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ARNE NYLANDSTED LARSEN Institute of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark

ABSTRACT The advent of epitaxial-growth techniques dedicated to the growth of epitaxial layers of group-IV semiconductors and their alloys has opened up new possibilities for novel types of diffusion experiments and thus for critical tests of existing diffusion theories. It is now possible to grow test structures for diffusion consisting of well-defined narrow distributions of tracer impurities in epitaxial layers of biaxial-strained or relaxed alloys of different compositions. This enables studies of alloy effects and/or strain effects on diffusion with varying strain energy for a well-defined type of strain (tensile or compressive). A review is presented on recently published results on impurity diffusion in strained and relaxed, epitaxial SiGe alloy layers. As almost all these studies have focused on boron and antimony diffusion, these two impurities will have the leading roles in this review.

INTRODUCTION There are at least two major reasons for studying impurity diffusion in SiGe alloys. One stems from a need to control, very accurately, the impurity profiles in integrated circuits encompassing SiGe alloy layers. SiGe alloy layers have the potential of extending the device-performance limits of Si [I]; however, for a successful implementation into Si device technology, a detailed knowledge of impurity diffusion in these materials is essential. The performance of the hetero-junction bipolar transistor, having a base consisting of a thin boron doped, strained Si,_ xGeX layer, depends critically on the exact B-depth profile relative to the base: An outdiffusion of boron might reduce the transistor's frequency performance, gain, and Early voltage. This explains, in part, the great interest in the understanding and modelling of B diffusion in strained Sil-xGex layers. The other major reason for studying impurity diffusion in SiGe alloy layers is of a more fundamental nature as the system offers a variety of physical parameters which can be varied almost independently. The lattice constant of Ge is 4.2% larger than that of Si. As a 187 Mat. Res. Soc. Symp. Proc. Vol. 532 0 1998 Materials Research Society

consequence, thin, epitaxial Ge layers on top of a Si substrate or vice versa will be biaxiallycompressive strained or biaxially-tensile strained, respectively. The critical thicknesses, however, for pseudomorphic, epitaxial growth of Ge or Si on Si or Ge substrates, respectively, are too small (_