Date on Master's Thesis/Doctoral Dissertation

12-2005

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Committee Chair

Sunkara, Mahendra K.

Author's Keywords

Gallium nitride; Nanowire; Single crystals

Subject

Crystal growth

Abstract

III-nitrides (InN, GaN, AlN) are some of the most promising materials for making blue light emitting diodes (LED), blue laser diodes (LD) and high power, high temperature field effect transistors (FET). Current techniques produce GaN films with defect densities on the order of 107/cm2 or higher. The performance and life-time of the devices critically depend upon the defect densities and high power, high frequency devices require the defect densities to be lower than 104/cm2. So, the need for new processes to produce large size GaN crystals with defect densities less than 107/cm2 is immediate. In addition to large area single crystals (or wafers), the nanowires also present as an alternating platform for making devices. So, the processes for controlled synthesis, modifying and integrating sub 100 nm nanowires into electronic devices are of great interest. This thesis presents a new concept of ‘self-oriented growth' of GaN platelets shaped crystals on molten gallium to produce near single crystal quality GaN films over large areas (> 1 cm²). The process involves direct nitridation of Ga films using nitrogen plasma at low pressures (few mTorr). GaN flakes with areas over 25 mm² have been successfully obtained. Raman spectra of the resulting GaN crystals show no stress and low native donor concentration on the order of 1017/cm³. XRD texture analysis showed an overall c-axis tilt of 2.2o between GaN domains within the flake. The cross-sectional TEM micrographs showed that the resulting GaN films are free from dislocation crops inside the grains but showed diffraction contrast due to small mis-orientation between the grains. The twist and tilt angles between adjacent columnar grains were determined using convergent beam electron diffraction technique to be less than 8o and 1o, respectively. HRTEM micrographs of the grain boundaries showed sharp interfaces resulting with both twisted and perfect attachments. This thesis also presents direct synthesis approach for GaN nanowires with control on growth directions using modified nitridation conditions. The nitridation in the presence of hydrogen or ammonia resulted in oxide sheath free GaN nanowires as thin as 20 nm and long as 100 Ìm in <0001> direction. The nitridation using low Ga flux in a vapor transport set-up resulted in sub 100 nm GaN nanowires with <10-10> growth direction. The difference in the nucleation and growth mechanism allowed control on the nanowire directions. Homo-epitaxial experiments onto pre-synthesized GaN nanowires with the above two growth directions using the vapor transport of Ga and dissociated ammonia exhibited different morphologies, e.g. micro hexagonal columns for <0001> nanowires and micro belts for nanowires with <10-10> growth direction. The results further illustrate a new phenomenon of enhanced surface diffusion on nanowires in general but more pronounced for wires with <0001> growth direction. The results from homo-epitaxy experiments suggest that the <10-10> direction wires could be used as "seeds" for growing large area GaN crystals in vapor phase homo-epitaxy schemes.

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