Research Fields
My research focuses on the synthesis of novel materials, which includes the deposition of crystalline and amorphous thin films (zinc oxide), growth of single crystals (zirconium tungstate and transition metal phosphides), fabrication of nanocomposite glasses, and exploration of electronic, magnetic and optical materials (dilute magnetic semiconductors and electro-optics).  My group characterizes these materials by performing X-ray diffraction, elemental analysis, ellipsometry/reflectometry, and measuring electrical and magnetic properties.
DMS Materials

Recently, much attention has been drawn to a class of materials called dilute magnetic semiconductors (DMS) due to the discovery of carrier-mediated ferromagnetic properties in Mn-substituted p-type GaAs.  Based on DMS materials, the possibility exists to fabricate electronics such as transistors which do not rely on the magnitude of the electrical current to distinguish on and off states, but instead rely on the spin of the electrons.  Understanding the interaction of the hole electronic carriers and the localized electronic states of the magnetic ions, which are typically Mn, Fe, Co, and Ni substituted into a semiconductor lattice at approximately 5% on the cation site, is of importance in order to find materials with ferromagnetic Curie temperatures greater than room temperature.  Specifically, we are focused on materials with larger bandgap energies (phosphides versus arsenides) which empirically demonstrate higher Curie temperatures.  We primarily synthesize binary and ternary phosphides using a sodium flux method or vapor transport for crystal growth in addition to conventional powder preparation methods.

Piezoelectric Thin Films

Piezoelectric thin films are used in microelectromechanical systems (MEMS) and resonator devices.  These films are on the order of one micron in thickness and must be crystalline with a high degree of texture, i.e. crystallographically oriented grains.  Our studies have focused on zinc oxide thin films deposited by sputtering.  We have found different mechanisms of deposition depending on whether the target is initially an oxide or a metal.  In addition, the morphological evolution of the thin films is driven by surface diffusion and can be described by a modified structure zone model.  Correlations between the crystallinity, texture, electrical conductivity, and piezoelectric response have been observed by varying the substrate temperature during deposition.  Higher temperatures around 700 ˚C lead to crystalline, oriented films, however, due to an increase of leakage current by over 8 orders of magnitude compared to the films deposited near room temperature, the piezoelectric response is diminished.  Surprisingly, a significant piezoelectric response was observed for films deposited near room temperature even tough the X-ray diffraction intensity and Raman absorption profiles indicate highly disordered films.  This was attributed to local order only observed by electron diffraction.  Our interests are now focused on ternary oxide piezoelectric materials.

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