Reduced Pressure - Chemical Vapor Deposition of Ge thick layers on Si(001) for microelectronics and optoelectronics purp
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Reduced Pressure - Chemical Vapor Deposition of Ge thick layers on Si(001) for microelectronics and optoelectronics purposes J.M. Hartmann (*), A.M. Papon, P. Holliger, G. Rolland and T. Billon, CEA-DRT, LETI / D2NT & DPTS, CEA – GRE, 17, Avenue des Martyrs 38054 Grenoble Cedex, France. M. Rouvière (1), L. Vivien and S. Laval, IEF (UMR 8622 CNRS), Bât. 220, Université Paris-Sud, 91405 Orsay Cedex, France. (1) : also STMicroelectronics, 38926 Crolles Cedex, France. Abstract : Ge-based photodetectors operating in the telecommunication wavelength range (1.31.6 µm) of silica fibers are highly desirable for the development of optical interconnections on SOI substrates. We have therefore investigated the structural and optical properties of Ge thick films grown directly onto Si(001) substrates using a production-compatible Reduced Pressure Chemical Vapor Deposition system. The thick Ge layers grown using a low-temperature / high temperature approach are in a definite tensile-strain configuration, with a threading dislocation density for as-grown layers of the order of 3x107 cm-2. The surface of those Ge thick layers is rather smooth, especially when considering the large lattice mismatch in-between Ge and Si. The root mean square roughness is indeed of the order of 2 nm only for as-grown layers. The layers produced are of high optical quality. An absorption coefficient α ≈10000 cm-1 @ 1.3µm (4500 cm-1 @ 1.55µm) has been found at room temperature for our Ge thick layers. A 30 meV bandgap shrinkage with respect to bulk Ge (0.77 eV 0.80 eV) is observed as well in those tensilestrained Ge epilayers.
I - Introduction With the increasing use of optical fibers in telecommunications, the demand for low-cost and efficient photo-detectors operating in the low loss windows (1.3-1.6 µm) of silica fibers is growing rapidly [1]. Moreover, existing electrical interconnection technologies limit the performance of high speed computers and switching systems. An alternative, based on optical interconnections, is being studied. It requires the development of high speed (≥10 GHz), reliable and compact optical terminal photodetectors compatible with the telecommunication wavelengths. Pure Ge grown directly onto Si is the best candidate for such photodetectors, due to its band gap of 0.8 eV at room temperature [2-5]. However, due to the large lattice mismatch with Si (4.2%), it is not easy to obtain Ge films with suitable characteristics, i.e. thickness appropriate for maximum efficiency, flatness compatible with sub-micron lithography, minimum defect content for high speed devices etc. Several routes have been explored since 1998 to obtain such films. One, which has been proposed initially by Colace et al. in 1998 [5], followed quite closely by Hernandez et al. [9], relies on the deposition of a low-temperature Ge layer, followed by the deposition of a high temperature Ge layer. The low-temperature (330°C – 450°C [5], [9-12]) adopted for the first Ge layer plastically relaxes the strain in the Ge film without the nucleation of any 3
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