High performance infrared focal plane arrays (FPAs) play a critical role in a wide range of imaging applications. However the high cost associated with the required cooling and serially processed die-level hybridization is major barrier that has thwarted Mid-wavelength Infrared (MWIR) detector technology from penetrating largevolume, low-cost markets. Under the Defense Advanced Research Projects Agency (DARPA) WIRED program, the HRL team has developed a wafer level integration schemes to fabricate large format Antimonidebased MWIR FPAs on Si Read Out Integrated Circuit (ROIC) as a means to achieve significant fab cost reduction and enhanced production scalability. The DARPA-hard challenge we are addressing is the thermal and stress management in the integration of two dissimilar materials to avoid detector and ROIC degradation and to maintain structure integrity at the wafer scale. In addition, a digital ROIC with extremely large well capacity was designed and taped-out, in order to increase the operating temperature of the FPAs. In this talk, we discuss our progress under the DARPA WIRED program.
We discuss our recent work in development of 1280 x 1024/12μm pitch bulk InAsSb MWIR/MWIR twocolor focal planes with cutoff wavelengths of 4.2μm and 5.1μm in the two bands as well as SWIR/MWIR focal planes with cutoff wavelengths of 3.0μm and 4.9μm. Barrier detectors based on the InAsSb materials system have recently been developed to realize substantial improvements in the performance of MWIR detectors operating in a single MWIR wavelength band, enabling FPA performance at operating temperatures as high as 150K. We have extended this detector architecture to encompass two-color detectors operating in a sequential mode utilizing back-to-back barrier devices. These detectors utilize the ternary alloy InAsSb materials system grown by molecular-beam epitaxy on GaAs substrates as a pathway to cost-effective production of large-area focal-plane arrays. Based on extensive FPA characterization, NEDT values of 18.3mK (Band-1) and 14.2mK (Band-2) were measured under f/2.3 illumination at an array operating temperature of T = 120K, with high NEDT operabilities (2x median) of 99.93% and 99.7% in Band-1 and Band-2, respectively. No significant performance degradation was observed in epoxystabilized hybrids after 500 thermal cycles between 300K and 110K. Finally, we discuss the progress that has been made in SWIR/MWIR array development and present measurements of 1280 x 1024 FPA performance for SWIR/MWIR focal planes with cutoff wavelengths of 3.0μm and 4.9μm at T = 120K. NEDT values (f/2.3 illumination) of 18.5mK (SWIR) and 15.0mK (MWIR) and high operabilities of 99.96% (SWIR) and 99.3% (MWIR) for cutoff wavelengths of 3.0μm and 4.9μm were measured.
The main driving force for High Operating Temperature (HOT) detectors is the strong need for low cost, compact IR imaging solution capable of supporting a wide range of military and civilian applications. In the HOT regime where imagers can be cooled with multi-stage thermoelectric coolers, the major portion of the cost is due to the die-level back-end process, from the chip hybridization to final packaging. We present here an approach to achieve significant cost reduction of MWIR imagers by monolithically integrating III-V devices directly on Silicon substrates for wafer-scale fabrication and packaging of focal plane arrays (FPAs). High quality InAs films can be grown on a blanket Silicon wafer by metal-organic chemical vapor deposition (MOCVD) in a low growth temperature regime that complies with the thermal budget of the Si-electronics. High Resolution Transmission Electron Microscopy reveals predominantly oriented, single-crystal-like InAs films, with Σ3(111) twin boundaries, which our band structure calculations predict to be electrically benign. More intriguingly, selective-area growth on SiO2-masked ROIC-like templates is demonstrated with single-crystal-like InAs film nucleation at small Si(001) openings, together with the suppression of unwanted deposition on the dielectric mask. High crystallinity lateral epitaxial overgrowth of the InAs islands and film coalescence is achieved, enabling the potential to fully cover the entire patterned substrate. MBE-grown MWIR devices (λcut-off = 4.1 μm) on blanket InAs/Si templates exhibit a dark current of 2x10-5 A/cm2 , a specific detectivity of 6x1011 Jones and a quantum efficiency (QE) above 60% at 100K. The QE remains constant at high temperatures (<200K) where the dark current approaches that of baseline single-crystal HOT devices grown on native substrates At 230K, it is 6x10-2 A/cm2, yielding a specific detectivity of 1010 Jones.
Barrier detectors based on III-V materials have recently been developed to realize substantial improvements in the performance of mid-wave infrared (MWIR) detectors, enabling FPA performance at high operating temperatures. The relative ease of processing the III-V materials into large-format, small-pitch FPAs offers a cost-effective solution for tactical imaging applications in the MWIR band as an attractive alternative to HgCdTe detectors. In addition, small pixel (5-10μm pitch) detector technology enables a reduction in size of the system components, from the detector and ROIC chips to the focal length of the optics and lens size, resulting in an overall compactness of the sensor package, cooling and associated electronics. To exploit the substantial cost advantages, scalability to larger format (2kx2k/10μm) and superior wafer quality of large-area GaAs substrates, we have fabricated antimony based III-V bulk detectors that were metamorphically grown by MBE on GaAs substrates. The electro-optical characterization of fabricated 2kx2k/10μm FPAs shows low median dark current (3 x 10-5 A/cm2 with λco = 5.11μm or 2.2 x 10-6 A/cm2 with λco = 4.6μm) at 150K, high NEdT operability (3x median value) >99.8% and >60% quantum efficiency (non-ARC). In addition, we report our initial result in developing small pixel (5μm pitch), high definition (HD) MWIR detector technology based on superlattice III-V absorbing layers grown by MBE on GaSb substrates. The FPA radiometric result is showing low median dark current (6.3 x 10-6 A/cm2 at 150K with λco = 5.0μm) with ~50% quantum efficiency (non-ARC), and low NEdT of 20mK (with averaging) at 150K. The detector and FPA test results that validate the viability of Sb-based bulk and superlattice high operating temperature MWIR FPA technology will be discussed during the presentation.
We describe our recent results in developing and maturing small pixel (5μm pitch), high definition (HD) mid-wave infrared (MWIR) detector technology as well as focal-plane-array (FPA) hybrids, and prototype 2.4 Megapixel camera development operating at high temperature with low dark current and high operability. Advances in detector performance over the last several years have enabled III-V high operating temperature (T≥150K), unipolar detectors to emerge as an attractive alternative to HgCdTe detectors. The relative ease of processing the materials into large-format, small-pitch FPAs offers a cost-effective solution for tactical imaging applications in the MWIR band. In addition, small pixel detector technology enables a reduction in size of the system components, from the detector and ROIC chips to the focal length of the optics and lens size, resulting in an overall compactness of the sensor package, cooling and associated electronics. An MBE system has been used to grow antimony-based detector structures with 5.1μm cutoff with low total thickness variation (TTV) across a 3” wafer, in order to realize high interconnect yield for small-pitch FPAs. A unique indium bump scheme is proposed to realize 5μm pitch arrays with high connectivity yield. Several 1kx2k /5μm hybrids have been fabricated using Cyan’s CS3 ROICs with proper backend processing and finally packaged into a portable Dewar camera. The FPA radiometric result is showing low median dark current of 2.3x10-5 A/cm2 with > 99.9% operability, and >60% QE (without AR coating).
InAsSb material with a cutoff wavelength in the 5 μm range has been grown on GaAs substrates. The MWIR InAsSb detector arrays were fabricated and hybridized to a ROIC to permit measurement of the electrical and optical properties of detectors. Detector arrays were fabricated in a 1024 x 1024 format on an 18 μm pitch. A fanout was utilized to directly acquire data from a set of selected detectors without an intervening read out integrating circuit (ROIC). Variable temperature Jdark vs Vd measurements have been made with the dark current density ~ 10-5 A/cm2 at 150 K. The external QE measured using a narrow band filter centered at ~ 4 μm had values in the 65 – 70 % range. Since the detectors were illuminated through a GaAs substrate, which has a reflectance of 29%, the internal QE is greater than 90%. A 1024 x 1024 ROIC on an 18 μm pitch was also designed and fabricated to interface with the barrier detectors. The ROIC operates at 30 Hz frame rate and has a well capacity of 20.7 M electrons. QE at 150 K for a 1024 x 1024 detector array hybridized to a ROIC had a median D* at 150 K under a flux of 1.07 x 1015 ph/(cm2/s) was 1.2 x 1011 cm Hz1/2 /W. The NEdT was 44 mK and imagery was obtained at 150 K using an f/2.3 MWIR lens.
InAsSb material with a cutoff wavelength in the 5 μm range has been grown on GaAs substrates. The MWIR
InAsSb detector arrays were fabricated and hybridized to fanouts and ROICs to permit measurement of the
electrical and optical properties of detectors. Detector arrays were fabricated in a 1024 x 1024 format on an 18
μm pitch. A fanout was utilized to directly acquire data from a set of selected detectors without an intervening
read out integrating circuit (ROIC). Variable temperature Jdark vs Vd measurements have been made with the
dark current density ~ 10-5 A/cm2 at 150 K. The external QE measured using a narrow band filter centered at ~ 4 μm had values in the 65 - 70 % range. Since the detectors were illuminated through a GaAs substrate
which has a reflectance of 29%, the internal QE is greater than 90 %.
A 1024 x 1024 ROIC on an 18 μm pitch was also designed and fabricated to interface with the barrier
detectors. The ROIC operates at 30 Hz frame rate and has a well capacity of 20.7 M electrons. QE at 150 K
for a 1024 x 1024 detector array hybridized to a ROIC had a median D* at 150 K under a flux of 1.07 x 1015
ph/(cm2/s was 1.2 x 1011 cm Hz1/2 /W. The NEdT was 44 mK and imagery was obtained at 150 K using an f/2.3 MWIR lens.
Mid-wavelength infrared (MWIR) InAsSb alloy barrier detectors grown on GaAs substrates were characterized as a function of temperature to evaluate their performance. Detector arrays were fabricated in a 1024 × 1024 format on an 18 μm pitch. A fanout was utilized to directly acquire data from a set of selected detectors without an intervening read out integrating circuit (ROIC). The detectors have a cutoff wavelength equal to ~ 4.9 μm at 150 K. The peak internal quantum efficiency (QE) required a reverse bias voltage of 1 V. The detectors were diffusion-limited at the bias required to attain peak QE. Multiple 18 μm × 18 μm detectors were tied together in parallel by connecting the indium bump of each detector to a single large metal pad on the fanout. The dark current density at -1 V bias for a set of 64 × 64 and 6 × 6 array of detectors, each of which were tied together in parallel was ~ 10-3 A/cm2 at 200 K and 5 × 10-6 A/cm2 at 150 K. The 4096 (64 × 64) and 36 (6 × 6) detectors, both have similar Jdark vs Vd characteristics, demonstrating high operability and uniformity of the detectors in the array. The external QE measured using a narrow band filter centered at ~ 4 μm had values in the 65 – 70 % range. Since the detectors were illuminated through a GaAs substrate which has a reflectance of 29%, the internal QE is greater than 90 %. A 1024 × 1024 ROIC on an 18 μm pitch was also designed and fabricated to interface with the barrier detectors. QE at 150 K for a 1024 × 1024 detector array hybridized to a ROIC matched the QE measured on detectors that were measured directly through a fanout chip. Median D* at 150 K under a flux of 1.07 × 1015 ph/(cm2/s was 1.0 x 1011 cm Hz1/2 /W.
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.
In InAs1-xSbx material alloy composition was adjusted to achieve 200K cutoff wavelengths in the 5 μm
range. Reflectance was minimized and absorption in the InAs1-xSbx material maximized by the use of
pyramid shaped structures fabricated in the InAs1-xSbx material which function as an AR coating.
Compound-barrier (CB) detectors were fabricated and tested for optical response and dark current density
versus bias measurements were acquired as a function of temperature. For 5 μm cutoff detectors, QE is
high, ~ 75 % between 4.0 μm and 4.6 μm and > 80 % between 2.0 μand 4.0 μm, demonstrating the
efficacy of the pyramids as photon trap structures and as a replacement for multi-layer AR-coatings. Jdark
in the low 10-3 A/cm2 range at 200 K and low 10-5 A/cm2 range at 150 K was measured at the bias at
which the QE peaked.
The Photon-Trap Structures for Quantum Advanced Detectors (PT-SQUAD) program requires MWIR
detectors at 200 K. One of the ambitious requirements is to obtain high (> 80 %) quantum efficiency over
the visible to MWIR spectral range while maintaining high D* (> 1.0 x 1011 cm √Hz/W) in the MWIR. A
prime method to accomplish the goals is by reducing dark diffusion current in the detector via reducing
the volume fill ratio (VFR) of the detector while optimizing absorption. Electromagnetic simulations
show that an innovative architecture using pyramids as photon trapping structures provide a photon
trapping mechanism by refractive-index-matching at the tapered air/semiconductor interface, thus
minimizing the reflection and maximizing absorption to > 90 % over the entire visible to MWIR spectral
range. InAsSb with bandgap appropriate to obtaining a cutoff wavelength ~ 4.3 μm is chosen as the
absorber layer. An added benefit of reducing VFR using pyramids is that no AR-coating is required.
Compound-barrier (CB) detector test structures with alloy composition of the InAsSb absorber layer
adjusted to achieve 200 K cutoff wavelength of 4.3 μm (InAsSb lattice-matched to GaSb). Dark current
density at 200 K is in the low 10-4 A/cm2 at Vd = -1.0 V. External QE ~ 0.65 has been measured for
detectors with a Si carrier wafer attached. Since illumination is through the Si carrier wafer that has a
reflectance of ~ 30 %, this results in an internal QE > 0.9.
We have evaluated selective doping techniques for the fabrication of type II LWIR superlattice planar
detectors. Ion-implantation and diffusion of dopants were evaluated for selective doping of the electrical
junction region in planar photodiodes. Residual damage remains when superlattice structures are implanted
with Te ions with an energy of 190 keV and a dose of 5x1013 cm-2, at room temperature. Controlled Zn
diffusion profiles with concentrations from 5x1016 to > 5x1018 cm-3 in the wide bandgap cap layer was
achieved through a vapor phase diffusion technique. Planar p-on-n diodes were fabricated using selective
Zn diffusion. The I-V characteristics were leaky due to G-R and tunneling in the homojunction devices, for
which no attempts were made to optimize the n-type absorber doping level. Work is underway for the
implementation of planar diodes with the n-on-p architecture through selective Te diffusion. Due to
increased minority carrier lifetimes for p-type InAs/GaSb superlattice absorber layers, planar device with
the n-on-p architecture have the potential to provide improved performance as compared to the p-on-n
counterparts.
J. Asbrock, S. Bailey, D. Baley, J. Boisvert, G. Chapman, G. Crawford, T. de Lyon, B. Drafahl, J. Edwards, E. Herrin, C. Hoyt, M. Jack, R. Kvaas, K. Liu, W. McKeag, R. Rajavel, V. Randall, S. Rengarajan, J. Riker
Advanced LADAR receivers enable high accuracy identification of targets at ranges beyond standard EOIR sensors. Increased sensitivity of these receivers will enable reductions in laser power, hence more affordable, smaller sensors as well as much longer range of detection. Raytheon has made a recent breakthrough in LADAR architecture by combining very low noise ~ 30 electron front end amplifiers with moderate gain >60 Avalanche Photodiodes. The combination of these enables detection of laser pulse returns containing as few as one photon up to 1000s of photons. Because a lower APD gain is utilized the sensor operation differs dramatically from traditional "Geiger mode APD" LADARs. Linear mode photon counting LADAR offers advantages including: determination of intensity as well as time of arrival, nanosecond recovery times and discrimination between radiation events and signals. In our talk we will present an update of this development work: the basic amplifier and APD component performance, the front end architecture, the demonstration of single photon detection using a simple 4 × 4 SCA and the design and evaluation of critical components of a fully integrated photon counting camera under development in support of the Ultra-Sensitive Detector (USD) program sponsored by AFRL-Kirtland.
Michael Jack, Jim Asbrock, Steven Bailey, Diane Baley, George Chapman, Gina Crawford, Betsy Drafahl, Eileen Herrin, Robert Kvaas, William McKeag, Valerie Randall, Terry De Lyon, Andy Hunter, John Jensen, Tom Roberts, Patrick Trotta, T. Dean Cook
Raytheon is developing HgCdTe APD arrays and sensor chip assemblies (SCAs) for scanning and staring LADAR systems. The nonlinear characteristics of APDs operating in moderate gain mode place severe requirements on layer thickness and doping uniformity as well as defect density. MBE based HgCdTe APD arrays, engineered for high performance, meet the stringent requirements of low defects, excellent uniformity and reproducibility. In situ controls for alloy composition and substrate temperature have been implemented at HRL, LLC and Raytheon Vision Systems and enable consistent run to run results. The novel epitaxial designed using separate absorption-multiplication (SAM) architectures enables the realization of the unique advantages of HgCdTe including: tunable wavelength, low-noise, high-fill factor, low-crosstalk, and ambient operation. Focal planes built by integrating MBE detectors arrays processed in a 2 x 128 format have been integrated with 2 x 128 scanning ROIC designed. The ROIC reports both range and intensity and can detect multiple laser returns with each pixel autonomously reporting the return. FPAs show exceptionally good bias uniformity <1% at an average gain of 10. Recent breakthrough in device design has resulted in APDs operating at 300K with essentially no excess noise to gains in excess of 100, low NEP <1nW and GHz bandwidth. 3D LADAR sensors utilizing these FPAs have been integrated and demonstrated both at Raytheon Missile Systems and Naval Air Warfare Center Weapons Division at China Lake. Excellent spatial and range resolution has been achieved with 3D imagery demonstrated both at short range and long range. Ongoing development under an Air Force Sponsored MANTECH program of high performance HgCdTe MBE APDs grown on large silicon wafers promise significant FPA cost reduction both by increasing the number of arrays on a given wafer and enabling automated processing.
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 cm2 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.
Michael Jack, James Asbrock, C. Anderson, Steven Bailey, George Chapman, E. Gordon, P. Herning, Murray Kalisher, Kim Kosai, V. Liquori, Valerie Randall, Joseph Rosbeck, Sanghamitra Sen, P. Wetzel, Maurice Halmos, Patrick Trotta, Andrew Hunter, John Jensen, Terence de Lyon, W. Johnson, B. Walker, Ward Trussel, Andy Hutchinson, Raymond Balcerak
HgCdTe APDs and APD arrays offer unique advantages for high-performance eyesafe LADAR sensors. These include: operation at room temperature, low-excess noise, high gain, high-quantum efficiency at eyesafe wavelengths, GHz bandwidth, and high-packing density. The utility of these benefits for systems are being demonstrated for both linear and area array sensors. Raytheon has fabricated 32 element linear APD arrays utilizing liquid phase epitaxy (LPE), and packaged and integrating these arrays with low-noise amplifiers. Typical better APDs configured as 50-micron square pixels and fabricated utilizing RIE, have demonstrated high fill factors, low crosstalk, excellent uniformity, low dark currents, and noise equivalent power (NEP) from 1-2 nW. Two units have been delivered to NVESD, assembled with range extraction electronics, and integrated into the CELRAP laser radar system. Tests on these sensors in July and October 2000 have demonstrated excellent functionality, detection of 1-cm wires, and range imaging. Work is presently underway under DARPA's 3-D imaging Sensor Program to extend this excellent performance to area arrays. High-density arrays have been fabricated using LPE and molecular beam epitaxy (MBE). HgCdTe APD arrays have been made in 5 X 5, 10 X 10 and larger formats. Initial data shows excellent typical better APD performance with unmultiplied dark current < 10 nA; and NEP < 2.0 nW at a gain of 10.
Since its initial synthesis and investigation more than 40 years ago, the HgCdTe alloy semiconductor system has evolved into one of the primary infrared detector materials for high-performance infrared focal-plane arrays (FPA) designed to operate in the 3-5 mm and 8-12 mm spectral ranges of importance for thermal imaging systems. Over the course of the past decade, significant advances have been made in the development of thin-film epitaxial growth techniques, such as molecular-beam epitaxy (MBE), which have enabled the synthesis of IR detector device structures with complex doping and composition profiles. The central role played by in situ sensors for monitoring and control of the MBE growth process are reviewed. The development of MBE HgCdTe growth technology is discussed in three particular device applications: avalanche photodiodes for 1.55 +m photodetection, megapixel FPAs on Si substrates, and multispectral IR detectors.
Maurice Halmos, Michael Jack, James Asbrock, C. Anderson, Steven Bailey, George Chapman, E. Gordon, P. Herning, Murray Kalisher, Louis Klaras, Kim Kosai, V. Liquori, Mike Pines, Valerie Randall, Robin Reeder, Joseph Rosbeck, Sanghamitra Sen, Patrick Trotta, P. Wetzel, Andrew Hunter, John Jensen, T. DeLyon, Charlie Trussell, James Hutchinson, Raymond Balcerak
Raytheon has recently been funded by DARPA to develop an FPA for single shot eyesafe ladar operation. The goal of the program is to develop new high speed imaging rays to rapidly acquire high resolution, 3D images of tactical targets at ranges as long as 7 to 10 kilometers. This would provide precision strike, target identification from rapidly moving platforms, such as air-to-ground seekers, which would enhance counter-counter measure performance and the ability to lock-on after launch. Also a goal is to demonstrate the acquisition of hidden, camouflaged and partially obscured targets. Raytheon's approach consists of using HgCdTe APD arrays which offer unique advantages for high performance eyesafe LADAR sensors. These include: eyesafe operation at room temperature, low excess noise, high gain to overcome thermal and preamp noise, Ghz bandwidth and high packing density. The detector array will be coupled with a Readout Integrated Circuit, that will capture all the information required for accurate range determination. The two components encompass a hybrid imaging array consisting of two IC circuit chips vertically integrated via an array of indium metal 'bumps'. The chip containing the PAD detector array and the silicon signal processing readout chip are independently optimized to provide the highest possible performance for each function.
Terence de Lyon, B. Baumgratz, G. Chapman, E. Gordon, Andrew Hunter, Michael Jack, John Jensen, W. Johnson, Blaine Johs, Kim Kosai, W. Larsen, Greg Olson, M. Sen, Burt Walker
Separate absorption and multiplication avalanche photodiode (SAM-APD) device structures, operating in the 1.1 - 1.6 micrometer spectral range, have been fabricated in the HgCdTe material system by molecular-beam epitaxy. These HgCdTe device structures, which offer an alternative technology to existing III-V APD detectors, were grown on CdZnTe(211)B substrates using CdTe, Te, and Hg sources with in situ In and As doping. The alloy composition of the HgCdTe layers was adjusted to achieve both efficient absorption of IR radiation in the 1.1 - 1.6 micrometer spectral range and low excess-noise avalanche multiplication. To achieve resonant enhancement of hole impact ionization from the split-off valence band, the Hg1-xCdxTe alloy composition in the gain region of the device, x equals 0.73, was chosen to achieve equality between the bandgap energy and spin-orbit splitting. The appropriate value of this alloy composition was determined from analysis of the 300 K bandgap and spin-orbit splitting energies of a set of calibration layers, using a combination of IR transmission and spectroscopic ellipsometry measurements. MBE-grown APD epitaxial wafers were processed into passivated mesa-type discrete device structures and diode mini-arrays using conventional HgCdTe process technology. Device spectral response, dark current density, and avalanche gain measurements were performed on discrete diodes and diode mini- arrays on the processed wafers. Avalanche gains in the range of 30 - 40 at reverse bias of 85 - 90 V and array-median dark current density below 2 X 10-4 A/cm2 at 40 V reverse bias have been demonstrated.
Significant progress has been made in the technology for MBE growth of HgCdTe infrared focal-plane arrays on Si substrates since the initial demonstration of MBE HgCdTe-on- Si heteroepitaxy in 1989. In 1995, the first all-MBE-grown detector arrays on Si were produced through direct MBE growth of (112)B-oriented II-VI films on Si without III-V initiation layers, culminating in detector performance comparable to LPE-grown detectors on bulk CdZnTe substrates. This achievement was enabled by the development of two key contributing technologies: CdTe on Si buffer layer growth and HgCdTe p-on-n double-layer heterojunction growth using p-type chemical doping with As. The MBE process for deposition of high crystalline quality CdTe buffer layers has been developed so that x-ray rocking curve FWHM less than 75 arc-sec and near-surface etch pit densities (EPD) of 2 multiplied by 106 cm-2 are routinely achievable for 9-micrometer-thick CdTe buffer layers. The dependence of CdTe EPD on ZnTe initiation layer thickness, insertion of CdTe/CdZnTe strained layer superlattices, and thermal cycling to cryogenic temperatures has been investigated and is reviewed. HgCdTe baselayers deposited by MBE on these CdTe/Si composite substrates exhibit x-ray FWHM as low as 72 arc-sec and EPD of 3 - 20 multiplied by 106 cm-2. To demonstrate the potential for MBE growth of large-area HgCdTe FPAs on Si, detectors with 78 K cutoff wavelength of 7.8 micrometer have been fabricated in this HgCdTe/Si epitaxial material with array-average R0A product of 1.64 multiplied by 104 (Omega) -cm2 (0 FOV).
HgCdTe MBE technology offers many advantages for the growth of multi-layer heterojunction structures for high performance IRFPAs. This paper reports data on major advances towards the fabrication of advanced detector structures, which have been made in MBE technology at Hughes Research Laboratories during the last couple of years. Currently device quality materials with desired structural and electrical characteristics are grown with the alloy compositions required for short-wavelength infrared (SWIR, 1 - 3 micron) to very long- wavelength infrared (VLWIR, 14 - 18 micron) detector applications. In-situ In (n-type) and As (p-type) doping developed at HRL have facilitated the growth of advanced multi-layer heterojunction devices. Thus, high performance IR focal plane arrays (128 X 128) with state-of-the-art performance have been fabricated with MBE-grown double-layer heterojunction structures for MWIR and LWIR detector applications. In addition, the growth of n-p-p-n multi-layer heterojunction structures has been developed and two-color detectors have been demonstrated. Recently, significant preliminary results on the heteroepitaxy growth of HgCdTe double-layer heterojunction structures on silicon have been achieved.
Molecular-beam epitaxy (MBE) has been utilized to deposit single crystal epitaxial films of CdTe(112)B and HgCdTe(112)B directly onto Si(112) substrates without the use of GaAs interfacial layers. The films have been characterized with x-ray diffraction and wet chemical defect etching, and IR detectors have been fabricated and tested. CdTe(112)B films are twin- free and have x-ray rocking curves as narrow as 72 arc-seconds and near-surface etch pit density (EPD) of 2 X 106 cm-2 for 8 micrometers -thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 92 arc-seconds and EPD of 8 - 30 X 106 cm-2. HgCdTe/Si infrared detectors have been fabricated with R0A equals 4.3 X 103 (Omega) -cm2 (f/2 FOV) and 7.8 micrometers cutoff wavelength at 78 K to demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.
Molecular-beam epitaxy (MBE) has been utilized to deposit single crystal films of ZnTe and CdZnTe/ZnTe onto Si(100) and Si(112) substrates. Parallel epitaxy of ZnTe(100) and CdZnTe(100)/ZnTe(100) has been observed for growth on Si(100) substrates misoriented from 0-8 degrees towards the [011] direction. With ZnTe initiation layers, high quality CdZnTe(100) films have been demonstrated on both 4° and 8° misoriented Si(100) with x-ray rocking curve FWHM as narrow as 158 arc-seconds, which is comparable to that obtained with GaAs/Si composite substrates. The observed surface morphologies are superior to those obtained on GaAs/Si composite substrates. HgCdTe(100) films with x-ray FWHM as low as 55 arcseconds and average etch pit densities of 5 x 106 cm2 have been deposited by liquid phase epitaxy on these MBE CdZnTe/ZnTe/Si(100) substrates. On vicinal Si(1 12) substrates, ZnTe films are observed to nucleate in either the (1 12) or its twin (552) orientation depending on the misorientation of the Si substrate away from (1 12). For Si(1 12) misorientations of 5° or 10° towards from the [1 1-1] direction, ZnTe nucleates in a parallel (1 12) orientation, while for misorientations of 0° or 5° away from the [1 1-1] direction, ZnTe is observed to nucleate in a (552) orientation. CdTe deposited on ZnTe/Si(112) is observed to nucleate in the same orientation as the ZnTe. CdTe(552) epilayers are of substantially higher quality than (1 12)oriented films. X-ray rocking curves as narrow as 1 10 arc-seconds have been observed for the CdTe(331) reflection in the case of (552)-oriented epitaxy.
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