Lincoln J. LauhonDepartment of Materials Science & Engineering
July 24, 2016
Establishing correlations between synthesis, nanostructure, and properties of nonplanar
semiconductor heterostructures
M&M Pre-Meeting Congress
Non-Planar Heterostructures and Devices3D Mapping of Quantum Wells in a
GaN-InGaN Core-Shell Nanowire LED Array
Nano Lett. 13, 4317 (2013)
Spatial Mapping of Efficiency of GaN/InGaNNanowire Array Solar Cells
Nano Lett. 13, 5123 (2013)
Reliable Analysis of the Indium Mole Fraction in InGaN Quantum Well LEDs
Appl. Phys. Lett. 104, 152102 (2014)
10 nm
SiGa
Al
High Performance δ-doped GaAs/AlGaAsCore/Shell Nanowire MODFETs
Nano Lett 15, 3295 (2015)
Tomography from the nanoscale to microscaleis needed to determine the confinement potentials in non-planar nanowire heterostructures.
Example: 2DEG in Planar Heterostructure
• Negligible misfit strain at GaAs- AlxGa1-xAs interfaces.• Electrons confined in GaAs.• Reduced impurity scattering.
SubstrateGaAs
AlGaAs
GateAlGaAs
GaAs
Si
Si
2DEG2DEG
Energy
EF
MBE-Grown δ-doped GaAs-AlGaAs Nanowire
GaAs
AlGaAs
Si
2DEG
Gregor Koblmüller et alWalter Schottky InstituteTU Munich
Nano Lett 15, 3295 (2015)
3D Mapping of Core-Shell Nanowire
Si δ-doping layer
GaAsAlGaAs
10 nmAlGa
Al Mole Fraction [%
]
70
60
50
40
30
20
10
010 nm
60 % Al
10 % Al
30 %
Al0.3Ga0.7As Shell
Nano Lett 15, 3295 (2015).
What properties can be correlated with variations in the shell composition?
Si C
once
ntra
tion
[ato
ms/
cm3 ]
Si
Silicon δ-Doping Creates an Electron Gas
10 nm
AlGaSi
Distance [nm]
Mole Fraction [%
]
AlGa
Dopant Concentration
Nano Lett 15, 3295 (2015).
Simulation of Electron Gas Distribution
(nextnano3)
Composition fluctuations in shell are large
Al Mole Fraction [%
]
70
60
50
40
30
20
10
0
10 nm
Al-rich stripes result from facet-driven segregation.
Beyond-random fluctuations arise in shell on {110} facets.
ACS Nano 9, 8335 (2015)
Composition fluctuations modulate potential AlM
oleFraction
[%]
2D Composition
Hsieh et al. Applied Physics Letters 68 1790 (1996)
Bandgap Variation with Al
Potential MinimumACS Nano 9, 8335 (2015)
Potential fluctuations may localize excitons
Bandgap Variation with Al
Hsieh et al. Applied Physics Letters 68 1790 (1996)ACS Nano 9, 8335 (2015)
Potential fluctuations may localize excitons
ACS Nano 9, 8335 (2015)
Ga –rich Cluster
GaAs Mole Fraction Isosurfaces
Ga –rich Cluster
GaAs Mole Fraction Isosurfaces
Confinement potential extracted from surfaceProximity Histogram of Interface
Cluster size and interface width enables modeling.
ACS Nano 9, 8335 (2015)
Ga –rich Cluster
Correlation of Clusters with Emission
Nextnano3
• Band of QD-like emission consistent with cluster size distribution.
• Tunneling between clusters is likely.
ACS Nano 9, 8335 (2015)
“Planar” InGaN Quantum Wells (QWs)
Oliver, et al. Appl. Phys. Lett. 103, 14114 (2013).
Masabuau, et al. J. Cryst. Growth. 386, 88 (2013).
• XS-TEM images are projection of 3D structure.
• X-ray diffraction averages over multiple QWs and over large areas.
Atom Probe Tomography (APT) provides a 3D perspective.
Sample Structure and Property Overview
14
• Two samples were grown by MOCVD:1. H2 dosed during GaN barrier layer growth.2. No H2 dosing.
• H2 dosed sample is 1.5 times brighter in photoluminescence (PL) measurement.
• AFM consistent with gaps in H2 dosed QWs.
200 nm
Work w/Dan Koleske, Sandia National Lab
APT of Continuous QWs Sample
• 3-D reconstruction: – Five quantum wells
and dilute InGaN underlayer
– Superlattice period matches XRD measurement
– Interface is not abrupt as was assumed during XRD fitting.
10 nm
Appl Phys Lett 107, 022107 (2015)
APT Reveals Asymmetric Composition Profile
• Proximity histograms used to measure indium mole fraction profile.– Asymmetric interface
abruptness
– Peak In mole fraction 0.18
• APT profiles used to constrain XRD fitting.
Growth
Direction
10 nm
0.5 nm bin
Appl Phys Lett 107, 022107 (2015)
Improving the Abrupt Interface Model
• Mean absolute error (MAE) was used to assess goodness of fit.
• APT profiles improve XRD fitting by reducing the intensity of higher order peaks.
𝑀𝑀𝑀𝑀𝑀𝑀𝑙𝑙𝑙𝑙𝑙𝑙 =1
𝑁𝑁 − 1� 𝑙𝑙𝑙𝑙𝑙𝑙 𝐼𝐼𝐸𝐸𝐸𝐸𝐸𝐸 − 𝑙𝑙𝑙𝑙𝑙𝑙 𝐼𝐼𝑆𝑆𝑆𝑆𝑆𝑆
Inte
nsity
(Cou
nts)
Omega (o)
Appl Phys Lett 107, 022107 (2015)
X-ray Diffraction of Discontinuous QWs Sample
• XRD fitting based on abrupt interface model:– 0.08 indium mole fraction
represents an ill-defined average value.
MeasurementSimulation
• Correlation with optical properties is challenging.
Site-specific APT analysis
Appl Phys Lett 107, 022107 (2015)
Photoluminescence is correlated with composition
• PL mapping was conducted relative to Pt marker lines.
• Lift-out was conducted in representative regions.
• Superlattice period is 11.9 ± 0.6 nm.
• Peak indium mole fraction is 0.16 ± 0.02.
(a) 448 nm
442 nm
4 µm
PL Map
10 nm
APT
In mole fraction determines emission wavelength.
Appl Phys Lett 107, 022107 (2015)
Correlating Composition with Optical Properties
𝑀𝑀𝑙𝑙,𝐼𝐼𝐼𝐼𝑥𝑥𝐺𝐺𝐺𝐺1−𝑥𝑥𝑁𝑁 𝑥𝑥 = 𝑀𝑀𝑙𝑙,𝐼𝐼𝐼𝐼𝑁𝑁 𝑥𝑥 + 𝑀𝑀𝑙𝑙,𝐺𝐺𝐺𝐺𝑁𝑁 1 − 𝑥𝑥 − 𝑏𝑏𝑥𝑥 1 − 𝑥𝑥
Compositionmeasurement
Bowing factor (eV)
XRD 1.0
XRD 3.2
XRD 4.1
RBS 3.5
Adapted from Yam et al Superlattice Miccrostruc. 43, 1 (2008)
Appl Phys Lett 107, 022107 (2015)
• Range of bowing factors is large when In compositions are determined by techniques that measure average value.
• APT analysis determines bowing factor to be 2.20 ± 0.15 eV.
H2 exposure influences indium incorporation
21
Excess indium incorporated into lattice during growth of GaN layer
H2 dosing can remove excess indium by forming volatile In-H compounds.
H2 dosing
In
GaN
InGaN
H2
In
GaN
InGaN
Appl Phys Lett 107, 022107 (2015)
22
In discontinuous QWs sample, indium distribution is more symmetric, having a less “diffusive tail”.
Excess indium incorporated into lattice during growth of GaN layer
H2 dosing can remove excess indium by forming volatile In-H compounds.
H2 dosing
H2
In
GaN
InGaN
In
GaN
InGaN
Appl Phys Lett 107, 022107 (2015)
H2 exposure influences indium incorporation
Summary
• Quantum dot-like emission associated with composition fluctuations in ternary alloy.‒ 3-D composition field used to generate potential
landscape experienced by charge carriers.
• PL emission from discontinuous quantum wells.‒ 1-D composition profiles used to constrain
correlated x-ray diffraction.‒ Local composition measurements help decompose
contributions of bandgap and confinement in spectrum.
Lauhon Research Group
NUCAPT, NUANCE
Group:Alex Henning (PD)Megan Hill (G-1)Michael Moody (G-1) Jack Olding (G-1)Sarah Rappaport (UG) Xiaochen Ren (G-4)Zhiyuan Sun (G-3)
Alumni:Nari Jeon Argonne National LabJames Riley Intel
Collaborators:Mark Hersam NorthwesternTobin Marks NorthwesternTeri Odom NorthwesternM. Kanatzidis Northwestern
Dan Koleske Sandia National LabGregor Koblmuller TU MunichYossi Rosenwaks Tel Aviv UniversityArunima Singh NISTFrancesca Tavazza NIST