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Interfacial strain analysis of Si, SiGe Strained-Layer Superlattice Structure
using Convergent Beam Electron Diffraction
N.V.V. Mogili 1, 2* , D.A. Tanner 1, 2 1, 3
, S. Nakahara
1 Materials & Surface Science Institute, University of Limerick, Ireland
2 Department of Design and Manufacturing Technology, University of Limerick, Ireland
3 Department of Physics & Energy, University of Limerick, Ireland
Vishnu.Mogili@ul.ie
Abstract
Due to the mechanical thinning of Strained-Layer
Superlattice (SLS) structure, two types of relaxation
occur. One type of relaxation arises between adjacent
layers of SLS and another between whole SLS and its
substrate. In the present work by combining Low Angle
Annular Dark Field (LAADF) imaging with Convergent
Beam Electron Diffraction (CBED), the relaxation that
appears between whole SLS and its substrate of Si,
SiGe structure has been studied.
Introduction
Understanding the interfacial strain phenomenon of
Strained-Layer Superlattices (SLS) and quantum well
semiconductors at nanometer level is very important in
designing advanced electronic devices. Due to the
difference in lattice parameters of epitaxially grown
materials, strain exists at adjacent layers of SLS and
also between whole SLS and the substrate. The nature
of the strain will be homogeneous when the SLS is in its
bulk form. But as the complete structure is thinned
down to electron transparency owing to elastic
relaxation phenomenon, strain state will be altered. To
quantitatively determine the local behavior of strain,
Convergent Beam Electron Diffraction (CBED) is an
excellent technique to study the layers of SLS with high
spatial resolution.
Material & Methods
The specimen analyzed in this work is a TEM
calibration sample known as MAG-I-CAL ® . It is a
cross sectional SLS structure with alternating
strained Silicon-Germanium (Si0.81Ge0.19) and Silicon
(Si) layers grown epitaxially on Si substrate. CBED
analysis was performed at a nominal voltage of 200kv in
JEOL 2100 FEG TEM and all strain measurements were
performed in [340] Zone Axis (ZA) which is 8° apart
from [011] ZA.
As the conical electron beam is used to illuminate the
specimen surface, CBED patterns consists of pairs of
deficit and excess High Order Laue Zone (HOLZ) lines
appear in its transmitted and diffracted disks. The
positions of these lines are very sensitive to the changes
in local lattice parameters and Transmission Electron
Microscope (TEM) accelerating voltage. Hence for the
quantitative study of strain nature, initially TEM
170
accelerating voltage has been calculated with accuracy
down to 0.05 kv by means of dynamical simulations
using JEMS electron microscopy software. And for
precise handling of electron beam on the specimen
surface, CBED has been performed in Scanning
Transmission Electron Microscope (STEM) mode.
Results & Discussion
When the electron beam sampled specimen surface
away from SLS and substrate interface, sharp HOLZ
lines have been obtained. This has been interpreted that
there is no strain gradient along the electron beam
direction. As the beam approached the interface, sharp
HOLZ lines appeared to be in split HOLZ lines and
Low Angle Annular Dark Field (LAADF) image taken
in this region has shown contrast variation. This has
been believed that at the interface, crystal lattice planes
are bent and produces non-uniform strain along beam
direction. With the help of kinematic simulation, from
the width of the split HOLZ lines, lattice plane bending
angle has been quantitatively determined and using
Finite Element Modeling, work is ongoing to correlate
strain variation in TEM specimen and bulk structure.
Fig. -1: Overlap of
simulated and expt.
HOLZ lines of [340]
transmitted disk @
202.405kv
Fig. -2: Experimental
split HOLZ lines in
transmitted disk of Si
[340] zone axis
Fig. -3: Kinematically
simulated [340] HOLZ
lines splitting @
202.405kv overlapped
on expt. pattern
References
[1] J.P. Mornirolli, “Large Angle Convergent Beam Electron
Diffraction (LACBED): Application to crystal defects, Taylor
& Francis, (2004).
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[5] D. V. S Rao et al. Journal of Electron Microscopy, 1, 0,
(2008)
[6] F. Usesugi et al. Jpn. J. Appl. Phys., 47, 5, 2008
[7] X.F. Duan et al. Appl. Phys. Lett. 61, 3,1992