The use of Raman spectroscopy to identify strain and strain relaxation in strained Si/SiGe structures

Strained silicon/SiGe alloy films have attracted interest in recent years because of their importance for high-speed microelectronic devices. In order to achieve such speed increases, devices are made from silicon with a tensile strain introduced by epitaxial deposition onto a SiGe alloy with a larger lattice parameter. In this study, strained silicon layers grown on SiGe virtual substrates have been investigated by Raman spectroscopy to determine their strain and if any strain relaxation has occurred. The Raman spectra of epitaxial Si1−xGex alloys (x = 0.1, 0.15…0.3) covered with a thin (∼ 10 nm) Si cap layer were analyzed with the aim of accurately measuring the Si cap peak position which is related to macrostrain. However, the signal obtained from the Si cap layer is very weak and shows considerable overlap with the Si in SiGe peak. In order to find the Si cap peak position, two methods were performed. The first method is to obtain a difference spectrum before and after selectively etching the Si cap and the second method involves peak-fitting and deconvolution by software. Both of these methods show very good agreement, however the peak fitting technique is a faster and non-destructive technique and is suitable for analyzing devices at various stages of processing. The generation of strain as a function of germanium content of the virtual substrate and the relaxation of this strain at defects such as dislocations is discussed.