Planar <Emphasis Type="Italic">n</Emphasis>-on-<Emphasis Type="Italic">p</Emphasis> HgCdTe FPAs for LWIR and VLWIR Applications

Mainly driven by space applications, mercury cadmium telluride (MCT) focal-plane arrays (FPAs) have been successfully developed for very long wavelengths (λ _CO > 14 μm at 55 K). For this purpose, the standard n-on-p technology based on MCT grown by liquid-phase epitaxy (LPE) and involving vacancy doping has been modified to extrinsic doping by a monovalent acceptor. Due to the planar diode geometry obtained by ion implantation, most of the carrier generation volume is located in the p-type region with a thickness of approximately 8 μm. According to our understanding, the Shockley–Read centers connected with the Hg vacancies are thus significantly reduced. This situation should lead to longer minority-carrier lifetimes and smaller generation rates under equilibrium conditions, therefore yielding lower dark current. We indeed observe a reduction by a factor of approximately 15 by using extrinsic doping. Recent dark current data obtained in the temperature range from 55 K to 85 K on 288 × 384 FPAs with λ _CO(60 K) = 12 μm, either intrinsically or extrinsically doped, corroborate this finding. These data, new results on a 112 × 112 pixel demonstrator array with λ _CO(55 K) = 14.4 μm, and earlier measurements are compared with Tennant’s Rule 07 established for p-on-n technology.     

HgCdTe <Emphasis Type="Italic">p</Emphasis>-on-<Emphasis Type="Italic">n</Emphasis> Focal-Plane Array Fabrication Using Arsenic Incorporation During MBE Growth

Extrinsic p-type doping during molecular-beam epitaxy (MBE) growth represents an essential generic toolbox for advanced heterostructures based on the HgCdTe material system: PiN diodes, mesa avalanche photodiodes (APD) or third-generation multispectral focal-plane arrays. Today, arsenic appears to be the best candidate to fulfill this role and our group is actively working on its incorporation during MBE growth, using an original radio frequency (RF) plasma source for arsenic. Such a cell is supposed to deliver a monatomic As flux, and as expected we observed high As electrical activation rates after annealing short-wave (SW), mid-wave (MW), and long-wave (LW) layers. At last, a couple of technological runs have been carried out in the MW range in order to validate the approach on practical devices. p-on-n focal-plane arrays (FPA) have been fabricated using a mesa delineated technology on an As-on-In doped metallurgical heterojunction layer grown on a lattice-matched CdZnTe layer (320 × 256, 30 μm pitch, 5 μm cutoff at 77 K). Observed diodes exhibit very interesting electro-optical characteristics: large shunt impedance, high quantum efficiency, and no noticeable excess noise. The resulting focal-plane arrays were observed to be very uniform, leading to high operabilities. Noise equivalent temperature difference (NETD) distributions are very similar to those observed with the As ion-implanted p-on-n technology, fabricated in our laboratory as well. In our opinion, those excellent results demonstrate the feasibility of our MBE in situ arsenic doping process. Good electrical activation rates and high-quality layers can be obtained. We believe that such an approach allows precise control of the p-doping profile in the HgCdTe layer, which is necessary for advanced structure designs.     

LWIR HgCdTe Detectors Grown on Ge Substrates

Long-wavelength infrared (LWIR) HgCdTe p-on-n double-layer heterojunctions (DLHJs) for infrared detector applications have been grown on 100 mm Ge (112) substrates by molecular beam epitaxy (MBE). The objective of this current work was to grow our baseline p-on-n DLHJ detector structure (used earlier on Si substrates) on 100 mm Ge substrates in the 10 μm to 11 μm LWIR spectral region, evaluate the material properties, and obtain some preliminary detector performance data. Material characterization techniques included are X-ray rocking curves, etch pit density (EPD) measurements, compositional uniformity determined from Fourier-transform infrared (FTIR) transmission, and doping concentrations determined from secondary-ion mass spectroscopy (SIMS). Detector properties include resistance-area product (RoA), spectral response, and quantum efficiency. Results of LWIR HgCdTe detectors and test structure arrays (TSA) fabricated on both Ge and silicon (Si) substrates are presented and compared. Material properties demonstrated include X-ray full-width of half-maximum (FWHM) as low as 77 arcsec, typical etch pit densities in mid 10⁶ cm⁻² and wavelength cutoff maximum/minimum variation <2% across the full wafer. Detector characteristics were found to be nearly identical for HgCdTe grown on either Ge or Si substrates.     

High-Performance MWIR/LWIR Dual-Band 640 × 480 HgCdTe/Si FPAs

HgCdTe grown on large-area Si substrates allows for larger array formats and potentially reduced focal-plane array (FPA) cost compared with smaller, more expensive CdZnTe substrates. The goal of this work is to evaluate the use of HgCdTe/Si for mid-wavelength/long-wavelength infrared (MWIR/LWIR) dual-band FPAs. A series of MWIR/LWIR dual-band HgCdTe triple-layer n-P-n heterojunction (TLHJ) device structures were grown by molecular-beam epitaxy (MBE) on 100-mm (211)Si substrates. The wafers showed low macrodefect density (<300 cm⁻²) and was processed into 20-μm-unit-cell 640 × 480 detector arrays which were mated to dual-band readout integrated circuits (ROICs) to produce FPAs. The measured 80-K cutoff wavelengths were 5.5 μm for MWIR and 9.4 μm for LWIR, respectively. The FPAs exhibited high pixel operabilities in each band, with noise equivalent differential temperature (NEDT) operabilities of 99.98% for the MWIR band and 99.6% for the LWIR band demonstrated at 84 K.     

Design and Modeling of HgCdTe nBn Detectors

An n-type mercury cadmium telluride (HgCdTe) unipolar nBn infrared detector structure is proposed as a means of achieving performance limited by intrinsic thermal carrier generation without requirements for p-type doping. Numerical modeling was utilized to calculate the current–voltage and optical response characteristics and detectivity values for HgCdTe nBn and p–n junction devices with a cut-off wavelength of 12 μm for temperatures between 50 K and 300 K. Calculations demonstrate similar dark current density, responsivity, and detectivity values within 10% for the long-wavelength infrared (LWIR) nBn detector compared with the p–n junction structure for temperatures from 50 K to 95 K. These results show that the HgCdTe nBn device may be a promising alternative for achieving high performance using a simplified device structure while circumventing issues related to p-type doping in current p–n junction technology such as achieving low, controllable doping concentrations, and serving as a basis for next-generation device structures.     

From Long Infrared to Very Long Infrared Wavelength Focal Plane Arrays Made with HgCdTe n + n ?/ p Ion Implantation Technology

This paper aims at studying the feasibility of very long infrared wavelength (VLWIR) (12–18 μm) focal plane arrays using n-on-p planar ion-implanted technology. To explore and analyze the feasibility of such VLWIR detectors, a set of four Cd _x Hg_1−x Te LPE layers with an 18 μ cutoff at 50 K has been processed at Defir (LETI/LIR–Sofradir joint laboratory), using both our “standard” n-on-p process and our improved low dark current process. Several 320 × 256 arrays, 30-μm pitch, have been hybridized on standard Sofradir readout circuits and tested. Small dimension test arrays characterization is also presented. Measured photonic currents with a 20°C black body suggest an internal quantum efficiency above 50%. Typical I(V) curves and thermal evolution of the saturation current are discussed, showing that standard photodiodes remain diffusion limited at low biases for temperatures down to 30 K. Moreover, the dark current gain brought by the improved process is clearly visible for temperatures higher than 40 K. Noise measurements are also discussed showing that a very large majority of detectors appeared background limited under usual illumination and biases. In our opinion, such results demonstrate the feasibility of high-performance complex focal plane arrays in the VLWIR range at medium term.     

Status of LWIR HgCdTe-on-Silicon FPA Technology

The use of silicon as an alternative substrate to bulk CdZnTe for epitaxial growth of HgCdTe for infrared detector applications is attractive because of potential cost savings as a result of the large available sizes and the relatively low cost of silicon substrates. However, the potential benefits of silicon as a substrate have been difficult to realize because of the technical challenges of growing low-defect-density HgCdTe on silicon where the lattice mismatch is ∼19%. This is especially true for long-wavelength infrared (LWIR) HgCdTe detectors where the performance can be limited by the high (∼5 × 10⁶ cm⁻²) dislocation density typically found in HgCdTe grown on silicon. The current status of LWIR (9 μm to 11 μm at 78 K) HgCdTe on silicon focal-plane arrays (FPAs) is reviewed. Recent progress is covered including improvements in noise equivalent differential temperature (NEDT) and array operability. NEDT of <25 mK and NEDT operability >99% are highlighted for 640 × 480 pixel, 20-μm-pitch FPAs.     

Characterization of HgCdTe MWIR Back-Illuminated Electron-Initiated Avalanche Photodiodes

This article reports new characterization data for large-area (250 μm ×  250 μm) back-illuminated planar n-on-p HgCdTe electron-initiated avalanche photodiodes (e-APDs). These e-APDs were fabricated in p-type HgCdTe films grown by liquid-phase epitaxy (LPE) on CdZnTe substrates. We previously reported that these arrays exhibit gain that increases exponentially with reverse bias voltage, with gain-versus-bias curves that are quite uniform from element to element, and with a maximum gain of 648 at −11.7 V at 160 K for a cutoff wavelength of 4.06 μm. Here we report new data on these planar e-APDs. Data from a third LPE film with a longer cutoff wavelength (4.29 μm at 160 K) supports the exponential dependence of gain on cutoff wavelength, for the same bias voltage, that we reported for the first two films (with cutoffs of 3.54 μm and 4.06 μm at 160 K), in agreement with Beck’s empirical model for gain versus voltage and cutoff wavelength in HgCdTe e-APDs. Our lowest gain-normalized current density at 80 K and zero field-of-view is 0.3 μA/cm² at −10.0 V for a cutoff of 4.23 μm at 80 K. We report data for the temperature dependence of gain over 80 K to 200 K. We report, for the first time, the dependence of measured gain on junction area for widely spaced circular diodes with radii of 20 μm to 175 μm. We interpret the variation of measured gain with junction area in terms of an edge-enhanced electric field, and fit the data with a two-gain model having a lower interior gain and a higher edge gain. We report data for the excess noise factor F(M) near unity for gains up to 150 at 196 K. We describe the abrupt breakdown phenomenon seen in most of our devices at high reverse bias.     

High-Performance LWIR MBE-Grown HgCdTe/Si Focal Plane Arrays

We have been actively pursuing the development of long-wavelength infrared (LWIR) HgCdTe grown by molecular beam epitaxy (MBE) on large-area silicon substrates. The current effort is focused on extending HgCdTe/Si technology to longer wavelengths and lower temperatures. The use of Si versus bulk CdZnTe substrates is being pursued due to the inherent advantages of Si, which include available wafer sizes (as large as 300 mm), lower cost (both for the substrates and number of die per wafer), compatibility with semiconductor processing equipment, and the match of the coefficient of thermal expansion with silicon read-out integrated circuit (ROIC). Raytheon has already demonstrated low-defect, high-quality MBE-grown HgCdTe/Si as large as 150 mm in diameter. The focal plane arrays (FPAs) presented in this paper were grown on 100 mm diameter (211)Si substrates in a Riber Epineat system. The basic device structure is an MBE-grown p-on-n heterojunction device. Growth begins with a CdTe/ZnTe buffer layer followed by the HgCdTe active device layers; the entire growth process is performed in␣situ to maintain clean interfaces between the various layers. In this experiment the cutoff wavelengths were varied from 10.0 μm to 10.7 μm at 78 K. Detectors with >50% quantum efficiency and R ₀ A ∼1000 Ohms cm² were obtained, with 256 × 256, 30 μm focal plane arrays from these detectors demonstrating response operabilities >99%. Work supported by the Missile Defense Agency (MDA) through CACI Technologies, Inc. subcontract no. 601-05-0088, NVESD technical task order no. TTO-01, prime contract no. DAAB07-03-D-C214, (delivery order no. 0016)     

LWIR Strained-Layer Superlattice Materials and Devices at Teledyne Imaging Sensors

Strained-layer superlattices (SLS) based on type II InAs/Ga(In)Sb materials are a rapidly maturing technology and are theoretically predicted to exceed the dark-current performance of state-of-the-art HgCdTe. A substantial effort is underway at Teledyne Imaging Sensors in the development of SLS materials for infrared focal-plane arrays. In this paper, we describe state-of-the-art materials, device research and characterization, along with testing results for long-wavelength infrared SLS devices based on double-heterostructure and p ⁺-B-n architectures, having n-on-p and p-on-n polarities, respectively. Detector materials exhibited excellent morphological and crystalline characteristics, and electro-optical characterization demonstrated performance comparable to the state of the art.     

HgCdTe MWIR Back-Illuminated Electron-Initiated Avalanche Photodiode Arrays

This paper reports data for back-illuminated planar n-on-p HgCdTe electron-initiated avalanche photodiode (e-APD) 4 × 4 arrays with large unit cells (250 × 250 μm²). The arrays were fabricated from p-type HgCdTe films grown by liquid phase epitaxy (LPE) on CdZnTe substrates. The arrays were bump-mounted to fanout boards and characterized in the back-illuminated mode. Gain increased exponentially with reverse bias voltage, and the gain versus bias curves were quite uniform from element to element. The maximum gain measured was 648 at −11.7 V for a cutoff wavelength of 4.06 μm at 160 K. For the same reverse-bias voltage, the gains measured at 160 K for elements with two different cutoff wavelengths (3.54 μm and 4.06 μm at 160 K) show an exponential increase with increasing cutoff wavelength, in agreement with Beck’s empirical model for gain versus voltage and cutoff wavelength in HgCdTe e-APDs. Spot scan data show that both the V = 0 response and the gain at V = −5.0 V are spatially uniform over the large junction area. To the best of our knowledge, these are the first spot scan data for avalanche gain ever reported for HgCdTe e-APDs. Capacitance versus voltage data are consistent with an ideal abrupt junction having a donor concentration equal to the indium concentration in the LPE film. U.S. Workshop on the Physics and Chemistry of II-VI Materials Newport Beach, California October 10–12, 2006.     

Planar p-on-n HgCdTe FPAs by Arsenic Ion Implantation

This paper reviews recent developments in the characterization of planar p-on-n photodiodes fabricated from long- and mid-wavelength Hg_1−x Cd _x Te at␣the Electronics and Information Technology Laboratory (LETI). The Hg_1−x Cd _x Te epitaxial layers were grown by both liquid-phase and molecular-beam epitaxy. Planar p-on-n photodiodes were fabricated by arsenic implantation into an indium-doped Hg_1−x Cd _x Te base layer. Electro-optical characterization on these p-on-n photodiodes showed low leakage currents (shunt resistance > 10 GΩ) and mean R ₀ A values comparable to the state of the art, i.e., equal to 5000 Ω cm² at λ _c = 9.3 μm (λ _c: cutoff wavelength). Results of focal-plane arrays operating in both the long-wavelength infrared (IR) and middle-wavelength IR bands are reported, with noise equivalent delta temperature and responsivity values at λ _c = 9.3 μm in excess of 99.64%. These results demonstrate the viability and technological maturity of both material growth and device processing.     

Fabrication and Characterization of Small Unit-Cell Molecular Beam Epitaxy Grown HgCdTe-on-Si Mid-Wavelength Infrared Detectors

Small 15 μm unit-cell mid-wavelength infrared (MWIR) detectors have been fabricated and characterized at Raytheon Vision Systems (RVS) to enable the development of high resolution, large format, infrared imaging systems. The detectors are fabricated using molecular beam epitaxy (MBE) grown 4-in. HgCdTe-on-Si wafers with a p-on-n double layer heterojunction (DLHJ) device architecture. Advanced fabrication processes, such as inductively coupled plasma (ICP) etching, developed for large format MBE-on-Si wafers and 20 μm unit-cell two-color triple layer heterojunction (TLHJ) focal plane arrays (FPAs) have been successfully extended and applied to yield high performance 15 μm unit-cell single color detectors that compare favorably with state-of-the-art detectors with larger pitch. The measured 78 K MWIR cut-off wavelength for the fabricated detectors is near 5.5 μm, and the current–voltage characteristics of these devices exhibit strong reverse breakdown and RoA performance as a function of temperature with diffusion limited performance extending to temperatures down to 120 K.     

Design and development of multicolor MWIR/LWIR and LWIR/VLWIR detector arrays

Multicolor infrared (IR) focal planes are required for high-performance sensor applications. These sensors will require multicolor focal plane arrays (FPAs) that will cover various wavelengths of interest in mid wavelength infrared/long wavelength infrared (MWIR/LWIR) and long wavelength infrared/very long wavelength infrared (LWIR/VLWIR) bands. There has been significant progress in HgCdTe detector technology for multicolor MWIR/LWIR and LWIR/VLWIR FPAs.^1–3 Two-color IR FPAs eliminate the complexity of multiple single-color IR FPAs and provide a significant reduction of weight and power in simpler, reliable, and affordable systems. The complexity of a multicolor IR detector MWIR/LWIR makes the device optimization by trial and error not only impractical but also merely impossible. Too many different geometrical and physical variables need to be considered at the same time. Additionally, material characteristics are only relatively controllable and depend on the process repeatability. In this context, the ability of performing “simulation experiments” where only one or a few parameters are carefully controlled is paramount for a quantum improvement of a new generation of multicolor detectors for various applications.     

Large-Format HgCdTe Dual-Band Long-Wavelength Infrared Focal-Plane Arrays

Raytheon Vision Systems (RVS) continues to further its capability to deliver state-of-the-art high-performance, large-format, HgCdTe focal-plane arrays (FPAs) for dual-band long-wavelength infrared (L/LWIR) detection. Specific improvements have recently been implemented at RVS in molecular-beam epitaxy (MBE) growth and wafer fabrication and are reported in this paper. The aim of the improvements is to establish producible processes for 512 × 512 30-μm-unit-cell L/LWIR FPAs, which has resulted in: the growth of triple-layer heterojunction (TLHJ) HgCdTe back-to-back photodiode detector designs on 6 cm × 6 cm CdZnTe substrates with 300-K Fourier-transform infrared (FTIR) cutoff wavelength uniformity of ±0.1 μm across the entire wafer; demonstration of detector dark-current performance for the longer-wavelength detector band approaching that of single-color liquid-phase epitaxy (LPE) LWIR detectors; and uniform, high-operability, 512 × 512 30-μm-unit-cell FPA performance in both LWIR bands.     

Numerical Analysis of Current–Voltage Characteristics of LWIR nBn and p-on-n HgCdTe Photodetectors

We performed numerical analysis of the current–voltage characteristics of long-wavelength infrared unipolar HgCdTe nBn photodetectors and compared those results with those of conventional p-on-n HgCdTe photodiodes. A computer program was applied to explain in detail the impact of the charge carrier generation and recombination processes on current densities. In our model the carrier diffusion, thermal generation–recombination, band-to-band tunneling, trap-assisted tunneling (via states located at mercury vacancies as well as dislocation cores), and impact ionization are included as potential limiting mechanisms. To validate the model, we compared the theoretical predictions with experimental data of high-quality p-on-n photodiodes published in the literature.     

Nonequilibrium Operation of Arsenic Diffused Long-Wavelength Infrared HgCdTe Photodiodes

We demonstrated a device with a unique planar architecture using a novel approach for obtaining low arsenic doping concentrations in long-wavelength (LW) HgCdTe on CdZnTe substrates. HgCdTe materials were grown by molecular beam epitaxy (MBE). We fabricated a p-on-n structure that we term P ⁺/π/N ⁺ where the symbol “π” is to indicate a drastically reduced extrinsic p-type carrier concentration (on the order of mid 10¹⁵ cm⁻³); P ⁺ and N ⁺ denote a higher doping density, as well as a higher energy gap, than the photosensitive base π-region. Fabricated devices indicated that Auger suppression is seen in the P ⁺/π/N ⁺ architecture at temperatures above 130 K and we obtained a saturation current on the order of 3 mA on 250-μm-diameter devices at 300 K with Auger suppression. Data shows that about a 50% reduction in dark current is achieved at 300 K due to Auger suppression. The onset of Auger suppression voltage is 450 mV at 300 K and 100 mV at 130 K. Results indicate that a reduction of the series resistance could reduce this further. A principal challenge was to obtain low p-type doping levels in the π-region. This issue was overcome using a novel deep diffusion process, thereby demonstrating successfully low-doped p-type HgCdTe in MBE-grown material. Near-classical spectral responses were obtained at 250 K and at 100 K with cut-off wavelengths of 7.4 μm and 10.4 μm, respectively. At 100 K, the measured non-antireflection-coated quantum efficiency was 0.57 at 0.1 V under backside illumination. Received November 7, 2007; accepted March 19, 2008     

Junction Stability in Ion-Implanted Mercury Cadmium Telluride

Ion implantation into HgCdTe results in the production of Hg interstitials, which can be subsequently driven into the HgCdTe by an annealing process. This diffusive drive-in of the Hg interstitials fills vacancies and kicks out group I impurities and results in the formation of an n–p junction. In this work we report on the production of interstitials during baking subsequent to the ion implantation process. Various concentrations of metal vacancies were first introduced into mid-wavelength infrared (MWIR, 3 μm to 5 μm) HgCdTe by annealing under tellurium-saturated conditions at various temperatures. Baking subsequent to planar implantation of boron produced n–p junctions whose depths were measured by defect etching. The results were modeled using a simple diffusion limited model from a fixed surface concentration. The surface concentration was allowed to decrease exponentially to zero after a time, found to be of the order of ∼80 h to 150 h. Exhaustion of the interstitials sources produced by the implantation was nearly complete after ∼400 h. The total number of mercury interstitials produced was approximately 50% of the implant dosage.     

p-on-n HgCdTe Infrared Focal-Plane Arrays: From Short-Wave to Very-Long-Wave Infrared

This paper reports on recent developments made at the DEFIR joint laboratory on fabrication of planar p-on-n arsenic (As)-ion-implanted HgCdTe photodiodes. Our infrared focal-plane arrays (IRFPAs) cover a wide spectral range, from the short-wave infrared (SWIR) to the very-long-wave infrared (VLWIR). Our planar p-on-n technology is a classical one based on ion implantation followed by diffusion and activation. The p-type doping is obtained by As implantation, and n-type indium (In) doping is achieved during the epilayer growth. Our p-on-n IRFPAs show state-of-the-art performance from the SWIR to VLWIR spectral range. Mid-wave infrared (MWIR) and long-wave infrared (LWIR) FPAs have been designed with a television (TV) format and 15 μm pixel pitch. Preliminary results of high-operating-temperature detectors obtained in the MWIR (λ _c = 5.3 μm at 80 K) have shown highly promising electrooptical performance above 130 K. For space applications, imagers dedicated to low-flux detection have first been produced as TV/4 focal-plane arrays, with 15 μm pitch in the SWIR range (2 μm). Finally, TV/4 arrays with 30 μm pixel pitch have been manufactured for the VLWIR range. The measured dark current fits the “Rule 07,” with homogeneous imagers.     

Gated IR Imaging with 128 × 128 HgCdTe Electron Avalanche Photodiode FPA

The next generation of infrared (IR) sensor systems will include active imaging capabilities. One example of such a system is a gated active/passive system. The gated active/passive system promises target detection and identification at longer ranges compared to conventional passive-only imaging systems. A detector that is capable of both active and passive modes of operation opens up the possibility of a self-aligned system that uses a single focal plane. The mid-wave infrared (MWIR) HgCdTe electron injection avalanche photodiode (e-APD) provides state-of-the-art 3 μm to 5 μm performance for the passive mode and high, low-noise, gain in the active mode, and high quantum efficiency at 1.5 μm. Gains of greater than 1000 have been measured in MWIR e-APDs with a gain-independent excess noise factor of 1.3. This paper reports the application of the mid-wave HgCdTe e-APD for near-IR gated-active/passive imaging. Specifically a 128 × 128 focal-plane array (FPA) composed of 40-μm-pitch MWIR cutoff APD detectors and custom readout integrated circuit was designed, fabricated, and tested. Median gains as high as 946 at 11 V bias with noise equivalent photon inputs as low as 0.4 photon were measured at 80 K and 1 μs gate times. This subphoton sensitivity is consistent with the high gains, low excess noise factor, and low effective gain normalized dark-current densities, near or below 1 nA/cm², that were achieved in these FPAs. A gated imaging demonstration system was designed and built using commercially available parts. High resolution and precision gating was demonstrated in this system by imagery taken at ranges out to 9 km.