Self-Assembling Formation of Silicon Quantum Dots by Low Pressure Chemical Vapor Deposition
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were slightly etched back by dipping into a 0.1% HF solution to change the surface chemical bonding features. Si nanocrystallites were deposited on as-prepared or HF-treated SiO 2/c-Si by the thermal decomposition of pure monosilane in the temperature range from 500 to 650°C. During the deposition, the gas pressure was maintained at 0.2 Torr. AFM observations were carried out in air using a Au-coated Si 3N4 probe to assess the nanocrystallite size. High resolution TEM images were taken for nanocrystallites deposited near a substrate edge in the cross sectional view and for samples whose substrates were removed locally by etching in a KOH solution. RESULTS AND DISCUSSION The formation of cap-shaped Si nanocrystals was confirmed at deposition temperatures above 525°C by high resolution TEM observations as shown in Fig. 1. Notice that the lattice images for Si( 111) planes were always observed at a tilt by ~30 degrees from the substrate surface normal. This indicates that the growth of Si dots proceeds so that a Si(100) plane becomes parallel to the substrate surface to minimize the surface free energy. The Si dot growth was also evidenced from the change in the X-ray excited Si2p spectrum measured at a photoelectron take-off angle of 5~30'. AFM images taken for Si dots grown on atomically flat Si0 2/c-Si(l 11), whose surface morphology maintains an initial biatomic step structure of Si(1 11), clearly show that Si nucleation occurs randomly irrespective of the atomic scale roughness of the SiO 2 surface as represented in Fig. 2. Indeed, there is no significant difference between the Si dot formation on atomically flat Si0 2/cSi(1 11) and on rough SiO 2/Si(100). Figure 3 shows an Arrhenius plot of the areal Si-dot density on as-grown SiO 2 evaluated by AFM observations. In the early stages of the dot formation, as the deposition temperature rises from 550 to 570'C, the dot density is dramatically increased with an activation energy of -5.4 eV until reaching a value of 2x10 11 cm 2 , at which point it tends to be saturated because of the coalescence growth as confirmed by TEM observations. Considering that the Si-O bond energy is estimated to be 4.85 eV from the atomization energy of Si0 2 [8], the Si-O bond breaking might play a role in the creation of nucleation sites. The size distribution of obtained Si dots evaluated from AFM images can be fitted to a log-normal function [9] as indicated in Fig. 4. It is obvious that higher temperature deposition provides a wider size distribution. To get a clear insight into the growth mechanism, the average dot diameter or dot height was determined at each deposition temperature by fitting a measured size distribution to a log-normal
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Fig. 1. TEM micrographs of a sample after 36 sec deposition at 600'C on as-grown SiO 2 which were taken at a tilt cross sectional view with a low resolution (a) and at a cross section with a high resolution (b).
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Fig. 2 AFM images of Si dots formed at 550°C for 12 sec on atomically-flat as-grown Si0 2/Si(1 11). An open squa
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