Growth and Structure of Si Epilayers on Porous Si

The epitaxial system Si—porous Si—Si substrate possesses unusual properties for microelectronic structures and is promising for use in instruments, especially after converting the three-layer system into a Si-on-insulator structure via oxidation of the porous sublayer [1-8]. Anodic etching regimes enabling the surface layer of p+-type Si to be converted into a porous material with a significant pore volume are known. The physical properties of porous Si, mainly mechanical and optical, differ significantly from those of single-crystalline Si. In particular, the Young’s modulus of Si with 54% porosity is 1.7.1010N/m2[9], which is an order of magnitude less than that of a single crystal. The Poisson coefficient is 0.09, which is also much less than for compact crystalline Si, which has a Poisson coefficient of 0.26. Thus, porous Si is much weaker than the starting compact substrate material. Its strength decreases with increasing porosity. However, the crystal structure of Si within the porous layer is nearly identical to that of Si without pores. It has been noted [9] that the x-ray rocking curves of porous and nonporous Si are very similar. Immediately after anodic etching the lattice constant of porous Si perpendicular to the interface is hundredths of a percent greater than that of the substrate and depends on the porosity and the average pore size, increasing as they increase.