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