Geometric and electronic changes during interface alloy formation in Cu/Pd bimetal layers

This study involves monitoring the interface evolution with increasing annealing temperatures in a Cu–Pd bimetal layer structure. The changes due to interdiffusion and ensuing charge transfer are monitored by extensive X-ray photoelectron spectroscopy (XPS) and glancing angle X-ray diffraction (GAXRD) studies. The Pd and Cu core level fingerprinting and change in lattice parameter provide evidence for alloy formation. The changes in the valence band features indicate the formation of new states that are different from the density of states of individual metal surfaces. The study demonstrates the possibility of tuning interface properties by alloy formation that may have specific applications in catalysis, hydrogen sensing and storag

Optimization of energy and fluence of N2+ ions in the conversion of Al2O3 surface into AlN at room temperature

The work presents a systematic study of energetic N2+ ion interaction with the clean Al2O3 surface at room temperature. Energetic N2+ ions with energies ranging from 0.1 to 5 keV were bombarded onto the c-plane Al2O3 surface in situ in a UHV system equipped with X-ray Photoelectron Spectroscopy. Survey scans and core level spectra of Al(2p), O(1s), N(1s) were recorded as a function of ion fluence. Survey scans of XPS are used for the compositional analysis, while deconvoluted core level spectra are used to identify the evolution of the chemical bonding. Energetic dependence of N2+ ions occupying interstitial and substitutional sites in Al2O3 lattice are probed to follow the surface evolution. Results show that maximum thickness of surface is nitride by 5 keV N2+ ion with an optimal fluence of 1.5 × 1015 ions/cm2. This modified surface can be used as a template for low defect III-nitrides growth, with enhanced lattice matching than on bare c-Al2O3

Effect of Pb adatom flux rate on adlayer coverage for Stranski–Krastanov growth mode on Si(1 1 1)7 × 7 surface

Lead (Pb) has been a prototypical system to study diffusion and reconstruction of silicon surfaces. However, there is a discrepancy in literature regarding the critical coverage at which island formation takes place in the Stranski–Krastanov (S–K) mode. We address this issue by studying the initial stages of evolution of the Pb/Si(1 1 1)7 × 7 system by careful experiments in ultra-high vacuum with in situ characterization by auger electron spectroscopy, electron energy loss spectroscopy and low-energy electron diffraction. We have adsorbed Pb onto clean Si(1 1 1 )7 × 7 surface with sub-monolayer control at different flux rates of 0.05 ML/min, 0.14 ML/min and 0.22 ML/min, at room temperature. The results clearly show that the coverage of the Pb adlayer before the onset of 3D Pb islands in the S–K mode depends on the flux rates. LEED results show the persistence of the (7 × 7) substrate reconstruction until the onset of the island formation, while EELS results do not show any intermixing at the interface. This suggests that the flux rates influence the kinetics of growth and the passivation of dangling bonds to result in the observed rate-dependent adlayer coverages

Optimization of energy and fluence of N2+ ions in the conversion of Al2O3 surface into AlN at room temperature

The work presents a systematic study of energetic N2+ ion interaction with the clean Al2O3 surface at room temperature. Energetic N2+ ions with energies ranging from 0.1 to 5 keV were bombarded onto the c-plane Al2O3 surface in situ in a UHV system equipped with X-ray Photoelectron Spectroscopy. Survey scans and core level spectra of Al(2p), O(1s), N(1s) were recorded as a function of ion fluence. Survey scans of XPS are used for the compositional analysis, while deconvoluted core level spectra are used to identify the evolution of the chemical bonding. Energetic dependence of N2+ ions occupying interstitial and substitutional sites in Al2O3 lattice are probed to follow the surface evolution. Results show that maximum thickness of surface is nitride by 5 keV N2+ ion with an optimal fluence of 1.5 × 1015 ions/cm2. This modified surface can be used as a template for low defect III-nitrides growth, with enhanced lattice matching than on bare c-Al2O3

Transport and optical properties of c-axis oriented wedge shaped GaN nanowall network grown by molecular beam epitaxy

The transport and optical properties of wedge-shaped nanowall network of GaN grown spontaneously on cplane sapphire substrate by Plasma-Assisted Molecular Beam Epitaxy (PAMBE) show interesting behavior. The electron mobility at room temperature in these samples is found to be orders of magnitude higher than that of a continuous film. Our study reveals a strong correlation between the mobility and the band gap in these nanowall network samples. However, it is seen that when the thickness of the tips of the walls increases to an extent such that more than 70% of the film area is covered, it behaves close to a flat sample. In the sample with lower surface coverage (≈40% and ≈60%), it was observed that the conductivity, mobility as well as the band gap increase with the decrease in the average tip width of the walls. Photoluminescence (PL) experiments show a strong and broad band edge emission with a large (as high as ≈ 90 meV) blue shift, compared to that of a continuous film, suggesting a confinement of carriers on the top edges of the nanowalls. The PL peak width remains wide at all temperatures suggesting the existence of a high density of tail states at the band edge, which is further supported by the photoconductivity result. The high conductivity and mobility observed in these samples is believed to be due to a “dissipation less” transport of carriers, which are localized at the top edges (edge states) of the nanowalls

Clustering and layering of In adatoms on low and high index silicon surfaces: A comparative study

Using the morphological differences of low and high index surfaces as templates for metal growth, several low dimensional overlayer structures with novel structural and electronic properties can be formed. We present here a first report on submonolayer adsorption and residual thermal desorption studies of In adatoms on reconstructed high index Si (5 5 12)−2 × 1 surface and compare it with the observations on planar Si (111)−7 × 7 surface. The study is done by using in-situ Ultra High Vacuum surface sensitive probes like Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These conventional wide area techniques provide an understanding of atomistic issues involved in the evolution of the interface. We have observed an anomalous growth mode during adsorption at room temperature (RT) above 2ML, which includes adatom layering and clustering on Si (111) surface. This is also manifested during the desorption experiments on both surfaces, and the subtle differences on the two surfaces are discussed. The observation of LEED pattern during the adsorption process shows formation of different superstructural phases on Si (111)−7 × 7 surface. On Si (5 5 12) 2 × 1 surface we observe the sequential 2× (225), 2× (337) and 2× (113) facet formation during adsorption/desorption, which include quasi 1D-nanowire/chain structures. A combination of lattice strain effects, presence of step-edge barrier and quantum size effects are employed to speculate the differences in adsorption and desorption

Giant enhancement in ferromagnetic properties of Pd nanoparticle induced by intentionally created defects

The important central question related to origin of ferromagnetic properties in the non-magnetic materials at nano-dimensions has been investigated by a novel approach of studying the evolution of magnetic properties by intentionally creating defects. The ferromagnetic response of Pd is found to increase by 20 times for nanoparticle (NP) dispersed in carbon matrix and increase by about 3.5 times in case of Pd nanoparticles dispersed in SiO2 matrix on exposure to swift heavy ion irradiation. Ferromagnetic response is found to increase by about 9.3 times on subjecting the Pd nanoparticles to hydrogen loading-deloading cycle. Ferromagnetic properties of Pd nanoparticles dispersed in carbon and SiO2 matrices, despite having same size and concentration, are observed to be vastly different due to matrix effect. These changes in ferromagnetic properties are correlated to the change in the electronic structure due to matrix, nanoparticle size, and creation of defects in the nanoparticle core and at NP-matrix interface during post deposition treatments. Giant enhancement in the magnetic properties and change in electronic properties point toward a core and surface magnetic structure in metal nanoparticle. (C) 2012 American Institute of Physics