Point of Contact: W' Alan Doolittle, Georgia Institute of Technology, 777 Atlantic Dr', Atlanta, GA,

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InN A Material with Photovoltaic Promise and
Challenges W. Alan Doolittle, Elaissa Trybus,
Walter Henderson, Gon Namkoong, Shawn Burnham,
Kyoung Lee - Georgia Tech Other Collaborators,
Ian Ferguson and Christiana Honsberg
 Point of Contact W. Alan Doolittle, Georgia
Institute of Technology, 777 Atlantic Dr.,
Atlanta, GA, 30332-0250, phone and fax
404-894-9884, alan.doolittle_at_ece.gatech.edu
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Outline

• Opportunities and Challenges for InN in
  Photovoltaics
• Point Counter Point The good and the bad
• Major Challenges
• Addressing the major challenges
• Addressing Epitaxy, Surface Composition and Phase
  Issues in InN
• Conclusions

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Opportunities and Challenges for InN in
Photovoltaics
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Point-Counter Point

• Advantages
• Massive span in PV energies for high efficiencies
  in one material system
• Nitrides generally offer recombination
  insensitivity to dislocations
• Strong band bending is perfect for low surface
  recombination velocity

• Disadvantages
• Massive span in PV energies is not in a lattice
  matched system
• High dislocation density
  .
• Strong band bending has resulted in inability to
  form a solid state junction
• P-type doping undemonstrated
• Tunnel junctions probably not possible
• P-type base is usually preferred due to higher
  mobility of minority electrons
• 3-junction high concentration solar cells are
  already in excess of 37.2 efficient
  (GaInP/GaAs/Ge Spectrolabs King et al)

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Optimal Energy Bandgaps
Efficiency and band gap for tandem solar cells at
1000X
0.66, 1.42, 1.88 for Ge/GaAs/Ga0.51In0.49P
A. Bennett and L. C. Olsen, Analysis of
Multiple-Cell Concentrator/Photovoltaic Systems,
Proceedings of the 13th Photovoltaic Specialists
Conference, 868 873, (1978).
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Comparison of Available Bandgaps and Solar
Spectrum
AlN
GaN
InN
Lattice Matched Span
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Comparison of Available Bandgaps and Solar
Spectrum
AlN
GaN
InN
Lattice Matched Span
Si Ge
ZnO
LGO
SiC, Sapphire
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Need for Tunnel Junction
Tunnel junction requires degenerate doping!
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Addressing Epitaxy, Surface Composition and Phase
Issues in InN
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First Demonstration of InN on Ge

• InN on Ge is oriented as modeled via computer
• (0002) InN (111) Ge
• InN on Ge has surprisingly good structural
  quality
• X-ray diffraction (0002) FWHM is 360 arcsec for
  a 0.4 um thick film
• Equivalent to others result from 5-8 um thick
  films.

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Al/Ge and Al/InN domain matching
Superposition of Al and InN showing the domain
matching.
Superposition of Al and Ge showing the domain
matching.
Ge
Al
Ge
Al
InN
Al
Every 4th unit cell of InN aligns to every 5th
unit cell of Al to within 1
Every 7th unit cells of Al aligns to within 0.21
to every 8th unit cells of Ge
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Characterization of X-ray and IV curves
5 ohm/cm2
10x10?m
0.5x0.5?m
IV curves on n-InN/p-Ge for 1?2mm (red) and
2?2mm (black).
(0002) ?-2? full width at half maximum (FWHM)
370 and 276 arcsec for InN on Ge and Al/Ge
substrates
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Elimination of the need for a Tunnel Junction?
InN or InGaN cell
Al
Ge cell
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Increased Surface Oxygen Very high surface
oxidation rate!
10000000
Oxygen
1000000
Al2O3
100000
CPS
InN
10000
1000
100
0
5
10
15
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SIMS using Cs ion 15 kV primary beam
Sputter Time
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Rapidly Changing tilt structure with thickness
0.4 um
0.2 um
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What happens in a Leaky MBE System?

• Evidence of hexagonal InN 30 degree rotated with
  Sapphire
• Cubic In2O3 also present! Not observed in w-2q
  curve.
• Other unknown 3-fold symmetric (possibly
  tetragonal) phase

N1456 nitrided sapphire buffer
lt102gt Hexagonal InN lt113gt Hexagonal
Sapphire lt422gt Cubic In2O3 Possibly tetragonal In
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What happens in a Leaky MBE System?

• Evidence of hexagonal InN 30 degree rotated with
  Sapphire
• Cubic In2O3 also present! Not observed in w-2q
  curve.
• Other unknown 3-fold symmetric (possibly
  tetragonal) phase

N1491 with AlN Buffer
lt102gt Hexagonal InN lt113gt Hexagonal
Sapphire lt422gt Cubic In2O3 Possibly tetragonal In
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What happens in a Leaky MBE System?

• Cubic In2O3 present in w-2q curve.
• No metallic In

N1533
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No Nitridation and No AlN leads to Indium

• Metallic In observed but not droplets

N1439 no buffer
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What happens Indium when annealed in oxygen?
No In2O3 as grown - only In
In2O3 after 650 degree anneal for 10 minutes
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What happens in a Leaky MBE System?

• Evidence of cubic InN
• May result from oxygen induced stacking faults as
  in GaN
• Evidence of cubic In2O3 for very high oxygen
• Metallic In is easily identified by a high flux
  (x-ray mirror) x-ray diffraction system and
  occurs when no buffer (nitrided sapphire or AlN)
  is used

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Conclusions

• InN is a promising and challenging PV material
• Lattice matching of tandems cells is an issue
• Tunnel junctions are an issue
• Epitaxial inter-metallic layers can help both the
  above
• InN has a non-stoichiometric, oxygen rich surface
  layer
• InN CAN contain cubic InN/In2O3, surface In

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Backup Slides
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No Nitridation and No AlN leads to Indium

• Metallic In observed

extended in psi indicating a crystal with a
fairly large tilt distribution in psi(and
presumably omega) possibly tying in to the
semi-amorphous material that weve been seeing
N1439
N1439
Cubic InN (200)/sapphire (104) position (2theta
36.40)
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Epitaxial relationship of InN on Si/Ge