Elastic strain sharing in silicon/silicon germanium nanomembranes

Controlling strain in the silicon/silicon-germanium hetero-system provides control over band offsets and carrier mobility. The main problem in the Si/SiGe system is that to obtain strained Si layers, SiGe relaxation, typically by dislocation motion, is required. Dislocations extending into the device layer degrade device performance.; This dissertation describes a new method of creating large-area, continuous, strained-Si films with no or very few dislocations. I have grown SiGe and Si films on ultra-thin Si-on-insulator (SOI) substrates and then released large continuous membranes of the film system by removing the oxide layer. The membranes reach the final strain state after release from the substrate. The strained-Si films are made by depositing a thin hetero-structure of Si on SixGe1-x on SOI substrates, using chemical vapor deposition (CVD), each layer being below its critical thickness for dislocation formation. Following CVD growth, the substrates are patterned with an array of small openings to expose the oxide. Large membranes of Si/SiGe/Si are released from the substrate by wet chemical etching of the oxide with hydrofluoric acid. Upon release, the strain in the system becomes distributed elastically between the layers, based on the relative thicknesses of the layers. Following release, the membranes are transferred in solution to a new substrate or allowed to settle onto the original handle wafer. Modifications of the basic release process to increase the amount of strain transferred to Si layers are described. The strain state of membranes following release is characterized using x-ray diffraction.; As one of the most exciting advantages of using membranes for strain relaxation is the lack of dislocations, the stability of membranes to dislocation motion is analyzed. The nature of the membrane release process results in physical bending of the membrane, adding local strain to the uniform strain of each layer in a membrane structure. This bending strain may hasten the onset of dislocation formation. The expected critical strain for different membranes structures is determined as it relates to bending radius.