Fabrication and characterization of two-color midwavelength/long wavelength HgCdTe infrared detectors

High-performance 20-μm unit-cell two-color detectors using an n-p⁺-n HgCdTe triple-layer heterojunction (TLHJ) device architecture grown by molecular beam epitaxy (MBE) on (211)-oriented CdZnTe substrates with midwavelength (MW) infrared and long wavelength (LW) infrared spectral bands have been demonstrated. Detectors with nominal MW and LW cut-off wavelengths of 5.5 μm and 10.5 μm, respectively, exhibit 78 K LW performance with >70 % quantum efficiency, reverse bias dark currents below 300 pA, and RA products (zero field of view, 150-mV bias) in excess of 1×10³ Ωcm². Temperature-dependent current-voltage (I–V) detector measurements show diffusion-limited LW dark current performance extending to temperatures below 70 K with good operating bias stability (150 mV ± 50 mV). These results reflect the successful implementation of MBE-grown TLHJ detector designs and the introduction of advanced photolithography techniques with inductively coupled plasma (ICP) etching to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance. These detector improvements complement the development of high operability large format 640×480 and 1280×720 two-color HgCdTe infrared focal plane arrays (FPAs) to support third generation forward looking infrared (FLIR) systems.     

Heteroepitaxy of HgCdTe(112) infrared detector structures on Si(112) substrates by molecular-beam epitaxy

High-quality, single-crystal epitaxial films of CdTe(112)B and HgCdTe(112)B have been grown directly on Si(112) substrates without the need for GaAs interfacial layers. The CdTe and HgCdTe films have been characterized with optical microscopy, x-ray diffraction, wet chemical defect etching, and secondary ion mass spectrometry. HgCdTe/Si infrared detectors have also been fabricated and tested. The CdTe(112)B films are highly specular, twin-free, and have x-ray rocking curves as narrow as 72 arc-sec and near-surface etch pit density (EPD) of 2 × 10⁶ cm⁻² for 8 μm thick films. HgCdTe(112)B films deposited on Si substrates have x-ray rocking curve FWHM as low as 76 arc-sec and EPD of 3-22 × 10⁶ cm⁻². These MBE-grown epitaxial structures have been used to fabricate the first high-performance HgCdTe IR detectors grown directly on Si without use of an intermediate GaAs buffer layer. HgCdTe/Si infrared detectors have been fabricated with 40% quantum efficiency and R₀A = 1.64 × 10⁴ Ωm₂ (0 FOV) for devices with 7.8 μm cutoff wavelength at 78Kto demonstrate the capability of MBE for growth of large-area HgCdTe arrays on Si.     

High-quality large-area MBE HgCdTe/Si

HgCdTe offers significant advantages over other similar semiconductors, which has made it the most widely utilized variable-gap material in infrared (IR) focal plane array (FPA) technology. HgCdTe hybrid FPAs consisting of two-dimensional detector arrays that are hybridized to Si readout circuits (ROIC) are the dominant technology for second-generation infrared systems. 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 have been limited to 30 cm² in production. This size limitation does not adequately support the increasing demand for larger FPA formats which now require sizes up to 2048×2048, and only a single die can be printed per wafer. Heteroepitaxial Si-based substrates offer a cost-effective technology that can be scaled to large wafer sizes and further offer a thermal-expansion-matched hybrid structure that is suitable for large format FPAs. This paper presents data on molecular-beam epitaxy (MBE)-grown HgCdTe/Si wafers with much improved materials characteristics than previously reported. We will present data on 4- and 6-in diameter HgCdTe both with extremely uniform composition and extremely low defects. Large-diameter HgCdTe/Si with nearly perfect compositional uniformity and ultra low defect density is essential for meeting the demanding specifications of large format FPAs.     

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.     

HgCdTe/Si materials for long wavelength infrared detectors

The heteroepitaxial growth of HgCdTe on large-area Si substrates is an enabling technology leading to the production of low-cost, large-format infrared focal plane arrays (FPAs). This approach will allow HgCdTe FPA technology to be scaled beyond the limitations of bulk CdZnTe substrates. We have already achieved excellent mid-wavelength infrared (MWIR) and short wavelength infrared (SWIR) detector and FPA results using HgCdTe grown on 4-in. Si substrates using molecular beam epitaxy (MBE), and this work was focused on extending these results into the long wavelength infrared (LWIR) spectral regime. A series of nine p-on-n LWIR HgCdTe double-layer heterojunction (DLHJ) detector structures were grown on 4-in. Si substrates. The HgCdTe composition uniformity was very good over the entire 4-in. wafer with a typical maximum nonuniformity of 2.2% at the very edge of the wafer; run-to-run composition reproducibility, realized with real-time feedback control using spectroscopic ellipsometry, was also very good. Both secondary ion mass spectrometry (SIMS) and Hall-effect measurements showed well-behaved doping and majority carrier properties, respectively. Preliminary detector results were promising for this initial work and good broad-band spectral response was demonstrated; 61% quantum efficiency was measured, which is very good compared to a maximum allowed value of 70% for a non-antireflection-coated Si surface. The R₀A products for HgCdTe/Si detectors in the 9.6-μm and 12-μm cutoff range were at least one order of magnitude below typical results for detectors fabricated on bulk CdZnTe substrates. This lower performance was attributed to an elevated dislocation density, which is in the mid-10⁶ cm⁻² range. The dislocation density in HgCdTe/Si needs to be reduced to <10⁶ cm⁻² to make high-performance LWIR detectors, and multiple approaches are being tried across the infrared community to achieve this result because the technological payoff is significant.     

Long wavelength infrared, molecular beam epitaxy, HgCdTe-on-Si diode performance

In the past several years, we have made significant progress in the growth of CdTe buffer layers on Si wafers using molecular beam epitaxy (MBE) as well as the growth of HgCdTe onto this substrate as an alternative to the growth of HgCdTe on bulk CdZnTe wafers. These developments have focused primarily on mid-wavelength infrared (MWIR) HgCdTe and have led to successful demonstrations of high-performance 1024×1024 focal plane arrays (FPAs) using Rockwell Scientific’s double-layer planar heterostructure (DLPH) architecture. We are currently attempting to extend the HgCdTe-on-Si technology to the long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR) regimes. This is made difficult because the large lattice-parameter mismatch between Si and CdTe/HgCdTe results in a high density of threading dislocations (typically, >5E6 cm⁻²), and these dislocations act as conductive pathways for tunneling currents that reduce the R_oA and increase the dark current of the diodes. To assess the current state of the LWIR art, we fabricated a set of test diodes from LWIR HgCdTe grown on Si. Silicon wafers with either CdTe or CdSeTe buffer layers were used. Test results at both 78 K and 40 K are presented and discussed in terms of threading dislocation density. Diode characteristics are compared with LWIR HgCdTe grown on bulk CdZnTe.     

Performance of molecular-beam epitaxy-grown midwave infrared HgCdTe detectors on four-inch Si substrates and the impact of defects

We are continuing development of the growth of midwave infrared (MWIR) HgCdTe by molecular-beam epitaxy (MBE) on 4-in. Si substrates and the fabrication of state-of-the-art detectors and focal plane arrays (FPAs). Array formats of up to 2048 × 2048 and unit cells as small as 20 μm have been made. We regularly measure response operability values in excess of 99% on these arrays. These values typically exceed expectations, with the number of outages corresponding to as-grown defect densities four times lower than what we measure. We have investigated this operability discrepancy and now can account for it. Comparisons of measured properties were used to establish trends between defect occurrence and pixel operability. These correlations show that a combination of defect removal and low-impact defects provide the explanation. Having this knowledge will allow for better operability predictions and assist in efforts to reduce defect impact on FPA performance.     

MOCVD of bandgap-engineered HgCdTe p-n-N-P dual-band infrared detector arrays

We report the implementation of recent advances in metalorganic chemical vapor deposition (MOCVD) for in situ growth of four-layer HgCdTe mid wave/ long wave (MW/LW) simultaneous dual-band 64 × 64 infrared detector arrays. This independently accessed, simultaneous, double-heterojunction p-n-N-P dualband detector has two back-to-back stacked photodiodes grown on CdZnTe (100) substrates. The LW photodiode is a p-on-n heterojunction grown on top of an MW N-on-P heterojunction photodiode. Secondary ion mass spectrometry depth profiles of these 28 μ m thick p-n-N-P dual-band films show four well-defined regions of alloy composition and doping, and agree well with the device design. 64 × 64 arrays of dual-band detectors were fabricated from these films using electron cyclotron resonance dry etching and CdTe passivation, and hybridized to a dual-band readout chip. Two bump inter-connects in each unit cell provide independent electrical access to the back-to-back MW and LW photodiodes, and allow the MW and LW photocurrents to be separate and independent. The dualband infrared focal plane arrays (IRFPAs) spectral response data at 78K are well-behaved and are fully consistent with that observed in individual singleband LW p-on-n and MW N-on-P heterojunction devices of the same design. The hybridized 64 × 64 duai-band FPAs have MW and LW average in-band quantum efficiencies of 79 and 67%, and median D* values of 4.8 × 10¹¹ and 7.1 × 10¹⁰ cm-√Hz/W, in the respective spectral bands at 78K. The data demonstrate that MOCVD has progressed significantly toward being a practical and viable vapor phase in situ growth technology for advanced bandgap-engineered HgCdTe detector arrays.     

TV/4 dual-band HgCdTe infrared focal plane arrays with a 25-εm pitch and spatial coherence

The purpose of this paper is to present the electro-optical performances of dual-band infrared detectors operating in a fully spatially coherent mode, with a small pixel pitch. The successive steps of device fabrication are first exposed, including molecular beam epitaxy (MBE), technological processing, and readout circuit design. It is shown that very high-quality multiple layer heterostructures of HgCdTe can be grown and processed into 256×256 arrays of 25-μm pitch mesas, each mesa including two photodiodes with different cutoff wavelengths ranging in the midwave infrared (MWIR). Characterization of these focal plane arrays (FPAs) shows very good homogeneity, low defect density, and operabilities usually above 99% for both response and noise equivalent thermal difference (NETD).     

Molecular beam epitaxy growth of HgCdTe on Ge for third-generation infrared detectors

The Leti-Lir has studied II–VI compounds for infrared (IR) detection for more than 20 years. The need to reduce the production cost of IR focal plane arrays (FPAs) sparked the development of heteroepitaxy on large-area substrates. Germanium has been chosen as the heterosubstrate for the third generation of IR detectors. First, we report on the progress achieved in HgCdTe growth on 3-in. and 4-in. (211)B CdTe/Ge. Then, we discuss the choice of a new machine for larger size and better homogeneity. Finally, we present the latest results on third-generation IR multicolor and megapixel devices. First-time results regarding a middle wavelength infrared (MWIR) dual-band FPA, with a reduced pitch of 25 μm, and a MWIR 1,280×1,024 FPA will be shown. Both detectors are based on molecular beam epitaxy (MBE)-grown HgCdTe on Ge. The results shown validate the choice of Ge as the substrate for third-generation detectors.     

Developments in the fabrication and performance of high-quality HgCdTe detectors grown on 4-in. Si substrates

We are continuing to develop our growth and processing capabilities for HgCdTe grown on 4-in. Si substrates by molecular beam epitaxy (MBE). Both short-wave and mid-wave infrared (SWIR and MWIR) double-layer hetero-junctions (DLHJs) have been fabricated. In order to improve the producibility of the material, we have implemented an in-situ growth composition-control system. We have explored dry etching the HgCdTe/Si wafers and seen promising results. No induced damage was observed in these samples. Detector results show that the HgCdTe/Si devices are state-of-the-art, following the diffusion-limited trend line established by other HgCdTe technologies. Focal-plane array (FPA) testing has been performed in order to assess the material over large areas. The FPA configurations range from 128×128 to 1,024×1,024, with unit cells as small as 20 μm. The MWIR responsivity and NEDT values are comparable to those of existing InSb FPAs. Pixel operabilities well in excess of 99% have been measured. We have also explored the role of growth macrodefects on diode performance and related their impact to FPA operability. The SWIR HgCdTe/Si shows similar results to the MWIR material. Short-wave IR FPA, median dark-current values of less than 0.1 e⁻/sec have been achieved.     

MBE HgCdTe on Alternative Substrates for FPA Applications

Results of first-principles calculations and experiments focusing on molecular beam epitaxy (MBE) growth of HgCdTe on the alternative substrates of GaAs and Si are described. The As passivation on (2 × 1) reconstructed (211) Si and its effects on the surface polarity of ZnTe or CdTe were clarified by examining the bonding configurations of As. The quality of HgCdTe grown on Si was confirmed to be similar to that grown on GaAs. Typical surface defects in HgCdTe and CdTe were classified. Good results for uniformities of full width at half maximum (FWHM) values of x-ray rocking curves, surface defects, and x values of Hg_1−x Cd _x Te were obtained by refining the demanding parameters and possible tradeoffs. The sticking coefficient of As₄ for MBE HgCdTe was determined. The effects of Hg-assisted annealing for As activation were investigated experimentally and theoretically by examining the difference of the formation energy of As_Hg and As_Te. Results of focal-plane arrays (FPAs) fabricated with HgCdTe grown on Si and on GaAs are discussed.     

Performance characteristics of planar ion-implantation isolated heterojunction,near-infrared,and short-wave infrared HgCdTe devices

Two-dimensional near-infrared (NIR) and short-wave infrared (SWIR) HgCdTe arrays have been produced using planar ion-implantation isolated heterojunction (PI3H) device technology. This paper is an extension of an earlier study in which focal plane arrays (FPAs) were fabricated based on heterojunction-mesa and ion-implanted planar device structures. The PI3H device structure is pursued in order to verify whether it can encompass both the superb multilayer characteristics of heterojunction detectors as well as the planar integrity of ion-implanted devices. The PI3H devices are characterized, and R₀A measurements are carried out at different temperatures and compared to those obtained from heterojunction-mesa and ion-implanted device structures. Data shows the PI3H devices to be superior to both heterojunctionmesa and ion-implanted detectors at temperatures between 130 K and 300 K. Performance characteristics of the thermoelectric (TE) cooled SWIR FPAs with 320 × 256 format, as well as NIR FPAs with 640×512 format based on the PI3H device structure are also discussed.     

Effects of Inductively Coupled Plasma Hydrogen on Long-Wavelength Infrared HgCdTe Photodiodes

Bulk passivation of semiconductors with hydrogen continues to be investigated for its potential to improve device performance. In this work, hydrogen-only inductively coupled plasma (ICP) was used to incorporate hydrogen into long-wavelength infrared HgCdTe photodiodes grown by molecular-beam epitaxy. Fully fabricated devices exposed to ICP showed statistically significant increases in zero-bias impedance values, improved uniformity, and decreased dark currents. HgCdTe photodiodes on Si substrates passivated with amorphous ZnS exhibited reductions in shunt currents, whereas devices on CdZnTe substrates passivated with polycrystalline CdTe exhibited reduced surface leakage, suggesting that hydrogen passivates defects in bulk HgCdTe and in CdTe.     

HgCdTe focal plane arrays for dual-color mid- and long-wavelength infrared detection

Raytheon Vision Systems (RVS, Goleta, CA) in collaboration with HRL Laboratories (Malibu, CA) is contributing to the maturation and manufacturing readiness of third-generation, dual-color, HgCdTe infrared staring focal plane arrays (FPAs). This paper will highlight data from the routine growth and fabrication of 256×256 30-μm unit-cell staring FPAs that provide dual-color detection in the mid-wavelength infrared (MWIR) and long wavelength infrared (LWIR) spectral regions. The FPAs configured for MWIR/MWIR, MWIR/LWIR, and LWIR/LWIR detection are used for target identification, signature recognition, and clutter rejection in a wide variety of space and ground-based applications. Optimized triple-layer heterojunction (TLHJ) device designs and molecular beam epitaxy (MBE) growth using in-situ controls has contributed to individual bands in all dual-color FPA configurations exhibiting high operability (>99%) and both performance and FPA functionality comparable to state-of-the-art, single-color technology. The measured spectral cross talk from out-of-band radiation for either band is also typically less than 10%. An FPA architecture based on a single-mesa, single-indium bump, and sequential-mode operation leverages current single-color processes in production while also providing compatibility with existing second-generation technologies.     

Fabrication of high-performance large-format MWIR focal plane arrays from MBE-grown HgCdTe on 4″ silicon substrates

We have developed the capability to grow HgCdTe mid-wave infrared radiation double-layer heterojunctions (MWIR DLHJs) on 4″ Si wafers by molecular beam epitaxy (MBE), and fabricate devices from these wafers that are comparable to those produced by mature technologies. Test data show that the detectors, which range in cutoff wavelength over 4–7 μm, are comparable to the trendline performance of liquid phase epitaxy (LPE)-grown material. The spectral characteristics are similar, with a slight decrease in quantum efficiency attributable to the Si substrate. With respect to R₀A, the HgCdTe/Si devices are closer to the theoretical radiative-limit than LPE-grown detectors. Known defect densities in the material have been correlated to device performance through a simple model. Slight 1/f noise increases were measured in comparison to the LPE material, but the observed levels are not sufficient to significantly degrade focal plane array (FPA) performance. In addition to discrete detectors, two FPA formats were fabricated. 128×128 FPAs show MWIR sensitivity comparable to mature InSb technology, with pixel operability values in excess of 99%. A 640×480 FPA further demonstrates the high-sensitivity and high-operability capabilities of this material.     

CdZnTe substrate producibility and its impact on IRFPA yield

The need for cost effective production of HgCdTe infrared detectors and focal plane assemblies has led to increased attention to the availability of high quality large-area CdZnTe substrates. Reasonable yield of large-area substrates (≥4 cm×6 cm format) is necessary for fabrication of focal plane assemblies (FPAs) now in production, and for future infrared (IR) detectors which are growing in size and complexity. Raytheon’s infrared materials producibility (IRMP) program has addressed this issue, after identifying critical drivers of FPA yield coming from substrates, and targeted certain improvements in substrate process steps for highest impact on large-area substrate yield. Three specific areas of improvements in the substrate process were addressed: (1) compounding of a large 6 kg charge of CdTe; (2) vertical Bridgman growth of 92 mm diameter CdZnTe boules in both quartz and pyrolytic boron nitride (PBN) crucibles; and (3) optimized Cd overpressure control during growth and cool-down of the boule. It was shown that the Cd overpressure and the cooling schedule had the strongest effects on defect populations. The resulting improvements include a 33% increase in wafer yield per unit starting weight, an estimated 50% reduction in substrate cost per cm², better morphology of epitaxial HgCdTe layers, and improved yield of satisfactory IR detectors. The criteria for selecting substrates have also improved as a result of this work. In addition, photovoltaic detectors were fabricated on wafers from a variety of sources, and tested. Results compare favorably with those on baseline (earlier process) substrates.     

Selective CdTe Nanoheteroepitaxial Growth on Si(100) Substrates Using the Close-Spaced Sublimation Technique Without the Use of a Mask

The development of HgCdTe detectors requires high sensitivity, small pixel size, low defect density, long-term thermal-cycling reliability, and large-area substrates. CdTe bulk substrates were initially used for epitaxial growth of HgCdTe films. However, CdTe has a lattice mismatch with long-wavelength infrared (LWIR) and middle-wavelength infrared (MWIR) HgCdTe that results in detrimental dislocation densities above mid-10⁶ cm^?2. This work explores the use of CdTe/Si as a possible substrate for HgCdTe detectors. Although there is a 19% lattice mismatch between CdTe and Si, the nanoheteroepitaxy (NHE) technique makes it possible to grow CdTe on Si substrates with fewer defects at the CdTe/Si interface. In this work, Si(100) was patterned using photolithography and dry etching to create 500-nm to 1-μm pillars. CdTe was selectively deposited on the pillar surfaces using the close-spaced sublimation (CSS) technique. Scanning electron microscopy (SEM) was used to characterize the CdTe selective growth and grain morphology, and transmission electron microscopy (TEM) was used to analyze the structure and quality of the grains. CdTe selectivity was achieved for most of the substrate and source temperatures used in this study. The ability to selectively deposit CdTe on patterned Si(100) substrates without the use of a mask or seed layer has not been observed before using the CSS technique. The results from this study confirm that CSS has the potential to be an effective and low-cost technique for selective nanoheteroepitaxial growth of CdTe films on Si(100) substrates for infrared detector applications.     

Recent developments of high-complexity HgCdTe focal plane arrays at leti infrared laboratory

In this article, we present recent developments of the research in France at LETI infrared laboratory in the field of complex third-generation HgCdTe IRCMOS focal plane arrays (FPAs). We illustrate this with three prototypes of FPAs made at LETI, which have involved some technological improvements from the standard process today in production at Sofradir. We present, using molecular-beam epitaxy (MBE) growth, a 128 × 128 dual-band infrared (photodetector)-complementary metal oxide semiconductor (IRCMOS) with a pitch of 50 μm operating within 2–5 μm. Using the more conventional liquid-phase epitaxy (LPE) growth, we show a new generation of high-performance long linear arrays (1500 × 2; pitch, 30 μm) operating in medium-wavelength infrared (MWIR) or long-wavelength infrared (LWIR) bands based on a modular architecture of butted HgCdTe detection circuit and SiCMOS multiplexers. Finally, we present for the first time a megapixel (1000 × 1000) FPA with a pitch of 15 μm operating in the MWIR band that exhibits a very high performance and pixel operability.     

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)