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.     

Large Format AlInAs–InGaAs Quantum-Well Infrared Photodetector Focal Plane Array For Midwavelength Infrared Thermal Imaging

We report the first large format (640times512) AlInAs-InGaAs quantum-well infrared photodetector (QWIP) focal plane array (FPA), and investigate the characteristics of AlInAs-InGaAs QWIPs both at pixel and FPA level. The measurements on the detectors fabricated with molecular beam epitaxy grown epilayer structure including 30 InGaAs quantum wells (26 Aring thick, N_D=2times10¹⁸ cm^-3) yielded very promising characteristics. The detectors with lambda_p=4.2 mum and Deltalambda/lambda_p=25% displayed a background-limited performance temperature as high as 105 K with f/2 aperture. The noise equivalent temperature difference of the FPA is as low as 23 mK (f/1.5) at 105-K sensor temperature with 99.6% operability. These results are comparable to the best results reported for AlGaAs-InGaAs midwavelength infrared QWIPs showing the promise of this material system for completely lattice-matched multiband QWIP FPAs.     

640×486 long-wavelength two-color GaAs/AlGaAs quantum wellinfrared photodetector (QWIP) focal plane array camera

We have designed and fabricated an optimized long-wavelength/very-long wavelength two-color quantum well infrared photodetector (QWIP) device structure. The device structure was grown on a 3-in semi-insulating GaAs substrate by molecular beam epitaxy (MBE). The wafer was processed into several 640×486 format monolithically integrated 8-9 and 14-15 μm two-color (or dual wavelength) QWIP focal plane arrays (FPAs). These FPAs were then hybridized to 640×486 silicon CMOS readout multiplexers. A thinned (i.e., substrate removed) FPA hybrid was integrated into a liquid helium cooled dewar for electrical and optical characterization and to demonstrate simultaneous two-color imagery. The 8-9 μm detectors in the FPA have shown background limited performance (BLIP) at 70 K operating temperature for 300 K background with f/2 cold stop. The 14-15 μm detectors of the FPA reaches BLIP at 40 K operating temperature under the same background conditions. In this paper we discuss the performance of this long-wavelength dualband QWIP FPA in terms of quantum efficiency, detectivity, noise equivalent temperature difference (NEΔT), uniformity, and operability     

Improved Defect and Fourier Transform Infrared Spectroscopy Analysis for Prediction of Yield for HgCdTe Multilayer Heterostructures

The ability to accurately predict HgCdTe focal plane array (FPA) performance using nondestructive, postgrowth wafer analysis is of great importance. These predictions, if accurate, reduce costs by screening the wafers prior to processing, and selecting only those wafers that are most likely to yield FPAs that meet program specifications. In this paper, we examine the use of a macrodefect inspection tool, the NSX 1255, from August Technology. This inspection tool has the ability to measure defects 0.5 μm and larger and store the location and size data to a file. We have then, through the use of custom written software, been able to analyze these data on a wafer by wafer basis. We have also incorporated the use of a thin film transmission matrix model to analyze room-temperature Fourier transform infrared spectroscopy (FTIR) transmission spectra. This technique, which is applied to the entire wafer surface, can be used to determine the individual layer thicknesses as well as their compositions. Then, using analytical expressions for bandgap, absorption, and index of refraction, we can predict responsivity and quantum efficiency. Through the use of these two inspection tools and our analysis software, we are able to overlay FPA die information and perform statistics on a die-per-die basis. This allows us to effectively “pass” or “fail” each FPA based on the program specifications. We are then able to set a minimum criterion for the number of FPAs that pass on any given wafer. That wafer is then sent off to processing if it meets this criterion. Furthermore, knowing why a wafer fails before it reaches processing allows for real time feedback to the epilayer growth process. This allows for run-to-run adjustments in order to keep as many wafers within specifications as possible and increases yield overall. (Received October 24, 2006; accepted Feburary 26, 2007)     

Quantum Dot Based Infrared Focal Plane Arrays

In the past decade, there has been active research on infrared detectors based on intersubband transitions in self-assembled quantum dots (QDs). In the past two years, at least four research groups have independently demonstrated focal plane arrays based on this technology. In this paper, the progress from the first raster scanned image obtained with a QD detector to the demonstration of a 640 512 imager based on self-assembled QDs is reviewed. In particular, emphasis will be placed on a novel quantum dots-in-a-well (DWELL) design, which represents a hybrid between a conventional quantum-well infrared photodetector (QWIP) and a quantum-dot infrared photodetector (QDIP). In the DWELL detectors, the active region consists of InAs quantum dots embedded in an InGaAs quantum well. Like QDIPs, the DWELL detectors have 3-D confinement and display normal incidence operation while demonstrating reproducible ldquodial-in recipesrdquo for control over the operating wavelength, like QWIPs. Moreover, the DWELL detectors also have demonstrated bias-tunability and multicolor operation in the midwave infrared (MWIR, 3-5 ), long-wave infrared (LWIR, 8-12 ), and very long wave infrared (VLWIR, ) regimes. Recently midformat 320 256 and 640 512 focal plane arrays (FPAs) with an NETD of 40 mK at have been reported. The paper will conclude with a perspective on the future directions on the research on QDIP FPA including enhanced functionality and higher operating temperatures.     

Analytical model used to calculate focal-plane-array parameters

The theory making it possible to rather accurately predict a complete set of characteristics (signals, noises, and photoelectric parameters) inherent to all components of the photodiode-based focal plane array (FPA) under design and optimize its parameters has been developed. The theory relies on a new approach to the determination of FPA irradiance, which ensures its calculations at any shape of the diaphragm in the light-insulating shield. Both staring and scanning FPAs, the latter of which operate under the condition of time delay and accumulation, are discussed. The theory has been verified using a 320 × 256 FPA. The calculated dependences of the signals and noises of photoelectric components are compared with the experimental values obtained at different accumulation times and background irradiation temperatures. The theoretical data are revealed to be in complete agreement with the experiment, confirming the model validity. The theory will be undoubtedly useful for designers, manufacturers, and users of FPAs. The model can easily be extended to systems with FPAs.     

Inductively coupled plasma etching for large format HgCdTe focal plane array fabrication

Inductively coupled plasma (ICP) using hydrogen-based gas chemistry has been developed to meet requirements for deep HgCdTe mesa etching and shallow CdTe passivation etching in large format HgCdTe infrared focal plane array (FPA) fabrication. Large format 2048×2048, 20-μm unit-cell short wavelength infrared (SWIR) and 2560×512, 25-μm unit-cell midwavelength infrared (MWIR) double-layer heterojunction (DLHJ) p-on-n HgCdTe FPAs fabricated using ICP processing exhibit >99% pixel operability. The HgCdTe FPAs are grown by molecular beam epitaxy (MBE) on Si substrates with suitable buffer layers. Midwavelength infrared detectors fabricated from 4-in. MBE-grown HgCdTe/Si substrates using ICP for mesa delineation and CdTe passivation etching demonstrate measured spectral characteristics, RoA product, and quantum efficiency comparable to detectors fabricated using wet chemical processes. Mechanical samples prepared to examine physical characteristics of ICP reveal plasma with high energy and low ion angle distribution, which is necessary for fine definition, high-aspect ratio mesa etching with accurate replication of photolithographic mask dimensions.     

256×1甚长波量子阱红外焦平面研究

GaAs/AlGaAs量子阱红外探测器由于其所依据的GaAs基材料较为成熟的材料生长和器件制备工艺,使其特别适合于高均匀性、大面积红外焦平面的应用。报道了甚长波256×1元GaAs/AlGaAs多量子阱红外焦平面器件的研制成果, 探测器的峰值波长为15 μm,响应带宽大于1.5 μm。在40 K工作温度下,器件的平均黑体响应率Rp=3.96×106 V/W, 平均黑体探测率为D*=1.37×109 cm·Hz1/2/W, 不均匀性为11.3%, 并应用研制的器件获得了物体的热像图。     

红外焦平面探测器阵列规格的发展

红外焦平面探测器阵列规格的发展是一个从疏到密、从小到大的过程,受到大面积探测器材料生长和小像元制备等因素的限制。战略焦平面阵列一般用于探测点源目标,而战术焦平面阵列则一般用于探测扩展源目标。从相关的基本概念出发,分析了焦平面阵列规格的发展过程,讨论了作用距离与焦平面阵列规格之间的关系。由于电视格式基本固定,在战术焦平面阵列实现全帧格式以后,其规格进一步增加的势头即便不是停止,也必将会趋缓。但是另一方面,因为焦平面阵列的规格越大,其居高临下而一次看到的面积就越广,所以战略焦平面阵列将会继续向超大规格发展。     

Design and development of multicolor MWIR/LWIR and LWIR/VLWIR detector arrays

Multicolor infrared (IR) focal planes are required for high-performance sensor applications. These sensors will require multicolor focal plane arrays (FPAs) that will cover various wavelengths of interest in mid wavelength infrared/long wavelength infrared (MWIR/LWIR) and long wavelength infrared/very long wavelength infrared (LWIR/VLWIR) bands. There has been significant progress in HgCdTe detector technology for multicolor MWIR/LWIR and LWIR/VLWIR FPAs.^1–3 Two-color IR FPAs eliminate the complexity of multiple single-color IR FPAs and provide a significant reduction of weight and power in simpler, reliable, and affordable systems. The complexity of a multicolor IR detector MWIR/LWIR makes the device optimization by trial and error not only impractical but also merely impossible. Too many different geometrical and physical variables need to be considered at the same time. Additionally, material characteristics are only relatively controllable and depend on the process repeatability. In this context, the ability of performing “simulation experiments” where only one or a few parameters are carefully controlled is paramount for a quantum improvement of a new generation of multicolor detectors for various applications.     

256×256 focal plane array midwavelength infrared camera based on InAs/GaSb short-period superlattices

An infrared camera based on a 256×256 focal plane array (FPA) for the second atmospheric window (3–5 μm) has been realized for the first time with InAs/GaSb short period superlattices (SLs). The SL detector structure with a broken gap type-II band alignment was grown by molecular beam epitaxy on GaSb substrates. Effective bandgap and strain in the superlattice were adjusted by varying the thickness of the InAs and GaSb layers and the controlled formation of InSb-like bonds at the interfaces. The FPAs were processed in a full wafer process using optical lithography, chemical-assisted ion beam etching, and conventional metallization technology. The FPAs were flip-chip bonded using indium solder bumps with a read-out integrated circuit and mounted into an integrated detector cooler assembly. The FPAs with a cut-off wavelength of 5.4 μm exhibit quantum efficiencies of 30% and detectivity values exceeding 10¹³ Jones at T=77 K. A noise equivalent temperature difference (NETD) of 11.1 mK was measured for an integration time of 5 ms using f/2 optics. The NETD scales inversely proportional to the square root of the integration time between 5 ms and 1 ms, revealing background limited performance. Excellent thermal images with low NETD values and a very good modulation transfer function demonstrate the high potential of this material system for the fabrication of future thermal imaging systems.     

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.     

Comparison of Long-Wave Infrared Quantum-Dots-in-a-Well and Quantum-Well Focal Plane Arrays

This paper reports on a comparison between a commercially available quantum-well infrared focal plane array (FPA) and a custom quantum-dot (QD)-in-a-well (DWELL) infrared FPA in the long-wave infrared (LWIR). The DWELL detectors consist of an active region composed of InAs QDs embedded in $ hbox{In}_{.15}hbox{Ga}_{.85}hbox{As}$ quantum wells. DWELL samples were grown using molecular beam epitaxy and fabricated into 320 $times$ 256 pixels FPA with a flip-chip indium bump technique. Both the DWELL and QmagiQ commercial quantum-well detector were hybridized to an Indigo ISC9705 readout circuit and tested in the same camera system. Calibrated blackbody measurements at a device temperature of 60 K with LWIR optics yield a noise equivalent change in temperature of 17 mK and 91 mK for quantum-well and DWELL FPAs operating at 0.95- and 0.58-V biases, respectively. The comparison of the DWELL and quantum-well FPA when imaging a 35 $^{circ}hbox{C}$ black body showed that the DWELL had a signal-to-noise ratio of 124 while the quantum-well FPA showed 1961. As well, the quantum-well FPA showed a higher collection efficiency of 1.3 compared to the DWELL.      

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.     

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.     

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.     

640 × 512 Pixels Long-Wavelength Infrared (LWIR) Quantum-Dot Infrared Photodetector (QDIP) Imaging Focal Plane Array

Epitaxially grown self-assembled InAs-InGaAs-GaAs quantum dots (QDs) are exploited for the development of large-format long-wavelength infrared focal plane arrays (FPAs). The dot-in-a-well (DWELL) structures were experimentally shown to absorb both 45deg and normal incident light, therefore, a reflection grating structure was used to enhance the quantum efficiency. The devices exhibit peak responsivity out to 8.1 mum, with peak detectivity reaching ~1times10¹⁰ Jones at 77 K. The devices were fabricated into the first long-wavelength 640times512 pixel QD infrared photodetector imaging FPA, which has produced excellent infrared imagery with noise equivalent temperature difference of 40 mK at 60-K operating temperature     

美国弹道导弹防御系统的红外系统与技术的发展

红外传感器是弹道导弹防御系统可靠监视、探测、识别、瞄准与拦截目标的关键。过去二十几年中,美国的弹道导弹防御系统逐渐从构想成为现实,其中一个重要原因就是由于红外焦平面阵列技术的迅速发展,用于弹道导弹防御的红外传感器取得了突破性的进展。目前用于弹道导弹防御系统的红外传感器和动能拦截器红外导引头已从过去的基于红外探测器线列或较小规模的红外焦平面阵列(64×64)的系统发展到基于较大规模的凝视红外焦平面阵列的系统,红外传感器与拦截器红外导引头的性能有了很大的提高。但由于弹道导弹防御本身的复杂性,目前的导弹防御系统仅是初步的,为此,美国还在继续发展完善其弹道导弹防御系统,包括进一步发展更先进的红外焦平面阵列,改进红外探测跟踪系统与动能拦截弹红外导引头。综述了美国弹道导弹防御系统中红外系统与技术的发展现状与趋势。     

固体浸没式红外超表面透镜设计

红外焦平面阵列在各类红外成像系统中发挥着巨大的作用。为提升红外焦平面的工作温度、量子效率和灵敏度,通常使用微透镜阵列作为红外焦平面的聚光器。当前微透镜阵列的制作材料通常与红外探测器材料不同,因此在集成装配时需要额外的工艺手段,工艺难度较大且效率较低。利用微纳光学超表面技术体系,可以在红外探测器衬底材料上直接制作平面式的固体浸没型微透镜阵列,实现前置微透镜与红外焦平面的单片集成。文中以红外探测领域最有潜力的锑化物Ⅱ类超晶格红外探测器为应用目标,设计了一种基于GaSb衬底的固体浸没式红外超表面透镜。设计的超表面透镜在中波红外波段工作,能适用于所有入射偏振。器件设计焦距为100 μm,理论上在目标波长下的最高聚焦效率达到70.7%,数值孔径(NA)达到1.15。该设计可以推动微透镜阵列向扁平、超薄、轻量的方向发展,简化微透镜阵列与红外焦平面阵列的集成工艺,有望提升红外焦平面的探测效率,并降低制造成本。     

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.