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.     

Influence of the Silicon Substrate on Defect Formation in MCT Grown on II-VI Buffered Si Using a Combined Molecular Beam Epitaxy/Metal Organic Vapor Phase Epitaxy Technique

II-VI buffer layers grown by molecular beam epitaxy (MBE) onto silicon exhibit a uniform, slightly faceted surface morphology. However, a number of surface defects are apparent and these are amplified by the subsequent growth of mercury cadmium telluride (MCT) by metal organic vapor phase epitaxy. Some of these defects have been traced to polishing damage present within the silicon substrate. A range of analytical techniques, including x-ray topography, have been used to track the defects from the substrate through to the buffer layer and into the MCT. Defects of this type will cause dead elements in the infrared focal plane arrays and will be a major cause of low operabilities.     

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.     

Molecular beam epitaxial HgCdTe material characteristics and device performance: Reproducibility status

Extensive material, device, and focal plane array (FPA) reproducibility data are presented to demonstrate significant advances made in the molecular beam epitaxial (MBE) HgCdTe technology. Excellent control of the composition, growth rate, layer thickness, doping concentration, dislocation density, and transport characteristics has been demonstrated. A change in the bandgap is readily achieved by adjusting the beam fluxes, demonstrating the flexibility of MBE in responding to the needs of infrared detection applications in various spectral bands. High performance of photodiodes fabricated on MBE HgCdTe layers reflects on the overall quality of the grown material. The photodiodes were planar p-on-n junctions fabricated by As ion-implantation into indium doped, n-type, in situ grown double layer heterostructures. At 77K, diodes fabricated on MBE Hg_1−xCd_xTe with x ≈ 0.30 (λ_co ≈ 5.6 μm), x ≈ 0.26 (λ_co ≈ 7 μm), x ≈ 0.23 (λ_co ≈ 10 μm) show R₀A products in excess of 1 x 10⁶ ohm-cm², 7 x 10⁵ ohm-cm², and 3 x 10² ohm-cm², respectively. These devices also show high quantum efficiency. As a means to assess the uniformity of the MBE HgCdTe material, two-dimensional 64 x 64 and 128 x 128 mosaic detector arrays were hybridized to Si multiplexers. These focal plane arrays show an operability as high as 97% at 77K for the x ≈ 0.23 spectral band and 93% at 77K for the x ≈ 0.26 spectral band. The operability is limited partly by the density of void-type defects that are present in the MBE grown layers and are easily identified under an optical microscope.     

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.     

Molecular-beam epitaxial growth of CdSe<Subscript>x</Subscript>Te<Subscript>1−x</Subscript> on Si(211)

We report on the first successful growth of the ternary-alloy CdSe_xTe_1−x(211) on 3-in. Si(211) substrates using molecular-beam epitaxy (MBE). The growth of CdSeTe was performed using a compound CdTe effusion source and an elemental Se effusion source. The alloy composition (x) of the CdSe_xTe_1−x ternary compound was controlled through the Se:CdTe flux ratios. Our results indicated that the crystalline quality of CdSeTe decreases as the alloy composition increases, which is possibly due to an alloy-disordering effect. A similar trend was observed for the CdZnTe ternary-alloy system. However, the alloy-disordering effect in CdSeTe was found to be less severe than that in CdZnTe. We have carried out the growth of CdSeTe on Si at different temperatures. An optimized growth window was established for CdSeTe on Si(211) to achieve high-crystalline-quality CdSeTe/Si layers with 4% Se. The as-grown layers exhibited excellent surface morphology, low surface-defect density (less than 500 cm⁻²), and low x-ray full width at half maximum (FWHM) values near 100 arcsec. Additionally, the CdSeTe/Si layer exhibited excellent lateral uniformity and the best etched-pit density (EPD) value on a 4% CdSeTe, measured to be as low as 1.4 × 10⁵ cm⁻².     

Uniform low defect density molecular beam epitaxial HgCdTe

This paper describes recent advances in MBE HgCdTe technology. A new 3 inch production molecular beam epitaxy (MBE) system, Riber Model 32P, was installed at Rockwell in 1994. The growth technology developed over the years at Rockwell using the Riber 2300 R&D system was transferred to the 32P system in less than six months. This short period of technology transfer attests to our understanding of the MBE HgCdTe growth dynamics and the key growth parameters. Device quality material is being grown routinely in this new system. Further advances have been made to achieve better growth control. One of the biggest challenges in the growth of MBE HgCdTe is the day-to-day control of the substrate surface temperature at nucleation and during growth. This paper describes techniques that have led to growth temperature reproducibility within + - 1°C, and a variation in temperature during substrate rotation within 0.5°C. The rotation of the substrate during growth has improved the uniformity of the grown layers. The measured uniformity data on composition for a typical 3 cm × 3 cm MBE HgCdTe/CdZnTe shows the average and standard deviation values of 0.229 and 0.0006, respectively. Similarly, the average and standard deviation for the layer thickness are 7.5 and 0.06 μm, respectively. P-on-n LWIR test structure photodiodes fabricated using material grown by the new system and using rotation during growth have resulted in high-performance (R₀)A, quantum efficiency) devices at 77 and 40K. In addition, 128 × 28 focal plane arrays with excellent performance and operability have been demonstrated.     

Control and growth of middle wave infrared (MWIR) Hg<Subscript>(1−x)</Subscript>Cd<Subscript>x</Subscript>Te on Si by molecular beam epitaxy

Middle wave infrared (MWIR) HgCdTe p-on-n double-layer heterojunctions (DLHJs) for infrared detector applications have been grown on 100-mm Si (112) substrates by molecular beam epitaxy (MBE) for large format 2,560×512 focal plane arrays (FPAs). In order to meet the performance requirements needed for these FPAs, cutoff and doping uniformity across the 100-mm wafer are crucial. Reflection high-energy electron diffraction (RHEED), secondary ion mass spectrometry (SIMS), Fourier transform infrared spectrometry (FTIR), x-ray, and etch pit density (EPD) were monitored to assess the reproducibility, uniformity, and quality of detector material grown. Material properties demonstrated include x-ray full width half maximum (FWHM) as low as 64 arc-sec, typical etch pit densities in mid-10⁶ cm⁻², cutoff uniformity below 5% across the full wafer, and typical density of macrodefects <1000 cm⁻². The detector quality was established by using test structure arrays (TSAs), which include miniarray diodes with the similar pitch as the detector array for easy measurement of critical parameters such as diode I-V characteristics and detector quantum efficiency. Typical I-V curves show excellent R₀A products and strong reverse breakdown characteristics. Detector quantum efficiency was measured to be in the 60–70% range without an antireflection coating.     

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.     

Threading dislocation removal from the near-surface region of epitaxial cadmium telluride on silicon by lithographic patterning of the substrate

(211) oriented silicon substrates were patterned and etched to give mesas of various sizes and shapes. Cadmium telluride epitaxial layers were deposited on the patterned substrates by molecular beam epitaxy (MBE). Dislocation termini in the epilayer were found to be concentrated in the trenches that formed the mesa boundaries. Mesa sizes up to 17 μm were found to be nearly free of threading dislocation termini. Threading dislocation termini are observed to congregate in lines parallel to the 〈321〉 crystallographic directions. Evidence of subsurface, horizontal dislocations running through the mesa is given.     

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.     

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.     

MBE growth of HgCdTe on silicon substrates for large format MWIR focal plane arrays

HgCdTe p-on-n double layer heterojunctions (DLHJs) for mid-wave infrared (MWIR) detector applications have been grown on 100 mm (4 inch) diameter (211) silicon substrates by molecular beam epitaxy (MBE). The structural quality of these films is excellent, as demonstrated by x-ray rocking curves with full widths at half maximum (FWHMs) of 80–100 arcsec, and etch pit densities from 1 10⁶ to 7 10⁶ cm⁻². Morphological defect densities for these layers are generally less than 1000 cm⁻². Improving Hg flux coverage of the wafer during growth can reduce void defects near the edges of the wafers. Improved tellurium source designs have resulted in better temporal flux stability and a reduction of the center to edge x-value variation from 9% to only 2%. Photovoltaic MWIR detectors have been fabricated from some of these 100mm wafers, and the devices show performance at 140 K which is comparable to other MWIR detectors grown on bulk CdZnTe substrates by MBE and by liquid phase epitaxy.     

Molecular beam epitaxy growth of high-quality HgCdTe LWIR layers on polished and repolished CdZnTe substrates

We report here molecular beam epitaxy (MBE) mercury cadmium telluride (HgCdTe) layers grown on polished and repolished substrates that showed state-of-the-art optical, structural, and electrical characteristics. Many polishing machines currently available do not take into account the soft semiconductor materials, CdZnTe (CZT) being one. Therefore, a polishing jig was custom designed and engineered to take in account certain physical parameters (pressure, substrate rotational frequency, drip rate of solution onto the polishing pad, and polishing pad rotational velocity). The control over these parameters increased the quality, uniformity, and the reproducibility of each polish. EPIR also investigated several bromine containing solutions used for polishing CZT. The concentration of bromine, as well as the mechanical parameters, was varied in order to determine the optimal conditions for polishing CZT.     

Status of the MBE technology at leti LIR for the manufacturing of HgCdTe focal plane arrays

This paper presents recent developments that have been made in Leti Infrared Laboratory in the field of molecular beam epitaxy (MBE) growth and fabrication of medium wavelength and long wavelength infrared (MWIR and LWIR) HgCdTe devices. The techniques that lead to growth temperature and flux control are presented. Run to run composition reproducibility is investigated on runs of more than 15 consecutively grown layers. Etch pit density in the low 10⁵ cm⁻² and void density lower than 10³ cm⁻² are obtained routinely on CdZnTe substrates. The samples exhibit low n-type carrier concentration in the 10¹⁴ to 10¹⁵ cm⁻³ range and mobility in excess of 10⁵ cm²/Vs at 77 K for epilayers with 9.5 μm cut-off wavelength. LWIR diodes, fabricated with an-on-p homojunction process present dynamic resistance area products which reach values of 8 10³ Ωcm² for a biased voltage of −50 mV and a cutoff wavelength of 9.5 μm at 77 K. A 320 × 240 plane array with a 30 μm pitch operating at 77 K in the MWIR range has been developed using HgCdTe and CdTe layers MBE grown on a Germanium substrate. Mean NEDT value of 8.8 mK together with an operability of 99.94% is obtained. We fabricated MWIR two-color detectors by the superposition of layers of HgCdTe with different compositions and a mixed MESA and planar technology. These detectors are spatially coherent and can be independently addressed. Current voltage curves of 60 × 60 μm² photodiodes have breakdown voltage exceeding 800 mV for each diode. The cutoff wavelength at 77 K is 3.1 μm for the MWIR-1 and 5 μm for the MWIR-2.     

小型化大面阵非制冷红外连续变焦光学系统设计

为适应新型1024×768元大面阵高分辨率非制冷红外探测器及红外热成像系统小型化的需求,采用机械正组补偿式连续变焦结构形式,并通过合理分配各透镜的光焦度,设计了一种焦距为25～75 mm、工作波段为8～12 μm、F数为1.2的非制冷红外连续变焦成像光学系统.该系统由五片透镜组成,总质量仅为255 g,光学总长度为12...