Transcript Slide 1

Electrochemical formation of Porous Silicon in Room Temperature Ionic Liquid
O. Raz, D. Starosvetsky and Y. Ein-Eli.
Corrosion & Applied Electrochemistry Laboratory (CAEL)
Department of Materials Engineering, Technion, Haifa 32000, Israel.
Introduction: Porous Silicon (PS) can exhibit rich morphological features depending on various experimental conditions (doping of the bulk material, electrolyte concentration, etching volume, and
temperature). Macro-porous silicon is defined as silicon having pores in the micro-metric range. This material was the first to have commercial applications in the capacitor technology and it is investigated as
a material for micro-system technology and serves as an electrode based material. PS is usually formed by the application of low anodic bias on the silicon substrate immersed in a solution containing HF. PS
is also formed in electrolytes containing HF and an organic solvent (water addition, originated from HF (49%), is always included in the electrolyte composition). Such as, acetonitrile (MeCN),
dimethylformamide (DMF), dimethylsulfoxide (DMSO), ethanol and others, as well.
A novel study on the electrochemical interaction between silicon and room temperature ionic liquids (RTIL’s) is reported. This novel research provides new insight on silicon electrochemistry in RTIL’s. The
research is in designed RTIL’s, with desired properties: wide electrochemical window and high conductivity. As a part of this study is understanding the electrochemical behavior of silicon in a new and novel
RTIL based on 1-ethyl 3-methyl imidazolium (EMI) cation with (HF)2.3F- anion. This RTILis non aqueous and exhibits high conductivity (100 mS cm-1), and is considered acidic and hydrophobic.
Formation of a Porous Silicon by application of a positive potential on both n and p type Si in EMIF(HF) 2.3 is presented. The effect of time and potential on Porous Silicon formation and structure will be
discussed These results are important towards the understanding of the unique silicon dissolution mechanism, by performing it in a non-aqueous media.
Research objectives: To study and understand the electrochemical behavior of silicon in EMIF(HF)2.3 Room Temperature Ionic Liquid..
Results: n-type silicon
160
160
2
0
100
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Current (mA/cm )
Potential (VPt)
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120
100
80
60
40
20
80
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8
10
Time (hr)
60
120
100
80
60
40
40
20
20
0
0
-2
-7
-6
-5
-4
-3
-2
0
-1
1
2
Log I (A/cm )
Time (hr)
Figure 1: Anodic potentiodynamic curve of n-Si in
EMIF(HF)2.3 , Scan rate 5 mV/sec, 25oC.
Figure 2: 8V anodic polarization potentiotiostatic
study of n-Si in EMIF(HF)2.3
(a)
(b)
10 µm
10 µm
(c)
(d)
10 µm
10 µm
Figure 4: SE micrographs of n-type Si (100) subsequent
to exposure to a potential of 8 V (anodic polarization) in
EMIF(HF)2.3; (a) 15 min, (b) 30 min, (c) 3 hr, (d) 10 hr.
(a)
5 µm
5 µm
0
2
(a)
5 µm
5 µm
(a)
(b)
Figure 7: SE micrographs of crooss section view of n-type
Si (100) subsequent to exposure to anodic polarization in
EMIF(HF)2.3 for 10 hr; (a) 2V, (b) 5V, (c) 10V.
Summery
2
Current (mA/cm )
60
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50
5 µm
5 µm
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40
(c)
30
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•
20
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5 µm
2
3
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5
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8
Time (hr)
Figure 8 : Potentiotiostatic profile of p-Si (100) in
EMIF(HF)2.3 performed at 8 V.
5 µm
5 µm
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1
10
(c)
Figure 6: SE micrographs in 52o tilt view of n-type Si
(100) subsequent to exposure to anodic polarization in
EMIF(HF)2.3 for 10 hr; (a) 2V, (b) 5V, (c) 10V.
10
8
(b)
5 µm
5 µm
p-type silicon
6
Figure 3: 8V anodic polarization potentiotiostatic
study of n-Si in EMIF(HF)2.3
(c)
Figure 4: SE micrographs in top view of n-type Si (100)
subsequent to exposure to anodic polarization in
EMIF(HF)2.3 for 10 hr; (a) 2V, (b) 5V, (c) 10V.
4
Time (hr)
(b)
5 µm
(c)
0
2
Discussion: Figure 1 presents potentiodynamic studies of n-Si in EMIF(HF)2.3 in anodic polarization. currents of up to 100 mA/cm2 were
developed. The high currents, generated in EMIF(HF)2.3 RTIL are mainly due to the high conductivity and low viscosity of this particular RTIL.
Potentiostatic curve obtained from polarizing n-type silicon at a potential of 8V in EMIF(HF)2.3 obtained at different polarization periods are
shown in Figure 2. The current increases to a maximum value of 150 mA/cm2 within 15 minutes followed by a gradual decrease, to 20 mA/cm2
after 3 hours. This current is maintained low during the rest of the experiment time. During the experiments, gaseous products are emitted and the
solution color changes, from transparent to brown. Figure 4 presents the formation of pores at the silicon interface. Within 15 minutes, pits are
being formed at the silicon surface, and as time progresses pores are being formed. At a longer time period, the silicon porous structure becomes
less uniform. Figure 3 presents potentiostatic curves obtained from polarizing n-type silicon at different potentials in EMIF(HF)2.3 for ten hours
period. In all the curves there is an increase in the current up to a maximum value followed by a gradual decrease. The maximum current is
increased as the potential applied is increased. Figure 5-7 shows top, tilted and cross section views of n-type silicon after ten hours study at
different potentials. 8V study shows less uniform structure with pores with ~1mm wide and ~10mm wide. 5V study tilt view shows less uniform
bundle structure than the 2V study. The cross section view shows longer pores length at the 2V samples (20 mm) compared to the 5V sample
(5mm), these pores are thinner as the pore is deeper.
Figure 8 presents potentiostatic curve obtained from anodic polarization of n-type silicon in EMIF(HF)2.3 at a potential of 8 V. It shows an
increase in the current value, until a maximum current value of ~60 mA/cm2 is reached after one hour. During the experiments gas is evolved and
the solution color is changed from transparent to brown. The recorded current is low, compared with experiments conducted with n-type silicon.
Figure 9 presents silicon surface morphology subsequent to different experiment time periods. After an hour texturing of the surface is observed.
This texturing is probably developed at a later stage (longer time periods). A cross section of these samples will be studies, as well. The difference
between the morphology developed with n and p-type silicon will be investigated in this research.
A possible mechanism for the formation of the porous structure is provided here and should be verified during this study; the oxidation product or
products of the anion are in-charge of the pore formation. This oxidation leads to a change in the hydrofluoric anion structure. Thus it is
reasonable to assume that some of the anion molecules are dissociated to produce free HF, which is “available” to react with the silicon surface,
leading to the formation of pores. Finding the mechanism for this reaction and the oxidation products which are formed and responsible for this
process is one of the main goals of this research
(a)
(b)
2V
5V
8V
140
2
2
Current (mA/cm )
140
6
160
Current (mA/cm 2)
8
Electrochemical dissolution of n-type silicon in
EMIF(HF)2.3 leading to the formation of porous silicon
under anodic polarization is presented.
Increasing experiments results in porous structure
formation, which becomes less uniform with time.
Increasing the potential from 2V to 5V results in deeper
pores formation, while 8V experiments show wider pores.
Also, cathodic polarization of p-type silicon in
EMIF(HF)2.3 leads to electrochemical dissolution
Finding the mechanism responsible for electrochemical
dissolution of silicon in EMIF(HF)2.3.
Further studies will be conducted in order to determine the
mechanism responsible for these results.
Acknowledgements:
Figure 9 :HRSEM obtained from p-type Si (100)
subsequent to exposure at a potential of 8V in
EMIF(HF)2.3; (a) 1 hr, (b) 5 hr, (c) 8 hr.