Fracture Behavior of Granular Polycrystalline Silicon Using Micro-scale and Macro-scale Indentation Techniques

Abstract

Three batches of polycrystalline silicon (polysilicon) produced with the fluidized bed reactor method were used to study the fracture behavior using micro-scale and macro-scale indentation techniques. The fracture toughness was determined to be 0.60, 0.74, and 0.86 MPa m 0.5 using Vickers microhardness tests for three samples with a high, medium, and low hydrogen content, respectively. In the same order of the samples, Rockwell hardness tests showed that they failed at applied loads of 45, 55, and 60 kg, respectively. These results show that the fracture behaviors of both techniques are in agreement; the highest toughness materials can withstand higher loads prior to fracture in the Rockwell hardness test. The microstructure of the granular product has been documented and the only significant difference between the samples is the solute hydrogen content. Therefore, a Rockwell hardness instrument can be used as a rapid way to assess the relative fracture toughness of the as-grown polysilicon for solar applications, and the technique is insensitive to small amounts of porosity in these materials.

New pulverization parameter derived from indentation and dynamic compression of brittle microspheres

“…The average crack size vs. applied load was plotted as shown in Figure 2, where equation 3 was used to fit the data for each material to determine T. The deviations at low loads are likely due to the difficult evaluation of the crack sizes at small loads, and the inhomogeneity of the product. One of the materials, polycrystalline silicon, does exhibit approximately 4% porosity, however prior studies have shown that the pores present in this material did not show any significant effect on the crack morphology nor the measured toughness [19]. The measured toughnesses of all the materials are listed in Table 2.…”

“…They identified a dimensionless attrition propensity parameter that is related to impact conditions (impact velocity) and material properties (particle density, particles size, hardness, and fracture toughness) [17,18]. Zbib and Bahr recently identified a similar dimensionless attrition propensity parameter for spherical granular polycrystalline silicon which is related to the shape (and therefore stresses at a fixed loading condition), hardness, and elastic modulus of the silicon [19], where lower toughness or larger diameter particles exhibited a higher attrition parameter. However, these attrition parameters only discuss the likelihood of particle size reduction through fracture and do not address the particle failure morphology explicitly.…”

Fracture strength of micro- and nano-scale silicon components

“…However, Zbib and Bahr demonstrated that the toughness of LPCVD-deposited granular polysilicon can be reduced substantially, down to a claimed 0.6 MPa m 1/2 , by incorporation of hydrogen (H) in the Si through deposition in a H-rich atmosphere. 228 A simple explanation in this case is that the incorporated H disrupted the Si-Si bonding by forming terminal, non-bonding, Si-H complexes such that the Si-Si bonds/area (n A above) was decreased. Most studies of toughness of polysilicon focus on the role of the polycrystalline grain structure in governing toughness, although, unlike engineered ceramics such as yttria-stabilized zirconia or polycrystalline alumina, 229,230 there are no reports for polysilicon of significant microstructural modifications that improve the toughness substantially, and the toughness value is typically in the vicinity of 1 MPa m 1/2 .…”