1. Atom Probe Tomography of Advanced GaN-based LEDs James Speck, University of California-Santa Barbara, DMR
Atom Probe Tomography
3-D atomic scale chemical maps
Sharp tip (pcrepared by FIB or etching)
DC E-field + laser pulse (or E-pulse)
Atom position via position on detector
Atom nature via flight time
Atom probe reconstruction: GaN LED
Recorded at UCSB on Imago HR 3000 LEAP atom probe
m-plane, non-polar LED million atom data set
(left to right)
GaN:Si/6X InxGa1-x N/GaN QW/AlyGa1-yN/GaN:Mg
Key results
Homogeneous InxGa1-x N quantum wells
Mg clustering in GaN:Mg layer
Color code:
Ga+N: green; In: purple; Al: aqua; Mg: red
Atom probe data on high performance UCSB m-plane nonpolar GaN-based LEDs
(nonpolar orientations of GaN are grown such that the polar c-axis of the wurtzite (hexagonal crystal) is parallel to the film substrate interface – thus avoiding electric polarization in the growth direction
Atom probe samples, data, and analysis all performed at UCSB on the Imago HR 3000 LEAP system acquired with the MRI grant.
Data shows abrupt interfaces (more details on the next slide) in the InxGa1-x N quantum wells – this addresses a key issue in InxGa1-x N alloys (homogeneous alloys vs. a tendency for clustering)
Unexpected Mg clustering in the p-type region (Mg is a the only effective acceptor that generates holes in GaN).
Related reference:
M. Schmidt et al. JJAP 46, L126 (2007).

2. Atom Probe Tomography of Advanced GaN-based LEDs James Speck, University of California-Santa Barbara, DMR
Free surface
In: purple; Al: aqua
Note that the sample direction is reversed (free surface toward left in this image)
Example of detailed information available in high quality atom probe data.
This tool, now fully operational, is available ot a broad user base through the open
Microscopy and Microanalysis Laboratory in the Materials Department and MRSEC at UCSB
Detailed analysis of multiple quantum well (MQW) region
Chemically homogeous InxGa1-xN in the quantum wells
Abrupt lower and upper interface with additional evidence of floating In layer that
leads to additional indium incorporation in barrier layers
Broader Impact
Example of nanoscale chemical analysis with atomic scale, 3D resolution
Unparalleled resolution and dynamic range (from ~1017 cm-3 to bulk composition)

3. Training Activities James Speck, University of California-Santa Barbara, DMR 0821168
Ravi Shivaraman
M.S. in Materials Science (ASU)
1+ year staff member at Imago Scientific
Temporary staff member – UCSB Microscopy and Microanalysis Facility
(Summer 2009)
Ph.D. student – UCSB Materials Department (current)
Nicholas Cunningham
MS in Mechanical Engineering (UCSB)
Ph.D student
Summer 2009 LEAP (atom probe) Training Sessions
5 Participants
6 Training Sessions
8 Potential/Current Users
This tool, now fully operational, is available ot a broad user base through the open
Microscopy and Microanalysis Laboratory in the Materials Department and MRSEC at UCSB.
The slide above reflects initial training for new users. 3

4. International Activities James Speck, University of California-Santa Barbara, DMR 0821168
UCSB-Oxford-Leoben
Collaboration
Round-robin characterization of a reference advanced high temperature and radiation damage tolerant nano-dispersion-strengthened alloy with far from equilibrium nm scale Y-Ti-O precipitate clusters
Excellent agreement between data
from three laboratories
Example of detailed information available in high quality atom probe data.
Collaboration between UCSB (Bob Odette), Oxford (George Smith), and Leoben 4