Nanoindentation-Induced phase transformations in amorphous Germanium
Date
2016
Authors
Deshmukh, Sarita
Journal Title
Journal ISSN
Volume Title
Publisher
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.
Description
Keywords
Phase transformations, Amorphous Germanium, Nanoindentation
Citation
Collections
Source
Type
Thesis (MPhil)
Book Title
Entity type
Access Statement
License Rights
Restricted until
Downloads
File
Description