13th International Conference on Defects--Recognition, Imaging and Physics in Semiconductors: Quantum Confined Structures
Program Organizers: Marek Skowronski, Carnegie Mellon University; Robert Stahlbush, Naval Research Laboratory; Michael Dudley, State University of New York at Stony Brook
Monday AM
September 14, 2009
Room: Glessner Auditorium
Location: Oglebay Resort and Conference Center
Session Chair: Michael Dudley, State University of New York at Stony Brook
8:30 AM Introductory Comments
8:35 AM Invited
Classification of Energy Levels in Quantum Dot Structures by Means of Depleted Layer Spectroscopy Methods: Maria Kaniewska1; Olof Engström2; Mariusz Kaczmarczyk1; 1Institute of Electron Technology; 2Chalmers University of Technology
The coexistence of quantum confined energy levels and defect energy levels in quantum dot (QD) structures may cause difficulties in distinguishing between their different origin when using DLTS. For the functioning of QDs in practical devices, it is important to be able to separate the influence of such energy levels from those of the QDs. Using InAs/GaAs QDs as demonstration vehicles, we present methodologies to obtain such a classification by DLTS. QD-related spectra measured as a function of repetition frequency of electrical pulses, f, or temperature, T, and reverse voltage, V, are depicted as contour plots on (f, V)- and (T, V)-planes, thus reflecting the complex thermal and tunneling emission of electrons from the ground and excited states. Defect-related levels in reference samples without QDs give rise to different contour patterns and undergo modification exhibiting double-peak structured emission when defects are agglomerated in the vicinity of the QD plane.
9:05 AM
Electrical Properties of Si Nanowire Devices Characterized with Scanning Probe Microscopy: Sung-Soo Bae1; Zhiyong Li2; Nathaniel Quitoriano2; Theodore Kamins2; Regina Ragan1; 1University of California, Irvine; 2Hewlett-Packard Laboratories
Fundamental studies of how localized charged defects affect current transport along Si nanowires in field effect transistor architecture were performed in order to gain insight into how to fabricate reproducible sensors on this material platform. A change in lateral electron conductivity along nanowires that results due to interactions of (bio)molecules on nanowire surfaces has been attributed to a change in surface charge on nanowires. Yet, it is not clear if sensitivity to biomolecular binding events (and also sensitivity to non-specific events) is enhanced by the presence of defects on the surface. Using scanning Kelvin probe microscopy, we investigated localized changes in electronic structure of Si nanowires and found that localized potential changes were correlated with defects and/or impurities. Nanowires fabricated with top-down and bottom-up methods were compared where the latter demonstrated less variation of surface potential.
9:20 AM
Cross-Sectional Scanning Tunneling Microscopy and Spectroscopy of InAs Quantum Dots in GaAs: Sandeep Gaan1; Randall Feenstra1; John Walker1; Elias Towe1; 1Carnegie Mellon University
We have studied InAs/GaAs quantum-dot heterostructures using cross-sectional scanning tunneling microscopy and spectroscopy. Samples were grown by molecular beam epitaxy on n-type GaAs substrates. Individual quantum dots are clearly resolved in the images, and tunneling spectra were acquired at various distances from the dots. We find that spectra acquired 3-4 nm from the dots show a clear peak located in the upper part of the GaAs bandgap, which we associate with the lowest confined state of the InAs conduction band. Spectra acquired directly on the dots display large broadening of this feature, however, apparently due to non-equilibrium occupation of the state by the tunneling electrons. From the spectra we directly estimate a lowest confined state energy of 0.16 ± 0.03 eV below the GaAs conduction band edge.
9:35 AM
Investigation of Optical Properties of InGaN/(AlIn)GaN Multi Quantum Wells for Blue Lasers by Cathodoluminescence: Ute Zeimer1; Uwe Jahn2; Veit Hoffmann1; Markus Weyers1; Michael Kneissl3; 1Ferdinand-Braun-Institut für Höchstfrequenztechnik; 2Paul-Drude-Institut für Festkörperelektronik; 3Technische Universität Berlin, Institut für Festkörperphysik
To expanded the wavelength range of semiconductor lasers based on InGaN/(AlIn)GaN multi quantum wells (MQWs) to 450 nm and beyond the indium concentration in the quantum wells has to be increased. We investigate the influence of both the reduction of growth temperature as well as the increase of the QW thickness on the crystalline quality and defect structure of MQWs grown on sapphire substrate by cathodoluminescence (CL) at 6 K, secondary electron (SE) imaging and atomic force microscopy (AFM). The CL investigations show a local wavelength distribution which is connected to growth spirals. This can be explained by a higher local QW thickness and/or a higher indium concentration in the centre of the spirals. Small randomly distributed surface pits attributed to dislocations show only very weak contrasts in monochromatic CL images. Special emphasis will be devoted to the role of dislocations on the nonradiative recombination processes in InGaN MQWs.
9:50 AM
Effects of Epitaxial Graphene Stacking, Strain, and Thickness Uniformity on Carrier Mobility: Joshua Robinson1; Joseph Tedesco2; Mark Fanton1; David Snyder1; Glenn Jernigan2; Paul Campbell2; Rachel Myers-Ward2; Charles Eddy2; D. Gaskill2; 1Penn State University EO Center; 2Naval Research Lab
We report results of Raman spectroscopy studies of large-area epitaxial graphene grown on SiC. Our work reveals unexpectedly large variation in Raman shift resulting from graphene strain inhomogeneity, which is shown to be correlated with physical topography by coupling Raman spectroscopy with atomic force microscopy. We show that graphene strain can vary over a distance shorter than 300nm, and may be uniform only over roughly 1 um. Additionally, we have examined epitaxial graphene with mobility values of 25 – 18,100 cm2/Vs, and show that Raman topography is a vital tool for rapid identification of high mobility material. The Hall mobility of epitaxial graphene on the Si-face of SiC is not only highly dependent on thickness uniformity, but also on mono-layer strain uniformity. High mobility epitaxial graphene grown on the C-face of SiC is dependent on graphene layer stacking.
10:05 AM Break