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The infrared optical properties of thick GeSn films (>500 nm) having 10% Sn concentration and of SiGeSn layers, utilized for the growth of strain-relieved, direct-gap GeSn films by molecular beam epitaxy, are investigated. Two growth methods are used: a graded-growth structure and a stepped-growth structure that help us to illustrate the properties of the GeSn and SiGeSn layers. Interestingly, there can be strong absorption in SiGeSn films throughout the infrared. We observe an increase in infrared absorption with increasing Sn concentration up to 21% Sn and in films, where the Sn is held constant at 18%, with increasing Si concentration up to 30%. Cavity effects in the infrared transmission measurement of stepped-growth structures are observed and associated with reflections at growth interfaces. Si–Si bond formation is proposed to occur at high Si concentrations in SiGeSn films, and the bandgap in SiGeSn films appears to decrease with increasing Si and Sn concentrations. [ABSTRACT FROM AUTHOR]

Inclusion of Si atoms to the growth surface during the molecular beam epitaxy of Ge and Sn to form a SiGeSn alloy was identified as a reactive surface species and as a means to compensate strain, which allowed for the subsequent growth of GeSn alloys with high Sn content. The development of a SiGeSn virtual substrate having a 15% Sn concentration and lattice parameter larger than 5.72 Å is demonstrated, using atomic force microscopy, x-ray reciprocal space mapping, and transmission electron microscopy, as a method for the direct growth of thick (>500 nm) fully relaxed GeSn alloys with greater than 10% Sn. This buffer layer enables the monolithic integration of GeSn with silicon for optoelectronic applications, as the SiGeSn virtual substrate allows for selective chemical etching of GeSn, which is important for device fabrication. [ABSTRACT FROM AUTHOR]

We demonstrate field emission from an integrated three-terminal device using a suspended planar graphene edge as the source of vacuum electrons. Energy spectra of the emitted electrons confirm the field-emission mechanism. The energy spectra produced by graphene grown by chemical vapor deposition and reduced graphene oxide are compared. The drain-source voltage required to produce a given drain current increases when negative voltages are applied to the gate, confirming field-effect transistor operation. The emission current rises exponentially with inverse voltage over the measured current range from 1 pA to 10 nA. The current-voltage characteristics are consistent with tunneling through barrier potentials calculated numerically from the device geometry. [ABSTRACT FROM AUTHOR]

The properties of reduced graphene oxide (GO) are reported from a non-chemical reduction method. Ultra-high vacuum annealing of GO films in the thickness of 1-80 nm was studied by XPS, AFM, UV-Vis-NIR, Raman and TEM to observe the controlled removal of oxygen. We observed the loss of hydroxyl (C-OH) at low temperatures (<600 °C) followed by the complete loss of carbonyls (C=O) and epoxy (C-O-C) species by 1200 °C. As oxygen was removed, we observed a decrease in the layer spacing between the GO sheets and a concurrent decrease in the film resistance. While the Raman spectroscopy showed no change with reduction, indicating no change in the overall defect density or the general structure of the GO, the transmission spectra showed a shift in the transmission minimum from 245 nm to 260 nm and a total decrease in transmission above 800 nm occurs as the films visibly darken. TEM indicated that there is turbostratic stacking of the graphene layers as the reduction occurs, leading us to conclude that at a certain threshold of reduction the film properties are similar to epitaxial graphene growth on the C-face of SiC, but that a reduction gone too far results in a layer spacing equivalent to graphite. [ABSTRACT FROM AUTHOR]

Intelligent engineering of graphene-based electronic devices on SiC(0001) requires a better understanding of processes used to deposit gate-dielectric materials on graphene. Recently, Al2O3 dielectrics have been shown to form conformal, pinhole-free thin films by functionalizing the top surface of the graphene with fluorine prior to atomic layer deposition (ALD) of the Al2O3 using a trimethylaluminum (TMA) precursor. In this work, the functionalization and ALD-precursor adsorption processes have been studied with angle-resolved photoelectron spectroscopy, low energy electron diffraction, and X-ray photoelectron spectroscopy. It has been found that the functionalization process has a negligible effect on the electronic structure of the graphene, and that it results in a twofold increase in the adsorption of the ALD-precursor. In situ TMA-dosing and XPS studies were also performed on three different Si(100) substrates that were terminated with H, OH, or dangling Si-bonds. This dosing experiment revealed that OH is required for TMA adsorption. Based on those data along with supportive in situ measurements that showed F-functionalization increases the amount of oxygen (in the form of adsorbed H2O) on the surface of the graphene, a model for TMA-adsorption on graphene is proposed that is based on a reaction of a TMA molecule with OH. [ABSTRACT FROM AUTHOR]

Presents silicon resonant interband tunnel diodes that demonstrate negative differential resistance at room temperature with peak-to-valley current ratios greater than 2. Use of low-temperature molecular-beam epitaxy to grow the structures; Atomic distribution profiles of the as-grown and annealed structures.

Segregation of Ge in the ‘‘leading edge’’ of Si1-xGex alloys grown by molecular beam epitaxy is investigated using x-ray photoelectron spectroscopy (XPS). Alloys of 5%, 10%, 20%, and 40% Ge were grown in varying thickness (0–20 nm) at 500 °C to observe segregation during the initial stages of alloy growth. The length of the leading edge was found to decrease with increasing Ge concentration (4.8, 2.8, 2.4, and 2.0 nm, respectively). The amount of segregated Ge was found to increase with Ge concentration. A complete monolayer of Ge was found on the surface for all Ge concentrations and an increasing amount of Ge (20%, 55%, 80%, and 95%, respectively) was found in the second atomic layer. [ABSTRACT FROM AUTHOR]

A novel method for creating bilayer graphene is described where single-layer CVD graphene grown on Cu is bonded to single-layer epitaxial graphene grown on Si-face SiC. Raman microscopy and x ray photoelectron spectroscopy demonstrate the uniqueness of this bilayer, as compared to a naturally formed bilayer, in that a Bernal stack is not formed with each layer being strained differently yet being closely coupled. Electrical characterization of Hall devices fabricated on the unusual bilayer show higher mobilities, and lower carrier concentrations, than the individual CVD graphene or epitaxial graphene layers. [ABSTRACT FROM AUTHOR]