1. Application of Nanostructures Porous Silicon in Optoelectronic Devices
Dr. Hasan A. Hadi
AL-Mustansiriyah University
Education Faculty-Physics Dep.
Baghdad, Iraq

2. Porous silicon
is a network of air holes within an interconnected silicon matrix.  The size of these air holes, called pores, can vary from a few nanometers to a few microns depending on the conditions of formation and the characteristics of the silicon
porosity : The void fraction into the volume
Dr. Hasan A. Hadi

3. Type of porous Si
According to IUPAC standards, we can classify porous silicon (PS) in three families in function with their average pore diameter: macroporous, mesoporous and microporous silicon
Ref:
1. M. Beale, N. Chew, M. Uren, A. Cullis, and J. Benjamin, « Microstructure and formation mechanism of porous silicon », Applied Physics Letters, vol. 46, p. 86 (1985).
2. V. Lehmann, R. Stengl and A. Luigart, « On the morphology and the electrochemical formation mechanism of mesoporous silicon », Mat. Sc. Eng. B, vol.69/70, p. 11 (2000).
Dr. Hasan A. Hadi 2015

4. Why it is interesting?
Four major reasons that have contributed to the continued widespread attention given to porous silicon are the following:
Its efficient visible luminescence give rise to the possibility of all silicon based optoelectronic devices >>>light emission in silicon .. Increases band gap behaves as a direct gap semiconductor
Its production is simple and inexpensive compared to current technique(lithography, C.V.D and epitaxial growth) used to produce low dimensional structures
Its compatibility with current silicon microelectronic processing makes integrated silicon based optoelectronic devises possibility…nanometer size effects
Its very large surface to volume ratio makes the porous silicon matrix an excellent host for chemical and biological species m2/cm3
Ref: 1.Claudio Vinegoni, Massimo Cazzanelli, L. Pavesi, “Porous silicon microcavities”, in "Silicon-Based Materials and Devices", Academic Press, VOL. 2 (2001) PAG
2. Grosman & Ortega C (1997) Chemical composition of ‘fresh’ porous silicon. In: Canham L (ed) Properties of porous silicon. INSPEC LONDON: Dr. Hasan A. Hadi 2015

5. Preparation methods of porous silicon
Electrochemical Etching
Photo electrochemical Etching
Bottom-wafer electrochemical cell
Laser-induced etching
Stain etching
Lateral-wafer electrochemical cell
Anodic polarization
forward biased if Si is p-type
reverse biased if Si is n-type`
Dr. Hasan A. Hadi 2015

6. How is it made? Photo-Electrochemical etching N-type Si holes must be provided in the material. Most of the time, this can be achieved by backside wafer illumination.
Laboratory equipment
Inject current (holes) into Si
• the etching path and rate can be controlled
• holes congregate at the tips of the etch pores.
Schematic of the single cell back-side setup to fabrication of porous silicon
Cross-sectional view of lateral anodization cell.
Dr. Hasan A. Hadi 2015

7. Schematic of dissolution mechanism of silicon proposed
moderate current densities, J < JPS… two-holes consuming reaction
high current densities, J > JPS… four-holes consuming reaction
Ref: V. Lehmann, « The Physics of Macropore Formation in Low Doped n-Type Silicon », J. of the Electrochemical Society, vol. 140, No. 10, p.2836 (1993
Dr. Hasan A. Hadi 2015

8. Main etching parameters
the morphology family (microporous, mesoporous or macroporous).
Ref: X. Zhang, « Morphology and formation mechanisms of porous silicon », J. of the Electrochemical Society, vol. 151, p. C69 (2004).
Dr. Hasan A. Hadi 2015

9. Morphological characteristics of porous silicon
Shape
orientation
shape of the pore bottom
fill of macropores
branching
depth variation of PS layers
Ref: X. Zhang, « Morphology and formation mechanisms of porous silicon », J. of the Electrochemical Society, vol. 151, p. C69 (2004).

10. Properties of porous silicon{OM,AFM,SEM, EM,FTIR,XRD, )
SEM, TEM :the pore shape
AFM images of micro/nanoporous Si
SEM photo-image of the cross-sectional view of the formed triple-layered porous-silic`on
Right inset show the optical micrograph of bulk silicon n-Si and PS surface
(a) XRD spectra of PS sample anodized on (b) n-type silicon
Dr. Hasan A. Hadi 2015

11. AFM Optical microscopy
Ref: H. A. Hadi, Fabrication and characterization of porous silicon photodetector,PhD Thesis, University of AL-Mustansiriyah Iraq (2012).

12. PHOTODETECTORS • Optoelectronic properties a function of skeleton size
large surface area and
texturing for trapping light
behaves as a direct gap
semiconductor
Schematic cross section of the fabricated sn/PS/p-Si/Al photo detector.
Porous Silicon (PS) Schottky Barrier Detector
The modification was made just to make the Sn on top of the PS in stead of Si
Schematic setup of spectral photo responsivity measurement

13. (I-V) Characteristics Un. Illumination
Ref: H. A. Hadi, Fabrication and characterization of porous silicon photodetector,PhD Thesis, University of AL-Mustansiriyah Iraq (2012).

14. Responsivity
responsivity
Dr. Hasan A. Hadi 2015

15. Quantum efficiency Dr. Hasan A. Hadi 2015
Ref: H. A. Hadi, Fabrication and characterization of porous silicon photodetector,PhD Thesis, University of AL-Mustansiriyah Iraq (2012).
Dr. Hasan A. Hadi 2015

16. Energy gap of porous silicon
Current density ( )
Energy gap
(ev) 50
2.2 40 2 30 20
1.9
Ref: H. A. Hadi, Fabrication and characterization of porous silicon photodetector,PhD Thesis, University of AL-Mustansiriyah Iraq (2012).

17. Gas sensor Control of pore size and depth Chemical sensors
Dr. Hasan A. Hadi 2015

18. Multi-Sensing Principle
Gas out
Porous silicon surface
Gas in

19. Dr. Hasan A. Hadi 2015

20. Applications
LED's is based on porous silicon (PSi) and manufactured by electrochemical etching of a previously formed pn diode structure.
The band-gap is widening as the crystal size is reduced (quantum confinement), enabling light emission in the visible range.
Difficulties
Indirect bandgap
Fast nonradiative recombination
Slow radiative routes
Low quantum efficiency
Processing techniques must be consistent with microelectronics (CMOS)
Dr. Hasan A. Hadi 2015

21. High luminescence efficiency due to:
(i) quantum confinement, leading to a larger bandgap and an increased recombination probability
(ii) spatial confinement of free carriers , preventing them from reaching non-radiative recombination points
(iii) reduction of the refractive index of the material, increasing the extraction efficiency.
Dr. Hasan A. Hadi 2015

22. THANK YOU!
Dr. Hasan A. Hadi 2015

23. Science is facts; just as houses are made of stones, so is science made of facts; but a pile of stones is not a house and a collection of facts is not necessarily science.
Dr. Hasan A. Hadi 2015