The Si/SiO2 Interface: A ClassicOne of the main reasons for the dominant role of silicon technology is the extremely high quality of the Si/SiO2 interface. The number of electrically-active defects at this interface is only one in 100,000 interface atoms. Despite highly-developed device applications of silicon, the actual atomic bonding at the Si/SiO2 interface is still uncertain. The reason is the disordered structure of SiO2, which makes it impossible to use standard crystallographic techniques. By observing the energy shifts of the silicon 2p level induced by the bonding partners, it is possible to reveal the different bonding configurations at the interface. The figure shows that silicon atoms bonding to 0, 1, 2, 3, and 4 oxygens can be distinguished. Surprisingly, the intermediate oxidation states 1+, 2+, 3+ are found, which are not stable in normal silicon compounds (red). At the interface, such a bond arrangement is forced upon the silicon atoms by the fact that they have to bond to silicon on one side of the interface and to oxygen on the other. The distribution of oxidation states changes with the preparation conditions and with the quality of the interface. Synchrotron radiation is essential for achieving the surface sensitivity that is necessary to detect the 1-2 monolayers of silicon atoms in the interface region.
A widely-cited reference: F.J. Himpsel et al., Phys. Rev. B 38, 6084 (1988).
Diamonds Forever !Wide-gap semiconductors, such as diamond, silicon carbide, and gallium nitride, are gaining interest for high power, high temperature devices and for light emission in the blue. The figure below shows the energy shift of the carbon 1s level between the sigma-bonded bulk atoms and the pi-bonded surface atoms.
Diamond, in particular, exhibits an unusual negative electron affinity when hydrogen-terminated. That makes it an efficient electron emitter. All the electrons trickle down to the conduction band minimum and can escape into vacuum from there.
A widely-cited reference: F.J. Himpsel et al., Phys. Rev. B 20, 624 (1979).
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