Identification of Defects and Secondary Phases in Reactively Sputtered Cu 2 ZnSnS 4 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

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 Cu 2 SnS 3 and ZnS Theoretically even a 2-3% compositional variation could lead to phase separation Ternary Phase Diagram Binary Phase Diagram Cu 2 SZnS SnS 2 Cu 2 SnS 3 Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System." Journal of Alloys and Compounds (2004): Print. CZTS

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, Cu 2 SnS 3, and ZnS

5 Experimental Approach Substrate Cu Zn Sn H2SH2SH2SH2S H2SH2S H2SH2S H2SH2S H2SH2S 4H 2 S + 2Cu + Zn + SnCu 2 ZnSnS 4 + 4H 2 CZTS 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 Reactive Sputtering

6 Characterization - XRD Cu 2 S ZnS SnS 2 Cu 2 SnS 3 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 (2004): Print.

7 Characterization - XRD Cu 2 S ZnS SnS 2 Cu 2 SnS 3 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 Cu x S phases can be seen Cu x S can be removed with KCN etch CuS Cu 2 S (101) (200) (220) (312) (112) Olekseyuk, I.D. "Phase Equilibria in the Cu2S-ZnS-SnS2 System." Journal of Alloys and Compounds (2004): 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 Cu x S phase shown by other groups at growth temperatures higher than 500C Varying Cu/(Zn+Sn) Ratio

500nm 9 Device Fabrication Glass Substrate 3000 µm Molybdenum Layer 1 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 ZnO 85 nm ZnO ZnO:Al CdS CZTS Mo SEM Image

10 Device Characterization I-V MeasurementEQE 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 500nm 200nm CZTS Second phase 5nm 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

12 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

13 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-ZnS (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): Print.

14 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

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

16 Acknowledgements US Department of Energy, Office of Basic Energy Sciences as part of an Energy Frontier Research Center Applied Quantum Technologies Local thin film solar startup

17 Questions Questions?