High Resolution Electron Microscopy (HREM) Study of Chemically Vapor Deposited Polycrystalline Si 1-x Ge x Thin Films
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High Resolution Electron Microscopy (HREM) Study of Chemically Vapor Deposited Polycrystalline Si1-xGex Thin Films W. Qin, D. G. Ast and T. I. Kamins+ Department of Materials Science & Engineering Cornell University, Bard Hall, Ithaca, NY 14853 + Hewlett-Packard Laboratories, Palo Alto, CA 94304 ABSTRACT The microstructure of polysilicon and Si0.69Ge0.31 thin films, grown by chemical-vapordeposition (CVD) on oxidized silicon wafers covered with a very thin polysilicon seed layer, was investigated using high-resolution electron microscopy (HREM). The plan-view HREM images showed that polysilicon films contained less substructure inside grains and had fewer multiple twins and more extended twin bands than Si0.69Ge0.31. On the other hand, only SiGe contained multiple twins with five-fold symmetry. The atomic model of the second-order symmetric twin boundary proposed for Si and based on the insertion of five-member and seven-membered rings was found to describe the atomic structures of second-order symmetric twin boundaries in Si0.69Ge0.31 as well. Within the accuracy of HREM, the repeat unit of the boundary was the same in Si0.69Ge0.31 and Si. INTRODUCTION The SiGe alloy system has been studied intensively as this material system offers opportunities to further shrink CMOS devices. Enabling features are an adjustable bandgap, compatibility with standard Si CMOS processing, and a lower process temperature than Si [1,2,3]. The application of SiGe alloys to fabricate thin film transistors (TFTs) on lowtemperature substrates has also been investigated [4]. Polycrystalline SiGe alloys have potential applications as sacrificial implant layers and as low resisistivity, raised S/D contacts. However, their microstructure has not been studied on the atomic level. Knowledge of microstructure is important because dopant diffusivity in polycrystalline Si varies greatly with microstructure. Corresponding variations are expected in polycrystalline SiGe films. The Si-Ge phase diagram [5] is one of complete mutual miscibility in the liquid and solid state. The crystal structure is diamond throughout. However, moving towards Ge, the lattice constant increases by 4.2%, as Ge is a larger atom than Si. Insertion of a Ge atom on a Si lattice site will, therefore, generate misfit stresses on the atomic scale. Aside from this effect, given the chemical and structural similarity of Si and Ge, one expects a similar microstructure in polycrystalline Si1-xGex and Si or Ge films. Diamond-type semiconductors readily twin on {111} due to the small energy required, ~ 0.2 eV/nm2 in Si, to form a first-order symmetric twin boundary [6]. This twinning operation preserves the first and second nearest neighbor arrangement, changing only the third nearest neighbor arrangement from staggered to eclipsed. Twins can form during growth at the solidliquid interface (grown twins) or during cool-down (deformation twins). Deformation twinning is an effective stress relieving mechanism at temperatures too low for dislocations to move. In the diamond structure, four
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