Dopants and Defects in InN and InGaN Alloys

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Dopants and Defects in InN and InGaN Alloys

• Wladek Walukiewicz
• Materials Sciences Division,
• Lawrence Berkeley National Laboratory,
• Berkeley, CA 94720

Supported by US DOE and DOD
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Fundamental Band Gap of InN
InN
MBE grown samples show strong PL and
well-resolved absorption edge at about 0.7
eV. Very smooth absorption curve no resonances,
no additional edges at higher energies. No
excitonic enhancement of the absorption edge
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Selected area electron diffraction shows only the
wurtzite structure InN pattern with no evidence
of any secondary phase. Stoichiometric from
Rutherford Backscattering (RBS)
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In1-xGaxN Alloys

• Small bowing parameter in In1-xGaxN b 1.43 eV
• The bandgap of this ternary system ranges from
  the infrared to the ultraviolet region!

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Potential Applications Full Solar Spectrum
Photovoltaic Materials

• The direct energy gap of InGaN spans nearly the
  entire energy range of the solar spectrum.
• Multijunction solar cell based on this single
  ternary system could have higher energy
  efficiencies.
• Advantages of using InGaN
• Flexibility in choosing the number and the
  bandgaps of the constituent junctions to optimize
  the solar cell performance.
• Fabrication process could be greatly simplified
  when only one single alloy system is involved.
• Graded-bandgap solar cells?

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Space Applications Irradiation Effects

Van Allen belts High density of electrons and
protons, up to 108 cm-2s-1 Energies from keV to
hundreds of MeV
Extremely hostile environment
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High Energy Particle Irradiation

Kirtland AFB Dynamitron 1 MeV electrons up to
1x1017 cm-2 LBNL van de Graaf accelerator 2 MeV
protons up to 2x1015 cm-2 2 MeV He up to 2x1015
cm-2 NRL model used to put electron, proton, and
alpha irradiation on same scale Non-ionizing
energy loss (NIEL) and displacement damage dose
(Dd)
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Effect of Irradiation on Photoluminescence
Intensity
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Origin of the Radiation Resistance

InN has electron affinity of 5.7 eV, larger than
any other semiconductor Fermi level
stabilization energy, EFS (average energy of
dangling bonds) is located well above the
conduction band edge Defects located in the
band gap are effective as non- radiative
recombination centers
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Irradition Model
irradiated EF at 0.9 eV above CBM Eglt0.7
as-grown Eg0.7
EF
CB
1.1 eV
CB
0.7
VB
VB
Irradiation with alpha particles creates
N-vacancy defects with an energy level in the
CB Donor-like defects result in conduction band
filling -- all the way up to EFS Extra
conduction electrons provide greater electronic
screening (0.2 eV) and exchange energy
(0.2 eV) resulting in a bandgap narrowing
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Effect of Irradiation on Electron Concentration

Electron concentration saturates when Fermi level
reaches EFS
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Irradiation-Induced Stabilization of EF

with EFEFS
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Electron Mobility

Mobility well explained with Coulomb
scattering by double charged donor defects (VN
?). Onset of resonant scattering for the Fermi
energy approaching the defect energy level
- starting material
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ECV measurementsSurface accumulation layer in InN

• Independent of bulk electron concentration,
  surface electron concentration is mid-1020 cm-3
• Surface accumulation region, 4-8 nm thick
• Surface electron density 2x1013 cm-2
• Consistent with Fermi level pinning at 1 eV
  above the CBM.

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Burstein-Moss Shift in Thin InN
In very thin samples the electron concentration
and Fermi energy are determined by the surface
accumulation with n 4x1020 cm-3 Burstein-Moss
shifted absorption edge at 1.7 eV
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Accumulation Layer in p-type InN
The thin n-type inversion layer on the
surface determines the Hall conductivity
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Evidence for p-type InN Electrochemical C-V

• Type change observed below surface in InN doped
  with Mg
• Surface is n-type but bulk appears to be p-type

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Irradiation-Induced Type Conversion in p-type InN
Irradiation produces donor defects that
compensate Mg acceptors converting the bulk of
the layers from p- to n-type. Provides strong
support for an existence of p-type conducting
layer in the bulk
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Burstein-Moss Shift in Irradiated InN

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Outlook

• Surface electron accumulation
• Surface modification
• chemical treatments
• grazing angle sputtering
• Heterointerface with a large work function
  semiconductor e. g. p-type CuInS2
• Composition graded GaInN p/n junction

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Acknowledgments

• Collaborators
• J. Wu, K. M. Yu, X. Li, R. Jones W. Shan, J. W.
  Ager, E. E. Haller, Z. Liliental-Weber and J.
  Denlinger
• Lawrence Berkeley National Laboratory,
• University of California at Berkeley
• W. Schaff and H. Lu, Cornell University
• A. Cartwright, University of Buffalo
• US Department of Energy
• NRO Director Innovative Initiative

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Unintentional Doping with O and H

Concentrations of O and H smaller than free
electron concentration.
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X-Ray Diffraction

30 40 50 60 70
80
2-theta (0)
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Transmission Electron Microscopy

200 nm
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Optical Absorption and Photoluminescence