contact persons: Jennifer Gray (firstname.lastname@example.org);
Chi-Chin Wu (email@example.com)
1. Stress relaxation in SiGe/Si epitaxial films: Tailoring
surface morphology and mapping relaxation regimes
Heteroepitaxial growth of semiconductor structures is
important for the fabrication of many types of devices. However, due to the
lattice mismatch, large amounts of strain develop in the epitaxial layer.
This strain may be reduced through the formation of islands on the surface
of the film1 when there is sufficient adatom mobility, and also
through the injection of misfit dislocations at a critical thickness2.
Strain-relieving islands can self-assemble into ordered two-dimensional
arrays on the film surface. There has been recent widespread interest in
using island morphologies produced by such strain relieving mechanisms as a
possible way to form quantum structures for new devices. In these
experiments we measure relaxation in a growth regime where both of these
mechanisms compete and interact as a function of the deposition variables.
By mapping out relaxation regimes as a function of growth parameters and
combining with detailed microscopy analysis, we can gain insight into how to
control the surface and defect structure of thin films in general.
The surface morphology that develops in SixGe1-x
films grown on Si is known to be strongly dependent on the composition and
growth temperature. Growth at temperatures above about 700°C follows the
widely observed morphological evolution in the Ge/Si system of the formation
of discrete hut clusters followed by a transition to dome shaped islands at
greater thicknesses 3,4. Growth at lower temperatures generally
produces a rippled surface morphology due to limitations on adatom
diffusion.5,6 However, we have found that for relatively low
growth temperatures (550°C, Si0.7Ge0.3), it is
possible to form ordered quadruplet arrays of SiGe nanoclusters, with
possible applications in quantum cellular automata architectures.7
In these experiments, Si0.7Ge0.3 films
are grown using molecular-beam epitaxy (MBE) on (001) Si substrates. The
MBE system is equipped with reflection high-energy electron diffraction (RHEED),
for monitoring the surface of the film during growth, and a multi-beam
optical stress sensor (MOSS) system8 that measures wafer
curvature which is related to stress through Stoney’s equation . The
in-situ wafer curvature measurements from films grown at 550°C and a rate of
0.9 Å/s indicate that the stress in these films remains relatively constant
until approximately 20 nm of growth (Region I in Fig. 1). Around this
thickness, relaxation of 10% or less of the initial stress in the film
occurs, as is determined by the change in slope of the stress-thickness
curve at this point. Further growth beyond the islanding transition leads
to a second major transition in the stress-thickness slope at approximately
50 nm. This rapid relaxation regime corresponds to the introduction of
misfit dislocations in the film as we have confirmed by plan-view
transmission electron microscopy.
Atomic force microscopy (AFM) shows the initial development
of small, shallow pyramidal surface pits (Fig. 2a) at film thicknesses of
5-15 nm, below the islanding transition. With further growth past the
islanding transition, nucleation of islands surrounding the edges of the
pits along the <100> directions (Fig. 2b) occurs, forming a structure that
we term “quantum fortresses”. This resembles the cooperative nucleation
process as described by Jesson, et al9. The sidewall angle of
the pits is close to that of the typical hut cluster island <501> facet.
After the introduction of dislocations the “quantum fortress” islands begin
to be broken up by a cross-hatch morphology caused by the dislocations along
the <110> directions. A similar island geometry has been observed
previously in the SiGe/Si system, using deliberate incorporation of C.10
However, the quantum fortress morphology in these experiments appears to be
intrinsically associated with the growth conditions.
When the growth rate is decreased to 0.15 Å/s, the in-situ
curvature measurements are similar, however, the resultant surface
morphology consists of initial small and densely packed square islands that
elongate into island ridges along the <100> directions as growth is
continued (Fig. 2c). For higher growth temperatures (650, 750 ºC) we do not
observe the quantum fortress morphology, only islands for these growth
rates. It thus appears that the role of growth rate upon surface morphology
is far more significant at lower growth temperatures in this system. If we
anneal films grown under the “quantum fortress” growth conditions, we can
get yet another morphology. By stopping growth just after pit formation and
annealing, we find that the pits rapidly elongate in one <100> direction to
become slits (Fig. 2d). It may be possible to tailor the surface morphology
further by repeated sequences of growth, followed by anneals.
These results show that under kinetically limited conditions,
growth rate has a large impact on the resulting surface morphology and can
be used to form complex ordered nano-architectures. More experiments are
currently being done to determine the temperature, growth rate and
composition ranges over which “quantum fortress” type structures can be
formed. The in-situ wafer curvature data will allow us to map out when
these relaxation transitions occur as well providing the quantitative
measurements for relaxation amounts. This combined with the microscopy data
will provide better understanding of the relaxation behavior of strained
heteroepitaxial systems and the control of surface morphology and defect
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2. Investigation of stress relaxation and dislocations for
periodic sinusoidal Si1-xGex /Si
Due to the need to release the great strains
and stresses induced by interface misfit, the evolution of the undulation
instabilities lead to a sinusoidal-like morphology. Any corrugation of the
SiGe surface is very detrimental in the fabrication of high-speed electronic
devices that require abrupt interfaces and flat layers. On the other hand,
this periodic sinusoidal morphology can be used as self-patterned templates
for selective growth of self-assembled nanostructures. Before achieving
that, there is an urgent need to perform extensive studies both in modeling/
simulations with calculation techniques and observe them experimentally to
understand the possible correlations among competing factors of strain
relaxation, morphology and dislocations.
In order to gain fundamental understandings about the strained layer
epitaxial systems for the study of dislocations and strain relaxation
kitnetics for semiconductor devices. The research is conducted through
experimental observations from state-of-the art transmission electron
microscopy to in-situ observe the dislocation movements as well as
undulation morphology and correlate this information with the stress
distributions and dislocation interactions from simulation and modeling with
finite element analysis techniques. This work involves extensive
collaborations with researchers at the IBM Watson Research Center in
Yorktown Heights, New York State. The research system mainly focuses on
Silicon-Germanium alloy thin film with various thickness and undulation
dimensions grown on single crystalline silicon substrate. The main tasks of
this research are to investigate the effect of epitaxial layer morphology
(such as sinusoidal curve, various types of islands or any other networks)
on the strain relaxation mechanism and dislocation interactions.
The correlations between the simulations of
dislocation interactions with complicated self-interactive programs, stress
modeling with finite element methods, and in-situ observations make this
research unique. With these tools and advanced research resources, it is
possible to analyze and better understand the strain relaxation mechanism
which is a crucial information for successful thin film growth in
semiconductor industry. With the IBM code, this research is able to simulate
where a specific existing dislocation moves at any interested time instant
and how it interacts with other dislocations inside the system. With in-situ
TEM experiments, we are able to observe dislocation movements and correlate
them with the stress relaxations in local regions. With models done by the
finite element analysis method, the preliminary results have demonstrated
that the strains and stresses can be relaxed by inducing surface undulations
in such a way that the stress is released in the top surface regions and
compressed in the trough regions, as shown in the figure below. The same
technique can also study the composition segregation and precisely correlate
such information with stress relaxations, which is otherwise difficult to
carry out in TEM experiments in satisfactory accuracy.