Identification of Defects and Secondary Phases in Reactively Sputtered Cu

2

ZnSnS

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Thin Films

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

Outline

• Motivation • Problems with characterization of thin films • Experimental approach • Results • X-ray diffraction • Raman Spectroscopy • Transmission Electron Microscopy • Scanning Auger Microscopy • Summary & Acknowledgements 2

Motivation – Phase Equilibrium

Ternary Phase Diagram

SnS 2

Binary Phase Diagram

3 Cu 2 SnS 3 Cu 2 S ZnS • CZTS is a line compound between Cu 2 SnS 3 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.

Motivation – Crystal Structure

Theoretical XRD Patterns of CZTS, Cu 2 SnS 3 , and ZnS

4 • 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

Experimental Approach

Reactive Sputtering Zn

H 2 S H 2 S H 2 S H 2 S H 2 S H 2 S

4H 2 S + 2Cu + Zn + Sn

CZTS

Substrate Cu 2 ZnSnS 4 + 4H 2

• Introduce H 2 S 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 5

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Characterization - XRD

SnS 2

Varying Zn/(Cu+Sn) Ratio

Cu 2 SnS 3 Cu 2 S 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.

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Characterization - XRD

Varying Cu/(Zn+Sn) Ratio

SnS 2 Cu 2 SnS 3 Cu 2 S ZnS CuS Cu 2 S • Cu/(Zn+Sn) ratio is varied while holding Zn/Sn ratio constant • Need to get very far off 2:1:1 stoichiometry before any Cu x S phases can be seen • Cu x S can be removed with KCN etch Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System."

Journal of Alloys and Compounds

. 368. (2004): 135-143. Print.

Characterization – Raman

Varying Zn/(Cu+Sn) Ratio Varying Cu/(Zn+Sn) Ratio

8 • Raman spectra show only minor changes even though composition is varied dramatically • No evidence of the Cu x S phase shown by other groups at growth temperatures higher than 500C

340 nm 85 nm 55 nm 1.75 µm 1 µm

Device Fabrication

CZTS Device Stack

Aluminum Grid ZnO:Al (n-type) ZnO CdS (n-type)

CZTS Absorber (p-type)

Molybdenum Layer

SEM Image ZnO:Al ZnO CdS CZTS

9 3000 µm Glass Substrate

500nm Mo

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

Device Characterization

I-V Measurement EQE Measurement

10 • First CZTS devices grown by a reactive sputtering process • • Efficiency = 1.35%

Degraded EQE clearly points to undetected defects in the absorber

500nm

Characterization - TEM

Second phase CZTS 11

CZTS

• Detrimental secondary phase interspersed in CZTS matrix • Stacking faults in the secondary phase point to a transition between cubic and hexagonal crystal structures Second phase

Cu Zn Sn

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Characterization - Auger

CZTS ZnS 2 um • 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)

Characterization - CdZnS

CdS – ZnS Phase Diagram XRD before and after CBD

13 C C + H H • 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.

Sn Cd

Characterization - Auger

CZTS CdZnS 2 um • Overlay Sn and Cd signal • Cd ion exchanges with Zn during bath deposition and penetrates the ZnS phase 14

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 15

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?

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