Research topics span :

• Thin Film Photovoltaics
• Semiconductor Nanocrystals
• Active Ferroelectric Oxide Thin Films
• Plasmonics - Nanophotonic Materials and Devices

Applications of these research topics include :

• Solar cells and LEDS
• Nonvolatile memory devices
• MEMS Devices
• Waveguides and modulators

Brief Description of Research Areas :

• Nanocrystal Electronic Materials and Devices
  In the mesoscopic size regime, semiconductor materials have size-tunable properties that are intermediate between those of single atoms and their corresponding bulk solids. We are studying optical and transport properties of group IV semiconductor (Si and Ge), group III-V (GaAs) and group II-VI (CdSe, PbSe) nanocrystals that behave electronically as 'quantum dots'. We are investigating the photophysics and carrier injection dynamics in these materials and a recent theme has been the use of field effect structures for carrier injection into nanocrystal arrays. Using field effect carrier injection into silicon nanocrystals arrays we have explored switching between radiative excitonic and nonradiative Auger emission from silicon nanocrystals, and recently we identified a new electrical injection process called field effect electroluminescence, in which field effect modulation of a metal/oxide/semiconductor structure enables sequential electron and hole injection into nanocrystals, followed by radiative excitonic emission, enabling programmed excitonic emission. We have also observed an overall enhancement of light emission from coupled arrays of metallic nanostructures and semiconductor nanocrystals, in which proximity between metal nanostructures and semiconductor quantum dots alters the rates for both photon absorption and excitonic emission.
• Nanophotonic Materials and Devices - Plasmonics
  Since the development of the light microscope in the 16th century, optical device size and performance has been limited by diffraction. Optoelectronic devices of today are much bigger than the smallest electronic devices for this reason. However the spatial confinement of light at dimensions less than 10% of the free-space wavelength is possible in plasmonic materials. Such plasmonic devices exploit light localization in thin metallic films or the dipole-dipole coupling at the plasmon frequency between nanoscale metal particles in particle chain arrays, and the guided wave dispersion relations are highly tunable. . Light can also be propagated around sharp corners and through nanoscale networks. Thus there appears to be no fundamental scaling limit to the size and density of photonic devices, and ongoing work is aimed identifying important device performance criteria in the subwavelength size regime. Ultimately it may be possible to design a class of subwavelength-scale optoelectronic components (waveguides, sources, detectors, modulators) that could form the building blocks of a chip-based optical device technology that is scaleable to molecular dimensions, with potential imaging, spectroscopy and interconnection applications in computing, communication and chemical/biological detection.
• Thin Film Photovoltaics: Si Thin Film, Si Nanorod and III-V Multijunction Cell Structures
  Photovoltaics(PV), the direct generation of electric power from sunlight, is currently enjoying both intensive scientific growth and large technology investments. While there are many possible options for photovoltaic materials and devices, the key performance metric is the cost per Watt of PV-generated electricity. Thus while photovoltaic solar cells are semiconductor devices, the cost of device processing per unit area must be several orders of magnitude less expensive than typical microelectronics processing. While most current solar cell manufacturing is done with wafer-based crystalline or multicrystalline silicon, the future of photovoltaics lies in development of inexpensive thin film and nanostructured devices and processes with potential for breakthroughs to ultrahigh efficiency (circa 50%). We are investigating several approaching to very low cost and ultrahigh efficiency photovoltaics, including thin film poly-Si and Si nanorod-based cells, and also ultrahigh efficiency multijunction solar cells fabricated with III-V compound semiconductor absorber layers.
• Active Ferroelectric Oxide Thin Films
  Ferroelectric thin film integration with Si and other integrated device substrates has the potential to enable new modes of integration of high work/volume piezoelectric devices for MEMS integration as well as a new class ferroelectric photonic devices. To date, most ferroelectrics integration efforts have focused either on growth of polycrystalline films with poor control of microstructure or on perovskite epitaxial growth Si or another single crystal substrate as a template. However, realistic integration schemes with electronic and photonic devices probably will not allow growth directly on a single crystal silicon substrate, but rather will require perovskite oxide integration on top of amorphous dielectrics. Thus we have focused on two approaches for integration of PbxBa1-xTiO3 and LiNbO3 on Si using biaxially-textured MgO templates and direct wafer bonding. Biaxially textured MgO is formed by ion-beam assisted deposition on amorphous silicon nitide, and films are obtained with a narrow dispersion in fiber texture (< 4 degrees) and also a narrow dispersion of in-plane orientations (< 7 degrees). The MgO films form a template for heteroepitaxy of perovskites. To date we have demonstrated epitaxial growth of PbxBa1-xTiO3/MgO/SiN/Si by a solution organic deposition synthesis and BaTiO3/MgO/SiN/Si by metallorganic chemical vapor deposition (MOCVD). An integration approach that yields truly single-crystal films is realized via wafer bonding and ion implantation-induced layer transfer. This technique enables fabrication of BaTiO3 and LiNbO3 single crystal thin films with excellent control of crystallographic and domain microstructure relative to other thin film fabrication methods. Current research focuses on the role of microstructure in determining ferroelectric film properties and development of BaTiO3 and LiNbO3single crystal thin films for new integrated photonic and MEMS device applications.