Professor Stefan K. Estreicher
Physics Department, Texas Tech University, USA

Vibrational dynamics of defects in semiconductors

Abstract
The identification of defects in semiconductors involves a wide variety of electrical, optical, and magnetic experimental techniques, as well as theoretical support... The interplay between theory and experiment has evolved over the decades. At first, theorists benefitted from the huge experimental database (mainly in Si) and used it to develop theoretical tools. Gradually, semiempirical parameters were eliminated and so-called ‘first-principles’ techniques appeared. Today, density-functional-based theory in periodic supercells can predict some defects properties with surprising accuracy. The method has known weaknesses and substantial theory developments are under way to overcome them. They include the use of hybrid functionals and many-body perturbation theory. These developments are already being applied to selected defect problems, but they are computationally demanding and/or difficult to implement.

This lecture will focus on the vibrational properties of defects in semiconductors (mostly Si) and on the connections between optical (and thermal) experimental techniques and first-principles theory. Examples will include the calculation of (i) local vibrational modes which can often be measured by FTIR or Raman spectroscopy; (ii) isotope effects in the Debye temperature of semiconductors; (iii) temperature- and isotope-dependence of vibrational lifetimes; and (iv) impurity-isotope effects in the predicted thermal conductivity of Si nanostructures.

After a brief overview of the most important developments in the theory of defects in semiconductors, I will describe the key ingredients of the ‘first-principle’ approach and mention its weaknesses and the ongoing theory developments. Then, I will focus on the calculation of dynamical matrices and the precious information contained in its eigenvalues and especially eigenvectors. They can be used to perform ab-initio molecular dynamics simulations without thermostat. Examples will include the calculation of LVMs of course, but also of specific heats and vibrational entropies, the prediction of vibrational lifetimes and of the impact of impurities on the (macroscopic) thermal conductivity. Our theoretical approach often mimics the experimental method used to measure the quantity we want to calculate.

 

Professor Estreicher received his PhD in theoretical physics from the University of Z├╝rich in 1982. He was a postdoc and then an instructor at Rice University in Houston (Texas). He then joined the Physics faculty at Texas Tech University in 1986, where he is now a Paul Whitfield Horn Professor. His research interests are in the theory of defects in semiconductors, with applications to electronics, photovoltaics, and energy-related issues. He has published about 180 papers, a number of which result from collaborations with experimentalists. Ongoing research deals with the thermal properties of semiconductors and semiconductor nanostructures containing impurities, the calculation of migration paths and activation energies, the properties of vacancy clusters, and on the role of H in Si photovoltaic materials. Although his interests extend to the history of wine, he has yet to find a way to get this aspect of his research supported. He is a Fellow of the American Physical Society and of the Institute of Physics, and won the Friedrich Wilhelm Bessel research award from the Alexander von Humboldt Foundation in 2001.