1. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Chemical etching effects in porous silicon layers Daniel Navarro Urrios Dpto. de Física Básica, University of La Laguna, Spain INFM and Dipartimento di Fisica, University of Trento, Italy

2. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Co-workers C. Pérez-Padrón, E. Lorenzo, N. E. Capuj Dpto. de Física Básica, University of La Laguna, Avda. Astrofísico Fco. Sánchez, La Laguna, 38071 Spain Z. Gaburro, C. J. Oton and L. Pavesi INFM and Dipartimento di Fisica, University of Trento, Via Sommarive 14, Povo, Trento 38050 Italy

3. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Index Introduction and motivations Interferometric measurements PL measurements Conclusions

4. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Introduction to porous silicon What is porous silicon? – Nanostructured spongeous silicon – Electrochemically etched with HF Properties of PS: – Luminescent (quantum confinement) – Optically homogeneous – Modulable refractive index with current – Promising for photonics

5. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Introduction Porous silicon formation is slightly non-homogeneous in depth I HF HF chemically etches off porous silicon (porosity increases slowly) Also luminescence properties depend on porosity (Quantum confinement)

6. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Motivations Study the changes induced by chemical etching: - Optical (in-situ refractive index monitoring) - Structural (electronic microscopy) - Light emission (PL)

7. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Index Introduction and motivations Interferometric measurements PL measurements Conclusions

8. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Photodetector HF Platinum electrode He-Ne laser Attenuation Filter Mirror c-Si PS l 2 Experimental setup Low light intensity!

9. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Optical path difference between the two rays During the anodization, the optical path increases Frequency  opt. path change rate What we can see l 2

10. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. After anodization, there is still an oscillating signal! Refractive index decrease The phase changes sign!

11. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. (out-diffusion of chemical species, micro-bubbles…) The signal frequency is the same before and after It is an irreversible chemical process, not a transient Could the post- etching oscillations be a transient? Short current pulse

12. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. During the post-etching we consider: - constant thickness - variations in refractive index (n PS ) Signal frequency: Frequency proportional to the sample thickness and to the refractive index change rate

13. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Results We studied samples with different anodization times: (increasing thickness) 400 s 800 s 1200 s 1600 s

14. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. They overlap The index change rate is independent on thickness Two different exponential decays: One fast (400 s) One slow (8200 s) Results Frequency vs. post-etching time Normalizing to the thickness

15. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. TEM micrographs Different post-etching times: (Sample thickness: 500 nm) 50 nm No post-etching 1200 sec 2400 sec Fast post etching process Beginning of the slow process

16. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. What is happening Chemical etching enlarges the pores and reduces the wall thickness

17. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Index Introduction and motivations Interferometric measurements PL measurements Conclusions

18. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Porous silicon luminescence is associated with quantum confinement in the nanostructures PL measurements What happens with the PL? Post-etching reduces the nanostructure size A blueshift is expected

19. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. We studied samples with the same anodization time (400s) (same thickness) PL measurements Different post-etching times 0 s 1200 s 1800 s 5200 s 12000 s 18000 s

20. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. PL measurements During post-etching, a short wavelength contribution appears, and then disappears First minutes It can be associated with the fast etching of a rough nanostructure on the walls of the pores

21. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Long post-etching times The main PL peak blue-shifts PL measurements (waterfall plot) Walls of the pores become thinner, increasing the quantum confinement

22. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Index Introduction and motivations Interferometric measurements PL measurements Conclusions

23. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Conclusions 1.HF chemically etches porous silicon homogeneously -Increasing porosity -Decreasing refractive index 2.For short times: -Fast index decrease -A blue component appears and disappears 3.For long times: - Slow index decrease - Slow blue-shift of the main PL peak

24. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003. Acknowledgements We acknowledge Spanish Ministry of Science and Technology Project MAT 2002-00044, the Canary Islands Government (Project No. PI2001/093, PI2001/074), the European project EC-SINERGIA and the Italian INFM project PAIS-SMOG. we thank Professors I. Martín and S. Gialanella

25. SPIE. Microtechnologies for the New Millenium 2003. 19-21 May 2003.