Amorphous Si 1-Y Ge Y :H, F films obtained by Low Frequency PECVD for uncooled microbolometers
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Amorphous Si1-YGeY :H, F films obtained by Low Frequency PECVD for uncooled microbolometers. R. Ambrosio1), A. Torres 1), A. Kosarev 1), A. S. Abramov 2)A. Heredia 1), M. Landa 1), M. Garcia 1) 1) Instituto Nacional de Astrofísica, Óptica y Electrónica, INAOE, Apdo. P. 51 and 216, P.O. Box. 72000 , Puebla, Mexico. 2) A.F Ioffe Phys. Techn. Institute St.- Petersburg, 194021, Russia.
ABSTRACT In this work, we report the composition, optical, and electrical properties of a- Si1-YGeY: H, F films to be used as sensing layer in uncooled microbolometers. The a-Si1-YGeY films where Y is Ge content in solid phase were deposited by low frequency PECVD from SiH4 and GeF4 feed gases, and H2 and Ar were used for dilution. The film composition, IR transmission and temperature dependence of conductivity were measured. The reduction of conductivity activation energy from 0.86 eV to 0.39 eV and the increase of room temperature conductivity from 1x10-9 to 2.1x10-3 Ohm-1cm-1 were observed with the change of Y from 0 (Si) to 1(Ge). These results demonstrate this material to be a good candidate as a sensing material in uncooled microbolometers, due to its high absorption in the range of λ= 10-13 µm , its relatively high activation energy, Ea=0.4 eV, consequently, a high temperature coefficient of resistance (TCR), and moderate resistivity at room temperature. INTRODUCTION The development of devices for night vision has been a subject of research for many decades. Night vision is usually achieved by using staring infrared focal plane detector arrays. Infrared focal plane arrays have both, commercial and defense applications. Security, fire, search and rescue services can benefit from low cost infrared images. Defense applications include night vision, low visibility target detection, tracking and remote earth observations. A bolometer is a thermal detector of IR radiation. The main characteristic of a bolometer is a change in the material resistivity due to the heating effect caused by absorbed radiation. In contrast to photoconductors, bolometers consist of almost any material exhibiting a temperature dependent resistivity. High temperature coefficient of resistance (TCR) which is defined as α=TCR=(1/R)(dR/dT), low noise, and the compatibility with standard IC fabrication processes is desirable for materials used in bolometers. A variety of materials have been suggested as active elements of bolometers; e.g. vanadium oxide, several metals, and amorphous and polycrystalline [1] semiconductors. Even though good results have been obtained with those materials, none of them can be considered as the optimum one. Vanadium oxide is not a standard material in integrated circuit (IC) technologies, whereas metals usually present a low TCR. On the contrary, amorphous semiconductors, such as a-Si:H, a-SiGe:H a-Si-B:H, are fully compatible with silicon technology. For room temperature operation, amorphous silicon (a-Si:H) has been used in
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commercial applications; however, it presents an undesirably high resistivity. Boron doping
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