Nanoindentation-Induced phase transformations in amorphous Germanium

Abstract

Semiconductors were traditionally considered to be classic ‘brittle’ materials, which under indentation load behave elastically until undergoing sudden and generally catastrophic failure via cracking. However, under certain conditions it is clear that many semiconductors also undergo considerable plastic deformation. Such plastic deformation mechanisms in semiconductor materials include defect generation and propagation, and under point loading, phase transformation. Germanium (Ge) is one of the most important semiconductors and is used in many technological applications. Crystalline Ge (c-Ge) has been reported to undergo a wide range of deformation mechanisms during point loading including twinning, defect generation as well as pressure-induced phase transformation. In this study amorphous Ge (a-Ge) is chosen as a starting material to explore the mechanisms of deformation that are excluded by the lack of long range order/crystallinity. In the literature there is some controversy as to what is the preferred indentation-induced deformation mechanism of Ge at room temperature. Some studies report twinning and defect generation while others report that a high-pressure phase transformation occurs. This thesis studies nanoindentation induced phase transformations in a-Ge. Ion implantation has been used to amorphize crystalline Ge in this study. This eliminates the competing deformation mechanisms of slip and twinning previously observed in c-Ge deformed via nanoindentation. Nanoindentation is now commonplace tool for the measurement of mechanical properties and also for inducing high-pressures required for phase transformation at small scales. In this study two different nanoindenter tips are used, spherical and Berkovich. Most of the work carried out using a spherical geometry to avoid cracking. A wide range of techniques are employed in this work to study the response of the indented a-Ge samples. These include micro-Raman spectroscopy, scanning electron microscopy, focussed ion beam milling and cross-sectional transmission electron microscopy. An interesting range of deformation responses is observed. Nanoindentation of the a-Ge samples shows that phase-transformation is readily induced, unlike c-Ge where phase transformations are only observed on occasion. Analysis of the nanoindentation curves from a-Ge shows that, above a threshold limit, a pop-in event occurs on loading. After the pop-in event the loading curves fall into two distinct deformation pathways. These have been named family ‘a’ and family ‘b’. In one case family ‘b’ the end-phase is predominantly observed to be diamond cubic Ge (dc-Ge) and the other case, the end-phase appears to be a rhombohedral phase with 8 atoms per unit cell (r8). The r8 phase is found to be unstable and transforms to hexagonal diamond Ge (hd-Ge) at room temperature within hours. The reason for these two different deformation pathways are related to the soft metallic (β-Sn)-Ge which forms on loading. It is proposed that if this metallic region is unconstrained by the indenter tip, the material is then extruded suddenly and during this process it transforms to dc-Ge. This behaviour is labelled as family ‘b’. Whereas, if the material is totally constrained under the tip, it transforms instead to unstable r8 structure which then further transform to hd-Ge. This pathway is referred to as family ‘a’. This work also examines the structure of the ion-implanted a-Ge as a function of annealing at temperatures below the recrystallization temperature. This so-called ‘structural relaxation’ is similar to that previously observed in amorphous silicon (a-Si). Moreover, similar to a-Si, the relaxation of a-Ge is shown here to lower its threshold for deformation via phase transformation. Finally, as previous studies on indentation-induced phase transformation in Ge have suggested that rate of loading and/or unloading may influence the deformation behaviour, this work also investigated this parameter. Slow loading rates are shown to mildly inhibit the phase transformation process of a-Ge. This work establishes a clear set of conditions under which phase transformations can be induced in Ge. In particular, the study shows that hd-Ge can be readily formed in a range of a-Ge film thicknesses. This finding enables these technologically-promising additional phases of Ge to be further studied and potential applications explored for the first time.