13th International Conference on Defects--Recognition, Imaging and Physics in Semiconductors: Electron Beam Imaging
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: Marek Skowronski, Carnegie Mellon University

10:30 AM  Invited
The Next Generation Electron Microscopes: Opportunities and Challenges beyond the Current State of the Art: Christian Kisielowski¹; ¹Lawrence Berkeley National Laboratory
     This paper addresses advances in electron microscopy that were accomplished over the past years with the incorporation of new electron optical components such as aberration correctors, monochromators or high brightness guns. Many of these developments are currently pursued within the DoE’s TEAM project.^1,2 As a result electron microscopy has reached 0.5 Å resolution. In this paper it is shown how the resolution improvement has helped to boost signal to noise ratios enabling a detection of single atoms across the Periodic Table of Elements. The described achievements allow for investigations of single point defects even if generated in two-dimensional sheets of materials such as graphene or single layers of BN. Further it is now possible to access depth information from single projections with a precision that has reached interatomic distances. Beam induced atom motion is found to be a significant source of noise in images of single atoms. ¹http://ncem.lbl.gov/team/TEAMpage/TEAMpage.html ²C. Kisielowski, B. Freitag, M. Bischoff, H. van Lin, S. Lazar, G. Knippels, P. Tiemeijer, M. van der Stam, S. von Harrach, M. Stekelenburg, M. Haider, H. Muller, P. Hartel, B. Kabius, D. Miller, I. Petrov, E. Olson, T. Donchev, E.A. Kenik, A. Lupini, J. Bentley, S. Pennycook, A.M. Minor, A.K. Schmid, T. Duden, V. Radmilovic, Q. Ramasse, R. Erni, M. Watanabe, E. Stach, P. Denes, U. Dahmen, /Microscopy and Microanalysis/ *14*, 454-462 (2008).

11:00 AM
Diffraction Contrast of Threading Dislocations in GaN and 4H-SiC Epitaxial Layers Using Electron Channeling Contrast Imaging: Mark Twigg¹; Yoosuf Picard¹; Joshua Caldwell¹; Charles Eddy¹; Michael Mastro¹; Ronald Holm¹; Philip Neudeck²; Andrew Trunek³; J. Powell⁴; ¹Naval Research Laboratory; ²NASA Glenn Research Center; ³OAI; ⁴Sest, Inc.
    Forescattered electron channeling contrast imaging (ECCI) offers the potential of imaging and analyzing extended defects in a scanning electron microscope (SEM). We have recorded and simulated ECCI images of a sample with features that are relatively easily studied and modeled: those based on specially engineered 4H-SiC mesa substrates. These mesas serve as substrates for both homoepitaxial 4H-SiC layers and heteroepitaxial GaN layers in which images of threading dislocations (TDs) have been recorded using ECCI and found to strongly resemble diffraction contrast simulations of TD intensity profiles. Burgers vector identification was confirmed through observations of the rotational direction of atomic step spirals associated with various screw dislocations. For the case of GaN layers, both threading edge dislocations (TEDs) and threading mixed dislocations (TMDs) are identified. It is also seen that TEDs mark low-angle grain boundaries in GaN layers, in accord with plan-view TEM observations.

11:15 AM
Identifying the Influence of Dislocations on 4H-SiC Substrate Step-Morphology and GaN Diode Performance Using Electron Channeling Contrast Imaging: Yoosuf Picard¹; Mark Twigg¹; Joshua Caldwell¹; Charles Eddy¹; Michael Mastro¹; Ronald Holm¹; Phillip Neudeck²; Andrew Trunek²; J. Powell²; ¹US Naval Research Lab; ²NASA Glenn Research Center
    Electron channeling contrast imaging (ECCI) is a scanning electron microscopy (SEM) technique capable of imaging individual dislocations in crystalline materials. Similar to transmission electron microscopy (TEM), ECCI employs diffraction contrast in order to allow direct dislocation imaging as well as Burgers vector identification. We employ ECCI to image screw dislocations that act as persistent atomic step sources in specially engineered 4H-SiC mesa substrates. Mesa substrates with no surface penetrating screw dislocations are nearly free of atomic steps. ECCI is also used to characterize heteroepitaxial GaN film-based devices deposited on these mesa substrates. The screw and edge dislocation density of individual GaN diodes (p-n junctions) are determined by ECCI and correlated to measured ultraviolet (UV) electroluminescence (EL) output. GaN deposited on nearly step-free 4H-SiC show an order of magnitude reduction in screw dislocation densities, yielding a 20-50% increase in UV-EL output.

11:30 AM
The Origin of Threading Dislocations in GaN Films: Michelle Moram¹; Carsten Ghedia¹; Menno Kappers¹; Colin Humphreys¹; ¹University of Cambridge
    It is presently unclear whether threading dislocations (TDs) in heteroepitaxial GaN films arise at island coalescence boundaries or in the initial film nucleation layer. To resolve this question, we studied a series of GaN films with thicknesses ranging from the nucleation layer to 500 nm. The TD densities and the degree of film coalescence were studied using cathodoluminescence, scanning electron microscopy, X-ray diffraction and atomic force microscopy. The density of a-type TDs at the film surface first decreases, then increases, whereas the density of (a+c)-type TDs increases slightly as the film coalesces. Although some TDs appear as the films coalesce, at least ~ 80% of TDs are present at the very start of growth and cannot appear due to island coalescence. X-ray diffraction data show that the initial islands are not misoriented by tilt and we conclude that TDs are instead predominantly formed in the GaN nucleation layer.

11:45 AM
Electron Beam Induced Current Contrast of Threading Edge Dislocation in n-Type 4H-SiC Epilayers: Ronen Berechman¹; Marek Skowronski¹; ¹Carnegie Mellon University
    Contrast of threading dislocations in 4H-SiC Schottky diodes was measured by Electron Beam Induced Current method (EBIC) as a function of the n-type background doping density. The doping concentrations were between 4.6×10¹⁴ cm^-3 and 7.2×10¹⁶ cm^-3 with the corresponding contrast change between 3 % and 17 %. Donolato’s expression for the recombination strength and the calculated minority carrier density within the dislocations space charge region were used to find the recombination rates. The recombination rate increased linearly with doping density. This result was interpreted within the framework of the Shockley-Read-Hall recombination statistics. The height of upward band bending at the dislocation, caused by the capture of electrons by bandgap traps in the dislocation core, and the trap level were extracted.

12:00 PM
Dislocations in Si-Doped LEC GaAs Revisited: A Spectrum Image Cathodoluminescence Study: Oscar Martínez¹; Juan Jiménez¹; ¹University of Valladolid
    Understanding the role of impurities is crucial to semiconductor device technology. The incorporation of these impurities to the lattice and the resulting free charge concentration depend on the interaction with native defects. Dislocations in Si-doped substrates were studied in the nineties using highly sensitive Diluted Sirtl applied with Light etching, Electron Beam Induced Current and micro-Photoluminescence techniques, aiming to understand the interaction between dislocations and the doped GaAs matrix. CL spectrum imaging allows revisiting this problem. By using a CCD multichannel detector it is possible to obtain the full spectral information over a selected area with submicrometric spatial resolution. The local spectra allow the identification of the defects responsible for the luminescence emission. The use of fitting routines allows mapping the distribution of the different defects and impurities, providing a full scenario of the Cottrell atmosphere. The CL images are complemented with etching depth images obtained by Phase Stepping Microscopy.

12:15 PM
Determination of Piezoelectric Fields across InGaN/GaN Quantum Wells by Means of Electron Holography: Masashi Deguchi¹; Shigeyasu Tanaka²; Takayoshi Tanji²; ¹Department of Electronics, Nagoya University; ²EcoTopia Science Institute, Nagoya University
    Electron holography (EH) was used to determine the piezoelectric fields across an InGaN/GaN quantum well structure in commercially available blue light emitting diodes. A wedge polishing technique was used for thinning samples. Thin samples prepared by this technique had wedge fronts nearly perpendicular to the interface, thus suitable for EH analysis. Holograms were taken under the condition that the sample was tilted such that the adjacent layers slightly overlapped. The tilting of the samples helps to avoid strong diffraction effects which cause an additional phase shift and make the analysis difficult. The phase changes in the overlapped regions were analyzed to determine the piezoelectric fields in each well. It was shown that the piezoelectric field is strongest at the center region of the quantum well structure. The field strength averaged over eight InGaN wells was approximately 2.2 MV/cm.