1. Atomic Resolved Study of Defects in GaSb Grown on Si
By:
Shahrzad Hosseini Vajargah
Supervisor:
Dr. G. A. Botton
Jan 27, 2012

2. Characterization Methods and Techniques Results
Outline
Introduction
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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3. Generation of carriers by p-n junction
Solar energy-power from the Sun
Adopted from :
Increasing world consumption of energy
Fossil fuel shortage
Global warming
Need for sustainable development
Photovoltaic Effect
Incident of Photons
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
Adopted from :

4. Physical properties & applications
Sb-based Compound Semiconductors
Adopted from :
Wide range of bandgap energies from 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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5. Crystal structure Silicon (Substrate) GaSb (Film) Diamond structure
Centrosymmetric
Advantages: low cost, large-scale integration and high quality
Zinc-Blende structure
Non- Centrosymmetric
Wide range of bandgap energies
Advantages: bandgap tunability
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6. Thin film growth technique
Molecular Beam Epitaxy (MBE)
Features
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)
Advantages
Abrupt interface
Highly precise controlling of doping levels
Adopted from:
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7. 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)
D. Cohen and C. B. Carter, Journal of Microscopy, 208(2), 84–99 (2002).

8. Reduction of efficiency of solar cell
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

9. APBs’ identification techniques with TEM
-200
200
Gowers, J. P. (1984). Applied Physics A Solids and Surfaces, 34(4),
200-type Superlattice Reflections
S. Y. Woo(2012) et al. (Submitted)
Two beam Condition Dark Field Imaging
A. Beyer, I. Ne´meth, S. Liebich, J. Ohlmann, W. Stolz, and K. Volz, J. of Appl. Phys. 109, (2011)
Convergent Beam Electron Diffraction (CBED)
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10. APBs’ identification techniques with TEM
High Resolution Transmission Electron Microscopy (HRTEM) images cannot be interpreted directly.
V. Narayanan, S. Mahajan , K.J. Bachmann, V. Woods, N. Dietz, Acta Materialia –1287 (2002)
Simulations show that the contrast highly depends on imaging condition
Defocus
Thickness
S. H. Huang, G. Balakrishnan, A. Khoshakhlagh, L. R. Dawson, and D. L. Huffaker, Appl. Phys. Lett. 93, (2008).
Misidentification of APB with twin
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11. Research objectives To understand: In order to:
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
MATLS 702

12. High Angle Annular Dark Field-STEM
Transmission Electron Microscopy
Z-contrast (High angle annular dark field – HAADF) Scanning Transmission Electron Microscopy (STEM)
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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13. Strained-layer superlattice (SLS)
Structure of the Epilayers
Layers
Thickness and Composition
Active Layer
1000 nm GaSb
SLS
25×10 nm GaSb
25×10 nm AlSb
GaSb Layer
1 μm GaSb
Buffer Layer
5 nm AlSb
Substrate
Si (001) Flat
(a)Experimental HAADF-STEM Image
(b) Multisclice Simulation of GaSb
(c) Multisclice Simulation of AlSb
GaSb
AlSb
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14. Polarity reversal
Top views
Side view
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15. Edge-on APB Si
GaSb
twin
APB
Mixed Nucleation
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16. 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 Local displacement field Strain Matrix
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17. Strain distribution x y
twin
APB
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18. 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.
MATLS 702

19. 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

20. Thank you !
Questions?