Amorphization and Conductivity of Silicon and Germanium Induced by Indentation

Abstract

We report the observation, by transmission electron microscopy, that single-crystal silicon and germanium are converted to an amorphous state at room temperature directly under both Vickers and Knoop indentations. The effect is seen for crystal orientations of [001], [Oil], and [111], and with applied loads between 0.1 and 0.5 N. We also observe that the materials become electrically conducting under load and that the process is reversible on subsequent unloading and reloading. Furthermore, the transformed phase is found to make Ohmic contact to the surrounding, untransformed, semiconductor.PACS numbers: 64.70.Dv, 64.70.Kb, 72.60.+g It has long been recognized that extremely high (hydrostatic and deviatoric) stresses are generated when a sharp indenter is loaded onto a flat surface of a solid. The exact details of the stress fields depend on the shape of the indenter and on the constitutive equations describing flow in the particular material, a subject that has now become the province of the theories of the hardness of materials and its measurement by indentation methods.' The existence of high hydrostatic stress has led investigators to suggest that the indentation experiment could be used to study high-pressure phase transformations, although it is recognized that the large deviatoric stresses make calculations difficult. Indeed Eremenko and Nikitenko, 2 and more recently Pirouz, Chaim, and Samuels, 3 report the formation of the hexagonal polymorph (Si IV) when silicon is indented at temperatures of 450-700 °C. Similarly, there is the suggestion by Gerk and Tabor 4 that silicon, germanium, and diamond may undergo a semiconductor-to-metal transition, as the pressures under an indenter are similar to those predicted for such a transition. We report here our findings of a rather different phase transformation in both silicon and germanium, the transition, through an electrically conducting state under load, to a metastable amorphous form after unloading, directly under an indentation. Such a crystailine-to-amorphous transition has not been observed in conventional high-pressure experiments.As part of our studies of cracks in brittle solids we have prepared samples containing arrays of small indentations together with their attendant cracks for transmission electron microscopy. Single crystals, mechanically ground and polished to a thickness of -100 /zm, were patterned with arrays of regularly spaced indentations with typically 100 indentations in a 3-mm-diam disk. The indentation impressions, produced by pyramidal diamond indenters having either the Vickers (148° included edge angle) or Knoop (asymmetric pyramid with included edge angles of 130° and 172.5°) configuration, were formed with loads in the range 0.1 and 0.5 N. In all cases the constant loading and unloading rates were 16.67 Ns" 1 . With the top, indented, surface protected by a glass cover slip, the samples were then ion thinned from the back until suitably thin for viewing in the transmission electron microscope. Figure 1 is a transmission electro...

A review on ductile mode cutting of brittle materials

“…(a) DMC by removing a ductile metallized layer resulted from the large contact pressure in cutting region; (b) BMC by material fracture leaving subsurface damages, of which the subsurface damage is as deep as 5-10 µm due to crack propagations in machining of silicon indentation process of brittle materials. The measurement results revealed a substantial increase in the conductivity of the material below the indenter that can be plastically deformed, which supports the theory of transition to a metallic state [25,26].…”

The martensitic transformation in silicon—I experimental observations

“…It consists of a heavy tangle of dislocations the density of which is so high that it is not possible to resolve individual dislocations. Clarke et al [ 14] have recently shown that room-temperature indentation of silicon results in a crystalline-to-amorphous transformation directly underneath the indenter. However, at the temperatures under investigation here, i.e.…”

“…the material simply tears apart to accommodate the indenter. An alternative accommodation mode is the decrease in the atomic volume 8 of the material underneath the indenter through high pressure phase transformations [25] and/or crystalline-to-amorphous transitions [ 14].…”

In situ TEM study of deformation-induced crystalline-to-amorphous transition in silicon

“…7,9,10 Stressinduced CAT in Si has in fact been widely observed under various mechanical loading conditions, such as indentation, 6,[10][11][12][13][14][15] ball milling, 16,17 scratching 2,7 and bending. 18,19 Clarke et al 6 firstly reported straining-induced amorphous Si (a-Si) through indentation experiments. On the basis of the sharp drop in the electric resistance measured during the loading process, they speculated that a possible path for the CAT is the transition from an intermediated highpressure crystalline phase (β-tin) to a-Si upon rapid unloading.…”