Epitaxial Si 1-x Ge x Films and Superlattice Structures Grown by CVD for Infrared Photodetectors
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Epitaxial Si1-xGex Films and Superlattice Structures Grown by CVD for Infrared Photodetectors L. Maddionaa, S. Coffaa , S. Lorentib, C. Bongiornoc a
Si-optoelectronics&post Silicon Technologies, Corporate R&D, ST Microelectronics, Catania, Italy DSG R&D, STMicroelectronics, Catania, Italy c Dipartimento di Scienze Chimiche,Università di Catania, Catania, Italy b
ABSTRACT Integration of photodetectors with high responsitivity in the near infrared (1.3-1.55 µm) on standard Si electronic circuits is important for a variety of applications in the field of on-chip, local area and long haul optical communications. In this work we report on a detailed structural and optical characterization of epitaxial Si1-xGex films and Si1-xGex /Si multilayers grown by chemical vapor deposition on (100) Si wafers. Cross-sectional transmission electron microscopy analyses show that metastable strained Si1-xGex films of few nanometer with x>40% can be deposited at low growth temperature and pressure. Absorption measurements on these films demonstrate the extension of the photo-response to 1.55 µm. Using these films as active layers Schottky integrated photodetectors have been fabricated. 1. INTRODUCTION A growing interest in optical communications has been induced by the need of faster data transmission through optical fibers, in free space or using integrated waveguides [1-4]. All these applications require fast and efficient photodetectors operating at different characteristic wavelengths: 1.54 µm for long haul transmission through fibers, 0.85 µm for free space connections and at a wavelength matched to the light source adopted in the transmission system for on-chip interconnects. Silicon is fully transparent at wavelengths above 1.1 µm and presents a very low absorption coefficients in the wavelength range 0.6-1.1 µm. Hence Si cannot be suitably used as active layers in photodetectors demanding fast response (and hence thin layers) and operation wavelengths extending in the near infrared. Pure Ge and SiGe films present a strong absorption that covers visible and near infrared region and extends up to 1.8 µm for a high Ge content. Hence epitaxial growth of SiGe layers on Si offers simultaneously the possibility to enhance the optical response of Si-based photodetectors while maintaining compatibility of integration with advanced Si integrated circuits. This is already achieved in modern high-speed bipolar transistors in which improvement of performances is achieved by integrating SiGe layers in the base of the transistors [5]. We report on the growth of SiGe films by chemical vapor deposition, structural and optical properties of these films. 2. GROWTH CONDITIONS The substrates used for the deposition of the SiGe films were p-type silicon wafers, (100) oriented, with a resistivity of 1.5 - 4 Ω cm. The sample were grown in a single wafer CENTURA AMAT LPCVD epitaxial reactor. Heating of the 150 mm Si wafers was achieved with 40 infrared lamps. The reactor was equipped with different gases: 2% diluted germane, pure silane and di
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