SURFACE SCIENCE

Unpinning Gallium Arsenide

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Science  05 Sep 2003:
Vol. 301, Issue 5638, pp. 1291
DOI: 10.1126/science.301.5638.1291b

The metal oxide semiconductor field-effect transistor (MOSFET) is made from silicon rather than GaAs, even though GaAs has a direct band gap and high carrier mobilities. The reason is that the insulating oxide layer that forms on the GaAs surface has a high density of states that “pin” the valence and conduction bands, rather than letting them move with respect to the Fermi level. The classical explanation is that oxidation liberates As atoms from the surface, and this “antisite” indirectly creates states above the valence band maximum.

Hale et al. have studied the adsorption of oxygen and Ga2O onto the GaAs(110)-c(28)/(24) via scanning tunneling microscopy (STM) and spectroscopy (STS) as well as via density functional calculations. They find that oxygen in fact creates donor and acceptor sites in the band gap directly from the Ga atom to which they are bonded. However, Ga2O bonds by inserting into As dimer pairs to form a c(22) unpinned surface. Chemisorption of GaO appears to restore surface Ga and As atoms to nearly their bulk charge states, whereas O2 creates charged Ga atoms that lead to pinning. — PDS

J. Chem. Phys. 10.1063/1.1601596.

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