April 12, 2006 - Researchers at the U. of Wisconsin-Madison have developed a method to transfer thin membranes of silicon of silicon or silicon germanium (SiGe), and transfer them to new substrates such as diamond, metal, and even plastic. Potential applications include flexible electronic devices, nanosize photonic crystals, and lightweight environmental sensors.
Layering materials with different crystal structures creates strain -- e.g., layering SiGe onto a thicker silicon substrate forces the germanium atoms closer together, compressing the SiGe layer. Controlling the compressive strain allows for further straining of silicon layers, which break up the crystal lattice and relax some of the strain. The process is time-consuming and expensive, however, and defects can scatter electrons and degrade performance.
Seeking to integrate silicon and SiGe and manage strain while minimizing defects, the researchers created a three-layer membrane (each tens of nanometers in thickness) composed of a SiGe layer sandwiched between two Si layers of similar thickness, all sitting atop a SiO2 layer in a silicon-on-insulator (SOI) substrate. The oxide layer was etched away with hydrofluoric acid, releasing the nanomembrane. Pulled by the SiGe, the silicon exhibited tensile strain, which can be adjusted by varying the thickness of the membrane layers, a technique dubbed "elastic strain sharing" -- since in the freed membrane, strain is balanced between the three layers.
The nanomembranes retain all the properties of the substrate in wafer form, and can be created in shapes ranging from flat to curved to tubular by varying the thickness. The team also showed that the strain produced by the technique traps electrons in the top silicon layer, which is the end goal for many devices that integrate silicon and silicon-germanium.
The process is not yet manufacturing-ready, as the nanomembranes need to be soaked in a solution before bonding to other materials, the researchers noted.
While the work focused on SOI, the new method should apply outside semiconductor materials, such as ferroelectric and piezoelectric materials -- wherever one layer, such as an oxide, can be removed to free a multilayer nanomembrane, according to Max Lagally, UW-M materials science and engineering professor and advisor to the researchers.
Details of the research were reported in the April 9 issue of Nature Materials.