The structure and property characteristics of amorphous/nanocrystalline silicon produced by ball milling

Abstract

The structural transformation of polycrystalline Si induced by high energy ball milling has been studied. The structure and property characteristics of the milled powder have been investigated by x-ray diffraction, scanning electron microscopy, high-resolution electron microscopy, differential scanning calorimetry, Raman scattering, and infrared absorption spectroscopy. Two phase amorphous and nanocrystalline Si has been produced by ball milling of polycrystalline elemental Si. The nanocrystalline components contain some defects such as dislocations, twins, and stacking faults which are typical of defects existing in conventional coarse-grained polycrystalline materials. The volume fraction of amorphous Si is about 15% while the average size of nanocrystalline grains is about 8 nm. Amorphous elemental Si without combined oxygen can be obtained by ball milling. The distribution of amorphous Si and the size of nanocrystalline Si crystallites is not homogeneous in the milled powder. The amorphous Si formed is concentrated near the surface of milled particles while the grain size of nanocrystalline Si ranges from 3 to 20 nm. Structurally, the amorphous silicon component prepared by ball milling is similar to that obtained by ion implantation or chemical vapor deposition. The amorphous Si formed exhibits a crystallization temperature of about 660 °C at a heating rate of 40 K/min and crystallization activation energy of about 268 kJ/mol. Two possible amorphization mechanisms, i.e., pressure-induced amorphization and crystallite-refinement-induced amorphization, are proposed for the amorphization of Si induced by ball milling.

High Refractive Index Si/SiOx Based Nanocomposites

“…Line broadening is clearly the biggest difference between bulk silicon and as milled sample ( Figure 3A and 3B respectively). This is in agreement with the thorough investigation of Shen et al [27], on the high energy ball-milling of Si, where nanocrystalline grains of Si ranging from 3 to 20 nr were obtained, based on high resolution TEM and XRD line broadening analysis. [27] A combination of pressure induced amorphization and crystalliterefinement-induced arnorphization was proposed to be responsible for this thermodynamically unfavorable process.…”

“…The broad a-Si Raman line, peaking at 475 cm-1 indicates considerable amorphization as a result of high-energy milling as reported by Shen et al [27] and concurred by the significant line broadening shown in Figure 3. Contrary to the expected, the 518 cm-1 c-Si Raman peak indicates a pressure induced crystallite-refinement, also witnessed by Shen et al [27] This was explained on the basis of elevated temperatures reached during high energy milling causing a small fraction of the crystals to grow to more perfect crystallites which are responsible for the up-shift of c-Si Raman peak to higher frequencies.…”

“…15%) character of the milled Si powder was distributed heterogeneously among the nanocrystalline portions while isolated nanocrystals were surrounded by a layer of amorphous silicon, surprisingly enough not oxidized by the adsorbed oxygen. [27] The XRD profile of the isolated nanosilicon in Figure 3C, although attained with significantly lower signal to noise ratio due to instrumentation shortcomings, resembles that of as milled powder with marginally increased line-broadening. Based on the work of Shen et al [27], and the work presented below, it is currently believed that these nanoparticles consist of nanocrystalline Si interior with an amorphous Si exterior, partially coated with surface oxide which assists formation of colloidal suspensions in polar solvents.…”

“…al. [27] reported the use of high energy milling for nanosizing silicon in order to attain crystal grain sizes ranging from 3 to 20 nm. This has prompted us to employ this technique in order to produce nanosized Si, in large quantities, for the fabrication of high refractive index nanocomposites.…”

“…[27] Electrochemical etching followed by electropolishing have been used to prepare porous silicon films of increased transparency with the index of refraction n calculated to be 2 in the IR. [28] Ultimately, high energy milling was chosen based on its environmental friendliness, low cost, and relative ease of nanoparticle separation.…”