, cumbersome, slow and expensive, and their resolution is limited
by bulky mechanical components such as mirrors and gratings. As such, low-cost, miniaturized
imaging spectrometers are of great interest. Microfabrication of micro-electro-mechanicalsystems
(MEMS)-based components opens the door for producing low-cost, reliable optical
systems. We present here our work on developing a miniaturized IR imaging spectrometer by
coupling a mercury cadmium telluride (HgCdTe)-based infrared focal plane array (FPA) with a
MEMS-based Fabry-Perot filter (FPF). The two membranes are fabricated from silicon-oninsulator
(SOI) wafers using bulk micromachining technology. The fixed membrane is a standard
silicon membrane, fabricated using back etching processes. The movable membrane is
implemented as an X-beam structure to improve mechanical stability. The geometries of the
distributed Bragg reflector (DBR)-based tunable FPFs are modeled to achieve the desired
spectral resolution and wavelength range. Additionally, acceptable fabrication tolerances are
determined by modeling the spectral performance of the FPFs as a function of DBR surface
roughness and membrane curvature. These fabrication non-idealities are then mitigated by
developing an optimized DBR process flow yielding high-performance FPF cavities. Zinc
Sulfide (ZnS) and Germanium (Ge) are chosen as the low and the high index materials,
respectively, and are deposited using an electron beam process. Simulations are presented
showing the impact of these changes and non-idealities in both a device and systems level.
Hyperspectral infrared imagers are of great interest in applications requiring remote identification of complex chemical agents. The combination of mercury cadmium telluride detectors and Fabry–Perot filters (FPFs) is highly desirable for hyperspectral detection over a broad wavelength range. The geometries of distributed Bragg reflector (DBR)-based tunable FPFs are modeled to achieve a desired spectral resolution and wavelength range. Additionally, acceptable fabrication tolerances are determined by modeling the spectral performance of the FPFs as a function of DBR surface roughness and membrane curvature. These fabrication nonidealities are then mitigated by developing an optimized DBR process flow yielding high-performance FPF cavities suitable for integration with hyperspectral imagers.
The development of a broadband IR focal plane array poses several challenges in the area of detector design, material, device physics, fabrication process, hybridization, integration and testing. The purpose of our research is to address these challenges and demonstrate a high-performance IR system that incorporates a HgCdTe-based detector array with high uniformity and operability. Our detector architecture, grown using molecular beam epitaxy (MBE), is vertically integrated, leading to a stacked detector structure with the capability to simultaneously detect in two spectral bands. MBE is the method of choice for multiplelayer HgCdTe growth because it produces material of excellent quality and allows composition and doping control at the atomic level. Such quality and control is necessary for the fabrication of multicolor detectors since they require advanced bandgap engineering techniques. The proposed technology, based on the bandgap-tunable HgCdTe alloy, has the potential to extend the broadband detector operation towards room temperature. We present here our modeling, MBE growth and device characterization results, demonstrating Auger suppression in the LWIR band and diffusion limited behavior in the MWIR band.
The combination of HgCdTe detectors and Fabry-Pérot filters (FPFs) is highly desirable for hyperspectral
detection in the infrared band over a broad wavelength range. The results of comprehensive modeling of distributed-
Bragg-reflector-based tunable FPFs that can be used with HgCdTe array detectors for hyperspectral imaging modules are
presented, focusing on the impact of FPF non-idealities on optical performance. The effects of surface and interface
roughness on the spectral resolution and transmissivity of the cavity was explored to determine if certain thin film
deposition techniques are suitable to economically fabricate FPFs. The impact of varying field-of-view (FOV) and
incident angles are also discussed. Finally, the impact of FPF bowing on spectral resolution is discussed.
Spatial noise and the loss of photogenerated current due material non-uniformities limit the performance of long
wavelength infrared (LWIR) HgCdTe detector arrays. Reducing the electrical activity of defects is equivalent to
lowering their density, thereby allowing detection and discrimination over longer ranges. Infrared focal plane arrays
(IRFPAs) in other spectral bands will also benefit from detectivity and uniformity improvements. Larger signal-to-noise
ratios permit either improved accuracy of detection/discrimination when an IRFPA is employed under current operating
conditions, or provide similar performance with the IRFPA operating under less stringent conditions such as higher
system temperature, increased system jitter or damaged read out integrated circuit (ROIC) wells. The bulk passivation of
semiconductors with hydrogen continues to be investigated for its potential to become a tool for the fabrication of high
performance devices. Inductively coupled plasmas have been shown to improve the quality and uniformity of
semiconductor materials and devices. The retention of the benefits following various aging conditions is discussed here.
We present in this study a theoretical and experimental investigation of the MWIR HgCdTe nBn device concept.
Theoretical work has demonstrated that the HgCdTe nBn device is potentially capable of achieving performance
equivalent to the ideal double layer planar heterostructure (DLPH) detector. Comparable responsivity, low current
denisty Jdark, and high detectivity *D values rival those of the DLPH device without requiring p-type doping. The
theoretical results suggests that the HgCdTe nBn structure may be a promising solution for achieving a simplified MWIR
device structure and addressing problems associated with reducing thermal generation in conventional p-on-n structures
and processing technology limitations such as achieving low, controllable in-situ p-type doping with MBE growth
techniques. Furthermore, the physical mechanisms for selective carrier conduction in the nBn structure may provide a
basis to incorporate into future device structures to suppress intrinsic Auger carrier generation. Likewise, the
experimental demonstration of the MWIR HgCdTe nBn devices introduces a promising potential alternative to
conventional high performance p-n junction HgCdTe photodiodes. The experiments described in this study illustrate the
successful implementation of a HgCdTe barrier-integrated structure. The measured current-voltage characteristics of
planar-mesa and mesa HgCdTe nBn devices exhibit barrier-influenced behavior and follow temperature-dependent
trends as predicted by numerical simulations. Optical measurements of the planar-mesa MWIR HgCdTe nBn device
indicate a bias-dependent spectral response. Further changes to MWIR HgCdTe nBn layer structure has shown an over
105 A/cm2 reduction in Jdark as well as a shift to a lower turn-on operation bias. This experimental investigation highlights
the potential for pursuing similar and related unipolar, type-I barrier devices for high performance infrared detector
applications.
The performance of leading HgCdTe p-n junction infrared (IR) device technology is limited by thermal generationrecombination
(G-R) mechanisms and material processing challenges associated with achieving low, controllable in-situ
p-type doping using molecular beam epitaxy (MBE) growth techniques. These aspects are addressed in the proposed
hybrid HgCdTe NBνN structure which relies on band gap engineered layers to suppress Shockley-Read-Hall (SRH) and
Auger G-R processes contributing to performance degradation. The unipolar NBνN architecture provides the desired
advantages of a simplified fabrication process, eliminating p-type doping requirements. Physics-based numerical device
simulations incorporating established HgCdTe material parameters and G-R mechanisms are used to study the
performance characteristics of a long wavelength infrared (LWIR) NBνN device with a 12 μm cut-off wavelength. The
calculated results are compared to those values obtained for an LWIR HgCdTe nBn device. Theoretical dark current
density (Jdark) values of the NBνN device are lower by an order of magnitude or more for temperatures between 50 K and
245 K. Calculated detectivity (D*) values of 2.367 x 1014 - 2.273 x 1011 cm Hz0.5/W for temperatures ranging from 50 K
and 95 K, respectively, are observed in the NBνN structure.
The Mid-wave infrared (MWIR) spectrum has applications to many fields, from night vision to chemical and biological
sensors. Existing broadband detector technology based on HgCdTe allows for high sensitivity and wide range, but lacks
the spectral decomposition necessary for many applications. Combining this detector technology with a tunable optical
filter has been sought after, but few commercial realizations have been developed. MEMS-based optical filters have
been identified as promising for their small size, light-weight, scalability and robustness of operation. In particular,
Fabry-Perot interferometers with dielectric Bragg stacks used as reflective surfaces have been investigated. The
integration of a detector and a filter in a device that would be compact, light-weight, inexpensive to produce and scaled
for the entire range of applications could provide spectrally resolved detection in the MWIR for multiple instruments.
We present a fabrication method for the optical components of such a filter. The emphasis was placed on wafer-scale
fabrication with IC-compatible methods. Single, double and triple Bragg stacks composed of germanium and silicon
oxide quarter-wavelength layers were designed for MWIR devices centered around 4 microns and have been fabricated
on Silicon-On-Insulator (SOI) wafers, with and without anti-reflective half-wavelength silicon nitride layers. Optical
testing in the MWIR and comparison of these measurements to theory and simulations are presented. The effect of film
stress induced by deposition of these dielectric layers on the mechanical performance of the device is investigated. An
optimal SOI substrate for the mechanical performance is determined. The fabrication flow for the optical MEMS
component is also determined. Part of this work investigates device geometry and fabrication methods for scalable
integration with HgCdTe detector and IC circuitry.
A nearly universal goal for infrared photon detection systems is to increase their operating temperature without
sacrificing performance. For high quality HgCdTe photovoltaic infrared detectors at elevated temperatures, the lowdoped
absorber layer becomes intrinsic, carrier concentrations are high and Auger processes typically dominate the
dark current. Device designs have been proposed to suppress Auger processes in the absorber by placing it between
exclusion and extraction junctions under reverse bias. In this work, we analyze the non-equilibrium operation of
very long wavelength infrared (VLWIR) HgCdTe devices and identify the performance improvements (operation
temperature, responsivity, detectivity) expected when Auger suppression occurs. We identify critical structure
design requirements that must be satisfied for optimal performance characteristics from VLWIR non-equilibrium
devices and compare these devices with current state of the art double layer planar heterostructure (DLPH) devices.
In the past decades, there have been several attempts to create a tunable optical detector with operation in the infrared.
The drive for creating such a filter is its wide range of applications, from passive night vision to biological and chemical
sensors. Such a device would combine a tunable optical filter with a wide-range detector. In this work, we propose
using a Fabry-Perot interferometer centered in the mid-wave infrared (MWIR) spectrum with an HgCdTe detector.
Using a MEMS-based interferometer with an integrated Bragg stack will allow in-plane operation over a wide range.
Because such devices have a tendency to warp, creating less-than-perfect optical surfaces, the Fabry-Perot interferometer
is prototyped using the SOI-MUMPS process to ensure desirable operation. The mechanical design is aimed at optimal
optical flatness of the moving membranes and a low operating voltage. The prototype is tested for these requirements.
An HgCdTe detector provides greater performance than a pyroelectic detector used in some previous work, allowing for
lower noise, greater detection speed and higher sensitivity. Both a custom HgCdTe detector and commercially available
pyroelectric detector are tested with commercial optical filter. In previous work, monolithic integration of HgCdTe
detectors with optical filters proved to be problematic. Part of this work investigates the best approach to combining
these two components, either monolithically in HgCdTe or using a hybrid packaging approach where a silicon MEMS
Fabry-Perot filter is bonded at low temperature to a HgCdTe detector.
High sensitivity HgCdTe infrared detector arrays operating at 77 K can be tailored for response across the infrared
spectrum (1 to 14 μm and beyond), and are commonly utilized for high performance infrared imaging applications.
However, the cooling system required to achieve the desired sensitivity makes them costly, heavy and limits their
applicability. Reducing cooling requirements and eventually operating at temperatures that could be reached with
thermoelectric coolers can lead to lighter and more compact systems. However, at these elevated temperatures, the
absorber layer becomes intrinsic, carrier concentrations are high and Auger processes typically dominate the dark current
and noise characteristics. Auger processes can be suppressed by placing the absorber layer between an exclusion junction
and an extraction junction at reverse bias. This reduces the minority carrier concentration in the absorber by several
orders of magnitude below thermal equilibrium. The majority carrier concentration also drops significantly below
thermal equilibrium to maintain charge neutrality, eventually reaching the extrinsic doping level. This device architecture
produces a lower dark current density and lower noise at non-cryogenic temperatures than standard p-n junction
photodiodes. Due to the precise control of the layer's thicknesses and compositions that could be achieved with
molecular beam epitaxy (MBE), this technique is the method of choice for implementing these novel non-equilibrium
devices. In this work, we analyze Auger suppression in HgCdTe alloy-based device structures and determine the
operation temperature improvements expected when Auger suppression occurs. We identified critical material (absorber
dopant concentration and minority carrier lifetime) requirements that must be satisfied for optimal performance
characteristics. Experimental dark current-voltage characteristics between 120 and 300 K are fitted using numerical
simulations. Based on this, the negative differential resistance (NDR) observed in experimental data is attributed to the
full suppression of Auger-1 processes and the partial suppression of Auger-7 processes. We will also present an analysis
and comparison of our theoretical and experimental device results in structures where Auger suppression was realized.
Photodetectors with high bandwidth and internal gain are required to detect highly attenuated optical signals for defense
application and long distance communication. IR avalanche photodiodes (APDs) are best suited for this purpose due to
their internal gain-bandwidth characteristics coupled with long range data transmission capability. For the past two
decades, HgCdTe has been the most successful material for infrared photodetector applications. Recent advances in
epitaxial growth techniques made possible the growth of advanced HgCdTe APD structures, but to the best of our
knowledge all are grown on expensive substrates (e.g. CdZnTe, CdTe). We report for the first time HgCdTe-based
MWIR (4.5 μm) p-i-n APD grown on Si substrate by molecular beam epitaxy (MBE). The devices were fabricated by
365nm UV photolithography and wet-etching technique. The diode had a junction area of 300μm diameter. The R0A of
the diode was 3 x 106 Ω-cm2 at 77K. Multiplication gains of 800 were measured at a reverse bias of 10 V in the linear
operation regime. The gain increased exponentially as the reverse bias was increased, indicating that only one carrier is
responsible for the impact ionization. Temperature dependence of the multiplication gain and of the breakdown voltage
further confirms that avalanche multiplication dominates high reverse bias I-V characteristics.
Intrinsic carriers play a dominant role especially in the long wavelength (8-12 μm cut-off) HgCdTe material near ambient temperatures due to high thermal generation of carriers. This results in low minority carrier lifetimes caused by Auger recombination processes. Consequently, this low lifetime at high temperatures results in high dark currents and subsequently high noise. Cooling is one means of reducing this type of detector noise. However, the challenge is to design photon detectors to achieve background limited performance (BLIP) at the highest possible operating temperature; with the greatest desire being close to ambient temperature operation. We have demonstrated a unique planar device architecture using a novel approach in obtaining low arsenic doping concentrations in HgCdTe. Results indicate Auger suppression in P+/π/N+ devices at 300K and have obtained saturation current densities of the order of 3 milli Amps-cm2 on these devices.
Inductively coupled plasma (ICP) chemistry based on a mixture of CH4, Ar, and H2 was investigated for the purpose of delineating HgCdTe mesa structures and vias typically used in the fabrication of second and third generation infrared
photo detector arrays. We report on ICP etching uniformity results and correlate them with plasma controlling
parameters (gas flow rates, total chamber pressure, ICP power and RF power). The etching rate and surface morphology
of In-doped MWIR and LWIR HgCdTe showed distinct dependences on the plasma chemistry, total pressure and RF
power. Contact stylus profilometry and cross-section scanning electron microscopy (SEM) were used to characterize the
anisotropy of the etched profiles obtained after various processes and a standard deviation of 0.06 &mgr;m was obtained for
etch depth on 128 x 128 format array vias. The surface morphology and the uniformity of the etched surfaces were
studied by plan view SEM. Atomic force microscopy was used to make precise assessments of surface roughness.
At the Army Research Laboratory (ARL), a new ternary semiconductor system CdSexTe1-x/Si(211) is being investigated as an alternative substrate to Bulk-grown CdZnTe substrates for HgCdTe growth by molecular beam epitaxy. Under optimized conditions, best layers show surface defect densities less than 400 cm-2 and full width at half maximum as low as 100 arcsec with excellent uniformity over 3 inch area. LWIR HgCdTe on CdTe/Si substrates have also been grown and characterized with optical, x-ray diffraction, etch pit etching and Hall effect measurements. Photo Voltaic devices fabricated on these LWIR material shows G-R limited performance at 78K indicating detector performance is not limited by the bulk properties of the grown material.
The cost and performance of hybrid HgCdTe infrared focal plane arrays are constrained by the necessity of fabricating the detector arrays on a CdZnTe substrate. These substrates are expensive, fragile, are available only in small rectangular formats, and are not a good thermal expansion match to the silicon readout integrated circuit. We discuss in this paper an infrared sensor technology based on monolithically integrated infrared focal plane arrays that could replace the conventional hybrid focal plane array technology. We have investigated the critical issues related to the growth of HgCdTe on Si read-out integrated circuits and the fabrication of monolithic focal plane arrays: (1) the design of Si read-out integrated circuits and focal plane array layouts, (2) the low temperature cleaning of Si(001) wafers, (3) growth of CdTe and HgCdTe layers on read-out integrated circuits, (4) array fabrication, interconnection between focal plane array and read-out integrated circuit input nodes and demonstration of the photovoltaic operation, and (5) maintenance of the read-out integrated circuit characteristics after substrate cleaning, molecular beam epitaxy growth and device fabrication. Crystallographic, optical and electrical properties of the grown layers are presented. Electrical properties for diodes fabricated on misoriented Si and read-out integrated circuit substrates are discussed. The fabrication of arrays with demonstrated I-V properties show that monolithic integration of HgCdTe-based infrared focal plane arrays on Si read-out integrated circuits is feasible and could be implemented in the 3rd generation of infrared systems.
Research on silicon based composite substrates is being conducted at the Army Research Laboratory. These substrates can be used to deposit HgCdTe alloys to fabricate large-format infrared photodetector arrays. Traditionally, composite structures are fabricated by growing CdZnTe buffer layers on Si substrates using molecular beam epitaxy process. Recently, we have demonstrated that composite structures using CdSeTe can also be used. The CdSeTe compound offers better surface morphology and control of composition. In this work we present our results on the Si-based substrate technology and its application in the use of substrate material for LWIR HgCdTe detector development. In this paper we also present our study of molecular beam epitaxy and characteristics of CdSexTe1-x ternary films on Si. A detailed study of the alloy composition and lattice structures were investigated. In general, we find that the crystalline quality of CdSeTe films on Si is superior to CdZnTe on Si. Best CdSeTe/Si samples had EPD as low as 1.4x105 cm-2. This study also discusses a comparison of cation versus anion mixing in chalcogenide compounds. Results of LWIR detectors on CdTe/Si are also presented as a precursor and rational for a need of better lattice-matched substrates other than the conventional CdZnTe/Si substrates.
The advantages of mercury cadmium telluride for 'HOT' IR detector applications are discussed. Molecular beam epitaxy (MBE) is used to grow advanced device structures for this purpose. MBE offers the potential to grow HgCdTe heterostructure layers on large silicon substrates leading to very large format and high performance IR focal plane array sin the future. Preliminary material and device properties achieved p+-v-n+ device structures grown on 3 inch oriented silicon wafers are discussed.
The epitaxial growth of Hg1-xCdxTe in the composition range 0.40 < x < 0.17 has been carried out on 3-inch CdTe/Si substrates mounted on indium-free molybdenum substrate holders. Because this mounting configuration prevents the effective use of a direct thermocouple contact to control the sample temperature, and because a dramatic change in the surface emissivity of the sample occurs during the onset of HgCdTe nucleation, an alternative method for controlling the surface temperature is developed. We utilize reflection high-energy electron diffraction (RHEED) and a thermocouple ramping sequence to maintain a constant HgCdTe surface temperature. Due to the narrowness of the HgCdTe growth window, small variations in the surface temperature produce a slight but observable change in the RHEED pattern. Through careful observation of the RHEED images, an optimized thermocouple ramping process is obtained such that the RHEED pattern remained constant from the onset of HgCdTe nucleation. Structural and electrical characterization of these samples demonstrate the usefulness of the temperature ramping methodology. For middle wavelength IR (MWIR) material, mobility measurements made on several n-type samples at 77 K range give values in the 2 X 104 - 4 X 104 cm2/Vsec range with doping levels in the low 1014 cm-3. Additionally, preliminary lifetime measurements made on one MWIR sample gives 2.8 microsecond(s) ec. For long wavelength IR material, mobility measurements made on several n-type samples at 77 K give values in the 3 X 105 to 5 X 105 cm2/Vsec range with doping levels in the mid 1015 cm-3. Electrical, structural and defect characterization along with device results are presented with a focus on the optimization of the thermocouple ramping process. In addition, the efficacy of Si-based composite substrates for the technological advancement of large format IR focal plane arrays will be discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.