Applied Surface Science 197±198 (2002) 674±678

Deposition dynamics of droplet-free Si nanoparticles
in Ar gas using laser ablation
D. Takeuchia,*, T. Mizutaa, T. Makimuraa, S. Yoshidaa, M. Fujitaa,
K. Hatab, H. Shigekawaa, K. MurakamiaaInstitute of Applied Physics, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8573, Japanb
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USAAbstract
Droplet-free deposition of Si nanoparticle ®lms has been studied applying time-resolved imaging of Si nanoparticles formedby laser ablation of Si targets in Ar gas. We found that Si nanoparticles can be deposited not only on substrates facing to thetargets but also on substrates placed beside the target. We further con®rmed using a scanning tunneling microscope (STM), Si
nanoparticles with sizes of 5±8 nm are deposited on substrates placed beside the target and using a scanning electron microscope(SEM) on the substrates, no droplets are observed.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Pulsed laser deposition; Droplet-free; Si nanoparticle; Time-resolved imaging

1. Introduction
Since nanostructured Si systems can emit lightef®ciently, it is possibly applied to novel opto-electronic materials and to integrated opto-devices onto Sidevices. Laser ablation can be used to fabricate such Si
nanostructures because surface modi®cation of thenanoparticles and impurity doping can be attainedby using the pulsed feature of laser ablation. On theother hand, deposition of droplets is a common problem of laser ablation. Therefore, extensive investigations have been carried out to deposit droplet-free®lms and demonstrated that the number of droplets
can be decreased by: (a) the eclipse method thatutilizes a spatial ®lter between targets and substrates[1,2]; (b) the off-axis method [3]; (c) a method using a*

Corresponding author. Tel.: 81-298-53-5574;
fax: 81-298-53-5574.
E-mail address: take@ims.tsukuba.ac.jp (D. Takeuchi).

velocity ®lter [4]; (d) a method choosing carefully
wavelength and ¯uence of ablation laser, and atmosphere gas [5,6] and (e) the porosity of a target [7]. Inspite of these investigations, complete elimination ofdroplets has not been achieved. Compared to Si ablation in He gas and Ne gas whose atoms are lighter thanSi atom, Si atoms and ions produced by laser ablationin Ar gas are con®ned in a limited region since Ar
atoms are heavier than Si atoms. Consequently, Si
nanoparticles are expected to deposit on substrates
placed beside the targets without droplets. So we haveinvestigated the growth dynamics of Si nanoparticles inAr gas utilizing a second laser [8]. The second laser wasirradiated to probe grown Si nanoparticles in inert gas.
In this work, we applied the second laser irradiation
method in order to observe time-resolved photoluminescence (PL) from Si nanoparticles formed by laser
ablation of Si targets in Ar gas. Based on the formation
dynamics of Si nanoparticles, we have tried to obtain
droplet-free ®lms of Si nanoparticles.

0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 4 3 9 - 7

D. Takeuchi et al. / Applied Surface Science 197±198 (2002) 674±678

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Fig. 1. A time-resolved 2-D spatial distribution of PL from Si nanoparticles at a delay time of 2.0 ms.

2. Experimental
Fig. 1 shows a typical two-dimentional (2-D) distribution of time-resolved PL from Si nanoparticles
and illustrate the experimental con®guration used. Forobserving time-resolved distributions of PL from Si
nanoparticles, p-type CZ±Si (8±12 O cm) targets (T)were ablated by a Q-switched Nd:YAG laser light(532 nm, 5 J/cm2, 7 ns) (L1) in 5 Torr Ar gas ®lled
in a vacuum chamber. Si nanoparticles were excited byirradiation with 355 nm pulsed light from another
Nd:YAG laser (L2) at 0.19 J/cm2. Resulting PL distributions were observed using an gated ICCD camera(Princeton Instruments, ITE/CCD-576-G/RB-E). As
shown in Fig. 1, 2-D spatial distributions of PL
were observed in the track of the sheeted second laserbeam.

is observed compared with the PL intensity without
previous ablations shown by the curve (b). This suggests that Si nanoparticles deposit backward on thetarget in 5 Torr Ar gas and that the deposited nanoparticles are desorbed by the following ablation
through processes such as thermal shock wave. Droplets are generally emitted radially from the ablated
spot and ¯y straight. Therefore, in Ar gas, droplet-free®lms of Si nanoparticles are expected to be depositedon substrates not facing to the target but coplanar withthe target plane.

3. Results and discussion
Fig. 2 shows the total intensity of the PL from Si
nanoparticles as functions of a delay time of thesecond laser from the initial ablation laser. The curve(a) was obtained from the measurements after thetarget has been previously ablated for several hundreds of shots. More intense PL from Si nanoparticles

Fig. 2. Total PL intensity of Si nanoparticles formed by laser
ablation in Ar gas for (a) a previously ablated Si target and (b) afresh target.

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D. Takeuchi et al. / Applied Surface Science 197±198 (2002) 674±678

Fig. 3. A typical STM image of Si nanoparticles deposited on an organic single molecule ®lm.

In order to con®rm the backward deposition, selfassembled monolayer (SAM) ®lms formed on Au®lms were placed 5 mm apart from the ablated spoton Si targets. Fig. 3 shows a typical scanning tunnelingmicroscope (STM) image of Si nanoparticles on theSAM ®lm after laser ablation of a Si target in 5 Torr Ar
gas [9]. We observed Si nanoparticles with diameter ina range of 5±8 nm, which are seen as bright spots onAu terrace.
Scanning electron microscope (SEM) observation
was performed in order to investigate the depositionof droplets on substrate (S) placed at different positions as illustrated in Fig. 4. When Si nanoparticles
were deposited at a conventional position (a) facing tothe target, droplets are deposited on the Si substrates
as shown in Fig. 4a. When Si nanoparticles weredeposited on substrates placed just beside a target(b), droplets are observed to have several trails in

directions emanating from the ablated spot. The trails
are tracks of ejected droplets that have shaved the®lms of Si nanoparticles deposited by previous ablations. In order to prevent the droplets, a Si wall with aheight of 1 mm was placed between a substrate and atarget as illustrated in Fig. 4c. We found that deposition of droplets is completely suppressed by the wall.
Since the wall is low enough compared with thedimension of ablation plume, Si nanoparticles formedin the gas phase can ¯y back to the substrate anddeposit on it. When a ®lm of Si nanoparticles wasdeposited on a substrate placed at a position (d) whichis 2 mm lower than the target, web-like ®lms of Si
nanoparticles were observed. This may be due to thelower annealing temperature of deposited ®lms of Si
nanoparticles exposed to ablation plasma because thesubstrate at position (d) is placed further from theplasma.

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677

Fig. 4. SEM images of droplets on ®lms of Si nanoparticles deposited at positions (a) opposite to a target, (b) beside a target, (c) beside atarget with a wall and (d) below a target.

4. Conclusion
In conclusion, we have investigated droplet-freedeposition of Si nanoparticles based on time-resolved

imaging of photoluminescence from Si nanoparticles.
We found that droplet-free ®lms of Si nanoparticles
can be backward deposited on substrates placed beside
the Si targets in Ar gas.

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D. Takeuchi et al. / Applied Surface Science 197±198 (2002) 674±678

Acknowledgements
This work is partly supported by the Priority Area
from the Ministry of Education, Culture, Sports,
Science and Technology.
References[1] M. Tachiki, M. Noda, K. Yamada, T. Kobayashi, J. Appl. Phys.
83 (1998) 5351.
[2] Z. Trajanovic, S. Choopun, R.P. Sharma, T. Venkatesan, Appl.
Phys. Lett. 70 (1997) 3461.

[3] S. Acquaviva, M.L. De Giorgi, L. Elia, M. FernaÂndez, G.
Leggieri, A. Luches, M. Martino, A. Zocco, Appl. Surf. Sci.
168 (2000) 244.
[4] E.V. Pechen, A.V. Varlashkin, S.I. Krasnosvobodtsev, B.
Brunner, K.F. Renk, Appl. Phys. Lett. 66 (1995) 2292.
[5] Y. Yonezawa, T. Minamikawa, K. Matsuda, K. Takezawa, A.
Morimoto, T. Shimizu, Appl. Surf. Sci. 127±129 (1998) 639.
[6] M. Ozegowski, K. Meteva, S. Metev, G. Sepold, Appl. Surf.
Sci. 138/139 (1999) 68.
[7] E. Agostinelli, J. Bohandy, W.J. Green, B.F. Kim, F.J. Adrian,K. Moorjani, J. Mater. Res. 5 (1990) 619.
[8] T. Makimura, T. Mizuta, K. Murakami, Appl. Phys. Lett. 76(2000) 1401.
[9] K. Hata, M. Fujita, S. Yoshida, T. Makimura, K. Murakami, H.
Shigekawa, Appl. Phys. Lett. 79 (2001) 692.