P. M. Thibado

Enhancing the Student‐Instructor Interaction Frequency

A 100-fold increase in the frequency of student–teacher interaction has been achieved in a large-enrollment classroom. Students answer in-class questions using personalized hand-held transmitters. Outside the classroom, personalized homework sets are generated and collected via the Internet.

Robust optical delivery system for measuring substrate temperature during molecular beam epitaxy

g an ate ate. nes he be the thod is ular . For for The phenomenal growth in wireless communications a optoelectronic technology is making III-V semiconduct based devices an increasingly important component of entire semiconductor market. 1 Unlike Si-based technology where devices are fabricated primarily by ion implantatio III-V device structures must be fabricated one atomic pla at a time. One of the most powerful and flexible grow methods for III-V structures is molecular beam epita ~MBE!. The ability of MBE to control the deposition of ex tremely thin films that is required for high-performance d vices, as well as the capability to grow device layers w arbitrary compositions and doping profiles, is unequal However, even MBE as traditionally used suffers from po day-to-day repeatability, and this is due, in part, to the la of any means to accurately sense and control one basic cess parameter, namely, the substrate temperature. This lem arises because the typical sensor used for temper control is a thermocouple, which cannot be in good therm contact with the substrate if one wants to produce hi quality, highly uniform material. The inaccuracies in su strate temperature, in turn, affect the overall progress wi the MBE community at large. The reason for this is that temperature profile used to produce a high-quality growth one institution cannot be transferred to other institutions. T MBE community has tried to minimize these difficultie through the implementation of optical pyrometers for su strate temperature determination. Unfortunately, the ac racy of pyrometers is limited by stray light from the sour ovens and from substrate heater filaments. In addition, rometer readings are affected by films deposited on the rometer viewport and by lack of knowledge of sample em sivity ~which in many cases is changing during the growth the structure !. Finally, if one uses direct radiative heating the substrate, the pyrometer becomes flooded with the in red radiation of the heater filaments, making the techni even less accurate. 2 Other workers have suggested the use

Self-organized platinum nanoparticles on freestanding graphene.

Freestanding graphene membranes were successfully functionalized with platinum nanoparticles (Pt NPs). High-resolution transmission electron microscopy revealed a homogeneous distribution of single-crystal Pt NPs that tend to exhibit a preferred orientation. Unexpectedly, the NPs were also found to be partially exposed to the vacuum with the top Pt surface raised above the graphene substrate, as deduced from atomic-scale scanning tunneling microscopy images and detailed molecular dynamics simulations. Local strain accumulation during the growth process is thought to be the origin of the NP self-organization. These findings are expected to shape future approaches in developing Pt NP catalysts for fuel cells as well as NP-functionalized graphene-based high-performance electronics.

Activation energy for Ga diffusion on the GaAs(0 0 1)-(2×4) surface: an MBE-STM study

The pure migration of individual Ga atoms on the technologically important GaAs(0 0 1)-(2]4) reconstructed surface has been studied as a function of substrate temperature using a combined molecular beam epitaxy and scanning tunneling microscopy (STM) ultra-high vacuum, multi-chamber facility. We have successfully deposited 1 10 of a plane of Ga atoms onto a pristine GaAs surface under a constant As 4 beam equivalent pressure of 10~6 Torr, at various substrate temperatures. After deposition the substrate was quenched to room temperature and transferred to the surface analysis chamber for STM imaging. A plot of the number density of islands formed as a function of deposition temperature follows an Arrhenius relationship. Assuming either a pure one-dimensional di!usion model or a pure isotropic two-dimensional di!usion model, the activation energy for di!usion is 2.3 or 1.7 eV, respectively. ( 1999 Elsevier Science B.V. All rights reserved.

Unusual ultra-low-frequency fluctuations in freestanding graphene

Intrinsic ripples in freestanding graphene have been exceedingly difficult to study. Individual ripple geometry was recently imaged using scanning tunnelling microscopy, but these measurements are limited to static configurations. Thermally-activated flexural phonon modes should generate dynamic changes in curvature. Here we show how to track the vertical movement of a one-square-angstrom region of freestanding graphene using scanning tunnelling microscopy, thereby allowing measurement of the out-of-plane time trajectory and fluctuations over long time periods. We also present a model from elasticity theory to explain the very-low-frequency oscillations. Unexpectedly, we sometimes detect a sudden colossal jump, which we interpret as due to mirror buckling. This innovative technique provides a much needed atomic-scale probe for the time-dependent behaviours of intrinsic ripples. The discovery of this novel progenitor represents a fundamental advance in the use of scanning tunnelling microscopy, which together with the application of a thermal load provides a low-frequency nano-resonator.

Monte Carlo derived diffusion parameters for Ga on the GaAs(001)- (2×4) surface: A molecular beam epitaxy–scanning tunneling microscopy study

The migration of individual Ga atoms on the technologically important GaAs(001)-(2×4) reconstructed surface has been studied as a function of substrate temperature and As4 pressure using a combined molecular beam epitaxy and scanning tunneling microscope ultrahigh vacuum multichamber facility. We have deposited 10% of a plane of Ga onto a GaAs(001) surface with a low defect density ( 0.5 μm) to avoid the influence of surface defects like step edges and vacancies. Both the island number density and the geometry are measured and compared to Monte Carlo solid-on-solid simulations. Basic diffusion parameters, such as the activation energy, directional hopping-rate ratio, directional sticking-probability ratio, etc., are reported.

Enabling in situ atomic-scale characterization of epitaxial surfaces and interfaces

A custom designed sample handling system which allows the integration of a commercially available scanning tunneling microscope (STM) facility with a commercially available molecular beam epitaxy (MBE) facility is described. No customization of either the STM imaging stage or the MBE is required to implement this design.

Origins of GaN(0 0 0 1) surface reconstructions

The reconstructions of the Ga polarity GaN(0 0 0 1) surface with and without trace amounts of arsenic and prepared by molecular beam epitaxy (MBE) have been studied with in situ reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). Various reconstructions are observed with RHEED by analyzing patterns while the substrate is exposed to a fixed NH3 flux or after depositing known amounts of Ga as a function of substrate temperature. In situ STM images reveal that only a few of these reconstructions yield long-range periodicity in real space. The controversial role of arsenic on Ga induced reconstructions was also investigated using two independent MBE chambers and X-ray photoelectron spectroscopy. � 2003 Elsevier B.V. All rights reserved.

Role of As4 in Ga diffusion on the GaAs(001)-(2×4) surface: A molecular beam epitaxy-scanning tunneling microscopy study

The role of As4 molecules in Ga diffusion on the GaAs(001)-(2×4) reconstructed surface has been studied using a combined molecular beam epitaxy and scanning tunneling microscopy multichamber facility. We deposited 10% of a plane of Ga atoms onto an otherwise pristine surface, while exposed to two separate As4 beam equivalent pressures of 10−5 and 10−6 Torr. The higher As4 flux resulted in the production of fewer and larger islands, indicating that increasing the As4 flux increases the total interrogation area available to the Ga atoms before forming islands.

Electronic transition from graphite to graphene via controlled movement of the top layer with scanning tunneling microscopy

A series of measurements using a technique called electrostatic-manipulation scanning tunneling microscopy (EM-STM) were performed on a highly oriented pyrolytic graphite surface. The electrostatic interaction between the STM tip and the sample can be tuned to produce both reversible and irreversible large-scale movement of the graphite surface. Under this influence, atomic-resolution STM images reveal that a continuous electronic transition from triangular symmetry, where only alternate atoms are imaged, to hexagonal symmetry can be systematically controlled. Density functional theory (DFT) calculations reveal that this transition can be related to vertical displacements of the top layer of graphite relative to the bulk. Evidence for horizontal shifts in the top layer of graphite is also presented. Excellent agreement is found between experimental STM images and those simulated using DFT.