The Center for Energy Nanoscience Emerging Materials For Solar Energy Conversion and Solid State Lighting
Nanostructures for energy device applications
P. Daniel Dapkus
University of Southern California University of Illinois at Urbana-Champaign
University of Michigan University of Virginia
Goals and Rationale of CEN
Develop the scientific understanding required to improve the performance and cost of solar cells and LEDs to make them the technologies of choice for clean energy generation and lighting. • Strong, complementary team of leaders in semiconductor
nanostructures and organic materials. • Sophisticated characterization tools to probe the materials
properties and physical mechanisms. • Atomistic simulations to address the fundamental issues that limit
acceptance of these technologies. • Solar cells and LEDs have synergistic requirements and limiting
issues that can be addressed by fundamental research on these emerging materials.
Multijunction Solar Cells
• Solar cell efficiency is a pacing factor in the adoption of solar energy because the energy source is dilute and neither land nor installation costs are free.
• To date only multijunction solar cells have produced solar cells with efficiencies greater than the maximum theoretical value for a single junction cell.
• 44.0 % Recently achieved using triple junction of InGaAsNSb/GaAs/InGaP on GaAs substrate
• How can we translate this expensive technology into a lower cost platform?
Tandem Design Elements
Monolithic
Solar Cells
Tunnel Diodes
Maximum Theoretical Efficiencies of Multijunction Solar Cells
No. of Cells
Optimal Bandgaps (eV) Max. Eff. %
Eg1 Eg2 Eg3 Eg4 Eg5 Eg6
1 1.31 31
2 0.97 1.70 42.5
3 0.82 1.30 1.95 48.6
4 0.72 1.10 1.53 2.14 52.5
5 0.66 0.97 1.3 1.7 2.29 55.1
6 0.61 0.89 1.16 1.46 1.84 2.41 57.0
Series Connected Cell at AM 1.5
“Limiting efficiencies of single and multijunction solar cells,” C.H. Henry, J. Appl. Phys. 51, 4494 (1980).
(GaAsP/Si, AlGaAs/Si, InGaP /Si )
(GaInP/InGaAs/Ge)
(InGaP/GaAsP/Si/Ge)
Nanowire Based Tandem Solar Cells
Traditional multijunction solar cell: III-V film-based tandem solar cells
are fabricated on Ge due to lattice-match constraints -- Very Expensive
Why Nanowires?
Natural Strain relief in the radial direction • Relax lattice-match requirement between materials
• Enable growth of III-V (GaAs, GaAsP, InGaP) nanowires on Si
(not possible to grow as films due to lattice and thermal mismatch )
• Eliminate expensive Ge substrates used in film-based multi-junction solar cells
• Leverage silicon solar cell infrastructure
Reduce dislocations by spatial filtering and elimination
Adjustable bandgap and absorption threshold of each cell
Optical Concentration of light
“Natural” Antireflection Properties
Device Design Issues
Vertically Stacked MJ NW cells Core-Shell MJ NW cells
“Limiting efficiencies of tandem solar cells consisting
of III-V nanowire arrays on silicon,”
Huang et. al., J. Appl. Phys. 112, p 064321 (2012)
• how does current matching constrain geometry? • are optimal band gap combinations the same as for bulk? • how much can material volume be reduced without sacrificing efficiency?
Methods
• Ultimate Efficiency: quantifies broadband absorption [Hu and Chen, Nano Lett. 7, 3249–3252 (2007)]
upper bound on efficiency, assuming one electron-hole pair per photon and perfect carrier collection
• Highly-accurate numerical calculations of absorption parallelized codes
o transfer matrix method
o finite-difference time domain method
I()A()310nm
g
gd
I()d310nm
4000nm
Conclusions: optical simulations
Silicon
Silicon
GaAs
Silicon
GaAs
Limit efficiency 40.88% 44.65%
Designed
efficiency
33.2% with GaAs(1.43eV)
30.74% with InP (1.2eV)
35.72% with AlGaAs(1.7eV)
36.03% with InGaP(1.8eV)
38.22%
with AlGaAs (2.0eV)
27.03%
with InGaP (1.8eV)
Optimization ongoing
44.65%
Optimization ongoing
GaAs
Selective Area Nanostructure Growth
• Localized growth rate controlled by vapor phase diffusion and surface migration to growth surface. • Unidirectional growth controlled by growth kinetics of facets. • Process conducted without intermediate metals. • Sharp transitions between layers in nanostructure.
Vapor Phase Diffusion Surface Migration
Nanowire SAG Growth Habits
Material Substrate Growth Plane
Crystal structure
GaAs GaAs (111)B
Zincblende
InAs GaAs or InP (111)B
Zincblende
InGaAs GaAs (111)B
Zincblende
GaP GaAs (111)B
Zincblende
InP InP or GaAs (111)A
Wurtzite
InAsP or InGaP No NW Growth -------
GaN (0001) Ga or N Wurtzite
Growth on Reconstructed (111)B Surface
• Surface reconstruction of (111)B – Electron counting model
• All group III dangling bonds must be empty • All group V dangling bonds must be filled
– In group V precursor over pressure, • Group V adatoms form trimers to stabilize
dangling bonds • GaAs shows 2×2 reconstruction
• Bond Dissociation Energy (BDE)
As trimers can be broken without breaking Ga-As bonds,
but Dissociation of P trimers affects In-P bonds.
Group V trimer
Surface group V atom
Bond BDE(kJ/mol) Bond BDE(KJ/mol)
P-P 201 As-As 146
In-P 197.9 8.4 Ga-As 202.5
Ga-P 227.9 12.6 In-As 20110
PS
PgroupV
PT
PS
PgroupV
PT
Window of crystal growth in group V partial pressure
(GaAs and InAs NS-SAG)
Growth suppression by trimers (InP NS-SAG)
2×2 reconstruction with group V trimers
Consequences of Surface Kinetics / Reconstruction
• Mixed alloys of InP will not grow vertically on (111B) orientation like all other III-V’s. e.g. InAsP, InGaP, InAlP?
• InP mixed alloys may grow radially on existing (111B) wires allowing InAlGaP to be used as a passivation layer on material with Eg up to ~ 2.5 eV.
GaAs Nanowire Arrays Grown by NS-SAG
• Growth conditions – Temperature: 650 C ~ 750 C – TMG partial pressure: 1.9~7.6×10-7 atm – AsH3 partial pressure: 0.6~2.4×10-4 atm
• GaAs nanowire arrays on (111)A – Nonuniform – Tilted toward nearest <111>B or
<112>B
• GaAs nanowire arrays on (111)B – Uniform and vertical – Zincblende phase down to 25 nm in
diameter – Low growth temperature and high precursor
partial pressure promote cluster formation and burying of nanowires in dense arrays
– High growth temperature and low precursor partial pressure suppress growth in vertical direction and degrade uniformity of sparse arrays.
[111]B
(111)A (111)B
HR-TEM
NW growth via in-situ formed metal droplet
In-situ deposition of Ga droplets Self-catalyzed growth Longer nanowires
• Relatively fast growth rate
• Relatively low growth temperature (600 oC)
• Growth from liquid metal droplet may reduce defects
in NW.
Dense GaAs NW Arrays on Si
•Oxide Removal
•Si Surface Reconstruction
•As treatment of surface
•Nucleation of GaAs “Seed”
•Nanowire Grwoth
GaAs nanowire arrays on Si as photoanodes and tandem
solar cells – A CEN / JCAP Collaboration
Scientific Achievement • GaAs-nanowire/Silicon-substrate multijunction structure
developed at USC / CEN • Demonstrated optical absorption, carrier generation and
electron transfer properties suitable for high-efficiency solar water splitting at Caltech / JCAP.
Significance and Impact A promising III-V / Si photoanode and tandem solar cell
a. High absorption and minimal reflection at both normal and off-normal incidence
b. Radial junction: ~100% carrier collection
c. Voc and Jsc at maximum values already
Research Details – Angle-dependent experimental and theory study for
enhanced absorption and in-coupling – GaAs nanowire-arrays on Si demonstrated – Near unity carrier-collection efficiencies – Open-circuit potential: 590±15mV – Short-circuit current density: 24.6±2.0mA cm-2 – Energy-conversion efficiency: ~ 8.1%
400 500 600 700 800 9000.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Spectral response
Absorption-corrected
spectral response
Wavelength (nm)
E (Fc+/Fc)
SC/Liquid junction
0
20
40
60
400 500 600 700 800 900
0
0.2
0.4
0.6
0.8
1
(nm) ()
Absorp
tion
25
20
15
10
5
0
Ph
oto
cu
rren
t d
en
sity J
(m
A c
m-2
)
-0.6 -0.4 -0.2 0.0 0.2
E (V vs. Solution)
nanowires
substrates “Optical, Electrical, and Solar Energy-Conversion Properties of Gallium Arsenide Nanowire-Array Photoanodes”, Shu Hu, Chun-Yung Chi, Katherine Fountaine, Maoqing Yao, Harry A. Atwater, P. Daniel Dapkus, Nathan S. Lewis and Chongwu Zhou, Energy Environ. Sci., 2013, 6, 1879.
e- V
% o
f p
ho
tocu
rren
t
% o
f ab
sorp
tio
n
Ab
sorp
tio
n
Axial Junction GaAs NR Solar Cells
Low Jsc and Voc limit efficiency
Excess Leakage Current
Short Diffusion Length
Field Assisted Carrier Collection
NW Surface Area Can Dominate Properties
• Surface Fermi level pinning can lead to carrier depletion.
• Recombination through surface states can lead to short carrier
lifetimes and diffusion lengths
AlGaAs and GaP surface cladding layers cause ~ 30X to 100X
increase in PL efficiency
AlGaAs / GaAs Core Shell NWs
Crystal Phase Amiguity
• Nanowires exhibit a high density of stacking faults and / or twin defects
• Stacking defects are a result of minimization of facet surface energy on the NW.
What is twin?
Twin plane: (111) plane
Carrier scattering or confinement effect
High resistivity
Shorter carrier life-time
Motivation
Improve crystal quality
Better electrical and optical properties
Grating Solar Cell
Effects of Twins on Electronic Transport
• Twin-scattering contribution to electron mobility estimated by density
functional theory
8kBT
e
m*1/ 2
dxx ln 1m* (v0 )2
2h2kBTx2
e
x2
1
ltwin
Local Kohn-Sham potential due to
a twin boundary
Intrinsic Core/Shell Structures in Nanowires
Z. Yuan et al., Appl. Phys. Lett. 100, 153116 (’12);
ibid. 100, 163103 (’12)
Hexagonal
core
Tetragonal
shell
ZnO
GaAs
GaAs
Slip-vector
field
Ad-atom energy
on (111)B
• Core/shell structure in
adatom energy
nucleation of the next
layer at a corner of the
nanowire top surface
Twin formation model
Ikejiri, K. et al. Growth characteristics of GaAs nanowires obtained by selective area metal-organic vapour-phase
epitaxy. Nanotechnology 19, 265604 (2008).
Two kinds of GaAs nucleations:
Both are tetrahedral in shape but one is 180
degree rotated with respect to the other.
Twins are caused by alternation of the two
forms of tetrahedron growth. The alternation of
the two orientations forms 6-fold symmetric
hexagonal nanowires on 3-fold symmetric GaAs
(111)B substrate.
Nano-Sheet
<11-2>
GaAs nano-sheets are grown on GaAs (111)B along
<11-2>
Nano-sheets share the same {-1-10} facets as <11-2>
tetrahedron nuclei; but keep two (1-10) and (111)B facets
as nanowires.
Growth rates on 3 inclined (110) planes and two (1-10)
planes are greatly reduced; growth only occurs on the
exposed (111)B surface.
Nanosheet can be seen as part of (11-2) tetrahedron
nucleation.
(-1-10) (0-1-1)
(-10-1) (111)B
Twin density in nanosheet
and nanowire
Cross-section TEM of nano-wire
Cross-section TEM of nano-sheet along <1-10>
zone axis
<111>B
<11-2>
(111)B GaAs
Nanosheet
Cross-section TEM of nano-sheet along <11-2> zone axis
(111)B GaAs
<111>B <1-10>
Nanosheet
No twins are observed within nano-sheet by TEM;
however high density of twins are observed within
most nano-wire structures.
Sail-like nanosheet
Arsine = 1 sccm; V/III = 26 Arsine = 0.25 sccm; V/III = 6
TMGa = 2.4 sccm; V/III= 2
Arsine = 17 sccm; V/III = 434
TMGa = 0.8 sccm; V/III= 6
Twin-free nanosheet growth condition
TMGa = 0.8 sccm; V/III= 6
Morphology change due to twins
1.Growth of (11-2) nanosheet terminated
with 3 (110) planes
2. Single twin forms at the tip of nanosheet
3. 180 degree rotated nanosheet initializes
on top of bottom nanosheet
4. Inclined planes of bottom nanosheet start
to grow
5. (111)A planes are formed
HRTEM for twinned nanosheet
20.62º
33.32º 18.52º
33.58º
68.07º
b
c
a
a
b
c
Stacking of two triangle growths
which are 180 degree rotated to
each other.
Boundary line between two triangle
growths which correspond to twin
plane.
FFT images show rotational twin
near the tip of nanosheet.
Summary
Twin-Free GaAs nanosheets were grown on <11-2> nano-stripe by
SAG
Low Arsine, high TMGa partial pressure can reduce driving force
for twin formation
Twinned nanosheet supports Ikejiri’s twinning model for NW
formation
Twinning can result in surface energy reduction
Two vertical <1-10> planes of nanosheet mitigate surface energy
reduction brought by twinning
http://www.lonelyconservative.com
Challenges for lighting applications
Strong piezoelectric field from strain
The spatial separation between the electron and hole wavefunctions reduces the radiative recombination efficiency.
Without strain Under compressive strain
Efficiency droop
Efficiency drops with increasing injection current
http://ssls.sandia.gov/research-thrusts/thrust1.html
High cost of nonpolar GaN bulk
http://www.semiconductor-today.com/news_items/2010/NOV/SEI_241110.htm
Fair light extraction efficiency
Air
GaN
Fresnel loss
Critical angle
loss
GaN nanorod arrays on c-plane substrates
• Nonpolar sidewall facets are exposed.
• Surface area of nopolar planes is proportional to the height of the GaN nanorods to potentially increase junction area.
Core-shell
InGaN/GaN quantum
well
Sapphire substrate (0001)
GaN bulk
<0001>
1100
c-direction
M - direction
• Core-shell InGaN/GaN quantum wells will be grown on the nonpolar facets by altering the growth mode.
• Light extraction can be enhanced from the nanostructures
http://dx.doi.org/10.1021/nl301307a
GaN nanosheet arrays on c-plane substrates
Sapphire substrate (0001)
GaN bulk
<0001>
c-direction
m-direction
1100
c-direction
<0001>
• Long parallel nonpolar sidewall facets are exposed.
• GaN nanosheet structure facilitates electrical conduction path for device fabrication.
• 3D junction morphology reduces current density.
Growth of 3-D Structure
• 3-D morphology is the result of growth competition between planes
• Planes with slower growth rate will dominate the shape
• Typical plane of GaN nano-structure: – Polar plane (0001), (c-plane)
– Non-plane {1-100}, (m-plane)
– Semi-polar plane {1-101}
GaN nanorods arrays grown with different pitches
•Uniform GaN nanorod arrays have been demonstrated in different spacings.
•All the sidewalls are non-polar planes serving as growth templates for InGaN/GaN
MQWs.
•Growth rates of > 5 m/hr achievable http://dx.doi.org/10.1021/nl301307a
250 nm
500 nm
750 nm
1 μm
GaN nanosheet arrays
Appl. Phys. Lett. 100 033119
(2012)
•The growth mask
is patterned along
<11-20> direction.
•Vertical sidewalls
are non-polar
planes, m-planes.
•Large connected
non-polar
sidewalls can be
used as MOW
growth templates.
InGaN/GaN core-shell MQWs on GaN nanorods
Semipolar
{1-101}
Polar
(0001
)
Non-polar
{1-100}
<11-20> zone
axis
TEM X-Sections of InGaN/GaN MQW on GaN Nanorods
43
CEN Annual Meeting
January 28 – 29, 2013
Cathodoluminescence analysis
CL mapping at different wavelengths
CL spectrum SEM image
• Dominant light emission from
non-polar planes is verified by
CL mapping.
• No light emission is observed
from the MQW grown on c-
plane due to the termination
of c-plane growth surface.
Cathodoluminescence analysis
1 2 3 4 5 6 7 8 9
SEM
image
CL spectrum CL spectra from specific regions
Both the height of nanrods and the pattern
spacing of growth masks must be carefully
designed to grow uniform MQWs on the m-planes.
Tuning the peak emission with patterns
•Samples were grown on the
same substrate with different
mask opening diameters.
•The peak positions shifted to
shorter wavelengths with
increasing opening diameter.
•Emission wavelength may
be controllable by careful
mask design and control of
the nonpolar plane surface
area.
143
157
173
209
243
GaN NanoLED structures
47
• Piezoelectric fields in MQWs grown
on the nonpolar planes eliminated.
• Thicker quantum wells can be
grown on the nonpolar planes to
enhance radiative recombination.
• Efficiency droop mitigated in the
thick quantum wells.
•Linked thin p-GaN •Coalescent thick p-GaN •300 μm ×300 μm NanoLED
MQWs grown
on nonpolar
planes
N-type GaN nanorods or GaN
nanosheets
A conducting p-type layer
SiNx mask
Electroluminescence of NanoLED
350 400 450 500 550 600 650 700
0
60
120
180
240
300
360
420
480
540
600
660
EL in
tens
ity (a
. u.)
Wavelength (nm)
90 mA
80 mA
70 mA
60 mA
50 mA
40 mA
30 mA
25 mA
20 mA
15 mA
10 mA
5 mA
0 20 40 60 80 100
460
465
470
475
480
485
490
495
500
EL p
eak
(nm
)
Current (mA)
Emission area: 4.5 ×10-4 cm2, Current injection:
CW
Spectral Broadening and Emission Peak Tuning affected by : Nanorod size variation
Current crowding effects along nanorod
Current spreading in p-GaN layer.
Composition variation and band filling.
Built-in electric field due to p-n junction.
L-I Characteristics
0 50 100 150 200
0
20
40
60
80
100
120
140
160
180
Inte
nsity
(a. u
.)
Current density (A/cm2
)
0 20 40 60 80 100 120 140 160 180 200
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Rela
tive
effic
ienc
y (a
.u.)
Current density (A/cm2
)
• The maximum efficiency occurs at 60 A/cm2.
• No heat sinking employed.
• Device structure and process requires optimization.
Light Intensity vs
injected current density
Relative efficiency vs
injected current density
Wide Spectral Emission Range
350 400 450 500 550 600 650 700 750 800
0.0
0.2
0.4
0.6
0.8
1.0
Norm
alize
d EL
inte
nsity
(a. u
.)
Wavelength (nm)
601 nm
589 nm
509 nm
494 nm
472 nm
Summary
•Ordered GaN nanorod and nanosheet arrays are successfully grown by
selective area growth. The exposed vertical sidewalls are nonpolar
planes.
•The growth of MQWs on the m-planes is confirmed by cross-sectional
TEM images of both nanorods and nanosheets.
•Light emission from regions of a InGaN / GaN nanorod investigated by
cathodoluminescence confirm the light emission is predominantly from
the MQWs grown on m-planes.
•NanoLEDs fabricated from nanorod arrays by p-layer overgrowth have
been demonstrated
•GaN nanorod and nanosheet arrays are candidates for enhancing LED
efficiency.
Contributors
Students Faculty Chun-Yung Chi Chongwu Zhou Maoqing Yao Michelle L. Povinelli Anuj Madaria Aiichiro Nakano Ting-Wei Yeh Steve Cronin
Yenting Lin Post Doctoral Student Chenxi Lin Shu Hu (Caltech) Ningfeng Huang Zaoshi Yuan Chia Chi Chang