Optical properties of IIInitride compounds

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Optical properties of III-nitride compounds

• Bernard Gil
• Groupe dEtude des Semiconducteurs
• Université Montpellier II
• 34095 Montpellier cedx5 - France

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Identification of the bandgap of GaN
Ilegems Dingle (1971)
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High quality heteroepitaxies come into the play
(mid nineties)
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The band gap of GaN and the lattice parameter
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Hypothesis a biaxial strain splits the valence
band states

• The crystal field splitting varies with c/a
• Gil, 1995
• Shikanai, 1995
• Kim 1997

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Treating it more seriously requires text book
quantum mechanics

• One parameter to account of the crystal-field
  splitting
• two parameters to account for the matrix elements
  
• of the spin-orbit interaction
• Four deformation potentials
• 
  
• 
  
  Igt -(px ipy)/2
• 
  
  I-gt (px - ipy)/2

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Evolution of band to band transitions with stress
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Excitonic corrections

• I?5gt I?5gt I?5 gt
• ?1 ?2 ?1 ?2 -1/2? -?
  0
• -? ?1 - ?2 ?1 ?2 -1/2? ?2?3
• 0 ?2?3 ?1 1/2?
• ?exc?5 ?I?5gt ? I?5gt ??5 gt
• ??exc?5 ?2 ( ? ? )2 / 2.

? 0.6 meV in GaN
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It works!
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The excitonic binding energies in GaN films

• Effective masses are anisotropic
• Schrödinger equation should include this
  anisotropy
• The hydrogenic series in 1/n2 is lost

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Orthorhombically strained GaN films in-plane
anisotropy
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Photoluminescence
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Photoluminescence and reflectance
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The excitonic polariton in GaN films
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Neutral-donor bound excitons / Rotational excited
statesCourtesy G.Neu
Almost strain free GaN
Free-standing
(HVPE)
Homoepitaxy
(MOCVD)
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GaN/SiC heterostructures (0001) Courtesy G.Neu

• ? resonant lines are observed over more than 10
  meV
• ?the energy difference between the resonant picks
  and the excitation is no more strictly constant
• a pick b is rapidly shifting
• by extrapolation it corresponds to D0XB

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Rotational excited states of D0XA Courtesy G.Neu
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Donor spectroscopy two electron transitions
Courtesy G.Neu
HVPE grown free-standing layer
2K - the D0X initial state is the ground state
- the final state is D0(n2,..)

• Above 2K
• The initial states are the
• D0X excited states

I2a (2p)
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Donor bound excitons in semiconductors a three
decade-long storyB.Gil et al , to be published
? Ed / 30
Jtot Lrot Jhole
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Wave functions of the rotator and two electron
transitions B.Gil , B. Monemar , to be published

• The angular momentum of the topmost valence band
  Bloch state J is 3/2.
• For the ground state 0,0gt, the total hole wave
  function has the symmetry of the valence band,
  Jtot 3/2. The ground state of the donor bound
  exciton complex has odd parity. When one of the
  electrons and the hole recombine leaving the last
  electron in a nS state, the ground state has even
  parity. This recombination process is dipole
  allowed and efficiently couples with the
  electromagnetic field.

• The first excited state correspond to the first
  excited state of the hole rotator. The total
  wave function of the complex is even and the most
  probable recombination process is a complex
  process leaving the remaining electron in a 2 p
  state.

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Zinc oxyde?
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Acknowledgements

• Olivier Briot,
• Nicolas Grandjean,
• Marc Ilegems,
• Mathieu Leroux,
• Bo Monemar,
• Hadis Morkoç,
• Gérard Neu,
• Shuji Nakamura,
• Sandra Ruffenach,
• Magloire Tchounkeu,
• Andenet Alemu