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PbTe IRFPAs for low-cost thermal imaging (Infrared Imaging)

Lead chalcogenides offer a low-cost alternative to II-VI and III-V materials for the fabrication of infrared focal plane arrays. Karim Alchalabi, Dmitri Zimin and Hans Zogg report on how they have monolithically integrated these with silicon read-out circuitry.
Up to now, the most sensitive infrared focal plane arrays (or IRFPAs) for thermal imaging were fabricated in a hybrid manner. A chip fabricated with the infrared sensitive material, in most cases a narrow-gap semiconductor (NGS) such as InSb or HgCdTe, or a quantum-well infrared photodetector (QWIP), contains the individual sensor pixels. This sensor chip is connected to a silicon read-out chip with integrated signal processing and multiplexing electronics. Typical arrays comprise 128 128 or more pixels, and electrical connections for each of the individual sensors are made with extremely small indium bumps. This bump bonding technique is now well established, but rather complicated and expensive; see for example Capper and Elliot. Monolithic integration with silicon A considerable cost saving would be possible if the narrow-gap infrared sensitive material could be grown epitaxially directly on the Si read-out chip on areas which are not occupied by the signal-processing circuits. Using a lead chalcogenide (IV-VI) NGS as the sensor material, we were able to demonstrate such a heteroepitaxial but monolithic IRFPA for the first time. To cover the 35 m wavelength range, we chose PbTe. The active Si substrate was fabricated using CMOS technology; the chips were designed by W Buttler, Ingenieurbro, Essen, and fabricated at the Fraunhofer Institute for Microelectronic Circuits, Duisburg, Germany. The array contains 96 128 pixels with a 75 m pitch. Each pixel includes a bare Si area where the NGS layer grows epitaxially, and an access transistor. The 96 rows are addressed serially with a shift register integrated into the chip, while the 128 lines are fed out in parallel to both sides where they are wire-bonded to two off-chip devices each containing 64 current integrators and amplifiers. A fill factor of about 40% is achieved by employing robust design rules. Higher fill factors are possible with finer design rules. Before MBE growth, the substrate is chemically cleaned to obtain a hydrogen-terminated surface in the pixel areas where epitaxial growth has to occur. The hydrogen desorbs when heating the substrate to the growth temperature. A 3 nm CaF2 buffer layer is grown first at 450 C, followed by a 34 m thick lead-chalcogenide layer at < 400 C. The standard Al metallization of the active Si substrate withstands these growth temperatures without problems. Photovoltaic sensors are then delineated in the IV-VI layers by wet- and dry-etch processes, and electrical connections between the IR-pixels and the read-out defined. shows a schematic cross-section of one pixel, and a micrograph of a part of the completed array. High pixel yield Up to 98% of the pixels were operative in our first, not yet optimized processes. As a result of using a direct bandgap semiconductor, quantum efficiencies are high (typically > 50%). The distribution of the sensitivities (differential resistances at zero bias Ro) of an arbitrarily chosen line had a mean value of about 4 M at 95 K, corresponding to a RoA product of 90 cm2. This sensitivity is considerably higher compared with our first results (K Alchalabi et al.) and much above the ambient temperature IR background fluctuation noise limit, thus allowing sensitive thermal imaging. We assembled a simple demonstration IR camera by connecting the 64 output lines at one side of the array to a 64-channel amplifier chip. An infrared image obtained in this way () demonstrates the functionality of the design. The integration time is 100 s/line and the noise equivalent temperature difference (NETD) resulting from the individual sensor and amplifier parameters is estimated to be below 100 mK. Low cost, high performance Fabrication costs of such IRFPAs are low since large standard prefabricated and tested active Si substrates containing many devices can be used. Also, the MBE growth is simple, and the final IR-pixel delineation can be performed with standard processes. While the bandgap of PbTe is fixed, ternary lead chalcogenides such as Pb1xEuxTe or Pb1xSnxTe allow tuning the bandgap to obtain cut-off wavelengths covering the full 35 and 812 m wavelength range of the atmospheric infrared windows. Lead chalcogenides are much easier to handle than II-VI compounds, while their theoretical performances are comparable. In addition, lead chalcogenides are "forgiving" materials, i.e. a rather high number of defects is tolerable. This allows us to grow layers of sufficient quality on Si substrates, despite the large lattice and thermal expansion mismatch. The dislocation densities (as high as 107 cm2) limit device performance, but performance is still high enough for most applications with ambient background temperatures. Such applications include fast imaging if the sensitivity of highest-quality hybrid IRFPAs is not required, or if the recording speed of uncooled microbolometer IRFPAs is not sufficient. Further reading K Alchalabi, D Zimin and H Zogg Electron. Dev. Lett. 2001 22(3) 110; Proc. SPIE 2001 436941. P Capper and C T Elliott eds 2001 Infrared Detectors and Emitters: Materials and Devices (Kluwer Academic).
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