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.     

HgCdTe: Recent Trends in the Ultimate IR Semiconductor

Since its introduction in the early 1970s, HgCdTe has become the general workhorse of the global infrared (IR) industry. Despite a significant lack of investment in the basic science of this materials system since 1990, huge advances have been made in HgCdTe focal-plane array (FPA) technology, resulting in the current availability of a wide variety of large-area FPAs, for all IR spectral bands, both monocolor and multicolor, with the capability for both passive and active imaging. The current status of the many technologies relevant to the continued good health of HgCdTe is discussed herein, with regard to both their present limitations and possible opportunities for improvement.     

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.     

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.     

碲镉汞红外探测器的前沿技术综述

红外光电探测器件在军事、民用和科学研究方面具有非常重要的应用。而碲镉汞(HgCdTe)由于自身的诸多优点在红外光电探测器的发展中起到了至关重要的作用,至今仍然是重要的战略战术应用中首选的材料体系。首先分析了针对新一代红外探测器所提出的SWaP3(Size,Weight,and Power,Performance and Price)概念,然后简略介绍了第三代红外焦平面研究背景下HgCdTe薄膜的衬底水平与材料生长情况,最后总结了大规模阵列器件、甚长波红外器件、高工作温度(High Operating Temperature,HOT)器件、超光谱探测器件、双色器件以及雪崩光电器件等前沿技术方面的研究进展。     

Effect of dislocations on performance of LWIR HgCdTe photodiodes

The epitaxial growth of HgCdTe on alternative substrates has emerged as an enabling technology for the fabrication of large-area infrared (IR) focal plane arrays (FPAs). One key technical issue is high dislocation densities in HgCdTe epilayers grown on alternative substrates. This is particularly important with regards to the growth of HgCdTe on heteroepitaxial Si-based substrates, which have a higher dislocation density than the bulk CdZnTe substrates typically used for epitaxial HgCdTe material growth. In the paper a simple model of dislocations as cylindrical regions confined by surfaces with definite surface recombination is proposed. Both radius of dislocations and its surface recombination velocity are determined by comparison of theoretical predictions with carrier lifetime experimental data described by other authors. It is observed that the carrier lifetime depends strongly on recombination velocity; whereas the dependence of the carrier lifetime on dislocation core radius is weaker. The minority carrier lifetime is approximately inversely proportional to the dislocation density for densities higher than 10⁵ cm⁻². Below this value, the minority carrier lifetime does not change with dislocation density. The influence of dislocation density on the R₀A product of long wavelength infrared (LWIR) HgCdTe photodiodes is also discussed. It is also shown that parameters of dislocations have a strong effect on the R₀A product at temperature around 77 K in the range of dislocation density above 10⁶ cm⁻². The quantum efficiency is not a strong function of dislocation density.     

MBE growth of HgCdTe on silicon substrates for large-area infrared focal plane arrays: A review of recent progress

We review the rapid progress that has been made during the past three years in the heteroepitaxial growth of HgCdTe infrared detector device structures on Si substrates by molecular-beam epitaxy. The evolution of this technology has enabled the fabrication of high performance, large-area HgCdTe infrared focal-plane arrays on Si substrates. A key element of this heteroepitaxial approach has been development of high quality CdTe buffer layers deposited on Si(112) substrates. We review the solutions developed by several groups to address the difficulties associated with the CdTe/Si(112) heteroepitaxial system, including control of crystallographic orientation and minimization of defects such as twins and threading dislocations. The material quality of HgCdTe/Si and the performance of HgCdTe detector structures grown on CdTe/Si(112) composite substrates is reviewed. Finally, we discuss some of the challenges related to composition uniformity and defect generation encountered with scaling the MBE growth process for HgCdTe to large-area Si substrates.     

Molecular beam epitaxy grown long wavelength infrared HgCdTe on Si detector performance

The use of silicon as a substrate alternative to bulk CdZnTe for epitaxial growth of HgCdTe for infrared (IR) 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 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. We have fabricated a series of long wavelength infrared (LWIR) HgCdTe diodes and several LWIR focal plane arrays (FPAs) with HgCdTe grown on silicon substrates using MBE grown CdTe and CdSeTe buffer layers. The detector arrays were fabricated using Rockwell Scientific’s planar diode architecture. The diode and FPA and results at 78 K will be discussed in terms of the high dislocation density (∼5×10⁶ cm²) typically measured when HgCdTe is grown on silicon substrates.     

CdZnTe substrate producibility and its impact on IRFPA yield

The need for cost effective production of HgCdTe infrared detectors and focal plane assemblies has led to increased attention to the availability of high quality large-area CdZnTe substrates. Reasonable yield of large-area substrates (≥4 cm×6 cm format) is necessary for fabrication of focal plane assemblies (FPAs) now in production, and for future infrared (IR) detectors which are growing in size and complexity. Raytheon’s infrared materials producibility (IRMP) program has addressed this issue, after identifying critical drivers of FPA yield coming from substrates, and targeted certain improvements in substrate process steps for highest impact on large-area substrate yield. Three specific areas of improvements in the substrate process were addressed: (1) compounding of a large 6 kg charge of CdTe; (2) vertical Bridgman growth of 92 mm diameter CdZnTe boules in both quartz and pyrolytic boron nitride (PBN) crucibles; and (3) optimized Cd overpressure control during growth and cool-down of the boule. It was shown that the Cd overpressure and the cooling schedule had the strongest effects on defect populations. The resulting improvements include a 33% increase in wafer yield per unit starting weight, an estimated 50% reduction in substrate cost per cm², better morphology of epitaxial HgCdTe layers, and improved yield of satisfactory IR detectors. The criteria for selecting substrates have also improved as a result of this work. In addition, photovoltaic detectors were fabricated on wafers from a variety of sources, and tested. Results compare favorably with those on baseline (earlier process) substrates.     

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.     

MBE Growth of HgCdTe on Large-Area Si and CdZnTe Wafers for SWIR,MWIR and LWIR Detection

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. Received December 7, 2007; accepted February 25, 2008     

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.     

32×32甚长波红外HgCdTe焦平面器件

甚长波红外波段富含大气湿度、CO2含量及云层结构和温度轮廓等大量信息,是大气遥感的重要组成部分。设计了一种3232甚长波红外焦平面阵列,采用在ZnCdTe衬底上液相外延生长的As掺杂p型材料上进行B+离子注入形成光敏元,通过铟柱倒焊技术和带有改进型背景抑制结构的读出电路互联,制成截止波长达到14 m的焦平面器件。该红外焦平面器件像元面积为60 m60 m,工作温度在50 K温度下。测试结果显示:读出电路性能良好,焦平面黑体响应率达到1。35107V/W,峰值探测率为2。571010 cmHz1/2/W,响应率非均匀性约为45%,盲元率小于12%。     

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.     

国外长波红外焦平面列阵现状

文中介绍了长波工外焦平面列阵的国外进展，这些列阵包括碲镉汞光电二极管、肖特基势垒器件、砷化镓／砷铝镓多量子阱光导体、高温超导体以及室温工作的焦平面列阵，其中非制冷的混合式热释电陶瓷和单片式的微型测辐射热计焦平面列阵已进入批理生产阶段。     

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.     

Performance Comparison of Long-Wavelength Infrared Type II Superlattice Devices with HgCdTe

The InAs/GaSb family of type II superlattices (T2SL) is the only known infrared (IR) detector material having a theoretically predicted higher performance than HgCdTe. The Auger lifetime has been predicted to be much longer, offering the possibility of much lower dark currents. In this paper the present state of the technology for long-wavelength infrared (LWIR) applications is evaluated by examining the dark current density in LWIR T2SL diodes at 78 K as a function of device cutoff wavelength, and comparing it with the HgCdTe benchmark known as Rule 07. The dark current density remains greater than Rule 07, but it has rapidly decreased in recent years with advancing technology, particularly due to innovative barrier structures.     

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.     

Processing of LPE-Grown HgCdTe for MWIR Devices Designed for High Operating Temperatures

Significant improvements of HgCdTe (MCT) detectors for the midwave infrared (MWIR) region with cutoff wavelength of about 5.2 μm at 77 K have been achieved. Optimizing the CdTe passivation proved to be a decisive step towards higher operating temperatures. The optimization was done by refining the interdiffusion process of the CdTe passivation layer with the liquid phase epitaxy-grown layer. The dark current density was reduced almost to the level of Rule 07, a common infrared detector benchmark. Additionally, improving the passivation process also decreased tunneling. These advancements also showed up in the focal-plane array (FPA) performance. A considerable reduction of the noise-equivalent temperature difference at temperatures above 130 K was attained. Based on these preliminary results, an operating temperature for these devices of more than 160 K is expected. Additionally, infrared (IR) pictures taken with a MWIR MCT-based FPA processed with the previous, slightly improved technology are presented. It is shown that good picture quality is attained at operating temperature of 140 K while retaining operability of 99.61%.