An overview of our recent works on one-dimensional nanostructures, with special emphasis on group III nitrides nanowires¹, nanobelts² and nanotips³ will be presented. All these 1D nanostructures exhibit interesting optical and electrical properties unmatched by their thin film or bulk counterparts. For instance, the InN nanobelts exhibit sharpest PL peak in the IR region reported to date for the InN system, and their power-dependent PL data also suggest lasing behavior. A "step edge" model has been proposed to explain the growth of solid nanotips of AlN and InN. The model predicts a set of apex angles for AlN and InN nanotips, which is in good agreement with the apex angles measured in InN nanotips⁴. Photoconductivity (PC) studies on arrayed GaN nanowires reveal responsivity and gain several orders of magnitude higher than those reported for thin films. A spatially separated electronic transport mechanism is proposed to explain the ultra-long carrier lifetime (>10^-3 s) observed in nanowires. A scheme that allows on-chip growth of an ensemble of nanowires for device application is also demonstrated. In addition, nanoscale engineering and strategies for either dispersing noble-metal nanoparticles on the nanotip arrays or encapsulating them inside dielectric nanowires, in a self-assembled manner, will be presented.

¹C.-C. Chen et al., J. Am. Chem. Soc. 123, 2791-2798 (2001).

²M. S. Hu et al., Adv. Func. Mater. 16, 537 (2006).

³C.H. Hsu et al., Nano Letters 4, 471 (2004).⁴ S. Chattopadhyay et al., An invited review article for Critical Reviews in Solid state and Material Science 31, 15-53 (2006).

GaN is a well known material for its potential electronic and optoelectronic applications. However, GaN could have structural defects arising due to mismatch in lattice parameters and thermal expansion coefficients between the GaN epilayer and its substrates. In order to reduce the number of defects, freestanding GaN samples have very recently been fabricated. The GaN samples are grown by hydride vapor phase epitaxy (HVPE) on sapphire substrates and then removed from the substrate by laser lift-off process. These samples are expected to have better crystalline quality and lesser defects due to the strain relaxation in the freestanding GaN films.

To determine the crystalline quality, strain distribution, and surface defect structure, high resolution x-ray topographs and rocking curves were obtained on free standing HVPE grown n-type GaN. Intrinsic rocking curves were measured for the (00l) reflections using a triple axis four-circle diffractometer. The full width at half maximum (FWHM) ranged between 60 and 88 arcseconds, indicating good crystalline quality.

High resolution x-ray topographs were obtained using double crystal geometry in which asymmetrically cut Si (111) was used to monochromatize and expand the incident x-ray beam. The topographic images revealed areas of high dislocation density, ridges, and inclusions. In addition, hexagonal cavities are also observed. Since the films were warped, the radius of curvature was measured and found it be of the order of 1 m.

Photoluminescence (PL) and Cathodoluminescence (CL) were applied to determine the nature of extended defects in these freestanding films. FWHM of the PL spectra was used as a figure of merit for the quality of the films and was found to be consistent with the FWHM measured by HRXRD. Also, presence of the hexagonal cavities was confirmed by the CL measurements.

GaN is a wide band gap (3.4 eV) semiconductor, which is of great interest for various applications, such as short wavelength optoelectronic device applications, as well as high power/high temperature electronic applications. In recent years, there has been increasing interest in the fabrication and characterization of GaN nanostructures.

In this work, GaN nanowires were fabricated on Si substrates coated with Au or Ni catalyst with a thermally-assisted chemical vapor deposition method by evaporating mixture of Ga(0.04g), GaN(0.04g), MnO₂ and carbon (single wall carbon nanotubes (SWCNT) or graphite) at 1100°C in ammonia gas flow. The fabricated nanostructures were characterized by scanning electron microscopy using LEO 1530 FESEM, energy dispersive X-ray spectroscopy, and X-ray diffractometry. We investigated the influence of the source of carbon and substrate temperature on the morphology and composition of the nanowires. We found that the nanowire composition is strongly affected by the substrate temperature, with oxygen incorporation increasing with decreasing substrate temperature. All nanowires exhibited very low Mn content, and nanowire composition and morphology were also dependent on whether graphite or carbon nanotubes were used as a source material. Growth mechanism and properties of obtained nanostructures are discussed.