Single-crystalline Tungsten Nanoparticles Produced by Thermal Decomposition of Tungsten Hexacarbonyl
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Jan-Olle Malm Inorganic Chemistry 2, Lund University, Box 124, S-221 00 Lund, Sweden (Received 18 October 1999; accepted 12 April 2000)
Nanometer-sized particles of W are of interest in semiconductor device research, where such particles may store electrons inside heteroepitaxially defined structures. In this paper, we present results concerning W particles produced by thermal decomposition of tungsten hexacarbonyl. By the described method, it was possible to produce size-selected, single-crystalline W particles in the size range between 15 and 60 nm. The sintering behavior of the particles was studied between ambient temperatures and 1900 °C. The particle morphology and structure were examined with high-resolution transmission electron microscopy and electron diffraction techniques. Particles sintered at the highest temperatures typically were single crystals, with well-developed facets. Some problems concerning a yield reducing charging mechanism are discussed.
I. INTRODUCTION
Nanometer-sized metal particles are of interest in semiconductor research (e.g., for creating buried electron-trapping layers in order to realize memory devices, or for use as building blocks in single electron devices). Lithographically defined W particles have been incorporated in GaAs structures, where the W particles trapped the free electrons, rendering the material semiinsulating.1 The main advantage of W over other metals is its great resistance to heat and many acids, making the structures stable during subsequent processing. It is difficult to produce size-selected particles of W (and other refractory metals) with conventional evaporation/ condensation methods, due to the high temperatures that would be needed for evaporation. Such high temperatures generally require a vacuum environment, because most gases would decompose and all gases conduct heat. Vacuum is also needed for electron beam evaporation. A carrier gas is necessary for size selection, and therefore a vacuum environment is not possible. Laser ablation of W in a gaseous environment has been attempted to overcome this limitation.2 We have chosen instead to work with tungsten hexacarbonyl, W(CO)6, which thermally decomposes to W and CO starting at 200 °C3, through the reaction W(CO)6 → W + 6CO
a)
.
(1)
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J. Mater. Res., Vol. 15, No. 7, Jul 2000 Downloaded: 11 Oct 2015
W(CO)6 has been widely used for chemical vapor deposition (CVD) of thin films of W. Reaction (1), being thermally activated, is driven further to completion at higher temperatures; consequently, W films deposited at low temperatures contain rather high amounts of C and O due to incomplete CO dissociation.4 There has been concern that the CO molecules might react with newly formed films at temperatures above 700 °C.3 In general, thermal cracking of the W(CO)6 is employed,4–6 but photolysis7,8 has also been used, as well as electric field9 and electron-beam-assisted decomposition.10 Nanome
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