Materials Science and Engineering C 19 Ž2002. 215–218 www.elsevier.comrlocatermsec
Nanostructured films composed of silicon nanocrystals G. Ledoux a,) , D. Amans a,1, J. Gong a , F. Huisken a , F. Cichos b, J. Martin b a
b
Max-Planck-Institut fur Bunsenstr. 10, D-37073 Gottingen, Germany ¨ Stromungsforschung, ¨ ¨ Laboratory of Optical Spectroscopy and Molecular Physics, Institute of Physics, TU Chemnitz, D-09107 Chemnitz, Germany
Abstract Crystalline silicon nanoparticles were synthesized by laser pyrolysis of silane in a flow reactor. Afterwards the nanocrystals were extracted through a nozzle and shaped as a well-collimated molecular beam propagating under high vacuum. The nanocrystals produced in this way show strong photoluminescence ŽPL. that can be clearly ascribed to quantum confinement effects. In order to produce structured films being composed of these nanocrystals, we used the directivity of the beam and interposed a mask that defines the pattern. The crystals were then deposited on a substrate placed directly behind the mask. To check the resolution that can be obtained by these means, we have taken holey carbon films normally used for transmission electron microscopy. We have been able to create structures with lateral dimensions down to 30 nm. In another experiment, we have produced regular patterns of micrometer-sized spots being composed of Si nanocrystals. The photoluminescence behaviour of these structured films has been studied by laser scanning confocal microscopy. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Silicon nanocrystals; Structuration; Atomic force microscopy; Laser scanning confocal microscopy
1. Introduction Since the discovery of the strong photoluminescence of porous silicon by Canham in 1990 w1x, the study of silicon nanostructures has become an active field of research. This is because silicon is the number one material of microelectronics, and, until the discovery of the photoluminescence of Si nanocrystals, it was not possible to design efficient light sources from silicon. Recently, it was also shown that silicon nanocrystals can act as an amplifier for a probe laser beam if, at the same time, they are pumped by another laser w2x. So, the idea of designing a laser out of this material is not unrealistic anymore. Another step towards all-silicon optoelectronic devices would be the ability to produce micro- and nanometer-sized structures made from these nanocrystals. It is the purpose of this paper to present first results obtained in this direction.
The particles were extracted by a conical nozzle into a high vacuum chamber and transferred into a well-collimated directed molecular beam of non-interacting species w5x Žsee Fig. 1.. Exploiting the correlation between velocity and mass, the particles could be selected according to their size by means of a fast-spinning molecular beam chopper with two 1-mm slits w6,7x. The particles were deposited at low energy onto various substrates mounted behind the chopper. Finally, micro- and nanostructured thin films
2. Experimental Silicon nanoparticles were produced via pulsed CO 2laser-induced dissociation of silane in a flow reactor w3,4x. ) Corresponding author. Tel.: q49-551-5176-578; fax: q49-551-5176607. E-mail address:
[email protected] ŽG. Ledoux.. 1 Permanent address: LEOM, Ecole Centrale de Lyon, F-69131 Ecully, France.
Fig. 1. Schematic of the setup used for the deposition of structured films. A CO 2 laser is used to decompose silane. Nanocrystals are formed and extracted through a nozzle Ža. and a skimmer Žb. to form a molecular beam of freely propagating particles. A mask Žc. is introduced into the beam so that a structured film can be deposited on the substrate Žd..
0928-4931r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 0 1 . 0 0 4 6 6 - 0
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report on two new achievements: the realization of structured thin films in the micro- and nanometer range and the implementation of the size selection technique to avoid the contamination of the nanostructures by larger particles.
3. Results In the first experiment of the present study, the mask was a TEM copper covered by a holey carbon foil as is normally used for electron microscopy. The grid area was approximately 2.35 mm in diameter and the separation between the 30-mm-thick copper wires was 120 mm. The TEM grid was fixed onto a mica substrate, which in turn was mounted on a substrate holder. When this holder was moved into the molecular beam, silicon nanoparticles with an average diameter of 3.85 nm and a full width at half maximum ŽFWHM. of 0.7 nm were allowed to fly through
Fig. 2. Deposition of silicon nanoparticles through the holes of a TEM carbon foil. The upper panel Ža. shows an optical microscope image of the photoluminescent deposit when exposed to the irradiation of UV lamp Ž lexc s 254 nm.. The diameter of the circular deposit is 2.35 mm. The lower panel Žb. shows an AFM image of the deposit on a mica substrate Ž6=6 mm2 .. White areas indicate islands of Si nanoparticles producing the PL that is seen in panel Ža..
were obtained by placing appropriate masks in front of the substrate. When the substrate holder was moved out of the beam, the size distribution of the nanoparticles could be determined in situ with a time-of-flight mass spectrometer ŽTOFMS.. For this purpose, the Si nanoparticles were ionized by the radiation of an ArF excimer laser w6x. Recently, we have demonstrated the deposition of light-emitting silicon nanoparticles through a copper foil with millimeter-sized structures w8,9x. The structured films exhibited a bright photoluminescence ŽPL. in the visible when illuminated by UV light at 254 nm. These experiments were carried out without size selection. Here, we
Fig. 3. SEM micrograph of the R1.2r1.3 Quantifoil used as a mask in the deposition experiment Ža.. The bottom panel Žb. shows the AFM image of the deposited structure of silicon nanoparticles Ž12=12 mm2 ..
G. Ledoux et al.r Materials Science and Engineering C 19 (2002) 215–218
the holes of the carbon foil and to be deposited on the substrate. The other particles hitting the foil were prevented from reaching the substrate. As a result, the grid and the structure of the holey carbon foil are imaged on the substrate. The coarse pattern produced by the deposited silicon nanoparticles can be easily visualized by observing their photoluminescence after excitation with the light from a laboratory UV lamp Ž l s 254 nm.. A photo of this luminescent structure is shown in Fig. 2a. This photograph was slightly overexposed, and therefore, structural details due to the holey carbon film were lost. One can only recognize the image of the copper grid as a structure in the luminescent film of Si nanoparticles. However, at lower exposition times, one can clearly see darker regions and bright spots corresponding to the largest holes in the carbon foil covering the copper grid. Fig. 2b shows a 6 = 6 mm2 atomic force microscopy ŽAFM. image of Si nanoparticles deposited on mica. The
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white islands are thin films Žup to 30 nm thick. being composed of Si nanoparticles. This image reflects the holey structure of the carbon foil, which consists of holes of many different sizes. The structures deposited have sizes varying from 50 nm to 1 mm. However, it should be noted that the resulting AFM image is the convolution of the true structure and the shape of the AFM tip. Therefore, the deposited structures are even smaller than shown in the figure. Thus, it follows that, with this technique, it is possible to produce nanostructures of the order of 50 nm and below. In another experiment, we have employed a regularly structured carbon foil as supplied by Quantifoil Micro Tools, Jena Žmodel R1.2r1.3.. It consists of a regular pattern of 1.2 mm diameter holes 2.5 mm apart Žseparation between centers.. The electron scanning micrograph depicted in Fig. 3a shows the holey carbon foil and the copper support. Using this foil as a mask, we succeeded in
Fig. 4. Laser scanning confocal microscopy images of the deposit of Si nanoparticles made through the R1.2r1.3 Quantifoil TEM grid. The three pictures have been obtained at different magnifications Ža: 1 = 1 mm2 , b: 125 = 125 mm2 , c: 12 = 12 mm2 .. The photon collection times per pixel were 0.6 ms Ža and c. and 0.15 ms Žb., respectively.
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producing a regular pattern of dots deposited on a mica substrate. The AFM image of this sample is reproduced in Fig. 3b. The circular dots are made from approximately two monolayers of silicon nanoparticles with an average diameter of 3.25 nm. This sample was further characterized using a laser scanning confocal microscope ŽZeiss LSM 510.. As exciting radiation, the 458-nm line of an argon ion laser was coupled to the LSM 510. The emission was collected by either a Zeiss Epiplan-Neofluar 10 = r0.3 NA ŽFig. 4a. or a Zeiss Epiplan-Neofluar 100 = r0.9 NA ŽFig. 4b and c. microscope objective. The straylight of the excitation laser was suppressed by using a long-pass filter with a cut-on wavelength of 560 nm. All measurements were performed under ambient conditions. Fig. 4 shows three images of the luminescent deposit obtained by this method. Fig. 4a was taken at coarse resolution, and we can recognize the AshadowB from the copper grid. Brighter spots correspond to the damaged areas in the carbon foil through which more particles have been deposited. There are also some darker spots corresponding to places where zooms have been made. The lower PL yield is the result of a bleaching process which occurs when the intensity of the exciting light is above a certain level. When zooming on the deposit ŽFig. 4b., the regular structure in the carbon foil becomes apparent. Once again, in the bottom left corner, one can see a place where the carbon foil was broken, giving rise to a stronger signal without any structure. Zooming even further ŽFig. 4c., we recognize very well the details of the deposited structure also apparent in the AFM image ŽFig. 3b.. However, this time the image is produced by the luminescent Si nanoparticles. It appears that the resolution of the confocal microscope is of the order of 300 nm.
4. Conclusion The present results show that the low-energy cluster beam deposition through appropriate mask is an excellent
means to produce very fine structures composed of silicon nanocrystals while keeping their photoluminescence properties. An interesting application of this method would be to build a photonic crystal, i.e. a regular structure at the scale of a fraction of the emission wavelength of the nanocrystals. This would allow to suppress the emission in the plane of the deposit while enhancing the emission perpendicular to the deposit. Combining this structure with two Bragg mirrors w10x should permit to build a very efficient silicon-based device to be pumped with photons. Theoretical and experimental work in this direction is in progress.
Acknowledgements We gratefully acknowledge the support of the Deutsche Forschungsgemeinschaft in the frame of its Schwerpunktprogramm Specific Phenomena in Silicon Chemistry. D.A. also thanks La Region Rhone-Alpes for financial support.
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