Optimised indentation conditions for large-area phase transformations in Silicon

Abstract

Silicon has long been the most important material in semiconductor technology, having a huge impact in the field. This is predominantly due to its highly useful electrical and optical properties. It has also been shown to phase transform under pressure, from the diamond cubic Si-I that is ubiquitous in our modern society, to several different phases. Of particular interest is the rhombohedral Si-XII phase which, while showing some similar optical and electrical properties to Si-I, has potentially even more useful properties. A metastable region of Si-XII, in conjunction with Si-III, can be generated through point loading pressure via nanoindentation. This study addresses the challenges of creating large uniform volumes of Si-XII and compares the electrical properties of such volumes to a similar region created through a line of overlapping indents. This is motivated by the desire to broaden the range of uses for Si-XII and to allow for greater flexibility in the creation of regions of this material. Nanoindentation of Si-I is conducted using two separate spherical indenters; a larger, ~60 um diameter, indenter with a non-uniform surface and a smaller, ~20 um diameter, indenter with a smooth surface. Indents are made across a large range of maximum load forces. The maximum loads are separated into deformation regimes based on the presence of events in the load/unload curves. The characteristics of each of the regimes are determined using Raman microspectroscopy, cross-sectional transmission electron microscopy (XTEM), and scanning electron microscopy (SEM). For the ~20 um diameter tip, the regime in which only phase transformation occurs extends across the maximum loads 350 - 500 mN. Indentation made to a maximum load above 500 mN still display phase transformation but also display other competing deformation mechanisms such as dislocations, slip and fracturing. This causes a reduction in the amount of phase transformation at these higher maximum loads. For the ~60 um diameter tip, the phase transformation only regime ranges from 550 - 750 mN. Indentation above 750 mN displays competing deformation mechanisms. A maximum load of 2500 mN (>>750 mN) creates a larger transformed region than 750 mN but causes a large amount of dislocations in the underlying substrate. In the phase transformation only regime, the smaller diameter indenter with a smoother tip creates larger regions of Si-III/XII with a more consistent shape than the larger diameter indenter with pitting on the tip. The thus obtained understanding of the optimal indentation conditions are utilised to extract the electrical properties of Si-XII. Therefore, amorphous silicon (a-Si) resistive bar structures that isolate two contact pads are bridged with a region of Si-III/XII generated by a single indent and the electrical properties of this region is measured. These measurements from a single indentation are comparable to those of a line of overlapping indents. This shows that the process of overlapping indentation areas do not negatively affect the electrical properties of the region. This allows for greater flexibility in the "writing" of Si-III/XII zones through overlapping indentation areas, thus promises greater potential for future technological application. Show more