Lattice Lattice-mismatched GaAs

mismatched GaAs

Cells Grown on Silicon by 

Cells Grown on Silicon by 

John F. Geisz, J.M. Olson

John F. Geisz, J.M. Olson

Romero, C.S. Jiang, A.G. N

Romero, C.S. Jiang, A.G. N

National Renewable Energy La

National Renewable Energy La

at the 

4th World Conference on Photovolta 4th World Conference on Photovoltaic Energy Conversio

ic Energy Conversio

NREL/PR-520-39847Presented at the 2006 IEEE 4th World Conference on Photovoltaic Energy Conversion (WMay 7-12, 2006 in Waikoloa, Hawaii.
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specific commercial product, process, or service by trade name, trademark, manufacturer, or othimply its endorsement, recommendation, or favoring by the Government or any agency thereof. authors and/or presenters expressed herein do not necessarily state or reflect those of MRI, the agency thereof. 
Motivation Motivation

Advantages of silicon substrates:

• Excellent solar cell material• Lower cost than III-V or Ge• Mature Si technology• Mechanically robust• Two-junction cell using Si bottom jun

optimal theoretical efficiency

- 34% efficiency at 1 sun AM0- 44% efficiency at 500 sun AM1.5G

1.

1.

1.

1.

tu

tu

GaAsP

• Monolithic, two (or more) junction Monolithic, two (or more) junction 

single crystal solar cell on Si single crystal solar cell on Si

• Old idea (SERI 1980 Old idea (SERI 1980’s)
Challenges for III Challenges for III

-V on Si 

V on Si 

• Silicon oxides Silicon oxides• Surface contamination Surface contamination• Antiphase domains Antiphase domains• Etching by sources Etching by sources • Interdiffusion Interdiffusion• Lattice Lattice

-mismatch mismatch

• Thermal expansion mismatc

Thermal expansion mismatc

New developments New developments• Improved characterization tec

Improved characterization tec

• Better understand for III Better understand for III

-V nuc

V nu

• Lattice Lattice

-matched GaNPAs ma

matched GaNPAs ma
P-on-N GaAsP Cell Stru

N GaAsP Cell Stru

• III-V grown by OMVPE V grown by OMVPE

• Smooth GaP nucleation Smooth GaP nucleation 

on Si

• Step graded GaAsP Step graded GaAsP 

buffer to reduce buffer to reduce dislocations dislocations

• Constant lattice GaAsP Constant lattice GaAsP 

junction with GaInP junction with GaInP passivation passivation

Ti

1.0 μm 

20 

0

0.25 or 

0.2 μ

1 - 2 μm G

0.2 μm 

50 nm 

GaAs 0.63P 0.

GaAs 0.07P 0.
GaP Nucleation on 

GaP Nucleation on 

1 

μ

m 1 

μ

m 

5.7 nm GaP 5.7 nm GaPRMS = 1.2 nm RMS = 1.2 nm

17 nm GaP 17 nm GaP33 nm GaNPRMS = 1.9 nm RMS = 1.9 nm

• Antiphase domains during GaP Antiphase domains during GaP nucleation r

nucleation r

• Growth of LM Growth of LM GaNP smoothes surface and 

smoothes surface and 
Step graded buffer la

Step graded buffer la

• 0.25 or 0

0.25 or 0

GaAs

x

P

1

• Composi

Composi

Δx = 0.07 

x = 0.07 

• 3.75 or 6

3.75 or 6

buffer thi

buffer thi

• Many dis

Many dis

grade, bu

grade, bu

active lay

active lay

Cross

-sectional TEM sectional TEM

GaP

nucleation nucleation
Composition and Str

Composition and Str

• Grown under Grown under 

compression because compression because growing growing epilayers epilayershave larger lattice have larger lattice constant constant

• Relaxes at Relaxes at 

T

g above critical thickness critical thickness

• Cools toward tension Cools toward tension• Residual compressive Residual compressive 

strain at strain at 

T

g results in results in less tension at RT (no less tension at RT (no 

cracks) cracks)

X

-ray dif

ray dif

224GI re

224GI re
Threading Dislocatio

Threading Dislocatio

Plan

-

•Electron Electron

-Beam

-Induced Induced 

Current (EBIC) Current (EBIC)

•9 x 10

7

- 1 x 10

8 cm

-2 

threading dislocations threading dislocations
GaAsP/Si Device Perfor

GaAsP/Si Device Perfor

• Single

-junction 8.7% efficiency AM1.5G 

junction 8.7% efficiency AM1.5G 

• Increase Increase 

Jsc with lower with lower 

E

g (need about 2

(need about 2

2

-junction current matching) junction current matching)

• Improve QE with better Improve QE with better passivation passivation, thin

, thin
Literature Comparis

Literature Comparis

GaAs/SiGe/Si ~ 10 GaAs/SiGe/Si ~ 10

6 cm-2

Ringel, Prog. PV 10, (2002), 417 . PV 10, (2002), 417

GaAsP/GaAs ~ 10 GaAsP/GaAs ~ 10

6 cm-2

Vernon, 19th PVSC, (1987),108 Vernon, 19th PVSC, (1987),108Wanlass Wanlass, 19th PVSC, (1987), 530 , 19th PVSC, (1987), 530

AlGaAs/Si ~10 AlGaAs/Si ~10

7 cm-2

Soga, J.Appl. Phys., 78, (1995), 4196 . Phys., 78, (1995), 4196

GaAs

GaAsP/S

GaAsP/S

Hayashi, 1st 

Hayashi, 1st 

Voc = 

Eg/q - 0.4

LM 

Ge

Ge

Voc is excellent is excellent 

measure of quality measure of quality 

for mismatched for mismatched 

solar cells solar cells 
Conclusions Conclusions

• Lattice Lattice-mismatched GaAsP so

mismatched GaAsP s

grown on Si grown on Si

• Compositional step grade red

Compositional step grade red

dislocations to ~ dislocations to ~108 cm-2

• Voc not ideal, but comparable 

not ideal, but comparable 

grown on Si with transparent 

grown on Si with transparent 

• Diffusion lengths better than L

Diffusion lengths better than L

• Want to reduce dislocations to 

Want to reduce dislocations to 
Extra Slides Extra Slides
Thermal Expansio

Thermal Expansio

• Can measure strain at RT with XRD Can measure strain at RT with XRD

• Would like to measure strain during 

Would like to measure strain during 

• Can calculate strain state at Can calculate strain state at Tg ass

ass

– change in in change in in

-plane lattice constant of plane lattice constant of epilayer

epilayer

by thick Si by thick Si

– no relaxation upon cool no relaxation upon cool

-down

• Linear coefficients of thermal expan

Linear coefficients of thermal expan

– Si: 3.7 x 10 3.7 x 10-6– GaP: 5.3 x 10-6– GaAs: 6.8 x 10-6– GaN: 6 x 10-6 

?

– InAs: 5.2 x 10-6

• Scales with Scales with Tg

• Biaxial strain energy Biaxial strain energy

U = Y 

ε

x

2

t
Cracking from Tensile Str

Cracking from Tensile Str

220DF

crack

1 

μ

m

20 

μ

m

Optical Optical Nomarski Nomarski

Cross

-sec

sec

Plan

-view SEM view SEM

Charging effect reveals cra

Charging effect reveals cr
Si junction Si junction

• Under current gr

more P than Ga from GaP

• Creates n-type e• Emitter passivat

interface defects

• Voc of silicon junction ~535mV

• Currently using CZ Si, but float-zone 

may be better

• BSF from annealed Al contact
GaNPAs/Si Tandem Solar C

GaNPAs/Si Tandem Solar C

• Working tandem Working tandem• Good current in Si junction, but Voc could 

Good current in Si junction, but Voc could 

• GaNPAs delivers half the current necessa

delivers half the current necessa

match tandem match tandem

Geisz et al., PVSC et al., PVSC 31 (2005) 695 (2005) 695
Step Grading for Mismatc

Step Grading for Mismatc

• Top layer strained to match next in Top layer strained to match next in

-plane 

lattice constant (residual strain) lattice constant (residual strain)

• Mostly relaxes while next layer growing Mostly relaxes while next layer growing

1 

μ

m

GaAsP step grade step grade

220DF

GaAs0.7

P0.3

GaP

XRD Recipr

XRD Recipr

XTEM
Lattice Lattice-matched matched Ga
GaNPAs/Si Tandem Solar Ce

GaNPAs/Si Tandem Solar Ce

0.5 

μ

m
GaAsP/Si Device Perfor

GaAsP/Si Device Perfor

• Single

-junction 8.7% efficiency AM1.5G w/o AR coat junction 8.7% efficiency AM1.5G w/o AR coat

• Increase Increase 

Jsc by lower by lower 

E

g (need about 20 mA/cm (need about 20 mA/cm

2 for 

• Improved QE with wide depletion region, but decent 

Improved QE with wide depletion region, but decent 

region

• GaAsP/Si better than GaNPAs/GaP GaAsP/Si better than GaNPAs/GaP

– Longer diffusion length even with 10 Longer diffusion length even with 10

8 cm-2 TD