On the intrinsic spacer layer in Si/SiGe heterojunction bipolar transistor grown by ultra high vacuum chemical vapor deposition

In this paper, we report the study on the intrinsic SiGe spacer layer in Si/SiGe HBT and a novel phenomenon in Si/SiGe HBT layer structure grown by ultra high vacuum chemical vapor deposition (UHVCVD) using Si₂H₆ and GeH₄. When intrinsic SiGe spacer layer width decreases from 100 A to 75 A, the BC junction turn-on voltage decreases from ~0.6 V to ~0.2 V, while the BE junction turn-on voltage remains ~0.6 V. As a result, the offset voltage of the HBT operated at common emitter configuration increases from nearly zero to ~0.4 V. Therefore, 100 A is used as the intrinsic SiGe spacer layer width. Furthermore, the Si/SiGe HBT layer structures with different SiGe spacer layer widths grown by UHVCVD were investigated by cross-sectional transmission electron microscopy (XTEM). It is shown that a Ge-rich very thin layer exists in the strained SiGe layer grown on Si, which is in parallel with the interface of SiGe and Si. When the SiGe spacer layer width decreases from 100 A to 75 A, the Ge-rich thin layer moves closer to the interface of SiGe base and Si collector, and makes the BC junction turn-on voltage decrease, the offset voltage of HBT, therefore, increases