Study of Germanium Epitaxial Recrystallization on Bulk-Si Substrates
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1252-I07-03
Study of Germanium Epitaxial Recrystallization on Bulk-Si Substrates Byron Ho, Reinaldo Vega, Tsu-Jae King-Liu Dept. of EECS, University of California at Berkeley, Berkeley, CA 94720-1770, U.S.A. ABSTRACT LPCVD Ge films are deposited onto bulk Si substrates and subjected to either a rapid thermal anneal (RTA) or furnace anneal (FA) at a temperature that is higher than the melting point of Ge in an attempt to induce epitaxial recrystallization. Spiking into the Si and voids in the Ge film are observed after the anneal. This is attributed to defect-assisted Ge diffusion into the Si substrate caused by strain at the Ge-Si interface. Simple diffusion theory using published diffusivity values predicts diffusion depths similar to the spiking depths observed by scanning electron microscopy and transmission electron microscopy. Approaches to reduce the strain at the interface are explored. It is found that the quasi-equilibrium nature of FA reduces spiking and that there is an area dependence. Grazing-incidence x-ray diffraction analysis suggests that this technique for epitaxial recrystallization does not result in single-crystalline Ge. INTRODUCTION Continued CMOS technology scaling has required the incorporation of advanced materials to sustain the historical pace of improvement in transistor performance. In this context, silicongermanium (Si1-xGex) embedded in the source and drain (S/D) regions of p-channel MOSFETs is now used to compressively strain the channel [1] and thereby enhance the hole mobility. Due to its higher carrier mobilities, Ge is also being considered as an alternative channel material for use in future high-performance integrated circuit applications. To date, Ge MOSFETs have been demonstrated on Si1-xGex substrates and Ge-on-insulator (GOI) substrates [2-4]. Epitaxial Geon-Si growth processes using molecular beam epitaxy or ultra-high-vacuum chemical vapor deposition (UHV-CVD) have been developed [5, 6], but these are relatively expensive. Recently, it has been shown that molten Ge can recrystallize epitaxially upon solidification on Si and may provide for a lower cost approach to fabricating Ge MOSFETs on Si or Si1-xGex S/D stressors [7]. There have been several investigations of this approach for silicon-on-insulator (SOI) substrates [8, 9]. In this work, the formation of epitaxial Ge on Si bulk substrates via melting and recrystallization is investigated, to assess the viability of this approach for forming S/D stressors and high-mobility channel regions. EXPERIMENTAL DETAILS In this study, Ge was selectively deposited onto (100) Si substrates in an LPCVD furnace, in a series of experimental splits. In Exp. 1, silicon dioxide was thermally grown on the Si substrates and patterned into 1um to 5um wide lines. Then, Ge films were selectively deposited onto the exposed Si regions at 425oC (400mT, 50sccm GeH4). A capping layer of lowtemperature silicon dioxide (LTO) was then deposited to prevent evaporation of Ge during annealing. These samples were subjected to a rapid thermal anneal (RTA
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