ABSTRACT
The effects of germanium (Ge) segregation on composition profiles across SiGe layers, grown on silicon by both molecular beam epitaxy (MBE) and chemical vapour deposition (CVD), are analysed by a number of transmission electron microscopy (TEM) techniques. A clear disparity in the relative sharpness of the interfaces on either side of the layer is observed, the SiGe-on-Si interface being much sharper than the Si-on-SiGe. SiGe/Si structures, with SiGe layer thicknesses approaching a few nanometres, are employed for electronic devices such as heterojunction bipolar transistors, heterostructure field effect transistors and infrared detectors. Device electronic properties depend critically on the composition, thickness, strain and interface morphology of the layers. The asymmetry was attributed to the action of Ge segregation and an attempt was made to simulate the Ge profiles employing a theoretical model and accounting for the resolution limitations intrinsic to energy-filtered TEM measurements. For MBE materials, an excellent agreement was found, whereas a significant discrepancy was found for the CVD wells.
