Growth of Multi-Layer Si/Si 1-x Ge x Structures using Rapid Thermal Chemical Vapour Deposition
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ween the two materials, which can be tailored by control of the germanium fraction, x. Heterojunction bipolar transistors (HBT's), incorporating Si-.,Gex base layers, offer superior performance to silicon-only bipolar transistors. Compared to other possible heterostructure combinations (e.g. SiC/Si), the Si/Si1 .•Gex system has the smallest lattice mismatch and poses the fewest problems in fabrication. However, relaxation of the strained Si-.xGe. layers and contamination (particularly with oxygen) must be avoided if optimal HBT performance is to be obtained. This paper describes the development of an in-situ growth process sequence typical of that required for the emitter-base structure of an HBT. Associated reactor development is also described. SYSTEM DESCRIPTION
The QUPLAS reactor, built at Queen's University, and illustrated schematically in Figure 1, is a rapid thermal chemical vapour deposition (RTCVD) system with a quartz process chamber. The single 4" process wafer is radiantly heated using an array of tungsten-halogen lamps with a total power capacity of 18kW. The system is non-UHV, but does include a turbo-pumped load-lock chamber, isolated from 187 Mat. Res. Soc. Symp. Proc. Vol. 326. @1994 Materials Research Society
quartz
tungsten--halogen chamber Ihlamp banks
gas in lettub
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pump
process waer
quartz boat
isolation valves
to roots/rotary to combination mrootsot
Figure 1: Schematic diagram of QUPLAS reactor the process chamber by a letter-box gate valve. During growth, the process chamber is
evacuated by a roots/rotary combination. Process gases are supplied via lines constructed exclusively of welded or metal-seal joints, and in-line gas purification is used to ensure high gas purity at the point of use. A gas switching facility allows gas flows to stabilize whilst flowing directly to exhaust, prior to switching into the process chamber. The process gases have been divided into 3 groups. The combined flow of each group is switched either directly to the pumping track (bypassing the chamber) or into a manifold where the gases from all 3 groups are mixed before entering the process chamber.
PRELIMINARY CHARACTERIZATION OF Si AND Sil.1Ge. GROWTH Before developing a multi-layer growth process, preliminary work was carried out in the QUPLAS reactor to obtain a basic understanding of both silicon and Sil-.Gex growth.
Epitaxial Silicon Growth Epitaxial silicon was grown on Si substrates in a SiH 4/H 2 gas ambient. Epitaxial alignment was observed (using transmission electron diffraction) at growth temperatures down to 590'C. However, the choice of pre-clean was found to have a profound effect on defect density. An ex-situ chemical oxidation followed by an in-situ high temperature H2 bake at 950°C consistently resulted in a high density of defects. This is attributed to uneven removal of the passivating oxide and subsequent substrate etching during the high temperature bake. The alternative approach of ex-situ hydrogen passivation was therefore investigated. The most successful hydrogen passivati
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