Influence of nanocrystalline structure of surface on boron gettering from silicon

Phenomenon of strong absorption of microwave radiation by porous layer after the sample was annealed in air at 400-500 °C was also detected at 37 GHz. This absorption is equivalent to high conductivity of layer (ρ = 10− 2-10− 3 Ω cm) and it is much higher (at least in 104 times) than of c-Si substrate. Removal of porous layer (thickness of ∼ 20 μm) from 400-μm substrate leads to where Q-factor of cavity is restored completely. The high conductivity of the porous layer comes mainly from conduction along the nanocrystalline grains, while SiO2 layer essentially insulates their interface. One possible effect explanation is that high concentration of positive charge states formed in SiO2 layer can lead to the creation of high density of free electrons in the SiO2/Si interface with formation of conductive channels along each nanocrystallite. On the other hand, appreciable microwave absorption is not detected on 9.4 GHz (X-band); therefore, we cannot exclude the existence of quasi-resonant microwave absorption that is related with nanocrystallites sizes, i.e. quantum confinement and electrons tunneling between grains.