Material considerations for third generation infrared photon detectors

In the paper, issues associated with the development and exploitation of materials used in fabrication of third generation infrared photon detectors are discussed. In this class of detectors two main competitors, HgCdTe photodiodes and quantum well photoconductors are considered. The performance figures of merit of state-of-the-art HgCdTe and QWIP focal plane arrays (FPAs) are similar because the main limitations come from the readout circuits. The metallurgical issues of the epitaxial layers such as uniformity and number of defected elements are the serious problems in the case of long wavelength infrared (LWIR) and very LWIR (VLWIR) HgCdTe FPAs. It is predicted that superlattice based InAs/GaInSb system grown on GaSb substrate seems to be an alternative to HgCdTe with good spatial uniformity and an ability to span cutoff wavelength from 3 to 25 μm. In this context the material properties of type II superlattices are considered more in detail.     

Assessment of HgCdTe photodiodes and quantum well infrared photoconductors for long wavelength focal plane arrays

Recent trends in infrared detectors are towards large, electronically addressed two-dimensional arrays. In the long wavelength infrared (LWIR) spectral range HgCdTe focal plane arrays (FPAs) occupy a dominant position. However, the slow progress in the development of large LWIR photovoltaic HgCdTe infrared imaging arrays and the rapid achievements of novel semiconductor heterostructure systems have made it necessary to foresee the future development of different material technologies in fabrication large FPAs. Among the competing technologies in LWIR are the quantum well infrared photoconductors (QWIPs) based on lattice matched GaAs/AlGaAs and strained layer InGaAs/AlGaAs material systems. This paper compares the technical merits of two IR detector arrays technologies; photovoltaic HgCdTe and QWIPs. It is clearly shown that LWIR QWIP cannot compete with HgCdTe photodiode as the single device especially at higher temperature operation (>70 K) due to fundamental limitations associated with intersubband transitions. However, the advantage of HgCdTe is less distinct in temperature range below 50 K due to problems involved in HgCdTe material (p-type doping, Shockley–Read recombination, trap-assisted tunnelling, surface and interface instabilities). Even though the QWIP is a photoconductor, several of its properties such as high impedance, fast response time, long integration time, and low power consumption, well satisfy the requirements of fabrication of large FPAs. Due to the high material quality at low temperature, QWIP has potential advantages over HgCdTe for very LWIR (VLWIR) FPA applications in terms of the array size, uniformity, yield and cost of the systems.     

New material systems for third generation infrared photodetectors

Third-generation infrared (IR) systems are being developed nowadays. In the common understanding, these systems provide enhanced capabilities-like larger numbers of pixels, higher frame rates, and better thermal resolution as well as multicolour functionality and other on-chip functions. In this class of detectors, two main competitors, HgCdTe photodiodes and quantum-well photoconductors, have being developed. Recently, two new material systems have been emerged as the candidates for third generation IR detectors, type II InAs/GaInSb strain layer superlattices (SLSs) and quantum dot IR photodetectors (QDIPs). In the paper, issue associated with the development and exploitation of multispectral photodetectors from these new materials is discussed. Discussions is focused on most recently on-going detector technology efforts in fabrication both photodetectors and focal plane arrays (FPAs). The challenges facing multicolour devices concerning complicated device structures, multilayer material growth, and device fabrication are described.     

Exploring novel methods to achieve sensitivity limits for high operating temperature infrared detectors

A review of high operating temperature (HOT) infrared (IR) photon detector technology vis-a-vis material requirements, device design and state of the art achieved is presented in this article. The HOT photon detector concept offers the promise of operation at temperatures above 120 K to near room temperature. Advantages are reduction in system size, weight, cost and increase in system reliability. A theoretical study of the thermal generation–recombination (g–r) processes such as Auger and defect related Shockley Read Hall (SRH) recombination responsible for increasing dark current in HgCdTe detectors is presented. Results of theoretical analysis are used to evaluate performance of long wavelength (LW) and mid wavelength (MW) IR detectors at high operating temperatures.     

QWIP or other alternative for third generation infrared systems

Choosing the right detector technology for third generation thermal imaging systems is directly derived from the requirements of these new generation infrared imaging systems.

It is now evident that third generation thermal imager will still need the higher resolution capabilities as well as capabilities in multispectral detection and polarization sensitivity. Four technologies candidates are analyzed; the field-proved HgCdTe (MCT), uncooled microbolometer technology, antimonide based materials and quantum well infrared photodetectors (QWIP). Taking into account the risks, maturity and technologies barrier of each technology, we claim that for non-strategic applications (not low background conditions), QWIP technology is the most favorite. The ternary and superlattice antimonide based materials group seems to be theoretically the best alternative, but are not recommended due to its immaturity and the high risk involved in this technology (passivation, doping control, etc.). We anticipate large penetration of the uncooled detectors to the low-end and medium-end market. The HgCdTe will still be in progress due to the inertia of the large funding and the strategic importance of this detectors technology.     

Detector and readout performance goals for quantum well and strained layer superlattice focal plane arrays imaging under tactical and strategic backgrounds

Advancements in III–V semiconductor based, Quantum-well infrared photodetector (QWIP) and Type-II Strained-Layer Superlattice detector (T2SLS) technologies have yielded highly uniform, large-format long-wavelength infrared (LWIR) QWIP FPAs and high quantum efficiency (QE), small format, LWIR T2SLS FPAs. In this article, we have analyzed the QWIP and T2SLS detector level performance requirements and readout integrated circuit (ROIC) noise levels for several staring array long-wavelength infrared (LWIR) imaging applications at various background levels. As a result of lower absorption QE and less than unity photoconductive gain, QWIP FPAs are appropriate for high background tactical applications. However, if the application restricts the integration time, QWIP FPA performance may be limited by the read noise of the ROIC. Rapid progress in T2SLS detector material has already demonstrated LWIR detectors with sufficient performance for tactical applications and potential for strategic applications. However, significant research is needed to suppress surface leakage currents in order to reproduce performances at pixel levels of T2SLS FPAs.     

Heterostructure infrared photovoltaic detectors

HgCdTe remains the most important material for infrared (IR) photodetectors despite numerous attempts to replace it with alternative materials such as closely related mercury alloys (HgZnTe, HgMnTe), Schottky barriers on silicon, SiGe heterojunctions, GaAs/AlGaAs multiple quantum wells, InAs/GaInSb strained layer superlattices, high temperature superconductors and especially two types of thermal detectors: pyroelectric detectors and silicon bolometers. It is interesting, however, that none of these competitors can compete in terms of fundamental properties. In addition, HgCdTe exhibits nearly constant lattice parameter which is of extreme importance for new devices based on complex heterostructures. The development of sophisticated controllable vapour phase epitaxial growth methods, such as MBE and MOCVD, has allowed fabrication of almost ideally designed heterojunction photodiodes. In this paper, examples of novel devices based on heterostructures operating in the long wavelength, middle wavelength and short wavelength spectral ranges are presented. Recently, more interest has been focused on p–n junction heterostructures. As infrared technology continues to advance, there is a growing demand for multispectral detectors for advanced IR systems with better target discrimination and identification. HgCdTe heterojunction detectors offer wavelength flexibility from medium wavelength to very long wavelength and multicolour capability in these regions. Recent progress in two-colour HgCdTe detectors is also reviewed.     

QWIP focal plane arrays on InP substrates for single and dual band thermal imagers

Alternative material systems on InP substrate provide certain advantages for mid-wavelength infrared (MWIR), long-wavelength infrared (LWIR) and dual band MWIR/LWIR quantum well infrared photodetector (QWIP) focal plane arrays (FPAs). While InP/InGaAs and InP/InGaAsP LWIR QWIPs provide much higher responsivity when compared to the AlGaAs/GaAs QWIPs, AlInAs/InGaAs system facilitates completely lattice matched single band MWIR and dual band MWIR/LWIR FPAs.We present an extensive review of the studies on InP based single and dual band QWIPs. While reviewing the characteristics of InP/InGaAs and InP/InGaAsP LWIR QWIPs at large format FPA level, we experimentally demonstrate that the cut-off wavelength of AlInAs/InGaAs QWIPs can be tuned in a sufficiently large range in the MWIR atmospheric window by only changing the quantum well (QW) width at the lattice matched composition. The cut-off wavelength can be shifted up to ～5.0 μm with a QW width of 22 Å in which case very broad spectral response (Δλ/λ_p = ～30%) and a reasonably high peak detectivity are achievable leading to a noise equivalent temperature difference as low as 14 mK (f/2) with 25 μm pitch in a 640 × 512 FPA. We also present the characteristics of InP based two-stack QWIPs with wavelengths properly tuned in the MWIR and LWIR bands for dual color detection. The results clearly demonstrate that InP based material systems display high potential for dual band MWIR/LWIR QWIP FPAs needed by third generation thermal imagers.     

Characterization of LWIR diodes on InAs/GaSb Type-II superlattice material

Long wavelength infrared (LWIR) focal plane arrays (FPAs) built on Type-II strained layer InAs/GaSb superlattice materials are emerging as an alternative to LWIR HgCdTe. We have made progress in the development of this technology in a collaborative effort between Raytheon Vision Systems and Jet Propulsion Laboratory, resulting in successful devices with LWIR cutoff wavelengths. We report here two investigations related to wafer processing and superlattice material characteristics. The critical interface between the superlattice and the silicon dioxide passivation was examined at the atomic scale by high resolution transmission electron microscopy (HRTEM), showing a conformal coating on an InAs/GaSb mesa sidewall, which undulates with the superlattice periodicity due to differential etching. Electron energy loss spectroscopy (EELS) showed that oxides of the superlattice elements were present but minimal, and some occasional arsenic precipitates were observed at the passivation interface. Our previous analysis of the current–voltage curves was extended further to reveal the minority carrier lifetimes responsible for producing the generation–recombination (GR) and the diffusion dark currents. Lifetimes at 78 K were found to be 6 and 20 ns in the GR and diffusion processes, respectively. Lifetimes from both mechanisms track together with temperature. A HgCdTe diode was analyzed in the same manner for comparison.     

Demonstration of 640 × 512 pixels long-wavelength infrared (LWIR) quantum dot infrared photodetector (QDIP) imaging focal plane array

We have exploited the artificial atom-like properties of epitaxially grown self-assembled quantum dots (QDs) for the development of high operating temperature long wavelength infrared (LWIR) focal plane arrays (FPAs). QD infrared photodetectors (QDIPs) are expected to outperform quantum well infrared detectors (QWIPs) and are expected to offer significant advantages over II–VI material based FPAs. We have used molecular beam epitaxy (MBE) technology to grow multi-layer LWIR dot-in-a-well (DWELL) structures based on the InAs/InGaAs/GaAs material system. This hybrid quantum dot/quantum well device offers additional control in wavelength tuning via control of dot-size and/or quantum well sizes. DWELL QDIPs were also experimentally shown to absorb both 45° and normally incident light. Thus we have employed a reflection grating structure to further enhance the quantum efficiency. The most recent devices exhibit peak responsivity out to 8.1 μm. Peak detectivity of the 8.1 μm devices has reached 1 × 10¹⁰ Jones at 77 K. Furthermore, we have fabricated the first long-wavelength 640 × 512 pixels QDIP imaging FPA. This QDIP FPA has produced excellent infrared imagery with noise equivalent temperature difference of 40 mK at 60 K operating temperature.     

HgCdTe avalanche photodiodes: A review

This paper presents a comprehensive review of fundamental issues, device architectures, technology development and applications of HgCdTe based avalanche photodiodes (APD). High gain, above 5×10³, a low excess noise factor close to unity, THz gain-bandwidth product, and fast response in the range of pico-seconds has been achieved by electron-initiated avalanche multiplication for SWIR, MWIR, and LWIR detector applications involving low optical signals. Detector arrays with good element-to-element uniformity have been fabricated paving the way for fabrication of HgCdTe-APD FPAs.     

QWIP detectors and thermal imagers

Standard GaAs/AlGaAs QWIPs (Quantum Well Infrared Photodetector) are now well established for long wave infrared (LWIR) detection. The main advantage of this technology is the duality with the technology of commercial GaAs devices. The realization of large FPAs (up to 640×480) drawing on the standard III–V technological process has already been demonstrated. The second advantage widely claimed for QWIPs is the so-called band-gap engineering, allowing the custom design of the quantum structure to fulfill the requirements of specific applications such as multispectral detection. QWIP technology has been growing up over the last ten years and now reaches an undeniable level of maturity. As with all quantum detectors, the thermal current, particularly in the LWIR range, limits the operating temperature of QWIPs. It is very crucial to achieve an operating temperature as high as possible and at least above 77 K in order to reduce volume and power consumption and to improve the reliability of the detection module. This thermal current offset has three detrimental effects: noise increase, storage capacitor saturation and high sensitivity of FPAs to fluctuations in operating temperature. For LWIR FPAs, large cryocoolers are required, which means volume and power consumption unsuitable for handheld systems. The understanding of detection mechanisms has led us to design and realize high performance ‘standard’ QWIPs working near 77 K. Furthermore, a new in situ skimmed architecture accommodating this offset has already been demonstrated. In this paper we summarize the contribution of THALES Research & Technology to this progress. We present the current status of QWIPs in France, including the latest performances achieved with both standard and skimmed architectures. We illustrate the potential of our QWIPs through features of Thales Optronique's products for third thermal imager generation. To cite this article: E. Costard et al., C. R. Physique 4 (2003).     

History of infrared detectors

This paper overviews the history of infrared detector materials starting with Herschel??s experiment with thermometer on February 11^th, 1800. Infrared detectors are in general used to detect, image, and measure patterns of the thermal heat radiation which all objects emit. At the beginning, their development was connected with thermal detectors, such as thermocouples and bolometers, which are still used today and which are generally sensitive to all infrared wavelengths and operate at room temperature. The second kind of detectors, called the photon detectors, was mainly developed during the 20^th Century to improve sensitivity and response time. These detectors have been extensively developed since the 1940??s. Lead sulphide (PbS) was the first practical IR detector with sensitivity to infrared wavelengths up to ??3 ??m. After World War II infrared detector technology development was and continues to be primarily driven by military applications. Discovery of variable band gap HgCdTe ternary alloy by Lawson and co-workers in 1959 opened a new area in IR detector technology and has provided an unprecedented degree of freedom in infrared detector design. Many of these advances were transferred to IR astronomy from Departments of Defence research. Later on civilian applications of infrared technology are frequently called ??dual-use technology applications.?? One should point out the growing utilisation of IR technologies in the civilian sphere based on the use of new materials and technologies, as well as the noticeable price decrease in these high cost technologies. In the last four decades different types of detectors are combined with electronic readouts to make detector focal plane arrays (FPAs). Development in FPA technology has revolutionized infrared imaging. Progress in integrated circuit design and fabrication techniques has resulted in continued rapid growth in the size and performance of these solid state arrays.     

Research topics at Thales Research and Technology: Small pixels and third generation applications

Recent results obtained on building blocks for future third generation infrared focal plane arrays (FPAs) are presented. Our approach concerning the FPA performance assessment and small pixels modelling is exposed. We also demonstrate the ability of the quantum well infrared photodetector technology to answer the needs for compact (20 μm pitch) polarimetric FPAs. Finally, we present our first results on mid-wave infrared detectors at wavelengths below 4.2 μm.     

Trade-offs and difficulties of the vertical photoconductor: a novel device structure suitable for HgCdTe two-dimensional infrared focal plane arrays

Recently Siliquini and Faraone [J.F. Siliquini, L. Faraone, Infrared Phys. Technol. 38 (1997) 205] have proposed vertical photoconductive device (PC) based two-dimensional long wavelength infrared region focal plane arrays (LWIR FPAs). In this note, we examine some trade-offs and difficulties of this proposed structure.     

HgCdTe technology in France

SOFRADIR is one of the leading companies worldwide for the production of second generation InfraRed (IR) detectors. This success is due to the top level quality of the unique and production oriented French HgCdTe technology for manufacturing IR focal plane arrays based on an HgCdTe array and a CMOS readout and multiplexed silicon array. This technology and main products are presented in this paper. Finally, in order to prepare for future military and industrial needs, SOFRADIR has been working in close relationship with CEA-LETI/LIR on third generation developments based on HgCdTe material using Molecular Beam Epitaxy (MBE) growth. To cite this article: P. Tribolet, C. R. Physique 4 (2003).     

Uncooled infrared photodetectors in Poland

The history and present status of the middle and long wavelength Hg_1-xCd_xTe infrared detectors in Poland are reviewed. Research and development efforts in Poland were concentrated mostly on uncooled market niche. Technology of the infrared photodetectors has been developed by several research groups. The devices are based on mercury-based variable band gap semiconductor alloys. Modified isothermal vapour phase epitaxy (ISOVPE) has been used for many years for research and commercial fabrication of photoconductive, photoelectromagnetic and other devices. Bulk growth and liquid phase epitaxy was also used. At present, the fabrication of IR devices relies on low temperature epitaxial technique, namely metalorganic vapour phase deposition (MOCVD), frequently in combination with the ISOVPE. Photoconductive and photoelectromagnetic detectors are still in production. The devices are gradually replaced with photovoltaic devices which offer inherent advantages of no electric or magnetic bias, no heat load and no flicker noise. Potentially, the PV devices could offer high performance and very fast response. At present, the uncooled long wavelength devices of conventional design suffer from two issues; namely low quantum efficiency and very low junction resistance. It makes them useless for practical applications. The problems have been solved with advanced 3D band gap engineered architecture, multiple cell heterojunction devices connected in series, monolithic integration of the detectors with microoptics and other improvements. Present fabrication program includes devices which are optimized for operation at any wavelength within a wide spectral range 1–15 μm and 200–300 K temperature range. Special solutions have been applied to improve speed of response. Some devices show picoseconds range response time. The devices have found numerous civilian and military applications. The paper presented there appears in Infrared Photoelectronics, edited by Antoni Rogalski, Eustace L. Dereniak, Fiodor F. Sizov, Proc. SPIE Vol. 5957, 59570K (2005).     

Large format voltage tunable dual-band QWIP FPAs

Third generation thermal imagers with dual/multi-band operation capability are the prominent focus of the current research in the field of infrared detection. Dual band quantum-well infrared photodetector (QWIP) focal plane arrays (FPAs) based on various detection and fabrication approaches have been reported. One of these approaches is the three-contact design allowing simultaneous integration of the signals in both bands. However, this approach requires three In bumps on each pixel leading to a complicated fabrication process and lower fill factor.If the spectral response of a two-stack QWIP structure can effectively be shifted between two spectral bands with the applied bias, dual band sensors can be implemented with the conventional FPA fabrication process requiring only one In bump on each pixel making it possible to fabricate large format dual band FPAs at the cost and yield of single band detectors. While some disadvantages of this technique have been discussed in the literature, the detailed assessment of this approach has not been performed at the FPA level yet. We report the characteristics of a large format (640 × 512) voltage tunable dual-band QWIP FPA constructed through series connection of MWIR AlGaAs–InGaAs and LWIR AlGaAs–GaAs multi-quantum well stacks, and provide a detailed assessment of the potential of this approach at both pixel and FPA levels. The dual band FPA having MWIR and LWIR cut-off wavelengths of 5.1 and 8.9 μm provided noise equivalent temperature differences as low as 14 and 31 mK (f/1.5) with switching voltages within the limits applicable by commercial read-out integrated circuits. The results demonstrate the promise of the approach for achieving large format low cost dual band FPAs.     

Demonstration of large format mid-wavelength infrared focal plane arrays based on superlattice and BIRD detector structures

We have demonstrated the use of bulk antimonide based materials and type-II antimonide based superlattices in the development of large area mid-wavelength infrared (MWIR) focal plane arrays (FPAs). Barrier infrared photodetectors (BIRDs) and superlattice-based infrared photodetectors are expected to outperform traditional III–V MWIR and LWIR imaging technologies and are expected to offer significant advantages over II–VI material based FPAs. We have used molecular beam epitaxy (MBE) technology to grow InAs/GaSb superlattice pin photodiodes and bulk InAsSb structures on GaSb substrates. The coupled quantum well superlattice device offers additional control in wavelength tuning via quantum well sizes and interface composition, while the BIRD structure allows for device fabrication without additional passivation. As a demonstration of the large area imaging capabilities of this technology, we have fabricated mid-wavelength 1024 × 1024 pixels superlattice imaging FPAs and 640 × 512 MWIR arrays based on the BIRD concept. These initial FPA have produced excellent infrared imagery.     

NIR, MWIR and LWIR quantum well infrared photodetector using interband and intersubband transitions

This paper presents the design, fabrication and characterization of a QWIP photodetector capable of detecting simultaneously infrared radiation within near infrared (NIR), mid wavelength infrared (MWIR) and long wavelength infrared (LWIR). The NIR detection was achieved using interband transition while MWIR and LWIR were based on intersubband transition in the conduction band. The quantum well structure was designed using a computational tool developed to solve self-consistently the Schrödinger–Poisson equation with the help of the shooting method. Intersubband absorption in the sample was measured for the MWIR and LWIR using Fourier transform spectroscopy (FTIR) and the measured peak positions were found at 5.3 μm and 8.7 μm which agree well with the theoretical values obtained 5.0 μm and 9.0 μm for the two infrared bands which indicates the accuracy of the self-consistent model. The photodetectors were fabricated using a standard photolithography process with exposed middle contacts to allow separate bias and readout of signals from the three wavelength bands. The measured photoresponse gave three peaks at 0.84 μm, 5.0 μm and 8.5 μm wavelengths with approximately 0.5 A/W, 0.03 A/W and 0.13 A/W peak responsivities for NIR, MWIR and LWIR bands, respectively. This work demonstrates the possibility of detection of widely separated wavelength bands using interband and intersubband transitions in quantum wells.