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

3

Motivation – Phase Equilibrium

• CZTS is a line compound between Cu2SnS3 and ZnS

• Theoretically even a 2-3% compositional variation could lead to phase separation

Ternary Phase Diagram Binary Phase Diagram

Cu2S ZnS

SnS2

Cu2SnS3

Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System." Journal of Alloys and Compounds. 368. (2004): 135-143. Print.

CZ

TS

4

Motivation – Crystal Structure

• 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

Theoretical XRD Patterns of CZTS, Cu2SnS3, and ZnS

5

Experimental Approach

Substrate

CuZn

Sn

H2S H2S

H2SH2S

H2S

H2S

4H2S + 2Cu + Zn + Sn Cu2ZnSnS4 + 4H2

CZTS

• 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

Reactive Sputtering

6

Characterization - XRD

Cu2S ZnS

SnS2

Cu2SnS3

Varying Zn/(Cu+Sn) Ratio

• Zn/(Cu+Sn) ratio is varied while holding Cu/Sn ratio constant• Impossible to determine difference between CZTS, CTS, and

ZnS from XRD pattern

(101

)

(200

)

(220

)

(312

)

(112

)

Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System." Journal of Alloys and Compounds. 368. (2004): 135-143. Print.

7

Characterization - XRD

Cu2S ZnS

SnS2

Cu2SnS3

Varying Cu/(Zn+Sn) Ratio

• 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

(101

)

(200

)

(220

)

(312

)

(112

)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

• 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

Varying Cu/(Zn+Sn) Ratio

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500nm

Device Fabrication

Glass Substrate3000 µm

Molybdenum Layer1 µm

1.75 µm

CdS (n-type)55 nm

ZnO:Al (n-type)340 nm

Aluminum Grid

CZTS Absorber (p-type)

CZTS Device Stack

• Zn-rich films incorporated into standard CIGS device stack for testing

ZnO85 nm ZnO

ZnO:Al

CdS

CZTS

Mo

SEM Image

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Device Characterization

I-V Measurement EQE Measurement

• First CZTS devices grown by a reactive sputtering process• Efficiency = 1.35%• Degraded EQE clearly points to undetected defects in

the absorber

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Characterization - TEM

500nm

200nm

CZTS

Secondphase

5nm

CZTS

Secondphase

• Detrimental secondary phase interspersed in CZTS matrix

• Stacking faults in the secondary phase point to a transition between cubic and hexagonal crystal structures

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Characterization - Auger

• 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)

2 um

Cu

Zn

Sn

CZTS

ZnS

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Characterization - CdZnS

• Cd penetration into ZnS lowers the cubic-hexagonal transition temperature

• Stacking faults in TEM images are created during CBD of CdS layer

CdS – ZnS Phase Diagram

(101

)

H-Z

nS

(220

)

(312

)

(112

)

XRD before and after CBD

C

C + H

H

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

Sn

Cd • Overlay Sn and Cd signal• Cd ion exchanges with Zn during

bath deposition and penetrates the ZnS phase

CZTS

CdZnS

2 um

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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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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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Questions

Questions?