Identification of nanoindentation-induced phase changes in silicon by in situ electrical characterization

In situ electrical measurements during nanoindentation of Czochralski grown p-type crystalline silicon (100) have been performed using a conducting diamond Berkovich indenter tip. Through-tip current monitoring with a sensitivity of ∼10pA and extraction of current-voltage curves at various points on the complete load-unload cycle have been used to track the phase transformations of silicon during the loading and unloading cycle. Postindent current-voltage curves prove to be extremely sensitive to phase changes during indentation, as well as to the final phase composition within the indented volume. For example, differences in the final structure are detected by current-voltage measurements even in an unloading regime in which only amorphous silicon is expected to form. The electrical measurements are interpreted with the aid of previously reported transmission electron microscopy and Raman microspectroscopy measurements.This work was funded by the Australian Research Council and WRiota Pty Ltd

Phase transformations induced by spherical indentation in ion-implanted amorphous silicon

The deformation behavior of ion-implanted (unrelaxed) and annealed ion-implanted (relaxed) amorphous silicon(a-Si) under spherical indentation at room temperature has been investigated. It has been found that the mode of deformation depends critically on both the preparation of the amorphous film and the scale of the mechanical deformation.Ex situmeasurements, such as Raman microspectroscopy and cross-sectional transmission electron microscopy, as well as in situ electrical measurements reveal the occurrence of phase transformations in all relaxed a-Si films. The preferred deformation mode of unrelaxed a-Si is plastic flow, only under certain high load conditions can this state of a-Si be forced to transform. In situ electrical measurements have revealed more detail of the transformation process during both loading and unloading. We have used ELASTICA simulations to obtain estimates of the depth of the metallic phase as a function of load, and good agreement is found with the experiment. On unloading, a clear change in electrical conductivity is observed to correlate with a “pop-out” event on load versus penetration curves

Effect of oxygen concentration on nanoindentation-induced phase transformations in ion-implanted amorphous silicon

The effect of the local oxygen concentration in ion-implanted amorphous Si (a-Si) on nanoindentation-inducedphase transformations has been investigated. Implantation of oxygen into the a-Sifilms has been used to controllably introduce an approximately constant concentration of oxygen, ranging from ∼10¹⁸ to ∼10²¹ cm⁻³, over the depth range of the phase transformed zones. Nanoindentation was performed under conditions that ensure a phase transformed zone composed completely of Si-III/XII in the nominally oxygen-free a-Si. The effect of the local oxygen concentration has been investigated by analysis of the unloading curves, Raman microspectroscopy, and cross-sectional transmission electron microscopy (XTEM). The formation of Si-III/XII is suppressed with increasing oxygen concentration, favoring a greater volume of a-Si within the zones. The Raman microspectroscopy and XTEM verify that the volume of Si-III/XII decreases with increasing O concentration. With the smaller volumes of Si-III/XII, the pop-out normally observed on load versus penetration depth curves during unloading decreases in magnitude, becoming more kinklike and is barely discernable at high concentrations of oxygen. The probability of forming any high pressure phases is reduced from 1 to ∼0.1 for a concentration of 10²¹ cm⁻³. We suggest that the bonding of O with Si reduces the formation of Si-III/XII during unloading through a similar mechanism to that of oxygen-retarded solid phase crystallization of a-Si.This project is funded by the Australian Research Council and WRiota Pty Ltd

Nanoscale characterization of energy generation from piezoelectric thin films

We report on the use of nanoindentation to characterize in situ the voltage and current generation of piezoelectric thin films. This work presents the controlled observation of nanoscale piezoelectric voltage and current generation, allowing accurate quantification and mapping of force function variations. We characterize both continuous thin films and lithographically patterned nanoislands with constrained interaction area. The influence of size on energy generation parameters is reported, demonstrating that nanoislands can exhibit more effective current generation than continuous films. This quantitative finding suggests that further research into the impact of nanoscale patterning of piezoelectric thin films may yield an improved materials platform for integrated microscale energy scavenging systems

Segregation and precipitation of Er in Ge

Although Er-doped Genanomaterials are attractive for photonic applications, very little is known about the basic properties of Er in Ge. Here, the authors study the annealing behavior of Geimplanted with keV Er ions to doses resulting in ≲1at.% of Er. Large redistribution of Er, with segregation at the amorphous/crystalline interface, starts at ≳500°C, while lower temperatures are required for material recrystallization. However, even at 400°C, Er forms precipitates. The concentration of Er trapped in the bulk after recrystallization decreases with increasing temperature but is independent of the initial bulk Er concentration for the range of ion doses studied here.Work at the ANU was supported by the ARC

High pressure crystalline phase formation during nanoindentation: Amorphous versus crystalline silicon

Phase transformations induced by indentation at different unloading rates have been studied in crystalline and amorphous silicon via Raman microspectroscopy and transmission electron microscopy. Unloading was performed at a “slow” rate of ∼0.9mN∕s which is known to create volumes of high pressure phases (Si-III and Si-XII) in crystalline silicon as well as “rapid” unloading (∼1000mN∕s), where amorphous phases are expected. Stark differences between the resulting structures are observed depending on whether the starting material is amorphous or crystalline silicon. Interestingly, amorphous silicon transforms to high pressure phases much more readily than crystalline silicon even after rapid unloading.This work was funded by the Australian Research Council and WRiota Pty Ltd

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

Solid-phase epitaxial regrowth of amorphous layers in Si(100) created by low-energy, high-fluence phosphorus implantation

Medium energy ion scattering has been used to study the kinetics of solid-phaseepitaxial regrowth (SPEG) of ultrathin amorphous layers formed by room-temperature implantation of 5keV energy phosphorus ions into Si (100). The implants create P distributions with peak concentrations up to ∼7×10²¹cm⁻³. SPEG has been driven by rapid thermal annealing, 475°C⩽TA⩽600°C, for times up to 2000s. At each temperature, the regrowth velocity is enhanced in the early stages due to the presence of phosphorus but then slows sharply to a value more than an order of magnitude below the intrinsic rate. The critical phosphorus concentration at the transition point for TA=475°C regrowth is ∼6×10²⁰cm⁻³ and increases steadily with anneal temperature. Time-of-flight secondary ion mass spectroscopy profiles confirm the onset of phosphorus push out, where the advancing recrystallization front enters the transition region. Supplementary cross-sectional transmission electron microscopy evidence confirms the existence of a local strain field.This work has been supported by the Natural Sciences and Engineering Research Council of Canada

Effect of hydrogen on nanoindentation-induced phase transformations in amorphous silicon

The effect of local hydrogen concentration on nanoindentation-inducedphase transformations has been investigated in ion-implanted amorphous silicon(a-Si). Elevated concentrations of H ranging from 5×10¹⁸ to 5×10²⁰ cm⁻³, over the depth of indentation-induced phase transformed zones have been formed in the a-Si by H ion-implantation. Indentation has been performed under conditions that result in phase transformed zones composed totally of Si-III/Si-XII in the “H-free” samples. Deformation during indentation and determination of phase transformation behavior has been examined by analysis of load/unload curves, Raman microspectroscopy, and cross-sectional transmission electron microscopy (XTEM). With increasing H content, the probability of forming Si-III/Si-XII and the volume fraction of Si-III/Si-XII decrease. XTEM shows that these reduced volumes are randomly distributed within the phase transformed zone and are surrounded by indentation-induced a-Si. For a H concentration of 5×10²⁰ cm⁻³, the probability of forming Si-III/Si-XII is reduced to 0.5 compared to 1 in “H-free” material and for those indents that exhibit the Si-III/Si-XII end phase the volume fraction is approximately 60 %. We suggest that the monohydride bonding configuration of Si and H in a-Si reduces the formation of the high pressure crystalline phases by retarding growth of the crystallites through a similar mechanism to that of hydrogen-retarded solid phase crystallization of a-Si to diamond cubic crystalline Si-I phase.The authors are grateful to funding from the Australian Research Council

Formation and growth of nanoindentation-induced high pressure phases in crystalline and amorphous silicon

Nanoindentation-induced formation of high pressure crystalline phases (Si-III and Si-XII) during unloading has been studied by Raman micro-spectroscopy, cross-sectional transmission electron microscopy (XTEM), and postindentation electrical measurements. For indentation in crystalline silicon(c-Si), rapid unloading (∼1000 mN∕s) results in the formation of amorphous silicon(a-Si) only; a result we have exploited to quench the formation of high pressure phases at various stages during unloading to study their formation and evolution. This reveals that seed volumes of Si-III and Si-XII form during the early stages of unloading with substantial volumes only forming after the pop-out event that occurs at about 50% of the maximum load. In contrast, high pressure phases form much more readily in an a-Si matrix, with substantial volumes forming without an observable pop-out event with rapid unloading. Postindentation electrical measurements have been used to further investigate the end phases and to identify differences between indentations which otherwise appear to be identical from the XTEM and Raman analyses.This research was funded by the Australian Research Council and by WRiota Pty. Ltd