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Infrared detectors: status and trends

This article reviews the history, the present status and possible future developments of HgCdTe ternary alloy for infrared (IR) detector applications. HgCdTe IR detectors have been intensively developed since the first synthesis of this material in 1958. This article summarizes the fundamental properties of this versatile narrow gap semiconductor, and relates the material properties to its successful applications as an IR photoconductive and photovoltaic detector material. An emphasis is put on key developments in the crystal growth and their influence on device evolution. Competitive technologies to HgCdTe ternary alloy are also presented. Recent advances of backside illuminated HgCdTe heterojunction photodiodes have enabled a third generation of multispectral instruments for remote sensing applications and have led to the practicality of multiband IR focal plane array technology. Finally, evaluation of HgCdTe for room temperature long wavelength IR applications is presented. (Some figures in this article are in colour only in the electronic version)

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HgCdTe infrared detector material: history, status and outlook

Quantum-well photodetectors fabricated from photonic metamaterials show enhanced room-temperature sensitivity to long-wavelength infrared radiation and produce gigahertz-frequency heterodyne signals when pumped with quantum cascade lasers. Current technologies for detecting infrared radiation in the long-wavelength regime (8–12 micrometres) do not lend themselves to low-cost, compact implementations—usually because of the need for low-temperature operation—which limits their breadth of applicability. Daniele Palaferri et al. show how photonic metamaterial ideas can be combined with quantum-well infrared photodetectors to enhance room-temperature sensitivity to the level that potentially opens up a range of applications, from thermal imaging and environmental remote sensing to laser-based free-space communication. Room-temperature operation is essential for any optoelectronics technology that aims to provide low-cost, compact systems for widespread applications. A recent technological advance in this direction is bolometric detection for thermal imaging1, which has achieved relatively high sensitivity and video rates (about 60 hertz) at room temperature. However, owing to thermally induced dark current, room-temperature operation is still a great challenge for semiconductor photodetectors targeting the wavelength band between 8 and 12 micrometres2, and all relevant applications, such as imaging, environmental remote sensing and laser-based free-space communication3,4,5, have been realized at low temperatures. For these devices, high sensitivity and high speed have never been compatible with high-temperature operation6,7. Here we show that a long-wavelength (nine micrometres) infrared quantum-well photodetector8 fabricated from a metamaterial made of sub-wavelength metallic resonators9,10,11,12 exhibits strongly enhanced performance with respect to the state of the art up to room temperature. This occurs because the photonic collection area of each resonator is much larger than its electrical area, thus substantially reducing the dark current of the device13. Furthermore, we show that our photonic architecture overcomes intrinsic limitations of the material, such as the drop of the electronic drift velocity with temperature14,15, which constrains conventional geometries at cryogenic operation6. Finally, the reduced physical area of the device and its increased responsivity allow us to take advantage of the intrinsic high-frequency response of the quantum detector7 at room temperature. By mixing the frequencies of two quantum-cascade lasers16 on the detector, which acts as a heterodyne receiver, we have measured a high-frequency signal, above four gigahertz (GHz). Therefore, these wide-band uncooled detectors could benefit technologies such as high-speed (gigabits per second) multichannel coherent data transfer17 and high-precision molecular spectroscopy18.

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Room-temperature nine-µm-wavelength photodetectors and GHz-frequency heterodyne receivers

The authors propose to exploit the unique properties of surface plasmons to enhance the signal-to-noise ratio of midinfrared photodetectors. The proposed photodetector consists of a slit in a metallic slab filled with absorptive semiconductor material. Light absorption in the slit is enhanced due to Fabry-Perot resonances. Further absorption enhancement is achieved by surrounding the slit with a series of periodic grooves that enable the excitation of surface plasmons that carry electromagnetic energy towards the slit. Using this scheme, they design and optimize a photodetector operating at 0=9.8 m with a roughly 250 times enhancement in the absorption per unit of volume of semiconductor material compared to conventional photodetectors operating at the same wavelength. © 2006 American Institute of Physics. DOI: 10.1063/1.2360896 Midinfrared photodetectors and imaging systems operating in the vicinity of 10 m wavelength are important in applications ranging from night vision to astronomy research. 1 To suppress noise caused by thermal fluctuation, usually these detection systems have to be cooled, 1‐3 which greatly increases the cost and limits the portability. To improve signal/noise ratio, it is desirable to reduce the active detector volume from which the noise arises without affecting the absorbed light power. In this letter, we computationally explore the use of surface plasmons in these systems to improve the signal/noise ratio. The proposed structure, shown in Fig. 1a, consists of a metallic slit, surrounded by a linear grating structure. The entire structure is placed on top of an insulating oxide. The slit is filled with absorptive semiconductor material, and has a width that is far smaller than the operating wavelength. The metal regions serve both as electrical contacts and as a concentrator that funnels light into the deep-subwavelength slit. 4‐6 To maximize the concentration effects, we combine two distinct mechanisms that relate to the presence of the slit and the grating structure. 7 A metal slit supports a propagating TE mode, even when the width of the slit is at deepsubwavelength scales. Moreover, due to the strong impedance mismatch between the modes in the slit and free-space propagating waves, strong reflection can occur at the entrances to the slit. Consequently, with an appropriate choice of the length, the slit forms a Fabry-Perot resonator, and light absorption in the slit is resonantly enhanced. The grating, on the other hand, enhances the light absorption in the slit by converting incident electromagnetic EM waves into surface plasmons on the metal surface that can be funneled into the slit. 8 The area from which EM energy is collected can be substantially larger than the slit width and gives rise to significant enhancements in the absorbed energy. With a careful choice of the geometric parameters of the structure, these two mechanisms can be made to occur at the same wavelength, resulting in an optimized efficiency of the devices. To simulate the response of this structure for a normally incident EM plane wave, we use a two-dimensional finitedifference frequency-domain method, 9,10 which directly uses experimental data for the frequency-dependent dielectric constant of metals such as gold, 11 including both the real and imaginary parts, with no further approximation. For the detective materials in the slit, we have chosen to use HgCdTe MCT with a frequency independent dielectric permittivity of =12.5+i1.75. 2,11 The substrate is a low-index oxide

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M. Grudzien

Papers

Near room-temperature IR photo-detectors

Composition and thickness control of CdxHg1-xTe layers grown by open tube isothermal vapour phase epitaxy

High capability, quasi-closed growth system for isothermal vapour phase epitaxy of (Hg, Cd)Te