Surface morphology of carbon-doped GaAs grown by MOVPE

The development of the surface structures of carbon-doped epitaxial GaAs layers grown by metalorganic vapor phase epitaxy was investigated by atomic force microscopy (AFM). Carbon-doped GaAs layers were grown using trimethyl gallium and a mixture of AsH₃/TMAs. The AFM micrographs were quantitatively analyzed through the determination of the height-height and height-difference correlation functions, which yields both the short and long range surface structures. The incorporation of carbon leads to the progressive roughening of the GaAs surface as well as an increase in surface correlation length. The high concentration of surface-adsorbed methyl radicals are suggested to lead to the diminution of growth rate and change in surface structure.     

Photochemical functionalization of gallium nitride thin films with molecular and biomolecular layers

We demonstrate that photochemical functionalization can be used to functionalize and photopattern the surface of gallium nitride crystalline thin films with well-defined molecular and biomolecular layers. GaN(0001) surfaces exposed to a hydrogen plasma will react with organic molecules bearing an alkene (C=C) group when illuminated with 254 nm light. Using a bifunctional molecule with an alkene group at one end and a protected amine group at the other, this process can be used to link the alkene group to the surface, leaving the protected amine exposed. Using a simple contact mask, we demonstrate the ability to directly pattern the spatial distribution of these protected amine groups on the surface with a lateral resolution of <12 mum. After deprotection of the amines, single-stranded DNA oligonucleotides were linked to the surface using a bifunctional cross-linker. Measurements using fluorescently labeled complementary and noncomplementary sequences show that the DNA-modified GaN surfaces exhibit excellent selectivity, while repeated cycles of hybridization and denaturation in urea show good stability. These results demonstrate that photochemical functionalization can be used as an attractive starting point for interfacing molecular and biomolecular systems with GaN and other compound semiconductors.     

Chemical Characterization of DNA-Immobilized InAs Surfaces Using X-ray Photoelectron Spectroscopy and Near-Edge X-ray Absorption Fine Structure

Single-stranded DNA immobilized on an III-V semiconductor is a potential high-sensitivity biosensor. The chemical and electronic changes occurring upon the binding of DNA to the InAs surface are essential to understanding the DNA-immobilization mechanism. In this work, the chemical properties of DNA-immobilized InAs surfaces were determined through high-resolution X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). Prior to DNA functionalization, HF- and NH(4)OH- based aqueous etches were used to remove the native oxide from the InAs surface. The initial chemical state of the surface resulting from these etches were characterized prior to functionalization. F-tagged thiolated single-stranded DNA (ssDNA) was used as the probe species under two different functionalization methods. The presence of DNA immobilized on the surface was confirmed from the F 1s, N 1s, and P 2p peaks in the XPS spectra. The presence of salt had a profound effect on the density of immobilized DNA on the InAs surface. To study the interfacial chemistry, the surface was treated with thiolated ssDNA with and without the mercaptohexanol molecule. An analysis of the As 3d and In 3d spectra indicates that both In-S and As-S are present on the surface after DNA functionalization. The amount of In-S and As-S was determined by the functionalization method as well as the presence of mercaptohexanol during functionalization. The orientation of the adsorbed ssDNA is determined by polarization-dependent NEXAFS utilizing the N K-edge. The immobilized ssDNA molecule has a preferred tilt angle with respect to the substrate normal, but with a random azimuthal distribution.     

Making the Nanoworld Comprehensible: Instructional Materials for Schools and Outreach

Journal of Nanoparticle Research -     

Controlled formation of GaAs pn junctions during hydride vapor phase epitaxy of GaAs

Interface formation in HVPE GaAs was investigated through the growth of multilayer test structures with alternately doped and undoped layers and subsequently, pn diode devices. Two growth procedures were used in device formation: continuous growth of all layers, and a growth interruption with simultaneous equilibration of new gas flows for subsequent layers. These junctions were probed using SIMS to determine the doping profiles and impurity incorporation near the interfaces and throughout the bulk of the layers. The junction I–V characteristics were measured with and without illumination to correlate the junction properties with the measured photovoltaic performance. It was discovered that the use of a growth interruption leads to doping transitions up to 6x narrower than samples grown without interruption. The growth interruption leads to an interfacial Si spike that is not observed in the uninterrupted samples during growth of GaAs doped with silane. This spike does not appear to degrade either the material quality or pn junction quality, and pn diodes grown with interruption have exhibited enhanced device efficiencies under solar simulation compared with devices grown without interruption, reaching efficiencies of up to 9.2% without the use of antireflective coatings.     

Stabilization of Copper Catalysts for Liquid‐Phase Reactions by Atomic Layer Deposition

Atomic layer deposition (ALD) of an alumina overcoat can stabilize a base metal catalyst (e.g., copper) for liquid‐phase catalytic reactions (e.g., hydrogenation of biomass‐derived furfural in alcoholic solvents or water), thereby eliminating the deactivation of conventional catalysts by sintering and leaching. This method of catalyst stabilization alleviates the need to employ precious metals (e.g., platinum) in liquid‐phase catalytic processing. The alumina overcoat initially covers the catalyst surface completely. By using solid state NMR spectroscopy, X‐ray diffraction, and electron microscopy, it was shown that high temperature treatment opens porosity in the overcoat by forming crystallites of γ‐Al₂O₃. Infrared spectroscopic measurements and scanning tunneling microscopy studies of trimethylaluminum ALD on copper show that the remarkable stability imparted to the nanoparticles arises from selective armoring of under‐coordinated copper atoms on the nanoparticle surface.