Transcript Atomic resolved study of defects in GaSb grown on Si

Atomic Resolved Study of Defects
in GaSb Grown on Si
By:
Shahrzad Hosseini Vajargah
Supervisor:
Dr. G. A. Botton
Jan 27, 2012
Outline
 Introduction
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Solar cell & Multijunctions
Physical Properties & Crystal Structure
Growth Techniques & Challenges
Importance of defects and their Identification Techniquces
 Characterization Methods and Techniques
 Results
 Identification of Polarity Reversal and Antiphase Boundaries
 Strain Analysis
 Summary & Acknowledgment
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Adopted from : http://eng3060.pbworks.com
Solar energy-power from the Sun
• Increasing world consumption of energy
• Fossil fuel shortage
• Global warming
• Need for sustainable development
Photovoltaic Effect
Incident of Photons
Adopted from : http://en.wikipedia.org/wiki
Generation of carriers by p-n junction
movement of electrons to the n-type side
and holes to the p-type side of junction
Generation of voltage
Efficiency: Ratio of number of carriers
collected by solar cell to photons of given
energy
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Physical properties & applications
Sb-based Compound Semiconductors
Adopted from : http://gorgia.no-ip.com/phd/html/thesis/phd_html/node4.html
 Wide range of bandgap energies
from 0.165 eV for InSb to 1.58 eV for
AlSb
 AlSb indirect and InSb and GaSb
direct bandgap
 High electron mobility and wide
range of bandgap offsets
Applications:
 Multijunction solar cells
 High speed electronic devices
 Thermophotovolatic applications
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Crystal structure
Silicon (Substrate)
 Diamond structure
 Centrosymmetric
 Advantages: low cost, large-scale
integration and high quality
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GaSb (Film)
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Zinc-Blende structure
Non- Centrosymmetric
Wide range of bandgap energies
Advantages: bandgap tunability
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Thin film growth technique
Molecular Beam Epitaxy (MBE)
Features
Advantages
 Abrupt interface
 Highly precise controlling of doping levels
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Adopted from: http://department.fzu.cz
 Ultra high vacuum and controlled temperature condition
 Effusion cells
 Heated substrate
 Different deposition ratio
 In-situ surface analysis with
Reflection High Energy Electron Diffraction
(RHEED)
Growth challenges
 Lattice mismatch between film and substrate
 Misfit dislocation
 Relaxation of film
 Planar Defects
 Twins
 Anti-Phase Boundaries (APB)
 Polar on non-polar growth
 Stoichiometric and non-Stoichiometric
 Lowest formation energy {110}-type APB
(Vanderbilt et al. 1992, Rubel et al. 2009)
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D. Cohen and C. B. Carter, Journal of Microscopy, 208(2), 84–99 (2002).
Why are defects so important?
• Uncompleted or dangling bonds in the core of dislocations generate states
near the middle of bandgap which are deep levels acting as recombination
centers.
• Elastic strain field of defects changes atomic distances and hence
electronic states, acting as a trap.
• Antiphase boundaries create non-radiative recombination centers.
Reduction of efficiency of solar cell
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APBs’ identification techniques with TEM
-200
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200-type Superlattice Reflections
Gowers, J. P. (1984). Applied Physics A Solids and
Surfaces, 34(4), 231-236.
•
Two beam Condition Dark Field Imaging
S. Y. Woo(2012) et al. (Submitted)
Convergent Beam Electron Diffraction
(CBED)
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A. Beyer, I. Ne´meth, S. Liebich, J. Ohlmann, W. Stolz, and K. Volz, J. of Appl. Phys. 109,
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083529 (2011)
APBs’ identification techniques with TEM
High Resolution Transmission Electron
Microscopy (HRTEM) images cannot be
interpreted directly.
Thickness
Defocus
Simulations show that the
contrast highly depends
Misidentification
on imaging
condition of
APB with twin
S. H. Huang, G. Balakrishnan, A. Khoshakhlagh, L. R. Dawson, and D. L.
Huffaker, Appl. Phys. Lett. 93, 071102 (2008).
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V. Narayanan, S. Mahajan , K.J. Bachmann, V. Woods, N. Dietz, Acta Materialia 50 1275–1287 (2002)
Research objectives
To understand:
 the atomic arrangements at antiphase boundaries
 origin of the APB at interface
 possible mechanism of APBs’ self-annihilation
In order to:
 prevent the APB formation, or
 make them to self-annihilate
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High Angle Annular Dark Field-STEM
 Transmission Electron Microscopy
Z-contrast (High angle annular dark field –
HAADF) Scanning Transmission Electron
Microscopy (STEM)
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High angle elastically scattered electrons
Annular detector
Composition sensitive
Less sensitive to thickness and focus
 Resolution is limited by lens aberrations:
1-Spherical (Cs)
2-Chromatic (Cc)
 Advantages of using Aberration correctors:
 Better Resolution
 Reduced Contrast Delocalization
 Sub-Å probe for spectroscopy
 Tuning capability of Cs
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Strained-layer superlattice (SLS)
Structure of the Epilayers
Layers
Thickness and Composition
Active Layer
1000 nm GaSb
25×10 nm GaSb
SLS
25×10 nm AlSb
GaSb Layer
1 μm GaSb
Buffer Layer
5 nm AlSb
Substrate
Si (001) Flat
GaSb
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AlSb
(a)Experimental HAADF-STEM Image
(b) Multisclice Simulation of GaSb
(c) Multisclice Simulation of AlSb
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Polarity reversal
Top views
Side view
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Edge-on APB
twin
APB
GaSb
Si
Mixed
Nucleation
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Strain measurement technique
Geometric Phase Analysis (GPA)
In an image of perfect crystal intensity at each position like (r) can be written as Fourier
sum which has amplitude and phase component.
Degree of contrast of a set of fringe
Lateral position fringes within image
(Geometric phase)
For a perfect crystal: Phase is constant across image
For an imperfect crystal: Any lattice distortion or displacement causes local shift of
fringes and consequently phase change or phase shift.
Phase variations
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Local displacement field
Strain Matrix
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Strain distribution
APB
twin
y
x
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Summary
• Polarity reversal due to the formation of antiphase boundaries has
identified directly for the first time with HAADF-STEM.
• The direct identification of polarity reversal with HAADF-STEM avoids the
misinterpretations in characterizing the planar defects.
• The APB has formed due to the mixed nucleation at interface in spite of
prior soaking with Sb.
• Different bonding length in anti-phase bonds compared to in-phase bonds
induces strain and lattice rotation at APB.
• Compensating the lattice rotation by lateral shift and faceting can play an
important role in the self-annihilation of the APBs.
• Simultaneous control of the substrate misorientation angle and prelayer
soaking step in growth can help to suppress the APB formation.
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Acknowledgment
• My supervisor: Prof. G. A. Botton
• Research Group Fellows for helpful
suggestions
• Canadian Centre for Electron Microscopy
(CCEM) staff
• Ontario Center of Excellence (OCE)
• Center of Emerging Device Technology for
providing me with samples
• Arise Technology for funding this project
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Thank you !
Questions?
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