High-quality mid-wave infrared (MWIR) double-layer heterojunction HgCdTe has been grown on 8-inch Si substrates using molecular beam epitaxy. We grew six 8-inch-diameter MWIR HgCdTe wafers on Si substrates and measured within-wafer and wafer-to-wafer variations of key parameters such as cutoff wavelength, HgCdTe thickness, macrodefect density including voids and microvoids, and arsenic doping uniformity. The results indicate that the growth was very successful and the process repeatable, well controlled, and ready for production.

The formation of impurity ‘hot spot’ macro-defects—localized high impurity level contaminates—is examined. The evolution of macro-defects through their critical stages: as-received CdZnTe substrate, molecular beam epitaxy (MBE) prep etch, Te/oxide desorption and stabilization at deposition temp in MBE chamber, thin (200 nm) HgCdTe deposition, and thick (> 4 μm) MBE HgCdTe deposition, is analyzed. As-received CdZnTe wafers have small and large agglomerations of residual SiO2 and Al2O3 polishing grit. Additionally, macro-defects composed of organic residue, Cd(Zn)Te particulates, and additional impurities are also observed. The MBE preparation etch does not significantly alter the types of defects found on the wafer. The Te/oxide desorption process adds a small number of defects due to colloidal graphite particles. HgCdTe deposition introduces MBE spit defects and Hg droplet residue. There is significant movement of polishing grit and organic macro-defects during the various processing steps. Energy dispersive X-ray spectroscopy (EDS) analysis of MBE spit defects shows they are Cd rich with some of the spit defects containing high concentrations of impurities. The vertical gradient in Te concentration associated with the overgrowth on a metal rich MBE spit defect acts as an impurity pipe for Cu to travel toward the surface to become a ‘hot spot’ defect.

In this work, impurity ‘hot spot’ macro-defects—high impurity level macro-defect contaminates were examined. ‘Hot spots’ have very high localized concentrations of: K, Mg, Ni, Cr, Mn, Ca, Al, Na, Fe, and Cu. For example, these ‘hot spot’ macro-defects can have Cu concentrations > 1 × 1018 cm−3. Focused ion beam scanning transmission electron microscopy analysis of four ‘hot spots’ was performed. The origin of ‘hot spot’ defects is unresolved—however, our analysis has shown ‘hot spots’ can arise due to molecular beam epitaxy spit defects and CdZnTe substrate defects. The estimated ‘hot spot’ density is ∼ 30 cm−2. The presence of impurity ‘hot spot’ macro-defects in HgCdTe/CdZnTe is confirming evidence for the occurrence of L. Bubulac’s impurity ‘pipe’ mechanism.

Dark current density data recorded over the past 14 years at Raytheon Vision Systems on short-wavelength infrared (SWIR) and medium-wavelength infrared (MWIR) devices were examined. This included HgCdTe detector arrays grown by liquid-phase epitaxy on CdZnTe and molecular beam epitaxy on both silicon and CdZnTe substrates. This study analyzed zero-bias resistance–area product and current–voltage measurements from test structure assemblies included on every detector wafer. The data span cutoff wavelengths from 1.7 μm to 7.5 μm and operating temperatures from 40 K to 300 K. A basis is derived for a simple manufacturing trend model for a wide range of cutoffs and temperatures. This model uses a function similar to Tennant’s Rule’07 but includes a generation–recombination (GR) term. Dark current densities below the test set limit are extrapolated assuming GR-limited performance. Model assumptions are tested using sensor chip assembly (SCA) median dark current density values at the same inverse cutoff–temperature products. This model allows probabilistic determination of array manufacturability and prediction of yield, and provides a statistical basis for Raytheon’s state-of-the-art performance.

There is a well-known direct negative correlation between dislocation density and optoelectronic device performance. Reduction in detector noise associated with dislocations is an important target for improvement of mercury cadmium telluride (Hg1−x Cd x Te)-based material in order to broaden its use in the very long-wavelength infrared (VLWIR) regime. The lattice mismatch and differences in physical properties between substrates and the epitaxial Hg1−x Cd x Te layers cause an increased threading dislocation density. As demonstrated in this work, the presence of arsenic impurities via p-type doping in molecular beam epitaxy (MBE)-grown epitaxial crystal structure increases the etch pit density (EPD) of Hg1−x Cd x Te grown on Si substrates but not on CdZnTe substrates. This EPD increase is not observed in indium n-type-doped Hg1−x Cd x Te grown on either Si or CdZnTe substrates. This trend is also seen in layers with different cadmium compositions. All of the EPD variations of the structures studied here are shown to be independent of the MBE machine used to grow the structure. The fundamentals of this higher EPD are not yet completely understood.

Mercury cadmium telluride (HgCdTe) grown on large-area silicon (Si) substrates allows for larger array formats and potentially reduced focal-plane array (FPA) cost compared with smaller, more expensive cadmium zinc telluride (CdZnTe) substrates. In this work, the use of HgCdTe/Si for mid-wavelength/long-wavelength infrared (M/LWIR) dual-band FPAs is evaluated for tactical applications. A number of M/LWIR dual-band HgCdTe triple-layer n-P-n heterojunction device structures were grown by molecular-beam epitaxy (MBE) on 100-mm (211)Si substrates. Wafers exhibited low macrodefect densities (< 300 cm−2). Die from these wafers were mated to dual-band readout integrated circuits to produce FPAs. The measured 81-K cutoff wavelengths were 5.1 μm for band 1 (MWIR) and 9.6 μm for band 2 (LWIR). The FPAs exhibited high pixel operability in each band with noise-equivalent differential temperature operability of 99.98% for the MWIR band and 98.7% for the LWIR band at 81 K. The results from this series are compared with M/LWIR FPAs from 2009 to address possible methods for improvement. Results obtained in this work suggest that MBE growth defects and dislocations present in devices are not the limiting factor for detector operability, with regards to infrared detection for tactical applications.

The crystalline structure and impurity profiles of HgCdTe/CdTe/alternate substrate (AS; Si and GaAs are possibilities) and CdTe/AS were analyzed by secondary-ion mass spectrometry, atomic force microscopy, etch pit density analysis, and scanning transmission electron microscopy. Impurities (Li, Na, and K) were shown to getter in as-grown CdTe/Si epilayers at in situ Te-stabilized thermal anneal (~500°C) interfaces. In HgCdTe/CdTe/Si epilayers, indium accumulation was observed at Te-stabilized thermal anneal interfaces. Impurity accumulation was measured at HgCdTe/CdTe and CdTe/ZnTe interfaces. Processing anneals were found to nearly eliminate the gettering effect at the in situ Te-stabilized thermal anneal interfaces. Impurities were found to redistribute to the front HgCdTe/CdTe/Si surface and p–n junction interfaces during annealing steps. We also investigated altering the in situ Te-stabilized thermal anneal process to enhance the gettering effect.

This paper presents the status of HgCdTe growth on large-area Si and CdZnTe substrates at Raytheon Vision Systems (RVS). The different technological tools that were used to scale up the growth from 4 inch to 6 inch diameter on Si and from 4 cm × 4 cm to 8 cm × 8 cm on CdZnTe without sacrificing the quality of the layers are described. Extremely high compositional uniformity and low macrodefect density were achieved for single- and two-color HgCdTe layers on both Si and CdZnTe substrates. Finally, a few examples of detector and focal-plane array results are included to highlight the importance of high compositional uniformity and uniformly low macrodefect density of the epitaxial layers in obtaining high operability and low cluster outages in single- and two-color focal-plane arrays (FPAs).

Six-inch HgCdTe-on-silicon wafer capability is important due to the increase in die size along with the reduction in pixel pitch. Successful manufacturing of 6-inch HgCdTe on silicon depends upon the availability of low-defect-density substrates, robust chemical processing, and in situ measuring techniques. Critical advances in wafer processing across 6-inch wafers have made the transition from development to production successful. The status of each of these steps is described in this paper, with emphasis on wafer uniformity and process reproducibility. Finally a short section is dedicated to implementation of statistical process control for optimal throughput and yield.

Molecular beam epitaxy (MBE) growth of HgCdTe on large-size Si (211) and CdZnTe (211)B substrates is critical to meet the demands of extremely uniform and highly functional third-generation infrared (IR) focal-panel arrays (FPAs). We have described here the importance of wafer maps of HgCdTe thickness, composition, and the macrodefects across the wafer not only to qualify material properties against design specifications but also to diagnose and classify the MBE-growth-related issues on large-area wafers. The paper presents HgCdTe growth with exceptionally uniform composition and thickness and record low macrodefect density on large Si wafers up to 6-in in diameter for the detection of short-wave (SW), mid-wave (MW), and long-wave (LW) IR radiation. We have also proposed a cost-effective approach to use the growth of HgCdTe on low-cost Si substrates to isolate the growth- and substrate-related problems that one occasionally comes across with the CdZnTe substrates and tune the growth parameters such as growth rate, cutoff wavelength (λ cutoff) and doping parameters before proceeding with the growth on costly large-area CdZnTe substrates. In this way, we demonstrated HgCdTe growth on large CdZnTe substrates of size 7 cm × 7 cm with excellent uniformity and low macrodefect density.

Factors such as lateral control of surface chemistry, reproducible etch depths and widths, and elimination of cross-contamination are vital to achieving world-class Hg1-X Cd X Te focal plane infrared detector arrays. Raytheon Vision Systems (RVS) has made significant progress toward a greater understanding of these factors. The current investigation applies time of flight–secondary ion mass spectroscopy (TOF-SIMS) to assess the manner in which Hg1-X Cd X Te surface chemistry is influenced by three different critical physical-chemical factors: (1) process chemistry, including baseline and four alternatives; (2) patterned photoresist format; and (3) etch geometry, including aspect ratio, trench depth, trench width, and unit cell spacing. The first of two patterned photoresist formats consisted of an array having 15-μm unit cells with 5-μm-wide and either 3.5-μm- or 6-μm-deep trenches. The second format consisted of a special diagnostic array of parallel dry-etched stripes having various permutations of trench depths (6 μm and 10 μm), trench widths (3 μm and 5 μm), and trench-to-trench separations (3 μm, 5 μm, and 20 μm). The surface chemistry results relative to etch-depth/width ratios, exposed Hg1−X Cd X Te area (etch widths), and trench separation distances show that these parameters have a measurable influence on cross-contaminant abundance and type and on the relative ranking of the process cleaning efficacies. Novel analytical methods for using TOF-SIMS data to qualitatively assess cleaning efficacy, geometry-dependent surface species distributions, the polar/hydrophilic and nonpolar/hydrophobic nature of the processed surface, and the dependence of cleaning efficacy on surface chemistry are also discussed. These methods are intended for use in a variety of studies.

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.

Raytheon Vision Systems (RVS) is developing two-color and large format single color FPAs fabricated from molecular beam epitaxy (MBE) grown HgCdTe triple layer heterojunction (TLHJ) wafers on CdZnTe substrates and double layer heterojunction (DLHJ) wafers on Si substrates, respectively. MBE material growth development has resulted in scaling TLHJ growth on CdZnTe substrates from 10cm(2) to 50cm(2), long-wavelength infrared (LWIR) DLHJ growth on 4-inch Si substrates and the first demonstration of mid-wavelength infrared (MWIR) DLHJ growth on 6-inch Si substrates with low defect density (< 1000cm(-2)) and excellent uniformity (composition < 0.1%, cut-off wavelength Delta center-edge < 0.1 mu m). Advanced FPA fabrication techniques such as inductively coupled plasma (ICP) etching are being used to achieve high aspect ratio mesa delineation of individual detector elements with benefits to detector performance. Recent two-color detectors with MWIR and LWIR cut-off wavelengths of 5.5 mu m and 10.5 mu m, respectively, exhibit significant improvement in 78K LW performance with > 70% quantum efficiency, diffusion limited reverse bias dark currents below 300pA and RA products (zero field-of-view, +150mV bias) in excess of 1x10(3) Omega cm(2). Two-color 20 mu m unit-cell 1280x720 MWIR/LWIR FPAs with pixel response operability approaching 99% have been produced and high quality simultaneous imaging of the spectral bands has been achieved by mating the FPA to a readout integrated circuit (ROIC) with Time Division Multiplexed Integration (TDMI). Large format mega pixel 20 mu m unit-cell 2048x2048 and 25 mu m unit-cell 2560x512 FPAs have been demonstrated using DLHJ HgCdTe growth on Si substrates in the short wavelength infrared (SWIR) and MWIR spectral range. Recent imaging of 30 mu m unit-cell 256x256 LWIR FPAs with 10.0-10.7 mu m 78K cut-off wavelength and pixel response operability as high as 99.7% show the potential for extending HgCdTe/Si technology to LWIR wavelengths.

Time of flight-secondary ion mass spectrometry (TOF-SIMS) is a Hg 1−x Cd x Te surface diagnostic tool with unprecedented analysis capabilities, including analyzing a 0.5-µm diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. The present investigation demonstrates the power of TOF-SIMS when coupled with optical interferometry in understanding process reproducibility and uniformity critical to the fabrication of Hg 1−x Cd x Te detector arrays at RVS. Previous published works and unpublished studies at RVS have shown that geometry and fluid dynamics influence the lateral uniformity of surface chemistry, topography, and etch rates. By combining a set of photolithographically delineated features having various relative areas of photoresist-coated and uncoated regions in varying proximity to each other with various wet etching chemistries, we have exploited TOF-SIMS interrogation along with optical interferometry to investigate physical-chemical drivers of etch rate variation with window geometry orientation with respect to vertical gravity etchant fluid draining direction and proximity to other structures. This study has given us the ability to deconvolve two important etch rate drivers (depletion of etchant species and cross-contamination of etched windows) and elucidate their roles in enhancing and diminishing etch rates for features having far and close proximities to neighboring structures, respectively. This information allows a more judicious optimization of processing technology.

Hg1-xCdxTe surface chemistry has been studied extensively with well-known tools such as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES) in order to advance detector array operability, performance, and yield. Raytheon Vision Systems has pioneered the first application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) as a Hg1-xCdxTe surface diagnostic tool to provide unprecedented analysis capability, including analyzing a 0.5-mu m-diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. In this study, data are presented illustrating surface chemistry geometry effects and photoresist redeposition due to common Hg1-xCdxTe processing steps including photolithography, bromine etching, and photoresist stripping.

Hg 1−x Cd x Te surface chemistry has been studied extensively with well-known tools such as electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES) in order to advance detector array operability, performance, and yield. Raytheon Vision Systems has pioneered the first application of time-of-flight secondary ion mass spectrometry (TOF-SIMS) as a Hg 1−x Cd x Te surface diagnostic tool to provide unprecedented analysis capability, including analyzing a 0.5-µm-diameter spot, high mass resolution, elemental and molecular composition scrutiny, applicability to insulators, and surface film sensitivity in the part per million range. In this study, data are presented illustrating surface chemistry geometry effects and photoresist redeposition due to common Hg 1−x Cd x Te processing steps including photolithography, bromine etching, and photoresist stripping.

Since its initial synthesis and investigation more than 40 years ago, the HgCdTe alloy semiconductor system has evolved into one of the primary infrared detector materials for high-performance infrared focal-plane arrays (FPA) designed to operate in the 3-5 mum and 8-12 mum spectral ranges of importance for thermal imaging systems. Over the course of the past decade, significant advances have been made in the development of thin-film epitaxial growth techniques, such as molecular-beam epitaxy (MBE), which have enabled the synthesis of IR detector device structures with complex doping and composition profiles. The central role played by in situ sensors for monitoring and control of the MBE growth process are reviewed. The development of MBE HgCdTe growth technology is discussed in three particular device applications: avalanche photodiodes for 1.55 mum photodetection, megapixel FPAs on Si substrates, and multispectral IR detectors.

Progress on achieving reproducible growth of high performance, dual-band IR detector structures in HgCdTe grown by molecular beam epitaxy (MBE) is described. The reproducibility achieved in the MBE growth of n–p–n device structures comprising HgCdTe epitaxial layers with different composition and doping characteristics was evaluated from the run-to-run precision in the alloy composition, dopant concentration and dislocation density. For a series of 25 growth runs, the standard deviation of the alloy composition in the n-type absorbing layer was 0.002; the yield for the in situ n- and p-type doping process was >95%; and the average dislocation density was <5×105cm−2. In situ optical diagnostics, including spectroscopic ellipsometry and an optical absorption flux monitor were used for the real-time determination of the alloy composition and Cd flux during MBE growth of the two-color device structures. Focal plane arrays with 128×128 elements were fabricated for the simultaneous detection of two sub-bands in the MWIR spectrum. Average RoA values exceeding 1×106 and 2×105Ωcm2 were measured at 77K for diodes operating at 4.0 and 4.5μm, respectively, and the quantum efficiency was greater than 70% in each band. These results on MBE growth and device performance demonstrate that HgCdTe MBE technology is poised for the modest-scale production of advanced IR devices.