D. Aurongzeb

Controlled growth of GaN nanowires by pulsed metalorganic chemical vapor deposition

Controlled and reproducible growth of GaN nanowires is demonstrated by pulsed low-pressure metalorganic chemical vapor deposition. Using self-assembled Ni nanodots as nucleation sites on (0001) sapphire substrates we obtain nanowires of wurtzite-phase GaN with hexagonal cross sections, diameters of about 100nm, and well-controlled length. The nanowires are highly oriented and perpendicular to the growth surface. The wires have excellent structural and optical properties, as determined by x-ray diffraction, cathodoluminescence, and Raman scattering. The x-ray measurements show that the nanowires are under a complex strain state consistent with a superposition of hydrostatic and biaxial components.

Ignition studies of Al/Fe2O3 energetic nanocomposites

We prepare energetic nanocomposites, which undergo an exothermic reaction when ignited at moderate temperature. The nanocomposites are a mixture of Al fuel and Fe2O3 oxidizer where Fe2O3 is in the form of an array of nanowires embedded in the thin Al film. We achieve a very high packing density of the nanocomposites, precise control of oxidizer–fuel sizes at the nanoscale level, and direct contact between oxidizer and fuel. We find that the flame temperature does not depend on ignition temperature.

Evolution of surface roughness of AlN and GaN induced by inductively coupled Cl2/Ar plasma etching

We study the effects of plasma etching on the evolution of surface roughness of GaN and AlN. The etch-induced roughness is investigated using atomic force microscopy by systematically varying plasma power, chamber pressure, and Cl2/Ar mixture gas composition. GaN etches three to four times more rapidly than AlN for identical plasma conditions. For both GaN and AlN, we find that the surface roughness is correlated to etch rate. Induced roughness remains comparable to the as-grown value provided etching is carried out below rates 400 (GaN) and 90 nm/min (AlN). Above these cutoff etch rates, the roughness increases in proportion to etch rate. This result is independent of plasma parameters varied to produce the higher etching rates. By analyzing the surface properties through the power spectral density (PSD), we correlate roughness with the formation of fine-scale features present as a consequence of more aggressive etching. The cutoff etch rates and spatial-frequency dependence of the PSD are interpreted us...

Self-assembly of faceted Ni nanodots on Si 111

We report the formation of Ni nanodots on Si(111). Island density is varied by annealing temperature and time and is studied using atomic force microscopy (AFM) and magnetic force microscopy. Activation energies of 0.09±0.02 and 0.31±0.05eV are observed for the formation of these islands. These are associated with Ni surface self-diffusion across the (111) and (110) Ni facets, respectively. For brief 500°C anneals, regular nanodots are observed with self-limiting sizes of height ∼16nm and area 180nm×260nm, while density exhibits a power-law time dependence with exponent 1.13±0.12. AFM analysis reveals a “truncated hut” shape consistent with (110) top and (111) sidewall surfaces.

Plasma etching transfer of a nanoporous pattern on a generic substrate

We describe a nonlithographic nanofabrication method for creating a nanoporous pattern on any substrate. The approach utilizes plasma etching through a nanoporous template to transfer the pore pattern onto the substrate. We demonstrate this method to transfer a porous alumina pattern consisting of a hexagonal array of 50 nm diam pores onto an aluminum layer. A nanoporous alumina template (0.6 μm) is initially created by electrochemical anodization of an aluminum film (1 μm) deposited on a substrate. Controlled plasma etching is then used to etch through the pores onto the aluminum layer below the pores. In this manner, we demonstrate the hexagonal array of 50 nm diam pores in the aluminum film.

Influence of surface/interface roughness and grain size on magnetic property of Fe/Co bilayer

In this work, we report the influence of surface roughness and cluster size on coercivity of Fe∕Co bilayer. Coercivity was tuned by thermal annealing. No systematic trend was found for temperature dependent annealing. However, after annealing at 350 °C, we find systematic increase in coercivity with anneal time. For as-deposited film, we find unusually low coercivity (0.39 Oe). By increasing annealing time, coercivity was tuned to values as high as 600 Oe. Surface characterization using atomic force microscopy showed uniform clusters at this temperature after 2 h of annealing. The observed magnetic properties are discussed in terms of cluster size and surface/interface roughness.

Influence of nanocrystal growth kinetics on interface roughness in nickel–aluminum multilayers

We study the layer morphology of Ni/Al multilayer structures, with 50 nm period, as deposited and following 10 min anneals up through the melting temperature of Al. X-ray reflectivity measurement of the as-deposited film shows interference fringes, characteristic of a well-defined multilayer stack, with ∼1 nm interface roughness. Over a narrow anneal range of 360–500 °C these fringes diminish in amplitude and disappear, indicating elevated interface roughening. However, fringes are observed for anneal temperatures both below and above this range, indicating the presence of well-defined layers with smooth interfaces. A model, in which nanocrystal domains of intermetallic nickel aluminides form at the interfaces, is developed to quantify the annealing induced interface roughness. This model agrees well with the experimental results.

Formation of nickel nanodots on GaN

We examine the annealing-induced formation of nickel nanodots on GaN substrates. The initial Ni layer thickness is 2 nm. Annealing temperatures range from 550 to 930 °C. The islands are well defined at the highest temperatures. Island formation kinetics provide an activation energy of 0.34±0.07eV. Time dependence of the nanodot island areas, annealed at 750 °C, is consistent with a t2∕3. These observations are indicative of diffusion-limited ripening as the primary formation mechanism. X-ray diffraction results show that nickel gallides form at anneal temperatures 750 °C and above.

Diffusion process and formation of super-spin-glass state in soft magnetic Fe/Pt system.

We report results on surface and micromagnetic structures of Fe thin films consisting of a Pt underlayer. We use atomic force microscopy to study the surface structure evolution of the Fe films as a function of annealing time at an annealing temperature of 800°C. Power spectral density analysis shows saturation in roughness exponent after 15min of annealing. However, lateral correlation length and roughness continue to increase for up to 45min. At high annealing temperature, the authors find two separate phase correlation lengths and a single surface correlation length indicating super-spin-glass state in the system.

Self-assembly of fractal nanowires and stripe magnetic domain on stretchable substrate

Magnetic properties of Ni thin films on polydimethylsiloxane (PDMS) are being studied. The author finds that coercivity of the films grown on PDMS is two to three times higher than their rigid counterpart. Magnetic force microscopy studies show that surface morphology and magnetic domains are quite different on elastic substrate. 5nm thick films do not exhibit planar morphology but rather form very long nanowires. The 50nm thick films exhibit in-plane and canted magnetizations with nonmagnetic disordered regions within the thin films. Study suggests that elastomers coupled with magnetic thin films can give rise to interesting material properties.