Annealing kinetics of nanoindentation-induced polycrystalline high pressure phases in crystalline silicon

Transformation kinetics of nanoindented zones in silicon containing high pressure crystalline phases (Si III and Si XII) during annealing (100 °C<T<450 °C) have been studied using Raman microspectroscopy and cross-sectional transmission electron microscopy. Signature peaks associated with Si III/XII in the Raman spectra were monitored to track the annealing of these phases to polycrystalline Si I as a function of annealing time and temperature. An overall activation energy for this transformation was found to be 0.67 eV. During annealing, Si XII disappeared faster than Si III, suggesting either that Si XII first converts to Si III or that Si XII transforms to polycrystalline Si I faster than Si III

Giant pop-ins and amorphization in germanium during indentation

Sudden excursions of unusually large magnitude (>1 μm), “giant pop-ins,” have been observed in the force-displacement curve for high load indentation of crystalline germanium(Ge). A range of techniques including Raman microspectroscopy, focused ion-beam cross sectioning, and transmission electron microscopy, are applied to study this phenomenon. Amorphous material is observed in residual indents following the giant pop-in. The giant pop-in is shown to be a material removal event, triggered by the development of shallow lateral cracks adjacent to the indent. Enhanced depth recovery, or “elbowing,” observed in the force-displacement curve following the giant pop-in is explained in terms of a compliant response of plates of material around the indent detached by lateral cracking. The possible causes of amorphization are discussed, and the implications in light of earlier indentation studies of Ge are considered

Temperature dependent deformation mechanisms in pure amorphous silicon

High temperature nanoindentation has been performed on pure ion-implanted amorphous silicon (unrelaxed a-Si) and structurally relaxed a-Si to investigate the temperature dependence of mechanical deformation, including pressure-induced phase transformations. Along with the indentation load-depth curves, ex situ measurements such as Raman micro-spectroscopy and cross-sectional transmission electron microscopy analysis on the residual indents reveal the mode of deformation under the indenter. While unrelaxed a-Si deforms entirely via plastic flow up to 200°C, a clear transition in the mode of deformation is observed in relaxed a-Si with increasing temperature. Up to 100°C, pressure-induced phase transformation and the observation of either crystalline (r8/bc8) end phases or pressure-induced a-Si occurs in relaxed a-Si. However, with further increase of temperature, plastic flow rather than phase transformation is the dominant mode of deformation. It is believed that the elevated temperature and pressure together induce bond softening and "defect"formation in structurally relaxed a-Si, leading to the inhibition of phase transformation due to pressure-releasing plastic flow under the indenter

The high pressure phase transformation behavior of silicon nanowires

Si nanowires of 80–150 nm and 200–250 nm diameter are pressurized up to 22 GPa using a diamond anvil cell. Raman and x-ray diffraction data were collected during both compression and decompression. Electron microscopy images reveal that the nanowires retain a nanowire-like morphology (after high pressure treatment). On compression, dc-Si was observed to persist at pressures up to 19 GPa compared to 11 GPa for bulk-Si. On decompression, the metallic b-Sn phase was found to be more stable for Si nanowires compared with bulk-Si when lowering the pressure and was observed as low as 6 GPa. For the smallest nanowires studied (80–150 nm), predominately a-Si was obtained on decompression, whereas for larger nanowires (200–250 nm), clear evidence for the r8/bc8-Si phase was obtained. We suggest that the small volume of the individual Si nanowires compared with bulk-Si inhibits the nucleation of the r8-Si phase on decompression. This study shows that there is a size dependence in the high pressure behavior of Si nanowires during both compression and decompressionL.Q.H. acknowledges her support from an Australian Government Research Training Program Scholarship. J.E.B. would like to acknowledge funding from the ARC Future Fellowship Scheme. A.L. acknowledges financial support from the Austrian Science Fund (FWF): Project No. P28175- N27 and e-beam lithography support by Manfred Reiche from the Max Planck Institute of Microstructure Physics, Halle, German

Experimental evidence of new tetragonal polymorphs of silicon formed through ultrafast laser-induced confined microexplosion

Ordinary materials can transform into novel phases at extraordinary high pressure and temperature. The recently developed method of ultrashort laser-induced confined microexplosions initiates a non-equilibrium disordered plasma state. Ultra-high quenching rates overcome kinetic barriers to the formation of new metastable phases, which are preserved in the surrounding pristine crystal for subsequent exploitation. Here we demonstrate that confined microexplosions in silicon produce several metastable end phases. Comparison with an ab initio random structure search reveals six energetically competitive potential phases, four tetragonal and two monoclinic structures. We show the presence of bt8 and st12, which have been predicted theoretically previously, but have not been observed in nature or in laboratory experiments. In addition, the presence of the as yet unidentified silicon phase, Si-VIII and two of our other predicted tetragonal phases are highly likely within laser-affected zones. These findings may pave the way for new materials with novel and exotic properties

Annealing of nanoindentation-induced high pressure crystalline phases created in crystalline and amorphous silicon

Thermally induced phase transformation of Si-III/Si-XII zones formed by nanoindentation has been studied during low temperature (200<T<300 °C) thermal annealing by Raman microspectroscopy and transmission electron microscopy. Two sizes of spherical indenter tips have been used to create substantially different volumes of phase transformed zones in both crystalline (c-Si) and amorphous silicon (a-Si) to study the zone size and starting matrix effects. The overall transformation is from Si-III/XII to poly- or nanocrystalline Si-I through intermediate phases of Si-XIII and Si-IV. Attempts have been made to determine the exact transformation pathways. Two scenarios are possible: either Si-XII first transforms to Si-III before transforming to Si-I through the intermediate phases or that Si-XII goes through the intermediate phases while Si-III transforms directly to Si-I. Finally, the phase transformations are slower in the larger indents and the starting matrix (crystalline or amorphous) has a substantial effect on the transformation kinetics of the small indents compared to the larger ones. We attribute this increased stability to both matrix effects (nucleation) and a difference in overall residual stress in indents made in a-Si compared to c-Si

Phase transformation pathways in amorphous germanium under indentation pressure

Nanoindentation-induced phase transformations have been studied in amorphous Ge thin films. These films initially tend to deform via plastic flow of the amorphous phase under load but at a critical pressure a sudden phase transformation occurs. This transformation, to a soft metallic (β-Sn-like)-Ge phase confined under the indenter, is signified by a "pop-in" event on loading. Following "pop-in," the indentation tests fall into two distinct types of behavior. In one case, the rate of deformation with increasing load after "pop-in" increases, and the observed end-phase following complete unloading is observed to be predominately diamond-cubic Ge. In the other case, the deformation rate (slope of the loading curve) remains the same as that before "pop-in," and the end phases following unloading are found to contain predominantly unstable r8 and more stable hexagonal Ge phases. The different transformation pathways for these two cases are shown to be related to the probability that the soft (β-Sn-like)-Ge phase volume, which suddenly forms at the transformation pressure, is either unconstrained by the indenter tip (the first case) or totally constrained under the indenter tip (in the latter case)

Indentation-Induced Damage Mechanisms in Germanium

The response of crystalline Ge to indentation has been studied over a range of maximum loads. At a certain load, an unusual 'giant pop-in' event occurs, in which a discontinuous extension of >1 μm is observed in the force-displacement curve. In such cas

Thermal stability of simple tetragonal and hexagonal diamond germanium

Exotic phases of germanium, that form under high pressure but persist under ambient conditions, are of technological interest due to their unique optical and electrical properties. The thermal evolution and stability of two of these exotic Ge phases, the simple tetragonal (st12) and hexagonal diamond (hd) phases, are investigated in detail. These metastable phases, formed by high pressure decompression in either a diamond anvil cell or by nanoindentation, are annealed at temperatures ranging from 280 to 320 C for st12-Ge and 200 to 550 C for hd-Ge. In both cases, the exotic phases originated from entirely pure Ge precursor materials. Raman microspectroscopy is used to monitor the phase changes ex situ following annealing. Our results show that hd-Ge synthesized via a pure form of a-Ge first undergoes a subtle change in structure and then an irreversible phase transformation to dc-Ge with an activation energy of (4.3 6 0.2) eV at higher temperatures. St12-Ge was found to transform to dc-Ge with an activation energy of (1.44 6 0.08) eV. Taken together with results from previous studies, this study allows for intriguing comparisons with silicon and suggests promising technological applications.This work was supported by the Australian Research Council under the Discovery Project Scheme. L.Q.H. is supported by an Australian Government Research Training Program Scholarship. J.E.B. acknowledges the ARC for the award of a Future Fellowship. B.H. was supported through a Weinberg Fellowship (ORNL) and the Neutron Scattering User Facilities (ORNL), supported by the U.S. Department of Energy, Office of Sciences, Basic Energy Sciences. The ORNL is funded under DOE-BES Contract No. DE-AC05-00OR22725 and the Alvin M. Weinberg Fellowship by the ORNL LDRD scheme under Project No. 7620

Formation of an r8-Dominant Si Material

The rhombohedral phase of Si (r8-Si), a promising semiconducting material, is formed by indentation together with the body-centered cubic phase (bc8-Si). Using a novel sample preparation method, x-ray diffraction is used to determine the relative volume of these phases in indented Si and allow observation of a distorted unit cell along the direction of indentation loading. Theoretical calculations together with these observations suggest the indent contains an intrinsic compression of ∼4  GPa that stabilizes the r8 phase.We would like to acknowledge and thank Beamline Scientist Ruqing Xu for his help in obtaining the X-ray data. J. E. B. would like to acknowledge the Australian Research Council (ARC) (FT130101355). B. H. gratefully acknowledges funding through a Weinberg Fellowship sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy and ORNL’s Neutron Facilities, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. A. M. acknowledges support from MINECO Project No. MAT2016-75586-C4-3-P (Spain)