1. Generalizing the correlation between electrical and structural properties in sputtered ZnO:Al Andrea Crovetto *, Tobias Ottsen, Eugen Stamate, Daniel Kjær, Jørgen Schou, Ole Hansen Technical University of Denmark (DTU) * : ancro@nanotech.dtu.dk

2. Introduction 1.compressive stress increases It is often reported that the electrical properties of sputtered AZO degrade as: Low resistivity High resistivity A. Bikowski et al., J. Appl. Phys. 2013, 114, 223716 K. Ellmer, J. Phys. D. Appl. Phys. 2000, 33, R17–R32 Problem: what is the general correlation between electrical and structural properties? 2.grain size decreases 3.(001) texture coefficient decreases …but inconsistent cases exist in the literature A. Bikowski et al., J. Appl. Phys. 2013, 114, 223716 C. Charpentier et al., EPJ Photovoltaics 2012, 2, 25002 S. H. Jeong et al., Thin Solid Films 2004, 447-448, 105–110

3. Design of experiment Radio-frequency sputtering from ceramic target Cross-sectional view Bottom view Different substrates for different characterization techniques Parameters explored: 1.Deposition pressure (in a low pressure regime) 2.Radial distance from center of target (from x 0 to x 3 )

4. Relationship between electrical and structural properties? Data analysis strategy Goal: to find as general correlations as possible (including both pressure- and positional effects) Why don’t we combine pressure- and spatial dependances into a single plot?

5. Electrical properties vs. stress ”universal” curves vs. compressive stress

6. Structural properties vs. stress Why is there a minimum for a finite value of the compressive stress?? Hypothesis: the minimum corresponds to Zone T in the structure zone diagram J. A. Thornton, Annu. Rev. Mater. Sci. 1977, 7, 239–260 Zone T: ”competitive growth of differently oriented grains” Expected to reduce grain size and texture coefficient

7. A model for structural properties A. Anders, Thin Solid Films 2010, 518, 4087–4090. P. Barna, M. Adamik, Thin Solid Films 1998, 317, 27–33

8. Note: how to get grain size from XRD? When evaluating grain size from XRD peak width… Filled: after deconvolution Open: before deconvolution of instrumental broadening and micro-strain broadening It’s very important to check if micro-strain and instrumental effects are affecting the conclusions factor of 8 Average error in our data: 43% (underestimation)

9. Conclusion 1.”General” relation between electrical properties and compressive stress For best electrical properties, compressive stress must be minimized 2.”General” relation between grain size/texture and compressive stress Minimum in grain size and (001) texture around 1GPa stress  zone T of structure zone diagrams 3.!Warning! If you need to estimate grain size from XRD: In-plane grain size and out-of plane grain size are not necessarily related If micro-strain is present in the film  necessary to deconvolute it from XRD peaks, otherwise huge errors are possible

10. Answers to initial problem 1.compressive stress increases It is often reported that the electrical properties of sputtered AZO degrade as: true 2.grain size decreases Only true if compressive stress is low (below ~ 1GPa) 3.(001) texture coefficient decreases Only true if compressive stress is low (below ~ 1GPa)

11. Acknowledgments Funding by: Micro-Hall measurements by: http://www.capres.com I am happy to discuss the results! ancro@nanotech.dtu.dk

12. Introduction Two models for spatial inhomogeneity of electrical properties: Bombardment by high-energy oxygen ions Inhomogeneity in oxygen reaching the substrate Often claimed that electrical properties degrade as: grain size decreases (001) texture coefficient decreases compressive stress increases

13. Goals 1.Origin of spatial inhomogeneity of electrical properties? 2.Relationship between electrical and structural properties?

14. Problem 1): Inhomogeneity

15. c-axis lattice shift proportional to in-plane stress proportional to (particle flux density) X (particle energy) 1/2 (if stress is compressive) Particle bombardment?

16. Problem 1): Inhomogeneity Evidence of particle bombardment: 1.Negative correlation between compressive stress and resistivity 2.Amorphization effects at the pressure-position with largest compressive stress 3.Drop in deposition rate at the pressure-position with largest compressive stress (resputtering)

17. Problem 1): Inhomogeneity Inhomogeneity in oxygen content?

18. Morphology

19. Conclusion 1.Origin of spatial inhomogeneity of electrical properties? a)Low/medium pressure: mostly particle bombardement b)High pressure: unclear c)Medium pressure: (small) effect of oxygen inhomogeneity on mobility only 2.Relationship between electrical and structural properties?

20. Al dopant content vs. stress Roughly linear increase in Al content with compressive stress. WHY? It’s not a direct relationship The more particle bombardment, the higher the actual temperature of the growing film Al content known to increase with growth temperature