Quantum-dot infrared photodetectors: Status and outlook

Novel InAs/InGaAs quantum dots-in-a-well (DWELL) infrared photodetectors are reviewed. These detectors, in which the active region consists of InAs quantum dots (QDs) embedded in an InGaAs quantum well, represent a hybrid between a conventional quantum well infrared photodetector (QWIP) and a QD infrared photodetector (QDIP). Like QDIPs, DWELL detectors display normal incidence operation without gratings or optocouplers while demonstrating reproducible ‘dial-in recipes’ for control over the operating wavelength, like QWIPs. Using femtosecond spectroscopy, long carrier lifetimes have been observed in DWELL heterostructures, suggesting their potential for high temperature operation. Moreover, DWELL detectors have also demonstrated bias-tunability and multicolour operation in the mid-wave infrared (3–5 µm), long-wave infrared (LWIR, 8–12 µm) and very long-wave infrared (>14 µm) regimes. We have recently developed LWIR 320 × 256 focal plane arrays operating at liquid nitrogen temperatures. One of the potential problems with these detectors is the low quantum efficiency, which translates into low responsivity and detectivity. Some solutions for mitigating these problems are suggested at the end of this paper.

Human knowledge of infrared (IR) radiation is about 200 years old. However it was in the late 20th century that we developed a wide range of smart technologies for detection and started to take advantage for our benefit. Today IR detector technology is in its 3rd generation and comes with challenging demands. Based on the propagation of IR radiation through free space it is divided into different transmission windows. The most interesting for thermal imaging are the mid-wave IR (MWIR) and the long-wave IR (LW IR). Infrared detectors for thermal imaging have a number of applications in industry, security, search & rescue, surveillance, medicine, research, meteorology, climatology and astronomy. Currently high-performance IR imaging technology is mainly based on epitaxially grown structures of the small-bandgap bulk alloy mercury-cadmium-telluride (MCT), indium antimonide (InSb) and GaAs based quantum-well infrared photodetectors (QWIPs), depending on the application and wavelength range. However, they operate at low temperatures requiring costly and bulky cryogenic systems. In addition there is always a need for better performance, which generates possibilities for developing new technologies. Some emerging technologies are quantum dot infrared photodetectors (QDIPs), type-II strained layer super-lattice, and QDIPs with type-II band alignment. In this report a brief review of the current and new technologies for high performance IR detectors, will be presented.

Quantum dot infrared photodetectors (QDIPs) have attracted significant attention due to selective photoresponse, high photoconductive gain, and numerous possibilities for nanoscale engineering of photoelectron processes, which control the detector characteristics. Our approach to improving QDIP performance is based on optimization of three dimensional nanoscale potential profile created by charged quantum dots (QDs). Nanoscale profile around QDs allows us to control photoelectron capture processes, which determines the photoelectron lifetime, detector operating speed, responsivity, the spectral density of noise, noise bandwidth, and the detector dynamic range. The nanoscale potential profile is determined by doping of QDs and inter-dot space. In this work, we study various ways of selective doping and its effects on characteristics of photodetectors. We investigate and compare intra-dot doping, inter-dot doping, and complex bipolar doping. To investigate effects of selective doping, we fabricated AlGaAs/InAs QD structures with n-doping of QD layers, structures with n-doping of barriers, and structures with p-doping of QD layers and n-doping of interdot space. We measured dark current, spectral photoresponse, voltage dependence of responsivity, and noise characteristic. The photoresponse is improved due to photon-electron coupling, which increases with QD filling by electrons. However, the noise current also increases due to increase in QD filling. Therefore, possibilities for improvement of QDIP structures with unipolar doping are very limited. Our results show that spectral photoresponse, responsivity, and detector sensitivity are substantially improved due to bipolar doping, which provides decoupled control of electron filling of QDs and the potential barriers around QDs.

Characteristics analysis of dark current in quantum dot infrared photodetectors

: This is the Final Report of ARO Grant No. DAADl9-02-l-0437. It is a report on the characteristics of(InGa)As/GaAs long-wave (8-12 um) quantum-dot infrared photodetector structures. Pcak responsivities and dctectivitics were measured using a calibrated black body source. The results of this work indicate that for the structures characterized here, the maximum operating temperature is limited to %O5 K. No effort was made in the original design of the device structure for higher temperature operation. For operation at temperatures higher than this, one must investigate other device structures; such structures have been identified and will be the subject of future work. The work reported here has paved the way to the identification of candidate device structures for high operating temperature (HOT) quantum-dot detector

The performance of quantum well infrared photodetectors (QWIPs) as compared to other types of semiconductor infrared (IR) detectors is examined utilizing the α/G ratio, where α is the absorption coefficient and G is the thermal generation. From the discussion results that long wavelength 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 because of the problems involved in an HgCdTe material (p-type doping, Shockley-Read recombination, trap-assisted tunneling, surface and interface instabilities). Due to the limited charge storage capacity of available readout circuits, a small photoconductive gain of QWIPs improves the noise equivalent difference temperature the sensor. The imaging performance of HgCdTe photodiodes and QWIP focal plane arrays can be said to be nearly equivalent when they are operated under their optimum conditions. Decision of the best technology is therefore driven by the specific needs of a system.

Mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) 1024 × 1024 pixel quantum well infrared photodetector (QWIP) focal planes have been demonstrated with excellent imaging performance. The MWIR QWIP detector array has demonstrated a noise equivalent differential temperature (NEΔT) of 17 mK at a 95 K operating temperature with f/2.5 optics at 300 K background and the LWIR detector array has demonstrated a NEΔT of 13 mK at a 70 K operating temperature with the same optical and background conditions as the MWIR detector array after the subtraction of system noise. Both MWIR and LWIR focal planes have shown background limited performance (BLIP) at 90 K and 70 K operating temperatures respectively, with similar optical and background conditions. In this paper, we will discuss the performance in terms of quantum efficiency, NEΔT, uniformity, operability and modulation transfer functions.

Mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) 1024x1024 pixel InGaAs/GaAs/AlGaAs based quantum well infrared photodetector (QWIP) focal planes have been demonstrated with excellent imaging performance. The MWIR QWIP detector array has demonstrated a noise equivalent differential temperature (NEΔT) of 17 mK at a 95K operating temperature with f/2.5 optics at 300K background and the LWIR detector array has demonstrated a NEΔT of 13 mK at a 70K operating temperature with the same optical and background conditions as the MWIR detector array after the subtraction of system noise. Both MWIR and LWIR focal planes have shown background limited performance (BLIP) at 90K and 70K operating temperatures respectively, with similar optical and background conditions. It is well known that III-V compound semiconductor materials such as GaAs, InP, etc. are easy to grow and process into devices. In addition, III-V compound semiconductors are available in large diameter wafers, up to 8-inches. Thus, III-V compound semiconductor based infrared focal plane technologies such as QWIP, InSb, and strain layer superlattices (SLS) are potential candidates for the development of large format focal planes such as 4096x4096 pixels and larger. In this paper, we will discuss the possibility of extending the infrared detector array size up to 16 megapixels.

For the majority of applications involving detection of weak signals or thermal imaging, the quantum well infrared photodetector (QWIP) is designed to have the highest possible detectivity and operating temperature. The device parameters, such as the doping density, are chosen accordingly. In a different direction, the intrinsic short carrier lifetime (~ 5 ps) makes QWIPs well suited for high speed and high frequency applications. In such cases, since lasers are normally used, a high dark current can be tolerated. The most important parameter is then the absorption efficiency. For system simplicity and potential wide use, room temperature or near room temperature (reachable by thermo-electric cooling) operations are desirable. This paper discusses the QWIP design for high absorption and elevated temperature operation, and present a systematic experimental study on a set of GaAs/AlGaAs QWIPs with different doping densities. High absorption (~ 100%) and up to room temperature operation are achieved in devices having high doping densities and 100 quantum wells.

The GaAs/AlGaAs based quantum well IR photodetectors (QWIPs) afford greater flexibility than the usual extrinsically doped semiconductor IR detectors because the wavelength of the peak response and cutoff can be continuously tailored over any wavelength between 6-20 micrometers . The spectral band width of these detectors can be tuned from narrow to wide allowing various applications. Also, QWIP offers multi-color IR cameras which is capable of simultaneously acquiring images in different IR bands. Each pixel of such array consists of vertically stacked, independently readable, QWIP detectors sensitive in different narrow IR bands. In this article, we discuss the result of a 10-16 micrometers large format broadband QWIP focal plane array IR bands. In this article, we discuss the results of a 10-16 micrometers large format broadband QWIP focal plane array (FPA). The size of the FPA is 640 X 512 and its pixel pitch is 25 microns. The highest operating temperature of the FPA is 45K, and it was determined by the charge storage capacity and the other features of the particular readout multiplexer used in this demonstration. Excellent imagery, with a noise equivalent differential temperature of 55 mK has been achieved. In addition, we will discus the developments and results of the 640 X 512 dual-band QWIP FPA.

Control of size and distribution of silicon quantum dots in silicon dielectrics for solar cell application: A review

N-on-p extrinsically doped MWIR HgCdTe material and photodiodes have been developed to benefit from the expected reduction of the Auger generation in the p-type absorbing layer. Samples with two doping levels have been characterized using dark current, current noise, Hall effect and PhotoLuminescence Decay (PLD) measurements. The dark current and PLD measurements are consistent with a reduction of the Auger generation quantified by the ratio between the Auger 1 and 7 recombination coefficients 𝛾 around 10. The corresponding dark current in the sample with the lowest doping level was slightly higher than in typically p-on-n photodiodes. The low frequency noise, characterized by a Tobin coefficient below 10-5, is lower than the values reported for other MWIR HgCdTe photodiodes at the same dark current density. The low dark current and dark current noise show on the high potential of such photodiodes to form focal plane array that can be operated at high operating temperature without degradation of the image quality.

Astronomical imaging with quantum well infrared photodetectors

The maturation of the III-V materials technology has provided an opportunity for the development of a producible and affordable class of IR detector arrays. Designs based on the GaAs compounds permit the realization of multiple quantum well IR photodetectors (QWIPs) which are useful for long wavelength focal plane arrays with sizes demonstrated up to 640 X 480. Similar designs using InP based materials can cover an even broader IR spectral region with lattice matched structures. QWIP demonstrations have been made for midwave detectors and very long wave detectors as well. New detector structures that improve optical performance and reduce bias current can lead to higher performance QWIPs which approach the performance of mercury cadmium telluride at moderate operating temperatures. These developments offer the possibility of practical, large, affordable IR focal plane arrays in the near future.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The performance of AlGaAs/GaAs quantum well infrared photodetectors (QWIPs) with and without NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> plasma treatment are investigated. It is demonstrated that the NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> plasma treatment not only gets rid of the oxide defects, such as Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , and As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> , but also prevents the formation of oxides on the GaAs surface when exposed to atmosphere for one month. The peak responsivity of the QWIPs without NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> plasma treatment is 0.48 A/W, and the detectivity is 3.13×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> cm-Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> /W at 1.5 V and 60 K. However, the QWIP after the 10-m NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> plasma treatment exhibits a better performance. The highest operation temperature can be increased from 60 to 90 K. At 90 K, the peak responsivity of the QWIP is 1.25 A/W and the detectivity is 3.54×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> cm-Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> /W at 1.5 V.