Influence of Precursor Chemistry on Synthesis of Silicon-Carbon-Germanium Alloys
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ABSTRACT We describe the synthesis and use of the novel molecular precursors C(SiH 3 )4 , CH 3GeH3, and SiH 3 CH 2GeH 3 to generate silicon-carbon-germanium materials by ultrahigh vacuum chemical vapor deposition. By using these precursors in reactions with SiH4 and GeH4 between 470 0 C and 650 0 C we obtained: 1) heteroepitaxial Sil-_yxGexCy (y=0.04-0.06) alloys with C(SiH 3)4 ; (2) polycrystalline alloys with carbon compositions ranging from 2-14 at.% with CH 3 GeH 3 ; (3) mixtures of diamond cubic nanocrystals (Ge, Sil-xGex) and amorphous SiC with SiH 3CH 2 GeH 3 . The effect of the precursor chemistry on composition, crystallinity, and microstructure of the materials as characterized by Rutherford backscattering spectroscopy (RBS) , secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM) is discussed.
INTRODUCTION The concept of bandgap engineering has been extensively applied to create fast transistors and a host of other novel optical and electronic devices using group 111-V semiconductor alloys. Recently, group IV bandgap-engineered Sii-xGex semiconductors have been used to develop heterojunction bipolar transistors (HBT) which achieve speeds much greater than those of traditional silicon devices [1-41. The major concern with growing device quality heteroepitaxial silicon-germanium films on Si is the compressive strain caused by the lattice mismatch between SiGe and Si which ultimately gives rise to stability problems and limits the film thickness. Addition of substitutional carbon into Sil-xGex is expected to reduce this lattice mismatch because the smaller size of carbon should compensate for the larger size of germanium in the lattice [5]. In addition to relieving strain, substitution of carbon in the SiGe system can provide an additional design parameter in bandstructure engineering on silicon. Optical bandgaps of the ternary semiconductor Sil-x-yGex C are likely to vary with lattice constant and composition from 5.5eV for pure carbon (diamond) to 0.66eV for pure germanium. For these reasons, growth of Sil-x-yGex Cy alloys via implantation of C into SiGe and by molecular beam epitaxial methods has become the focus of considerable recent research 16-81. However, since the maximum solubility of carbon in Si is only 2x10- 3 at. % and negligible in Ge, formation of undesirable carbide phases has been a considerable problem especially at the high temperatures that are needed to repair the implantation damage. Also, the concentration of substitutional carbon obtained by these methods was limited to only low levels (1-2 at.%), evidently not enough to substantially reduce the lattice strain and to influence the bandgap. Most recently, chemical vapor deposition (using Sill4 , GeH 4 , and various hydrocarbons as feedstock gases) has been used to generate pseudomorphic Sit xyGex Cy alloys with y=:0.01-0.02 [9]. Attempts to increase the carbon incorporation have resulted in SiC precipitation and formation of amorphous hydrogenated SiGeC materials. 529
Mat. Res. Soc. Symp. Proc. Vol. 377 ©
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