Progress of MCT Detector Technology at AIM Towards Smaller Pitch and Lower Dark Current

We present our latest results on cooled p-on-n planar mercury cadmium telluride (MCT) photodiode technology. Along with a reduction in dark current for raising the operating temperature (T _op), AIM INFRAROT-MODULE GmbH (AIM) has devoted its development efforts to shrinking the pixel size. Both are essential requirements to meet the market demands for reduced size, weight and power and high-operating temperature applications. Detectors based on the p-on-n technology developed at AIM now span the spectrum from the mid-wavelength infrared (MWIR) to the very long wavelength infrared (VLWIR) with cut-off wavelengths from 5 μm to about 13.5 μm at 80 K. The development of the p-on-n technology for VLWIR as well as for MWIR is mainly implemented in a planar photodetector design with a 20-μm pixel pitch. For the VLWIR, dark currents significantly reduced as compared to ‘Tennant’s Rule 07’ are demonstrated for operating temperatures between 30 K and 100 K. This allows for the same dark current performance at a 20 K higher operating temperature than with previous AIM technology. For MWIR detectors with a 20-μm pitch, noise equivalent temperature differences of less than 30 mK are obtained up to 170 K. This technology has been transferred to our small pixel pitch high resolution (XGA) MWIR detector with 1024 × 768 pixels at a 10-μm pitch. Excellent performance at an operating temperature of 160 K is demonstrated.     

Two-Dimensional Long-Wavelength and Very Long-Wavelength Focal-Plane Arrays at AIM

In recent years AIM has expanded its portfolio of standard infrared (IR) focal-plane arrays in the 3 μm to 5 μm (mid-wavelength infrared, MWIR) and 8 μm to 10 μm (long-wavelength infrared, LWIR) spectral ranges with two-dimensional IR detectors, sensitive in the 0.9 μm to 2.5 μm (short-wavelength infrared, SWIR) and especially in the 10 μm to 15 μm (very long-wavelength infrared, VLWIR) spectral ranges. This paper reports on the latest technological advancements that will benefit not only prototype applications for which they are demonstrated but a wide range of AIM products. A reduction of the pixel pitch from 24 μm to 15 μm is the result of increasing demands for compact detection modules with reduced weight, size, power consumption, and cost efficiency. Performance characterization for such a reduced-pitch 640 × 512 module in the LWIR (cut-off 10.5 μm at 67 K) yields mean noise equivalent temperature difference of 32.2 mK and defective pixel rate of only 0.5%. Extending the detection wavelength further into the VLWIR is of major interest for space applications such as the Meteosat Third Generation, which poses challenging requirements for sensor material homogeneity and dark-current density. To meet this requirement, an extrinsic doping approach is utilized on a 256 × 256 mercury cadmium telluride (MCT) focal-plane array with ～14 μm cut-off wavelength at 55 K operating temperature, and a dark- current density of about 1 pA/μm² is demonstrated.     

Large-Format and Long-Wavelength Infrared Mercury Cadmium Telluride Detectors

Due to continuous improvement of traditional liquid-phase epitaxy n-on-p technology, excellent mid-wavelength infrared HgCdTe (MCT) detectors with 1280 × 1024 pixels and 15 μm pitch size have been produced at AIM. At an operating temperature (T _OP) of 80 K, a noise equivalent temperature difference (NETD) of 17.8 mK and an operability of 99.96% have been obtained. At T _OP = 130 K, the operability remains high at 99.58%. On the basis of these results, the sensitivity of this detector design has been extended into the long-wavelength infrared (LWIR) region, obtaining a detector with an NETD of 30.4 mK and an operability of 99.81% at T _OP = 80 K. In parallel, the growing maturity of molecular-beam epitaxy (MBE) growth of MCT on GaAs substrates has led to an improvement of the performance of the MWIR 640 × 512, 15-μm-pitch detector from an operability of 99.30% in 2011 to 99.83% recently.     

Improvement of RTS Noise in HgCdTe MWIR Detectors

Random telegraph signal (RTS) noise is present in all bands of the infrared spectrum from λ _c = 2.5 μm (short-wavelength infrared) to λ _c = 15.75 μm (very long-wavelength infrared) and decreases the performance of infrared photodetectors. The main features of RTS noise such as the jump amplitude and RTS frequency are defined, and their dependence as a function of focal-plane array (FPA) temperature was measured for all bands of the infrared spectrum. Both of these features comply with a Boltzmann activation law \( \left( { \propto {\hbox{e}}^{{\frac{{ - E_{\rm{a}} }}{k_{\rm B} T}}} } \right) \), and their activation energies scale with the bandgap. Comparison of three different HgCdTe mid-wavelength infrared photodetector technologies was also performed, showing that the optimized n-on-p improvement of operability (AOP) and p-on-n high-operating-temperature technologies show a reduced number of pixels exhibiting RTS noise (by about two decades) in comparison with standard n-on-p technology.     

Ultralow-Dark-Current CdHgTe FPAs in the SWIR Range at CEA and Sofradir

We report the first results of work carried out at CEA and Sofradir to build ultralow-dark-current focal-plane arrays (FPAs) in the short-wave infrared range (SWIR) for space applications. These FPAs are designed to detect very low flux in the 2-μm wavelength range. To this end, Sofradir has designed a source follower per detector read-out circuit (ROIC, 384?×?288, 15?μm pitch). This ROIC has been hybridized on different HgCdTe diode configurations processed at CEA-LETI, and low-flux characterizations have been carried out at CEA-IRFU at low temperature (from 60?K to 160?K). Both ion-implanted p/n and n/p diodes have been evaluated. The metallurgical nature of the absorbing layer has also been examined, and both molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE) have been applied. Dark-current measurements are discussed in comparison with previous results from the literature. State-of-the-art dark currents are recorded for temperatures higher than 120?K. At temperatures lower than 100?K, the decrease in dark current flattens out for both technologies. In this region, currents between 0.4?e^–/s/pixel and 0.06?e^–/s/pixel are reported.     

Performance of 12-μm- to 15-μm-Pitch MWIR and LWIR HgCdTe FPAs at Elevated Temperatures

Infrared (IR) focal-plane arrays (FPAs) with higher operating temperatures and smaller pitches enable reduced size, weight, and power in infrared systems. We have characterized a large number of medium- and long-wavelength IR (MWIR and LWIR) FPAs as a function of temperature and cutoff wavelength to determine the impact of these parameters on their performance. The 77-K cutoff wavelength range for the MWIR arrays was 5.0 μm to 5.6 μm, and 8.6 μm to 11.3 μm for the LWIR. The dark currents in DRS’s high-density vertically integrated photodiode (HDVIP)^® FPAs (based on a front-side- illuminated, via-interconnected, cylindrical-geometry N+/N/P architecture) are dominated by Auger-7 recombination from 120 K to 200 K for the MWIR and 70 K to 100 K for the LWIR. In these temperature ranges the FPA operability is generally limited not by dark current defects but by noise defects. Pixels with high 1/f noise should produce a tail in the root-mean-square (rms) noise distribution. We have found that the skewness of the rms noise distribution is the simplest measure of an array’s 1/f noise, and that the rms noise skewness typically shows little variation over these temperature ranges. The temperature dependence of the defect counts in normal arrays (wet etched prior to CdTe interdiffusion) increases as n _i, while nonstandard arrays (ion milled or plasma etched prior to CdTe interdiffusion) can have high 1/f noise and defect counts that vary as n _i ² . Our models indicate that, if the dominant dark current is due to diffusion, then the 1/f noise varies as n _i ² , whereas if depletion current dominates, then the 1/f noise varies as n _i. Systemic 1/f noise is not an issue for DRS’s standard MWIR FPAs at 110 K to 160 K, or for standard LWIR FPAs at 77 K to 100 K.     

Effect of Dislocations on VLWIR HgCdTe Photodiodes

The effects of dislocations on very-long-wavelength infrared (VLWIR) HgCdTe photodiodes (cutoff wavelength >14 μm at 40 K) have been determined experimentally and analyzed. The photodiodes are in the back-illuminated configuration, fabricated from HgCdTe p-on-n double-layer heterostructure (DLHJ) films grown at BAE Systems by liquid phase epitaxy (LPE) onto lattice-matched (111) CdZnTe substrates. Arrays were hybridized to silicon ROICs to form focal plane arrays (FPAs). After characterization for dark current and response, the arrays were dehybridized and stripped of their metals and passivation layers. Dislocations were revealed using a Hähnert and Schenk (H&;S) etch. Pixel traceability was maintained throughout the analysis, permitting one-to-one correlation between photodiode performance and dislocation density measured within that photodiode. We found that response and dark current were correlated to etch pit density (EPD), which we assumed to be equal to dislocation density. Our results support earlier dislocation studies on larger-bandgap HgCdTe, which showed response was only weakly impacted by EPD, while dark current was strongly affected by EPD. Measured EPD values ranged from low 10⁵ to low 10⁷ cm^?2. Potential causes for this range in EPD are discussed.     

High-Operating-Temperature MWIR Detector Diodes

The high-operating-temperature (HOT) midwave infrared (MWIR) n-on-p detector has been pursued using the high-density vertically integrated photodiode (HDVIP^®) architecture. In this device, arsenic-doped HgCdTe grown by liquid-phase epitaxy (LPE) is used, passivated on both surfaces with interdiffused CdTe. Dark current densities on these diodes as low as 2.5 mA/cm² normalized to a 5 μm cutoff at 250 K have been demonstrated. 1/f noise at 1 Hz, measured at 250 K, is found to be 6 × 10^?11 A/rHz-cm measured on a cutoff of 4.43 μm. These results agree with the theoretical predictions for the devices made.     

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.     

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.     

MTF Issues in Small-Pixel-Pitch Planar Quantum IR Detectors

The current trend in quantum infrared (IR) detector development is the design of very small-pixel-pitch large arrays. From the previous 30 μm pitch, the standard pixel pitch today is 15 μm and is expected to decrease to 12 μm in the next few years. Furthermore, focal-plane arrays (FPAs) with pixel pitch as small as 10 μm have been demonstrated. Such ultrasmall-pixel pitches are very small compared with the typical length ruling the electrical characteristics of the absorbing materials, namely the minority-carrier diffusion length. As an example, for low-doped n-type HgCdTe or InSb material, this diffusion length is on the order of 30 μm to 50 μm, i.e., three to five times the targeted pixel pitches. This has strong consequences for the modulation transfer function (MTF) of planar structures, where the lateral extension of the photodiode is limited by diffusion. For such aspect ratios, the self-confinement of neighboring diodes may not be efficient enough to maintain an optimal MTF. Therefore, this issue has to be addressed to take full advantage of the pixel pitch reduction in terms of image resolution. The aim of this work is to investigate the evolution of the MTF of HgCdTe and InSb FPAs when decreasing the pixel pitch below 15 μm. Both experimental measurements and finite-element simulations are used to discuss this issue. Different scenarios are compared, namely deep mesa etch between pixels, internal drift, surface recombination, and thin absorbing layers.     

Short-Wave Infrared HgCdTe Avalanche Photodiodes

Short-wave infrared (SWIR) HgCdTe avalanche photodiodes (APDs) have been developed to address low-flux applications at low operating temperature and for laser detection at higher temperatures. Stable multiplication gains in excess of 200 have been observed in homojunction APDs with cutoff wavelengths down to 2.8???m and operating temperatures up to 300?K, associated with low excess noise F?<?1.3 and low 1/f noise. The measured dark current density at 200?K of 6.2???A/cm² is low enough to enable high-sensitivity single-element light detection and ranging (lidar) applications and time-of-flight imaging. Corresponding APD arrays have been hybridized on a readout integrated circuit (ROIC) designed for low-flux low-SNR imaging with low noise and frame rates higher than 1500?frames/s. Preliminary focal-plane array characterization has confirmed the nominal ROIC performance and showed pixel operability above 99.5% (pixels within ±50% of average gain). The bias dependence of the multiplication gain has been characterized as a function of temperature, cadmium composition, and junction geometry. A qualitative change in the bias dependence of the gain compared with mid-wave infrared (MWIR) HgCdTe has motivated the development of a modified local electric field model for the electron impaction ionization coefficient and multiplication gain. This model gives a close fit to the gain curves in both SWIR and MWIR APDs at temperatures between 80?K and 300?K, using two parameters that scale as a function of the energy gap and temperature. This property opens the path to quantitative predictive device simulations and to estimations of the junction geometry of APDs from the bias dependence of the gain.     

Development and Production of Array Barrier Detectors at SCD

XBn or XBp barrier detectors exhibit diffusion-limited dark currents comparable with mercury cadmium telluride Rule-07 and high quantum efficiencies. In 2011, SemiConductor Devices (SCD) introduced “HOT Pelican D”, a 640 × 512/15-μm pitch InAsSb/AlSbAs XBn mid-wave infrared (MWIR) detector with a 4.2-μm cut-off and an operating temperature of ～150 K. Its low power (～3 W), high pixel operability (>99.5%) and long mean time to failure make HOT Pelican D a highly reliable integrated detector-cooler product with a low size, weight and power. More recently, “HOT Hercules” was launched with a 1280 × 1024/15-μm format and similar advantages. A 3-megapixel, 10-μm pitch version (“HOT Blackbird”) is currently completing development. For long-wave infrared applications, SCD’s 640 × 512/15-μm pitch “Pelican-D LW” XBp type II superlattice (T2SL) detector has a ～9.3-μm cut-off wavelength. The detector contains InAs/GaSb and InAs/AlSb T2SLs, and is fabricated into focal plane array (FPA) detectors using standard production processes including hybridization to a digital silicon read-out integrated circuit (ROIC), glue underfill and substrate thinning. The ROIC has been designed so that the complete detector closely follows the interfaces of SCD’s MWIR Pelican-D detector family. The Pelican-D LW FPA has a quantum efficiency of ～50%, and operates at 77 K with a pixel operability of >99% and noise equivalent temperature difference of 13 mK at 30 Hz and F/2.7.     

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.     

Improvements of MCT MBE Growth on GaAs

In recent years, continuous progress has been published in the development of HgCdTe (MCT) infrared (IR) focal plane arrays (FPAs) fabricated by molecular beam epitaxy on GaAs substrates. In this publication, further characterization of the state-of-the art 1280 × 1024 pixel, 15-μm pitch detector fabricated from this material in both the mid-wavelength (MWIR) and long-wavelength (LWIR) IR region will be presented. For MWIR FPAs, the percentage of defective pixel remains below 0.5% up to an operating temperature (T _OP) of around 100 K. For the LWIR FPA, an operability of 99.25% was achieved for a T _OP of 76 K. Additionally, the beneficial effect of the inclusion of MCT layers with a graded composition region was investigated and demonstrated on current–voltage (IV) characteristics on test diodes in a MWIR FPA.     

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.     

Impact of CdZnTe Substrates on MBE HgCdTe Deposition

The highest sensitivity, lowest dark current infrared focal plane arrays (IRFPAs) are produced using HgCdTe on CdZnTe substrates. As-received state-of-the-art CdZnTe 6 × 6 and 7 × 7.5 cm substrates were analyzed using Nomarski phase contrast microscopy, Auger electron spectroscopy, scanning electron microscopy/energy dispersive spectroscopy, and scanning profilometry. On all CdZnTe substrates tested, we observed as-received large area macro-defect contamination. Using a defect specification limit of 50 contiguous defective pixels, we identified non-compliant 1280 × 720 12-μm pitch focal plane arrays due to as-received substrate macro-defect contamination. Using the above specification, up to 20% IRFPA wafer yield loss is due to state-of-the-art as-received CdZnTe substrate macro-contamination.     

Short-Wave Infrared HgCdTe Electron Avalanche Photodiodes for Gated Viewing

Short-wave infrared (SWIR) HgCdTe electron avalanche photodiodes (eAPDs) with different doping profiles have been characterized for use in SWIR gated viewing systems. Gated viewing offers enhanced image contrast in scenes with clutter from the foreground or background. HgCdTe-based eAPDs show exponential gain–voltage characteristics and low excess noise and are, therefore, well suited for active imaging applications. The gain achievable at a fixed reverse voltage varies with the bandgap of the Hg_1?xCd_xTe detector material. We analyze current–voltage and gain–voltage plots measured on SWIR Hg_1?xCd_xTe eAPDs with x?=?0.45, corresponding to a cutoff wavelength of 2.55 μm at 150 K. The cutoff has been chosen as a trade-off between achievable APD gain and operating temperature for SWIR gated-viewing systems with target distances of about 1000 m. Focal plane arrays with a readout-integrated circuit featuring a fast internal clock have been built and their performance with respect to gated viewing applications has been evaluated on a laboratory demonstrator for short distances. Future plans for a field demonstrator for distances up to 1000 m are described briefly at the end.     

Substrate-Removed HgCdTe-Based Focal-Plane Arrays for Short-Wavelength Infrared Astronomy

Removal of the CdZnTe substrate offers several performance benefits for near-infrared (NIR, 1.7 μm) and short-wave infrared (SWIR, 2.5 μm) focal-plane arrays (FPAs). Among these are visible wavelength detection, improved infrared sensitivity and uniformity, and greatly reduced susceptibility to the effects of ionizing radiation. Data for substrate-removed NIR FPAs fabricated for the Wide Field Camera 3 (WFC3) upgrade to the Hubble Space Telescope (HST) are presented, including detailed data from delivered units and summary trends for additional units. The WFC3 flight FPA is expected to improve discovery speed by 3.5–120× over current HST instruments. For SWIR FPAs, selected results are presented from the 16 megapixel mosaic delivered to Observatories of Carnegie Institution in Washington which serve as an example of the current state of the art.