Atomic Resolved Study of Defects in GaSb Grown on Si

By: Shahrzad Hosseini Vajargah

Supervisor:Dr. G. A. Botton

Jan 27, 2012

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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

• 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

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Solar energy-power from the Sun

Physical properties & applications

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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Sb-based Compound Semiconductors

Crystal structure

Silicon (Substrate) Diamond structure Centrosymmetric Advantages: low cost, large-scale

integration and high quality

GaSb (Film) Zinc-Blende structure Non- Centrosymmetric Wide range of bandgap energies Advantages: bandgap tunability

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Thin film growth techniqueMolecular 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

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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)

Growth challenges

D. Cohen and C. B. Carter, Journal of Microscopy, 208(2), 84–99 (2002).

• 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.

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Reduction of efficiency of solar cell

Why are defects so important?

200-type Superlattice Reflections

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APBs’ identification techniques with TEM-200 200

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)

A. Beyer, I. Ne´meth, S. Liebich, J. Ohlmann, W. Stolz, and K. Volz, J. of Appl. Phys. 109, 083529 (2011)

Convergent Beam Electron Diffraction (CBED)

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High Resolution Transmission Electron Microscopy (HRTEM) images cannot be interpreted directly.

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APBs’ identification techniques with TEM

MATLS 702 V. Narayanan, S. Mahajan , K.J. Bachmann, V. Woods, N. Dietz, Acta Materialia 50 1275–1287 (2002)

Simulations show that the contrast highly depends on imaging condition

Defocus

S. H. Huang, G. Balakrishnan, A. Khoshakhlagh, L. R. Dawson, and D. L. Huffaker, Appl. Phys. Lett. 93, 071102 (2008).

Misidentification of APB with twin

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Research objectives

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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

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

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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)

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(a)Experimental HAADF-STEM Image(b) Multisclice Simulation of GaSb(c) Multisclice Simulation of AlSb

GaSb AlSb

Structure of the EpilayersLayers Thickness and Composition

Active Layer 1000 nm GaSb

SLS25×10 nm GaSb25×10 nm AlSb

GaSb Layer 1 μm GaSbBuffer Layer 5 nm AlSb

Substrate Si (001) Flat

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Polarity reversal

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Top views

Side view

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Edge-on APB

twin

APB

Si

GaSb

Mixed Nucleation

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 imageFor 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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Strain distribution

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x

y

twin

APB

• 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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Summary

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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

Thank you !

Questions?

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