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.     

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.     

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.     

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.     

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.     

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).     

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.     

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.     

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.     

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.     

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.     

LWIR HgCdTe on Si detector performance and analysis

We have fabricated a series of 256 pixel×256 pixel, 40 μm pitch LWIR focal plane arrays (FPAs) with HgCdTe grown on (211) silicon substrates using MBE grown CdTe and CdSeTe buffer layers. The detector arrays were fabricated using Rockwell Scientific’s double layer planar heterostructure (DLPH) diode architecture. The 78 K detector and focal plane array (FPA) performance are discussed in terms of quantum efficiency (QE), diode dark current and dark current operability. The FPA dark current and the tail in the FPA dark current operability histograms are discussed in terms of the HgCdTe epitaxial layer defect density and the dislocation density of the individual diode junctions. Individual diode zero bias impedance and reverse bias current-voltage (I-V) characteristics vs. temperature are discussed in terms of the dislocation density of the epitaxial layer, and the misfit stress in the epitaxial multilayer structure, and the thermal expansion mismatch in the composite substrate. The fundamental FPA performance limitations and possible FPA performance improvements are discussed in terms of basic device physics and material properties.     

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.     

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.     

Hydrogenation of HgCdTe epilayers on Si substrates using glow discharge plasma

Preliminary results of a study of the hydrogenation of HgCdTe epilayers grown by molecular beam epitaxy on Si substrates using a glow-discharge plasma are presented. The aim of the program is to employ H to passivate the detrimental opto-electronic effects of threading dislocations present in the HgCdTe epilayers. Secondary ion mass spectroscopy depth profiling has been performed to characterize ¹H and ²H incorporation. It has been found that H can be controllably incorporated in HgCdTe epilayers to levels in the 10¹⁴ cm⁻³ to 10¹⁸ cm⁻³ range while maintaining the sample at temperatures lower than 60°C. Profiles indicate that H accumulates in regions of known high defect density or in highly strained regions. Analysis of the H depth profile data indicates that the current density-time product is a good figure of merit to predict the H levels in the HgCdTe epilayer. There are progressive differences in the ¹H and ²H uptake efficiencies as a function of depth. Magneto-Hall measurements show consistently higher mobilities at low temperatures for majority carriers in hydrogenated samples.     

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.     

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.     

Influence of Substrate Orientation on the Growth of HgCdTe by Molecular Beam Epitaxy

An empirical study is reported, wherein HgCdTe was deposited simultaneously on multiple CdZnTe substrates of different orientations by molecular beam epitaxy. These orientations included the following vicinal surfaces: (115)B, (113)B, (112)B, and (552)B. Additionally, growth on (111)B was explored. Growth conditions found to be nearly optimalfor the standard (112)B orientation were selected. Through a series of growth runs, substrate temperature was varied, and the physical properties of the resulting HgCdTe epilayers were measured. These measurements included Nomarski microscopy, infrared transmission, x-ray diffraction, and defect decoration etching. The properties of HgCdTe epilayers as a function of temperature were roughly similar for all vicinal surfaces. Namely, as the temperature increased, the dislocation density decreased. At some critical temperature, the density of void defects increased dramatically. This critical temperature varied with orientation, the (115)B exhibiting the lowest critical temperature and the (112)B and (552)B exhibiting the highest. The (115)B, (113)B, and (112)B orientations exhibited “needlelike” defects on the as-grown HgCdTe surface. The density of these defects decreased with increasing temperature. The (552)B surface exhibited no such defects and growth behavior nearly identical to the (112)B growthsurface.     

MBE growth and device processing of MWIR HgCdTe on large area Si substrates

The traditional substrate of choice for HgCdTe material growth has been lattice matched bulk CdZnTe material. However, as larger array sizes are required for future devices, it is evident that current size limitations of bulk substrates will become an issue and therefore large area Si substrates will become a requirement for HgCdTe growth in order to maintain the cost-efficiency of future systems. As a result, traditional substrate mounting methods that use chemical compounds to adhere the substrate to the substrate holder may pose significant technical challenges to the growth and fabrication of HgCdTe on large area Si substrates. For these reasons, non-contact (indium-free) substrate mounting was used to grow mid-wave infrared (MWIR) HgCdTe material on 3″ CdTe/Si substrates. In order to maintain a constant tepilayer temperature during HgCdTe nucleation, reflection high-energy electron diffraction (RHEED) was implemented to develop a substrate temperature ramping profile for HgCdTe nucleation. The layers were characterized ex-situ using Fourier transform infrared (FTIR) and etch pit density measurements to determine structural characteristics. Dislocation densities typically measured in the 9 10⁶ cm⁻² to 1 10⁷ cm⁻² range and showed a strong correlation between ramping profile and Cd composition, indicating the uniqueness of the ramping profiles. Hall and photoconductive decay measurements were used to characterize the electrical properties of the layers. Additionally, both single element and 32 32 photovoltaic devices were fabricated from these layers. A RA value of 1.8 10⁶-cm² measured at −40 mV was obtained for MWIR material, which is comparable to HgCdTe grown on bulk CdZnTe substrates.