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