In-situ electrical characterization of Si during nanoindentation
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In-situ electrical characterization of Si during nanoindentation J. E. Bradby,∗ J. S. Williams,∗ M. V. Swain,† ∗ Department of Electronic Materials Engineering, RSPhysSE, The Australian National University, Canberra, ACT 0200, Australia E-mail address: [email protected] † Biomaterials Science Research Unit, Department of Mechanical and Mechatronics Engineering and Faculty of Dentistry, University of Sydney, Eveleigh, NSW 1430, Australia ABSTRACT A novel in-situ electrical characterization technique is used to study the deformation behavior of silicon during nanoindentation. The method involved the formation of a Schottky contact on high resistivity epitaxial Si that is converted to an ohmic contact when Si transforms from the familiar semiconducting Si-I to a metallic Si-II phase. This behavior leads to substantial changes in the current measured across the sample. The Si conductivity used (epitaxial 5 Ωcm on 6 x 10−3 Ωcm) provides particular sensitivity to the onset of a phase transformation directly under the indenter. On unloading, a reverse transformation from ohmic to Schottky contact was observed. This configuration was used to correlate the observed changes in the electronic properties with features in nanoindentation load-unload curves. The onset of the transformation to the metallic phase was observed to occur during loading using both spherical and Berkovich indenters. Interestingly, the onset of the transformation was detected before the observed discontinuity on loading (the so-called ‘pop-in’ event). This observation is consistent with our previous suggestion that the pop-in event is a result of the onset of flow of the ductile metallic phase beyond the constraint of the indenter. These changes were consistently observed after repeated indentation on the same position in the sample, indicating that small volumes of Si-III and Si-XII crystalline phases as well as amorphous Si (a-Si), which form on unloading, can transform back to the metallic Si-II phase on reloading. A strong decrease in the measured electrical current across the sample occurred as soon as the unloading cycle commenced and prior to the observation of the pop-out event. Overall, these in-situ measurements have provided much insight into pressure-induced transformation in Si under nanoindentation. INTRODUCTION The complex mechanisms of mechanical deformation in crystalline silicon have been a topic of extensive research over the past decade [1–12]. High pressure diamond-anvil studies have shown that Si undergoes a series of phase transformations under isostatic loading [1, 2]. At a pressure of 11.3–12.5 GPa, diamond-cubic Si-I transforms to the metallic β − Sn phase, Si-II [1]. As this phase is not stable at ambient pressure further transformations occur on pressure release [2]. Nanoindentation induces such phase transformations in Si [3–6] with load–unload curves showing discontinuities on both the loading and unloading [5–9]. A discontinuity on loading (a ‘pop-in’ event) has only been conclusively observed for indents made
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