For example, the hillock produced in air/vacuum at N = 100 on Si(111) surface is 42%/29% lower than that on Si(100) surface. The hillocks produced at N = 200 show the similar results. It was also noted that the hillock produced in air was a little lower than that in vacuum, which may be to some extent ascribed to the protective effect of surface oxide layer on the Baf-A1 molecular weight silicon surface . Since less silicon oxide layer was observed on the hillock surface VX-680 when scratched in vacuum than
that in air, taller hillocks would be created in vacuum . In summary, because of the anisotropic properties of silicon surfaces, the friction-induced hillocks on Si(100) surface were the highest, but those on Si(111) surface were the lowest under the same conditions. The reasons responsible to the difference will be further discussed in the next section. Figure 4 Comparison of the (a) height ( h ) and (b) volume ( V ) of the friction-induced hillocks. The hillocks were produced at F n = 50 μN and N = 100 in air and in vacuum, respectively. Discussions Effect of the mechanical property on the hillock formation The transmission electron microscope observation indicated that the friction-induced hillock on Si(100) surface contained a thin superficial oxidation layer and a thick disturbed (amorphous and deformed) layer in the subsurface [17, 18]. It was suggested
that the mechanical interaction through amorphization was the key contributor to hillock formation Dichloromethane dehalogenase on Si(100) surface. Although the silicon wafers with various selleck screening library crystal planes present different elastic modulus, all these wafers consist of Si-I phase (diamond-like structure) regardless of crystallographic orientations. During the sliding process, the transformation of Si-I to amorphous structure may occur on three silicon crystal planes, which will further induce the formation of hillock on these silicon surfaces. However, under the same loading conditions, the height of hillock on various silicon crystal planes
was different as shown in Figures 2, 3 and 4. The results suggested that the crystal plane orientation of silicon had a strong impact on the friction-induced nanofabrication on the silicon surface. Due to the anisotropic mechanical properties of monocrystalline, the tip-sample contact may be different on three silicon crystal planes during scratching. When the scratch test was performed at F n = 50 μN, the maximum shear stress on the contact area was estimated as 2.6 GPa on Si(100), 3.1 GPa on Si(110), and 3.3 GPa on Si(111) with the Hertzian contact model, respectively . Since all the shear stress was below the yield stress of silicon (approximately 7 GPa), the deformation during the scratch process on the three silicon crystal planes was assumable to be elastic according to the Tresca yield criterion . However, the repeated scanning under low load may lead to the deformation of silicon matrix, i.e.