James Bangs

Status of two-color and large format HgCdTe FPA technology at Raytheon Vision Systems

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 10cm2 to 50cm2, 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 Δcenter-edge<0.1μm). 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 5.5μm and 10.5μm, 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 1×103 Ωcm2. Two-color 20μm unit-cell 1280×720 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 20μm unit-cell 2048×2048 and 25μm unit-cell 2560×512 FPAs have been demonstrated using DLHJ HgCdTe growth on Si substrates in the short wavelength infrared (SWIR) and MWIR spectral range. Recent imaging of 30μm unit-cell 256×256 LWIR FPAs with 10.0-10.7μm 78K cut-off wavelength and pixel response operability as high as 99.7% show the potential for extending HgCdTe/Si technology to LWIR wavelengths.

I-V modeling of current limiting mechanisms in HgCdTe FPA detectors

This paper details significant improvements in current-voltage (I-V) modeling capabilities using an automated iterative non-linear fitting program. The properties of a particular infrared (IR) detectors I-V curve are dependent upon the current limiting mechanisms in the device which depend upon the temperature, applied bias, and cutoff wavelength or detector bandgap. This model includes ideal diode diffusion, generation-recombination, band-to-band tunneling, trap-assisted tunneling, shunt resistance, and avalanche breakdown as potential current limiting mechanisms in an IR detector diode. The modeling presented herein allows one to easily distinguish, and more importantly to quantitatively compare, the amount of influence each current limiting mechanism has on various detectors I-V characteristics. Modeling of the trap-assisted-tunneling mechanism leads to an estimate of the density of occupied trap states at a given temperature. This model is now routinely applied to Raytheon Vision Systems’ test structures to better understand detector current limitations.

Status of HgCdTe-MBE technology for producing dual-band infrared 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 x 10 5 cm -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 x 128 elements were fabricated for the simultaneous detection of two sub-bands in the MWIR spectrum. Average R o A values exceeding 1 x 10 6 and 2 x 10 5 Ω cm 2 were measured at 77 K for diodes operating at 4.0 and 4.5 μm, 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.

High-Performance M/LWIR Dual-Band HgCdTe/Si Focal-Plane Arrays

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.

Large format high-operability SWIR and MWIR focal plane array performance and capabilities

High-performance large-format detector arrays responsive to the 1-5μm wavelength range of the infrared spectrum fabricated using large area HgCdTe layers grown on 6-inch diameter (211) silicon substrates are available for advanced imaging applications. This paper reviews performance and capabilities of Raytheon Vision Systems (RVS) HgCdTe/Si Focal Plane Arrays (FPA) and shows 2k x 2k format MWIR HgCdTe/Si FPA performance with NEdT operabilities better than 99.9%. SWIR and MWIR detector performance for HgCdTe/Si is comparable to established performance of HgCdTe/CdZnTe wafers. HgCdTe devices fabricated on both types of substrates have demonstrated very low dark current, high quantum efficiency and full spectral band fill factor characteristic of HgCdTe. HgCdTe has the advantage of being able to precisely tune the detector cutoff via adjustment of the Cd composition in the MBE growth. The HgCdTe/Si detectors described in this paper are p-on-n mesa delineated architecture and fabricated using the same mature etch, passivation, and metallization processes as our HgCdTe/CdZnTe line. Uniform device quality HgCdTe epitaxial layers and application of detector fabrication processes across the full area of 6-inch wafers routinely produces high performing detector pixels from edge to edge of the photolithographic limits across the wafer, offering 5 times the printable area as costly 6×6cm CdZnTe substrates. This 6-inch HgCdTe detector wafer technology can provide applications demanding very wide FOV high resolution coverage the capability to produce a very large single piece infrared detector array, up to a continuous image plane 10×10 cm in size. Alternatively, significant detector cost reduction through allowing more die of a given size to be printed on each wafer is possible, with further cost reduction achieved through transition towards automated detector fabrication and photolithographic processes for both increased yields and reduced touch labor costs. RVS continues to improve its FPA manufacturing line towards achieving low cost infrared FPAs with the format, size, affordability, and performance required for current and future infrared applications.

Performance of MWIR and SWIR HgCdTe-based focal plane arrays at high operating temperatures

Raytheon Vision Systems (RVS) is producing large format, high definition HgCdTe-based MWIR and SWIR focal plane arrays (FPAs) with pitches of 15 μm and smaller for various applications. Infrared sensors fabricated from HgCdTe have several advantages when compared to those fabricated from other materials -- such as a highly tunable bandgap, high quantum efficiencies, and R0A approaching theoretical limits. It is desirable to operate infrared sensors at elevated operating temperatures in order to increase the cooler life and reduce the required system power. However, the sensitivity of many infrared sensors, including those made from HgCdTe, declines significantly above a certain temperature due to the noise resulting from increasing detector dark current. In this paper we provide performance data on a MWIR and a SWIR focal plane array operating at temperatures up to 160K and 170K, respectively. The FPAs used in the study were grown by molecular beam epitaxy (MBE) on silicon substrates, processed into a 1536x1024 format with a 15 μm pixel pitch, and hybridized to a silicon readout integrated circuit (ROIC) via indium bumps to form a sensor chip assembly (SCA). This data shows that the noise equivalent delta temperature (NEDT) is background limited at f/3.4 in the SWIR SCA (cutoff wavelength of 3.7 μm at 130K) up to 140K and in the MWIR SCA (cutoff wavelength of 4.8 μm at 115K) up to 115K.

Advances in HgCdTe-based infrared detector materials: the role of molecular-beam epitaxy

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 mm and 8-12 mm 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 +m photodetection, megapixel FPAs on Si substrates, and multispectral IR detectors.

Higher Dislocation Density of Arsenic-Doped HgCdTe Material

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−xCdxTe)-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−xCdxTe 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−xCdxTe grown on Si substrates but not on CdZnTe substrates. This EPD increase is not observed in indium n-type-doped Hg1−xCdxTe 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.

Impurity Gettering in (112)B HgCdTe/CdTe/Alternate Substrates

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.

Detection comparisons between LWIR and MWIR polarimetric sensors

Polarimetry sensor development has been in work for some time to determine the best use of polarimetry to differentiate between manmade objects and objects made by nature. Both MWIR and LWIR Focal Plane Arrays (FPAs) have been built at Raytheon Vision Systems each with exceedingly higher extinction ratios. This paper compares field imagery between MWIR and LWIR micro-grid polarimetric sensors independently and during simultaneous image collects. LWIR polarimetry has the largest polarimetric signal level and an emissive polarimetric signature which allows detection at thermal crossover and is less dependent on sun angles. Polished angled glass and metal objects are easily detected using LWIR polarimetry. While LWIR polarimetry has many advantages its resolution is not as good as MWIR. MWIR polarimetry has higher resolution than LWIR. With good sun angles plastic drums, and wet surfaces provide good polarization signatures. With poor sun angles detection can be challenging. To gain acceptance polarimetric sensors must provide intelligence signatures that are better than existing nonpolarimetric Infrared sensors. This paper shows several examples of images without polarimetric processing and identical images with MWIR and/or LWIR polarimetric fusion onto the non-polarized images to show the improvement of detection using polarimetric sensors. It is the authors belief that the fastest way to gain acceptance of polarimetric remote sensing is through field demonstration as shown in Figure 1.