MECHANICALLY AND THERMALLY STABLE SI-GE FILMS AND HETEROJUNCTION BIPOLAR-TRANSISTORS GROWN BY RAPID THERMAL CHEMICAL VAPOR-DEPOSITION AT 900-DEGREES-C

Traditional techniques for growing Si-Ge layers have centered around low-temperature growth methods such as molecular-beam epitaxy and ultrahigh vacuum chemical vapor deposition in order to achieve strain metastability and good growth control. Recognizing that metastable films are probably undesirable in state-of-the-art devices on the basis of reliability considerations, and that in general, crystal perfection increases with increasing deposition temperatures, we have grown mechanically stable Si-Ge films (i.e., films whose composition and thickness places them on or below the Matthews-Blakeslee mechanical equilibrium curve) at 900-degrees-C by rapid thermal chemical vapor deposition. Although this limits the thickness and the Ge composition range, such films are exactly those required for high-speed heterojunction bipolar transistors and Si/Si-Ge superlattices, for example. The 900-degrees-C films contain three orders of magnitude less oxygen than their limited reaction processing counterparts grown at 625-degrees-C. The films are thermally stable as well, and do not interdiffuse more than about 20 angstrom after 950-degrees-C for 20 min. Therefore, they can be processed with standard Si techniques. At 900-degrees-C, the films exhibit growth rates of about 15-20 angstrom/s. We have also demonstrated the growth of graded layers of Si-Ge, and have determined that a strain gradient exists in these layers.