  Graphene has many unique properties which make it an attractive material for
fundamental study as well as for potential applications. In this paper, we
report the first experimental study of process-induced defects and stress in
graphene using Raman spectroscopy and imaging. While defects lead to the
observation of defect-related Raman bands, stress causes shift in phonon
frequency. A compressive stress (as high as 2.1 GPa) was induced in graphene by
depositing a 5 nm SiO2 followed by annealing, whereas a tensile stress (~ 0.7
GPa) was obtained by depositing a thin silicon capping layer. In the former
case, both the magnitude of the compressive stress and number of graphene
layers can be controlled or modified by the annealing temperature. As both the
stress and thickness affect the physical properties of graphene, this study may
open up the possibility of utilizing thickness and stress engineering to
improve the performance of graphene-based devices. Local heating techniques may
be used to either induce the stress or reduce the thickness selectively.
