Research

Semiconductor Nanowires

Semiconductor nanowires provide a unique avenue to build upon existing electronic and optoelectronic device technology in a geometry that opens up new possibilities for band structure engineering and device integration. Our research is focused on the synthesis of semiconductor nanowires using vapor-liquid-solid (VLS) growth. In the VLS method, a metal is used as a solvent to mediate crystallization and preferential wire growth in one direction. We utilize gaseous chemical sources, typically used for chemical vapor deposition (CVD), as precursors for nanowire synthesis. Our research in this area combines basic studies of the nanowire growth process with detailed structural characterization to elucidate fundamental process-property relationships relevant to the synthesis of nanoscale materials. In collaboration with the Mayer, Dickey and Mohney groups, we are studying silicon and III-V nanowires for potential applications as high speed transistors in future nanoelectronic circuits. This work is supported by the NSF through a Nanoscale Interdisciplinary Research Team grant and by the Penn State MRSEC Center for Nanoscale Science. In collaboration with the Mallouk group, we are exploring the use of nanowire arrays in alternative energy applications for hydrogen generation and solar energy conversion.

Redwing Research Group

Text Box: Superconducting Thin Films Magnesium diboride (MgB2) is a recently discovered superconductor with a transition temperature of ~39K. There is growing interest in the use of MgB2 for high speed electronics and in superconducting magnet applications, including magnetic resonance imaging (MRI). These applications require the ability to form thin films and coatings of MgB2 with well-controlled properties. Our work in this area, which is carried out in collaboration with the Xi group at Penn State, has focused on the development of a chemical vapor deposition-based approach to fabricate MgB2 which offers unique advantages compared to competing deposition methods in terms of film properties and process flexibility. Utilizing computational fluid dynamics simulations in combination with experimental studies, we are developing a model of the MgB2 deposition process and exploring the effects of process conditions and reactor design on film properties and uniformity. We are also investigating new precursor chemistries for controlled impurity incorporation and the formation of MgB2 tri-layer structures for Josephson junction applications. This work is supported by ONR and NSF.

Text Box: Wide Bandgap Materials The group III-nitrides (GaN, AlN, InN) are compound semiconductors with bandgap energies that span the range from the UV (AlN, Eg=6.2eV) to the near IR (InN, Eg»0.7 eV). Nitride materials are used in a variety of electronic and optoelectronic devices such as high brightness blue/green light emitting diodes, which are used in displays and general illumination, and microwave power transistors which are of interest for radar and wireless communication systems. Our research is focused on the synthesis of group III-nitride thin films by metalorganic chemical vapor deposition (MOCVD). One of the current challenges associated with the deposition of GaN thin films is the lack of a native substrate. Consequently, GaN films are typically grown on lattice-mismatched substrates such as sapphire and SiC. The differences in lattice constant and thermal expansion between the GaN film and the substrate introduce stress and defects into the film which degrade device performance. We are utilizing laser reflectance techniques to directly monitor the film stress during GaN, AlGaN and InN growth on lattice mismatched substrates in order to correlate observed changes in stress with the evolution of microstructure as determined by post-growth transmission electron microscopy characterization. Our research is focused both on understanding the fundamental origins of film stress in the nitride material system and on developing practical approaches to mediate film stress and reduce dislocation density. This work is done in collaboration with the Wide Bandgap Materials group at the Penn State Electro-Optics Center and the Dickey group at Penn State with financial support provided by the NSF, AFRL and the Lehigh Center for Optical Technologies.