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The galvanostatic electrodeposition of epitaxial FexNi1-x films on n-GaAs(001) substrates from aqueous metal ammonium sulfate solutions is reported. Structural measurements using X-ray diffraction and transmission electron microscopy indicate that the films have single crystalline or highly oriented body-centered cubic (bcc) or face-centered cubic (fcc) structure at Fe and Ni-rich compositions, respectively. The preparation of the substrate surface via ammonium hydroxide as well as the usage of an ammonium sulfate electrolyte buffer facilitates the initial nucleation of aligned metal islands on GaAs. The ratio of the Fe atomic concentration in the fcc films to that in the electrolyte is close to unity (1.1), whereas a preferential incorporation occurs for the bcc phase such that this ratio increases linearly with Fe atomic composition at a rate of 3.7. bcc FexNi(1-x)/GaAs diodes have uniform Schottky barrier heights as measured by current-voltage measurements independent of Fe composition Consistent with a high interfacial state density. The observed magnetic properties of FexNi1-x are consistent with single crystalline material with distributed inhomogeneities. The epitaxy via electrodeposition suggests the importance of ammonium sulfate for technical applications. [ABSTRACT FROM AUTHOR]

The hot electron transport through Au/Pd or Au/Ag/molecular layer/n-GaAs diodes has been studied by using ballistic electron emission microscopy (BEEM). The monolayers consisted of either octanedithiol (HS–(CH2)8–SH) or hexadecanethiol (HS–(CH2)15–CH3) linear alkane chains and were deposited from ethanol solutions onto molecular beam epitaxially grown GaAs (001) surfaces. The metals were electron-beam evaporated onto the molecular layer. Similar to previous results for Au diodes, Au/Pd metallization onto an octanedithiol monolayer resulted in a large reduction in the BEEM signal, compared to Au and Au/Pd reference diodes (without monolayers). However, unlike Au/octanedithiol diodes, the BEEM signal was stronger and the threshold was found to be 1.2 eV instead of 1.4 eV. We attribute this to ballistic electron transmission rather than to the photonic signal that we previously reported for Au/octanedithiol. In comparison, the Au/Pd/hexadecanethiol diodes showed little change from the reference Au/Pd diodes, suggesting monolayer disruption by interdiffusion of Pd or Au through the molecules to the interface. Ag/GaAs or Au/Ag/GaAs diodes, with or without a molecular layer, did not result in reliable tunneling, likely the result of a tip-induced reaction. The octanedithiol layer on GaAs provides a free thiol group for bonding with the top metal contact, minimizing Au or Pd penetration and the displacement of the molecule-GaAs thiol bond but not completely eliminating these effects in the case of Pd. [ABSTRACT FROM AUTHOR]

Epitaxial Bi/GaAs diodes have been formed by electrodeposition from bismuth nitrate and ammonium sulfate ((NH4)2SO4) aqueous solutions. Bi grows (0001) oriented on both GaAs (111)B and (001) substrates while it tilts 16° to a (011_8) surface orientation for (011) GaAs. The metal orients in all cases with its {112_0} planes parallel the GaAs {110} planes. Diodes prepared on (001), (111)B, and (011) wafers have current-voltage barrier heights [uppercase_phi_synonym]BIV that vary from 0.74, to 0.76, to 0.83 eV (n=1.01–1.11), respectively. These barrier heights straggle values from earlier reports for polycrystalline Bi deposited by ultrahigh vacuum techniques or electrodeposition. Barrier heights measured from high frequency, capacitance-voltage characteristics are higher than the [uppercase_phi_synonym]BIV results, 0.06–1.5 eV, as a function of the GaAs orientation, increasing in value in order of (011), (001), to (111)B. This is explained by a combination of image force lowering and field emission corrections, and interface state/dipoles that are likely dependent on the GaAs orientation and on the degree of (0001) Bi alignment. These results are supported by cross-sectional transmission electron microscopy investigations indicating abrupt Bi/GaAs interfaces without evidence of a significant interfacial oxide or reacted layer. [ABSTRACT FROM AUTHOR]

The shape evolution of interface voids in copper metallization systems under electromigration stress is modeled including instabilities driven by both capillary and electron wind forces. The model employs a three-dimensional, axisymmetric, finite-difference method combined with a boundary-element electrical potential. With zero electric field or with small fields applied, a large void experiences a capillary instability which leads to open circuit failure. As the electric field becomes larger, the growth of this instability is suppressed and the void shape stabilizes. Thus, for a typical electromigration stress condition, a large copper void elongates its shape along the interface parallel to the electric field, suggesting a mechanism for the delayed open circuit failure observed in copper integrated circuit metallization system. [ABSTRACT FROM AUTHOR]

Channeling of low energy (25 to 35 keV) focussed He ions transmitted through crystalline Si (001) nanomembranes (50 nm) has been observed using a He ion microscope. Planar (110) and (100) channeling was detected with critical incident angles of 1.0° at 35 keV. Beam steering of up to 2° occurs. The technique has potential for He ion diffraction and femtometer-scale detection of interstitial atoms and impurities. [ABSTRACT FROM AUTHOR]

Electron beam induced current (EBIC) measurements were carried out in situ in the scanning electron microscope on free-standing GaAs/Fe core-shell nanowires (NWs), isolated from the GaAs substrate via a layer of aluminum oxide. The excess current as a function of the electron beam energy, position on the NW, and scan direction were collected, together with energy dispersive x-ray spectroscopy. A model that included the effects of beam energy and Fe thickness predicted an average collection efficiency of 60%. Small spatial oscillations in the EBIC current were observed, that correlated with the average Fe grain size (30 nm). These oscillations likely originated from lateral variations in the interfacial oxide thickness, affecting the resistance, barrier potentials, and density of minority carrier recombination traps.

The growth of heteroepitaxially strained semiconductors at the nanoscale enables tailoring of material properties for enhanced device performance. For core/shell nanowires (NWs), theoretical predictions of the coherency limits and the implications they carry remain uncertain without proper identification of the mechanisms by which strains relax. We present here for the Ge/Si core/shell NW system the first experimental measurement of critical shell thickness for strain relaxation in a semiconductor NW heterostructure and the identification of the relaxation mechanisms. Axial and tangential strain relief is initiated by the formation of periodic a/2 <110> perfect dislocations via nucleation and glide on {111} slip-planes. Glide of dislocation segments is directly confirmed by real-time in situ transmission electron microscope observations and by dislocation dynamics simulations. Further shell growth leads to roughening and grain formation which provides additional strain relief. As a consequence of core/shell strain sharing in NWs, a 16 nm radius Ge NW with a 3 nm Si shell is shown to accommodate 3% coherent strain at equilibrium, a factor of 3 increase over the 1 nm equilibrium critical thickness for planar Si/Ge heteroepitaxial growth.

Infrared spectroscopic ellipsometry was used for determination of molecular orientation and for lateral homogeneity studies of organic monolayers on GaAs and Au, the organic layer being either octanedithiol or hexadecanethiol (HDT). The laterally resolved measurements were performed with the infrared mapping ellipsometer at the synchrotron storage ring BESSY II. The molecular orientation within the monolayers was determined by optical model simulations of the measured ellipsometric spectra. Different tilt angles were obtained for the monolayers of HDT and octanedithiol on GaAs: 19 degrees and >30 degrees , respectively. The tilt angle of the methylene chains for HDT on Au substrate (22 degrees ) is similar to the 19 degrees tilt which was obtained for the HDT monolayers on GaAs, thus suggesting similar molecular ordering of the thiolates on both substrates.

Epitaxial growth of vertical GaAs nanowires on Si(111) substrates is demonstrated by metal-organic chemical vapor deposition via a vapor-liquid-solid growth mechanism. Systematic experiments indicate that substrate pretreatment, pregrowth alloying temperature, and growth temperature are all crucial to vertical epitaxial growth. Nanowire growth rate and morphology can be well controlled by the growth temperature, the metal-organic precursor molar fraction, and the molar V/III ratio. The as-grown GaAs nanowires have a predominantly zinc-blende crystal structure along a <111> direction. Crystallographic {111} stacking faults found perpendicular to the growth axis could be almost eliminated via growth at high V/III ratio and low temperature. Single nanowire field effect transistors based on unintentionally doped GaAs nanowires were fabricated and found to display a strong effect of surface states on their transport properties.