Laser-Ablated Particles From Porous Silicon
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Mat. Res. Soc. Symp. Proc. Vol. 354 ©1995 Materials Research Society
0 CurrentPt
Teflon cell
.~
(ncult)voltage
2
'..regulator F F
oj
..HF Et .OH
i
~Si Si 0
0
Fig. 1 Anodization of c-Si using single cell. P-type Si(1 11) wafer was anodized at current density of-10 mAcm-2 for 10 minutes.
200
400 Binding
(gold-plated)
600
800
1000
energy (eV)
Fig. 2 X-ray photoelectron spectrum of PSi. Peaks from impurities (0, C, F) adsorbed on pore surface are observed.
*C in order to get Ohmic contact at the interface. The electrolyte was a mixture of an equal volume of hydrofluoric acid (HF) and ethanol. A reference electrode containing potassium chloride (KC1) aqueous solution separated by a ceramic salt bridge was employed to determine the potential of the system. The anodization was carried out at a typical current density of 10 mA.cm- 2 for 10 minutes. The anodized area of the wafer was a circle of 18 mm in diameter and was immediately set in the vacuum after dipping in deionized water. The conditions mentioned above were selected so as to obtain PSi with the smallest size of micropores (-nm) according to the literature3 . The anodizing current beyond 30 mA.cm"2 resulted in peeling the PSi off the wafer. Though the anodized area looked brownish gray in color, no obvious structure was observed in the scanning electron microscope analysis. In Fig.2 the X-ray photoelectron spectrum of the PSi is shown. As usually expected for PSi, impurity peaks by oxygen, carbon, and fluorine were observed. The both c-Si and PSi were ablated with ultraviolet lasers in a ultrahigh vacuum (2) ions had no reproducible intensity. When either the bias voltage of the sample or the laser fluence was changed, the peak energy estimated from the elapsed time shifted toward the higher energy (i.e., they reached the detector earlier than expected). In Fig.8, the sample bias dependence of the excess translational energy (Eex) of the Si+ ions is plotted for various laser fluences. The larger fluence resulted in 2 the greater Eex for a constant sample bias. For the fluence of 8.2 Jcm- which was just above 2 the threshold for the ablation (6 - 7 Jcm- ), no significant Eex was observed under the resolution of the system. The peaks for Si2+ showed the similar behavior. 20 5 0 C C C
C C
"*
0
-5
S-20
-10 -15
S-40
-20 -60
-25 0
50 100 150 Elapsed time (4s)
200
Fig. 5 Ion TOF spectrum for grounded c-Si. Derived temperature of plasma estimated from Si+ peak is 2x10 5 K (30 eV).
100 25 50 75 Elapsed time (Its) Fig. 6 Ion TOF spectrum for c-Si biased to +500 V. Spectrum in Fig.5 was a convolution of signals for Si+ and Si 2 +. d) •
0
V/
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i
.
i
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i
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n- : 8.2 Jcn-2 >1
50
16. Jc -2 2
30
16 6Jcm
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20
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0
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.
30 Z
100
V
2
0
10 20 30 40 50 Elapsed time (gs)
Fig. 7 Laser fluence dependence of relative signal intensity for Si+ and Si2 +. C-Si was biased to +500 V. Increase of Si2 + ratio and peak shifts are observed for higher fluence.
594
.9
2.
Jcm '0
2
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250 500 0\750 10001250 Sampl
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