Semiconductor Surfaces/Interfaces

The Si/SiO₂ Interface: A Classic

One 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.
For details see: 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. Diamond, in particular, exhibits an unusual negative electron affinity, which makes it an efficient electron emitter. The undulator beam lines at the Advanced Light Source (ALS) are optimized for studies of the carbon 1s level. The figure shows the energy shift of the 1s level for carbon atoms at the diamond surface, which are arranged into pi-bonded chains.

Atomic layer epitaxy (ALE): Control is Everything

This technique provides the ultimate control in growing semiconductor structures. Every atomic layer is deposited separately in a two-step process. A self-saturating monolayer is adsorbed first and then activated for adsorption of the next layer. Depositing alternating silicon and carbon layers could be used for tailoring the band gap in silicon carbide superlattices. The key problem is the controlled deposition of carbon, which is traditionally done at high temperature where the atomic layers inter-diffuse. We found that compounds with carbon atoms bonded to more than one silicon have the best chance to remain at the surface during the activation process, whereas single-bonded hydrocarbon groups desorb. Today, atomic layer epitaxy is an essential ingredient for fabricating the next generation of "high k" gate oxides in MOSFETs. These are necessary to contain the leakage current across the gate of the transistors, which is currently the biggest threat to Moore's law, the exponential growth of silicon technology.
For details see: D.G.J. Sutherland et al., J. Appl. Phys. 82, 3567 (1997).

Luminescent Polymers: A Display Made of Plastic?

The discovery of conducting polymers with efficient luminescence has opened prospects for fabricating cheap displays, with the ultimate possibility of a "paper-like" display. The orbital structure of these materials can be determined by a combination of photoemission, fluorescence, and absorption spectroscopy. Changes in the bonding are observed under irradiation in oxygen, an effect that is limiting the lifetime of the displays. Organic monomers can be used as well, such as aluminum quinolate (Alq3).
Polymers (MEH-PPV): D.G.J. Sutherland et al., Appl. Phys. Lett. 68, 2046 (1996).
Monomers (Alq3): A. Curioni et al., Appl. Phys. Lett. 72, 1575 (1998).

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