Nanometer strain profiling through Si/SiGe quantum layers

Abstract

For the first time, nanometer-scale strain resolution is demonstrated using conventional Raman spectroscopy to profile strain through thin epitaxial Si/SiGe layers used as high mobility metal oxide field effect transistor channels. The strained layers were grown using ultrahigh vacuum chemical vapor deposition on relaxed SiGe virtual substrates. We observe how strain varies through the layer with 1.2 nm depth resolution. This is achieved by shallow angle (0.7°) bevelling. Tensile strain is found to be maximum at the buried Si/SiGe interface and decreases toward the surface. The partial surface strain relaxation is considered to be due to the imminence of the critical thickness. The bevel process has been characterized and does not impact results. SiGe composition and strained layer thickness are also determined and are in excellent agreement with secondary ion mass spectroscopy and x-ray diffraction data. The technique is proven to have 1 nm resolution in thickness measurements. Strain throughout the epitaxial layer stack has also been investigated. We show that the undulating surface morphology characteristic of relaxed SiGe alloys generated using compositional grading relates to periodic fluctuations in the strain fields in the SiGe virtual substrate, which are transferred to the overlying tensile strained Si. The resulting peak-peak variation in the tensile strained Si is determined to be 0.1%.