Ultrafast Carrier Transport and Space-Charge Field Formationin Semiconductors

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Materials growth and fabrication techniques now allow, for example, the introduction of barriers, wells dopants and strain in a controlled fashion and allow the growth of all- binary ordered materials (as an alternative to ternary alloys). Moreover, ultrafast laser techniques have been developed that now allow the measurement of charge transport over micron and nanometer dimensions with picosecond and femtosecond temporal resolution. The objective of this proposal, simply stated, is to take advantage of these advances in materials growth and in ultrafast laser spectroscopy to study, measure, optimize and control charge transport and space-charge field formation in semiconductor structures over nanometer spatial dimensions. The intent is not only to improve the transport, but simultaneously to enhance the magnitudes and speeds of the optical nonlinearities that result from such transport. Future advances in the development of photonic and electronic systems, and interconnects between the two, may well rely on, and be limited by, the ability to measure and control charge formation, screening and changes in optical properties that accompany such transport. Carrier transport over these dimensions will occur on sub-picosecond time scales and may even be ballistic. Moreover, even modest voltages applied across such small dimensions will produce huge fields and may result in hot carriers. While much is known about the relaxation of nonequilibrium carriers in energy within the bands, little is known about transport under these conditions of the effects of that transport on the nonlinear optical response. Since the idea is to engineer the structure to improve the transport and to enhance the optical nonlinearities that result from such transport, these studies will require the growth, investigation and optimization of several unique and non-traditional structures and materials for which growth techniques have not been extensively developed. Moreover, we will be engaged in an active iterative process in which the optical-based transport information will be correlated with structural characterization and fed back into the structural design and growth to obtain material and structures with improved transport and transport- related optical properties. In this way, structures can be deterministically designed and fabricated with a firm fundamental understanding of the influence of the characteristics under our control (strain, piezo-electric fields, dopants, interfaces, barriers, order, etc.) on the useful properties of the resulting structure.