Materials simulation from solid-state physics

Electronic structure calculations of materials are increasingly useful nowadays with new algorithms and computational methods, and advances in solid state theory. Many properties of materials can now be determined directly from first-principles calculations, providing new insightful and critical information in physics and materials science. Today’s talk will focus on two topics, Diluted Magnetic Semiconductors (DMS) for spintronics and semiconductor nanostructures for hot carrier solar cells.

DMS in which cations are partially replaced by transition metals could incorporate ferromagnetism into semiconductors. The physics based on the interplay of the electronic charge and spin in a single substance would spawn a new field of electronics known as “Spintronics” [1]. A search for the room temperature ferromagnetism in DMS is currently one of the most intensely studied topics [2]. However, from both theoretical and experimental point of view, there are still unanswered questions as to the origin of this ferromagnetism, from magnetic ions actually substituting in the lattice or from secondary magnetic phases or metal precipitates; and the mediation mechanism of the long ranged ferromagnetism, by electrons or holes? First principles calculations were performed in various DMS systems to explore the physical mechanism of magnetism in DMS.

The energy loss rate of a carrier (thermalization rate) is determined by both the rate at which the carrier’s energy is lost by optical-phonon emission and the rate at which the carrier gains energy from optical-phonon absorption. The latter rate can be significant in low dimensional quantum structures since the phonon emitted by energetic carriers can accumulate due to the phonon spatial confinement. When the phonon densities in confined semiconductors are typically well above the equilibrium distribution (called hot), phonons will be reabsorbed by the carriers. The phenomenon of re-absorption of hot phonons is referred to as the ‘hot-phonon-bottleneck effect’, which is an important effect for the performance of many low dimensional semiconductor electronic devices. In the new concept of hot carrier photovoltaics (HCPV)[3], if the hot carriers with high kinetical energy can be collected before their thermalization, the work per one absorbed photon can be increased greatly. Experimentally, it has been found that the hot carrier cooling rate can be much slower in GaAs quantum wells compared to bulk GaAs at very high illumination intensities[4]. Experimental and theoretical studies searching for good hot carrier solar cell absorber materials with slowed carrier cooling rate up to nanoseconds without applying high illumination intensities have been under way [5]. Phononic engineering using low dimensional confined structures and mass difference were used to design materials and quantum-structures for HCSC.

1. I. Zutic, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004); Semiconductor Spintronics and Quantum Computation, Edited by D. D. Awschalom, D. Loss, and N. Samarth, Springer, Berlin.
2. T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science, 287, 1019 (2000).
3. Robert T. Ross, Arthur J. Nozik, J. Appl. Phys. 53, 3813 (1982).
4. A. J. Nozik, C. A. Parsons, D. J. Dunlary, B. M. Keyes, R. K. Ahrenhiel, Solid state Comm. 75, 247 (1990).
5. G. J. Conibeer, D. Konig, M. A. Green, J. F. Guillemoles, Thin Solid Films, 516, 6948 (2008).