Silicon and Si1-x Gex layers have been epitaxially grown on silicon < 100 substrates in a radiantly heated rapid thermal LPCVD reactor using SiH4/GeH4/H2 gas chemistry with the addition of B2H6 and PH3 for in-situ doping. Oxygen-free Si1-xGex layers (0.05<x<0.2) have been grown at temperatures down to 530°C, whereas oxygen-free silicon growth has only been achieved at temperatures above 690°C. The difference is attributed to the formation of volatile GeO during Si1-x Gex growth.
The target application for these Si1-x Gex layers has been as the base of heterojunction bipolar transistors. In-situ boron-doped Si1-x Gex layers with a doping level of 5xl018cm-3 and thicknesses down to 30nm are flanked by undoped Si,.xGex spacers, typically of 15nm in width, to accommodate boron out-diffusion. These have been grown sequentially at 600°C, preceded by silicon buffer growth at 710°C and followed by silicon cap growth also at 710°C.
The process and system development necessary for the production of ultra-thin multiple layer structures are described, addressing the issues of compositional and dopant profile control.