Alloys & Semiconductor Systems

A major focus of my research for many years has been the testing of the assumptions behind the most successful theory of disordered alloys, the coherent potential approximation (CPA) which treats random alloys as an ordered array of average scattering potentials. We have shown that assumptions made in the CPA concerning both physical structure and electronic structure are questionable. This has proved controversial but has contributed to advances in CPA theory so as to remove some of its major limitations. This research has employed a variety of experimental techniques most notably photoelectron and Auger spectroscopies using both laboratory instruments and synchrotron sources and also involved the development of potential models for the separation of initial state and final state effects in the interpretation of the results of electron spectroscopy.

Major achievements have been made in relating Auger spectral profiles to local electronic structure particularly in establishing the validity of the Cini-Sawatzky theory and in extending the Hubbard Hamiltonian to include off-site interactions. We have also been able to quantify the link between the physical and electronic structure of the classic CuPd alloy system and made the first observation of disorder broadening of photoelectron lines in disordered alloys by charge transfer effects. This latter work demonstrates the need to distinguish between the Madelung energy and the Madelung potential in disordered systems and provides a new link between the results of electron spectroscopy and the results of total energy calculations of the electronic structure of alloys.

The development of a potential model lead to a new definition of electronegativity and put this important chemical concept on a firm quantum mechanical basis. This resolved the long standing problem of how to define and measure charge transfer, gave insight into the nature of electron screening in metals and semiconductors and yielded the first determination of the charge transfer across interfaces; the As/Si(100) 2x1 and As/Si(111) 1x1 interfaces which are important intermediates in attempts to integrate Si and GaAs based semiconductor technologies. This approach also lead to the first determination of the activation level of a delta doped layer in a semiconductor using electron spectroscopy; Sb in Si, yielding a result in agreement that determined from conventional transport measurements.

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