TABLE OF CONTENTS In this section we shall examine the deposition of heteroepitaxial films in greater depth using various reactors. Focus is placed on systems that have successfully demonstrated devices. High quality pseudomorphic Si 1 ? xGe x, Si 1 ?yC y and Si 1 ? x ?yGe xC y alloy layers with a carbon concentration up to 7% have been prepared by MBE [36] and other techniques. MBE is preferred for research work because of its flexibility, but chemical vapor deposition systems are more convenient for production consistency.
Molecular beam epitaxy, a physical deposition process, uses an ultrahigh vacuum chamber with a base pressure of about 10 ?10 Torr. Cryopumps provide an oil-free evacuation system. A modern MBE system used for Si heteroepitaxy is shown in figure 2.4. A load-lock is used for wafer loading to prevent exposing the main deposition chamber to the atmosphere. Bean and his coworkers [37] used an argon sputter cleaning cycle to etch 10 nm from the surface of the wafer. The etch was followed by an 850 C anneal before lowering to the deposition temperature, between 500 and 750 C. It has been demonstrated that sputter cleaning leads to degradation in the minority carrier lifetime by heavy metal contamination sputtered from the chamber onto the surface of the wafer [38]. Because of the ultra high vacuum conditions, medium temperature (< 850 C) bakeouts may be sufficient to cause...
