Large silicon-on-insulator (SOI) structures have been obtained using a new two-step oxidation technique on a silicon molecular epitaxial film grown onto a porous silicon sample. The first low-temperature oxidation step oxidizes the large-pore silicon underlayer to avoid the collapse of the porous structure and thus secure the lateral oxygen supply channels in the porous oxide. In the second step, a higher temperature is used to oxidize the remaining porous silicon and the bottom part of the Si molecular beam epitaxy overlayer with the oxygen supplied through the oxidized porous silicon underlayer. A new porous silicon structure used is composed of a top porous silicon layer which has a smaller porosity for the ease of subsequent silicon epitaxial growth, and a second layer with a larger porosity which not only gives oxygen supply channels during the SOI oxidation but also allows the volume expansion of the porous silicon oxide to be accommodated. SOI sizes of 325 μm×2 mm have been successfully fabricated. The breakdown voltage of the SOI structures is about 400 V and the leakage current densities between the SOI layer and the substrate are below 3 nA cm-2 with a 10-V bias. n-channel metal-oxide-silicon field-effect transistors have been fabricated on the SOI structures with good characteristics, indicating that this technology may be suitable for very large scale integrated circuits applications.
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