| Strain significantly affects the electronic properties of Si by altering band structure. However it has not been possible to measure the strain dependence of the band structure directly because properly strained Si has not been available. Due to the high sensitivity of band structures to strain, it is not possible perform measurements if samples contain strain nonuniformities because of dislocations. This limitation has been resolved via the creation of dislocation-free Si nanomembranes which strain can be applied elastically, resulting in uniform strain.;Strained Si nanomembranes are created by epitaxial growth of Si/SiGe/Si trilayer structures on SOI substrates and then releasing the layer by selective removal of the buried SiO2 layer. After release, strain is transferred to the Si layers from the of SiGe stressor layer, without forming dislocations. To ensure the quality, the strained Si nanomembranes are inspected for dislocations using low-energy electron microscopy (LEEM). In-situ LEEM observations made during high temperature annealing show that the membranes are dislocation free and stable against dislocation formation at temperature up to 1100°C. The observations also show the increased driving force for dislocation nucleation resulting from the core spreading effect during the Si/SiGe/Si trilayer growth on SOI, and from strain experienced by bending during the release-transfer process. Furthermore, LEEM observations show that preexisting dislocations at the Si/SiO2 interface that are created during trilayer growth can be removed during the releasing process. These findings are used to improve the fabrication process and identify important structural parameters of the trilayer membrane for minimization of the driving force for dislocation formation.;Band structure measurements on strained Si nanomembranes with (001) and (110) surface orientations are carried out by x-ray absorption spectroscopy (XAS) using synchrotron x ray with 10meV energy resolution and 5 nm depth resolution. XAS spectra acquired from Si membranes at different strain levels reveals shifts and splittings of Delta, L1 and L3 conduction band valleys as a function of strain. The analysis also shows that tensile strain shifts the conduction band downward relative to the vacuum level, the direction consistent with the most recent theoretical predictions. |
