Identification of Defects and Secondary Phases in Reactively Sputtered Cu2 ZnSnS4 Thin Films

1. Identification of Defects and Secondary Phases in Reactively Sputtered Cu2ZnSnS4 Thin Films

Vardaan Chawla, Stacey Bent, Bruce Clemens
April 7th, 2010
Center on Nanostructuring for Efficient Energy Conversion
Materials Science and Engineering
Stanford University

2.

Outline
• Motivation
• Problems with characterization of thin films
• Experimental approach
• Results
X-ray diffraction
Raman Spectroscopy
Transmission Electron Microscopy
Scanning Auger Microscopy
• Summary & Acknowledgements
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3.

Motivation – Phase Equilibrium
Ternary Phase Diagram
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Binary Phase Diagram
SnS2
Cu2S
CZTS
Cu2SnS3
ZnS
• CZTS is a line compound between Cu2SnS3 and ZnS
• Theoretically even a 2-3% compositional variation could
lead to phase separation
Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System." Journal of Alloys and Compounds. 368. (2004): 135-143. Print.

4.

Motivation – Crystal Structure
Theoretical XRD Patterns of CZTS, Cu2SnS3, and ZnS
• Crystal structures of secondary phases similar to CZTS
• All primary peaks overlap and hard to separate
• Low intensity peaks cannot be seen easily in thin films
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5.

Experimental Approach
Reactive Sputtering
Zn
H2S
H2S
H2S
H2S
H2S
H2S
4H2S + 2Cu + Zn + Sn
CZTS
Cu2ZnSnS4 + 4H2
Substrate
• Introduce H2S into chamber during sputter deposition
• Sulfur is incorporated into the film in one step (no anneal)
• Expect to see higher densities and improved film quality
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Cu2S
(312)
(101)
Cu2SnS3
(220)
SnS2
(200)
Varying Zn/(Cu+Sn) Ratio
(112)
Characterization - XRD
ZnS
• Zn/(Cu+Sn) ratio is varied while holding Cu/Sn ratio constant
• Impossible to determine difference between CZTS, CTS, and
ZnS from XRD pattern
Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System." Journal of Alloys and Compounds. 368. (2004): 135-143. Print.

7.

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Cu2S
(312)
Cu2SnS3
(220)
(101)
SnS2
(200)
Varying Cu/(Zn+Sn) Ratio
(112)
Characterization - XRD
ZnS
• Cu/(Zn+Sn) ratio is varied while
holding Zn/Sn ratio constant
• Need to get very far off 2:1:1
stoichiometry before any CuxS phases
can be seen
• CuxS can be removed with KCN etch
CuS
Cu2S
Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System." Journal of Alloys and Compounds. 368. (2004): 135-143. Print.

8.

Characterization – Raman
Varying Zn/(Cu+Sn) Ratio
Varying Cu/(Zn+Sn) Ratio
• Raman spectra show only minor changes even though
composition is varied dramatically
• No evidence of the CuxS phase shown by other groups at
growth temperatures higher than 500C
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9.

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Device Fabrication
CZTS Device Stack
SEM Image
Aluminum Grid
340 nm
ZnO:Al (n-type)
85 nm
ZnO
55 nm
CdS (n-type)
1.75 µm
ZnO:Al
ZnO
CdS
CZTS Absorber (p-type)
CZTS
1 µm
3000 µm
Molybdenum Layer
Glass Substrate
500nm
Mo
• Zn-rich films incorporated into standard CIGS device
stack for testing

10.

Device Characterization
I-V Measurement
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EQE Measurement
• First CZTS devices grown by a reactive sputtering process
• Efficiency = 1.35%
• Degraded EQE clearly points to undetected defects in
the absorber

11.

Characterization - TEM
CZTS
Second
phase
• Detrimental
secondary phase
interspersed in
CZTS matrix
• Stacking faults in the
secondary phase
point to a transition
between cubic and
hexagonal crystal
structures
CZTS
500nm
200nm
5nm
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Second
phase

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Characterization - Auger
Cu
CZTS
ZnS
Zn
2 um
Sn
• Raster beam over sputtered surface
of sample and scan for Cu, Zn, Sn
• Overlay Cu, Zn, Sn signal
• Composition variation points to
CZTS / ZnS (Zn-rich)

13.

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Characterization - CdZnS
XRD before and after CBD
(312)
C
(220)
C+H
(101)
H
H-ZnS
(112)
CdS – ZnS Phase Diagram
• Cd penetration into ZnS lowers the cubic-hexagonal
transition temperature
• Stacking faults in TEM images are created during CBD
of CdS layer
Chen et al.“Solid State Phase Equilibria in the ZnS-CdS System." Materials Research Bulletin. 23. (1988): 1667-1673. Print.

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Characterization - Auger
CZTS
Sn
CdZnS
2 um
Cd
• Overlay Sn and Cd signal
• Cd ion exchanges with Zn during
bath deposition and penetrates the
ZnS phase

15.

Summary
• CZTS thin films were grown using Reactive
Sputtering
• Films were characterized using X-ray Diffraction
and Raman Spectroscopy
• Full devices have been grown and tested but
are limited due to secondary phases in the films
• Transmission Electron Microscopy and
Scanning Auger Microscopy can be used to
identify these secondary phases
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16.

Acknowledgements
• US Department of Energy, Office of Basic
Energy Sciences as part of an Energy Frontier
Research Center
• http://www.er.doe.gov/bes/EFRC/index.html
• Applied Quantum Technologies
• Local thin film solar startup
• http://www.aqtsolar.com
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17.

Questions
Questions?
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