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23 October 2006


Modeling metal-induced reconstruction of surfaces

Researchers at the Chinese Academy of Sciences (CAS) Institute of Physics (IOP), in collaboration with physicists at the US National Renewable Energy Laboratory, Oak Ridge National Laboratory and the University of Tennessee,
have proposed a generalized electron-counting model to serve as the guiding principle in understanding metal-induced surface reconstruction of compound semi- conductors (Lixin Zhang et al, Phys. Rev. Lett. 97 (2006), 126103).

On a typical semiconductor surface, atoms in the top layers often rearrange themselves to form a reconstructed surface. For compound semiconductors such as gallium arsenide and zinc selenide, a simple electron-counting (EC) model has proven to be instrumental in identifying the various forms of surface reconstruction. Since its proposal, the EC model has been applied successfully to many homogeneous semiconductor systems. It has also been
invoked extensively in determining the structures of surface defects such as vacancies, steps, and islands formed during homoepitaxial growth.

However, for the growth of metal on a semiconductor, in the earliest stages the adsorption of a submonolayer of metal often leads to the appearance of much richer surface reconstruction patterns than that in the corresponding
homogeneous case. Because the reconstruction influences many important properties of the metal/semiconductor contacts such as the Schottky-barrier heights, it is vital to understand the precise form of reconstruction for a given system. More recently, such reconstructions have also been shown to play an important role in influencing the growth of diluted magnetic semiconductors at the growth front.

To date, the determination of metal-induced reconstruction of compound semiconductor surfaces has primarily relied on a trial-and-error approach, typically using scanning tunneling microscopy (STM) or other techniques for structural characterization, and results from extensive first-principles calculations as inputs on a case-by-case basis.

Based on theoretical analysis, first-principles calculations, and experimental observations, CAS IOP doctoral student Zhang Lixin, under the guidance of tutor En'ge Wang, has established a generic guiding principle, embodied in generalized electron counting (GEC), that governs the surface reconstruction of compound semiconductors induced by different metal adsorbates. Within the GEC model, the adsorbates serve as an electron bath,
donating or accepting the right number of electrons as the host surface chooses a specific reconstruction that obeys the classic electron-counting model. In the paper, the predictive power of the GEC model is illustrated for a wide range of metal adsorbates.